Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review

SoilUse and Management doi: 10.1111/j.1475-2743.2010.00266.x

Soil Use and Management, June 2010, 26, 94–107

REVIEW ARTICLE

Soil tillage methods to control phosphorus loss and potential side-effects: a Scandinavian review B . U l e´ n 1 , H . A r o n s s o n 1 , M . B e c h m a n n 2 , T . K r o g s t a d 3 , L . Ø y g a r d e n 2 & M . S t e n b e r g 1 1

Department of Soil and Environment, Swedish University of Agricultural Sciences, Box 7014, SE-750 07 Uppsala, Sweden, Bioforsk, Frederik A. Dahls vei 20, 1432 A˚s, Norway, and 3Institute of Environment and Bioscience, University of Life Sciences, Box 5003, 1432 A˚s, Norway 2

Abstract In Scandinavia high losses of soil and particulate-bound phosphorus (PP) have been shown to occur from tine-cultivated and mouldboard-ploughed soils in clay soil areas, especially in relatively warm, wet winters. The omission in the autumn of primary tillage (not ploughing) and the maintenance of a continuous crop cover are generally used to control soil erosion. In Norway, ploughing and shallow cultivation of sloping fields in spring instead of ploughing in autumn have been shown to reduce particle transport by up to 89% on highly erodible soils. Particle erosion from clay soils can be reduced by 79% by direct drilling in spring compared with autumn ploughing. Field experiments in Scandinavia with ploughless tillage of clay loams and clay soils compared to conventional autumn ploughing usually show reductions in total P losses of 10–80% by both surface and subsurface runoff (lateral movements to drains). However, the effects of not ploughing during the autumn on losses of dissolved reactive P (DRP) are frequently negative, since the DRP losses without ploughing compared to conventional ploughing have increased up to fourfold in field experiments. In addition, a comprehensive Norwegian field experiment at a site with high erosion risk has shown that the proportion of DRP compared to total P was twice as high in runoff water after direct drilling compared to ploughing. Therefore, erosion control measures should be further evaluated for fields with an erosion risk since reduction in PP losses may be low and DRP losses still high. Ploughless tillage systems have potential side-effects, including an increased need for pesticides to control weeds [e.g. Elytrigia repens (L.) Desv. ex Nevski] and plant diseases (e.g. Fusarium spp.) harboured by crop residues on the soil surface. Overall, soil tillage systems should be appraised for their positive and negative environmental effects before they are widely used for all types of soil, management practice, climate and landscape.

Keywords: Direct drilling, shallow cultivation, deep ploughing, nutrient leaching, soil erosion, no ploughing

Introduction Transport of phosphorus (P) via surface runoff or tile drainage from arable land to lakes and seas is a major problem in the Scandinavian countries (Rekolainen et al., 1996) and in other parts of the world (Carpenter et al., 1998). In Scandinavia the problem is especially serious in warm and wet winters, which can produce twice the P loads of dry and snowy winters (Puustinen et al., 2007). Development of P risk indices Correspondence: B. Ule´n. E-mail: [email protected] Received February 2010; accepted after revision February 2010

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for the identification of critical source areas (CSA) for P is important for decisions where appropriate countermeasures can be best applied (e.g. Sweden: Djodjic & Bergstro¨m, 2005; Denmark: Andersen et al., 2007; Norway: Bechmann et al., 2007). The concept of a P risk index was first introduced in the US (e.g. Coale, 2000) and has since received great attention in the Scandinavian countries and other parts of the world (e.g. Heckrath et al., 2008). Soil texture, soil structure, soil P chemistry, slope and rate of water flow are important factors used as predictors for a high risk of P loss to water. Phosphorus losses from agricultural soils occur both in particulate (PP) and dissolved reactive form (DRP). The

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Soil tillage methods to control phosphorus

latter can contribute in Scandinavia a widely varying proportion (9–93%) of the total P in agricultural runoff water (Ule´n et al., 2007). Much attention has been paid to reducing erosion, but decreasing the loss of suspended solids (SS) and PP is complex since this depends on the physicochemical processes controlling particle size enrichment, runoff-amount through different pathways and other factors (Sharpley et al., 1992). In addition, decreased runoff from non-ploughed areas may not always result in a reduction in bioavailable P. Knowledge on the availability of eroded P for algae and bacteria is still limited (Ekholm & Krogerus, 2003) and actual availability depends on the recipient water environment. Soil tillage is a major factor contributing to an increased risk of soil erosion and PP losses by water (Lundekvam & Skøien, 1998). Tillage affects P mineralization and mobilization in variable and site-specific ways depending on, for example, the inherent susceptibility of the soil to structural degradation or improvement (Puustinen et al., 2005; Withers et al., 2007). Furthermore, the risk of transport of soil particles and PP loss depends on the slope and soil texture. The risk of soil erosion is highest during the autumn and winter and hence direct drilling or undisturbed stubble, compared to mouldboard ploughing in autumn, is an important option to mitigate P and soil losses in arable cropping. In addition, the presence of crop residues and an intact root system may act as a filter for particles containing P. Ploughless tillage is practised to decrease surface runoff losses of PP in Canada, the US and many European countries (e.g. Baker & Richards, 2002). However, without total topsoil inversion, fertiliser and manure incorporation into the soil is limited. A stratified layer of P builds up on or near the soil surface (Logan et al., 1991), increasing the risk of P release and subsequent transport of DRP via surface runoff (Sharpley & Smith, 1994) and tile drain water (Gaynor & Findlay, 1995). Studies from the northern Mississippi area of the US concluded that even though losses of total P were considerably reduced when soil tillage was omitted, losses of DRP were eightfold higher with no-till compared to conventional ploughing (McDowell & McGregor, 1984). Addiscott & Thomas (2000) strongly recommend interrupting periods of ploughless tillage with conventional ploughing in order to dilute P concentrations in the uppermost part of the soil. They also suggest that soil inversion and mixing of the soil increase adsorption of DRP to soil particles and reduce DRP in horizontal and lateral water movements. An important factor in decreasing P leaching may be the presence of a well-developed cash or catch crop. The amount of P removed annually by catch crops in Scandinavia (15– 50 kg ⁄ ha) is much smaller than the amount of P readily available for the crop in the topsoil (100–500 kg ⁄ ha) (Bergstro¨m et al., 2007). However, the plant cover and improved root development may itself increase soil stability against erosion, though any improvement in crop and root

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development limits the possibilities for soil tillage. The combined effects of crop and soil management are complex and difficult to quantify, since they may be masked by chemical and physical processes in the soil. In addition, in systems with no autumn soil management, there is a contribution of P released from plants (weeds, catch crops, germinated cereals) damaged by frost (e.g. Bechmann et al., 2005), though there is a lack of knowledge regarding the P release between newly germinated fresh plant material and plant regrowth, and how plants translocate P to their roots as they mature. The desired outcome of a change in tillage practices is to have a soil structure that permits the gradual absorption of rain or snowmelt. In contrast, a dense soil or soil layer enhances the risk of surface runoff by reducing the infiltration capacity or the hydraulic conductivity. Furthermore, a dense soil allows rapid channel flow in the soil. Improving soil structure through strengthening soil aggregates and improving water infiltration can thus result in decreased PP losses by decreasing erosion. In addition, improving the soil structure can improve soil–water interaction and thus decrease the DRP concentration in soil water. Severe topsoil compaction can ensue when coarsetextured soils with poor structure are cultivated without ploughing (Davies & Finney, 2002). Moreover, various sandy soils and other structurally unstable soils need to be loosened by regular deep tillage. Clay soils should be ploughed in dry conditions to avoid unnecessary wear of the soil aggregates which might increase particle and P loss. This paper evaluates the effects of various Scandinavian soil tillage practices on losses of PP and DRP via surface runoff and tile drainage (subsurface runoff). The study focuses on loamy clays (including silty loam clays) to clay soils (>30% clay), primarily with erosion problems. Potential side-effects of the various soil tillage practices are also reviewed.

Climate, runoff, soil type, tillage and subsidies for no-till Climate and runoff There are great variations in climatic conditions between the four Scandinavian countries of Denmark, Finland, Norway and Sweden but also within these countries. Precipitation is generally very high on the west coast of Norway, where runoff exceeds 1000 mm ⁄ yr, but is much lower in eastern parts. In Sweden high precipitation may occur on the west coast. These two countries generally have a greater documented range of runoff from arable land (Table 1) than Finland and Denmark. The main runoff periods in Scandinavia are autumn and winter, with the 8-month period of September–April contributing 85% or more of total runoff according to runoff monitoring data from small agricultural catchments.

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Table 1 The dominant soil types classified according to FAO (2006) and based on Rasmussen (1999) and climatic conditions for the vicinities of Copenhagen, Helsinki, Oslo and Stockholm, represented by the USDA hardiness zone* (scale 1–11) with the range for minimum temperature (Min. Temp.), average percentage of days with snow per year (snow days), and annual runoff as average and range for agricultural land in different parts of the country based on small monitored catchments Soil

Climatic conditions

Runoff

Hardiness zone Country

Classification

Zone

Denmark Finland Norway Sweden

Eutric Fluvisols Gleyic Cambisols Stagnic Luvisols Gleyic Cambisols

7b 5 6 7a

Min. Temp. (C) )12.2 )26.1 )20.6 )15.0

to to to to

Snow days (%)

Average (mm)

Range

2 28 30 3

270 290 580 200

90–510 130–500 80–1200 70–1200

)15.5 )28.9 )23.3 )17.0

*Hardiness zone is a geographical area where a category of plants is capable of growing, largely defined by temperature extremes.

Soil type and tillage Although very variable, the soils in central areas of the Scandinavian countries have been classified into three major soil types (Table 1). In south eastern Norway and central areas of Sweden and Finland clay soils dominate, as do erosion problems (Boardman & Poesen, 2006). By tradition, the Gleyic Cambisols and Stagnic Luvisols are ploughed in the autumn with mouldboard ploughs. However, direct drilling and ploughing in spring are applicable to most clay soils in Scandinavia, and many clay loam and silty soils may be

ploughed in spring instead of the autumn. In addition, a number of Scandinavian field experiments have shown that when the early summer is dry, shallow cultivation on clays and clay loams gives substantially higher yields than regular ploughing (e.g. Aura, 1999). Cropping systems without ploughing are receiving great attention in Scandinavia and world-wide both for economic reasons, including possible reduction in labour and energy consumption, and for soil improvements (Holland, 2004). A soil tillage system with direct drilling or shallow cultivation (Table 2) excludes inversion of the whole topsoil.

Table 2 Soil tillage systems in Scandinavia, with corresponding American definitions in italics according to ASAE (2006), impact down to soil depth (Depth), commonly used equipment and time of practice: early autumn (e au) after harvest or before winter drilling; late autumn (l au); and spring (sp). Tillage sequence is a–c Tillage system

Depth (cm)

Tillage systems without ploughing Direct drilling ⁄ conservative tillagea (a) Crop sown in a single operation 5–10 Mulching effect from crop residues Shallow cultivation instead of ploughing ⁄ reduced tillageb (a) Shallow cultivation 5–10 (b) Harrowing 3–5 Mulching effect from crop residues Deep cultivation (rarely practised) (a) Deep cultivation 10–15 (b) Harrowing 3–5 Tillage systems with ploughing Conventional tillage (a) Shallow cultivation 5–10 (b) Conventional ploughing 18–23 (c) Harrowing 3–5 Tillage systems with deep soil inversion (rarely practised) (a) Shallow cultivation 5–10 (b) Deep ploughing 23–28 (c) Harrowing 3–5 a

Commonly used equipment

Time

Direct-drill, commonly with cultivator in front

e au ⁄ sp

Cultivator, disc harrow, rotovator Harrow

e au ⁄ l au ⁄ sp e au ⁄ l au ⁄ sp

Deep cultivator Harrow

e au ⁄ sp e au ⁄ sp

Cultivator, disc harrow, rotovator Mouldboard plough Harrow

e au e au ⁄ l au ⁄ sp e au ⁄ l au ⁄ sp

Cultivator, disc harrow, rotovator Mouldboard plough Harrow

e au e au ⁄ l au ⁄ sp e au ⁄ l au ⁄ sp

More than 30% of crop residues are left on the soil surface after planting. bMore than 15% of crop residues cover the soil surface year round.

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Soil tillage methods to control phosphorus

Such methods have a mulching effect, since crop residues are left on the soil surface or in the uppermost soil layer. The term mulching can also refer to covering the soil surface with material imported from beyond the field, e.g. straw and leaves, as commonly used in horticulture. In Scandinavia, direct drilling is most commonly used in early autumn for winter crops, especially when establishing winter wheat (Triticum sp. L.), after oilseed rape (Brassica sp. L.) or other dicotyledonous crops (Myrbeck & Stenberg, 2008). In several areas in central and northern Scandinavian, clay loam soils suitable for spring ploughing are still ploughed in autumn, partly as a result of tradition, partly to control weeds and pests and partly due to lack of time for ploughing in the short spring. In contrast, light-textured soils in Denmark and southern Sweden are commonly spring-ploughed in combination with a catch crop with the capability for vigorous growth during the autumn, e.g. undersown perennial ryegrass (Lolium perenne L.) or fodder radish (Raphanus sativus L.) to reduce nitrate leaching.

Subsidies for tillage practices other than conventional ploughing in autumn Norway is the only Scandinavian country with widespread subsidies for not ploughing in autumn in order to reduce erosion and consequently has good statistics on this soil management approach. Several studies have shown reduced erosion, soil loss and P loss by avoiding autumn tillage (Øygarden, 2000; Lundekvam, 1997). Government subsidies are awarded in relation to erosion risk for every field or area based on an erosion risk map (Lundekvam et al., 2003). As a result conventional mouldboard ploughing in autumn has shifted to mouldboard ploughing in spring in many areas. In 2006, 43.3% of the area cultivated with cereals in Norway was ploughed in autumn. In the same year, 4.6% of the area was only shallow harrowed in autumn, usually as preparation for sowing winter wheat (Triticum sp. L.). No autumn soil management was carried out on 50.8% of the arable area of Norway (Øygarden et al., 2008). In Norwegian agricultural catchments with serious water eutrophication problems, special restrictions have been placed on farmers. They receive no subsidies for production unless they avoid autumn ploughing in any field with erosion risk class 3 or 4. This has led to changes in tillage practices for 59–65% of the cerealgrowing areas in two catchments with P erosion problems. On the west coast of Norway, subsidies are available for deep ploughing in autumn in areas with potato cropping, since heavy potato harvesting machinery often creates a plough pan which causes a high risk of erosion. Soils in Denmark and southern Sweden are often loamy sands or sands, and spring ploughing in these areas is subsidized in order to reduce the risk of N leaching. Several Danish and Swedish studies have shown that this is one of the most important ways of controlling N leaching on soils

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with a clay content below 20% (Wallgren & Linde´n, 1994; Djurhuus & Olsen, 1997; Møller Hansen & Djurhuus, 1997).

Phosphorus erosion, release and losses based on field experiments and the ERONOR model This review of different tillage practices and the resulting P losses is based on recent results from three experimental sites in Finland, seven in Norway and three in Sweden. From Denmark no recent results are available. One of the sites in Finland was an experimental field with measurements of surface and subsurface runoff in the south of the country, the results from which were statistically analyzed with the Wilcoxon ranked test (Koskiaho et al., 2002). At the second site, four experimental plots in SW Finland, also including both surface and subsurface runoff, were evaluated using ANOVA (Uusitalo et al., 2007). At the third site, plough layer P runoff from 12 experimental plots close to Helsinki was evaluated using the Wilcoxon ranked test (Puustinen et al., 2005) while that from two small agricultural catchments was evaluated using univariate analysis (Puustinen et al., 2007). The results from Sweden are based on studies of surface runoff from 15 plots in SW Sweden (Ule´n, 1997), nine plots in central and western Sweden (Ule´n & Kalisky, 2005), and 14 subsurface experimental fields in SW Sweden using ANOVA (Stenberg et al., 2009). For six of the Norwegian sites (Table 3), a total of 45 plots representing different tillage systems and soils were used. Surface runoff was used to evaluate erosion from sloping land (on average 13%) (Lundekvam & Skøien, 1998). Most of these sites are very sensitive to erosion and only total phosphorus (TOTP) losses are recorded. The TOTP concentration in the runoff water has been demonstrated using regression analysis to be closely related to the concentration of SS in water and to the P concentration in the surface soil (R2 = 0.96%) (Lundekvam, 2002). One Norwegian long-term study consisted of two sites comparing different soil management methods for winter wheat (Grønsten et al., 2007a,b,c). On sloping soils (12–13%) both PP and DRP were reduced by shallow cultivation compared to ploughing before sowing winter wheat (Table 4). Twice as high DRP concentrations were recorded from the water without ploughing compared to conventional ploughing. The empirical and dynamic ERONOR model has been developed based on the long-term plot studies of soil erosion in Norway (Lundekvam, 2002). This model is a national modification of the Universal Soil Loss Equation (USLE), with hydrological factors adapted for the region and with parameterisation for soil erodibility, slope length and slope steepness, together with a relative cropping factor. Model simulations have revealed very clear effects of different soil tillage methods (Figure 1). According to model estimations and plot studies, omitting conventional ploughing in autumn has the potential to reduce soil erosion

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B. Ule´n et al.

Table 3 Experimental field results on soil tillage as a measure to reduce losses of dissolved reactive phosphorus (DRP) and particulate-bound phosphorus (PP) from clay soils (>30% clay) or clay loam soils with surface runoff or subsurface lateral movements to drains Clay

Clay loam

Clay and clay loamy

Surface or subsurface loss relative to conventional ploughing Option Tillage without ploughing Direct drilling in autumn Direct drilling in spring Deep cultivation in autumn Shallow cultivation, early autumn Shallow cultivation, late autumn Shallow cultivation, spring Tillage with ploughing Ploughing early autumn Ploughing in spring Contour ploughing Avoiding soil compaction* Deep ploughing

DRP

PP

1.9d

++ + + 0.5d

1.7d

1.2d

0.7e 1.4e 1.8g 1.1g ++ +++ ++

0.9e 0.9e 0.7g 0.9g 0.2b ++ +

DRP

PP

3.5c 4.0f 1.4c 1.0f 1.3c 0.5f 4.4a 1.9a 1.4g 1.0g

0.3c 0.4f 1.0c 0.8f 0.6b 0.8c 0.9f 0.4a 0.9a 1.3g 0.9g

+

0.2b

1.2c 1.1f 1.0c +++ ++

0.7b 0.6b 0.5c 0.5f 0.8c ++ +

Surface runoff

Subsurface runoff

DRP ⁄ TOTP (%)

64c 53f 18c 14f 20c 7f 48a 14g

33d 16a 11g 19d 27e

37c 22e 23f 26g 17c

19e 16g

DRP, dissolved reactive P; PP, particulate-bound phosphorus; TOTP, total phosphorus. Results from clay are expressed as relative values where P losses from conventional ploughing in mid-autumn are set to 1. Positive effects (reduced losses) without measured values are indicated by +, ++ or +++. Ratio of DRP to total phosphorus (DRP ⁄ TOTP) (%) is given for each field experiment. *Refers to ploughing system. Values based on aKoskiaho et al. (2002) (PP is presented as the difference between TOTP and DRP); bLundekvam (1993) (all TOTP values estimated to be PP); cPuustinen et al. (2005) (runoff above plough pan is here presented as surface runoff); dStenberg et al. (2009) and unpublished (all TOTP-DRP values is estimated to be PP); eUle´n (1997) (short slopes); fUle´n & Kalisky (2005) gUusitalo et al. (2007).

and losses of PP from clay loams and clay soils in Scandinavia. A reduction was observed in 21 of 24 observations from experiments representing different types of tillage, soil and runoff. However, in at least 15 of these cases DRP losses increased. These were the sites with the greatest slope and were most prone to erosion. The positive or negative effect of every specific tillage practice thus needs to be further discussed.

Efficiency of specific soil tillage practices in controlling phosphorus erosion and leaching Soil tillage without ploughing: direct drilling Definition. In this context, direct drilling means that the crop is sown in a single operation with or without shallow cultivation by separate tines or discs in front of the drill tines.

Positive effects Several soil properties are improved by direct drilling under Scandinavian conditions. Soil structure, including soil aggregation, is preserved, which enables good infiltration and percolation of water (Eich & Børresen, 1997). More stable soil aggregates with low erodibility can develop in the

absence of tillage (Rasmussen, 1999). Direct drilling of soils lowers soil susceptibility to surface sealing and provides a better surface for tractor traffic due to the more consolidated and uniform soil structure (Rasmussen, 1999). A soil surface cover with crop residues can increase soil moisture retention and water storage capacity (Puustinen et al., 2005; Schjønning & Thomsen, 2006), resulting in more uniform water infiltration into the topsoil. If traffic is well planned and avoided during unfavourable conditions, direct drilling results in less compaction in the soil profile (Ekenberg & Riley, 1997; Chamen et al., 2003). In such cases, development of a plough pan is prevented and there may also be more biological activity due to an increase in organic matter in the uppermost soil layer and promotion of species such as earthworms (Rasmussen, 1999). In a 9-yr direct drilling field experiment in Finland, crop cover during winter decreased PP losses in plough layer runoff by 70% compared with autumn ploughing and establishment of winter wheat (Triticum sp. L.) (Puustinen et al., 2005). In addition to this direct effect on PP erosion, direct-drilled winter wheat has been demonstrated to have deeper roots in regions with a cool autumn climate, which might increase the resistance to erosion (Qin et al., 2004). A Norwegian study on two sloping sites (12–13%) (Grønsten et al., 2007a,b) showed a reduced or equal concentration of

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Soil tillage methods to control phosphorus

Table 4 Mean concentrations (2002–2007) of suspended solids (SS), total phosphorus (TOTP) dissolved reactive phosphorus (DRP) and the ratio between DRP ⁄ TOTP for three soil management strategies before sowing of winter wheat at a site with moderate and a site with high erosion risk in Norway (Grønsten et al., 2007a) SS

TOTP

DRP

(mg ⁄ L) Moderate erosion risk (1) Direct drilling (2) Shallow cultivation (3) Ploughing High erosion risk (1) Direct drilling (2) Shallow cultivation (3) Ploughing

DRP ⁄ TOTP

250 420 630

0.55 0.80 1.31

0.38 0.50 0.85

0.7 0.6 0.7

330 1800 2000

1.05 1.39 2.37

0.64 0.82 0.74

0.6 0.6 0.3

DRP, dissolved reactive P; SS, suspended solids; TOTP, total phosphorus.

Subsurface runoff Surface runoff

la

re tu Pa s

illi ct

ire

til

D

ge

in

dr

sp

tu au in

ng

g

m

rin

n

ng hi ug ge la al

lo w

til Sh

al lo w

Sh

W in

te

rw

he

at

Au t

af

um

te

n

rp

pl

lo

ou

gh

Po t

in

at

g

o

1600 1400 1200 1000 800 600 400 200 0

Figure 1 Computer-simulated annual total phosphorus (TOTP) losses (g ⁄ ha) (ERONOR model) in surface and subsurface runoff for different soil management systems (H. Lundekvam, pers. comm. 2007) for a clay loam with 3% slope in southern Norway with a yearly precipitation of ca. 900 mm. Surface runoff was estimated at 156 mm and subsurface runoff at 350 mm.

soil particles, total P and DRP in runoff from winter wheat with direct drilling compared with ploughing before sowing (Table 4). Furthermore, in long-term studies in Norwegian conditions, direct drilling in early autumn has been shown to improve soil cover by straw and reduce erosion and PP losses by up to 85% compared with autumn ploughing, while direct drilling in spring reduced particle erosion from clay soils by up to 79% compared with autumn ploughing (Lundekvam,

2002). Overall, particle losses in Norway have been estimated to be reduced by 70–90% with direct drilling compared with conventional autumn tillage in many cases (Lundekvam, 1993; Lundekvam & Skøien, 1998; Øygarden et al., 2006). On the basis of long-term studies on sloping land, changes in tillage have been estimated to be highly effective in reducing soil erosion, especially on artificially levelled areas (with topsoil and subsoil redistributed in order to reduce the unevenness of agricultural land) and other erosion-sensitive soils (Table 5).

Negative effects Although bulk density is often higher in systems with direct drilling (Schjønning et al., 1995), soil pores in no-till systems seem to be more effective in transmitting water than those in ploughed soil due to formation of a larger number of continuous macropores (Shipitalo et al., 2000). This implies an increased risk of P transport through soil macropores in no-till systems. With the Scandinavian climate, a soil that is covered with crop residues is generally cool during spring, which has been demonstrated to decrease cereal yields in some years under plough-less tillage (Riley et al., 2005). Uneven or late seedbed preparation, especially by an untrained operator, can also cause variable germination and reduced yields (Ule´n & Kalisky, 2005). Cropping systems with no ploughing are less effective in reducing weeds than conventional tillage. Couchgrass [Elytrigia repens (L.) Desv. ex Nevski] tends to increase under no-till (Chandler et al., 1994; Henriksen, 2006), and this increases the need for herbicides such as glyphosate, Table 5 Cropping factors comparing erosion of particles for five tillage systems on three types of soil (12–13% slope) under Norwegian conditions (Lundekvam, 2007). The different systems are expressed as relative values where ploughing in autumn and harrowing in spring are set to 1. The factors are based on long-term measurements in plots with levelled silty clay loams of extremely high erodibility, clay soils of high erodibility and loam soils of low erodibility

Tillage systems without ploughing (1) Direct drilling in spring (2) Shallow cultivation in autumn and spring (3) Permanent grassland Tillage systems with ploughing (4) Ploughing and shallow cultivation in spring (5) Cultivating across slopes (6) Ploughing in autumn and harrowing in spring

Levelled silty clay loam

Clay

– 0.47

0.19 0.21

– –

0.05

–

0.18

0.11

–

0.59

0.70 1

– 1

– 1

Loam

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100 B. Ule´n et al. which quickly kills off the weeds or any catch crops, causing risk of release of P from the plant material. In two direct-sown field experiments on silty soil, one in western Sweden and one in SE Norway, frequent weed treatments with glyphosate increased losses of DRP via surface runoff, with the DRP probably released from the plant material (Ule´n & Kalisky, 2005; Bakkega˚rd et al., 2007). In the latter 8-yr study, total DRP losses were 1.3 kg ⁄ ha from the direct-drilled plot compared with 0.3 kg ⁄ ha from three plots with conventional ploughing or deep cultivation. However, PP losses were lower (1.1 kg ⁄ ha) than those from plots that were ploughed or deep cultivated in autumn (2.1 kg ⁄ ha). In tillage systems without autumn ploughing or deep cultivation, the straw and stubble on the soil surface pose a risk of re-infection by plant diseases and pests surviving from 1 yr to the next. The plant disease leaf net blotch (Pyrenophoros teres Drechsl. ex Fries) is one such problem (Stenrød et al., 2007). Fungal diseases, e.g. varieties of Fusarium, in cereal crops are a particular problem. These diseases produce mycotoxins in harvested grain and can be a serious problem in direct-drilled systems. Outside Scandinavia, non-inversion of the total topsoil has been demonstrated to result in higher soil moisture and higher N2O emissions compared with ploughing (Robertson et al., 2000; Six et al., 2004). However, the latter have been estimated to only increase by 17–40 kg CO2 equivalents ⁄ ha, or 3–7% of N2O emissions from conventional ploughing.

Conclusion 1 Direct drilling has great potential for reducing soil erosion and losses of PP from unstable, erodible clay loams and clay soils, but poses an increased risk of DRP loss since more plant material is left on the soil surface which may release P since this accumulates in the topsoil. Additionally, DRP loss may increase because of increased surface runoff, while the concentration of DRP in surface runoff does not increase in the no-till systems. Dissolved reactive P has a higher ecological impact than PP due to its higher bioavailability and therefore this countermeasure should preferable be allocated to erosion-prone sites as identified using the CSA concept. With direct drilling, the risk of infestation by pests and weeds and the need for chemical control must be considered. Scientific evidence about the possible effects of increased use and leaching of herbicides on aquatic wildlife remains inconclusive.

Soil tillage without ploughing: shallow cultivation Definition. Shallow cultivation, which is also referred to as reduced tillage (Table 2), involves no inversion of the total topsoil soil layer. Tillage is to a depth of 5–10 cm and leaves the soil covered with some crop residues.

Positive effects Shallow cultivation compared to mouldboard ploughing can decrease soil susceptibility to surface sealing. The conditions for uniform hydraulic conductivity are also improved, as a result of plough pan development being avoided (Batey, 2009). Fifteen-year studies in Sweden have also revealed less detrimental compaction in the long-term (Rydberg, 1992). Improved water storage and a reduction in water losses by evaporation may follow (Aura, 1999). Increased biological activity including number of worms and improved soil structure due to an increased amount of organic matter in the surface layers are other benefits (Rasmussen, 1999). However, the potential for reduced sediment and PP losses in systems with shallow cultivation is lower than for direct drilling, where soil disturbance by tillage is minimized. Slightly reduced N leaching has been reported after shallow cultivation in early autumn compared with conventional ploughing. For example, leaching via subsurface runoff has been reported to be reduced by 0.2 kg N ⁄ ha ⁄ yr from a clay loam (Koskiaho et al., 2002).

Negative effects Under wet autumn conditions, use of a cultivator on clay soil can make the soil cloddy, resulting in soil compaction with associated soil structural problems, higher runoff volumes, soil erosion, PP losses and ultimately reduced crop yields (Børresen & Njøs, 1993). Late autumn ploughing has also been reported to increase PP losses by 40% compared with early autumn ploughing (Lundekvam, 1993). Plant cover on the soil surface by shallow tillage potentially reduces the risk of sediment and PP losses compared with autumn ploughing (Lundekvam, 1997). Soil P status and soil chemistry may influence the effect of shallow tillage on DRP losses. Accordingly, a 5-yr study on a clay soil in southern Finland reported four times higher DRP losses via surface runoff under shallow cultivation compared with conventional tillage (Koskiaho et al., 2002). This was explained by P accumulation in the non-inverted topsoil. In another Finnish study, DRP release from senescent plant tissues was mentioned (Puustinen et al., 2005). In a Swedish lysimeter study, a similar finding for soils under shallow cultivation was explained by macropores in the topsoil being formed by repeated freeze ⁄ thaw cycles with the resultant fracturing of soil aggregates and hence release of P (Djodjic et al., 2002). In that study, ploughing was no better at reducing stratification and shallow macropore formation. Moreover, a 5-yr Swedish study on a clay soil showed large variations in P losses between years, but no significant differences between shallow cultivation and mouldboard ploughing in September (Aronsson et al., 2006). In a 2-yr study on a

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Soil tillage methods to control phosphorus 101

heavy clay soil in central Sweden, PP losses were much higher after conventional autumn ploughing than after shallow cultivation in autumn with discs and tines during the second year. The DRP losses under conventional autumn ploughing were much lower than the PP losses (Figure 2). The major part of TOTP was in particulate bound form. In the same study, more DRP moved laterally to the tile drains after shallow cultivation (November 2006 and December 2007) compared to before soil management.

Conclusion 2 Based on the field experiments reviewed here (Table 3), it can be concluded that shallow cultivation is an important measure in reducing PP losses in CSA areas with high erosion risks. However, some of these studies show that losses of DRP may increase in such tillage systems, especially via surface runoff. Omitting autumn ploughing is therefore appropriate on soils where structure problems have already been alleviated and where the erosion risk is high.

(a) DRP (kg/ha) 0.06 0.05

Cultivated Ploughed

0.04 0.03 0.02

20

06 20 -07 06 -01 -0 20 9-0 06 1 20 -11 07 -01 20 0107 01 20 0307 01 20 0507 01 20 0707 01 20 0907 01 20 1108 01 20 -01 08 -01 20 0308 01 -0 501

0.01 0.00

(b) PP (kg/ha) 0.40 0.30

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0.20 0.10

10 -0 1 20 107 01 20 0307 01 20 0507 01 20 0707 01 20 0907 01 20 1108 01 20 -01 08 -01 20 0308 01 -0 501

20

07

-1

01 20

06

9-0

06

20

20

06

-0

7-

01

0.00

Figure 2 Losses of (a) dissolved reactive phosphorus (DRP) and (b) particulate-bound phosphorus (PP) subsurface runoff (lateral movements to tile drains) from a clay (40%) soil in central Sweden in 2006–2008 with two treatments without replicates. One plot was shallow-cultivated, while the other was ploughed. Different scales are used for sub-surface losses of DRP and PP.

Soil tillage with ploughing: spring ploughing instead of autumn ploughing Definition. The soil is ploughed in the spring and the whole topsoil is inverted by mouldboard ploughing.

Positive effects By delaying ploughing until the spring the bare soil is not exposed over the winter and stubble and weeds provide some surface cover. Roots and fungal hyphae can enmesh soil particles while realigning them and releasing organic compounds that hold the particles together (Bronick & Lal, 2004). Therefore, when the roots of the harvested crop are left intact, they better hold the soil together and reduce dispersion of soil particles. Ploughing usually increases soil macroporosity in the topsoil. With autumn ploughing, more macropores may be present in the topsoil in winter and vigorous preferential flow down through the soil profile may occur (Petersen et al., 1997). This effect is avoided when ploughing is delayed until spring. In studies in SE Norway, delaying ploughing until spring has been shown to greatly reduce P losses from high to medium erosion risk arable fields (50–75%), whereas changes in soil tillage on low erosion risk fields may have varying effects on soil erosion and P loss (Lundekvam, 1997). Results from a silty soil in western Sweden showed a 50% reduction in PP losses when this soil was tilled in spring compared with the autumn (Ule´n & Kalisky, 2005). These findings are in agreement with observations from a clay loam soil in southern Norway (Lundekvam & Skøien, 1998). In Norway ploughing and shallow cultivation of sloping fields in spring instead of ploughing in the autumn has been shown to reduce particle transport by up to 89% on highly erodible soils (Table 3). In another Norwegian study Stenrød et al. (2007) found that particle losses from a loam soil through surface runoff and leaching were substantially reduced when the soil was ploughed in spring instead of being rotovated at 10 cm depth in the autumn. Figure 3 shows the impact of any autumn tillage on a heavy clay soil compared with no cultivation or growing winter wheat over 3 yr on a farm in SW Sweden (Nylinder et al., 2009; Stenberg et al., 2009). In this study, ploughing took place very late in December after the very wet autumn of 2006. Significant differences (P < 0.05) in flow-weighted PP concentrations between different soil covers were found in several autumn and winter periods in the last two wet winters, with lower PP concentrations from soils with more plant cover. In contrast, no significant differences were found for DRP. By leaving the stubble undisturbed by direct drilling in the spring there was a significant reduction in PP loss via tile drains by an average of 42% over 3 yr. In addition, preparation for sowing by cultivation and harrowing reduced by 9% PP losses the following winter

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102 B. Ule´n et al.

Ploughed Winter wheat Uncultivated

DRP (mg/l) 0.30

PP (mg/l) Ploughed Winter wheat Uncultivated

2.0 1.5

0.20

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0.5 0.0

0.00 s o n d j

f m a m j

s o n d j

2005/2006 DRP (mg/l) 0.30

f m a m j

2005/2006

Ploughed Winter wheat Uncultivated

0.20

PP (mg/l)

Ploughed Winter wheat Uncultivated

2.0 1.5 1.0

0.10

0.5 0.0

0.00 s o n d j f m a m j 2006/2007 DRP (mg/l) 0.30

Ploughed Winter wheat Uncultivated

s o n d j f m a m j 2006/2007 PP (mg/l) 2.0

Ploughed Winter wheat Uncultivated

1.5

0.20

1.0 0.10

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0.00

0.0 s o n d j f m a m j 2007/2008

s o n d j f m a m j 2007/2008

by growing winter wheat compared with conventional ploughing and leaving the soil bare over winter. In Scandinavia, the practice of avoiding tillage during autumn is regarded as an important measure against N leaching. Ploughing in early autumn results in increased microbial activity and N mineralization which constitute a significant risk of elevated N leaching. Studies on sandy soils in Denmark and southern Sweden have demonstrated that N leaching is reduced when spring ploughing is practised instead of autumn ploughing (Djurhuus & Olsen, 1997; Stenberg et al., 1999).

Negative effects The time available for spring ploughing is usually short in Scandinavia and in some years the spring is wet. Ploughing, or even discing in a wet spring can damage soil structure for many years (Riley et al., 2005). The P in above-soil crop residues and senescent tissues of weeds may be frozen to affect potential increases in P loss from spring-ploughed compared to autumn-ploughed soil. This may give rise to

Figure 3 Flow-weighted concentrations of dissolved reactive phosphorus (DRP) and particulate-bound phosphorus (PP) in tile drain water from 16 drained plots with ploughed or shallow cultivated soil left bare for the winter, winter wheat (after cultivation and harrowing), unploughed soil or uncultivated soil for the winter. Different scales are used for sub-surface losses of DRP and PP. Significantly (P < 0.05) different PP concentrations were observed for December 2005 to January 2006 and December 2007 to January 2008.

enhanced DRP losses, as demonstrated in surface runoff in SW Norway and SW Sweden (Børresen & Njøs, 1993; Ule´n, 1997) (Table 3). Since surface runoff may increase under spring tillage, the concentrations of DRP may still be lower for spring tillage compared to autumn tillage.

Conclusion 3 Spring rather than autumn ploughing for a spring crop is an important method for reducing erosion and P loss from sloping soils. In many cases concentrations of PP and DRP are reduced by spring ploughing compared to the autumn. However, increased surface runoff sometimes causes increased losses of DRP from spring ploughed soils. The method should be applied to many clay loams and other clayey fine-textured soils since it does not pose a risk to soil compaction and soil structural damage. Increased DRP losses have been demonstrated in some field experiments and this negative effect has to be combined with the positive effect of reduced PP, depending on the area of receipt. In addition, the combination of spring ploughing and cultivation of catch

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Soil tillage methods to control phosphorus 103

crops has not been thoroughly evaluated with respect to DRP loss from a range of soil types.

Soil tillage with ploughing: contour ploughing Definition. Contour ploughing means ploughing, cultivating and seed drilling perpendicular to slopes and along contours.

Positive effects Contour ploughing increases water storage in soil furrows to thus reduce surface runoff. The ponding that can occur allows greater time for water infiltration and potentially reduces surface runoff (Sibbesen et al., 1994). With contour ploughing there are no downslope-orientated furrows which easily collect water and develop concentrated surface flow paths. Outside Scandinavia, for example in the UK, positive trends have been noted for reduced PP losses with contour ploughing but in practice effects have been very variable (Stevens et al., 2009). In Norway, Lundekvam (1997) estimated a 50% reduction in soil erosion for ploughing across the slope compared with downslope ploughing. In another plot study on different tillage methods, ploughing across the slope reduced soil losses by 50% compared with ploughing up and down the slope (Grønsten et al., 2007c).

Negative effects During prolonged rainfall, breakthrough flow may occur even with contour ploughing. For steeply sloping fields with complex slope patterns, attempts at cultivation across the slope often lead to channelling of runoff. Runoff along tramlines has been demonstrated to cause severe rill erosion in the UK (Stevens et al., 2009). In Denmark, contour ploughing has been reported to be unsuccessful due to such channelling of water (Schjønning et al., 1995).

Conclusion 4 In Scandinavia, contour ploughing is applicable to CSA areas with a high risk of surface runoff and erosion. The importance of rill erosion needs to be stressed. Eroded particles in surface runoff may be filtered through grass and allowed to settle in depressions in fields with undulating topography. Therefore, grass-covered waterways in combination with contour ploughing are desirable.

Measures to avoid soil compaction Definition. We recognize two types of soil compaction, topsoil compaction and subsoil compaction. The latter includes compaction of both the plough pan and any unloosened subsoil.

Negative effects of soil compaction Both topsoil and subsoil compaction reduce infiltration which can result in fast channel flow and severe erosion (e.g. Fleige & Horn, 2000). High PP losses may accompany mobilization of P-enriched soil particles. Topsoil compaction can also be associated with a surface crust, further reducing the opportunities for water interception (Ule´n & Kalisky, 2005). On occasion topsoil compaction can occasionally be followed by high PP erosion. In contrast, subsoil compaction can lead to more permanent problems with PP erosion, since such compaction has been found to be long-term and difficult to correct (Alakukku et al., 2003).

Avoiding soil compaction Appropriate soil moisture conditions during tillage operations are important in avoiding soil compaction (Chamen et al., 2003). In particular, ploughing (or even discing) in spring or in late autumn under wet conditions can damage clay soil structure and reduce water infiltration, thereby increasing surface runoff and the risk of P losses (Withers et al., 2007). More or less horizontal flow above the plough pan may also occur, with high PP losses (Lundekvam, 2007). Ploughing in late autumn or in spring instead of early autumn is considered an important measure against N leaching and is commonly practised in Scandinavia, often in combination with catch crops. Thus for clay soils, this measure must be carried out during appropriate soil moisture conditions in order not to damage soil structure and not to increase P loss. Soil compaction can be avoided by ploughing and cultivating the soil under dry conditions, avoiding traffic with heavy machinery and limiting movement to pre-determined tramlines. In addition, larger tyres with reduced pressure can be used to avoid unnecessary compaction (Alakukku et al., 2003). Soil compaction is also lessened by avoiding the use of heavy rollers (tilth packers) as has been demonstrated in southern Sweden (Na¨tterlund, 2007). Avoiding both topsoil and subsoil compaction is an important strategy for maintaining a satisfactory rate of infiltration. In addition, a lack of soil compaction ensures crop development and crop P uptake (Arvidsson, 1999), to thus foster root development and reduction in erosion, PP loss and consequent DRP leaching.

Conclusion 5 In Scandinavia, tillage to avoid soil compaction is possibly a very important measure in reducing PP losses and to some extent DRP losses, both via surface and subsurface flow from fine-textured soil. However, the effects are hard to quantify and can only be postulated on a conceptual basis.

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Deep ploughing ⁄ subsoiling and other countermeasures against subsoil compaction Definition. Deep ploughing is soil inversion to a deeper depth (23–28 cm) than conventional ploughing. Subsoiling means inversion to an even deeper depth of 30–45 cm.

Positive effects When the plough pan is broken by deep ploughing or subsoiling, any macropores present can no longer act as continuous vertical channels for preferential flow and associated PP loss down the soil profile (Withers et al., 2007).

Duration of effect and complementary countermeasures After the plough pan is broken up by deep ploughing, the soil usually becomes re-compacted rather soon and therefore deep ploughing can be combined with liming in order to prolong the effect (Westlin, 2007). The Ca supplied via liming combines with soil organic matter and results in the formation of Ca-humus complexes which significantly improve aggregate stability and soil structure (Alakukku & Aura, 2006). In addition, topsoil and subsoil conditions can be improved by planting a soil-loosening catch crop. Deep-rooting crops such as oil radish (Raphanus sativus L.) are commonly cultivated in southern Sweden, but in trials the most successful deeprooting crops have been shown to be lucerne (Medicago sativa L.) and chicory (Chichorium intybus L.) (Lo¨fkvist, 2005). However, this biological loosening effect may not be sufficient to alleviate severe plough pan compaction if the compacted layer exceeds 6–8 cm in thickness (Birka´s et al., 2004).

Conclusion 6 Deep ploughing should be complemented with other options in order to break up any compacted subsoil. Such combined measures are possibly highly relevant for reducing PP and DRP losses from compacted Scandinavian clay soils, but have not been quantified.

General discussion and conclusion A general conclusion about P losses that can be drawn from the Scandinavian studies is that the positive factors influencing crop growth such as good crop establishment, efficient plant nutrient uptake and well developed root systems which stabilize soil have received less attention than factors related to different tillage methods and timing. Omitting ploughing significantly reduces PP losses from CSAs and sloping soils. The positive effects may partly be short-term for the winter, but continuous use of different tillage practices can alter soil structure. Such structural effects have to be evaluated for most of the measures discussed here. In addition, it is important to

take into account the effectiveness of overall P control and not just where soil erosion occurs, since P losses are not necessarily associated with erosion and may also be associated with, for example manure application. Large P losses can occur from manured soils even during periods with rainfall of low intensity and low associated erosion loss, and therefore the concepts of a P risk index and CSA should be adopted. Unfavourable weather conditions, especially a wet autumn followed by a mild winter, can also counteract P load decreases as achieved by changes in tillage practice. Furthermore, improved tillage should be combined with efficient sub-surface drainage to minimize surface runoff. Omitting ploughing may, as demonstrated, increase DRP losses in different parts of Scandinavia compared to conventional soil management (Table 3). The DRP concentration in runoff depends partly on the release of P as DRP from soil particles as they erode. Soil particles with a high soil P content may release P in water and reduce erosion with particle loss also reducing DRP losses. This form of P constitutes a large proportion of the total loss from some of the tillage systems and should be of special concern since this P form is more or less totally bioavailable and poses the greatest risk of eutrophication. Tillage without soil inversion has been shown to have very good effects in reducing P losses from most sloping clay loam and clay soils in Scandinavia, especially on areas of high erosion risk as in Norway. However, tillage systems should be carefully evaluated for the following adverse effects: 1. Increases in crop residues left on or near the soil, accumulated surplus P fertilizer and accumulation of organic matter pose a clear risk of P release and an increased proportion of DRP in surface and subsurface water. 2. Tillage without ploughing may result in decreased crop yields and more weeds. 3. Tillage without ploughing may increase the risk of Fusarium infection which may result from an increased use of pesticides and resultant leaching.

Acknowledgements We thank the EU for funding COST Action 869 ‘Mitigation options for nutrient reduction in surface water and groundwater’ and for making collaboration and exchanges of ideas possible through this network.

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