Elsevier

Agriculture, Ecosystems & Environment

Volume 237, 16 January 2017, Pages 181-193
Agriculture, Ecosystems & Environment

Assessing the farm-scale impacts of cover crops and non-inversion tillage regimes on nutrient losses from an arable catchment

https://doi.org/10.1016/j.agee.2016.12.034Get rights and content

Highlights

  • A three-year farm-scale cover crop and non-inversion tillage trial was conducted.

  • Oilseed radish cover crop reduced soil water nitrate concentrations by 75–97%.

  • Corresponding reductions in riverine nitrate concentrations were not observed.

  • Non-inversion tillage alone was ineffective at reducing nutrient leaching losses.

  • Mitigation trial profit margins were comparable with conventional farm practice.

Abstract

The efficacy of cover crops and non-inversion tillage regimes at minimising farm-scale nutrient losses were assessed across a large, commercial arable farm in Norfolk, UK. The trial area, covering 143 ha, was split into three blocks: winter fallow with mouldboard ploughing (Block J); shallow non-inversion tillage with a winter oilseed radish (Raphanus sativus) cover crop (Block P); and direct drilling with a winter oilseed radish cover crop (Block L). Soil, water and vegetation chemistry across the trial area were monitored over the 2012/13 (pre-trial), 2013/14 (cover crops and non-inversion tillage) and 2014/15 (non-inversion tillage only) farm years. Results revealed oilseed radish reduced nitrate (NO3-N) leaching losses in soil water by 75–97% relative to the fallow block, but had no impact upon phosphorus (P) losses. Corresponding reductions in riverine NO3-N concentrations were not observed, despite the trial area covering 20% of the catchment. Mean soil NO3-N concentrations were reduced by ∼77% at 60–90 cm depth beneath the cover crop, highlighting the ability of deep rooting oilseed radish to scavenge nutrients from deep within the soil profile. Alone, direct drilling and shallow non-inversion tillage were ineffective at reducing soil water NO3-N and P concentrations relative to conventional ploughing. Applying starter fertiliser to the cover crop increased radish biomass and nitrogen (N) uptake, but resulted in net N accumulation within the soil. There was negligible difference between the gross margins of direct drilling (£731 ha−1) and shallow non-inversion tillage (£758 ha−1) with a cover crop and conventional ploughing with fallow (£745 ha−1), demonstrating farm productivity can be maintained whilst mitigating diffuse pollution. The results presented here support the wider adoption of winter oilseed radish cover crops to reduce NO3-N leaching losses in arable systems, but caution that it may take several years before catchment-scale impacts downstream are detected.

Graphical abstract

The impact of cover crops and non-inversion tillage regimes on soil and riverine nutrient concentrations is assessed at the farm-scale.

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Introduction

Diffuse nutrient pollution from intensive arable agriculture is a major driver behind the eutrophication of freshwater environments and leads to an array of detrimental economic (Dodds et al., 2009, Smith and Schindler, 2009) and environmental (Skinner et al., 1997, Némery and Garnier, 2016) impacts. As naturally limiting nutrients of plant growth in aquatic systems, the enhanced land-to-river transfer of fertiliser derived nitrogen (N) and phosphorus (P) fuels blooms of phytoplankton, periphyton and neuro-toxin secreting cyanobacteria colonies which can dramatically lower species diversity and lead to a fundamental breakdown of ecosystem functioning (Smith et al., 1999, Hilton et al., 2006). Treating eutrophic water also incurs significant economic costs, with water companies having to remediate problems with taste, colour and odour whilst lowering concentrations of contaminants in order to make the water potable (Pretty et al., 2000). In the United Kingdom, the total costs of eutrophication have been estimated at £75–114 million per year (Pretty et al., 2003). Consequently, on-farm mitigation measures are required to help reduce land-to-river nutrient transfers, with such schemes being financially incentivised through agri-environmental stewardship programmes (Kay et al., 2009, Deasy et al., 2010).

The efficacy of two commonly applied mitigation measures at reducing nutrient losses from arable land, cover crops (Snapp et al., 2005, Tonitto et al., 2006, Valkama et al., 2015) and non-inversion tillage (Tebrügge and Düring, 1999, Stevens and Quinton, 2009, Soane et al., 2012), have been widely studied for several decades. Cover crops are typically non-cash crops sown in the autumn to provide winter groundcover when the field would otherwise be fallow, thereby reducing the risk of soil nutrient losses from leaching and erosion (Dabney et al., 2001, Hooker et al., 2008). A range of species can be grown, including N fixing leguminous (e.g. clover, vetch and pea) and non-leguminous (e.g. rye, sorghum and brassicas) varieties. Cover crops have primarily been used to minimise NO3 leaching by scavenging highly soluble residual soil NO3 and converting it into relatively immobile organic N (Aronsson and Torstensson, 1998, Beaudoin et al., 2005, Premrov et al., 2014). However, they have also been shown to protect surface soils from erosive flows, increase soil organic matter content, enhance soil structure, suppress weeds and improve soil moisture balance (Lu et al., 2000, Dabney et al., 2001, Stevens and Quinton, 2009). Unfortunately, an array of negative agronomic impacts of cover crops have also been reported and include the cost of establishment, difficulty in destroying the cover crop prior to sowing the subsequent cash crop, the harbouring of insect pests and the complexity of predicting the release of mineralised N as the cover crop residues degrade (Snapp et al., 2005, Deasy et al., 2010).

The main objective of non-inversion, or conservation, tillage systems is to improve soil structure and stability (Holland, 2004, Lal et al., 2007). In conventional tillage systems, the soil is typically inverted to a depth of >20 cm using a mouldboard plough prior to secondary cultivation to create a seedbed into which the subsequent cash crop is sown (Morris et al., 2010). However, under non-inversion tillage systems the soil is either disturbed to a lesser degree (i.e. shallow non-inversion tillage to a depth of <10 cm using discs or tines) or not disturbed at all, with sowing occurring directly into the residue of the previous crop (i.e. direct drilling) (Morris et al., 2010). By improving soil structure, non-inversion tillage methods have been shown to reduce soil erosion, increase organic matter content, improve drainage and water holding capacity and increase microbial and earthworm activity (Deasy et al., 2009, Soane et al., 2012, Abdollahi and Munkholm, 2014). However, the lack of inversion can increase pest populations and lead to an accumulation of nutrients near the soil surface which can be readily mobilised by surface flows and thus pose a risk to freshwater environments (Holland, 2004, Bertol et al., 2007, Stevens and Quinton, 2009).

To date, much of the research into the effectiveness of cover crops and non-inversion tillage at reducing arable nutrient losses has come from small, controlled plot scale studies (e.g. Catt et al., 1998, Bakhsh et al., 2002). Whilst such studies are typically able to yield definitive conclusions as to the effectiveness of certain measures by controlling for the multiple sources of variability that exist within agroecosystems, they are unable to demonstrate how effective these measures would be when applied in real world situations on large, commercial, arable farms. Specifically, plot-scale studies typically fail to account for the impacts of mitigation measures upon crop yields, farm profit margins, catchment-scale nutrient losses, or the practicalities for the farmer of deploying such measures. Consequently, there is a need for more farm- and catchment-scale approaches to help better inform government decision making on agri-environmental policy, particularly in the UK (Kay et al., 2009). Addressing this deficiency, in 2010 the UK government launched the Demonstration Test Catchment (DTC) research platform to evaluate the extent to which on-farm mitigation measures could cost-effectively reduce the impacts of diffuse agricultural pollution on river ecology whilst maintaining food production capacity (McGonigle et al., 2014). Across the UK, three DTCs were established with each concentrating on a different farming system. This paper focuses upon the intensive arable River Wensum DTC in Norfolk, UK, where cover crops and non-inversion tillage methods were trialled as diffuse pollution mitigation measures on a large, commercial arable farm over a three-year period (Wensum Alliance, 2016).

The primary objectives of this paper are as follows:

  • (i)

    To assess the effectiveness of cover crops and non-inversion tillage regimes at reducing N and P losses at the farm-scale;

  • (ii)

    To examine the impact of cover crops and non-inversion tillage methods on soil fertility;

  • (iii)

    To assess the sub-catchment scale impacts of the mitigation measures by monitoring river water chemistry downstream of the trial area;

  • (iv)

    To compare the economic viability and farm practicalities of cover crops and non-inversion tillage operations with those of conventional farm practice.

Section snippets

Study location

This study focuses upon the large (20 km2) commercial Salle Park Estate located within the Blackwater sub-catchment of the lowland calcareous River Wensum, Norfolk, UK (52°47′09″N, 01°07′00″E). The estate is situated 40–50 m above sea level with gentle slopes (<1°) meaning that subsurface leaching rather than surface runoff is the dominant pollution pathway. Intensive arable cropping comprises 79% of the land use and is managed with a seven-year rotation of winter wheat, winter and spring barley,

Nitrate

During the pre-trial period (2012/13) when all blocks were under either winter wheat or spring barley, there were no significant differences in mean field drain NO3-N concentration between Blocks L (5.5 mg N L−1), P (6.4 mg N L−1) and J (9.6 mg N L−1) (Fig. 3; Table 2). Concentrations of 10.0 mg N L−1 were observed in the normal practice Block N, with the two fields in this block under winter barley and spring beans.

However, during the cover crop and non-inversion tillage period (2013/14), pronounced

Effectiveness of the cover crop

The oilseed radish cover crop proved to be highly effective at reducing soil water NO3-N levels, thereby minimising NO3-N leaching losses and lowering diffuse pollution risk. Concentrations in the 90 cm depth porous pots were reduced by 96–97% in late winter (February 2014) and by 79–80% in mid-spring (April 2014) compared to the fallow control block, whilst concentrations were reduced by 75–87% in the 100–150 cm depth field drains across the 2013/14 farm year. This beneficial effect compares

Conclusions

To date, the majority of research into the efficacy of on-farm measures for mitigating diffuse agricultural pollution has come from controlled plot scale studies which typically fail to account for the impacts of measures upon crop yields, farm profit margins, catchment-scale nutrient losses, or the practicalities for the farmer of deploying such measures. Here, we have addressed these issues by assessing the impacts of cover crops and non-inversion tillage regimes at the farm-scale. The key

Acknowledgements

This research was funded by the UK Department for Environment, Food and Rural Affairs (Defra) under the Demonstration Test Catchments initiative (WQ0212/WQ0225/LM0304). ZH acknowledges support from the Kurdistan Regional Government and Human Capacity Development Program. The authors would like to thank the Salle Park Estate for their cooperation with the field trials and Carl Gudmundsson of Väderstad for technical advice and machinery support. Paul Brown of Frontier Agriculture Limited provided

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