EARTH SURFACE PROCESSES AND LANDFORMS Earth Surf. Process. Landforms 35, 699–706 (2010) Copyright © 2010 John Wiley & Sons, Ltd. Published online 2 February 2010 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/esp.1894
Hillslope scale surface runoff, sediment and nutrient losses associated with tramline wheelings Martyn Silgram,1* D.R Jackson,1 Alison Bailey,3 John Quinton2 and Carly Stevens2 ADAS UK Ltd., ‘Woodthorne’, Wergs Road, Wolverhampton WV6 8TQ, UK 2 Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK 3 Department of Agriculture, University of Reading, Reading RG6 6AR, UK
1
Received 9 December 2007; Accepted 7 July 2009 *Correspondence to: Martyn Silgram, ADAS UK Ltd., ‘Woodthorne’, Wergs Road, Wolverhampton WV6 8TQ, UK. E-mail:
[email protected]
ABSTRACT: Research on arable sandy loam and silty clay loam soils on 4° slopes in England has shown that tramlines (i.e. the unseeded wheeling areas used to facilitate spraying operations in cereal crops) can represent the most important pathway for phosphorus and sediment loss from moderately sloping fields. Detailed monitoring over the October–March period in winters 2005–2006 and 2006–2007 included event-based sampling of surface runoff, suspended and particulate sediment, and dissolved and particulate phosphorus from hillslope segments (each ~300–800 m2) established in a randomized block design with four replicates of each treatment at each of two sites on lighter and heavier soils. Experimental treatments assessed losses from the cropped area without tramlines, and from the uncropped tramline area, and were compared to losses from tramlines which had been disrupted once in the autumn with a shallow tine. On the lighter soil, the effects of removal or shallow incorporation of straw residues was also determined. Research on both sandy and silty clay loam soils across two winters showed that tramline wheelings represented the dominant pathway for surface runoff and transport of sediment, phosphorus and nitrogen from cereal crops on moderate slopes. Results indicated 5·5–15·8% of rainfall lost as runoff, and losses of 0·8–2·9 kg TP ha−1 and 0·3–4·8 t ha−1 sediment in tramline treatments, compared to only 0·2–1·7% rainfall lost as runoff, and losses of 0·0–0·2 kg TP ha−1 and 0·003–0·3 t ha−1 sediment from treatments without tramlines or those where tramlines had been disrupted. The novel shallow disruption of tramline wheelings using a tine once following the autumn spray operation consistently and dramatically reduced (p < 0·001) surface runoff and loads of sediment, total nitrogen and total phosphorus to levels similar to those measured in cropped areas between tramlines. Results suggest that options for managing tramline wheelings warrant further refinement and evaluation with a view to incorporating them into spatially-targeted farm-level management planning using national or catchment-based agri-environment policy instruments aimed at reducing diffuse pollution from land to surface water systems. Copyright © 2010 John Wiley & Sons, Ltd. KEYWORDS: runoff; phosphorus; sediment; tramlines; mitigation
Introduction This paper addresses the need for practical, affordable, and targeted management of fields with cereal crops to help reduce losses of soil, phosphorus and nitrogen from land to water courses. Inputs of nitrogen (N) and phosphorus (P) from agricultural land are a significant contribution to the riverine loads in UK rivers (Defra, 2002, 2006; Environment Agency, 2007) and at the European scale (Kronvang et al., 2005). The critical concentration of total phosphorus entering water bodies in relation to eutrophication is recognized by a range of organizations to be 0·1 mg l−1 (Withers and Sharpley, 1995). Phosphorus has low solubility, and is strongly bound by the soil, being associated primarily with the finer soil fractions (Quinton et al., 2001). Runoff preferentially transports the finer fractions of the soil (Quinton et al., 2001), and agricultural activities are thought to be responsible for 30% of phosphorus
inputs to surface waters in the UK (White and Hammond, 2006). Enrichment of surface waters with sediment, phosphorus, and nitrogen is problematic as siltation restricts fish spawning and excess nutrients encourage the development of algal blooms toxic to animals. As a result, reducing these losses from land to water is a priority associated with implementation by the European Union (EU). Member States of the Nitrates Directive (91/676/EEC), Water Framework Directive (WFD) (2000/60/EC), Habitats Directive (92/43/EEC), and Freshwater Fish Directive (2006/44/EC). Reducing nutrient losses from fields to surface waters benefits farmers as well as the environment by retaining plant nutrients on fields, thereby maintaining soil fertility and limiting the need for fertilizer inputs. EU Member States have implemented a wide range of agri-environment policies aimed at achieving these policy objectives. In England, the Environmental Stewardship scheme (Defra, 2005) awards
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points for specific environmentally-beneficial activities, ranging from maintaining hedgerows to the use of vegetated headlands around field margins. The scheme has initially focused on improving biodiversity and limiting the ecological impacts of land management. However, such agri-environment schemes could be modified to incorporate practical and cost-effective management techniques for controlling diffuse pollution to water. The research outlined in this paper explores specific potential pollution mitigation options for reducing surface losses under cereal cropping systems which have the potential to be incorporated into commercial farming practices. Tramline wheelings are the unseeded lines left bare in autumn which are used to facilitate spraying operations, for example in cereal cropping systems. These tramlines comprise a pair of adjacent parallel wheel marks often around 30–35 cm wide, which in UK agriculture are typically spaced 18–24 m apart depending on the type of machinery used. Recent research has identified that tramlines can represent an important pathway in the loss of sediment and phosphorus from a silty clay loam soil under arable management on a 7° slope in western England (Silgram, 2004), and this is supported by Withers (2006) and Silgram et al. (2007). The associated compaction caused by tramlines is also of agronomic significance as it can increase lodging risk in the adjacent crop (Scott et al., 2005). Conventional agricultural systems usually attempt to maintain surface infiltration (as this aids water conservation and reduces runoff risk), and keep soil compaction to a minimum by cultivating tramline wheeling areas during the post-harvest field operations. This contrasts to some versions of Controlled Traffic Farming (CTF) which use a gantry system (e.g. Chamen et al., 2003) and leave the wheeled area as a semi-permanent ‘roadway’ which remains uncultivated over successive cropping years. The effect of such CTF methods on runoff and diffuse pollution losses are not clear, although it seems logical that the increased compaction in the tramline from successive CTF passes with farm machinery will lead to reduced infiltration in this uncropped wheeled area which could increase the risk of surface runoff and associated losses of soil and surface-applied materials (such as slurries and pesticides) over the longer term. The potentially important role of tramlines has recently been recognized in broad assessments of potential mitigation options for limiting diffuse pollution from land to water (e.g. Cuttle et al., 2007; Mainstone et al., 2008). Tramlines have also been included in conceptual representations of runoff, sediment and phosphorus loss processes in models such as PSYCHIC (Davison et al., 2008), although there appears to be a shortage of robust, event-based data covering different soils, slopes and land uses to characterize and predict such losses with confidence. The limited work which has been reported on tramline effects either focus on impacts on agronomic issues (Dickson
and Ritchie, 1996; Chamen et al., 2003), or if they do consider impacts on runoff and diffuse pollution risk they tend to cover only a single site or year, or involve small bounded plot-scale methods with their associated limitations in terms of representativeness and edge effects (e.g. Withers, 2006). The project reported here extends the body of literature concerning the role of tramlines to test (i) whether tramline wheelings are important in influencing surface runoff, sediment, nitrogen and phosphorus losses across a broader range of soil textures, over multiple years, and on unbounded hillslope-scale sections on moderate slopes; and (ii) whether relatively simple tramline management and/or crop residue management can function as effective mitigation techniques suitable for integrating into agri-environment schemes aimed at reducing erosion and loss of sediment and phosphorus from land to surface water systems to help achieve water quality targets. The scope of this paper focuses on annual losses (as patterns with respect to rainfall duration, intensity and timing will be considered in a subsequent paper). The work reported here is relevant to a wide variety of stakeholders, including scientists involved in developing and evaluating pollution mitigation options, and government and agency staff concerned with making, implementing and evaluating agrienvironment policy measures at farm and catchment.
Methodology This paper focuses on the first two years of results from two experimental sites in a project exploring practical mitigation options for limiting phosphorus and sediment loss from fields under cereal cropping systems. Unbounded long hillslope segments were established to quantify surface runoff and diffuse pollutant loss on a uniform 4° slope with erodible sandy loam soils of the Bromyard and Middleton series at Old Hattons farm in Staffordshire, England (52°38′50″N, 2°10′15″W). Similar unbounded hillslope segments were established at a second site at Rosemaund in Herefordshire, England, in a slightly wetter area 100 km southwest of the first site on a uniform 5° slope and contrasting silty clay loam soils of the Salwick series (52°7′43″N, 2°38′5″W). All hillslope sections were 3·5 m wide at both sites, and were 270 m long at Hattons and 100 m long at Rosemaund. Fields were drilled up and down the slope, as is the usual practice at these commercial farms. An overview of the two study sites and experimental treatments are shown in Table I. Surface runoff from each 3·5 m wide section of hillslope was intercepted using metal gutters installed oblique to the slope, which channelled water via guttering and plastic pipes. Given the large hillslope area contributing to flow, it was necessary to collect only a representative subsample of the total runoff volume from each hillslope section in each collection tank. This
Table I. Site characteristics and treatment summary for the two study sites Experimental treatments Mean annual rainfall (mm)
Altitude (m)
Slope angle (deg)
Slope length (m)
Organic matter (%)
Olsen P (mg l−1)
Rosemaund
660
100
5
100
2·6
Old Hattons
600
110
4
270
3·6
Site name
Copyright © 2010 John Wiley & Sons, Ltd.
Year 1
Year 2
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No Tramline Tramline Chop/spread straw Bale/remove straw
No Tramline Tramline Disrupted tramline
43
No Tramline Tramline Disrupted tramline Offset tramline
No Tramline Tramline Disrupted tramline Offset tramline
Earth Surf. Process. Landforms, Vol. 35, 699–706 (2010)
SURFACE RUNOFF, SEDIMENT AND NUTRIENT LOSSES FROM CEREALS
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Figure 1. Runoff from upslope monitoring area is collected by metal gutters which channel runoff into piping (top left); piping diverts runoff downslope (2–3° angle) into tipping bucket sample splitters, which direct one eighth of runoff into covered sample tanks for analysis (the remaining runoff is directed to waste) (top right); close-up of sample splitter (bottom left); close-up of sample tanks from unwheeled areas and from areas with tramlines (bottom right).
was achieved by piping the runoff into novel tipping bucket devices which diverted one eighth of the total runoff into collection tanks (with the remainder diverted to waste). The experimental layout is shown in Figure 1. The representativeness of these sample splitters was verified through a calibration process. Event-based samples of runoff in the collection tanks were analysed for total phosphorus (TP), total dissolved phosphorus, particulate phosphorus, total nitrogen (TN), total dissolved nitrogen, and suspended sediment (SS). Statistical analysis of results was undertaken on a treatment basis for individual sites and years using Analysis of Variance (ANOVA) and Duncan’s multiple range test (Duncan, 1955) using the software Genstat (version 10.1, 2007, VSN International). In Year 1 at Hattons, treatments included areas where postharvest cereal straw residues had either been baled and removed, or had been chopped and incorporated into the soil. Losses from hillslope sections which included tramline wheelings, and from hillslope sections which included only the conventionally planted area between tramline wheelings were monitored separately. The resulting split-plot design consisted of four treatments, each with four replicates. Two hypotheses could be tested: whether runoff and diffuse pollution losses from areas including compacted unvegetated tramline wheelings were significantly different from areas without these tramline wheelings; and whether chopping and spreading cereal straw (rather than baling and removing it) significantly reduced runoff and diffuse pollution losses due to the physical protection of the soil surface from the erosive energy in incident precipitation. In Year 1 at Rosemaund, treatments also measured surface losses from areas including tramlines and from the conventionally planted area between tramlines. In addition, a third treatment explored whether using a cultivator fitted with a tine to disrupt the compacted surface of the wheeling (to 6 cm depth) after its establishment in the late autumn had a signifiCopyright © 2010 John Wiley & Sons, Ltd.
cant effect in reducing surface runoff and diffuse pollution losses (by loosening near-surface soil compaction and increasing surface roughness and infiltration rates). There were four replicates of each treatment. In Year 2, treatments were identical at both sites, and involved: (i) vegetated areas between tramlines (‘no tramline’); (ii) areas with conventional tramlines (‘tramline’); (iii) areas where a cultivator fitted with a tine were used to disrupt (to 6 cm depth) the compacted surface of the tramline wheeling (‘disrupted tramline’); and in Year 2 only, (iv) areas where the crop sprayer was run over the emerging crop during the autumn (‘offset tramline’), rather than running on the unseeded tramline area. This was undertaken to identify whether any effect of the presence of tramlines was driven primarily by the compaction caused by the sprayer machinery used following crop emergence in the autumn, or by the lack of vegetation cover in the wheeled areas.
Results Year 1 – Hattons site At Hattons, seven events were monitored between October 2005 and March 2006. This paper focuses on total over-winter losses, but to demonstrate that the pattern of results was evident within individual events, the characteristics of individual events and event-based results for this site and year are shown in Figure 2. Overall results across these events are summarized in Table II. There was a consistent trend for treatments receiving 2·5 t ha−1 straw chopped and incorporated to have lower surface runoff, sediment, TP and TN fluxes than those where straw had been baled and removed. However, this effect was not statistically significant (p > 0·05) as it was overwhelmed by the dominating influence of the presence or absence of tramline wheelings. Earth Surf. Process. Landforms, Vol. 35, 699–706 (2010)
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Hattons – 2005–2006 Event date Total rainfall Duration Peak runoff rate Mean runoff rate
Event mm h l/min l/min
1 31/10/05 42·9 45·0 34·1 0·110
2.0
2 02/11/05 10·9 5·0 9·5 0·130
3 07/11/05 24·3 7·5 15·2 0·110
4 02/12/05 32·4 18·8 20·1 0·320
5 05/12/05 10·0 6·3 7·7 0·450
6 03/01/06 15·8 12·0 9·1 0·300
7 13/03/06 15·0 12·0 8·9 0·004
Bale/Remove - No Tramline Bale/Remove - Tramline Chop/Spread - No Tramline Chop/Spread - Tramline
1.8 1.6
Runoff (mm)
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 Event 1
Event 2
Event 3
Event 4
160
Event 6
Event 7
Bale/Remove - No Tramline Bale/Remove - Tramline Chop/Spread - No Tramline Chop/Spread - Tramline
140 Sediment load (kg/ha)
Event 5
120 100 80 60 40 20 0 Event 1
Figure 2.
Event 2
Event 3
Event 4
Event 5
Event 6
Event 7
Characteristics of Hattons Year 1 (2005–2006) rainfall events, and associated event-based treatment results
Table II. Total over-winter losses and standard error of the mean for Hattons, winter 2005–2006
Runoff (mm) Means Baled/removed Baled/removed Chopped/spread Chopped/spread
No tramline Tramline No tramline Tramline p
Standard error of the mean Baled/removed No tramline Baled/removed Tramline Chopped/spread No tramline Chopped/spread Tramline
Percentage rainfall as runoff
Sediment (kg ha−1)
TP (kg ha−1)
TN (kg ha−1)
0·4a 8·4b 0·2a 6·4b
