Intensive management in grasslands causes diffuse water pollution at the farm scale

Hydro-chemical responses at different scales in a rural catchment, UK, and implications for managing the unintended consequences of agriculture

Diffuse pollutant transfers from agricultural land often constitute the bulk of annual loads in catchments and storm events dominate these fluxes. There remains a lack of understanding of how pollutants move through catchments at different scales. This is critical if the mismatch between the scales used to implement on-farm management strategies, compared to those used for assessment of environmental quality, is to be addressed. The aim of this study was to understand how the mechanisms of pollutant export may change when assessed at different scales and the corresponding implications for on-farm management strategies. A study was conducted within a 41 km2 catchment containing 3 nested sub-catchments, instrumented to monitor discharge and various water quality parameters. Storm data over a 24-month period were analysed and hysteresis (HI) and flushing (FI) indices calculated for two water quality variables that are typically of environmental significance; NO3-N and suspended sediment (SSC). For SSC, increasing spatial scale had little effect on the mechanistic interpretation of mobilisation and the associated on-farm management strategies. At the three smallest scales NO3-N was chemodynamic with the interpretation of dominant mechanisms changing seasonally. At these scales, the same on-farm management strategies would be recommended. However, at the largest scale, NO3-N appeared unaffected by season and chemostatic. This would lead to a potentially very different interpretation and subsequent on-farm measures. The results presented here underscore the benefits of nested monitoring for extracting mechanistic understanding of agricultural impacts on water quality. The application of HI and FI indicates that monitoring at smaller scales is crucial. At large scales, the complexity of the catchment hydrochemical response means that mechanisms become obscured. Smaller catchments more likely represent critical areas within larger catchments where mechanistic understanding can be extracted from water quality monitoring and used to underpin the selection of on-farm mitigation measures.

Current advisory interventions for grazing ruminant farming cannot close exceedance of modern background sediment loss - Assessment using an instrumented farm platform and modelled scaling out

Water quality impairment by elevated sediment loss is a pervasive problem for global water resources. Sediment management targets identify exceedance or the sediment loss 'gap' requiring mitigation. In the UK, palaeo-limnological reconstruction of sediment loss during the 100-150 years pre-dating the post-World War II intensification of agriculture, has identified management targets (0.20-0.35 t ha^-1 yr^-1) representing 'modern background sediment delivery to rivers'. To assess exceedance on land for grazing ruminant farming, an integrated approach combined new mechanistic evidence from a heavily-instrumented experimental farm platform and a scaling out framework of modelled commercial grazing ruminant farms in similar environmental settings. Monitoring (2012-2016) on the instrumented farm platform returned sediment loss ranges of 0.11-0.14 t ha^-1 yr^-1 and 0.21-0.25 t ha^-1 yr^-1 on permanent pasture, compared with between 0.19-0.23 t ha^-1 yr^-1 and 0.43-0.50 t ha^-1 yr^-1and 0.10-0.13 t ha^-1 yr^-1and 0.25-0.30 t ha^-1 yr^-1 on pasture with scheduled plough and reseeds. Excess sediment loss existed on all three farm platform treatments but was more extensive on the two treatments with scheduled plough and reseeds. Excessive sediment loss from land used by grazing ruminant farming more strategically across England, was estimated to be up to >0.2 t ha^-1 yr^-1. Modelled scenarios of alternative farming futures, based on either increased uptake of interventions typically recommended by visual farm audits, or interventions selected using new mechanistic understanding for sediment loss from the instrumented farm platform, returned minimum sediment loss reductions. On the farm platform these were 2.1 % (up to 0.007 t ha^-1 yr^-1) and 5.1 % (up to 0.018 t ha^-1 yr-1). More strategically, these were up to 2.8 % (0.014 t ha^-1 yr^-1) and 4.1 % (0.023 t ha^-1 yr^-1). Conventional on-farm measures will therefore not fully mitigate the sediment loss gap, meaning that more severe land cover change is required.

Sediment loss in response to scheduled pasture ploughing and reseeding: The importance of soil moisture content in controlling risk

Soil water regimes have been shown to have important implications for the erosion risks associated with land management decisions. Despite this, there remains a paucity of information on soil moisture thresholds for farm management operations including the periodic ploughing and reseeding of improved pasture used for ruminant farming. Against this background, this study analysed sediment loss monitored on a heavily instrumented farm platform, in SW England, over four phases of ploughing and reseeding. Precipitation and sediment yields were highly variable between the ten different field scale catchments on the experimental platform after reseeds. Post-plough period rainfall ranged between 461-1121 mm and corresponding sediment yields between 0.20 - 3.13 t. ha^-1 yr^-1. The post-plough and reseeding periods accounted for a very high proportion (mean 28.8 %) of monitored sediment fluxes over the study (2012-2019) despite only covering an average of 10.9 % of the 2002 days of flume monitoring. Post-plough sediment yields were highest (2.57 t. ha^-1 yr^-1 and 3.13 t. ha^-1 yr^-1) when two catchments were ploughed in autumn months and soils were saturated. The yields for the same catchments after summer ploughing were far lower (0.72 t. ha^-1yr^-1and 0.73 t. ha^-1yr^-1). Thresholds of 35-38 % soil moisture were identified at which ploughing represented a highly elevated erosion risk. Whilst pinpointing thresholds for the clay loam soils with slowly permeable drainage in the study area, the results serve to illustrate the wider need for robust scientific data on soil moisture status to help guide the timing of farm management operations for improving production, to help reduce negative environmental consequences.

Research on Livestock Carrying Capacity of Arid Pastoral Areas Based on Dynamic Water–Forage–Livestock Balance in OtogBanner, China

There are nonequilibrium characteristics of grassland ecosystems driven by water, and constraints on the development scale of artificially irrigated grassland caused by the lack of water resources in arid pastoral areas. Based on the interaction of water, forage, and livestock, this study built a model of livestock-carrying capacity within the dynamic water–forage–livestock balance, to analyze the livestock carrying capacity of arid pastoral areas. The results showed that compared with the fixed livestock carrying capacity of 1.0898 million sheep units with a dynamic forage–livestock balance, the livestock carrying capacity based on the dynamic water–forage–livestock balance of OtogBanner were in a multi-equilibrium state due to the fluctuation of rangeland productivity caused by a change in precipitation conditions and the adjustment of the tame grassland irrigation scale caused by the change in water demand of other water users in the pastoral area. Under the conditions of the wet, normal, and dry years, the livestock carrying capacity was 1.632 million standard sheep units under the 26.5 thousand hm2 tame grassland developing areas, 1.3037 million standard sheep units under the 25.9 thousand hm2 tame grassland developing areas, and 0.9155 million standard sheep units respectively under 22.4 thousand hm2 tame grassland developing areas. This fluctuation change was more prominent in the pastoral areas with rangeland as the key field. Besides this, the model could effectively identify the predicament of water and forage resources. At present, the overload of forage resources and water resources coexisted in the pastoral area of OtogBanner, and an important reason for this was that the distribution of water and forage resources was poorly matched with the mode of animal husbandry production. The value of 1.3037 million sheep units was recommended to the livestock-carrying capacity of OtogBanner according to the model. This study could provide a new method for the calculation of livestock carrying capacity, and offered a scientific basis for the protection of the grassland ecological environment and the sustainable development of animal husbandry in the arid pastoral area of OtogBanner.

Phosphorus leaching from riparian soils with differing management histories under three grass species

Plants release carbon-based exudates from their roots into the rhizosphere to increase phosphorus (P) supply to the soil solution. However, if more P than required is brought into solution, additional P could be available for leaching from riparian soils. To investigate this further, soil columns containing a riparian arable and buffer strip soil, which differed in organic matter contents, were sown with three common agricultural and riparian grass species. The P loads in leachate were measured and compared with those from unplanted columns, which were 0.17 ± 0.01 and 0.89 ± 0.04 mg kg^-1 for the arable and buffer strip soil, respectively. A mixture of ryegrass and red fescue significantly (p ≤ .05) increased dissolved inorganic P loads in leachate from the arable (0.23 ± 0.01 mg kg^-1 ) and buffer strip soil (1.06 ± 0.05 mg kg^-1 ), whereas barley significantly reduced P leaching from the buffer strip soil (0.53 ± 0.08 mg kg^-1 ). This was dependent on the dissolved organic C released under different plant species and on interactions with soil management history and biogeochemical conditions, rather than on plant uptake of P and accumulation into biomass. This suggested that the amount and forms of P present in the soil and the ability of the plants to mobilize them could be key factors in determining how plants affect leaching of soil P. Selecting grass species for different stages of buffer strip development, basing species selection on root physiological traits, and correcting soil nutrient stoichiometry in riparian soils through vegetative mining could help to lower this contribution.

Phosphorus in Agriculture: A Review of Results from 175 Years of Research at Rothamsted, UK

Insight into the role of phosphorus (P) in soil fertility and crop nutrition at Rothamsted, UK, and its involvement in associated environmental issues, has come from long-term field experiments initially started by J. B. Lawes in 1843 and continued by others, together with experiments on different soils. Results from the 1940s confirmed that residues of P applied in fertilizers and manures build up reserves of P in soil. There is a strong relationship between crop yield and plant-available P (Olsen P), and a critical level of Olsen P can be determined. For soils near the critical level, P-use efficiency is high when the P applied and offtake by the crop is nearly equal. Soil inorganic P is associated with various soil components and is held there with a range of bonding energies so that when no P is applied, the decline in Olsen P follows a smooth curve. We conceptualize inorganic soil P as being in four pools of vastly varying size, availability for uptake, and extractability by reagents used in routine soil analysis, and with reversible transfer of P between pools. For very disparate soils at Rothamsted and in the United States, there is a strong relationship between the change in Olsen P and P removal/input ratios, suggesting an underlying similarity in inorganic P behavior. Maintaining soil near the critical level should optimize yield and the use of the global P resource while minimizing the risk of transfer of large amounts of P to the aquatic environment.

Phosphorus and nitrogen losses from temperate permanent grassland on clay-loam soil after the installation of artificial mole and gravel mole drainage

Mole (M) and gravel-mole (GM) drainage systems improve the permeability of soils with high clay contents. They collect and carry away infiltrating water during episodic rainfall events. Characterisation of nutrient fluxes (concentration and flows) in overland flow (OF) and in mole drain flow (MF) across sequential rainfall events is important for environmental assessment of such drainage systems. The objective of this study is to assess the impact of drainage systems on soil nutrient losses. Three treatments were imposed on grazed permanent grassland on a clay loam soil in Ireland (52°30'N, 08°12'W) slope 1.48%: undrained control (C), mole drainage (M) and gravel mole drainage (GM). Plots (100 m × 15 m) were arranged in a randomized complete block design with four replicated blocks. Nitrogen (N) and phosphorus (P) concentrations in OF, MF and groundwater (GW) were measured from each plot over 15 consecutive rainfall events. The results showed that M and GM (P < 0.05) deepened the watertable depth and decreased OF. M and GM increased losses of nitrate-N (22%) and ammonium-N (14%) in GW. Nitrate-N concentrations from all the flow pathways (mean and standard error (s.e.): 0.99 s.e. 0.10 mg L^-1) were well below the 11.3 mg L^-1 threshold for drinking water. Ammonium-N concentrations from all the flow pathways (mean: 0.64 s.e. 0.14 mg L^-1) exceeded drinking water quality standards. On the other hand M and GM lowered total P losses (mean annual losses from C, M and GM: 918, 755 and 853 s.e. 14.1 g ha^-1 year^-1) by enhancing soil P sorption. Hence M and GM can be implemented on farms under similar management to that described in the present study with a minor impact on N (increased concentration on averaged 18% to GW) and P (reduced by on avenged 114 g ha^-1 year^-1).

Determining the sources of nutrient flux to water in headwater catchments: Examining the speciation balance to inform the targeting of mitigation measures

Diffuse water pollution from agriculture (DWPA) is a major environmental concern, with significant adverse impacts on both human and ecosystem health. However, without an appropriate understanding of the multiple factors impacting on water, mitigation measures cannot be targeted. Therefore, this paper addresses this gap in understanding, reporting the hydrochemical monitoring evidence collected from the UK Government's Demonstration Test Catchments (DTC) programme including contrasting chalk and clay/mudstone catchments. We use data collected at daily and sub-daily frequency over multiple sites to address: (1) How does the behaviour of the full range of nitrogen (N) species and phosphorus (P) fractions vary? (2) How do N species and P fractions vary inter- and intra-annually? (3) What do these data indicate about the primary pollution sources? And (4) which diffuse pollution mitigation measures are appropriate in our study landscapes? Key differences in the rates of flux of nutrients were identified, dependent on catchment characteristics. Full N speciation and P fractionation, together with dissolved organic carbon (DOC) enabled identification of the most likely contributing sources in each catchment. Nitrate (NO₃-N) was the dominant N fraction in the chalk whereas organic and particulate N comprised the majority of the load in the clay/mudstone catchments. Despite current legislation, orthophosphate (PO₄-P) was not found to be the dominant form of P in any of the catchments monitored. The chalk sub-catchments had the largest proportion of inorganic/dissolved organic P (DOP), accompanied by episodic delivery of particulate P (PP). Contrastingly, the clay/mudstone sub-catchments loads were dominated by PP and DOP. Thus, our results show that by monitoring both the inorganic and organic fractions a more complete picture of catchment nutrient fluxes can be determined, and sources of pollution pin-pointed. Ultimately, policy and management to bring nutrient impacts under control will only be successful if a multi-stressor approach is adopted.

Roles of instrumented farm-scale trials in trade-off assessments of pasture-based ruminant production systems

For livestock production systems to play a positive role in global food security, the balance between their benefits and disbenefits to society must be appropriately managed. Based on the evidence provided by field-scale randomised controlled trials around the world, this debate has traditionally centred on the concept of economic-environmental trade-offs, of which existence is theoretically assured when resource allocation is perfect on the farm. Recent research conducted on commercial farms indicates, however, that the economic-environmental nexus is not nearly as straightforward in the real world, with environmental performances of enterprises often positively correlated with their economic profitability. Using high-resolution primary data from the North Wyke Farm Platform, an intensively instrumented farm-scale ruminant research facility located in southwest United Kingdom, this paper proposes a novel, information-driven approach to carry out comprehensive assessments of economic-environmental trade-offs inherent within pasture-based cattle and sheep production systems. The results of a data-mining exercise suggest that a potentially systematic interaction exists between 'soil health', ecological surroundings and livestock grazing, whereby a higher level of soil organic carbon (SOC) stock is associated with a better animal performance and less nutrient losses into watercourses, and a higher stocking density with greater botanical diversity and elevated SOC. We contend that a combination of farming system-wide trials and environmental instrumentation provides an ideal setting for enrolling scientifically sound and biologically informative metrics for agricultural sustainability, through which agricultural producers could obtain guidance to manage soils, water, pasture and livestock in an economically and environmentally acceptable manner. Priority areas for future farm-scale research to ensure long-term sustainability are also discussed.

The role of mobilisation and delivery processes on contrasting dissolved nitrogen and phosphorus exports in groundwater fed catchments

Diffuse transfer of nitrogen (N) and phosphorus (P) in agricultural catchments is controlled by the mobilisation of sources and their delivery to receiving waters. While plot scale experiments have focused on mobilisation processes, many catchment scale studies have hitherto concentrated on the controls of dominant flow pathways on nutrient delivery. To place mobilisation and delivery at a catchment scale, this study investigated their relative influence on contrasting nitrate-N and soluble P concentrations and N:P ratios in two shallow groundwater fed catchments with different land use (grassland and arable) on the Atlantic seaboard of Europe. Detailed datasets of N and P inputs, concentrations in shallow groundwater and concentrations in receiving streams were analysed over a five year period (October 2010-September 2015). Results showed that nitrate-N and soluble P concentrations in shallow groundwater give a good indication of stream concentrations, which suggests a dominant control of mobilisation processes on stream exports. Near-stream attenuation of nitrate-N (-30%), likely through denitrification and dilution, and enrichment in soluble P (+100%), through soil-groundwater interactions, were similar in both catchments. The soil, climate and land use controls on mobilisation were also investigated. Results showed that grassland tended to limit nitrate-N leaching as compared to arable land, but grassland could also contribute to increased P solubilisation. In the context of land use change in these groundwater fed systems, the risk of pollution swapping between N and P must be carefully considered, particularly for interactions of land use with soil chemistry and climate.

Assessing the risk of phosphorus transfer to high ecological status rivers: Integration of nutrient management with soil geochemical and hydrological conditions

Agriculture has been implicated in the loss of pristine conditions and ecology at river sites classified as at 'high ecological status' across Europe. Although the exact causes remain unclear, diffuse phosphorus (P) transfer warrants consideration because of its wider importance for the ecological quality of rivers. This study assessed the risk of P loss at field scale from farms under contrasting soil conditions within three case-study catchments upstream of near-pristine river sites. Data from 39 farms showed P surpluses were common on extensive farm enterprises despite a lower P requirement and level of intensity. At field scale, data from 520 fields showed that Histic topsoils with elevated organic matter contents had low P reserves due to poor sorption capacities, and received applications of P in excess of recommended rates. On this soil type 67% of fields recorded a field P surplus of between 1 and 31kgha^-1, accounting for 46% of fields surveyed across 10 farms in a pressured high status catchment. A P risk assessment combined nutrient management, soil biogeochemical and hydrological data at field scale, across 3 catchments and the relative risks of P transfer were highest when fertilizer quantities that exceeded current recommendations on soils with a high risk of mobilization and high risk of transport as indicated by topographic wetness index values. This situation occurred on 21% of fields surveyed in the least intensively managed catchment with no on-farm nutrient management planning and soil testing. In contrast, the two intensively managed catchments presented a risk of P transfer in only 3% and 1% of fields surveyed across 29 farms. Future agri-environmental measures should be administered at field scale, not farm scale, and based on soil analysis that is inclusive of OM values on a field-by-field basis.

Eurasian beaver activity increases water storage, attenuates flow and mitigates diffuse pollution from intensively-managed grasslands

Beavers are the archetypal keystone species, which can profoundly alter ecosystem structure and function through their ecosystem engineering activity, most notably the building of dams. This can have a major impact upon water resource management, flow regimes and water quality. Previous research has predominantly focused on the activities of North American beaver (Castor canadensis) located in very different environments, to the intensive lowland agricultural landscapes of the United Kingdom and elsewhere in Europe. Two Eurasian beavers (Castor fiber) were introduced to a wooded site, situated on a first order tributary, draining from intensively managed grassland. The site was monitored to understand impacts upon water storage, flow regimes and water quality. Results indicated that beaver activity, primarily via the creation of 13 dams, has increased water storage within the site (holding ca. 1000m³ in beaver ponds) and beavers were likely to have had a significant flow attenuation impact, as determined from peak discharges (mean 30±19% reduction), total discharges (mean 34±9% reduction) and peak rainfall to peak discharge lag times (mean 29±21% increase) during storm events. Event monitoring of water entering and leaving the site showed lower concentrations of suspended sediment, nitrogen and phosphate leaving the site (e.g. for suspended sediment; average entering site: 112±72mgl^-1, average leaving site: 39±37mgl^-1). Combined with attenuated flows, this resulted in lower diffuse pollutant loads in water downstream. Conversely, dissolved organic carbon concentrations and loads downstream were higher. These observed changes are argued to be directly attributable to beaver activity at the site which has created a diverse wetland environment, reducing downstream hydrological connectivity. Results have important implications for beaver reintroduction programs which may provide nature based solutions to the catchment-scale water resource management issues that are faced in agricultural landscapes.

Spatial variation in soil properties and diffuse losses between and within grassland fields with similar short-term management

One of the major challenges for agriculture is to understand the effects of agricultural practices on soil properties and diffuse pollution, to support practical farm-scale land management. Three conventionally managed grassland fields with similar short-term management, but different ploughing histories, were studied on a long-term research platform: the North Wyke Farm Platform. The aims were to (i) quantify the between-field and within-field spatial variation in soil properties by geostatistical analysis, (ii) understand the effects of soil condition (in terms of nitrogen, phosphorus and carbon contents) on the quality of discharge water and (iii) establish robust baseline data before the implementation of various grassland management scenarios. Although the fields sampled had experienced the same land use and similar management for at least 6 years, there were differences in their mean soil properties. They showed different patterns of soil spatial variation and different rates of diffuse nutrient losses to water. The oldest permanent pasture field had the largest soil macronutrient concentrations and the greatest diffuse nutrient losses. We show that management histories affect soil properties and diffuse losses. Potential gains in herbage yield or benefits in water quality might be achieved by characterizing every field or by area-specific management within fields (a form of precision agriculture for grasslands). Permanent pasture per se cannot be considered a mitigation measure for diffuse pollution. The between- and within-field soil spatial variation emphasizes the importance of baseline characterization and will enable the reliable identification of any effects of future management change on the Farm Platform.

HIGHLIGHTS

Quantification of soil and water quality in grassland fields with contrasting management histories.Considerable spatial variation in soil properties and diffuse losses between and within fields.Contrasting management histories within and between fields strongly affected soil and water quality.Careful pasture management needed: the oldest pasture transferred the most nutrients from soil to water.

A new emphasis on root traits for perennial grass and legume varieties with environmental and ecological benefits

Grasslands cover a significant proportion of the agricultural land within the UK and across the EU, providing a relatively cheap source of feed for ruminants and supporting the production of meat, wool and milk from grazing animals. Delivering efficient animal production from grassland systems has traditionally been the primary focus of grassland-based research. But there is increasing recognition of the ecological and environmental benefits of these grassland systems and the importance of the interaction between their component plants and a host of other biological organisms in the soil and in adjoining habitats. Many of the ecological and environmental benefits provided by grasslands emanate from the interactions between the roots of plant species and the soil in which they grow. We review current knowledge on the role of grassland ecosystems in delivering ecological and environmental benefits. We will consider how improved grassland can deliver these benefits, and the potential opportunities for plant breeding to improve specific traits that will enhance these benefits whilst maintaining forage production for livestock consumption. Opportunities for exploiting new plant breeding approaches, including high throughput phenotyping, and for introducing traits from closely related species are discussed.

Identifying contrasting influences and surface water signals for specific groundwater phosphorus vulnerability

Two groundwater dominated catchments with contrasting land use (Grassland and Arable) and soil chemistry were investigated for influences on P transfer below the rooting zone, via the aquifer and into the rivers. The objective was to improve the understanding of hydrochemical process for best management practise and determine the importance of P transfer via groundwater pathways. Despite the catchments having similar inorganic P reserves, the iron-rich soils of the Grassland catchment favoured P mobilisation into soluble form and transfer to groundwater. Sites in that catchment had elevated dissolved reactive P concentrations in groundwater (>0.035 mg l(-1)) and the river had flow-weighted mean TRP concentrations almost three times that of the aluminium-rich Arable catchment (0.067 mg l(-1) compared to 0.023 mg l(-1)). While the average annual TRP flux was low in both catchments (although three times higher in the Grassland catchment; 0.385 kg ha(-1) compared to 0.128 kg ha(-1)), 50% and 59% of TRP was lost via groundwater, respectively, during winter periods that were closed for fertiliser application. For policy reviews, slow-flow pathways and associated time-lags between fertiliser application, mobilisation of soil P reserves and delivery to the river should be carefully considered when reviewing mitigating strategies and efficacy of mitigating measures in groundwater fed catchments. For example, while the Grassland catchment indicated a soil-P chemistry susceptibility, the Arable catchment indicated a transient point source control; both resulted in sustained or transient periods of elevated low river-flow P concentrations, respectively.

Contrasting temperature responses of dissolved organic carbon and phenols leached from soils

BACKGROUND AND AIMS

Plant-derived phenols are a major input to the terrestrial carbon cycle that might be expected to contribute substantially to dissolved organic carbon (DOC) losses from soils. This study investigated changes in DOC and phenols in leachates from soil treated with individual plant litter types under seasonal temperature change.

METHODS

Senescing grass, buttercup, ash and oak litters were applied to soil lysimeters. Leachates were collected over 22 months and analysed for DOC and phenols. Phenols in litter and DOC were analysed using on-line thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide (TMAH).

RESULTS

Mass loss differed between litter type (buttercup>ash>grass>oak). Phenol concentrations in the senescing litters (<2 % TOC) were small, resulting in minor losses to water. Seasonal soil temperature positively correlated with DOC loss from litter-free soils. An initial correlation between temperature change and total phenol concentration in grass and ash litter treatment leachates diminished with time. Dissolved phenol variety in all litter-amended soil leachates increased with time.

CONCLUSIONS

Plant-derived phenols from senescing litter made a minor contribution to DOC loss from soils. The strength of the relationship between seasonal temperature change and phenol type and abundance in DOC changed with time and was influenced by litter type.