Mechanisms of nitrogen transfer in a model clover-ryegrass pasture: a 15N-tracer approach.

Purpose: Nitrogen (N) transfer from white clover (Trifolium repens cv.) to ryegrass (Lolium perenne cv.) has the potential to meet ryegrass N requirements. This study aimed to quantify N transfer in a mixed pasture and investigate the influence of the microbial community and land management on N transfer. Methods: Split root 15N-labelling of clover quantified N transfer to ryegrass via exudation, microbial assimilation, decomposition, defoliation and soil biota. Incorporation into the microbial protein pool was determined using compound-specific 15N-stable isotope probing approaches. Results: N transfer to ryegrass and soil microbial protein in the model system was relatively small, with one-third arising from root exudation. N transfer to ryegrass increased with no microbial competition but soil microbes also increased N transfer via shoot decomposition. Addition of mycorrhizal fungi did not alter N transfer, due to the source-sink nature of this pathway, whilst weevil grazing on roots decreased microbial N transfer. N transfer was bidirectional, and comparable on a short-term scale. Conclusions: N transfer was low in a model young pasture established from soil from a permanent grassland with long-term N fertilisation. Root exudation and decomposition were major N transfer pathways. N transfer was influenced by soil biota (weevils, mycorrhizae) and land management (e.g. grazing). Previous land management and the role of the microbial community in N transfer must be considered when determining the potential for N transfer to ryegrass. Supplementary Information: The online version contains supplementary material available at 10.1007/s11104-022-05585-0.

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

Arable land use is generally assumed to be the largest contributor to agricultural diffuse pollution. This study adds to the growing evidence that conventional temperate intensively managed lowland grasslands contribute significantly to soil erosion and diffuse pollution rates. This is the first grassland study to monitor hydrological characteristics and multiple pollutant fluxes (suspended sediment [SS] and the macronutrients: total oxidized nitrogen-N [TON], total phosphorus [TP], and total carbon [TC]) at high temporal resolution (monitoring up to every 15 min) over 1 yr. Monitoring was conducted across three fields (6.5-7.5 ha) on the North Wyke Farm Platform, UK. The estimated annual erosion rates (up to 527.4 kg ha), TP losses (up to 0.9 kg ha), and TC losses (up to 179 kg ha) were similar to or exceeded the losses reported for other grassland, mixed land-use, and arable sites. Annual yields of TON (up to 3 kg ha) were less than arable land-use fluxes and earlier grassland N studies, an important result as the study site is situated within a Nitrate Vulnerable Zone. The high-resolution monitoring allowed detailed "system's functioning" understanding of hydrological processes, mobilization- transport pathways of individual pollutants, and the changes of the relative importance of diffuse pollutants through flow conditions and time. Suspended sediment and TP concentrations frequently exceeded water quality guidelines recommended by the European Freshwater Fisheries Directive (25 mg L) and the European Water Framework Directive (0.04 mg soluble reactive P L), suggesting that intensively managed grasslands pose a significant threat to receiving surface waters. Such sediment and nutrient losses from intensively managed grasslands should be acknowledged in land management guidelines and advice for future compliance with surface water quality standards.

Understanding spatial variability of soil properties: a key step in establishing field- to farm-scale agro-ecosystem experiments.

RATIONALE: The spatial variability of soil properties is poorly understood, despite its importance in designing appropriate experimental sampling strategies. As preparation for a farm-scale agro-ecosystem services monitoring project, the 'North Wyke Farm Platform', there was a need to assess the spatial variability of key soil chemical and physical properties. METHODS: The field-scale spatial variability of soil chemical (total N, total C, soil organic matter), soil physical properties (bulk density and particle size distribution) and stable isotope ratios (δ(13) C and δ(15) N values) was studied using geostatistical approaches in an intensively managed grassland. RESULTS: The scales over which stable isotopes vary (ranges: 212-258 m) were larger than those of the total nutrients, soil organic matter and bulk density (ranges: 84-170 m). Two visually and statistically distinct areas of Great Field (north and south) were identified in terms of co-occurring high/low values of several soil properties. CONCLUSIONS: The resulting patterns of spatial variability suggest lower spatial variability of stable isotopes than that of total nutrients, soil organic matter and bulk density. Future sampling regimes should be conducted in a grid with <85 m distance between sampling locations to sufficiently capture the spatial variability of the measured soil properties on the 'North Wyke Farm Platform'. Consultation of the management histories of the sampled field revealed that it had previously comprised two fields with contrasting management histories, suggesting an effect of management legacy (>5 years) on the patterns of spatial variability.