A novel approach to evaluating relationships between soil test and runoff P at landscape scales by integrating farmer knowledge on soil drains

Abstract

Dissolved phosphorus loss pathways are often overlooked relative to erosion P losses but contribute under certain soil land use and management combinations and may be exacerbated by subsurface drainage. Measures of P desorption to water are commonly made in laboratory soil testing and related to an agronomic P test using stronger extractants. We aimed to relate soil drainflow P concentrations as an integrative measure of a range of landscape source and transport processes to soil test P-status and examine other influential soil co-variates. Grassland and cropland sites (n=28)were sampled across Scotland involving farmers to incorporate local knowledge relating drains to influencing soils and gain important management data not often available. Using quality controls (on-site filtration, guidance and pro-forma data collection) farmers took their own samples and posted them on for laboratory analysis. Soils were analysed for CNP parameters, reactive complexes and soil agronomic P status by Modified Morgan’s P (MMorgP). Drain waters were analysed for total dissolved P (TDP), dissolved molybdate reactive (DRP) and unreactive P (DUP). Grasslands (n=11) differed from cropland (n=17) with significantly greater drain concentrations of Total P, DUP, DOC:TDN and soil total C, N and P, oxalate-extractable Fe (Feox) and C:P. Conversely, cropland had greater drain NO3, DRP:TDP, soil pH, Pox and reactive surface P-saturation. Significant relationships were gained between both TDP and DUP with MMorgP across all sites, although relationships showed less scatter for grassland than cropland sites. A group of moderate soil P status soils, mostly cropland, with low organic matter status had proportionally greater DRP losses, with soil N content being a significant inverse predictor of DRP concentrations or DRP:TDP ratios in drain water. The results indicated soil test P (MMorgP) as a main control of TDP concentrations via drainage pathways, but that organic matter quality affected dissolved P speciation, but not concentrations. There was greater risk of DRP losses from cropland soils low in organic matter. The study outcomes benefitted from involving the farming community in terms of coupling soil to drain samples and local management knowledge. Compared to batch laboratory results the additional processes integrated by using field drains as a landscape-scale measure of dissolved P losses will give more realistic relationships for change in P leaching between soils differing in P status spatially, or in time following management.