Treading compaction during winter grazing can increase subsequent nitrate leaching by enhancing drainage

Abstract

Nitrate leaching from agricultural production systems threatens groundwater and surface water quality. Grazing of forage crops can result in high inputs of urinary-N and soil compaction from livestock treading. However, the effects of soil compaction from winter grazing on nitrate leaching are poorly understood. The objective of this study was to test the hypothesis that the effects of soil compaction on nitrate leaching after winter grazing depend on rainfall (amount and temporal pattern) and evaporation-regulated soil water content. HYDRUS-1D was used to simulate post-winter grazing drainage and nitrate leaching with five degrees of soil compaction and four factors (i.e. forage crop establishment method, fallow period, climate, and urine addition). Empirical data were obtained from a previous experiment conducted on a moderately well-drained soil (Templeton silt loam, Udic Haplustept) in Canterbury, New Zealand. The results suggested that soil compaction can increase nitrate leaching, and this was more likely to occur in cultivated soils, following high input of urea N, during longer fallow periods and where rainfall is higher during the fallow periods (winter/spring). The increase in nitrate leaching from compaction was mainly attributed to greater drainage as a result of reduced evaporation and a lower capacity to retain water before drainage was triggered. A conceptual model was developed to explain the effect of compaction on drainage during two different phases of soil evaporation that depend on soil water content. This study highlighted the importance of soil water content to determining drainage and nitrate leaching. Therefore, management practices that help to promote soil drying by increasing evaporation (e.g. use no-tillage to establish forage crops to reduce compaction, shallow tillage after winter grazing) and transpiration (e.g. sowing a catch crop earlier) are essential to reducing the risk of drainage and nitrate leaching after winter grazing.