How much does NH-N contribute to mineral-N losses in N-impacted acid soils under grassland in the UK? A microcosm study

Abstract

It is hypothesised that NH-N is more mobile in soils than often thought. However, because litter layers and live vegetation can modify N species transformation in soils, intact microcosms were employed to investigate NH-N, NO-N and total dissolved inorganic N (DIN) leaching in soils from across an unfertilised and N-impacted grassland in York, UK. Simulated rain of diverse compositions was applied to five intact core microcosms, and leachate samples were collected at regular intervals. Leaching with simple simulated rain, consisting of appropriately dilute NaCl solution, showed that broadly comparable NH-N and NO-N leaching would occur from freely draining profiles. Profiles of heavy textured soils result in marginal DIN losses, primarily as NH-N because of denitrification, but possibly substantial losses as NH-N via overland flow during heavy precipitation. Further sequential leaching experiments on freely draining profiles with rain containing supplementary NH-N showed enhanced NH-N and NO-N leaching in two of three pairs of microcosms. However, one profile showed consistently lower NH-N leaching associated with the development of a consistently higher nitrification rate. The marked difference between profiles was thought to be due to the presence of a more distinct surface litter layer in one profile, compared with observable litter incorporation into subsoils for other profiles. Effects on mineral N species mobilities of drying and rewetting were also assessed. This confirmed that a delay of several days is required to re-establish the microbial population and its activities. Final phases, in which NO-N and then NH4NO3 were added via the rain, confirmed the mobility of N pollutant inputs through the freely draining soils.