Disruption of the naturally evolved N conservation strategy in soil under grassland at a sports field in York, UK

Water- and KCl-extractable ammonium-N and nitrate-N concentrations have been monitored at approximately monthly intervals over a year in soils from 0-10 and 10-20 cm depths under permanent grass at a sports field in York, UK. Measurements were made on both fresh, field-moist soils and after the same soils had been incubated for 7 days at ambient outdoor temperatures, to assess seasonal changes in the capacity of the soils to produce mineral-N species in the absence of plant uptake and other effects. Water extracts allowed potential mobility of N species to be assessed. Comparison of seasonal trends in mineral-N species concentrations in pre- and post-incubation soils confirmed depletion of exchangeable ammonium-N from the winter to summer. Mineral-N in fresh and incubated soils displayed summer minima and also low production in winter, associated with the effects of low temperature on nitrate production and probably microbial immobilization of nitrate produced by residual senescent plant litter with a higher C:N ratio from the previous autumn. The results support the concept that plant/soil systems co-evolved under more pristine conditions to conserve soil N by matching the dynamics of soil mineral N production and plant N uptake, but now N pollution has resulted in a dynamic mismatch.