Litter effects on ammonium dynamics in an acid soil under grassland

►A microcosm study was conducted outdoors at ambient temperatures in York, UK, to investigate the effects of seasonal temperature changes on NH4+-N dynamics associated with litter mineralization. Litter decomposition and associated N mineralization contributed substantially towards NH4+-N production in the soil profiles, especially at 0-10 cm depths, even at low winter temperatures. Increase in temperature from winter to summer substantially increased KCl-extractable and water-soluble NH4+-N concentrations via enhanced organic matter mineralization. The difference between KCl-extractable and water-soluble NH4+-N increased over time as well, as much of the NH4+-N produced and mobilized in the litter layer was retained in the soil profiles. However, the presence of litter, especially in the mixed litter treatment, markedly reduced mineral-N concentrations in the leachate in winter. Nitrate-N concentrations in leachate were substantially lowered in summer too, in spite of the absence of vegetation uptake in summer. This indicates that microbial N immobilization in soil and/or abiotic N retention linked with soil organic matter, and not just plant-N uptake, reduce nitrate leaching to freshwaters in summer months. The study has particular significance for management perspective in areas receiving reduced N inputs from the atmosphere.