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. Temporal variations in the effects of a surface litter layer and litter mixed with near-surface mineral soils on the mobility of NH4+–N were assessed over 7 months from the start of winter. Litter decomposition and associated N mineralization contributed significantly towards NH4+–N production in the soil profiles, especially at 0–10 cm depths, even at low winter temperatures. Increase in temperature over time 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 much of the NH4+–N produced and mobilized in the litter layer was retained in the soil profiles. Surprisingly, 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.