Increased soil nitrogen supply enhances root-derived available soil carbon leading to reduced potential nitrification activity

Nitrogen (N) immobilisation by heterotrophic microorganisms is critical for reducing N losses from soils and ensuring a long-term supply of N to plants in grassland ecosystems. The supply of carbon (C) available to soil microbes may stimulate heterotrophic N immobilisation by reducing the availability of ammonium to autotrophic nitrifiers and, hence, for nitrification activity. The main source of available C to soils is rhizodeposition, but its effects on nitrification activity remain unclear as rhizodeposition differs between plant species and varying N availabilities. The aim of this work was to investigate the role of root-derived C on nitrification activity for five different grassland plant species. Cichorium intybus (chicory), Lolium perenne (perennial ryegrass), Plantago lanceolata (ribwort plantain), Raphanus raphanistrum and R. sativus (wild and cultivated radish), and an unplanted control were grown for nine weeks under controlled environmental conditions and treated either with a low (no urea-N) or a high rate of additional N (550 kg urea-N ha−1). Plant biomass, water-extractable C concentration and ammonia-oxidising bacteria (AOB) abundance increased in the planted high N treatments. The high N addition to planted soils resulted in increased C available for microbial activity and led to decreased potential nitrification activity compared to those for the low N treatments. An increase in water-extractable C concentration was associated with a decrease in potential nitrification activity, suggesting that the increase in available C for microbial activity may have stimulated heterotrophic NH4+ uptake and thus N immobilisation. This study highlights that N addition can be used to manipulate root-derived available C and, with the tight coupling of soil C and N cycling processes, can be used to identify management practices that will promote N retention and reduce losses from grassland soils.