Causes of temporal variations in N species transformations and mobility under acid grassland in York: The multi-functional role of plant litter

Nitrogen cycling has been studied in soils from Hob Moor, an unimproved, unfertilized and N-impacted acid grassland near York, UK. Preliminary assessment of 7 soil profiles sampled to 60 cm depth indicated considerable N mineralization and nitrification in sub-soils. Later soils from a freely draining area of the grassland were sampled from 2 superficially similar profiles to 1 m depth to explore interactions between N species transformations and soils properties. The spatial heterogeneity of the 2 soil profiles was not anticipated, and was attributed to different fates of litter inputs to the soils. When N mineralization was expressed on a soil C basis, both profiles followed similar trends. These freely drained acidic profiles showed incredible potential for N mineralization and mobility well below the root zone which potentially could contaminate surface and/or ground-waters. This was confirmed using intact core microcosms with vegetation and litter layers. Episodic drainage water analysis revealed considerable and sustained NH4+-N and NO3--N concentrations from 3 freely draining soil profiles. One profile apparently had litter incorporated into sub-soils and behaved differently compared to the other two with more surficial litter presence. This led to design of a microcosm study to assess the litter effects on drainage water inorganic nitrogen (DIN), organic nitrogen (DON) and organic carbon (DOC) concentrations and fluxes over a natural seasonal temperature gradient over 7 months from early winter to mid summer. Litter mineralization resulted in substantial NH4+ production. Temperature apparently showed strong effects on NH4+ production. Mobile NH4+ from litter layers raised the extractable NH4+ concentrations in underlying subsoils. When litter was incorporated within subsoils, it greatly reduced NO3- concentrations in the drainage water, especially in winter when NO3- is many fold more mobile due to lower biological uptake. Extractable NH4+ concentrations correlated positively with water-soluble DOC, suggesting a role of DOC in NH4+ dynamics. In contrast, extractable NO3- concentrations were correlated negatively with DOC, indicating a role for DOC in NO3- immobilization by acting as substrate for microorganisms. Litter manipulations significantly altered concentrations and fluxes of DIN, DON and DOC in the drainage water. Concentrations of NH4+ increased substantially after freeze-thaw events, which facilitated NH4+ mobilization. Litter layers produced sustainable DOC and DON in the drainage water. In summer, increase in temperature significantly enhanced DOC and DON concentrations for the control and surface litter treatments; however, the reverse behaviour was observed for the subsurface litter treatment. Cumulative fluxes indicated that DON formed a significant component of total dissolved nitrogen (TDN), 42, 46 and 62% for the control, surface litter and subsurface litter treatments respectively. Cumulative fluxes showed net NH4+ retention in each treatment and significant reduction in NO3- flux associated with subsurface litter placement.