An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils

Abstract

Surface and subsurface litter fulfil many functions in the biogeochemical cycling of C and N in terrestrial ecosystems. These were explored using a microcosm study by monitoring dissolved inorganic nitrogen (DIN) (NH4 +–N + NO3 −–N), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations and fluxes in drainage water under ambient outdoor temperatures. Subsurface litter remarkably reduced the DIN concentrations in winter, probably by microbial N uptake associated with higher C:N ratio of added litter compared with soil at 10–25 cm depth. Fluxes of DIN were generally dominated by NO3 −–N; but NH4 +–N strongly dominated DIN fluxes during freeze–thaw events. Appreciable concentrations of NH4 +–N were observed in the drainage from the acid grassland soils throughout the experiment, indicating NH4 +–N mobility and export in drainage water especially during freeze–thaw. Litter contributed substantially to DOC and DON production and they were correlated positively (p < 0.01) for all treatments. DOC and DON concentrations correlated with temperature for the control (p < 0.01) and surface litter (p < 0.001) treatments and they were higher in late summer. The subsurface litter treatment, however, moderated the effect of temperature on DOC and DON dynamics. Cumulative N species fluxes confirmed the dominance of litter as the source of DON and DOC in the drainage water. DON constituted 42, 46 and 62% of cumulative TDN flux for control, surface litter and subsurface litter treatments respectively.