Nitrogen Dynamics in an Australian Semiarid Grassland Soil

We conducted a four-week laboratory incubation of soil from a Themeda triandra Forsskal grassland to clarify mechanisms of nitrogen (N) cycling processes in relation to carbon ( C) and N availability in a hot, semiarid environment. Variation in soil C and N availability was achieved by collecting soil from either under tussocks or the bare soil between tussocks, and by amending soil with Themeda litter. We measured N cycling by monitoring: dissolved organic nitrogen (DON), ammonium (NH4+), and nitrate (NO3-) contents, gross rates of N mineralization and microbial re-mineralization, NH4+ and NO3- immobilization, and autotrophic and heterotrophic nitrification. We monitored C availability by measuring cumulative soil respiration and dissolved organic C (DOC). Litter-amended soil had cumulative respiration that was eightfold greater than non-amended soil ( 2000 compared with 250 mu g C/g soil) and almost twice the DOC content ( 54 compared with 28 mu g C/ g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 mu g N/g soil) and lower gross N rates (1-4 compared with 13-26 mu g N-.[g soil](-1.)d(-1)) and NO3- accumulation (0.5 compared with 22 mu g N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks ( 300 compared with 175 mu g C/g soil), and greater DOC content (33 compared with 24 mu g C/ g soil). However, unamended soil from under tussocks had lower gross N rates (3-20 compared with 17-31 mu g N-.[g soil](-1.)d(-1)) and NO3- accumulation (18 compared with 25 mu g N/g soil) relative to soil from between tussocks. We conclude that N cycling in this grassland is mediated by both C and N limitations that arise from the patchiness of tussocks and seasonal variability in Themeda litterfall. Heterotrophic nitrification rate explained > 50% of total nitrification, but this percentage was not affected by proximity to tussocks or litter amendment. A conceptual model that considers DON as central to N cycling processes provided a useful initial framework to explain results of our study. However, to fully explain N cycling in this semiarid grassland soil, the production of NO3- from organic N sources must be included in this model.