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 μg C/g soil) and almost twice the DOC content (54 compared with 28 μg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 μg N/g soil) and lower gross N rates (1-4 compared with 13-26 μg$\text{N}\cdot [\text{g}\text{soil}]^{-1}\cdot \text{d}^{-1}$) and NO₃⁻ accumulation (0.5 compared with 22 μg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 μg C/g soil), and greater DOC content (33 compared with 24 μg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3-20 compared with 17-31 μg$\text{N}\cdot [\text{g}\text{soil}]^{-1}\cdot \text{d}^{-1}$) and NO₃⁻ accumulation (18 compared with 25 μg 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 &gt50% 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.