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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 (NH 4 ++ ), and nitrate (NO 3 −− ) contents, gross rates of N mineralization and microbial re-mineralization, NH 4 ++ and NO 3 −− 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 μμg C/g soil) and almost twice the DOC content (54 compared with 28 μμg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 μμg N/g soil) and lower gross N rates (1––4 compared with 13––26 μμg N··((g soil)) −−1 ··d −−1 ) and NO 3 −− accumulation (0.5 compared with 22 μμg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 μμg C/g soil), and greater DOC content (33 compared with 24 μμg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3––20 compared with 17––31 μμg N··((g soil)) −−1 ··d −−1 ) and NO 3 −− accumulation (18 compared with 25 μμg 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 NO 3 −− from organic N sources must be included in this model.
Ecology – Ecological Society of America
Published: Aug 1, 2006
Keywords: Australia ; autotrophic/heterotrophic nitrification ; dissolved organic matter ; nitrogen cycle ; nitrogen mineralization ; semiarid environment
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