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Nitrogen dynamics in an Australian semiarid grassland soil

Cookson, W.R., M, C., O'Brien, P.A., Murphy, D.V. and Grierson, P.F. (2006) Nitrogen dynamics in an Australian semiarid grassland soil. Ecology, 87 (8). pp. 2047-2057.

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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 (NH4+), and nitrate (NO3"12) contents, gross rates of N mineralization and microbial re-mineralization, NH4+ and NO3"12 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 ;Cg C/g soil) and almost twice the DOC content (54 compared with 28 ;Cg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 ;Cg N/g soil) and lower gross N rates (1 134 compared with 13 1326 ;Cg N 22[g soil]"121 22d"121) and NO3"12 accumulation (0.5 compared with 22 ;Cg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 ;Cg C/g soil), and greater DOC content (33 compared with 24 ;Cg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3 1320 compared with 17 1331 ;Cg N 22[g soil]"121 22d"121) and NO3"12 accumulation (18 compared with 25 ;Cg 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"12 from organic N sources must be included in this model.

Item Type: Journal Article
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Publisher: Ecological Society of America
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