title: Nitrogen dynamics in an Australian semiarid grassland soil.
abstract: 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 microg C/g soil) and almost twice the DOC content (54 compared with 28 microg C/g soil). However, litter-amended soils had only half as much DON accumulation as non-amended soils (9 compared with 17 microg N/g soil) and lower gross N rates (1-4 compared with 13-26 microg N x [g soil](-1) x d(-1)) and NO3- accumulation (0.5 compared with 22 microg N/g soil). Unamended soil from under tussocks had almost twice the soil respiration as soil from between tussocks (300 compared with 175 microg C/g soil), and greater DOC content (33 compared with 24 microg C/g soil). However, unamended soil from under tussocks had lower gross N rates (3-20 compared with 17-31 microg N x [g soil](-1) d(-1)) and NO3- accumulation (18 compared with 25 microg 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.
authors.affiliation: Centre for Land Rehabilitation, School of Earth and Geographical Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. wcookson@cyllene.uwa.edu.au
authors.forename: W R
authors.initials: WR
authors.lastname: Cookson
authors.forename: C
authors.initials: C
authors.lastname: Müller
authors.forename: P A
authors.initials: PA
authors.lastname: O'Brien
authors.forename: D V
authors.initials: DV
authors.lastname: Murphy
authors.forename: P F
authors.initials: PF
authors.lastname: Grierson
mesh_terms: D002244:Carbon / Q000032:analysis*; D002980:Climate; D009566:Nitrates / Q000032:analysis; D009584:Nitrogen / Q000032:analysis* / Q000378:metabolism; D006109:Poaceae; D000644:Quaternary Ammonium Compounds / Q000032:analysis; D012987:Soil / Q000032:analysis; D012988:Soil Microbiology; D014904:Western Australia
chemical_list: D009566:Nitrates; D000644:Quaternary Ammonium Compounds; D012987:Soil; D002244:Carbon; D009584:Nitrogen