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Published on: July 24, 2018
Nitrogen dynamics in an Australian semiarid grassland soil
W R Cookson1, C Müller, P A O'Brien
1Centre 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
Nitrogen cycling in semiarid grasslands is limited by carbon and nitrogen availability, influenced by plant tussocks and litterfall. Dissolved organic nitrogen plays a key role, but nitrate production from organic sources is also crucial for a complete model.
Area of Science:
- Soil Science
- Ecology
- Biogeochemistry
Background:
- Semiarid grasslands face unique challenges in nutrient cycling due to environmental conditions.
- Themeda triandra grasslands are important ecosystems where understanding nitrogen cycling is critical for management.
- Spatial heterogeneity, including tussock presence and litter distribution, can significantly impact soil processes.
Purpose of the Study:
- To elucidate nitrogen (N) cycling mechanisms in a hot, semiarid Themeda triandra grassland.
- To investigate the influence of carbon (C) and N availability on N cycling processes.
- To assess the impact of soil microsite variation (under tussocks vs. between tussocks) and litter amendment on N cycling.
Main Methods:
- Laboratory incubation of grassland soil over four weeks.
- Manipulation of C and N availability by collecting soil from different microsites and amending with Themeda litter.
- Monitoring of key N cycling indicators: dissolved organic nitrogen (DON), ammonium (NH4+), nitrate (NO3-), gross N mineralization, microbial re-mineralization, N immobilization, and nitrification rates.
- Measurement of C availability through cumulative soil respiration and dissolved organic C (DOC).
Main Results:
- Litter amendment significantly increased soil respiration and dissolved organic carbon (DOC) but reduced dissolved organic nitrogen (DON) accumulation and gross N cycling rates.
- Soil from under tussocks exhibited higher respiration and DOC than soil from between tussocks, but lower gross N rates and nitrate accumulation.
- Heterotrophic nitrification constituted over 50% of total nitrification, irrespective of tussock proximity or litter amendment.
Conclusions:
- Nitrogen cycling in this grassland is constrained by both C and N limitations, driven by tussock patchiness and seasonal litterfall.
- A conceptual model incorporating dissolved organic nitrogen (DON) is useful but requires the inclusion of nitrate (NO3-) production from organic N sources for a comprehensive understanding.
- Understanding these complex interactions is vital for managing nutrient dynamics in semiarid grassland ecosystems.
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