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Discussion papers | Copyright
https://doi.org/10.5194/soil-2018-5
© Author(s) 2018. This work is distributed under
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Original research article 24 Apr 2018

Original research article | 24 Apr 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal SOIL (SOIL).

Continental drivers of ammonium and nitrate in Australian soil under different land uses

Juhwan Lee1, Gina M. Garland2, and Raphael A. Viscarra Rossel1 Juhwan Lee et al.
  • 1CSIRO Land and Water, GPO Box 1700, Canberra ACT 2601, Australia
  • 2Agroscope, Zurich 8046, Switzerland

Abstract. Soil N is an essential element for plant growth, but its mineral forms are subject to loss to the environment by leaching and gaseous emissions. Despite its importance for the soil-plant system, factors controlling soil mineral N concentrations over large spatial scales are not well understood. We used NH4+ and NO3 concentrations (0–30cm depth) from 469 sites across Australia, and determined soil controls on their regional variation. Soil mineral N varied regionally but depended on the different land uses. In the agricultural region of Australia, NH4+ tended to be depleted (4.9±4.8 vs. 5.6±9.0mgNkg−1) and NO3 was significantly enriched (6.0±9.2 vs. 3.8±9.9mgNkg−1), compared to the non-agricultural ecological region. The relative importance of soil controls on mineral N in the agricultural region, identified by the model trees algorithm Cubist, showed that NH4+ was affected by total N, cation exchange capacity (CEC) and pH. In the ecological region, NH4+ was affected by CEC and pH, but also organic C and total P. In each of the regions, NO3 was primarily affected by CEC, with more complex biophysical controls. In both regions, correlations between mineral N and soil C:N:P stoichiometry suggest that more NH4+ was found in P-depleted soil relative to total C and total N. However, our results showed that only in the other ecological region, NO3 was sensitive to the state of C and its interaction with N and P. The models helped to explain 36–68% of regional variation in mineral N. Although soil controls on high N concentrations was highly uncertain, we found that region-specific interactions of soil properties control mineral N concentrations and therefore it is essential to understand how they alter soil mechanisms and N cycling at large scales.

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Short summary
Soil nitrogen (N) is an essential element for plant growth, but its plant-available forms are subject to loss to the environment by leaching and gaseous emissions. Still, factors controlling soil mineral N concentrations over large spatial scales are not well understood. We determined and discussed primary soil controls over the concentrations of NH4+ and NO3 at the continental scale of Australia, while considering specific dominant land use patterns on a regional basis.
Soil nitrogen (N) is an essential element for plant growth, but its plant-available forms are...
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