Continental soil drivers of ammonium and nitrate in Australia

Abstract. Soil N is an essential element for plant growth, but its mineral forms are subject to loss from the environment by leaching and gaseous emissions. Despite its importance for the soil-plant system, factors controlling soil mineral N contents over large spatial scales are not well understood. We used NH4+ and NO3- contents (0–30 cm 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 similar (median 4.0 vs. 3.5 mg N kg−1) and NO3- was significantly enriched (3.0 vs. 1.0 mg N kg−1), compared to the non-agricultural region. The 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 non-agricultural region, NH4+ was affected not only by CEC and pH, but also by 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 non-agricultural region was NO3- 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 contents were highly uncertain, we found that region-specific interactions of soil properties control mineral N contents. It is therefore essential to understand how they alter soil mechanisms and N cycling at large scales.