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 and 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, tended to be similar (median 4.0 vs. 3.5 mg N kg−1) and 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 was affected by total N, cation exchange capacity (CEC) and pH. In the non-agricultural region, was affected not only by CEC and pH, but also by organic C and total P. In each of the regions, 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 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 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.