Soil fertility in a large dryland floodplain: Patterns, processes and the implications of water resource development

Abstract

1. Primary production on semiarid floodplains supports a diverse local and regional fauna. Reduced flooding from water resource development (WRD) may affect floodplain production by decreasing water and nutrient supply. 2. We investigated the effects of reduced wetting on soil fertility by performing a regional soil survey across a gradient of flood frequency. Soil nitrogen (N), phosphorus (P) and carbon (C) were recorded over a soil-wetting event where heavy rainfall and flooding coincided. 3. Soil nutrient concentrations indicate N limits plant growth and P does not. 4. No spatial patterns in soil P were detected across the floodplain, suggesting that the principal mechanism controlling P fertility is the concentration of P in floodplain source material, and that flood mediated import and export of P are minor processes. 5. Soil N concentrations rose following rainfall and flooding and the greatest increased occurred in flooded areas. Flood deposition of N accounted for only 9% of the boost in soil N in flooded areas, and N concentrations continued to rise when the floods and rains ceased. Elevated soil N levels do not appear to persist because at the start of a growth cycle, when soils were dry, soil N did not vary significantly with flood frequency. This suggests most of the boost in soil N was due to N-fixation, with the subsequent loss of N likely to have resulted from in situ processes such as denitrification. 6. Agricultural export of nutrients appears to not be a significant process in the context of the high phosphorus fertility of floodplain source material and the apparently high rates of in situ processing of nitrogen. 7. Synthesis and applications. Our data suggest that floodplain soil fertility is controlled by mechanisms other than flood frequency or agricultural export, meaning that WRD is unlikely to affect soil fertility; however, the biological implications of the brief pulse in soil N associated with wetting need further investigation.