Modelling long-term changes in soil phosphorus and carbon under contrasting fertiliser and grazing management in New Zealand hill country

Soil carbon (C) stocks under permanent New Zealand pastures vary with slope and aspect due to differences in primary production, animal behaviour and nutrient return. A new nutrient transfer model was built using a web-based, general-purpose modelling tool to simulate long-term changes in soil phosphorus (P) and C in hill country under contrasting fertiliser and grazing management. Three self-contained farmlets under different P fertiliser and sheep grazing management were examined: LFNF = no P applied, LFLF = 125 kg single superphosphate (SSP) ha-1 year-1, HFHF = 375 kg SSP ha-1 year-1, since 1980. The new spatial model was able to simulate P and C distribution with varying slopes and aspects. For example, the mean annual changes in soil P and C were greater at low slopes and east aspects than at the other two slope and aspect positions, consistent with observed changes in these nutrients. The model identified both gaps in current knowledge and key factors influencing change in soil C stocks. Understanding the spatial patterns of soil C across the landscape is critical in the design of soil C monitoring regimes, should soil C stocks be considered at a national level as a sink or source of CO2 emissions.