The influence of land use and management on soil carbon levels for crop-pasture systems in Central New South Wales, Australia
Changes in land use and management have been proposed as a way to increase soil organic carbon (SOC) in crop and pasture systems. Some of the proposed activities to improve SOC are the introduction of pasture phases in cropping systems, stubble retention, no-till cropping, improved fertilisation, introduction of more productive pasture species and grazing management. There is also growing interest in novel farming systems, such as pasture cropping (intercropping cereal crops with established perennial pastures), which may improve SOC. However, there have been few broad scale surveys to determine whether these land management changes have an impact on commercial farms. In this study, comparisons of land use were established for mixed farming and pasture cropping systems in the slopes region (average annual rainfall: 500-650mm) and for cropping and pasture in the plains region (average annual rainfall: 300-500mm) of Central West NSW, Australia. The survey aimed to determine the difference in SOC stocks (Mg C ha-1) and the composition of three soil organic carbon fractions (particulate - POC, humus - HUM and resistant - ROC). The influences of management actions and pasture composition were also assessed across pasture and cropping land uses. Cropping systems had lower SOC stocks in the soil than pasture systems in each region, but pasture cropping was not different from perennial pasture. Generally, there were larger differences in the POC due to land use and management than the other SOC fractions. Management practices in cropping systems explained greater variability in SOC than in pastures. For cropping systems, higher amounts of P fertiliser were associated with higher SOC, POC and ROC while higher amounts of N fertiliser were associated with lower SOC, POC and ROC. For pastures, the proportion of bare ground was associated with lower SOC and POC. These associations indicate there is an opportunity to increase SOC by converting cropping land to permanent pasture, increasing the frequency of pasture phases, changing crop fertiliser regimes and reducing bare ground in pastures, but further work is needed to verify the causality behind these associations.