Assessing the response of soil carbon in Australia to changing inputs and climate using a consistent modelling framework

Abstract

Land use and management practices affect the response of soil organic carbon (C) to global change. Process-based models of soil C are useful tools to simulate C dynamics, but it is important to bridge any disconnect that exists between the data used to inform the models and the processes that they depict. To minimise that disconnect, we developed a consistent modelling framework that integrates new spatially explicit soil measurements and data with the Rothamsted carbon model (Roth C) and simulates the response of soil organic C to future climate change across Australia. We compiled publicly available continental-scale datasets and pre-processed, standardised and configured them to the required spatial and temporal resolutions. We then calibrated Roth C and ran simulations to estimate the baseline soil organic C stocks and composition in the 0-0.3m layer at 4043 sites in cropping, modified grazing, native grazing and natural environments across Australia. We used data on the C fractions, the particulate, mineral-associated and resistant organic C (POC, MAOC and ROC, respectively) to represent the three main C pools in the Roth C model's structure. The model explained 97%-98% of the variation in measured total organic C in soils under cropping and grazing and 65% in soils under natural environments. We optimised the model at each site and experimented with different amounts of C inputs to simulate the potential for C accumulation under constant climate in a 100-year simulation. With an annual increase of 1MgCha-1 in C inputs, the model simulated a potential soil C increase of 13.58 (interquartile range 12.19-15.80), 14.21 (12.38-16.03) and 15.57 (12.07-17.82)MgCha-1 under cropping, modified grazing and native grazing and 3.52 (3.15-4.09)MgCha-1 under natural environments. With projected future changes in climate (+1.5, 2 and 5.0°C) over 100 years, the simulations showed that soils under natural environments lost the most C, between 3.1 and 4.5MgCha-1, while soils under native grazing lost the least, between 0.4 and 0.7MgCha-1. Soil under cropping lost between 1 and 2.7MgCha-1, while those under modified grazing showed a slight increase with temperature increases of 1.5°C, but with further increases of 2 and 5°C the median loss of TOC was 0.28 and 3.4MgCha-1, respectively. For the different land uses, the changes in the C fractions varied with changes in climate. An empirical assessment of the controls on the C change showed that climate, pH, total N, the C:N ratio and cropping were the most important controls on POC change. Clay content and climate were dominant controls on MAOC change. Consistent and explicit soil organic C simulations improve confidence in the model's estimations, facilitating the development of sustainable soil management under global change.