Examining historical impacts and mitigation potential of soil tillage practices in the Community Land Model

Soil tillage is a ubiquitous management practice on croplands globally, and is thought to have historical impacts on climate through accelerated decomposition of soil C and increased CO₂ emissions on agricultural land. Conservation tillage (e.g. 'no-till') has been proposed as a mechanism for mitigating climate change through increased soil C storage on croplands. The magnitude of these effects remains unknown and likely varies through space and time. Earth system models (ESMs) are useful tools to understand the spatial and temporal impacts of tillage, though few studies have examined different soil tillage practices in ESMs. ESMs tend to underestimate historical C fluxes and aggregate biogeochemical impacts of land management practices on climate relative to empirical records. Here we use the Community Land Model (CLM), the land component of the Community Earth System Model, to assess 1) biogeochemical effects of soil tillage by testing the global sensitivity of soil C stocks in CLM to intensive conventional tillage practices of different intensities over the historical time period (1850-2014), and 2) climate change mitigation potential of conservation tillage practices by evaluating the sensitivity of CLM soil C stocks to lower levels of tillage intensity associated with conservation tillage practices for a future climate scenario (2014-2100). Results indicate that increasing tillage intensity increases soil C decomposition rates; total losses in soil C due to intensive tillage practices are 17 Pg C for the historical time period, or 11% of estimated historical C emissions from land use change. Larger differences in tillage intensity and lower decomposition rates for conservation tillage relative to intensive tillage results in greater soil C storage over the future climate scenario. Historical losses are tightly coupled temporally and spatially with land use change to crops and losses due to tillage increase over time with cropland expansion. These results show that carbon impacts of tillage depend on parameters governing the decomposition rates when tillage is applied, as lower tillage intensities have smaller soil carbon impacts.