Great Plains Climate and Land-Use Effects on Soil Organic Carbon

Soil organic C (SOC) is essential to agricultural productivity and sustainability in response to climate and land-use change. Here, we examine 14 sites across the U. S. Great Plains to determine the sensitivity of important SOC fractions to climatic gradients (temperature and precipitation) and land-use change (native, conservation reserve program [CRP], and cropped). We measured particulate organic matter C (POM-C), soil mineral associated C (Cmin-C), soil microbial biomass C (SMB-C), and soil aggregate stability (% AggStab). All fractions (POM-C, SMB-C, and % AggStab) except Cmin-C were affected by land-use and decreased from native > CRP > cropped sites. Total SOC and all the fractions decreased with increasing temperature and increased with greater clay content. Surprisingly, only the Cmin-C as well as the % AggStab increased with increasing mean annual precipitation (MAP)/ evapotranspiration (ET) ratio; the more labile fractions POM-C and SMB-C did not. Despite greater % AggStab, there was not POM-C protection, suggesting that aggregate turnover and subsequent C loss was greater in sites with more moisture. The Cmin-C fraction comprised the majority of total SOC (70%) and also comprised most of the whole soil response to temperature and moisture gradients. The POM-C fraction could be used as an easily measured indicator of land-use impacts in soil, since POM-C and SMB-C were highly correlated. Conservation practices that promote soil aggregation and reduce disturbance and erosion will be critical in maintaining mineral soil C and ameliorating soil C loss though increased temperatures. These practices have additional benefit of building the POM-C fraction and SMB-C.