Subsurface Vs. Surface Soil Carbon Change in Long-Term Maize Systems.

The role of soil organic carbon (SOC) sequestration at depth on mitigating greenhouse gas emissions from agriculture is poorly understood, and guidelines for low-cost assessment of SOC change beyond the surface are lacking.  In this study, we measured the distribution and variability of total SOC, labile C (permanganate oxidizable C, POXC), total soil nitrogen (TSN), bulk density (BD), percent rock fragments (RF), and soil texture to a 75 cm depth in several long-term (8- to 40-year) maize cropping systems in the Northeast, Midwest, and Great Plains regions of the U.S..  We evaluated: 1) the influence of management practices (tillage, crop rotation, and irrigation) and their duration on changes in SOC stocks calculated on both a fixed depth and equivalent soil mass basis, and 2) the contribution of SOC changes at depth on whole-profile SOC stocks in relation to sampling requirements and associated costs. Significant regional and management effects on SOC stocks were documented, but in almost all cases the impact of management was confined to the upper profile (e.g., above 40 cm), with no significant effects in deeper layers.  In those few cases where SOC increases at lower depths were detected, the magnitude of impact this had on whole-profile SOC stock was small and would not likely justify the required high sampling costs except over decadal time periods. The use of geostatistical models for predicting SOC at depth from upper profile measurements, and the use POXC and TSN as proxy-measure indicators of small changes in SOC will be discussed. High RF percent soils had greater variability (increasing sampling requirements), and less capacity for C sequestration as SOC concentration in the fine earth fraction and depth of SOC change in response to management did not compensate for reduced fine earth volume in the upper profile.  We show the importance of reporting SOC stocks on an equivalent soil mass basis because the inverse relationship between BD and SOC can mask management effects when calculated to a fixed depth. Results will be discussed in relation to new decision support tools for low-cost SOC assessment strategies.