Vertical distribution of soil organic carbon and nitrogen under warm-season native grasses relative to croplands in west-central Indiana, USA

Establishment of grasslands can be an effective means of sequestering soil organic carbon (SOC) and reducing atmospheric CO2 that is believed to contribute to global warming. This study evaluated the vertical distribution and overall sequestration of SOC and total nitrogen (N) under warm-season native grasses (WSNGs) planted 6–8 years earlier relative to a corn (Zea mays L.)–soybean (Glycine max L.) crop sequence, and switchgrass (Panicum virgatum) relative to tall mixed grasses of big bluestem (Andropogon gerardi), indiangrass (Sorghastrum nutans), and little bluestem (Andropogon scoparius). Paired soil samples from 0–15, 15–30, 30–60 and 60–100 cm depth increments were taken from WSNGs and adjoining croplands at 10 locations, and from switchgrass and adjoining tall mixed grasses at four locations in three major soil types of alfisols, mollisols, and entisols in Montgomery County, Indiana. Significant differences in SOC and N concentrations of WSNGs and croplands were limited to the surface 30 cm. On average, SOC concentrations in the surface 15 cm depth were higher in WSNGs than croplands (average: 22.4 and 19.8 g kg−1 C, respectively) but significant differences were observed in just 4 of 10 locations. Similarly, surface soil SOC concentrations were not different for switchgrass (22.1 g kg−1) relative to tall mixed grasses (21.4 g kg−1). Soil N concentrations never differed significantly among land use treatments. On average, SOC mass calculated to 1.0 m depth was 9.4% higher under WSNGs than cropland (P < 0.058), and 8.1% higher in switchgrass relative to tall mixed grass (P < 0.054), but soil N mass was the same for both WSNGs and cropland. Vertical distribution under WSNGs of SOC mass was 26, 21, 28, and 25%, and of total N mass was 31, 25, 28 and 16%, in the 0–15, 15–30, 30–60, and 60–100 cm depth intervals, respectively. Even though we acknowledge the potential influence of soil variability or prior landscape processes on our results at some locations, we estimated that WSNGs sequestered an average 2.1 Mg C ha−1 yr−1 more than the corn–soybean sequence.