Relationships between soil organic carbon and precipitation along a climosequence in loess-derived soils of the Central Great Plains, USA

Understanding the effects of varying average annual precipitation on organic carbon in loess-derived soils can assist the reconstruction of paleoclimates from buried paleosol sequences as well as facilitate the prediction of soil organic carbon (SOC) change in response to future climate forcings. The goal of this study was to examine trends in SOC and its relationships to particle-size distribution (PSD) and soil aggregation in surface loess along a precipitation gradient in the Central Great Plains of the United States. Soil cores were collected from undisturbed portions of seven pioneer cemeteries to a depth of 50 cm across a transect spanning northwest Kansas into western Missouri. Pioneer cemeteries were selected to minimize any effect from land use and disturbance. Soil cores were cut into 2.5-cm segments, which were prepared and analyzed for bulk density, SOC, and aggregated and disaggregated PSD from laser diffraction. Annual precipitation, depth, and PSD were used in the multivariate analyses to explain the distribution of SOC and complexed organic carbon (COC)-an indicator of soil physical quality-as well as bulk density. Precipitation was the variable that most explained SOC, COC, and bulk density. A proxy for microaggregation, geometric mean shift (GMS), was developed for this study and defined as the difference between the geometric mean of the aggregated and disaggregated PSDs. Microaggregation occurred below an OC:clay ratio of 0.163 in the upper 50 cm of the loess-derived soils analyzed. Among the variables considered, COC had the highest coefficient of determination (R-2 = 0.871). Our findings indicate that future climate forcings resulting in precipitation changes may have an effect on COC and, thus, soil physical quality of loess-derived soils. (C) 2015 Elsevier B.V. All rights reserved.