Vertical Distribution and Stratification of Organic Carbon Fractions in a Semi-Arid Calcareous Soil.

Changes in the size and distribution of soil carbon (C) and nitrogen (N) pools in response to land use and management are known to affect soil quality and biogeochemistry, particularly in dryland ecosystems where soil-related constraints limit crop production. Improved understanding of such changes is critical to maximize the potential of semi-arid soils for C storage and ecosystem recovery. Our objectives were (i) to evaluate the influence of land use on the concentration, vertical distribution and stratification ratio (SR) of soil organic carbon (SOC), and total nitrogen (TN); (ii) to separate SOC fractions, permanganate oxidizable carbon (POXC), particulate organic carbon (POC), and mineral organic carbon (MOC) and (iii) to determine any relationships among these C fractions within a Judith Clay Loam (fine-loamy, carbonatic Typic Calciustolls) from central Montana. Soil samples were collected from 0–10, 10–20, 20–30, and 30-40 cm in croplands under no till (NT) and strip till (ST), and of the adjoining soil under conservation reserve program (CRP) and native vegetation (NV). Land use change primarily altered the amount and distribution of labile (POXC and POC) and also some of the stable (MOC) soil carbon pools. The pool size of SOC and TN decreased with land use classification in the direction NV>CRP>Ag-NT=Ag-ST. In contrast, concentrations of POXC and POC fractions in soil under CRP were closer to those under NV. Stratification ratios increased with soil depth and were minimally and variably affected by land management (i.e. NT, ST, or CRP). POXC was positively related to SOC, and was more closely related to the mineral fraction MOC than the coarse fraction POC. Croplands under NT or ST exhibited a similar SOC gradient from surface to sub-soil layers. In contrast, CRP offers great potential to restore SOC fractions previously depleted by agriculture.