Drainage and Land Use Govern Soil Organic Carbon Stocks and Their Mid-Infrared Spectral Expression

Land use and soil drainage influence soil organic carbon (SOC) dynamics by impacting organic matter accumulation, decomposition and redox conditions. A total of 158 A horizon samples were collected from Alfisols under forest, pasture, and crop-pasture rotation. The soil samples were scanned using a MIR spectrometer (4000-400 cm(-1)). Aerial imagery was used to determine cultivation periods and the age and extent of forest cover. Poorly drained soils had higher SOC stocks and concentrations, bulk density (BD), soil inorganic carbon (SIC) and A horizon thickness (AHT). Moderately well drained soils had lower SOC stock, SOC concentration, BD and AHT. Soils under short-term (12-year) pasture and crop-pasture rotation areas had higher SOC concentration than other land uses. SIC exhibited large variation across land use and drainage classes. After > 75 years, converting forest to cropland increased SOC stock from 34.6 to 62.6 Mg ha(-1) due to a thicker A horizon, but it decreased SOC concentration from 21.1 to 19.5 g kg(-1) compared to soils under forest. Over time, SOC stock and SOC concentration decreased in pasture areas. Soils under forest differed from other soils in some regions of MIR spectra because of specific clay mineral peaks (3695, 3620 and 916 cm(-1)), alkyl groups (2930 and 2850 cm(-1)), carbonates (2517 cm(-1)), amides (1640 cm(-1)), aromatics (1510 cm(-1)), carbohydrates (1050 cm(-1)), and quartz (811, 790 and 693 cm(-1)). The spectral points at 3695, 3620 and 1050 cm(-1) were associated with differences in soil drainage. It was concluded that SOC stock and SOC concentration are influenced by long-term land use and drainage conditions, and the MIR spectrometer can be a valuable tool for identifying their impacts on soil.