Cultivation and Slope Position Effects on Soil Organic Matter

Abstract Our objective was to determine the effect of land use and slope position on the organic matter (total and active) of two related soils with different A‐horizon thicknesses. To accomplish this, we sampled the surface 7.5 cm of soil from top, mid and bottom slope positions of paired native grassland and wheat‐fallow cropland toposequences of the Ascalon (Aridic Argiustoll) and Renohill (Ustollic Haplargid) soil series. We measured total organic matter (organic C and Kjeldahl N), and active organic matter (microbial biomass, and mineralized N and respired C in 20‐d laboratory incubations) concentrations. Concentrations of all measured properties were much lower in cultivated than in native sites, and smaller fractions of the total organic matter were in microbial biomass or in forms mineralized during incubation. Total organic matter concentrations increased downslope in the Renohill soil, but decreased downslope in the Ascalon soil. Active organic matter concentrations differed less between between slope positions than between land uses or soil series. Concentrations of active organic matter were equal in the two soils, even though the Renohill soil had been tilled longer (>25 yr vs. 1 yr) and had a thinner native A horizon (13 vs. 32 cm). One year of cultivation of the Ascalon soil reduced both its active organic matter concentrations and the fractions of the total organic matter in active forms by approximately the same amount as did 25 yr of cultivation of the Renohill soil.