Abstract

"Agricultural practices influence the dynamics of soil organic matter (MOS) and its fractions. In this study, the following labile fractions of soil organic matter were determined: free light fraction (FLL), intra-aggregate light fraction (FLI), microbial biomass and mineralized C-CO2, on volcanic soil with different rotations, and later their use was evaluated as biological indicators of the impact of agricultural practices on the soil. The study was carried out in an eight year field experiment, with different productive systems (rotations) in a randomized complete block design. The free light fraction (FLL) was determined by density fractionation with NaI (1.8 g cm-3), and the intra-aggregate light fraction (FLI) was obtained by sonication (1,500 J s-1). Microbial biomass was quantified using the chloroform fumigation-incubation (FI) technique, and the basal soil respiration (C-CO2 evolution) was determined by incubation for a 10-day period. Increased soil use intensity decreased (P ≤ 0.05) C and N FLL contents, from 1.69 g C-FLL kg-1 soil (5-year rotation with alfalfa, Medicago sativa L.), to 0.49 g C-FLL kg-1 (annual crop rotation). However, these contents in FLI did not show a clear and consistent tendency (P ≤ 0.05). Soil biomass C and N decreased (P ≤ 0.05) with higher soil use intensity, from 551 to 264 μg C-CO2 g soil-1 and from 106 to 35 μg N-(NO3- + NH4+) g soil-1, respectively. The three studied indices were appropriate indicators to determine changes in soil organic matter quality as a result of agricultural practices."