Sensitivity of early indicators for evaluating quality changes in soil organic matter

Labile soil carbon fractions and enzymatic activity are typically more sensitive to changes in soil management practices or environmental conditions than total soil organic matter, and consequently they are well-established as early indicators of change due to crop residue incorporation, soil use or pollution. However, less information has been reported for volcanic soils under different cropping systems where there are unique C stabilization mechanisms that promote faster accumulation of soil organic matter. The aim of our study was to compare the influence of three types of crop rotations (annual, arable crop and crop-pastures rotation) on total soil C and N as well as on several early indicators (mineralizable C and N, soil microbial biomass (C(mic)), particulate organic matter (POM) and dehydrogenase activity), using a long-established experiment in volcanic soil (medial, amorphic, thermic Humic Haploxerands) of the central valley of Chile. The results found that the sensitivity to management changes varied between treatments for microbial biomass C and N, mineralized C and N, and dehydrogenase activity, and that in general these responses were proportionately greater than those found for total C. However, differences in POM-C content were less consistent Our results confirm the value of these labile fractions and this enzyme as sensitive indicators for detecting changes in SOM in the short term both for annual arable crop and crop-pasture rotations. In conclusion, microbial biomass and dehydrogenase activity were the indicators most sensitive to changes in soil-crop management increasing in 1-2 times their sensitivity in comparison with other indicators. However, increases in microbial biomass C in all treatments were proportionally lower than increases in organic C that resulted in low values for the microbial quotient (C(mic) to C(org)) (although there were different between treatments) suggesting that the stabilizing mechanisms acting in volcanic soils can result in proportionally larger amounts of stabilized organic matter that are contained in the total organic C of the soil. POM was not an appropriate indicator of changes in soil organic matter (SOM) for this volcanic soil under distinct crop rotations. The inclusion of pastures (5 years of alfalfa or white clover) in the annual crop rotation promotes the increase of both total and labile fractions of soil organic matter in comparison with annual arable crop rotations, while the inclusion of short rotation pastures (2 years of red clover) had a minimum effect on biological parameters. (C) 2009 Elsevier B.V. All rights reserved.