Microbial indicators related to soil carbon in Mediterranean land use systems

Soil C mineralization activity and microbial indices were measured in agricultural, forest and grassland soils, characterized by different managements in two adjacent fields: agricultural (conventional vs. organic), forest (conifer vs. broadleaf), grassland (naturally grazed grass vs. alfalfa ungrazed). The aim of the study was to determine if the land use and the management practices modified C mineralization activity and kinetics, microbial biomass size, microbial and metabolic quotients (C-mic/C-org ratio and qCO(2)). Land use induced significant changes in microbial biomass content, in most of the microbial indices, and in the cumulative CO2 production which showed the highest values for agricultural soils (300 mu g C-CO2 g(-1) 28 d(-1)) and the lowest for grassland soils (120 mu g C-CO2 g(-1) 28 d(-1)). In agricultural soils, a large availability of potentially mineralizable C (C-0) was determined. Forest soils mineralization activity was mainly dependent on environmental factors such as aboveground tree species and soil pH which probably induced changes in microbial community structure and/or functionality. This could also explain the significant differences found on chemical, biochemical and microbiological properties of the two forest soils under the two managements. Grassland soils were characterized by a high stability of soil organic matter (SOM) and consequently a low mineralization activity. Although total nitrogen and soil ON ratio varied between the two soils, the management practices did not affect C mineralization activity. In conclusion the results show that microbially mediated processes can be largely affected by land use confirming its role as a significant driver of soil C changes. (c) 2007 Elsevier B.V. All rights reserved.