Importance of land use change in altering soil microbial communities in California grasslands

A disturbance intensity gradient, represented by a set of different land use histories, across several sites on the same soil type, was used to study the effects of past human land use practices on soil biology and nutrient cycling. All sites are located in the grassland uplands or agricultural fields of Salinas and Carmel Valleys in Monterey County, CA. Soil microbial communities, based on PLFA analysis, differ markedly between grasslands and cultivated fields. The microbial communities of the relict native perennial grasslands are different from the perennial grassland oldfields. The soil microbial community does not return to a ‘pre-cultivation composition’, suggesting that above and below ground linkages may still be affected by land use history despite recovery in soil C and N. Also, even when the same perennial bunchgrass species (Nassella pulchra) is present, microbial communities differ between sites, suggesting differentiation due to local variation in environmental conditions. To restore native perennial bunchgrasses in exotic annual grassland, one of the few successful methods is to use tillage and herbicide for two to three years to reduce populations of non-native annuals before seeding with native perennials. A few years later, the microbial community composition of the restored perennial grassland is generally similar to the long-term annual grassland, despite increased biodiversity of native plant species. Understanding the effects of past land use history may be crucial when attempting restoration of ecosystem services and microbial and plant communities, especially if these effects limit or impede the restoration process.