The effects of C, N and P additions on soil microbial activity under two remnant tree species in a tropical seasonal pasture

Although the availability of soil nutrients can constraint microbial activity, little is known about the interactive effects on soil microbial activity in tropical soils of nutrient addition and plant species. To test this effect, incubation experiments were carried out to assess the effects of C, N and P addition in soil samples from under two remnant tree species (Caesalpinia eriostachys and Cordia elaeagnoides) and a dominant grass species (Panicum maximum) in a tropical seasonal pastures in western Mexico. Substrate-induced respiration method was used to determine microbial activity in soil samples from dry and rainy seasons. In the dry season, the addition of C, N and P had no effect on the soil microbial activity, but this activity was strongly influenced by the plant species. The soil associated with C. elaeagnoides had a higher C mineralization and a lower net C immobilization in microbial biomass than the soil associated with C. eriostachys. In contrast, in the rainy season soil microbial activity vias influenced by the interaction between the nutrient addition treatments and plant species. The addition of P enhanced microbial activity under C. elaeagnoides, while under C. eriostachys, added N increased accumulation of C in the microbial biomass. The differential response of soil microbial populations under both remnant tree species was explained by the soil's pH buffering capacity and by the amount of microbial biomass. However, under the grass species the addition of CNP and N increased C mineralization, but C and CNP addition treatments decreased net C immobilization. These results suggest that nutrient addition increased the decomposition of soil organic C, but the released C was not stabilized in the pasture soils. Thus, the fertilization of pasture soils is not a reliable soil management practice, because it does not contribute to sustaining the availability of soil C and nutrients during the growing season. The utilization of silvo-pastoral systems can be a better alternative for the management of the tropical pastures, but the selection of tree species is critical for successful results. (C) 2003 Elsevier B.V. All rights reserved.