Remnant tree effects on soil microbial carbon and nitrogen in tropical seasonal pasture in western Mexico

Selection of plant species for agro-silvo-pastoral or ecological reclamation programs must be based on a deeper knowledge of the existing relationships between plant species and soil nutrient dynamics in each ecosystem. We evaluated the seasonal pattern of soil microbial carbon (C) and nitrogen (N) under two remnant tree species (Caesalpinia eriostachys and Cordia elaeagnoides) in a tropical seasonal pasture dominated by Panicum maximum in western Mexico. Soil samples were taken from under two arboreal species and P. maximum in rainy and dry seasons. The soil C:N ratio was higher under P. maximum [17] than under both tree species [15]. The soil microbial C (Cm) was higher under C. elaeagnoides than under C. eriostachys and P. maximum. Magnitude and direction of effect of the two remnant tree species on soil biogeochemistry changed with seasonal rainfall. The interaction of plant species and seasonal rainfall did have an effect on soil microbial N (Nm). Soil samples from April and July had the lowest microbial N concentrations under the three plant species, increasing four fold in September under C. elaeagnoides and P. maximum. At the end of the wet season, C. elaeagnoides clearly had the highest Nm values (130 mu g N g(-1)), suggesting that this tree species has a higher capacity to protect soil N within microbial biomass than C. eriostachys, because under C. elaeagnoides the soil had more organic matter due a higher input of litter and root chemical quality. Therefore, C. elaeagnoides would be the best plant species to implement in agro-silvo-pastoral programs or ecological reclamation of TDF pastures. (C) 2008 Elsevier Masson SAS. All rights reserved.