Soil nitrogen heterogeneity in a Dehesa ecosystem

The Dehesa are savannah-like ecosystems that occupy extensive areas in the mediterranean regions of Southern Spain and Portugal. The changes in the so il nitrogen (N) potential mineralisation rate, microbial biomass-N and avai lable mineral N in soils were studied over 1 year in a Dehesa ecosystem, in the SW of Spain. The temporal variability of soil samples was compared by using the coefficient of variation of samples taken on a monthly basis. Spa tial variability of the soil samples was measured by comparing the variogra ms generated with geostatistical techniques. The potential N mineralisation reached a maximum in the spring and summer samples, and a minimum in the a utumn. However, the soil NH4+, NO3- and microbial biomass-N showed maximum levels in winter, compared with minimum levels in the spring and summer. Th e potential mineralisation rate showed a higher temporal variability in the samples taken under tree canopies, due to highest maximum and similar mini mum levels reached during the sampling period. The potential mineralisation rate, microbial biomass-N and soil mineral N were higher under the tree ca nopies than in the open grassland. The spatial distribution of the soil org anic matter and available NH4+ was strongly influenced by tree canopy size. The semivariograms were adjusted to a spherical model, indicating a maximu m NH4+ variability at a certain distance from the tree crown diameter. The same was not the case for the spatial distribution of the soil NO3- from th e tree crown. Our results indicate that the tree component of a Dehesa ecos ystem is a major influence in the spatial and temporal heterogeneity of soi ls.