SPATIAL RELATIONSHIPS OF SOIL MICROBIAL BIOMASS AND C AND N MINERALIZATION IN A SEMIARID SHRUB-STEPPE ECOSYSTEM

Microbial mineralization of both C and N in the shrub-steppe soils of central Washington influence plant productivity and ecosystem stability. Microbial processes in arid ecosystems are in turn influenced by heterogeneously-spaced plants and abiotic variables. Our objective was to determine the spatial relationships of C and N mineralization and microbial biomass to plant location. We measured inorganic N pools, microbial biomass and C and N mineralization potentials (C(o), N(o)) from 205 soil samples positioned around five Artemisia tridentata shrub plants. Most variables showed log normal distributions and all were significantly correlated to each other. Microbial biomass had the highest positive correlation with C mineralization and soluble C. The metabolic C quotient (qCO2) was twice that of other natural forest and grassland ecosystems. In addition the metabolic N quotient (qN) was lower in the shrub-steppe soil compared to other ecosystems. The coupled metabolic quotients indicate the shrub-steppe soil has low substrate quality with a high N immobilization capacity. Geostatistical analysis of spatial relationships showed that samples were spatially related with each other to a distance of 0.5-1.0 m. At sample locations where microbial biomass was high C(o) was also high. In contrast, N(o) was low in these areas. Cross-correlation with plants showed that C(o) was spatially related to shrubs and not to grass plants and that N(o) was not related to any plant location. Cross-correlation analysis revealed that variables that are linearly correlated may not necessarily be spatially correlated. Our study showed that the resource island effect of nutrients and microbial biomass in the shrub-steppe ecosystem is important when estimating microbial processes at the landscape level.