Spatial patterns of nitrogen availability, mineralization, and immobilization in northern Patagonia, Argentina

The spatial variation of nitrogen availability, mineralization, and immobilization was studied in heavily grazed areas of the Larrea divaricata and Stipa spp. steppe in northern Patagonia in order to identify the interactions between the nitrogen pool and vegetation. For this purpose, four of the,nest predominant types of vegetation patches and neighboring bare soil patches were selected for study. Soil samples were taken, during 1 year, in the external crown of each patch type, and inorganic N, in situ mineralization, and N in microbial biomass were analyzed. Nitrogen stores in the soil were related to canopy characteristics. Soil beneath undisturbed patches of vegetation held higher stores of available N and N retained in microbial biomass than bare soil. Small patches with incipient vegetation cover represented an intermediate situation between undisturbed patches and bare soil. In situ N mineralization rates were low, and no significant differences were detected among patches. This was attributed to the limiting water conditions prevailing during the study. Labile N retained in microbial biomass was an important source of N readily mineralizable in the soil of undisturbed vegetated patches, under optimal laboratory conditions of soil temperature and moisture. This characteristic makes undisturbed patches and incipient vegetated patches key structures in the dynamics of these ecosystems. Information on future structural and functional changes that may operate under different environmental and managerial scenarios could be inferred from these trends between N availability and vegetation dynamics. The N vegetation trends can also be used for monitoring purposes, as indicators of the state of conservation of these ecosystems.