Microbial community properties in relation to land management and nitrogen deposition in desert soils of southern California

To examine the effects of three land management practices including organic farming, integrated crop management and conventional farming system on microbial community properties, a polyphasic approach by analyzing soil microbial biomass, population densities of culturable bacteria (CFU), and community level substrate utilization (CLSU) patterns with Biolog (TM) EcoPlates, functional redundancy, soil respiration, and soil dehydrogenase activity was used. Results showed that the different agricultural practices had no effects on soil microbial biomass, colony forming unit (CFU) counts, soil dehydrogenase, and soil respiration. CLSU data showed that different farming system had significant effects on microbial metabolic response. However, there was no consistent effect of farming practice on functional redundancy for selected substrates. The effects of nitrogen deposition on soil carbon storage and soil fungal: bacterial ratios were studied in southern California coastal sage scrub soils along a nitrogen deposition gradient. Three sites with high, medium and low nitrogen depositions were fertilized with 50 kg N ha-1 yr-1 monitored for 20 months. Soil extractable nitrate and ammonia, soil total carbon and nitrogen and PLFA fungal: bacterial ratios were measured seasonally. Results showed nitrogen fertilization significantly increased soil nitrate content in high nitrogen deposition sites, but did not affect soil nitrate content at the low deposition site, which suggested nitrogen saturation in the high nitrogen deposition area. Soil total carbon was decreased by nitrogen fertilization especially at the low nitrogen deposition site. Soil F/B ratio was mostly affected by sampling date and location. In these soils F/B increased during the dry summer season and decreased in the rainy season, which corresponded with seasonal changes in soil carbon content. The effect of nitrogen fertilization on soil F/B ratio was marginal and varied with locations. In the location with high N deposition, the F/B ratio decreased after 20 months of fertilization at a rate higher than 100 kg N ha-1 yr-1 . The overall results suggest that increased inputs of nitrogen from atmospheric deposition, here simulated by fertilization, does not cause significant short term changes in carbon storage or carbon processing pathways in the semiarid soils of S. California.