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We used microbial lipid analysis to analyze microbial biomass and community structure during 6 years of experimental treatment at the Jasper Ridge Global Change Experiment ( JRGCE), a long-term multi-factor global change experiment in a California annual grassland. The microbial community fingerprint and specific biomarkers varied substantially from year to year, in both control and experimental treatment plots. Possible drivers of the variability included plant growth, soil moisture, and ambient temperature. Surprisingly, background variation in the microbial community was of a larger magnitude than even very significant treatment effects, and this variation appeared to constrain responses to treatment. Microbial communities were mostly not responsive or not consistently responsive to the experimental treatments. Both arbuscular mycorrhizal fungi biomarker abundance (16 : 1 ω5c) and the fungal to bacterial ratio were lower under nitrogen addition in most years. Bacterial lipid biomarker abundances (15 : 0 iso and 16 : 1 ω7c) were higher under nitrogen addition in 2002, the year of largest microbial biomass, suggesting that bacteria could respond more to nitrogen addition in years of better growth conditions. Nitrogen addition and warming led to an interactive effect on the Gram-positive bacterial biomarker and the fungal to bacterial ratio. These patterns indicate that in California grassland ecosystems, microbial communities may not respond substantially to future changes in climate and that nitrogen deposition may be a determinant of the soil response to global change. Further, year-to-year variation in microbial growth or community composition may be important determinants of ecosystem response to global change. [ABSTRACT FROM AUTHOR]

It is increasingly recognized that a detailed understanding of the impacts of land use on soil carbon pools and microbially mediated carbon dynamics is required in order to accurately describe terrestrial carbon budgets and improve soil carbon retention. Toward this understanding, we analyzed the levels of biomarkers including phospholipid fatty acids, amino acids, monosaccharides, amino sugars, and several indicators of labile and stabilized carbon in soil samples from a long-term agricultural field experiment. Our results imply that the composition of soil organic carbon (SOC) strongly depends on both the applied fertilization regime and the cultivated crop. In addition, our approach allowed us to identify possible mechanisms of microbial growth and contributions to soil carbon storage under different long-term agricultural management regimes. Amino acids and monosaccharides were quantitatively the most dominant biomarkers and their levels correlated strongly positively with microbial biomass. The relative contributions of the studied biomarkers to the total SOC varied only slightly among the treatments except in cases of extreme fertilization and without any fertilizer. In case of extreme fertilization and with alfalfa as crop type, we found evidence for accumulation of microbially derived monosaccharides and amino acids within the labile OC pool, probably resulting from soil C saturation. Interestingly, we also found an accumulation of microbially derived monosaccharides and amino acids in completely unfertilized plots, which we assumed to be caused by the smaller pore space volume and subsequent oxygen limitation for microbial growth. Mineral fertilization also had substantial effects on soil organic N when applied to plots containing alfalfa, a leguminous plant. Our results demonstrate that over-fertilization, fertilizer type, and the cultivated crop type can have major impacts on the turnover and composition of soil organic carbon, and should be considered when assessing management effects on soil C dynamics. [ABSTRACT FROM AUTHOR]