Jasper Ridge Global Change Experiment

Anthropogenic activities have dramatically increased the inputs of reactive nitrogen (N) into terrestrial ecosystems, with potentially important effects on the soil microbial community and consequently soil C and N dynamics. Our analysis of microbial communities in soils subjected to 14 years of 7 g N m-2 year-1 Ca(NO3)2 amendment in a Californian grassland showed that the taxonomic composition of bacterial communities, examined by 16S rRNA gene amplicon sequencing, was significantly altered by nitrate amendment, supporting the hypothesis that N amendment- induced increased nutrient availability, yielded more fast-growing bacterial taxa while reduced slow-growing bacterial taxa. Nitrate amendment significantly increased genes associated with labile C degradation (e.g. amyA and xylA) but had no effect or decreased the relative abundances of genes associated with degradation of more recalcitrant C (e.g. mannanase and chitinase), as shown by data from GeoChip targeting a wide variety of functional genes. The abundances of most N cycling genes remained unchanged or decreased except for increases in both the nifH gene (associated with N fixation), and the amoA gene (associated with nitrification) concurrent with increases of ammonia-oxidizing bacteria. Based on those observations, we propose a conceptual model to illustrate how changes of functional microbial communities may correspond to soil C and N accumulation. The JRGCE was designed to assess grassland responses to single and multiple components of global change, which includes elevated CO2, warming, nitrate amendment and enhanced precipitation. Each experimental unit at the JRGCE was circular, 2 m in diameter, and equally divided into four quadrants of 0.78 m2. The CO2 and warming treatments are operated at the plot level and N and water treatments were operated at the quadrants level in a factorial design. This creates 16 treatments with 8 replicates each. However, an experimental fire burned four replicate blocks in 2011 to simulate the wildfire effect, thus only the other four replicate blocks (a total of 64 samples) were used in this study.