Soil Warming Plus Nitrogen Addition Experiment at Harvard Forest since 2006

Climate warming and N deposition are occurring on a global scale with unknown long-term effects on soil microbial communities and the biogeochemical processes they perform. Few studies have examined the interactive effects of elevated temperatures and N additions on soil microbial community structure and function. The overall objective of this study is to investigate whether warming and N additions restructure microbial communities and alter the response of soil C pools to these two stressors. A related study is examining the interactive effects of warming and N additions on plant and ant diversity. This research is being carried out at the Soil Warming x Nitrogen Addition Study at the Harvard Forest which includes four treatments: control, warming (heating to 5 deg C above ambient), warming x N, and N additions only (addition of 50 kg N/ha/yr). Soil respiration measurements have been made monthly since the beginning of the experiment in 2006. In 2010 and 2011, two different methods were compared: static chamber measurements and instantaneous field IRGA assessments. Soil samples (~0-10 cm) have been sampled annually for total C and N, N mineralization, and microbial community composition. Most recently, soils were collected in October 2011 from across the entire profile (0-50 cm) to access potential changes in soil C and N pools with depth. First, 20 x 20 cm forest floor samples were collected. Mineral soils were then collected in 10 cm depth increments to ~50 cm. Samples are currently being analyzed for total C and N, microbial biomass and community composition and fungal gene expression (transcriptomics). Additionally, long-term incubations are being conducted to measure labile and recalcitrant C fractions. Additional soil physical (texture) and chemical (pH, inorganic N) are being measured. Field season measurements of soil respiration indicate that both warming and N additions continue to stimulate CO2 flux, with warming treatments having a stronger effect on respiration than N additions alone. Where warming x N occur together, warming appears to moderate the negative effects of N additions alone on soil respiration. Microbial biomass estimates show the greatest biomass within the warming x N treatment. Declines in microbial biomass with warming or N additions alone support similar findings from the same assessment in 2007 and 2009. Forest floor mass show small declines under warmed conditions. Soil C storage in the mineral soil showed modest changes with warming (-10% difference compared to control). Genomic and transcriptomic pipelines optimized in the Frey Lab are being used to measure the diversity, composition, and function (via gene expression) of the active fungal community in response to warming and N additions. Recent transcriptomic work has demonstrated that chronic N fertilization results in a decrease in expression of several transcripts of fungal laccase and glycoside hydrolases which are enzymes involved in lignocellulose degradation. Overall, our recent results suggest that anthropogenic stressors and seasonal changes continue to interact to affect soil microbial communities and biogeochemical cycles.