Microbial decomposition of soil organic matter is driven in part by soil nitrogen (N) concentration. This ecosystem service is being jeopardized by changes in N availability and soil acidity that have resulted from a tripling of atmospheric N deposition in the last century. Experiments simulating N deposition indeed report the accumulation of organic matter, an indication that microbial activity has slowed. We hypothesize that the magnitude of organic matter accumulation among fertilization experiments can be explained by background N deposition levels. To test this, we sampled soils at seven eastern US hardwood forests where background N deposition rates varied from 3.2 – 12.6 kg N ha-1 y-1, five of which also received N fertilization (25 – 50 kg N ha-1 y-1) to simulate future N-deposition levels. Our objectives were to: 1) measure soil fungal and bacterial relative abundance and activity in response to simulated and natural N deposition, and 2) compare experimental responses along a natural N deposition gradient. We measured microbial abundance by phospholipid fatty acid analysis (PLFA), microbial activities key to ecosystem processes (C and N mineralization rates), and soil pH.
Results/Conclusions
Soil fungal and bacterial abundance responded to simulated N deposition differently depending on background N deposition levels. Where background N deposition was low, fertilizer increased fungal abundance by 20% and bacterial abundance by 25%. Where N deposition was high, fertilizer decreased fungal abundance by 16% and bacterial abundance by 12%. In fertilized plots, the decline in abundance with background deposition was stronger for fungi (R2 = 0.30) than bacteria (R2 = 0.25). While soil acidity varied among sites and along the atmospheric N deposition gradient, it was unchanged by N fertilization treatments and did not affect fungal or bacterial abundance. Carbon-mineralization rates were reduced along the background N deposition gradient but were not altered by N addition. In contrast, N-mineralization rates did not vary across the gradient or between experimental treatments. In summary, microbial abundance and C-mineralization activity were reduced by background N-deposition at the regional scale and declines in microbial abundance due to simulated N deposition were greater when background N deposition was high. Reductions in C-mineralization and microbial abundance, especially fungi which decompose complex organic matter, explain why soil C accumulation occurs in hardwood forests experiencing elevated N-deposition.