Professor Cornell University Ithaca, New York, United States
Abstract: Soil is the largest terrestrial carbon stock, and processes that result in losses from this pool have important implications for the global carbon cycle. One such loss mechanism is soil microbial respiration, which is driven by the decomposition of soil organic matter. Microbial decomposition can be limited by nitrogen availability, and therefore, it might be expected that N availability should constrain microbial growth and respiration. However, many experiments have documented suppressed soil respiration in response to increased N. This discrepancy could be explained in part by nitrogen’s role as both a limiting nutrient and acidifier. When nitrogen is a limiting nutrient in soils, alleviating N limitation should stimulate microbial growth and respiration. In contrast, acidification can inhibit microbial growth and so N additions that acidify may suppress respiration. We test these hypotheses by measuring microbial respiration from soils sampled at three depths (forest floor, 0-3 cm, and 3-10 cm mineral soils), collected from a replicated nitrogen-pH manipulation study (+N, +pH; +N, -pH; -pH, control) in mixed temperate forests. We quantified the effects of nitrogen availability and soil acidity on microbial respiration by incubating soils from each depth in centrifuge tubes and measuring the accumulation of headspace CO2 over 24 hours. We expected that treatment effects would be consistent among the soil depths, but that the magnitude of the effects would decrease as soil depth increased.
In the forest floor, all three treatments tended to suppress microbial respiration. Deacidifying N addition was expected to increase respiration; instead, suppression of respiration by this treatment indicates that microbial activity is limited by neither pH nor N availability. In surface mineral soils, increased N availability did not affect respiration, and to our surprise, acidification alone substantially stimulated respiration. This unexpected acidification response may have been caused by an increase in belowground carbon allocation by trees, as acidification can reduce soil nutrient availability. Greater belowground carbon allocation by plants would increase the availability of substrate for microbial respiration, which could explain the increased flux. Overall, we found no support for the hypothesis that nutrient N stimulates decomposition and respiration by increasing microbial growth. Further, acidification alone suppressed respiration in the forest floor, but stimulated it in mineral soils, suggesting that acidification may have impacted respiration by both plant- and microbially-mediated mechanisms that vary with soil depth.
