Functional response of an Austrian forest soil to N addition

Abstract

Elevated atmospheric reactive nitrogen (NR) deposition is considered one of the key components of human induced global change, threatening biodiversity and possibly altering carbon sequestration, one of the forest’s key ecosystem services. Carbon sequestration is the net result of plant production and of soil organic matter (SOM) decomposition. Ignoring the impact of N deposition on plant growth, decomposition or any major physical, biological or anthropogenic process that alters the rate of conversion of soil organic matter to atmospheric CO2 (decomposition) will have profound implications for the global C budget and consequently climate change. Soil nitrogen cycling is predicted to change as a result of increased atmospheric N deposition and mineralization due to temperature increases. However, experimental results on the effects of increased N input on SOM decomposition in the field are inconsistent, reporting positive, negative and neutral responses of SOM to N input. We set out to test the impacts of elevated reactive nitrogen NR addition, specifically on the soil processes in the field, independently of forest production effects. Using a suite of conventional, natural abundance and isotope pool dilution methods in situ, we traced C and N transformations of soil microbial and gaseous pools and monitored concomitant changes in gross mineralization and nitrification rates, as well as enzymatic activity. Over a number of growing seasons in a spruce dominated Austrian forest we found evidence to suggest N addition significantly reduces gross N mineralization rates and enzyme activity, in-line with an emerging consensus that N deposition reduces soil fungal abundance and activity, ultimately resulting in greater stocks of soil organic carbon. Simulated elevated nitrogen deposition decelerated SOM decomposition and consequently increased soil carbon storage, an N input effect on soil processes independent of the effect of N on tree growth and forest production.