Soil carbon cycling response to long-term nitrogen addition varies across ecosystem types on nutrient-poor soils

Background/Question/Methods

Human activity has accelerated global inputs of biologically reactive nitrogen twelve fold since 1860. A recurring ecological question is how increased nitrogen availability will affect soil carbon cycling. As the largest terrestrial pool of carbon, even small shifts in soil carbon stocks could have large implications for atmospheric concentrations of CO₂. Despite considerable work aimed at addressing this question, the effects of increased nitrogen deposition on soil carbon cycling remain unclear. A recent meta-analysis by Janssens et al. (2010) reported increased soil carbon storage with nitrogen deposition in forested systems. However, as the authors noted, the effects of nitrogen addition in nutrient-poor soils and across different ecosystem types remain unclear. Our goal was to test the effects of sustained nitrogen deposition on soil carbon stocks and cycling across different ecosystem types at a nutrient-poor site. We used a long-term nitrogen fertilization experiment at the Cedar Creek Ecosystem Science Reserve in Minnesota, which includes grassland, conifer forest, and deciduous forest sites. Since 1999, NH₄NO₃ has been applied three times annually, totaling 10g N/m²/year. Soil cores were taken in October 2011. Microbial respiration was determined using a year-long respiration incubation and soil carbon was measured using dry combustion analysis. 

Results/Conclusions

In contrast to previous studies, we did not observe evidence of increased soil carbon storage with nitrogen addition. Instead, we found an interacting effect of site and nitrogen treatment on microbial respiration (cumulative mg C/ initial g C, p<0.05) and a near-significant interacting effect on soil percent carbon (p=0.08). This interaction was driven by two of the eight sites – an aspen stand and a pine stand – where nitrogen increased cumulative microbial respiration and decreased percent soil carbon. The mechanisms behind these interactions are unknown. One possible explanation could be that the form of carbon is different at these sites, however another pine stand in the study did not show the same response to nitrogen addition, which suggests there may be something else differentiating these sites other than carbon substrate. This work highlights the variability of nitrogen effects on soil carbon stocks and cycling, even among seemingly similar systems. Future work aimed at teasing apart the mechanisms behind observed responses will be necessary to properly understand how global increases in nitrogen deposition may or may not affect the soil carbon cycle.