COS 120-2 - Disentangling nitrogen’s role as a limiting nutrient and acidifier: nutrient nitrogen availability, not acidification, suppresses soil respiration

Abstract: Soil respiration is the second largest terrestrial carbon flux, vastly exceeding fossil fuel emissions. It is composed of two separate fluxes: autotrophic respiration, which is driven by plant respiration, and heterotrophic respiration, which comes predominantly from microbial decomposition. Many studies have demonstrated that increasing nitrogen availability can suppress soil respiration, but the mechanisms underpinning this response are difficult to discern. This challenge arises in part because each component flux can respond differently to changes in nitrogen availability, both through its role as a limiting nutrient and as an acidifying agent. 1) As a limiting nutrient, increasing nitrogen availability could reduce soil respiration by decreasing plant belowground carbon allocation to roots and microbial symbionts; however, supplying nitrogen to nutrient-limited soil microbes could offset this effect by increasing saprotroph biomass. (2) Alternatively, as an acidifying agent, nitrogen may decrease soil decomposer biomass and increase belowground carbon allocation. Previous studies have generally been unable to disentangle the roles of these potential mechanisms in soil respiration responses because most nitrogen fertilization studies apply forms of nitrogen that acidify, and nitrogen deposition gradients often co-occur with acidification. Here, we test these hypotheses (1, 2) by measuring respiration fluxes from fixed collars in a replicated, ten-year, nitrogen x pH manipulation (+N, +pH; +N, -pH; no N, -pH; no N, no pH change) study in mixed temperate forests in central New York.

Both of the nitrogen additions (acidifying and de-acidifying) led to large reductions in soil respiration. Further, we found that annual soil respiration was strongly negatively associated with nitrogen availability across all treatments. These findings are consistent with the hypothesis that increased nitrogen availability suppresses soil respiration through nitrogen’s role as a limiting nutrient, likely by way of reducing belowground C allocation by trees. Though we expected that acidification would slow decomposition by reducing decomposer biomass, it surprisingly did not affect total soil respiration in this study. However, it is possible that acidification both promoted increased belowground carbon allocation by trees and reduced microbial decomposer biomass, offsetting effects on the total flux. Overall, our results suggest that increasing N availability suppresses soil respiration by alleviating nutrient limitation and reducing belowground carbon allocation by trees. In contrast, acidification did not suppress soil respiration, and ongoing work seeks to clarify if offsetting heterotrophic and autotrophic respiration responses led to this null effect.