PS 40-97 - Nitrogen Deposition is not an Indicator of Continental-Scale Soil C Sequestration

Abstract: Soils are the largest terrestrial carbon (C) stock and therefore provide one of the most promising opportunities for enhanced C sequestration. Soils could potentially offset as much as 35% of anthropogenic C emissions, but increased global nitrogen (N) deposition presents uncertainties for future soil C storage. Previous studies simulating N deposition have reported both soil C gains and soil C losses. Thus, there is no consensus on how N deposition affects soil C stocks, and there are few, if any, experiments aiming to explain these contradicting results. We hypothesized that N addition has opposing effects on particulate organic matter (POM) and mineral-associated organic matter (MAOM) pools and that differences in pool sizes across sites explain previous conflicts in results. We utilized established long-term N fertilization experiments across North America and measured changes in POM and MAOM pools, along with extractable inorganic N and soil pH. We found that soil C responses to N deposition are site dependent. And while there was no universal soil C response to N deposition, soils systematically responded to N-induced changes in soil pH, with larger declines in pH resulting in greater POM accumulation.

Yet, soil acidification was ultimately caused by N addition. Therefore, we employed a lab incubation to separate the effects of N addition and acidification. Control soils from the aforementioned fertilization experiment were subjected to one of two treatments: 50 kg N/ ha/ yr, or, the hydrogen ion equivalent that would be produced if all 50 Kg N were nitrified. Once again, N addition produced conflicting POM responses across sites, but acid addition caused declines in pH, and the magnitude of acidification correlated with the magnitude of POM accumulation. Additionally, pH declines resulted in reduced CO₂-C respiration, while the change in extractable inorganic N indicated no correlation with microbial respiration rates. Ultimately, soil pH, not N availability, controls soil C fate across large spatial scales and total soil C responses are dependent on changes in POM and MAOM pools.