Amounts of substrate carbon and nitrogen control the decomposition of soil organic matter

Abstract:

Climate change, mainly caused by rising atmospheric CO₂ and nitrogen (N) deposition due to human activities, is postulated to increase energy and nutrient inputs to soils that can accelerate or retard soil organic matter (SOM) decomposition, a phenomenon called the priming effect. Yet, the direction and magnitude of priming in response to different amounts of energy and nutrient inputs remain elusive. Here we examined the effects of carbon (C) and N additions on priming, microbial turnover, extracellular enzyme activities, CO₂ fluxes, and C accumulation in four different ecosystems. We applied low and high C (¹³C glucose; 350 and 1000 µg C g^-1 wk^-1) and C with N (NH₄NO₃; 35 and 100 µg N g^-1 wk^-1) for five weeks. We found: 1) high C and high C+N stimulated weekly priming in the first two weeks and then leveled off, indicating soil microorganisms may have a short-term of accelerated growth and activity but quickly adapt to frequent inputs of high substrate amounts, 2) high C induced greater cumulative priming, microbial turnover, and β-glucosidase activities than low C, 3) high C+N had significantly lower cumulative priming, turnover, and β-glucosidase activities than high C, 4) high C and high C+N stimulated greater CO₂ fluxes and C accumulations than low substrate inputs. These results suggest that the amount of substrate (energy and nutrient) was a determinant factor in modulating the rate of SOM decomposition, microbial turnover, enzyme activities, and C balance. Overall we demonstrate that increased energy inputs can quickly accelerate SOM decomposition, but concurrent nutrient inputs can suppress such process, which could have a huge impact on terrestrial C storage and global biogeochemical C cycling under climate change.