Response of fine root decomposition to different forms of N deposition in a temperate grassland

Despite the importance of plant litter decomposition for ecosystem nutrient cycling and soil fertility, it is still largely unknown how this biogeochemical process is affected by different forms of nitrogen (N). Numerous studies have investigated the effects of exogenous N addition on leaf litter decomposition, while the response of decomposing roots and their microbial communities to externally applied N is rarely studied. Fine roots, however, represent a key input to soil organic matter and understanding their decomposition under elevated atmospheric N deposition is important for predicting soil carbon (C) dynamics in response to changes in climatic conditions. In this study, we decomposed fine roots of five dominant grassland species for two years in field plots fertilized with different forms of N in a typical temperate grassland in Inner Mongolia. Ammonium nitrate was selected as inorganic N (IN), while urea and glycine were chosen as organic N (ON). Equal amounts of N (10 g N.m(-2).yr(-1)) with different ratios of IN: ON (control, 10 : 0, 7 : 3, 5 : 5, 3 : 7, and 0 : 10) were added to the soil. Our results showed that all exogenous N additions, either IN or ON forms, stimulated the decomposition rates across species. Furthermore, the treatment with a mixture of IN and ON fertilizers led to the strongest responses in decomposition rate, which were, on average, 20% higher than control, and 12% higher than using just IN addition across the five studied species. Our results suggest that we need to consider the different components in N deposition when examining nitrogen deposition effects on terrestrial ecosystem carbon and nutrient cycles.