Nitrogen addition slows litter decomposition accompanied by accelerated manganese release: A five-year experiment in a subtropical evergreen broadleaf forest

Litter decomposition is a key process for mediating ecosystem carbon (C) and nutrient cycling and is sensitive to increasing atmospheric nitrogen (N) deposition. However, the response of long-term litter decomposition to elevated N input remains unclear, especially in (sub) tropical regions with high ambient N deposition. We conducted a five-year field experiment in an evergreen broadleaf forest in the West China Rain Zone, an area with one of the highest levels of atmospheric N deposition. Three levels of N addition: control (ambient N deposition), low-N (+50 kg N ha(-1) year(-1)), and high-N (+50 kg N ha(- 1) year (-1)); were applied monthly from April 2013. The decomposition process of two leaf litters, Schima sinensis and Lithocarpus hancei, was then investigated from November 2013 to December 2018. For both litters, we observed two distinct decomposition stages: an early stage of rapid decomposition (the first 2 years); and a late stage of slow decomposition (the following 3 years). Nitrogen addition did not affect the early-stage but significantly inhibited late-stage mass loss, thus decreasing decomposition rate or extent. Cellulose degradation was accelerated in the early stage but suppressed in the late stage, while lignin degradation was inhibited throughout the decomposition process. Nitrogen addition affected the concentration and/or release of nutrients during decomposition, and especially significantly accelerated manganese (Mn) release, with a significant decrease in Mn concentration and Mn/lignin ratio. The reduction in Mn concentration may explain the inhibition of litter decomposition after N addition, because it retards the degradation of lignin through reducing ligninolytic enzyme activity. The slowed litter decomposition in this forest is predicted to decrease soil CO2 emissions and increase soil C content.