Net nitrogen mineralization and enzyme activities in an alpine meadow soil amended with litter tannins

Abstract

Condensed tannins in plant litter play an important role in soil nitrogen (N) cycling at the plant–soil interface. However, how soil net N mineralization is affected by litter tannins through their interactions with soil enzymes remains unclear. The aims of our study are to explore the effects of litter tannins with different structures on soil net N mineralization and on the activities of soil enzymes that are directly or indirectly related to N mineralization. Soils were collected from a Tibetan alpine meadow community dominated by a tannin‐rich forb species, Polygonum viviparum. Condensed tannins purified from the litter were separated into the light‐fraction and heavy‐fraction tannins, with the former having a lower mean degree of polymerization and a higher prodelphinidin:procyanidin (PD:PC) monomer ratio in the structure than the latter. Soils were amended with the light‐fraction, heavy‐fraction, and unfractionated tannins, and then incubated for four weeks. The dynamics of soil enzyme activities and soil net N mineralization were monitored during the incubation. The results show that among the enzyme activities tested, the activities of laccase and peroxidase were significantly inhibited by the three tannins, whereas the activity of polyphenol oxidase was only significantly suppressed by the heavy‐fraction tannins. The overall activity of urease was not significantly affected by the tannin additions. In contrast, the activity of N‐acetyl‐β‐D‐glucosaminidase was significantly enhanced by the three tannins. These results suggest that the interactions between litter tannins and soil enzymes rely on both the structure of the tannins and the type of enzymes. Soil net N mineralization was significantly decreased by the three tannins, which could be attributed to the decreased net ammonification and increased immobilization of inorganic N. The light‐fraction tannins caused a more pronounced inhibitory effect on soil net N mineralization than the heavy‐fraction and unfractionated tannins, probably due to the stronger interaction of the PD monomers with soil enzymes and soil organic N compounds than the PC monomers. Overall, this study indicates that litter tannins with different structures could exert different effects on the activities of soil enzymes as well as on soil net N mineralization, which is important for interpreting the role of different litter tannins in N cycling at the plant–soil interface.