Condensed tannin addition decreased soil nitrate but increased soil enzyme activities in subtropical forest soil

PurposeTannin is the fourth most abundant biochemical compound in vascular plants. Due to its protein-binding capacity, tannin can interfere with soil nitrogen (N) biogeochemical cycling processes and potentially contribute to soil N conservation. Yet little is known about the effect and mechanism of tannin on regulating soil N transformation process.Materials and methodsWe devised microcosm study to evaluate the effects of different concentration and types of tannin on N-cycling processes. Soils were collected from subtropical Chinese fir plantation and incubated for 28 days. Two incubation experiments were carried out simultaneously; one was treated with different concentrations of condensed tannin (CT), including low (1 mg center dot g-1), middle (5 mg center dot g-1), and high (10 mg center dot g-1) concentrations of tannin, while the other was treated with different types of tannin including tannin acid (TA), catechin (CA), and tannin acid + catechin (TC). Soil enzyme activities, MBC, MBN, DOC, DON, soil N transformation rate and respiration were measured in both experiments.Results and discussionIn both incubation experiments, tannin amendment significantly lowered soil nitrate content compared with control soil, as well as soil net nitrification and mineralization rates. In contrast, we observed consistent increase of soil enzyme activities involved in N cycling (amidase, N-acetyl-beta-D-glocosaminidase, polyphenol oxidase, peroxidase, urease and laccase) under tannin treatments. This stimulating effect on soil enzyme activities is likely to due to the fact that tannin may serve as a carbon substrate for soil microorganisms rather than a toxic compound, as indicated by enhancing soil respiration with tannin addition. However, the stimulating effect on soil respiration eclipsed as the incubation proceeded. Furthermore, soil net mineralization rate, on average, decreased by 7.8%, 38.2%, and 69.1%, respectively, under low, middle, and high concentrations of tannin-added soils compared with control.ConclusionsThis study highlights the distinct role of tannin with different concentrations and types in regulating soil N cycling in a subtropical forest. Addition of tannin generally increased soil enzyme activities and soil respiration rate, whereas, the effects of different tannin type on soil N mineralization and nitrification remain largely unpredictable.