Response of the subtropical forest soil N transformations to tannin acid-organic nitrogen complexes

It is well known that the hydrolysable tannin (tannin acid) and condensed tannins in a plant can influence the availability of soil nitrogen (N) through the formation of tannin–organic N complexes. However, the lack of research on the effect of tannin acid–organic N complexes on soil N hinders our understanding of the subsequent role of these complexes. In this line, an incubation experiment was carried out with the addition of tannin acid-arginine and tannin-bovine serum albumin (BSA) complexes, where tannin acid was represented as the hydrolysable tannin. For the necessary comparisons, tannin acid, arginine, or BSA was added to the soil samples. The results showed that tannic acid addition quickly decreased protein contents at the beginning of incubation and decreased soluble organic N (SON) after 3 days to produce the inorganic N in the soil as compared to control. Tannin-arginine complexes increased NH4+–N compared with control and this increased degree was found lower than arginine treatment. Moreover, the addition of tannin–BSA complexes did not significantly increase soil NH4+–N as BSA alone treatment did. NO3−–N and N2O emission increased in each treatment compared with control indicating nitrification was not limited. The increase of NH4+–N was mainly attributed to the decrease of SON after 3 days of incubation. These results suggested that tannin acid quickly formed the complexes with protein to impact soil N. While, the effects of tannin-arginine and tannin–BSA complexes on soil N transformation were quite dissimilar. Thus, this study highlighted the subsequent role of tannin–organic N complexes under the influence of tannin. Tannin and tannin–organic N complexes naturally coexist in the ecosystem to impact soil N dynamics and may interact with each other as well.