Response of bacterial communities, enzyme activities and dynamic changes of soil organic nitrogen fractions to six-year different application levels of biochar retention in Northeast China

Abstract

Composition and availability of soil organic nitrogen play a key role in soil N retention and supply. The application of traditional chemical fertilizers can improve soil production and crop yields. However, it also leads to low N utilization efficiency and soil solidification. Previous studies have shown that fertilizers added with biochar can improve soil fertility and facilitate soil nutrient transformation by regulating soil biological indicators. However, whether biochar used with fertilizers can promote nutrient cycling in the long term, which remains to be tested. Therefore, this study aims to explore the long-term effects of biochar with different levels used with fertilizers on soil fertility, N supply potential of organic N, and microbial communities, thus, providing a theoretical reference for the rational management of biochar applications. In this study, a field experiment was carried out from 2013 to 2018 assess dynamic changes in organic N fractions and diversity and richness of microbial communities under the impacts of chemical fertilizers combined with different levels of biochar. Biochar with no amendment (CK), biochar with 15.75 t ha−1 (BC1), biochar with 31.5 t ha−1 (BC2), and biochar with 47.25 t ha−1 (BC3) are four biochar levels that were tested in this study. The results show that compared with CK, those other three biochar levels can increase soil organic matter, C/N, and pH values. Acid-hydrolysable N (AHN) makes a major contribution to organic N in soil. Biochar retention for six years has significantly affected AHN levels, as well as its four fractions, namely, amino acid N (AAN), acid-hydrolysable ammonium N (AN), amino sugar N (ASN), and hydrolysable unknown N (HUN). Some vital biological characteristics, with which each AHN fraction is primarily regulated, can be altered with the application of biochar. That is, AAN is mainly controlled by enriched Verrucomicrobia, Cyanobacteria, Glucosylbacteria, and Nitrospira, and reduced soil urease, while AN is mainly controlled by enriched Verrucomicrobia, Bacteroidetes, Gemmatimonadetes, Acidobacteria, and Proteobacteria. ASN is regulated by enriched Acidobacteria and depleted soil protease, while HUN is regulated by Gemmatimonadetes and enriched soil nitrate reductase. It has been found that microbial and soil enzyme activities are involved in N cycle processes, indicating that the formation of AHN fractions is closely associated with biochar-induced specific microbial and enzyme activities. This study has provided new insights into the influences of biochar with different levels on distribution of soil organic N fractions, and shifts in soil enzyme activities and microbial communities.