Changes of diazotrophic communities in response to cropping systems in a Mollisol of Northeast China

Changes of diazotrophic communities in response to cropping systems in a Mollisol of Northeast China Jiaxun Zou1,2,*, Qin Yao1,*, Junjie Liu1, Yansheng Li1, Fuqiang Song2, Xiaobing Liu1 and Guanghua Wang1 1 Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin, China 2 College of Life Science, Heilongjiang University, Harbin, China * These authors contributed equally to this work. ABSTRACT Submitted 6 January 2020 Accepted 24 June 2020 Published 15 July 2020 Corresponding author Guanghua Wang, Nitrogen-fixing microorganisms play important roles in N cycling. However, knowledge related to the changes in the diazotrophic community in response to cropping systems is still rudimentary. In this study, the nifH gene was used to reveal the abundance and community compositions of diazotrophs in the cropping systems of continuous cropping of corn (CC) and soybean (SS) and soybean-corn rotation for growing corn (CSC) and soybean (SCS) in a black soil of Northeast China. The results showed that the abundance of the nifH gene was significantly higher in cropping soybean than in cropping corn under the same cropping system, while remarkably increased in the rotation system under the same crop. The Shannon index in the CC treatment was significantly higher than that in the other treatments, but the OTU number and Chao1 index had no significant change among the four treatments. Bradyrhizobium japonicum was the dominant diazotrophic species, and its relative abundance was at the lowest value in the CC treatment. In contrast, Skermanella sp. had the highest relative abundance in the CC treatment. A PCoA showed that the diazotrophic communities were separated between different cropping systems, and the variation caused by continuous corn cropping was the largest. Among the tested soil properties, the soil available phosphorus was a primary factor in determining diazotrophic community compositions. Overall, the findings of this study highlighted that the diazotrophic communities in black soils are very sensitive to cropping systems. Academic editor Blanca Landa Subjects Ecology, Microbiology, Soil Science Keywords Black soil, Bradyrhizobium sp., Continuous cropping, Crop rotation, Diversity, nifH gene Additional Information and Declarations can be found on page 15 INTRODUCTION DOI 10.7717/peerj.9550 Copyright 2020 Zou et al. Distributed under Creative Commons CC-BY 4.0 The black soil zone of Northeast China is one of the four large Mollisol regions in the world and plays an important role in maintaining food security in China (Liu et al., 2008). In this region, soybean and corn are two major crops that are either continuously cropped or grown annually in rotation with each other. It is well known that continuous cropping can decline soil quality, which seriously decreases the quantity and quality of crop products. Previous studies have shown that continuous cropping has led to several How to cite this article Zou J, Yao Q, Liu J, Li Y, Song F, Liu X, Wang G. 2020. Changes of diazotrophic communities in response to cropping systems in a Mollisol of Northeast China. PeerJ 8:e9550 DOI 10.7717/peerj.9550 problems in soils, for example, deficiencies in soil nutrition (Ashworth et al., 2018), decreases in soil enzyme activity (Chavarría et al., 2016), increases in the autotoxicity of root exudates (Huang et al., 2013), increases in pests and diseases (Torres et al., 2018) and imbalances in soil microbial communities (Bennett et al., 2012). Although the problems caused by continuous cropping of corn is less harmful than that of continuous cropping of soybean (Xu, Li & Li, 2004), several studies have revealed that continuous cropping of corn has caused the lodging and surge of pests, which have damaged corn yield (Jirak-Peterson & Esker, 2011; Liang et al., 2017). The cause of the barriers of continuous cropping are very complex, and some biotic and abiotic factors are commonly related to crop-yielding decline. Among these factors, biotic factors, such as changes in soil microbial communities, are often considered to be the major reason for barriers to continuous cropping (Dias, Dukes & Antunes, 2015). Several studies have shown that continuous cropping destroyed the intrinsic balance of soil microorganisms and increased abundance of crop pathogens such as soil-borne Fusarium spp. (Xiong et al., 2015; Zhu et al., 2018). Biological nitrogen fixation (BNF) is the main route of inputting nitrogen (N) in natural ecosystems. Approximately 52–130 Tg of N is input into terrestrial ecosystems annually through the BNF method (Davies-Barnard & Friedlingstein, 2020). BNF is carried out by a wide range of microorganisms containing nitrogen-fixing enzymes. The enzymes are regulated by multiple genes, and the nifH gene is one of the most conserved functional genes and is widely used as a biomarker gene for studying nitrogen-fixing microorganisms or diazotrophic communities in soils (Coelho et al., 2009). A plethora of studies have shown that the diazotrophic community is highly sensitive to variations in soil factors such as pH (Fan et al., 2018), organic matter (Calderoli et al., 2017) and available nutrient content (Collavino et al., 2014). Since soil factors are greatly affected by different cropping systems (Jha, Prasad & Misra, 2004; Moisander et al., 2012), whether different cropping systems have a direct or indirect impact on diazotrophic communities in soils, especially in black soils, is rarely reported. Using molecular fingerprinting methods, such as polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) and the terminal restriction fragment length polymorphism (T-RFLP) method, several studies have investigated the changes in diazotrophic communities in response to cropping systems. For example, Pereira E Silva − et al. (2013) revealed that NHþ 4 -N, NO3 -N, and soil pH were the determining factors in shifting the diazotrophic community structure by using the PCR-DGGE method. Using the T-RFLP method, Xiao et al. (2010) revealed that diazotrophic community diversity in continuous cropping of soybean fields was different from that in rotational cropping. However, the low resolutions of molecular fingerprinting methods restricted in-depth data analysis and did not allow us to fully understand the changes in diazotrophic community structures. Recently, high-throughput sequencing (HTS) has become the mainstream method for studying diazotrophic communities (Wang et al., 2017a; Hu et al., 2018). Using this method, Wang et al. (2017b) revealed that soil pH and nutrient availability had a cooperative effect on diazotroph abundance, while soil nutrient Zou et al. (2020), PeerJ, DOI 10.7717/peerj.9550 2/19 availability was the main factor. Noticeably, most studies of using HTS method to analyze the changes in microbial community structures in response to different cropping systems have mainly focused on bacter (...truncated)