Influence mechanisms of long-term fertilizations on the mineralization of organic matter in Ultisol

Highlights

• Fertilizations stimulated the mineralization of SOM in Ultisol.

• Fertilizations increased the quantity and quality of SOM and hydrolase activity.

• Fertilizations increased the relative abundance of dominant microbial phyla.

• Fertilizations stimulated SOM mineralization mainly by increasing hydrolase activity.

Abstract

The influence mechanisms of long-term fertilizations with chemical fertilizer (NPK), a mixture of chemical fertilizer and straw (NPKS), animal manure (AM), and no fertilizer (Ck) on the mineralization of soil organic matter (SOM) in an Ultisol was studied in this study. The cumulative amount and rate of SOM mineralization were stimulated by the examined fertilizations, especially AM treatment. The order Ck ≤ NPK < NPKS < AM was found in the content of the soil total organic carbon, dissolved organic carbon, microbial biomass carbon, and organic carbon in each pool. The proportion of Fe/Al-associated organic carbon in the soil total organic carbon showed the opposite order. The fertilizations, especially the AM and NPKS treatments, increased the relative abundance of Actinobacteria, Proteobacteria and Ascomycota as well as the activity of β-glucosidase and invertase. Structural equation modeling showed that the activity of the hydrolases, the content of labile organic carbon, and the composition and diversity of the microbes were positive factors in controlling SOM mineralization. Among the positive factors, the activity of the hydrolases was the most important, and the content of labile organic carbon was more important than the composition and diversity of the microbes. The information implied that the stimulation of fertilizations on the mineralization of organic matter in Ultisol progressed mainly through the increase in microbial activity.

Introduction

Soil organic matter (SOM) is closely related to the emission of greenhouse gas, cultivation of soil fertility, and mobility of soil pollutants. Mineralization is an important decomposition process of organic matter in the soil. A clear understanding on SOM mineralization is very important in regulating the state and function of SOM.

The mineralization of SOM is generally regulated by soil inherent attributes and environmental factors. The quantity and quality of organic matter, the composition, diversity, activity of the microbes, the interaction of organic matter with minerals are important intrinsic factors and the temperature and moisture are the important environmental factors that govern the mineralization of SOM (Brookes et al., 2017; Garcia-Pausas and Paterson, 2011; Jiang et al., 2017; Qiu et al., 2018; Sun et al., 2016; Tardy et al., 2015; Wang et al., 2016). Labile fractions of organic matter with small sizes can be easily mineralized, while recalcitrant fractions with large sizes have lower turnover rates (Jiang et al., 2017). The low structural complexity of labile organic matter also facilitates the mineralization of SOM (Jiang et al., 2017). Garcia-Pausas and Paterson (2011) found that microbial community abundance and structure were the determinants of SOM mineralization. Copiotrophic microorganisms such as Actinobacteria and Proteobacteria contribute directly to the mineralization of SOM (Tardy et al., 2015; Xun et al., 2016; Zheng et al., 2017). However, Xiao et al. (2017b) found that the influence of microbial community composition on the mineralization of SOM was highly dependent on the chemistry of particulate organic matter. Dai et al. (2017) also showed that substrate quality had closer relationship with SOM mineralization than soil microbial communities. Soil minerals, especially oxides, protect organic matter against biodegradation through the formation of organo-mineral complexes (Mikutta et al., 2007; Saidy et al., 2015). Therefore, the role of organic matter, microbes and minerals in SOM mineralization could be different, but the contributions of these components to the mineralization of SOM are not systematically studied.

Ultisol is a typical soil in the tropical zone and is characterized by low fertility (Xu et al., 2018). Organic or inorganic fertilizations are normal means in improving soil fertility. The fertilizations usually increase the content of SOM and change the fraction of soil organic carbon (Ghosh et al., 2018; He et al., 2018; Li et al., 2018; Wu et al., 2019). Long-term fertilizations can alter the diversity and composition of the microbes and the association of organic matter with soil minerals (Tian et al., 2017; Veloso et al., 2019). All these influences can further affect the mineralization of SOM. Dai et al. (2017) showed that the mineralization of organic carbon in paddy soil tended to increase at the higher rates of long-term inorganic N, P, and K fertilization. Similarly, Xie et al. (2017) found that inorganic fertilization and inorganic fertilization plus dairy manure significantly increased the mineralization of organic carbon in an intensively cultivated Anthrosol. However, the influence mechanisms of fertilizations on the mineralization of SOM, especially in Ultisol, are not well understood until today.

The aims of the present study are to investigate the influence mechanisms of fertilizations on the mineralization of SOM in Ultisol by accessing the role of organic matter, microbes, and oxides in the mineralization of SOM.