Dissolved organic carbon (DOC), a small and labile forms of soil organic carbon, plays an important role in various biogeochemical processes (Taylor and Townsend 2010). Its composition directly provides information on the production or degradation of organic matter (Schmidt et al. 2011). It primarily originates microbial metabolites, plant biomass and organic matter (Malik and Gleixnerm 2013; Roth et al. 2019). Its production and turnover are affected by biotic factors (e.g., microbial diversity and enzyme activity) and abiotic factors (e.g., moisture and temperature). Among all factors, the forms of soil clay minerals appear to be one of most significant factors because they affect the decomposition of soil organic carbon by the physical and chemical protection (Yu et al. 2017). For example, exchangeable mineral ions preferentially bind organic acid molecules that are highly unsaturated or rich in oxygen by ionic or covalent bonds (Yu et al. 2012). The latest information about DOC have shown that various forms of soil mineral ions affect the transformation of DOC by selectively retaining the DOC carbon groups (e.g., carboxylate bound to the surface of calcium carbonate) or by regulating the abundances of microorganisms that affect DOC turnover (Leinemann et al. 2018; Wen et al. 2014).
Soil aggregates influences the DOC composition and turnover by mediating the kinetics processes of soil organic matter (Xu et al. 2021). Mineral-bound organic molecules can be retransformed through the destruction or construction of soil aggregates (Zhao et al. 2020). Aggregate stabilization processes (e.g., cementation and agglomeration) can alter remobilized DOC molecules or retransformation of carbon-containing groups by spatial isolation (Xu et al. 2019). Those changes in DOC are accompanied by variations in soil physicochemical properties. Soil physicochemical properties (e.g., soil nutrient availability, pH and cation exchange capacity) not only influence the molecular properties of DOC (Smebye et al. 2016) but also affect the efficiency of the microbial metabolism in degrading carbon-containing groups (Chen et al. 2017). The microbial community probably assembles new molecules or groups in the consumption and degradation of DOC (Wu et al. 2018).
Besides biotic factors and abiotic factors, the agronomic practices such as tillage (Bongiorno et al. 2019), irrigation (Said-Pullicino et al. 2016) and fertilization (Li et al. 2019) also influence DOC production and turnover. Fertilization is one of the primary factors influencing DOC turnover in agronomic practices. Chemical nitrogen fertilizer application promotes the preferential mineralization of saturated carbon groups (e.g., alkanes) and accumulation of unsaturated carbon (e.g., aromatic groups) in DOC by changing the ratio of carbon to nitrogen (C/N) in soils (Wang et al. 2020). Meanwhile, a long-term fertilization experiment indicates that aromatic carbon content increases with nitrogen addition (Li et al. 2019). The stepwise degradation of cellulose to extended aromatic compounds, an important contributor to DOC production, has shown to be more efficient in nitrogen-fertilized than non-fertilized plots (Yuan et al. 2018).
The current understanding of fertilization on DOC turnover suggests that fertilization directly influence the amount or ratio of labile group and inert group in DOC by the inherent inputs of exogenous matter (Singh et al. 2014), or indirectly impact DOC composition by regulating microbial activity and community (Wang et al. 2015; Zang et al. 2017). However, recent results reveal that DOC composition and turnover may be more susceptible to the influential factors of its surrounding environment (e.g., nutrient availability, aggregate stability and mineral forms) than to fertilization (Li et al. 2019; Wang et al. 2021). Furthermore, one study found that long-term fertilization influences DOC composition mainly via regulating soil aggregate size, not direct effect (Xu et al. 2021).
Gray desert soil, developed from loess parent material, is an important zonal soil in temperate desert areas with rich in the minerals of calcium and magnesium (Wang et al. 2016). The area where it is located has less precipitation and a high rate of DOC production and turnover. The increased mineralization of organic matter caused by the rapid turnover of DOC has become one of the main obstacles to the development of local agriculture and animal husbandry (Wang et al. 2016). The relationships between DOC properties and minerals of calcium or magnesium have been reported (Minick et al. 2017; Zhu et al. 2019), but the underlying mechanisms of how mineral forms of calcium or magnesium act as mediators to regulate the DOC turnover are still largely unknown.
Applying the Fourier transform infrared (FTIR) spectroscopy and 13C nuclear magnetic resonance (13C-NMR) techniques has allowed us to capture the variation in carbon-containing groups of DOC (Hayes et al. 2017). This provides a more detailed molecular-level understanding of DOC-group turnover. In this study, we conducted a long-term field experiment comparing different fertilization managements and used FTIR spectroscopy and 13C-NMR to quantitatively determine how the different fertilization strategies affected the composition and chemical properties of DOC. We hypothesized that soil physicochemical properties are the key agents for the DOC turnover driven by fertilization practices. We characterized the chemical composition of DOC and analyzed the implications of fertilization on the turnover of DOC groups and the role of soil physical and chemical properties in the turnover process. We expect that the results will contribute to understanding the DOC turnover and help to predict carbon cycling in agro-ecosystems.