Abstract
Root exudates can greatly modify microbial activity and soil organic matter (SOM) mineralization. However, the mechanism of root exudation and its stoichiometric ratio of C/N controlling upon paddy soil C mineralization are poorly understand. In this study, we used a mixture of glucose, oxalic acid, and alanine as root exudate mimics, employing three C/N stoichiometric ratios (CN6, CN10, and CN80) to explore the underlying mechanisms involved in C mineralization. The input of root exudates enhanced CO2 emission by 1.8–2.3-fold than that of the control. Artificial root exudates with low C/N ratios (CN6 and CN10) increased the metabolic quotient (qCO2) by 12% over those obtained at higher stoichiometric ratios (CN80 and C-only), suggesting a relatively high energy demand for microorganisms to acquire organic N from SOM by increasing N-hydrolase production. The stoichiometric ratios of enzymes (β-1,4-glucosidase to β-1,4-N-acetyl glucosaminidase) promoting organic C degradation compared to those involved in organic N degradation showed a significant positive correlation with qCO2; the stoichiometric ratios of microbial biomass (MBC/MBN) were positively correlated with carbon use efficiency. This suggests that root exudates with higher C/N ratios entail an undersupply of N for microorganisms, triggering the release of N-degrading extracellular enzymes. This in turn decreases SOM mineralization, implying the C/N ratio of root exudates to be a controlling factor. Our findings show that the C/N stoichiometry of root exudates controls C mineralization by the specific response of the microbial biomass through the release of C- and N-releasing extracellular enzymes to adjust for the microbial C/N ratio.
KEYWORDS: Root exudates – Stoichiometric ratios – Microbial biomass – Extracellular enzyme – Metabolic quotients – Carbon use efficiency
INTRODUCTION
Plants can dramatically modify the soil environment through the rhizosphere, either by the release of C from plant roots (e.g., root exudates) or via the rapid uptake of their associated microorganisms by the soil (Kuzyakov, 2002; Jones et al., 2009; Fisk et al., 2015; Liu et al., 2019; Xiong et al., 2019). Across different plant species, around 1–10% of photoassimilated C is released into the soil as root exudates (Jones et al., 2004; Phillips et al., 2011; Qiao et al., 2014; Yin et al., 2014), consisting primarily of sugars, while also containing organic acids, amino acids, phenolics, and other secondary metabolites (Haichar et al., 2014; Yuan et al., 2017). In addition to acting as direct substrates for microorganisms and possess nutritional value, the stoichiometric ratio of the C and N-containing are considered to have an impact on their utilization by microorganisms (Wild et al., 2014; Liu et al., 2020). Thus, elucidating the role and underlying mechanisms of action of different exudate C/N ratios on microbial substrate utilization is of key importance for understanding soil C and N cycling, and likewise for soil C sink strength.
Several mechanisms have been proposed to explain the changes in the microbial decomposition of soil organic matter (SOM) because of root exudate addition, namely: (i) root exudates provide energy for the stimulation of SOM decomposition and change the chemical and physical properties of the soil environment (Qiao et al., 2016; Zhu et al., 2018; Mehnaz et al., 2019; Du et al., 2020); (ii) labile C promotes microbial growth, which in turn increases the N demand and likewise microbial N mining from SOM (Manzoni et al., 2010; Dijkstra et al., 2013; Qiao et al., 2016; Zhu et al., 2018); and (iii) microbial C and N demands cause community shifts that alter microbial-mediated C decomposition (Phillips et al., 2011; Wild et al., 2014; Yuan et al., 2017; Fang et al., 2020; Wei, 2020). Moreover, impacts of nutrients on stoichiometry also need to be considered (such as the addition of C substrates like glucose, accompanied by several levels of N applications), which provide a proportion of C that is incorporated into the microbial biomass at the expense of CO2 emission, and becomes stabilized as soil organic carbon (SOC) (Creamer et al., 2014).
Recently, research on the stoichiometry of root exudate compounds combined with mineral N [e.g., (NH4)2SO4] has shown that higher C/N ratios increase CO2 emissions due to high microbial N demand (Du et al., 2020; Liu et al., 2020). Subsequently, resource stoichiometry alters microbial community composition to gain the required elements and increase SOM mineralization (Zhu et al., 2018; Wei, 2020). Extracellular enzyme activities, such as C- and N-acquiring enzymes, can also reflect the resource demands of microbial communities (Schimel, 2003; Hill et al., 2012). β-1,4-glucosidase (BG) and β-1,4-xylosidase (XYL) are the largest contributors to the degradation of cellulose and hemicellulose, respectively. Similarly, β-1,4-N-acetyl glucosaminidase (NAG) plays a role in the degradation of chitin and is involved in the organic N pool. The enzyme activities have similar stoichiometries, which could further relate to the elemental stoichiometry of the microbial biomass for microbial nutrient assimilation and growth (Sinsabaugh et al., 2008; Sinsabaugh and Follstad Shah, 2012). To date, the understanding of the influence of root exudates with different stoichiometric compositions on microbial growth and activities is still in its infancy, especially with respect to the interaction between microbial biomass stoichiometry and extracellular enzyme stoichiometry depending on the root exudate compositions.
In the present study, we applied different C:N stoichiometric ratios of artificial root exudates to soil to investigate how they modify microbial activities (including extracellular enzyme production and microbial biomass stoichiometric ratio) and likewise influence SOM mineralization. The stoichiometric ratios of artificial root exudates were regulated by combining different amounts of glucose, oxalic acid, and alanine, which represented low-molecular-weight organic compounds, namely, sugars, organic acids, and amino acids, respectively. We determined the activities of extracellular enzymes involved in C and N decomposition, microbial biomass C (MBC) and N (MBN), along with the associated CO2 emissions after the addition of root exudates. We hypothesized that (1) the application of C-only artificial root exudates leads to an imbalance in resource stoichiometry, thereby inhibiting microbial activity and SOM mineralization; and (2) the inclusion of N-containing root exudates meets microbial stoichiometric requirements, thus promoting microbial growth and increasing SOM mineralization.
MATERIALS AND METHODS