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