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.

Fertilization is a common approach to increase or sustain soil fertility, but its impact on microbial biomass and community structure remains controversial, particularly in paddy soils. In this study, we investigated the effect of different long-term fertilization strategies, beginning in 1986, namely no fertilization, mineral fertilization, mineral fertilization combined with rice straw or chicken manure, on microbial biomass and community composition at four soil depths (0–10, 10–20, 20–30, and 30–40 cm). The extracted soil phospholipid fatty acids (PLFAs) were pooled into gram-positive (G+) bacteria, gram-negative (G−) bacteria, fungi, and actinomycetes groups. Results showed that irrespective of the fertilization type, the abundance of PLFAs decreased with soil depth in the following order due to nutrient decrease along soil profiles: fungi > G− bacteria > G+ bacteria > actinomycetes. Mineral fertilization induced G+ bacteria more than G− bacteria and actinomycetes, which suggested that the inorganic nutrients in mineral fertilizers are utilized more by G+ bacteria than by other microbial groups. Partial replacement of mineral fertilizer with manure further stimulates G+ bacteria at all depths. Redundancy analysis showed obvious microbial separation at the 0−20 and 20−40 cm soil depths due to the rhizodeposition effect and also revealed that the microbial communities were significantly correlated with nutrient content (soil organic carbon and available N) and pH. Overall, our findings highlight microbial community shifts due to different fertilizer types, which provides basic information for understanding how substrate availability controls the structure of soil microbial communities in paddy soil systems.

Microbial biomass (MB) production and turnover strongly affect soil organic carbon (SOC) accumulation. Microbial carbon use efficiency (CUE) and MB turnover in paddy soil were determined using a novel substrate-independent H218O labeling approach and the effect of long-term fertilization with mineral (NPK) or combined (NPK+OM (manure)) amendments in 0-10, 10-20, and 20-30 cm depths were investigated. Long-term fertilization increased microbial C uptake, CUE, and growth rates, and all indexes were the highest in the NPK+OM treatment. The CUE ranged between 0.07 and 0.23 and showed variable behavior with depth: it reduced in the control treatment, indicating that more C was allocated to energy production than biomass growth, and increased in fertilized soils, showing the shift of C usage for biomass growth. The highest CUE was observed at 20-30 cm in NPK and NPK+OM and indicated that microorganisms overcome the nutrient deficiency in deep soil layers by keeping high C uptake rates at a constant CUE. MBC turnover was more rapid in NPK (10-70 d) and NPK+OM (40-65 d) compared to control (80 d) and intensified with the depth. These findings highlight that under long-term fertilization MB turnover can be controlled by CUE. These shifts in the strategies of microorganisms functioning can explain the accumulation of SOC in heavily fertilized paddy soils.