Alternate wetting–drying combined with controlled-release/stable fertilizer enhances soil N availability by altering organic N utilization in rice-microbial system and its dominant microbes

Context

Alternate wetting-drying (AWD) combined with controlled-release/stable fertilizers demonstrate promising potential for enhancing rice yield and N uptake. However, current research paradigms predominantly focus on inorganic N, while the critical role of soil organic N (ON) utilization within rice-microbe-soil continuum remains unexplored.

Objective

This study aims to investigate the shifts in soil N pools composition and availability, along with the underlying mechanism through ON partitioning within the rice-microbial system and its key microbial communities under different fertilization and irrigation regimes.

Methods

Based on the five-year field experiment with two irrigation (flood irrigation, FI; AWD) and five N fertilization treatments (zero N, N₀; conventional N, PUN₁₀₀; 80 % of conventional N, PUN₈₀; 80 % of conventional N applied as controlled-release N and urea, CRN₈₀; and 80 % of conventional N applied as stable compound N and urea, SFN₈₀), an incubation experiment was performed to investigate ON utilization by rice and microbes, using ¹³C,¹⁵N-labelled glycine and ¹³C-phospholipid fatty acids (PLFA) techniques.

Results

Compared to FI, AWD combined with SFN₈₀ and CRN₈₀ significantly enhanced rice yield, N uptake and N use efficiency through optimized soil fertility and microbial activity. This improvement was demonstrated by increased soil nitrification rate, extractable total N (ETN), NO₃^- and free amino acids (FAAs), along with elevated microbial carbon and N entropy (qMBC, qMBN). Furthermore, enzyme activities, including β-glucosidase (BG), invertase (Inv), N-acetyl-β-D-glucosaminidase (NAG) and urease (SUE) were also significantly enhanced. ¹⁵N uptake analysis revealed significantly higher values in CRN₈₀ and SFN₈₀ compared to PUN₁₀₀ and PUN₈₀, with AWD promoting the assimilation of both intact and mineralized N forms by rice and microbes. Notably, AWD increased the proportion of ¹⁵N-glycine uptake by rice while reducing its utilization by microbes. Subsequent correlation analysis established positive relationships between rice yield/NUE and glycine uptake parameters. The synergistic effect of AWD and optimized fertilization increased total ¹³C-PLFA contents and Fungi:Bacteria ratio, while reducing the Gram-positive:Gram-negative ratio. Gram-positive and Gram-negative bacteria, and general FAME (fatty acid methyl ester) groups have been identified as primary competitors with rice for soil ON. Redundancy analysis highlighted NO₃^-, FAAs, and qMBN as key drivers governing microbial composition and ON partitioning within the rice-microbial system.

Conclusions

This study demonstrated that AWD enhanced soil N availability and rice competitive advantage for ON uptake over soil microbes, particularly in CRN₈₀ and SFN₈₀. The improved soil bacterial community and Fungi:Bacteria ratio, together with increased nitrification rate and FAAs contents, created the optimal condition for ON utilization in rice-microbial system, which simultaneously improves soil N availability and ultimately enhances both rice yield and NUE.

Implications

This study highlights the importance of distinct N allocation strategy within rice-microbial system and ON utilization for rice growth under optimized fertilization and irrigation management. However, the accurate quantification of soil ON availability over long-term cultivation, as well as the functional mechanisms of specific microbial communities, warrant further investigation.