Priming effects of maize growth and photosynthetic substrate supply on soil N mineralization-immobilization turnover

Soil nitrogen (N) mineralization-immobilization turnover (MIT) regulates the inorganic N supply in terrestrial ecosystems. Much research has been done to understand the factors that control the MIT in soils, but how plants-soil-microbe interactions influence the MIT remains to be further explored.

A series of ¹⁵N tracing experiments, including with and without maize (Zea mays L.) planting, maize shading, and maize root endophytes inoculation, were conducted to investigate the drivers of the change in MIT by the presence of plant.

Soil gross N mineralization (M), especially the mineralization of recalcitrant organic-N to NH₄⁺ (M_Nrec, 0.51 mg N kg⁻¹ d⁻¹) was significantly improved by maize compared with the control treatment (without maize) (0.07 mg N kg⁻¹ d⁻¹). M_Nrec significantly decreased after removing maize or covering maize with a black box (preventing photosynthesis). Soil dissolved organic carbon (DOC) concentration significantly decreased after preventing photosynthesis, and showed a significant positive relationship with M_Nrec, confirming that photosynthetic substrate supply was the dominating factor in stimulating M_Nrec. Simultaneously, the release of absorbed NH₄⁺ on the cation exchange sites increased with decreasing M_Nrec when photosynthesis was prevented. Microbial N immobilization (I), especially NO₃⁻ immobilization rate (I_NO3), was significantly stimulated in all maize treatments compared to the control. The I_NO3 of unsterilized soil applied with unsterilized plant endophytes was significantly higher than unsterilized soil applied with sterilized endophytes, indicating a close relationship between endophytes and microorganisms on I_NO3.

M_Nrec was stimulated by the photosynthetic substrate supply, and the increasing microbial N immobilization induced by plant significantly increased the ratio of I to M in the presence of maize, which was beneficial to soil N retention and reduced N loss. Our results provide new insights of the MIT for better understanding the productivity of agricultural systems and its driving factors.