Harnessing moringa seed extract for control of soil nitrate accumulation and nitrous oxide emissions on the Loess Plateau

Biological nitrification inhibitors play a vital role in mitigating soil nitrate (NO₃⁻-N) accumulation, improving nitrogen (N) utilization efficiency, and reducing environmental pollution. Moringa (Moringa oleifera Lam.) seed extract (MSE) has a great potential to inhibit soil nitrification as it contains antioxidant bioactive compounds (e.g., polyphenols, flavonoids). However, the specific inhibitory effect of MSE on nitrification in different types of agricultural soils are still elusive. Here, grain field and greenhouse vegetable soils were chosen to investigate the effects of MSE on soil nitrification, microbial NO₃⁻-N immobilization, and nitrous oxide (N₂O) emissions. A laboratory incubation experiment was conducted with five treatments: no N fertilizer (control), N fertilizer alone, N fertilizer with MSE, N fertilizer with 3,4-dimethylpyrazole phosphate (DMPP), and N fertilizer with glucose. Compared to N application alone, MSE addition resulted in significantly higher NH₄⁺-N concentrations between 156 and 1274 mg kg⁻¹ in the soils, with low NO₃⁻-N concentrations <20 mg kg⁻¹ throughout the incubation period. While DMPP only suppressed the abundance of ammonia-oxidizing bacteria (AOB), MSE reduced the abundances of both ammonia-oxidizing archaea (AOA) and AOB. MSE addition also increased the concentration of microbial biomass carbon (MBC) in the soils and decreased the cumulative N₂O emissions by 63–74 %. The findings uncover that MSE inhibited soil nitrification by suppressing AOA and AOB activities and concurrently enhanced microbial immobilization of inorganic N. The results provide a promising new material and idea to reduce NO₃⁻-N accumulation in agricultural soils.