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Straw returning may stimulate soil microbial activity, thereby influencing microbial-mediated soil nitrification, which can lead to nitrate leaching and nitrogen (N) loss. However, its effects under long-term nitrogen fertilization remain unclear. At an experimental station with 34 years of fertilizer application (0, 135, and 270 kg ha−1 N), we investigated how nitrogen fertilization and straw returning affected the soil potential nitrification rate (PNR) and ammonia-oxidizing microorganisms (AOM). Our results suggest that N fertilization concurrently inhibits soil PNR, but this inhibition can be alleviated by straw returning, particularly with low nitrogen fertilization (p < 0.05). Long-term N fertilization significantly decreased the abundance of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and complete ammonia-oxidizing bacteria cladeB (CAOB-cladeB). Straw returning increased AOA abundance and diversity, especially with low or no fertilization (p < 0.05). Furthermore, the partial least squares path model demonstrated that AOA abundance affected soil PNR by altering the AOA community. According to random forest analysis, soil pH and AOA beta diversity were the primary factors affecting soil PNR (explaining 10.76% and 10.03% of the variation, respectively). Overall, our findings highlight the importance of straw returning and AOA in soil nitrification under long-term nitrogen fertilization, emphasizing the need to consider these interactions for sustainable agriculture. [ABSTRACT FROM AUTHOR]

This study investigated the influence of green manure planting on the spectroscopic properties of dissolved organic matter (DOM) in saline–alkali soil under freshwater leaching conditions at different soil depths. The UV254, UV253/UV203, α300, α355, SUVA254, SUVA260, and SR ultraviolet parameters indicated reductions in the content of large molecular substances, benzene ring substitution degree, colored dissolved organic matter, aromaticity, and hydrophobic components in the soil leachate DOM with an increasing soil depth. Compared with the non-green manure treatment control, green manure planting mitigated the leaching of dissolved organic matter in soil during saline irrigation, with rape green manure demonstrating superior effectiveness. Utilizing three-dimensional fluorescence combined with parallel factor analysis, this study analyzed three fluorescent components of soil leachate DOM: C1 (visible-light fulvic acid), C2 (humic acid), and C3 (tyrosine-like protein). The combined contribution of the two humic substance components (C1 + C2) was approximately 70%, indicating the dominance of humic substances in leachate DOM. The fluorescence parameters of soil leachate DOM included an average of the fluorescence index (FI) values between 1.4 and 1.9, low humification index (HIX) values consistently below 4, and biological index (BIX) values ranging from 0.8 to 1.0, suggesting a mixed source, low humification degree, poor stability, and moderate self-source characteristics. Compared with the non-green manure treatment control, both the green manure treatments exhibited a relatively higher proportion of biogenic sources and humification degree in soil leachate DOM. This suggests that planting green manure can reduce the relative DOM content under freshwater leaching conditions, increase the proportion of biogenic sources in soil leachate DOM, and enhance soil humification. Planting rapeseed green manure can diminish the leaching of DOM from land sources and augment soil humification. [ABSTRACT FROM AUTHOR]

Straw return with nitrogen (N) fertilizer can generally regulate soil N cycling by affecting the microbial community, thereby influencing crop productivity. Bacteria and fungi play a key role in nitrification, while the effects of soil microbial interactions that include protists under the condition of straw return and N fertilizer remain uncertain. Here, we evaluated the importance of the soil microbiome, particularly protists and ammonia‐oxidizing bacteria (AOB), in regulating the soil N transformation potential and wheat yields under different fertilization practices. The results showed that the activity of β‐1,4‐N‐acetylglucosaminidase (NAG) and the potential nitrification rate (PNR) increased significantly by 156.4% ~ 312.3% and 183.1% ~ 402.6% with the addition of N fertilizer and straw, respectively. The effect on PNR was greater for straw than fertilizer addition. The fertilization practices changed the microbial community structure, and there were stronger responses from the protists and AOB. The soil microbiome formed three closely co‐occurring modules (Modules 0, 1 and 2). Among them, Module 0 showed strong positive correlations with the soil total N, NAG, PNR and crop yield and may be the key ecological cluster. Moreover, the predatory protists Cercozoa, Conosa and Ochrophyta in Module 0 were significantly positively associated with the activity of NAG and the PNR, yields and AOB. The predatory protists in Module 0 may change the soil potential nitrification rate and enhance the yield of wheat by preying on the competitors of AOB. The results suggest that the incorporation of straw promotes the N transformation potential and wheat yields linked to trophic interactions between predatory protists and AOB in fluvo‐aquic soil. [ABSTRACT FROM AUTHOR]