Abstract:

Nutrient degradation in saline-alkali soil in the Yellow River Delta, particularly the decline in nitrogen availability, has severely constrained the sustainable development in the regional agriculture. Organic nitrogen, the primary form of total nitrogen in soil, comprises components whose distribution and transformation (such as acidogenic ammonia nitrogen and amino acid nitrogen) directly impact the nitrogen supply capacity. Salt stress may further disrupt these cycles. Owing to its distinctive structural and functional properties, such as high adsorption capacity and carbon sequestration potential, biochar has been shown to enhance soil nitrogen activation by increasing the levels of acid-soluble ammonium and amino acid nitrogen. Returning straw to the field contributes to nitrogen retention and release by adding organic matter, stimulating enzyme activity, and improving the stability of aggregates. However, there is a lack of systematic research on the differences in the mechanisms of the two approaches in regulating the organic nitrogen components of saline-alkaline soil, particularly their effects on key enzymes of the nitrogen cycle, microbial processes, and the aggregate-nitrogen coupling relationship. This study focused on moderately saline-alkaline soils in the Yellow River Delta. By examining the effects of three treatments—no corn stalk return (control group, CK), direct corn stalk return (ST), and application of corn stalk-derived biochar (BI)—on soil organic nitrogen components, enzyme activities, microbial diversity, and soil aggregates, the objective was to assess their influence on soil nitrogen mineralization, enzymatic functions, and microbial diversity, thereby elucidating the mechanisms by which biochar and straw return improve saline-alkali soils. The results indicated that both ST and BI treatments significantly increased total total nitrogen (TN) and three forms of inorganic nitrogen. Notably, BI treatment resulted in significantly greater increases in TN, ammonium nitrogen, and fixed ammonium than the ST treatment (p<0.05). ST treatment, on the other hand, had a more pronounced effect on enhancing the total inorganic nitrogen and nitrate nitrogen content (p<0.05). Both treatments markedly improved the levels of organic nitrogen components in saline soil, with the BI treatment showing a more substantial effect (p<0.05). Compared to the control group, BI increased the acid-hydrolyzable total nitrogen (TAHN), non-acid-hydrolyzable nitrogen (NHN), acid-hydrolyzable ammonium nitrogen (ASN), amino acid nitrogen (AAN), amino sugar nitrogen (AMN), and hydrolyzable unknown nitrogen (HUN) by 38.86%, 75.62%, 35.05%, 34.38%, 39.61%, and 59.19%, respectively. Both ST and BI treatments significantly enhanced the activities of soil urease, protease, alkaline phosphatase, sucrase, and catalase, with the BI treatment showing a notably stronger effect (p<0.05), resulting in activity increases of 139%, 104%, 43.4%, 50.2%, and 76.8%, respectively. BI treatment significantly increased soil microbial diversity (p<0.05), and the richness index, Simpson diversity index, and Shannon diversity index increased by 15.0%, 9.57%, and 20.8%, respectively, compared with the ST treatment. BI had a more significant effect on increasing the acid-hydrolyzable nitrogen content of aggregates of the four particle sizes (p<0.05), especially in aggregates with particle sizes of 0.053-0.25 mm. Correlation analysis revealed that the organic nitrogen components in saline-alkali soil were significantly and positively associated with both soil enzyme and microbial activity. This suggests that straw and straw-derived biochar primarily modify the organic nitrogen profile by stimulating enzymatic processes and enhancing microbial function, thereby promoting the transformation and retention of nitrogen. TAHN, a relatively active component of the soil nitrogen pool, serves as a direct indicator of improved nitrogen supply potential resulting from soil amendment. The distribution of acid-hydrolyzable organic nitrogen components followed the order AAN>AMN>HUN>ASN, with AAN and AMN identified as the primary sources of mineralizable nitrogen. Biochar treatment increased the concentrations of AAN and ASN by 34.38% and 39.61%, respectively, indicating enhanced accumulation of active nitrogen owing to improved microbial nitrogen fixation capacity. In contrast, NHN exhibited minimal changes in response to the treatments, further supporting the conclusion that biochar and straw primarily enhance nitrogen availability by regulating the levels of active nitrogen components. Overall, returning straw biochar to the field had a more significant effect on increasing the total nitrogen in saline-alkali soil, whereas returning straw had a more significant effect on increasing nitrate-nitrogen. Both treatments significantly increased the organic nitrogen components in the soil, with the enhancement effect of biochar on TAHN, AMN, ASN, AAN, and HUN being more pronounced. Biochar had a more significant effect than straw return in increasing the nitrogen contribution rate of the large and medium aggregates. Biochar is conducive to retaining acid-hydrolyzable nitrogen content in the soil microaggregates of saline-alkali soils. Both straw and straw biochar return significantly increased the enzyme activity in saline-alkali soil; however, straw return had a more pronounced effect, especially on urease. Furthermore, the return of straw and straw biochar significantly enhanced microbial activity in saline-alkali soil, with straw biochar exhibiting a stronger effect. This suggests that straw application markedly enhances soil enzyme activity, facilitating nitrogen transformation. The microbial diversity index, along with the Simpson and Shannon indices, exhibited the greatest increase under biochar treatment, indicating that biochar improved the microbial habitat, boosted microbial abundance and activity, and consequently strengthened the soil’s nitrogen mineralization and retention capacity. Owing to its porous structure and ability to stimulate microbial processes, biochar significantly improves nitrogen transformation in soil, alters the distribution of organic nitrogen fractions in saline-alkali soils, and contributes positively to nitrogen retention and stability. The organic nitrogen fractions in saline-alkali soil showed significant positive correlations with both soil enzymes and microbial activity, suggesting that straw and straw-derived biochar primarily influence organic nitrogen composition by enhancing these biological factors. In summary, both straw biochar and straw application offer distinct advantages in improving the nitrogen cycle in saline-alkali soils. Future studies should focus on developing an integrated application model that combines the strengths of both approaches to enhance soil fertility and ecological function more effectively, thereby providing a scientific foundation for the sustainable management of saline-alkali-affected lands.

Key words: biochar, straw, saline-alkali soil, organic nitrogen components, soil enzyme activity, soil microbial diversity

摘要： 施用生物炭和秸秆还田是改良盐碱土的重要措施，但盐碱土中土壤有机氮组分的变化特征及其对生物炭和秸秆还田的响应机理尚不清楚。以黄河三角洲盐碱土为研究对象，探讨不添加外源物质（CK）、秸秆还田（ST）和施用生物炭（BI）对土壤有机氮组分、土壤酶活、微生物多样性和土壤团聚体的影响。结果表明：ST和BI均改变了盐渍土有机氮组分含量，其中BI改变效果更为显著（p<0.05）。与对照相比，BI提高了酸解性总氮、非酸解性氮、酸解性氨态氮、氨基酸态氮、氨基糖态氮和酸解性未知氮，幅度分别为38.86%、75.62%、35.05%、34.38%、39.61%和59.19%。ST和BI均显著增加了土壤中脲酶、蛋白酶、碱性磷酸酶、蔗糖酶和过氧化氢酶的含量，其中BI对土壤酶含量提升效果更显著（p<0.05），增幅分别为139%、104%、43.4%、50.2%、76.8%。BI显著提高了土壤微生物多样性（p<0.05），其丰富度指数、辛普森多样性指数、香农多样性指数均高于ST处理，分别高出15.0%、9.57%和20.8%。BI显著提高土壤微团聚体中酸解氮含量（p<0.05），而ST显著提高了土壤团聚体中非酸解氮含量（p<0.05）。此外，有机氮各组分与土壤酶活和微生物多样性均呈显著正相关，说明生物炭和秸秆还田主要通过促进土壤酶活性和改变土壤团聚体结构，改变土壤有机氮组分含量。综上，在盐碱土中添加生物炭和秸秆，通过提高盐碱土壤中酶含量、团聚体酸解氮含量和微生物活性而改善了土壤有机态氮组成，促进了土壤质量的提升。

关键词: 生物炭, 秸秆, 盐碱土, 有机态氮组分, 土壤酶活性, 土壤微生物活性