Long-term Straw Return with Moderate Nitrogen Levels Reshapes Soil Bacterial Communities in a Vertisol

Zichun GUO ^1* Rui Qian ^1,2,3* Wei Li ^4* Tianyu Ding ^1,3 Lei Gao ^1,5* Xinhua Peng ^6*

• ¹ State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences (CAS), Nanjing, China
• ² Key Laboratory of Mollisols Agroecology, Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (CAS), Harbin, Heilongjiang Province, China
• ³ University of Chinese Academy of Sciences, Beijing, Beijing, China
• ⁴ Crop Research institute, Anhui Academy of Agricultural Science, Hefei, China
• ⁵ University of the Chinese Academy of Sciences, Nanjing, China
• ⁶ State Key Laboratory of Efficient Utilization of Arid and Semi-Arid Arable Land in Northern China, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing, China

Incorporating straw into the soil is a sustainable practice that can mitigate some of the adverse effects of excessive N fertilization on soil structure degradation and microbial diversity reduction. This objective of this study was to determine the combined effects of straw management (straw return and straw removal) and N fertilization (0, 360, 450, 540, 630, and 720 kg N ha-1 yr-1) on crop yields, soil properties, and soil microbial communities in a long-term wheat-maize cropping system. The results showed that moderate N application (N450–N540) with straw return optimized wheat (283.5 kg ha⁻¹) and maize (346.5 kg ha⁻¹) yields, whereas higher N fertilization (N630, N720) led to soil acidification (pH decline of 0.51–1.67 units), irrespective of straw management. Straw return increased soil organic carbon (SOC), total nitrogen (TN), nitrate (NO₃⁻-N), and available potassium (AK), but decreased ammonium (NH₄⁺-N). Bacterial diversity increased at moderate N rates but decreased at higher N rates. Fungal diversity was generally higher under straw removal, with Chaetomiaceae increasing under straw return, whereas Mortierellaceae and Trichocomaceae declined at high N levels. The Mantel test showed a strong correlation between soil pH and bacterial diversity, while fungal composition was influenced by SOC, TN, and NO₃⁻-N. Partial Least Squares Path Modeling (PLS-PM) demonstrated that N fertilization directly and indirectly increased wheat yield through improved soil properties, while straw return enhanced bacterial diversity, indirectly supported wheat yield. This study highlights the importance of balanced N fertilization and straw incorporation in maintaining bacterial community structure, fertility, and long-term crop productivity in intensive cropping systems on Vertisol.