Straw return strategies for soil microbial quotient enhancement: Insights from DOM vertical distribution characteristics

Abstract

The soil microbial quotient (SMQ), a critical indicator of agricultural soil quality, exhibits significant responsiveness to dissolved organic matter (DOM). However, how the heterogeneity of DOM characteristics regulates the SMQ remains unclear, despite DOM being a critical energy source for microorganisms. We conducted a field experiment with four treatments: conventional tillage (CT), no-tillage with straw mulch (NTS), conventional tillage with straw mulch and incorporation (CTS), and deep tillage with straw incorporation (DTS), with soil samples collected at four depth intervals (0–10, 10–20, 20–30, and 30–40 cm) to investigate how DOM characteristics influence SMQ. The results indicated that SMQ declined with soil depth. Straw return enhanced SMQ, with effectiveness ordered as CTS > NTS > DTS > CT. Compared to CT, straw return increased the content and humic-like components of DOM. Additionally, NTS and CTS accelerated depth-dependent variations in protein-like substances, whereas DTS promoted those in humic-like substances. Linear regression analysis showed that SMQ was positively correlated with both dissolved organic carbon and humic-like components of DOM. Random Forest analysis showed that soil enzyme activities and nutrients dominated SMQ regulation in the 0–20 cm layer, while DOM characteristics governed SMQ in the 20–40 cm layer. Among all treatments, CTS was identified as the most effective in enhancing SMQ. These findings elucidate the DOM-mediated mechanism of SMQ regulation under straw return, providing a scientific basis for optimizing agricultural management practices.