Effects of Cotton Stalk Returning on Soil Enzyme Activity and Bacterial Community Structure Diversity in Cotton Field with Long-term Saline Water Irrigation

Abstract

Long-term saline water irrigation will increase soil salinity, adversely affect soil physical and chemical properties, and change the diversity of soil bacteria. Straw returning can improve the soil microenvironment and subsequently affect soil enzyme activity and bacterial community structure diversity. This experiment used two types of irrigation water salinity:fresh water (FW, 0.35 dS·m-1) and saline water (SW, 8.04 dS·m-1). Under each irrigation water salinity, the amount of cotton straw applied was 0 and 6000 kg·hm-2 (represented by FWST and SWST, respectively). The results showed that:compared with those under fresh water irrigation, saline water irrigation significantly increased the soil salt, bulk density, total carbon, and available phosphorus but significantly decreased available potassium content. Under saline water irrigation, straw returning significantly increased the soil total carbon, total nitrogen, available potassium, and available phosphorus but reduced soil bulk density. Compared with those under fresh water irrigation, soil sucrase, alkaline phosphatase, and catalase activities under saline water irrigation decreased by 57.24%, 35.15%, and 3.91%, respectively, whereas urease activity increased by 26.73%. However, straw returning significantly increased sucrase, alkaline phosphatase, and catalase activities but decreased urease activity. Saline water irrigation decreased the relative abundance of Acidobacteriota, Actinobacteriota, Bacteroidota, Verrucomicrobiota, and Firmicutes and increased the abundance of Gemmatimonadota and Myxococcota. Under saline water irrigation, straw returning significantly increased the relative abundance of Actinobacteriota, Bacteroidetes, Firmicutes, Crenarchaeota, Sphingomonas, Dongia, and Steroidobacter. NMDS results also showed that saline water irrigation and straw returning changed the bacterial community structure. In conclusion, straw returning can improve soil nutrient content, reduce soil bulk density and salinity, and then change soil enzyme activity and bacterial community structure diversity. The change in soil bacterial community composition was mainly affected by soil salinity and bulk density. Therefore, straw returning can improve soil fertility and help maintain the health of soil ecosystem. This study revealed a relationship between soil enzyme activities and bacterial communities, which provides a theoretical basis and mechanism for applying cotton stalk to regulate the soil enzyme and micro-ecological environment.