Effects of reclaimed water irrigation levels on soil salinity and composition of soil bacteria community

Abstract: Reasonable reuse of reclaimed water is of great significance to alleviate the conflict between supply and demand of fresh water resources and reduce the risk of ecological environment pollution caused by waste sewage discharge. This stud aimed to reveal the effect of different irrigation levels of reclaimed water on soil salinity, total nitrogen, total phosphorus and bacteria community structure. The reclaimed water was collected from a sewage treatment plant. The designed irrigation water sources included reclaimed water and tap water. For each water source, 2 levels of irrigation were designed: full irrigation (90% of field water holding capacity) and deficit irrigation (70% of full irrigation). The soil column experiment was carried out from August 1, 2017 to May 28, 2018. The irrigation amount of full irrigation was 9.70 L per time, and that of deficit irrigation was 6.80 L per time. Water was irrigated every 20 d, and 15 times of irrigation were accumulated in the whole experiment period. Soil sample at 0-60 cm layer was collected after experiment which lasted for 300 days for determination of electrical conductivity, total nitrogen, total phosphorus and bacterial community structure. Soil bacteria diversity index was calculated. The results showed that: 1) Compared with tap water, reclaimed water irrigation significantly increased 0-60 cm soil salinity, total phosphorus content and 0-30 cm soil total nitrogen content, soil bacteria diversity and operational taxonomic units (OUT) were reduced under reclaimed water irrigation. Compared with deficit irrigation of reclaimed water, full irrigation of reclaimed water increased salinity in deep soil, the bacteria diversity and species number in deep soil were reduced under full irrigation of reclaimed water. 2) The soil bacteria under different treatments were mainly actinobacteria and proteobacteria. At deficit irrigation level, compared with tap water, the proportion of soil actinobacteria, chloroflexi, acidobacteria and firmicutes were increased but the proportion of soil proteobacteria were reduced under reclaimed water irrigation. Under full irrigation level, compared with tap water, the proportion of soil actinobacteria and nitrospirae were increased but the proportion of soil proteobacteria, chloroflexi, firmicutes and acidobacteria were reduced under reclaimed water irrigation. Regardless of irrigation levels, the irrigation with reclaimed water promoted soil actinobacteria and inhibited proteobacteria. Compared with deficit irrigation of reclaimed water, full irrigation of reclaimed water promoted soil actinobacteria and proteobacteria, and inhibited soil chloroflexi, acidobacteria and firmicutes bacteria. The high irrigation level of reclaimed water would favor the growth of dominant microorganisms in soil. 3) Membrane transport, carbohydrate metabolism and amino acid metabolism accounted for the largest proportion of bacteria metabolic pathways in all the treatments. Reclaimed water irrigation with high level greatly promoted the bacteria membrane transport, carbohydrate metabolism and amino acid metabolism of surface soil. Therefore, reclaimed water with high irrigation level would promote the cycle of material and energy in soil, and actively mediate the process of soil bacteria metabolism and reproduction.