Effects of Caragana compost and tillage depth on bacterial community composition and metabolic functions in continuous cucumber soils

Objectives We studied the effects of Caragana compost combined with different tillage depths on soil bacterial community and key environmental factor for soil quality. We aimed to provide a theoretical basis for utilising agricultural waste resources and sustainable greenhouse production in northwest China.

Methods A field experiment was conducted in a solar greenhouse in 2018 with five treatments across four successive cropping seasons. The treatments include traditional chicken manure with tillage depth of 15 cm (CK), Caragana compost with no-tillage (T0), and with tillage depth of 15 cm (T15), 35 cm (T35), and 45 cm (T45). Soil samples were collected at 0–20 cm to determine soil nutrient characteristics and bacterial community structure.

Results Compared with traditional chicken manure, Caragana compost treatment did not reduce cucumber yield. Increasing tillage depth combined with Caragana compost application could alleviate soil acidification and secondary salinization. Conversely, no-tillage (P<0.05) increased available N accumulation and decreased soil C/N ratio. The Caragana compost was the leading factor of bacterial community change, followed by the tillage depth. Compared with CK treatments, T15, T35 and T45 treatment significantly increased the Shannon and Simpson diversity index and Pielou evenness index. The relative abundance of Chloroflexi and Gemmatimonadetes under Caragana compost increased with increasing tillage depth. The relative abundance of Actinobacteria and Firmicutes increased in no-tillage. Compared with T0, deep tillage (T35 and T45) significantly reduced the utilization of amino acids, amines, and aromatic compounds, and T45 significantly increased the utilization of carboxylic acids. The KEGG annotation results showed that T35 promoted bacterial community metabolism and genetic information processing. The RDA results showed that soil pH, EC, and available N all had a direct or indirect (P<0.05) impact on the structure and function of the bacterial community. Soil pH was the dominant factor, and the decrease of pH was adverse to increasing soil bacteria's construct and functional diversity.

Conclusions Caragana compost with suitable deep tillage (T35) effectively promotes the construct and functional diversity of soil bacterial communities, conducive to the sustainable development of farmland soil ecosystems, in spite of the smaller adverse yield effect.