Connecting soil dissolved organic matter to soil bacterial community structure in a long-term grass-mulching apple orchard

This study aims to clarify the relationship between soil dissolved organic matter (DOM) components and bacterial communities in long-term grass mulching management apple orchards (Malus domestica). The key role of the transformation of DOM components and soil nutrient cycling in improving orchard grass mulching management strategies was assessed. Four treatments, including legumes, such as white clover (WC) and crown vetch (CV); Gramineae orchardgrass (OG); and conventional tillage (CT) as the control, were employed to explore DOM components, the bacterial community structure and their relationship. In all the management systems, the DOM identified included two humic-like components and one protein component. The protein component was dominant in leguminous mulching (43.04-77.13 %), whereas the humic-like component was dominant in graminaceous mulching (42.17-72.11 %). Component 2 accounted for less than 1% in CT, CV and WC. The three types of grass mulching increased the proportion of copiotrophic bacteria and reduced that of oligotrophic bacteria, especially Proteobacteria (34.27-28.83%) and Acidobacteria (20.96- 28.42%) at the phylum level. Legumes were beneficial for DOM transformation, and Gramineae species improved soil nutrition and bacterial communities. In conclusion, mulching of different grasses in orchards significantly changes the soil DOM components and bacterial community structure through different types of organic inputs, and the connection of these communities is closely coordinated with significant effects on soil nutrient cycling and orchard grass mulching strategies.