1 Introduction
Returning farmland to forest is an effective means to protect the ecological environment and prevent soil erosion; moreover, it effectively improves vegetation coverage, species diversity, and primary productivity (Wu et al., 2021). In Northeast China, afforestation account for 70% of the total forest area (Yang et al., 2010).Larix(L.) gmelinii has been widely used as a plantation species because of its rapid growth and excellent wood quality (Li et al., 2020; Yu and Liu, 2020). The response of soil microbial communities to such returning farmland to forest little attention. It is of particular importance to understand the effects of such conversion on soil microbial community, which will further affect the maintenance of ecosystem services and functions.
Soil microorganisms fundamentally determine soil organic carbon (SOC) accumulation and renewal and affect soil carbon mineralization (Jiang et al., 2018; Maron et al., 2018). Soil active microbial communities play a central role in subsurface processes, especially in mediating soil organic decomposition and nutrient cycling in forest ecosystems (Jiao et al., 2018; Wagg et al., 2019). The interaction between forest SOC decomposition and soil microbial communities is key to understanding the feedback of terrestrial ecosystem processes to global climate change (Cavicchioli et al., 2019; Geisen et al., 2021; Zheng et al., 2019). The majority of soil microorganisms obtain carbon by decomposing different types of SOC, which have different decomposition abilities and mechanisms (Zhong et al., 2018).
In soil systems, the source and composition of dissolved organic matter (DOM) change with varying soil depth, and the source may change from plant to microbial with an increase in depth (Roth et al., 2019). DOM can combine with fine soil particles to form mineralized SOC; it can also stimulate soil microorganism activity and promote SOC decomposition (Gross and Harrison, 2019; Kleber et al., 2021). The vertical movement of DOM plays a crucial role in soil development, microbial metabolism, carbon and nutrient cycling, and distribution (Ye et al., 2020), and it drives the basic biogeochemical processes in forest soils (Kaiser and Kalbitz, 2012). DOM is an important carbon source for soil microorganisms. Solid organic macromolecules are decomposed into molecules with relatively small water solubility under the action of extracellular enzymes. Soluble substrates are a prerequisite for microbial diffusion through cell membranes (H. Liu et al., 2021; Roth et al., 2019). DOM is the most bioavailable organic matter in the soil, and its composition is highly dynamic and sensitive to environmental change (Fan et al., 2020). In addition, some studies have revealed that microorganisms are unable to utilize all carbon resources and exhibit a significant substrate preference (Huang et al., 2021).
Plantations play a vital role in supporting the timber industry, promoting local economic development, and mitigating global climate change (Tong et al., 2020; Zhu et al., 2017). With reduced logging and effective wildfire control, plantations are increasing in age and, thus, are storing more carbon. Most of the current studies on the change of soil carbon focus on the surface layer, that is, 0-20 cm depth (Cheng et al., 2017). However, the balance between soil carbon mineralization and decomposition in the sub-deep layer (>20 cm) may play an important role in the long-term carbon sequestration potential of SOC. Therefore, it is necessary to quantify the vertical change in soil carbon stability in plantations (Hou et al., 2019; Mayer et al., 2020; Yang et al., 2020). Returning farmland to forest can significantly change soil properties and nutrient status, and further soil microbial community (Liu et al., 2020). However, there are strong interactions between soil physico-chemical and biological properties. Thus, the combined analysis of all these factors during the returning farmland to forest can provide useful information for comprehensive evaluation the effects of plantations on soil quality and functions.
In this study, the diversity in the composition of soil dissolved organic carbon (DOC) in three different stands along with the soil layers (0-40 cm) and explored the relationships between DOM composition and bacterial and fungal communities (i.e., diversity, species composition, and microbial abundance) by absorption and fluorescence spectroscopy. The objectives of this study were (1) to determine the effects in three different stands along with the soil layers (0-40 cm) on soil chemical properties, (2) to compare and evaluate the influence of returning farmland to forest on soil microbial community, and (3) to determine whether DOM quality plays a predominant role in structuring the soil microbial community.