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.