Biological soil crusts (BSCs) hold promise for reducing soil erosion in subtropical forest plantations, and microorganisms profoundly affect the formation and development of BSCs. The effects of biochar as a soil conditioner on the diversity and structure of soil microbial communities in BSCs are largely unknown. Therefore, our aim was to determine how biochar might improve microbial community composition and BSC function. Herein, a field experiment was conducted in a P. massoniana plantation; the addition of biochar was the treatment, and no biochar addition was the control (CK). Soil microbial communities associated with moss BSCs (in and beneath BSCs) with and without the addition of biochar were analyzed by Illumina sequencing technology. The results showed that Acidobacteria (28.35%), Proteobacteria (22.53%), Actinobacteria (17.41%), and Chloroflexi (16.74%) were the dominant bacterial phyla, whereas Basidiomycota (70.00%) and Ascomycota (22.76%) were the dominant fungal phyla in BSCs. The soil bacterial and fungal OTU number and richness in BSCs were higher than those beneath BSCs. The relative abundances of Acidobacteria, Chloroflexi, and Basidiomycota were higher in BSCs than beneath BSCs, whereas the relative abundances of Actinobacteria, Firmicutes, Ascomycota, and Chytridiomycota showed the opposite trend. Beneath BSCs, biochar addition increased the soil bacterial OTU number and richness (ACE index and Chao1) but decreased the soil fungal OTU number and richness. Biochar had little effect on soil microbial community structures in BSCs; however, beneath BSCs, it significantly increased the relative abundances of Acidobacteria, Chloroflexi, and Basidiomycota and significantly decreased the relative abundances of Actinobacteria, Firmicutes, Ascomycota, and Chytridiomycota. Biochar-induced changes in soil microbial communities were related to soil environmental factors, especially urease activity, organic matter content, pH, total nitrogen content, and sucrase activity. We demonstrated the different effects of biochar on soil microbial communities in and beneath the BSCs of subtropical forest plantations; these findings provided new insights into soil stabilization with BSCs below the forest canopy in subtropical regions.

During forest vegetation rehabilitation, changes in aboveground litter and underground root inputs affect soil microbial communities. Clarifying the effects of forest ecosystem carbon inputs on soil microbial community structure can provide a theoretical basis for the microbial driving mechanism of soil fertility evolution and ecosystem rehabilitation of vegetation rehabilitation in degraded red soil. Our research focuses on a Schima superba pure forest recovered from eroded and degraded red soil in a subtropical region. Five treatments were set as follows: control treatment (CT), mycorrhiza (M), root + mycorrhiza (RM), litter + root + mycorrhiza (LRM), and double litter + root + mycorrhiza (DLRM). We used Illumina HiSeq technology to study the effects of different organic carbon inputs on soil microbial community structure. The results showed that all organic carbon input treatments reduced the total population of soil bacteria by 55–79%; M, RM, and DLRM treatments increased the quantity of operational taxonomic units (OTUs) by 25–37%, ACE index by 25–34%, and Chao1 index by 28–39%. Acidobacteria, Proteobacteria, and Actinobacteria were the dominant bacteriophyta in the Schima superba pure forest soil. The relative abundance of Alphaproteobacteria decreased by 55% under LRM treatment, and Thermoleophilia increased by 81% under M treatment. The dominant fungal phyla were Basidiomycota and Ascomycota. RM, LRM, and DLRM treatments reduced the relative abundance of Sordariomycetes by 46–64% and increased the relative abundance of Mortierellomycetes by 251–615%. The order of effects of different organic carbon inputs on the bacterial community composition at the phylum level was LRM > RM > M > DLRM and that on the fungal community composition was DLRM > LRM and RM > M. Alphaproteobacteria, Thermoleophilia, Sordariomycetes, and Mortierellomycetes were the main microbial groups affected by changes in organic carbon inputs. Soil organic carbon and total nitrogen were the key factors affecting the change of Mortierellomycetes. The bacterial community mainly affected the activity of soil acid invertase, while the fungal community affected the activities of various enzymes, with positive or negative effects. We concluded that the organic carbon inputs changed the species and quantity of soil microorganisms in the Schima superba forest, and the influence of organic carbon input on the fungal community structure was greater than that of bacteria.

Low inner leaves in the thick canopy of dense Chinese fir plantations frequently show premature senescence and dieback regardless of age. To elucidate the underlying mechanisms, a 28-day growth chamber experiment was conducted under dark conditions to induce leaf senescence. Changes in leaf area, photosynthetic performance, and the responses of carbon metabolism and the antioxidant defense system were analyzed. Leaf area decreased significantly with time in darkness. The photosystem II reaction center was damaged, and fluorescence parameters and chlorophyll contents decreased, resulting in reduced light energy capture and conversion efficiencies. Photosynthetic rate, apparent quantum yield, stomatal conductance, transpiration rate, and light use efficiency all decreased, while the light compensation point and intercellular CO₂ concentration increased. Antioxidant enzyme activities initially increased but eventually collapsed as the stress continued and H₂O₂ and malondialdehyde accumulated, causing membrane conductivity, i.e., membrane permeability, to increase by 122%. Meanwhile, reduced non-structural carbohydrates, especially total non-structural carbohydrates content, decreased by 45.32%, triggering sugar starvation and accelerating aging. Our study provided new physiological evidence for light-stress response mechanisms in Chinese fir. Specifically, it revealed that dark-induced leaf senescence was mainly caused by irreversible damage to the photosynthetic apparatus and oxidative stress, which together led to carbon starvation and ultimately death.