Soil organic carbon (SOC) is critical for carbon cycling and sequestration in forest ecosystems. However, how stand age affects SOC components and stability still remains poorly understood. Here, soil samples (0–20 cm) were collected from Cryptomeria japonica var. sinensis (L. f.) D. Don plantations of seven stand ages (6, 12, 23, 27, 32, 46, 52 a) in the rainy area of western China. SOC fractions, including soil particulate organic carbon (POC), easily oxidizable carbon (EOC), labile organic carbon (LOC), recalcitrant organic carbon (ROC), and light fraction organic carbon (LFOC), were determined to explore the nature of carbon components and stability across a chronosequence of C. japonica plantation. Soil carbon fractions first increased and then trended to be stable with an increase in stand age. SOC concentrations were the largest in mature forests (27 or 32 a), but the concentrations of other carbon components often peaked in early over-mature forests (46 a). The concentrations of all carbon fractions were the lowest in the young forests (6 a). The ratios of ROC/SOC increased and LOC/SOC decreased with increasing stand age. Almost all carbon fractions were positively correlated with soil bulk density and negatively correlated with soil moisture. The allometric exponent of ROC or HFOC and soil physicochemical properties was higher as compared to LOC and LFOC. The results noted in this study indicate that SOC components often accumulate fast over the first 20 years of afforestation and SOC stability increases with increasing stand age for C. japonica plantation in this specific region.

Afforestation is one of the most important forestry practices, but its impact on soil microbial communities remains poorly understood. In this study, we sampled the soil from 0–15 cm and 15–30 cm soil depths of 7-, 13-, 24-, 33-, and 53-year-old Chinese cedar (Cryptomeria japonica var. sinensis) plantations. To investigate the effect of stand age on soil microbial communities and their potential drivers, we measured phospholipid fatty acids (PLFAs) and soil physicochemical properties. At the 0–15 cm soil depth, the biomass of total PLFAs and functional microbial groups such as bacteria (B), fungi (F), Gram-negative bacteria (GN), Gram-positive bacteria (GP), actinomycetes (ACT), and arbuscular mycorrhizal fungi (AMF) increased sharply in 7- to 13-year-old stands, but then gradually leveled off in older stands. On the other hand, the biomass of total PLFAs and functional microbial groups at the 15–30 cm soil depth peaked in the 33-year-old stand. The biomass of total PLFAs and functional microbial groups was strongly influenced by stand age and soil depth, and was significantly lower at the 15–30 cm soil depth than at the 0–15 cm soil depth except for the 7-year-old stand. The F/B and fungi/total PLFAs ratios of both soil depths were markedly lower in the 13-year-old stand than in the remaining four stand ages, while the proportions of the bacterial group (GP and GN) showed contrasting trends. The biomass of all functional microbial groups and the GP/GN ratio were mainly mediated by soil organic carbon (SOC) concentration and the soil organic carbon to total phosphorus (C/P) ratio at the 0–15 cm soil depth, but primarily affected by ammonium nitrogen (NH₄⁺-N) concentration at the 15–30 cm soil depth. The F/B ratio of the two soil depths was prominently affected by nitrate nitrogen (NO₃⁻-N) concentration. Our results highlighted that SOC concentration and mineral N (i.e., NH₄⁺-N and NO₃⁻-N) concentration mainly drove changes in the soil microbial biomass and community composition with stand age in Chinese cedar plantations, and that the 13-year-old stand may be the key period for management.

Forest litter accumulation can regulate the soil microclimate and alter nutrient distribution, but the effects of litter quality and seasonal differences on soil nitrogen (N) mineralization are still uncertain. The effects of litter change on the rates of net N mineralization, nitrification, and ammonification were studied through in situ incubation experiments in coniferous, mixed, and broad-leaved forests in the eastern Qinghai–Tibetan Plateau. Two litter treatments were established, one to allow the litter to enter the soil normally (remain litter) and the other to prevent the litter from entering the soil (remove litter). Soil samples were collected at the freezing (FS), thawing (TS), early growing (EGS), late growing (LGS), and early freezing (EFS) seasons during the 1.5-year incubation period. Compared to coniferous forests, the effects of litter removal on the net ammonification, nitrification, and N mineralization rates were more pronounced in broad-leaved forests, mainly during the growing and thawing seasons. Structural equation modeling indicated that microbial biomass N (MBN) was a common factor affecting the net ammonification, nitrification, and N mineralization rates in the three forest soils. The coniferous forest microbial biomass carbon (MBC), mixed forest soil moisture, broad-leaved forest soil N concentration, and C:N ratio were the unique influencing factors of the different forest types. The results showed that the effect of litter distribution on the soil net N mineralization mainly depended on forest type and season, suggesting that the litter composition and productivity in different seasons and forest types may alter the soil N cycling processes in subalpine forest ecosystems.

Base cation transfer from stemflow is an important process for nutrient transfer and plays a key role in maintaining the balance of soil nutrient pools. To research the differences of stemflow chemistry in mixed plantations, we conducted a continuous field experiment in the rainy zone of Western China from December 2016 to November 2017. Three representative mixed plantations, including a conifer–broadleaved mixed plantation, a deciduous broadleaved mixed plantation and a multispecies mixed plantation, were selected to investigate the concentration and flux characteristics of K⁺, Na⁺, Ca²⁺ and Mg²⁺ in stemflow. The results showed that: (1) the K⁺, Na⁺, Ca²⁺ and Mg²⁺ fluxes ranged from 1.75 to 2.44 kg ha⁻¹ year⁻¹, 0.14 to 0.24 kg ha⁻¹ year⁻¹, 1.25 to 2.11 kg ha⁻¹ year⁻¹, and 0.40 to 0.60 kg ha⁻¹ year⁻¹ in these mixed plantations during the one-year observation, and the annual or seasonal (i.e., rainy or dry season) base cation fluxes in the stemflow varied slightly with the plantation types; (2) broadleaved trees had a higher average stemflow base cation contribution rate and flux-based enrichment ratio than coniferous trees, and the enrichment ratios showed a decreasing tendency with increasing trunk diameter; (3) the stemflow base cation concentration was higher in the dry season, while flux was observed to be higher in the rainy season. These results suggested that increasing the proportion of broadleaved species in mixed plantations might improve soil nutrient content and benefit material cycling in subtropical forest ecosystems.

Changes in the microenvironment induced by forest gaps may affect litter decomposition, yet it is unclear how the gap effects respond to altitudinal and seasonal differences. Here, a four-year litterbag decomposition experiment along an elevation gradient (3000, 3300, 3600 m) was conducted in an Abies faxoniana Rehd. subalpine forest of southwestern China, to assess the potential seasonal effects of forest gaps (large: ≈250 m², middle: ≈125 m², small: ≈40 m² vs. closed canopy) on litter mass loss and carbon release at different elevations. We found that the A. faxoniana litter mass loss and carbon release reached 50~53 and 58~64% after four years of decomposition, respectively. Non-growing seasons (November to April) had a greater decline than the growing seasons (May to October). Litter in the forest gaps exhibited significantly higher mass loss than that under the closed canopy, and the decomposition constant (k) exhibited a gradually declining trend from large gaps, middle gaps, small gaps to closed canopy. Moreover, more significant differences of gap on both carbon content and release were observed at the 3600 m site than the other two elevations. Our findings indicate that (i) a rather high mass loss and carbon release during the decomposition of A. faxoniana litter was observed at high elevations of the subalpine forest subjected to low temperatures in the non-growing seasons and (ii) there were stimulative effects of forest gaps on litter mass loss and carbon release in early decomposition, especially in the non-growing seasons, driven by fewer freeze–thaw cycles when compared to the closed canopy, which diminished at the end of the experiment. The results will provide crucial ecological data for further understanding how opening gaps as a main regeneration method would induce changes in carbon cycling in subalpine forest ecosystems.