Effects of stand condition and root density on fine-root dynamics across root functional groups in a subtropical montane forest.

Fine roots play key roles in belowground C cycling in terrestrial ecosystems. Based on their distinct functions, fine roots are either absorptive fine roots (AFRs) or transport fine roots (TFRs). However, the function-based fine root dynamics of trees and their responses to forest stand properties remain unclear. Here, we studied the dynamics of AFRs and TFRs and their responses to stand conditions and root density in a subtropical montane mixed forest based on a 2-a root window experiment. Mean (± SE) annual production, mortality, and turnover rate of AFRs were 7.87 ± 0.17 m m-2 a-1, 8.13 ± 0.20 m m-2 a-1and 2.96 ± 0.24 a-1, respectively, compared with 7.09 ± 0.17 m m-2 a-1, 4.59 ± 0.17 m m-2 a-1, and 2.01 ± 0.22 a-1, respectively, for TFRs. The production and mortality of fine roots were significantly higher in high root-density sites than in low-root density sites, whereas the turnover of fine roots was faster in the low root-density sites. Furthermore, root density had a larger positive effect than other environmental factors on TFR production but had no obvious impact on AFR production. Tree species diversity had an apparent positive effect on AFR production and was the crucial driver of AFR production, probably due to a complementary effect, but had no evident impact on TFR. Both tree density and tree species diversity were positively correlated with the mortality of AFRs and negatively related to the turnover of TFRs, suggesting that higher root density caused stronger competition for rooting space and that plants tend to reduce maintenance costs by decreasing TFR turnover. These findings illustrated the importance of root functional groups in understanding root dynamics and their responses to changes in environmental conditions. Supplementary Information: The online version contains supplementary material available at 10.1007/s11676-022-01514-0.

Linking soil organic carbon stock to microbial stoichiometry, carbon sequestration and microenvironment under long-term forest conversion.

Forest conversion can drastically impact carbon (C) and nutrient processes and microbial stoichiometry, which will modify soil organic C (SOC) stock. However, SOC stock dynamics and its underlying mechanisms induced by long-term forest conversion remain unclear. Three well-protected plantations converted from natural forests for 36 years were compared, i.e., Cryptomeria fortunei (CF), Metasequoia glyptostroboides (MG) and Cunninghamia lanceolata (CL), with a natural forest (NF) as a control. SOC stock size and stability across three soil depths (0-10, 10-30 and 30-60 cm) were examined with aggregate-based method. Forest floors and fine roots were treated as C and nutrient inputs while soil respiration (Rs) was treated as C output. Soil microbial biomass C, nitrogen and phosphorus were measured to calculate microbial stoichiometry, as well as microenvironment and soil physicochemical properties. The relationships between SOC stock (size and stability) and these factors were explored using structural equation model. The results showed that microbial stoichiometry had strong or strict homeostasis at each soil depth. At 0-10 cm soil deep, SOC stock size varied with tree species (following the rank of CL > NF ≈ CF > MG) but its stability increased in all forest conversion types, regulated by forest floor quantity and quality associated with Rs; at 10-30 cm soil deep, the SOC stock sizes decreased in CF and MG, but SOC stock stability increased in MG, jointly driven by fine root quality and microenvironment; at 30-60 cm soil deep, SOC stock size decreased but its stability increased in MG, whereas both its size and stability had few changes in CF or CL, modified by soil physicochemical property associated with microbial stoichiometry and Rs. Overall, the effects of microbial stoichiometry and microenvironment on SOC stock were not pronounced. Thus, SOC stock size changed with soil depth and tree species but its stability tended to be steady at all depths varying with tree species. These results suggest that SOC stock size and stability are mainly determined by self-regulation process of forest ecosystems over more than three-decade after forest conversion, which will help us more accurately assess C sequestration strategies regarding long-term forest conversion.

Higher carbon sequestration potential and stability for deep soil compared to surface soil regardless of nitrogen addition in a subtropical forest.

BACKGROUND: Labile carbon input could stimulate soil organic carbon (SOC) mineralization through priming effect, resulting in soil carbon (C) loss. Meanwhile, labile C could also be transformed by microorganisms in soil as the processes of new C sequestration and stabilization. Previous studies showed the magnitude of priming effect could be affected by soil depth and nitrogen (N). However, it remains unknown how the soil depth and N availability affect the amount and stability of the new sequestrated C, which complicates the prediction of C dynamics. METHODS: A 20-day incubation experiment was conducted by adding 13C labeled glucose and NH4NO3 to study the effects of soil depth and nitrogen addition on the net C sequestration. SOC was fractioned into seven fractions and grouped into three functional C pools to assess the stabilization of the new sequestrated C. RESULTS: Our results showed that glucose addition caused positive priming in both soil depths, and N addition significantly reduced the priming effect. After 20 days of incubation, deep soil had a higher C sequestration potential (48% glucose-C) than surface soil (43% glucose-C). The C sequestration potential was not affected by N addition in both soil depths. Positive net C sequestration was observed with higher amount of retained glucose-C than that of stimulated mineralized SOC for both soil depths. The distribution of new sequestrated C in the seven fractions was significantly affected by soil depth, but not N addition. Compared to deep soil, the new C in surface soil was more distributed in the non-protected C pool (including water extracted organic C, light fraction and sand fraction) and less distributed in the clay fraction. These results suggested that the new C in deep soil was more stable than that in surface soil. Compared to the native SOC for both soil depths, the new sequestrated C was more distributed in non-protected C pool and less distributed in biochemically protected C pool (non-hydrolyzable silt and clay fractions). The higher carbon sequestration potential and stability in deep soil suggested that deep soil has a greater role on C sequestration in forest ecosystems.