Atmospheric nitrogen (N) deposition has a significant influence on soil organic carbon (SOC) accumulation in forest ecosystems. Microbial residues, as by-products of microbial anabolism, account for a significant fraction of soil C pools. However, how N deposition affects the accumulation of soil microbial residues in different forest biomes remains unclear. Here, we investigated the effects of six/seven-year N additions on microbial residues (amino sugar biomarkers) in eight forests from tropical to boreal zone in eastern China. Our results showed a minor change in the soil microbial residue concentrations but a significant change in the contribution of microbial residue-C to SOC after N addition. The contribution of fungal residue-C to SOC decreased under low N addition (50 kg N ha⁻¹ yr⁻¹) in the tropical secondary forest (−19%), but increased under high N addition (100 kg N ha⁻¹ yr⁻¹) in the temperate Korean pine mixed forest (+21%). The contribution of bacterial residue-C to SOC increased under the high N addition in the subtropical Castanopsis carlesii forest (+26%) and under the low N addition in the temperate birch forest (+38%), respectively. The responses of microbial residue-C in SOC to N addition depended on the changes in soil total N concentration and fungi to bacteria ratio under N addition and climate. Taken together, these findings provide the experimental evidence that N addition diversely regulates the formation and composition of microbial-derived C in SOC in forest ecosystems.

We investigated whether local-scale decontamination (removal of the litter layer, superficial soil layer, and understory) in a secondary forest contaminated by the Fukushima nuclear power plant accident reduced 137Cs contamination of the soil and litter. We also measured 137Cs concentrations in plants and in the web-building spider Nephila clavata (Nephilidae: Arachnida), as an indicator species, to examine 137Cs contamination in arthropods. One month after decontamination, the total 137Cs contamination (soil + litter) was reduced by 20% (100 kBq·m−2) relative to that in an adjacent untreated (i.e., contaminated) area, which was however not statistically significant. Four months after decontamination, 137Cs in the decontaminated area had increased to a level similar to those in the untreated area, and the air radiation dose in the decontaminated area was about 2.1 μSv·h−1, significantly higher than that in the untreated area (1.9 μSv·h−1). This may have been attributed to a torrential rain event. Although no statistically significant reduction was observed, most spiders had a lower 137Cs contamination than that before the decontamination. This implied that the decontamination may have reduced 137Cs transfer from soil via litter to N. clavata through the detrital food chains, but may not have reduced the amount of 137Cs transfer through grazing food chains because the concentration of 137Cs in living tree leaves was not reduced by the decontamination. In autumn, about 2 kBq·m−2 of 137Cs was supplied from foliage to the ground by litterfall. The results suggested that removal of the litter and superficial soil layers in a contaminated forest may be ineffective. The present study suggests that the local-scale decontamination in a secondary forest had no effect on the reduction of 137Cs contamination in the treated area.

The emergence of rock outcrops is very common in terrestrial ecosystems. However, few studies have paid attention to their hydrological role in the redistribution of precipitation, especially in karst ecosystems, in which a large proportion of the surface is occupied by carbonate outcrops. We collected and measured water received by outcrops and its subsequent export to the soil in a rock desertification ecosystem, an anthropogenic forest ecosystem, and a secondary forest ecosystem in Shilin, China. The results indicated that outcrops received a large amount of water and delivered nearly half of it to nearby soil patches by means of runoff. No significant difference was found in the ratio of water received to that exported to the soil by outcrops among the three ecosystems annually. When the outcrop area reaches 70 % of the ground surface, the amount of water received by soil patches from rock runoff will equal that received by precipitation, which means that the soil is exposed to twice as much precipitation. This quantity of water can increase water input to nearby soil patches and create water content heterogeneity among areas with differing rock emergence.

Studying the influence of topography and litter and soil nutrients on soil enzymes and microbial biomass is important to the understanding of soil nutrient transformation and cycling, but these relationships in heterogeneous soils of karst ecosystem remains poorly understood. We determined environment factors influencing the urease (URS) and alkaline phosphatase (ALP) activity and microbial biomass C and N (MBC and MBN) with advancing vegetation succession. The results showed that ALP increased but URS decreased with the advancing vegetation succession. The MBC and MBN were highest in shrubland, but both were lowest in grassland. The URS was positively correlated with the surface cover of rock outcrops (SRO) but negatively correlated with litter N, and soil available N and pH. Conversely, ALP was positively correlated with litter N, soil organic carbon (SOC), and soil available N and pH, but negatively correlated with soil total N. The MBC was positively related to litter quantities and SOC but negatively related to soil pH; the MBN was positively related to slope gradient (SLG), SOC, and soil total P and available P. Additionally, the trends of the index URS/MBN were grassland > secondary forest > shrubland > primary forest, but the index ALP/MBN increased with advancing vegetation succession. It indicated that soil microorganism mainly exudate extracellular URS and ALP to soils. We also found the interactions of topography (SLG and SRO), litter (nutrients and quantity), and soil (nutrients and pH) explained 42.00, 87.00, and 66.00% of the variations in URS, ALP, and microbial biomass, respectively. Path analysis showed that the topography had a directly positive effect on litter nutrients and quantities, but not on soil nutrients; the litter nutrients and quantities had direct positive effect on soil nutrients, which had direct effect on soil enzymes and microbial biomass; the relationships (R²) between the independent variable and enzymes activities and microbial biomass increased with advancing successions. Thus, it suggested that high SLG and SRO are good for collecting litters back to soils and then the topography, litter, and soil factors increased its controlling effect on soil enzymes activities and microbial biomass with advancing successions in karst ecosystem.

Arbuscular mycorrhizal (AM) fungi and nitrogen-fixing bacteria play important roles in plant growth and recovery in degraded ecosystems. The desertification in karst regions has become more severe in recent decades. Evaluation of the fungal and bacterial diversity of such regions during vegetation restoration is required for effective protection and restoration in these regions. Therefore, we analyzed relationships among AM fungi and nitrogen-fixing bacteria abundances, plant species diversity, and soil properties in four typical ecosystems of vegetation restoration (tussock (TK), shrub (SB), secondary forest (SF), and primary forest (PF)) in a karst region of southwest China. Abundance of AM fungi and nitrogen-fixing bacteria, plant species diversity, and soil nutrient levels increased from the tussock to the primary forest. The AM fungus, nitrogen-fixing bacterium, and plant community composition differed significantly between vegetation types (p < 0.05). Plant richness and pH were linked to the community composition of fungi and nitrogen-fixing bacteria, respectively. Available phosphorus, total nitrogen, and soil organic carbon levels and plant richness were positively correlated with the abundance of AM fungi and nitrogen-fixing bacteria (p < 0.05). The results suggested that abundance of AM fungi and nitrogen-fixing bacteria increased from the tussock to the primary forest and highlight the essentiality of these communities for vegetation restoration.

In mountainous areas, rock fragments (RFs) are a common feature on the soil surface and in topsoil. Few studies, however, have investigated the spatial distribution of RFs and the relevant mechanisms underpinning their distribution on steep hillslopes, especially in karst regions. We have collected and measured the RF cover, size, and content at the soil surface and within the topsoil of secondary forest, man-made forest, and non-forest land hillslopes in a karst region in Yunnan Province, southwest China. The results revealed no significant relationships between slope position and mean total RF coverage, median diameter (D₅₀), and mean total volumetric RF in topsoil within the three karst hillslopes covered by different types of vegetation. A limited effect of vegetation on the spatial distribution of RFs on the hillslopes was identified. However, the variation in RFs in the topsoil between the top and bottom slopes was greater than that at the surface between the top and bottom slopes, implying that underground leakage was greater than surface runoff.