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We aimed to reveal the plant material decomposition rate and abundance of bacteria and fungi in serpentine and karst sites of cool-temperate forests. Given the relatively higher soil pH, we hypothesized a higher decomposition rate of cellulose, lower decomposition rate of lignin-containing plant materials, and lower fungi-to-bacteria ratio in serpentine and karst sites than in neighboring non-serpentine/non-karst sites. This study was conducted on serpentine and mudstone sites in deciduous oak forests on Mount Oe and karst and sandstone sites on Mount Ibuki, Japan. We performed a decomposition experiment over 5 months using three chemically distinct materials: cellulose filter, lignin-containing coffee filter, and green tea leaves. These common materials enabled us to compare the in-situ utilization of different carbon substrates by microbial communities between sites. The abundance of bacteria and fungi and their ratio in topsoils and materials that decomposed for 1 month were assessed using quantitative PCR. In contrast to our hypothesis, the decomposition rate was higher in the mudstone site for cellulose and did not differ for the coffee filter compared to the serpentine site. Green teas with higher lignin content than coffee filters decomposed slower at the serpentine site only during the early period of decomposition. The serpentine site showed higher fungi-to-bacteria ratios in the decomposed materials and soil. The karst site also showed different decomposition patterns than our prediction, presenting no clear difference in the cellulose filter compared to the sandstone site. In the karst site, coffee filters decomposed faster at the early period; however, green teas decomposed slower as we expected, especially at the later period of decomposition. The karst site had a lower fungi-to-bacteria ratio in the topsoil, but similar fungi-to-bacteria ratios in the decomposed materials compared with the sandstone site. Our results suggest that the decomposition patterns in the serpentine and karst soils are not simply predicted based on soil pH and other distinct characteristics (e.g., soil moisture content and heavy metal concentration) would more strongly contribute to decomposition and microbial community composition in the study mountains.

Topographic positions within a natural forest can considerably influence litter traits, soil microbial characteristics, and nitrogen (N) mineralization, causing plant–soil feedbacks. Despite the high abundance of coarse litter (woody debris and coarse roots) in forest ecosystems, most studies have focused on linkages between fine litter (leaves and fine roots) and N dynamics and/or the soil microbial community. We investigated the association of fine and coarse litter with soil microbial biomass, community structure, and N mineralization at upper and lower slope positions on sedimentary rocks in a temperate forest dominated by Fagus crenata. Greater coarse litter abundance and litter C-to-N ratio, and lower soil microbial biomass, bacterial abundance, and N mineralization potential were found in upper positions than in lower positions. Among litter traits, coarse litter abundance and litter C-to-N ratio were the best predictors of the microbial biomass and fungal-to-bacterial dominance, possibly due to differences in climatic stress among plant communities. Microbial traits were the best predictors of N mineralization potential. Fine litter traits and coarse litter abundance are likely linked to soil microbial characteristics and N mineralization in natural forests with variable topography.

Volcanic deposits increase soil organic carbon storage. However, little is known about the effect of volcanic deposits on forest soil nitrogen (N) dynamics and microbial communities. We explored gross and net N transformation rates and microbial community structure using a phospholipid fatty acid (PLFA) method across eight forests with soils derived from different parent material in Japan. Volcanic mineral soils had approximately three-fold greater total N and inorganic N contents and gross nitrification, ammonium immobilization, and nitrate immobilization rates that were one order of magnitude higher than in non-volcanic soils. Moreover, volcanic mineral soils had a 1.7-fold lower N turnover rate, which was estimated as net N mineralization rate per soil N. This was likely caused by a higher gross N immobilization rate, potentially due to the mineral traits of volcanic soils. Volcanic mineral soils had approximately four- and three-fold greater microbial biomass-N and bacterial PLFA contents, respectively, and the fungal:bacterial PLFA ratio was lower in volcanic mineral soils than in non-volcanic soils. Microbial community structure, analyzed using non-metric multidimensional scaling, was distinct between volcanic and non-volcanic soils, and was significantly affected by soil characteristics. This study demonstrates that aside from soil carbon storage, volcanic deposits are related to increases in soil N immobilization, N content, bacterial biomass, and N transformation rates in forest ecosystems.

The soil microbial community and nitrogen (N) dynamics change seasonally due to several factors. The microbial community structure (MCS) can regulate N dynamics. However, there is insufficient information on seasonal changes in MCS and the relationship between MCS and N dynamics. We investigated MCS and N dynamics in forest soils with two different fertilities throughout a year. MCS, measured with phospholipid fatty acid (PLFA) analysis, showed a consistent seasonal trend, regardless of the fertility. Microbial indices (particularly the Saturated-/monounsaturated-PLFA ratio; Sat/mono) indicated a major PLFA shift among seasons, with temperature likely the most important factor. The fungal-/bacterial-PLFA ratio in the dormant season (December–April) was approximately 1.3 times greater than in the growing season (June–November). The trend in N dynamics showed that in summer (June–August), the gross N mineralization potential was greater than immobilization, whereas in winter (December–April), immobilization was dominant. The net mineralization potential in the growing season was approximately 1.6 times higher than in the dormant season. Moreover, a relationship was found between Sat/mono and N transformation potentials. We highlight the microbial sensitivity to seasonal dynamics which can be associated with temperature, as well as carbon and N dynamics.