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Key message The quantity of root exudate carbon produced by Quercus crispula Blume was strongly influenced by the amount of solar radiation 1 day before collection. We aimed to investigate seasonality in the quantity of root exudate produced, and the factors affecting exudate quantity, in mature Quercus crispula Blume trees throughout the growing season. We also aimed to understand the timespan of exudation, from production of photosynthate to release as exudate. We measured the amount of root exudate C produced by Q. crispula , as well as environmental factors including solar radiation and soil and air temperature throughout the growing season. The model best explaining the amount of exudate C, based on the lowest Akaike’s information criterion, was selected to determine the environmental factors that affect exudation. The quantity of exudates did not show clear seasonality, and instead varied widely among sampling dates. A regression model with daily solar radiation for 1 day before sample collection as the sole factor affecting the quantity of exudates was selected as the best model. Solar radiation, which fluctuated greatly among days, more strongly affected the quantity of exudates produced than factors that fluctuated seasonally, such as average temperature. Furthermore, most root exudates from Q. crispula are released within 1 day of being generated by photosynthesis, rather than over several days.

Aims 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. Methods 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 . Results 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. Conclusions Fine litter traits and coarse litter abundance are likely linked to soil microbial characteristics and N mineralization in natural forests with variable topography.

Microbial products, largely the necromass, are key contributors to stable soil organic matter (SOM) in terrestrial systems, and microbial communities may differ in the stabilization. Plants can control microbial communities through litter quality and entry sites of plant inputs (above- or below-ground). However, whether soil mineral fractions (due to the characteristics such as soil texture) can also control the microbial communities, remains unclear. We conducted two model soil incubation experiments (E1 and E2) in order to simulate the four field soils. Materials included were plant materials, such as general plant inputs in soil systems, and four mineral materials derived from different field soils (i.e., “the four original field soils”), with their SOM removed by combustion. E1 was undertaken to simulate root exudate using oxalate and glucose, and E2 was undertaken to simulate plant litter using broad leaves, coniferous leaves, branches, and fine roots. The microbial (mainly bacterial) community structure from phospholipid fatty acid (PLFA) across E1 and E2 showed differences due to the mineral materials, and the differences after accounting for plant materials are similar to the difference in the four original field soils. Additionally, the abundance of bacterial PLFAs in E2 increased with silt and clay content and was correlated with the abundance of the bacterial PLFAs in the four original field soil. This study implies that even in the case of this experiment under such conditions as a broad variety of plant inputs, mineral fractions strongly influence bacterial communities, in a manner consistent with field soil systems.

BackgroundsA large-scale ecological restoration project was initiated in the 1990s in southwest China, which is one of the largest areas of rocky desertification globally. However, the influence and potential mechanisms of vegetation restoration on soil carbon(C) sequestration in karst and non-karst regions remains unclear.MethodsBased on field investigation and multi-source data synthesis, the mechanisms of soil C sequestration were investigated to determine the most important variables affecting the rate of soil C change (Rs) in southwest China.ResultsOur results show significant differences in soil C sequestration between karst and non-karst regions with faster and longer C sequestration occurring in karst regions. In these areas, Rs was approximately 31% higher than in non-karst soils. The Rs of shrubland, grassland and cropland sites in karst areas was significantly higher than that observed in non-karst areas. We found that temperatures were the primary factor inhibiting soil C sequestration instead of precipitation. The total effect of nitrogen (N) on Rs was positive in both karst and non-karst regions.ConclusionsPhosphorus was the dominant factor limiting the use of N by vegetation in karst regions and then resulting in limitation of C sequestration. The results indicated that soil C storage may increase intermittently due to combination of karst environment and climate change in southwest China in future.

Nitrogen (N) is one of the most common limiting nutrients for primary production in terrestrial ecosystems. Soil microbes transform organic N into inorganic N, which is available to plants, but soil microbe activity in drylands is sometimes critically suppressed by environmental factors, such as low soil substrate availability or high salinity. Tamarisk (Tamarix spp.) is a halophytic shrub species that is widely distributed in the drylands of China; it produces litter enriched in nutrients and salts that are thought to increase soil fertility and salinity under its crown. To elucidate the effects of tamarisks on the soil microbial community, and thus N dynamics, by creating “islands of fertility” and “islands of salinity,” we collected soil samples from under tamarisk crowns and adjacent barren areas at three habitats in the summer and fall. We analyzed soil physicochemical properties, inorganic N dynamics, and prokaryotic community abundance and composition. In soils sampled beneath tamarisks, the N mineralization rate was significantly higher, and the prokaryotic community structure was significantly different, fromsoilssampledinbarrenareas, irrespective of site and season. Tamarisks provided suitable nutrient conditions for one of the important decomposers in the area, Verrucomicrobia, by creating “islands of fertility,” but provided unsuitable salinity conditions for other important decomposers, Flavobacteria, Gammaproteobacteria, and Deltaproteobacteria, by mitigating salt accumulation. However, the quantity of these decomposers tended to be higher beneath tamarisks, because they were relatively unaffected by the small salinity gradient created by the tamarisks, which may explain the higher N mineralization rate beneath tamarisks.

To investigate the effect of reduced snow cover on fine root dynamics in a cool-temperate forest in northern Japan because of decreases in snowfall at high latitudes due to global warming, we monitored root length, production, and mortality before and after snow removal with an in-ground root scanner. We measured root dynamics of both overstory deciduous oak (Quercus crispula) and understory evergreen dwarf bamboo (Sasa nipponica), the two major species in the forest. Snow removal advanced the timing of peak root production by a month both in total and in Sasa, but not in oak. There was a significant interaction between snow removal and plant form on root production; this indicates that enhanced Sasa root production following snow removal might increase its ability to compete with oak. In contrast, snow removal did not enhance root mortality, suggesting that the roots of these species tolerate soil freezing. The earlier snow disappearance in the snow removal plot expanded the growing season in Sasa. We speculate that this change in the understory environment would advance the timing of root production by Sasa by extending the photosynthetic period in spring. We propose that different responses of root production to reduced snow cover between the two species would change the competitive interactions of overstory and understory vegetation, influencing net primary production and biogeochemistry (e.g., carbon and nitrogen cycles) in the forest ecosystem.

Aims In this study, we investigated the effects of reduced snow depth on plant phenology, productivity, nitrogen (N) cycling, and N use in canopy and understory vegetation. We hypothesized that decreased snow depth would hasten the timing of leaf flushing and N uptake in understory vegetation, increasing its N competitive advantage over canopy trees. Results Snow removal did not directly affect the phenology of either canopy or understory vegetation. Understory vegetation took up more N in the snow removal plots than in the control plots, particularly in the mid- to late-growing season. Leaf production and N uptake in canopy trees also did not differ between the control and snow removal plots, but N resorption efficiency in the snow removal plots (57.6%) was significantly higher than those in control plots (50.0%). Conclusions Increased N uptake by understory plants may induce N limitation in canopy trees, which in turn may cause canopy trees to increase their N use efficiency. Such competitive advantage of understory vegetation over canopy trees against snow reduction may affect N cycling via litter quality and quantity not only just after the growing season but also in subsequent seasons.

Forest mycorrhizal type is getting more attention as a potentially significant factor controlling carbon (C) and nitrogen (N) cycling. Ectomycorrhizal (ECM) forests are frequently reported to have lower N availability and higher soil C storage than arbuscular mycorrhizal (AM) forests. However, it is still unclear whether such characteristics stem from the low organic matter quality inherent in the ECM forest or other biotic and abiotic factors, such as competition for N between ECM fungi and free-living microbes. We conducted soil and litter reciprocal transplant experiments between AM-symbiotic black locust and ECM-symbiotic oak forests to separate the effects of organic matter quality and forest type (i.e., factors including ECM fungal presence and soil physicochemical properties) on decomposition rates and N availability. We hypothesized that the forest type, rather than organic matter quality, is a more determinant factor for available N content but not organic matter decomposition rate. Forest type had a more substantial effect not only on nitrate content but also on decomposition rate than organic matter quality. Since the litter decomposition rate was higher when placed in the oak forest, the higher soil C accumulation in the oak than in the black locust forests may be caused by greater C input rather than the slower decomposition in the oak than black locust forest. In addition, nitrate content was determined by forest type, suggesting the suppression of nitrate content by ECM fungal presence. This study suggests that the forest type with different mycorrhizal associations can affect biogeochemical cycling independent of organic matter quality.

Recent global warming models project a significant change in winter climate over the next few decades. The decrease in snowpack in the winter will decrease the heat insulation function of the snowpack, resulting in increased soil freeze–thaw cycles. Here, we examined the impact of winter freeze–thaw cycles on year-round dissolved nitrogen (N) and carbon (C) dynamics and their relationship with dissolved organic matter and microbial biomass in soil by conducting an in situ experimental reduction in snowpack. We investigated dissolved inorganic N ( N H 4 + and N O 3 − ), dissolved organic N (DON), dissolved organic carbon (DOC), inorganic N leaching, soil microbial biomass, and microbial activities (mineralization and nitrification) in the surface soil of a northern hardwood forest located in Japan. Experimental snowpack reduction significantly increased the number of soil freeze–thaw cycles and soil frost depth. The N H 4 + content of the surface soil was significantly increased by the amplified soil freeze–thaw cycles due to decreased snowpack, while the soil N O 3 − content was unchanged or decreased slightly. The gravimetric soil moisture, DON and DOC contents in soil and soil microbial biomass significantly increased by the snowpack removal in winter. Our results suggest that the amplified freeze–thaw cycles in soil increase the availability of DON and DOC for soil microbes due to an increase in soil freezing. The increases in both DON and DOC in winter contributed to the enhanced growth of soil microbes, resulting in the increased availability of N H 4 + in winter from net mineralization following an increase in soil freeze–thaw cycles. Our study clearly indicated that snow reduction significantly increased the availability of dissolved nitrogen and carbon during winter, caused by increased soil water content due to freeze–thaw cycles in winter.

The use of heavy forestry machines for clear-cutting and site preparation causes soil compaction, which can decrease forest productivity. This process becomes complicated in forests with high levels of precipitation due to the erosion and deposition of surface soil. Here, we investigated how soil compaction and slash dispersal on compacted soil affect soil nitrogen (N) mineralization in a high precipitation area with erodible volcanic soil in southern Japan. The physical and chemical properties of the soil were measured inside and outside the ruts of work roads in the presence and absence of dispersed slash in three Cryptomeria japonica plantations 9–10 months after clear-cutting and site preparation. We found that the soil N mineralization rate, particularly the soil nitrification rate, was lower in compacted soil, but the dispersal of slash after soil compaction enhanced the soil N mineralization and nitrification rates. Soil compaction also led to a low soil water permeability and high volumetric soil water content and was associated with the erosion and deposition of surface soil, with soil deposition including organic matter, being observed under dispersed slash. Additionally, the soil carbon (C) and N concentrations were lower in compacted soil but improved under dispersed slash. Principal component analysis showed that soil compaction and the soil C and N concentrations were closely related to each other on the first principal component (PC1), while the soil C/N ratio was separated from other factors on PC2. Furthermore, the scores of both PC1 and PC2 were related to soil N mineralization. These results suggest that soil compaction by forestry machines has a negative impact on soil N mineralization under high precipitation, but slash dispersal on the compacted soil is an effective approach for maintaining the soil N mineralization. The soil C/N ratio is likely related with N mineralization in the impacted soils, but the negative relationship between soil compaction and soil C and N concentrations through the movement of surface soil containing these elements should also be considered to fully understand the changes in soil N mineralization that occurs in forests under high precipitation.