Studying carbon and nitrogen stocks in different types of larch forest ecosystems is of great significance for assessing the carbon sink capacity and nitrogen level in larch forests. To evaluate the effects of the differences of forest type on the carbon and nitrogen stock capacity of the larch forest ecosystem, we selected three typical types of larch forest ecosystems in the northern part of Daxing’an Mountains, which were the Rhododendron simsii-Larix gmelinii forest (RL), Ledum palustre-Larix gmelinii forest (LL) and Sphagnum-Bryum-Ledum palustre-Larix gmelinii forest (SLL), to determine the carbon and nitrogen stocks in the vegetation (trees and understories), litter and soil. Results showed that there were significant differences in carbon and nitrogen stocks among the three types of larch forest ecosystems, showing a sequence of SLL (288.01 Mg·ha−1 and 25.19 Mg·ha−1) > LL (176.52 Mg·ha−1 and 14.85 Mg·ha−1) > RL (153.93 Mg·ha−1 and 10.00 Mg·ha−1) (P < 0.05). The largest proportions of carbon and nitrogen stocks were found in soils, accounting for 83.20%, 72.89% and 64.61% of carbon stocks and 98.61%, 97.58% and 96.00% of nitrogen stocks in the SLL, LL and RL, respectively. Also, it was found that significant differences among the three types of larch forest ecosystems in terms of soil carbon and nitrogen stocks (SLL > LL > RL) (P < 0.05) were the primary reasons for the differences in the ecosystem carbon and nitrogen stocks. More than 79% of soil carbon and 51% of soil nitrogen at a depth of 0–100 cm were stored in the upper 50 cm of the soil pool. In the vegetation layer, due to the similar tree biomass carbon and nitrogen stocks, there were no significant differences in carbon and nitrogen stocks among the three types of larch forest ecosystems. The litter carbon stock in the SLL was significantly higher than that in the LL and RL (P < 0.05), but no significant differences in nitrogen stock were found among them (P > 0.05). These findings suggest that different forest types with the same tree layer and different understory vegetation can greatly affect the carbon and nitrogen stock capacity of the forest ecosystem. This indicates that understory vegetation may have significant effects on the carbon and nitrogen stocks in soil and litter, which highlights the need to consider the effects of understory in future research into the carbon and nitrogen stock capacity of forest ecosystems.

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Changes in above-ground litterfall can influence below-ground biogeochemical processes in forests, which substantially impacts soil nitrogen (N) and nutrient cycling. However, how these soil processes respond to the litter manipulation is complex and poorly understood, especially in the N-limiting boreal forest. We aimed to examine how soil N dynamics respond to litter manipulations in a boreal larch forest. A litter manipulation experiment including control, litter exclusion, and litter addition was performed in the Larix gmelinii forest on the north of the Daxing’an Mountains in China. Monthly soil inorganic N, microbial biomass and the rate of net N mineralization in both 0–10 cm and 10–20 cm layers, and N2O flux were analyzed from May 2018 to October 2018. In 0–20 cm soil layer the average soil inorganic N contents, microbial biomass N (MBN) contents, the rate of net N mineralization (Rmin), and the soil N2O emission in the litter addition plot were approximately 40.58%, 54.16%, 128.57%, and 38.52% greater, respectively than those in the control. While litter exclusion reduced those indexes about 29.04%, 19.84%, 80.98%, and 31.45%, respectively. Compared with the dynamics of the 10–20 cm soil layer, the N dynamics in 0–10 cm soil were more sensitive to litter manipulation. Rmin and N2O emissions were significantly correlated with MBN in most cases. Our results highlight the short-term effects of litter manipulations on soil N dynamics, which suggests that the influence of litter on soil N process should be considered in the future defoliation management of the boreal larch forest.

Alterations in plant litter inputs into the soil are expected to significantly affect soil greenhouse gas (GHG) emissions. However, the influence on boreal forest soils is not clear, given the large amount of accumulated soil organic matter that may buffer the impacts from the input of fresh litter. In this study, we conducted a litter manipulation experiment to explore the effects of the litter layer on soil GHG fluxes in a Dahurian larch (Larix gmelinii) forest ecosystem in northeastern China. Three litter treatments were implemented, namely aboveground litter removal (LR), litter double (LD), and unchanged litter input (CK). The associated microclimate, litter characteristics, and soil properties were also measured. The results showed that this larch forest soil acts as a source of CO2 and N2O but acts as a sink for CH4 for all litter manipulation treatments. LD increased the soil CO2 and N2O fluxes by 15% and 34%, while LR decreased them by 8% and 21%, respectively. However, soil CH4 uptake decreased by 34% in LD treatment and increased by 22% in LR treatment, respectively. Litter manipulation treatments can not only affect soil GHG fluxes directly but also, via their effects on soil MBC, NH4+−N, and NO3−−N content, indirectly affect variations in soil CO2, CH4 and N2O fluxes, respectively. Our study highlights the importance of the plant litter layer in regulating soil GHG between the atmosphere and soil in a Dahurian larch forest ecosystem, especially for litter addition. Considering the natural increase in litter quantity over time, this important regulatory function is essential for an accurate estimation of the role of boreal forests in mitigating future climate change.

IntroductionPlant species composition in forest ecosystems can alter soil greenhouse gas (GHG) budgets by affecting soil properties and microbial communities. However, little attention has been paid to the forest types characterized by understory vegetation, especially in boreal forests where understory species contribute significantly to carbon and nitrogen cycling.MethodIn the present study, soil GHG fluxes, soil properties and bacterial community, and soil environmental conditions were investigated among three types of larch forest [Rhododendron simsii-Larix gmelinii forest (RL), Ledum palustre-Larix gmelinii forest (LL), and Sphagnum-Bryum-Ledum palustre-Larix gmelinii forest (SLL)] in the typical boreal region of northeast China to explore whether the forest types characterized by different understory species can affect soil GHG fluxes.ResultsThe results showed that differences in understory species significantly affected soil GHG fluxes, properties, and bacterial composition among types of larch forest. Soil CO2 and N2O fluxes were significantly higher in LL (347.12 mg m−2 h−1 and 20.71 μg m−2 h−1) and RL (335.54 mg m−2 h−1 and 20.73 μg m−2 h−1) than that in SLL (295.58 mg m−2 h−1 and 17.65 μg m−2 h−1), while lower soil CH4 uptake (−21.07 μg m−2 h−1) were found in SLL than in RL (−35.21 μg m−2 h−1) and LL (−35.85 μg m−2 h−1). No significant differences between LL and RL were found in soil CO2, CH4, and N2O fluxes. Soil bacterial composition was mainly dominated by Proteobacteria, Actinobacteria, Acidobacteria, and Chloroflexi among the three types of larch forest, while their abundances differed significantly. Soil environmental variables, soil properties, bacterial composition, and their interactions significantly affected the variations in GHG fluxes with understory species. Specifically, structural equation modeling suggested that soil bacterial composition and temperature had direct close links with variations in soil GHG fluxes among types of larch forest. Moreover, soil NO3−−N and NH4+ − N content also affected soil CO2, CH4, and N2O fluxes indirectly, via their effects on soil bacterial composition.DiscussionOur study highlights the importance of understory species in regulating soil GHG fluxes in boreal forests, which furthers our understanding of the role of boreal forests in sustainable development and climate change mitigation.