Responses of Soil Carbon and Nitrogen Dynamics and GHG Fluxes in Forest Ecosystems to Climate Change and Human Activity

Abstract

Forest soils are considered the largest carbon and nitrogen pools in soil organic matter among terrestrial ecosystems, and soil carbon and nitrogen dynamics and greenhouse gas (GHG) emissions are normally affected by climate change and human activity. The collection of recent research on this scientific theme would provide a basis for understanding the responses of soil carbon and nitrogen dynamics and GHG fluxes in forest ecosystems to climate change and human activity. A Special Issue was, thus, organized to discuss recent research achievements, including a total of nine research articles and one review. This Special Issue includes the effects of climate changes such as changes in throughfall, snow cover, and permafrost degradation; human activities such as nitrogen and/or phosphorus addition and the use of biochar; and soil–plant interactions on soil carbon and nitrogen dynamics and GHG fluxes in forest ecosystems. Although this collection of papers reflects only a small part of this scientific theme, it can, to some extent, provide a basis for understanding some important research aspects related to the future of forest soil carbon and nitrogen dynamics and GHG fluxes in a changing world, thereby enabling sustainable development and the mitigation of climate change.

As the largest carbon and nitrogen sinks among terrestrial ecosystems, forest ecosystems account for more than 60% of carbon and nitrogen pools in terrestrial soil organic matter at 0–100 cm depths [1]. A small change in soil carbon and nitrogen pools due to climate change and human activity can even lead to an important change in the carbon and nitrogen balance of forest ecosystems, probably resulting in impacts on global carbon and nitrogen cycling, atmospheric greenhouse gas (GHG) concentration, and the climate system [2,3,4]. Both climate change (e.g., changes in precipitation and snowfall, global warming, and changes in dry–wet and freeze–thaw processes) and human activity (e.g., increased atmospheric nitrogen deposition, forest management, deforestation and afforestation, land use, and increased carbon dioxide and ozone concentrations) can affect the construction and functions of vegetation in forest ecosystems as well as the soil environment via its physical, chemical, and biological processes [5,6,7,8] (Figure 1). These variations following climate change and human activity and the associated soil–plant interactions can have a substantial impact on soil carbon and nitrogen dynamics and GHG fluxes in forest ecosystems [9,10,11,12,13,14,15,16] (Figure 1). In this Special Issue, a collection of articles has been published covering a wide range of topics related to forest soil carbon and nitrogen dynamics and GHG fluxes under climate change and human activity, which can, to some extent, improve our understanding these dynamics and fluxes in forest ecosystems in the changing world.

Changes in precipitation and global climate warming can affect the structure and functions of forest ecosystems by impacting forest vegetation and soil environments (Figure 1). In the last three decades, precipitation manipulation experiments via throughfall transferring have been conducted to study the dynamics of soil respiration and soil nutrient status in forest ecosystems in different climate zones around the world [11,17,18,19,20,21,22]. In a study by Xu [23], a meta-analysis was performed to explore the effects of experimental categories such as climate types, forest types, soil texture, and the area size of changes in throughfall manipulation on the changes in altered throughfall-induced annual soil carbon dioxide emissions. Based on the results of the meta-analysis, it can be reasonably concluded that the responses of annual soil carbon dioxide emissions to the altered throughfall become more sensitive in temperate forests than those in tropical and subtropical forests. Moreover, a relatively large positive response in soil carbon dioxide emissions to increased throughfall occurs in Mediterranean forests [23]. It was pointed out that fine root biomass and soil microbial biomass, together with the changes in annual precipitation, would substantially affect the altered throughfall-induced annual soil carbon dioxide emissions in global forest ecosystems [23]. Besides changes in precipitation manipulation in the context of climate change, some experimental simulated apparatus such as soil heating cables, infrared heaters, and open-top chambers have been widely used to perform in situ soil warming experiments to study the responses of soil carbon and nitrogen dynamics and GHG fluxes to globe climate warming in forest ecosystems [13,24]. Compared with these artificial, simulated apparatus manipulation experiments, translocation experiments with a small gradient in micrometeorological factors (e.g., temperature and rainfall) can be used to simulate climate change in nature as much as possible. In a study by Xu et al. [25], four-year field measurement of in situ soil column translocation experiments indicated that a small increase in climate warming would increase soil carbon dioxide emission and methane uptake in the temperate forests of northeastern China, especially during the non-growing season. A robust estimation illustrated that a moderate warming of 1 °C would increase annual soil heterotrophic respiration and annual methane uptake in the temperate forests in northeastern China by a magnitude of 44.536 and 0.0168 Tg carbon, respectively [25], which should be incorporated into soil-carbon-cycling-related models to reasonably predict the responses of soil carbon fluxes at the soil–atmosphere interface in forest ecosystems to global climate warming.

Global climate warming can affect the growth of vegetation and the structure of vegetation communities and also results in an increase in the active layer’s thickness of permafrost zones and changes in surface soil hydrothermal conditions [26,27]. These variations would, in turn, impact soil carbon and nitrogen dynamics and GHG fluxes in terrestrial ecosystems [28,29,30]. Using a regression model, TTOP model, and related datasets, Song et al. [30] analyzed the spatial distribution of permafrost and soil organic carbon (SOC) density at 0–30 cm depths in northeast China in the recent four decades, as well as annual changes in SOC stocks. The results reported by Song et al. [30] indicate that permafrost degradation due to global climate warming significantly reduces SOC stocks in northeast China in the short term but increases SOC stocks in the long term, mainly resulting from changes in land use and land cover. This study would provide a valuable scientific reference for understanding the impacts of alterations in permafrost degradation and land use on SOC changes in northeast China and other regions of the world.

Terrestrial ecosystems at high-latitude and/or high-altitude regions are currently experiencing variations in soil freezing and thawing events caused by reduced winter snowfall under global climate warming [31]. Soil freezing and thaw alterations, and their combination with litter decomposition and soil moisture can, to some extent, affect soil carbon and nitrogen dynamics and GHG fluxes (Figure 1) [15,32,33,34]. In a study by Xu et al. [34], simulations of autumn freeze–thaw, winter freeze, spring freeze–thaw, and the growing season were sequentially performed under laboratory conditions using undisturbed large soil columns collected from two adjacent temperate forest stands in northeastern China, to explore how the presence of litter and winter snow cover can influence carbon dioxide emissions from forest soils and its δ¹³C values. The results reported by Xu et al. [34] indicated that the impacts of snow cover on soil heterotrophic respiration and its δ¹³C values in cool temperate forests would vary with the different seasons of the year and the presence of aboveground litter.

As one of the main anthropogenic activities, chemical fertilizers and organic fertilizers (and their combination) can be selected as one of main effective management pathways to improve forest productivity and mitigate global warming potential in plantation forest ecosystems. Wang et al. [35] conducted a 12-month field study to investigate the impacts of biochar-based fertilizer, chemical fertilizer, and their combination on GHG fluxes from a subtropical typical bamboo plantation. The results reported by Wang et al. [35] indicate that the application of biochar-based fertilizers can be considered an environmentally friendly soil management method to mitigate GHG fluxes from subtropical bamboo plantations. Following the long-term application of chemical fertilizers such as nitrogen-based fertilizers and/or under enhanced atmospheric nitrogen deposition, both nutrient imbalance and phosphorus limitation normally occur in many tropical and subtropical forest ecosystems around the world, which can result in changes in nitrous oxide and methane fluxes from forest soils. In a study by Li et al. [36], manipulation experiments lasting one and half years were conducted to explore the effects of nitrogen and phosphorus addition at different levels on soil nitrous oxide and methane fluxes in a nitrogen-rich and phosphorus-limited subtropical Chinese fir plantation. The results reported by Li et al. [36] suggest that the impacts of nitrogen and phosphorus addition on soil nitrous oxide and methane fluxes may depend on the soil nutrient status and plant–microbial competition for soil nutrients in subtropical forests. Besides changes in soil GHG fluxes, enhanced atmospheric nitrogen deposition and/or extra nitrogen input may have impacts on SOC stability and fractions in forest ecosystems. Based on the measurement of a 7-year nitrogen addition experiment, Wu et al. [37] pointed out whether and how nitrogen addition of different durations affects soil particulate organic carbon and mineral-associated organic carbon in a subtropical Phyllostachys edulis forest. The results reported by Wu et al. [37] indicate the key role of microbial characteristics (e.g., microbial biomass, microbial carbon utilization efficiency, and soil enzyme activity) in regulating SOC stability and fractions following different durations of nitrogen addition in subtropical forests; this deserves further investigation at more specific sites around the world in the future. Following the addition of extra nitrogen, changes in leaf litter decomposition and the release of nutrients into the soil may complicate the changes in SOC density and stability in forest ecosystems with different nitrogen limitations (Figure 1). Based on the measurement of leaf litter decomposition and soil and litter characteristics in the Yimeng mountainous forests of China, Kong et al. [38] demonstrated that nitrogen addition can alter leaf litter decomposition, nutrient cycling, and SOC density in arid and barren mountainous regions with the limitation of available nitrogen, which can be closely associated with the tree species and leaf litter characteristics. This study highlights the importance of litter decomposition and nitrogen input in regulating soil nutrient dynamics and SOC sequestration in forest ecosystems with poor nutrient status.

Soil microbial properties such as soil enzyme activity and microbial biomass are closely associated with soil carbon and nitrogen dynamics and GHG fluxes in the changing world (Figure 1). A study by Kong et al. [39] showed that different land use types in the rocky, mountainous region of north China would have an important impact on the activities of soil enzymes such as urease, catalase, β-glucosidase, and protease, along with relatively large values in farmland cultivation and Platycladus orientalis (L.) Franco plantation. To improve soil fertility and enhance the adaptability of trees, the results reported by Kong et al. [39] indicate that Platycladus orientalis would be selected as the dominant tree species for plantation to achieve sustainable development in the barren, rocky, mountainous region of north China. Along elevation gradients, changes in vegetation properties and soil hydrothermal conditions are normally proposed to have an important impact on the availability of soil substances and the complicate plant–soil interactions in alpine ecosystems. Based on field investigations and laboratory analysis, Feng et al. [40] reported changes in vegetation diversity and soil biophysical and biochemical properties at three soil layers as well as their relationships along the five elevation gradients in the southern Taihang mountains, China. A study by Feng et al. [40] strongly indicates that the contents of active soil substances in alpine ecosystems would be greatly influenced by elevation gradients, soil layers, and plant–soil relationships.

In summary, this Special Issue contains some recent research results in the field of soil carbon and nitrogen dynamics and GHG fluxes in forest ecosystems following climate change and human activity. Although this collection only reflects a small part of this important scientific theme, it is our aim to promote international scientific researchers to further study these dynamics and fluxes in the changing world for sustainable development and the mitigation of climate change. Long-term perspectives are essential because changes in soil carbon and nitrogen pools, especially mineral-associated pools, are normally slow and the full influence of climate change and human activities (e.g., enhanced atmospheric nitrogen deposition and changes in land management practices) on soil carbon and nitrogen dynamics and GHG fluxes in terrestrial ecosystems may take decades to become obvious [1,11,41,42,43]. Furthermore, additional studies should be conducted to explore soil carbon responses to the varying nitrogen availability in forest ecosystems around the world, including carbon responses at different soil layers [10,41,43,44]. Regarding the related conceptual model (Figure 1), to date, there have been few studies focusing on the differences in these dynamics and fluxes under short-term and long-term manipulation of climate change and human activities [42,44] or under natural gradient changes and anthropogenic abrupt change conditions [45]. The upscaling of methods from sites to ecosystem and regional scales is also lacking [46,47,48]. These study aspects should be fully taken into consideration in the future.

As guest editor, I would like to thank all the authors for their valuable contributions to this Special Issue. I also wish to express my sincere appreciation to all reviewers for their insightful comments, which improved the quality of earlier versions of each paper. I would also like to sincerely thank the editorial office of Forests for their valuable assistance throughout the publication process.