Effects of afforestation on soil CH4 and N2O fluxes in a nsubtropical karst landscape

A meta-analysis of afforestation impacts on soil greenhouse gas emissions

Afforestation is a natural climate solution and a key strategy to mitigate climate change. While tree planting primarily achieves this mitigation via above-ground carbon sequestration, soils also play a dual role as sources and sinks of greenhouse gases (GHG). Understanding afforestation impacts on soil GHG flux is essential for leveraging afforestation to combat global warming. In this research, we conducted a global meta-analysis of 157 studies to assess the effects of afforestation on soil GHG emissions across different prior land uses and to identify key emission drivers. Our results indicated that afforestation significantly reduced CO2 emissions in former grasslands and deforested land and decreased CH4 emissions across most prior land uses. However, soil N2O flux was mostly unaffected by afforestation. The type of forest planted also influenced soil GHG emissions. Hardwood planting reduced CH4 emissions, but no clear trends emerged for N2O emissions from either softwood or hardwood forests. Tree planting density had no significant effect on GHG fluxes. GHG responses to afforestation also changed over time and were influenced by environmental factors. CO2 emissions correlated positively with soil organic carbon, mean annual precipitation, C:N ratio, and soil temperature. N2O flux increased with soil NO3- and microbial nitrogen and decreased with soil organic carbon and moisture. Additionally, soil microbial biomass carbon and soil organic carbon were positively correlated with CH4 emissions. These findings highlight the importance of selecting tree species, site conditions, and environmental factors to optimize afforestation's GHG mitigation potential.

Strip clear-cutting transformations increase soil N2O emissions in abandoned Moso bamboo forests

Forest transformation can markedly impact soil greenhouse gas emissions and soil environmental factors. Due to increasing labor costs and declining bamboo prices, the abandonment of Moso bamboo forests is sharply escalating in recent years, which weakens the carbon sequestration capacity and decreases the ecological function of forests. To improve the ecological quality of abandoned Moso bamboo forests, transformations of abandoned bamboo forests have occurred. However, the impact of such transformations on N2O emissions remains elusive. To bridge the knowledge gap, we conducted a 23-month field experiment to compare the effects of various forest management practices on soil N2O emissions and soil environmental factors in abandoned Moso bamboo forests in subtropical China. These practices included uncut abandonment as a control, intensive management, three intensities (light, moderate, and heavy) of strip clear-cutting with replanting local tree species, and clear-cutting with replanting transformation. During the experimental period, the mean soil N2O flux in abandoned Moso bamboo forests was 13.2 ± 0.1 μg m-2 h-1, representing a 44% reduction compared to intensive management forests. In comparison to the uncut control, light, moderate, and heavy strip clear-cutting and clear-cutting transformations increased soil N2O emission rates by 20%, 43%, 64%, and 94%, respectively. Soil temperature (69-71%), labile C (2-6%) and N (3-8%) were the main factors that explain N2O emissions following the transformation of abandoned Moso bamboo forests. Additionally, replanting could decrease soil N2O emissions by increasing the contribution of soil moisture. Overall, the light strip clear-cutting transformation is suggested to convert abandoned Moso bamboo forests to mitigate N2O emissions.

Soil adsorption potential: Harnessing Earth's living skin for mitigating climate change and greenhouse gas dynamics

Soil adsorption, which could be seen as a crucial ecosystem service, plays a pivotal role in regulating environmental quality and climate dynamics. However, despite its significance, it is often undervalued within the realms of research and policy frameworks. This article delves into the multifaceted aspects of soil adsorption, incorporating insights from chemistry and material science, ecological perspectives, and recent advancements in the field. In exploring soil components and their adsorption capacities, the review highlights how organic and inorganic constituents orchestrate soil's aptitude for pollutant mitigation and nutrient retention/release. Innovative materials and technologies such as biochar are evaluated for their efficacy in enhancing these natural processes, drawing a link with the sustainability of agricultural systems. The symbiosis between soil microbial diversity and adsorption mechanisms is examined, emphasizing the potential for leveraging this interaction to bolster soil health and resilience. The impact of soil adsorption on global nutrient cycles and water quality underscores the environmental implications, portraying it as a sentinel in the face of escalating anthropogenic activities. The complex interplay between soil adsorption mechanisms and climate change is elaborated, identifying research gaps and advocating for future investigations to elucidate the dynamics underpinning this relation. Policy and socioeconomic aspects form a crucial counterpart to the scientific discourse, with the review assessing how effective governance, incentivization, and community engagement are essential for translating soil adsorption's functionality into tangible climate change mitigation and sustainable land-use strategies. Integrating these diverse but interconnected strata, the article presents a comprehensive overview that not only charts the current state of soil adsorption research but also casts a vision for its future trajectory. It calls for an integrated approach combining scientific inquiry, technological innovation, and proactive policy to leverage soil adsorption's full potential to address environmental challenges and catalyze a transition towards a more sustainable and resilient future.

Elucidation of the dominant factors influencing N2O emission in water-level fluctuation zones in a karst canyon reservoir, southwest China

The water-level fluctuations zones (WLFZs) are crucial transitional interfaces within river-reservoir systems, serving as hotspots for N2O emission. However, the comprehension of response patterns and mechanisms governing N2O emission under hydrological fluctuation remains limited, especially in karstic canyon reservoirs, which introduces significant uncertainty to N2O flux assessments. Soil samples were collected from the WLFZs of the Hongjiadu (HJD) Reservoir along the water flow direction from transition zone (T1 and T2) to lacustrine zone (T3, T4 and T5) at three elevations for each site. These soil columns were used to conduct simulation experiments under various water-filled pore space gradients (WFPSs) to investigate the potential N2O flux pattern and elucidate the underlying mechanism. Our results showed that nutrient distribution and N2O flux pattern differed significantly between two zones, with the highest N2O fluxes in the transition zone sites and lacustrine zone sites were found at 75 % and 95 % WFPS, respectively. Soil nutrient loss in lower elevation areas is influenced by prolonged impoundment durations. The higher N2O fluxes in the lacustrine zone can be attributed to increased nutrient levels resulting from anthropogenic activities. Furthermore, correlation analysis revealed that soil bulk density significantly impacted N2O fluxes across all sites, while NO3-and SOC facilitated N2O emissions in T1-T2 and T4-T5, respectively. It was evident that N2O production primarily contributed to nitrification in the transition zone and was constrained by the mineralization process, whereas denitrification dominated in the lacustrine zone. Notably, the annual N2O efflux from WLFZs accounted for 27 % of that from the water-air interface in HJD Reservoir, indicating a considerably lower contribution than anticipated. Nevertheless, this study highlights the significance of WLFZs as a vital potential source of N2O emission, particularly under the influence of anthropogenic activities and high WFPS gradient.

Appraisal of different land use systems for heterotrophic respiration in a Karst landscape

Soil respiration, particularly heterotrophic respiration (RH), is a potent source of carbon dioxide (CO₂) in the atmosphere. The current research focuses on the evaluation of RH for six land use systems including sloping cropland (SC), shrub land (SD), grassland (GD), shrub & grassland (SGD), newly abandoned cropland (NC) and afforested forest (AF). Heterotrophic respiration showed a diverse seasonal pattern over a year long period that was affected by various soil properties and climatic variables across the six land use systems in a subtropical Karst landscape. The lowest RH scores were found in the SD site (annual cumulative soil CO₂ flux: 2447 kg C ha^-1), whereas the maximum heterotrophic respiration occurred in the SF site (annual cumulative soil CO₂ 13597 kg C ha^-1). The values of RH were: SC site: 3.8-191.5 mg C m^-2 h^-1, NC site: 1.04-129 mg C m^-2 h^-1, GD site: 3.6-100.7 mg C m^-2 h^-1, SGD site: 0.3-393.5 mg C m^-2 h^-1, SD site: 3-116 mg C m^-2 h^-1, and SF site: 10.6-398.2 mg C m^-2 h^-1. Highly significant (p ≤ 0.01) and positive correlations between RH rate and soil temperature were found for the studied land use types (correlation coefficients as follows; SC: 0.77, NC: 0.61, GD: 0.283, SGD: 0.535, SD: 0.230, SF: 0.85). However, water filled pore space (WFPS), NH₄⁺, NO₃^-, dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations showed varied (positive and negative) correlations with RH. The overall results show that soil temperature can be considered as the most limiting factor for RH among all the sites studied in the present research. In these environments, soil heterotrophic respiration significantly correlated with soil temperature, highlighting the significance of climate on heterotrophic respiration.

Thinning intensity but not replanting different species affects soil N2O and CH4 fluxes in Cunninghamia lanceolata plantation

Thinning and replanting are effective forest management measures to improve the stand structure and species composition of artificial forests. However, the effects of thinning and replanting on soil N₂O and CH₄ fluxes and their associations with changes in soil environment factors have been poorly understood in plantation forests. A 36-month field experiment was conducted to elucidate the effects of thinning and replanting different species on soil N₂O and CH₄ fluxes and related environmental factors in Cunninghamia lanceolata plantation on shallow soil. The experiment consisted of five treatments, uncut control (CK), moderate thinning + replanting evergreen seedlings (MTE), moderate thinning + replanting deciduous seedlings (MTD), heavy thinning + replanting evergreen seedlings (HTE), heavy thinning + replanting deciduous seedlings (HTD). Compared with the control, moderate and heavy thinning increased cumulative N₂O emissions by 12.4% and 21.4%, respectively, and reduced CH₄ cumulative uptake by 35.4% and 38.8%, respectively. However, the effects on soil N₂O and CH₄ fluxes replanting deciduous or evergreen seedlings were insignificant. The results showed that thinning increased N₂O emissions and decreased CH₄ uptake due to the increased soil temperature, labile C and N concentrations. Soil temperature was the dominant factor, and mineral N was a contributing factor affecting N₂O and CH₄ fluxes. The study concludes that thinning increased the global warming potential with N₂O contributing more than CH₄ (113.5%: -13.5%). Our findings highlight that thinning increased N₂O emissions and decreased CH₄ uptake with the increasing intensity and the replanting had no different effects between deciduous and evergreen seedlings on the fluxes of N₂O and CH₄ during the early years following thinning.

Asymmetric response of soil methane uptake rate to land degradation and restoration: Data synthesis

Land degradation and restoration profoundly affect soil CH₄ uptake capacity in terrestrial ecosystems. However, a comprehensive assessment of the response of soil CH₄ uptake to land degradation and restoration at global scale is not available. Here, we present a global meta-analysis with a database of 228 observations from 83 studies to investigate the effects of land degradation and restoration on the capacity of soil CH₄ uptake. We found that land degradation significantly decreased the capacity of soil CH₄ uptake, except the conversion of pasture to cropland where the soil CH₄ uptake rate showed no response. In contrast, all types of land restoration significantly increased the capacity of soil CH₄ uptake. Interestingly, the response of soil CH₄ uptake rate to land degradation and restoration was asymmetric: the increased soil CH₄ uptake rate in response to the land restoration was smaller compared to the decrease in CH₄ uptake rate induced by the land degradation. The effect of land degradation on soil CH₄ uptake rate was not dependent on the time since land use change, but the CH₄ sink strength increased with the time since land restoration. The response of soil CH₄ uptake rate to both land degradation and restoration was predominantly regulated by changes in the soil water-filled pore space, soil bulk density, and pH, whereas alterations in the substrate quantity and quality had negligible effect. Additionally, the effects of land degradation and restoration on soil CH₄ uptake were strongly related to the mean annual precipitation and soil texture. Overall, our results provide novel insights for understanding of how land degradation and restoration can affect the CH₄ sink strength of upland soils, and more importantly, our findings are beneficial to take measures to enhance the potential of soil CH₄ uptake in response to global land use change.

Developing an Automated Gas Sampling Chamber for Measuring Variations in CO2 Exchange in a Maize Ecosystem at Night

The measurement of net ecosystem exchange (NEE) of field maize at a plot-sized scale is of great significance for assessing carbon emissions. Chamber methods remain the sole approach for measuring NEE at a plot-sized scale. However, traditional chamber methods are disadvantaged by their high labor intensity, significant resultant changes in microclimate, and significant impact on the physiology of crops. Therefore, an automated portable chamber with an air humidity control system to determinate the nighttime variation of NEE in field maize was developed. The chamber system can automatically open and close the chamber, and regularly collect gas in the chamber for laboratory analysis. Furthermore, a humidity control system was created to control the air humidity of the chamber. Chamber performance test results show that the maximum difference between the temperature and humidity outside and inside the chamber was 0.457 °C and 5.6%, respectively, during the NEE measuring period. Inside the chamber, the leaf temperature fluctuation range and the maximum relative change of the maize leaf respiration rate were −0.3 to 0.3 °C and 23.2015%, respectively. We verified a series of measurements of NEE using the dynamic and static closed chamber methods. The results show a good common point between the two measurement methods (N = 10, R² = 0.986; and mean difference: ΔCO₂ = 0.079 μmol m−2s−1). This automated chamber was found to be useful for reducing the labor requirement and improving the time resolution of NEE monitoring. In the future, the relationship between the humidity control system and chamber volume can be studied to control the microclimate change more accurately.