The Effect of Land-Use Change on Soil CH4 and N2O Fluxes: A Global Meta-Analysis

Resilience of aerobic methanotrophs in soils; spotlight on the methane sink under agriculture

Aerobic methanotrophs are a specialized microbial group, catalyzing the oxidation of methane. Disturbance-induced loss of methanotroph diversity/abundance, thus results in the loss of this biological methane sink. Here, we synthesized and conceptualized the resilience of the methanotrophs to sporadic, recurring, and compounded disturbances in soils. The methanotrophs showed remarkable resilience to sporadic disturbances, recovering in activity and population size. However, activity was severely compromised when disturbance persisted or reoccurred at increasing frequency, and was significantly impaired following change in land use. Next, we consolidated the impact of agricultural practices after land conversion on the soil methane sink. The effects of key interventions (tillage, organic matter input, and cover cropping) where much knowledge has been gathered were considered. Pairwise comparisons of these interventions to nontreated agricultural soils indicate that the agriculture-induced impact on the methane sink depends on the cropping system, which can be associated to the physiology of the methanotrophs. The impact of agriculture is more evident in upland soils, where the methanotrophs play a more prominent role than the methanogens in modulating overall methane flux. Although resilient to sporadic disturbances, the methanotrophs are vulnerable to compounded disturbances induced by anthropogenic activities, significantly affecting the methane sink function.

Carbon and nitrogen fractions control soil N2O emissions and related functional genes under land-use change in the tropics

Converting natural forests to managed ecosystems generally increases soil nitrous oxide (N2O) emission. However, the pattern and underlying mechanisms of N2O emissions after converting tropical forests to managed plantations remain elusive. Hence, a laboratory incubation study was investigated to determine soil N2O emissions of four land uses including forest, eucalyptus, rubber, and paddy field plantations in a tropical region of China. The effect of soil carbon (C) and nitrogen (N) fractions on soil N2O emissions and related functional genes was also estimated. We found that the conversion of natural forests to managed forests significantly decreased soil N2O emissions, but the conversion to paddy field had no effect. Soil N2O emissions were controlled by both nitrifying and denitrifying genes in tropical natural forest, but only by nitrifying genes in managed forests and by denitrifying genes in paddy field. Soil total N, extractable nitrate, particulate organic C (POC), and hydrolyzable ammonium N showed positive relationship with soil N2O emission. The easily oxidizable organic C (EOC), POC, and light fraction organic C (LFOC) had positive linear correlation with the abundance of AOA-amoA, AOB-amoA, nirK, and nirS genes. The ratios of dissolved organic C, EOC, POC, and LFOC to total N rather than soil C/N ratio control soil N2O emissions with a quadratic function relationship, and the local maximum values were 0.16, 0.22, 1.5, and 0.55, respectively. Our results provided a new evidence of the role of soil C and N fractions and their ratios in controlling soil N2O emissions and nitrifying and denitrifying genes in tropical soils.

Deciphering nitrous oxide emissions from tropical soils of different land uses

Tropical regions are hotspots of increasing greenhouse gas emissions associated with land-use change. Although many field studies have quantified soil fluxes of nitrous oxide (N₂O; a potent greenhouse gas) from various land uses, the driving mechanisms remain uncertain. Here, we used tropical soils of diverse land uses and actively manipulated the soil moisture (35%, 60%, and 95% water-filled pore space [WFPS]) and substrate supply (control, nitrate, and nitrate plus glucose) to investigate the responses of N₂O emissions with short-term incubations. We then identified key factors regulating N₂O emissions out of a series of soil physicochemical and biological factors and explored how these factors interacted to drive N₂O emissions. Land-use changes from primary forest to oil palm or Acacia plantation risks emitting more N₂O, whereas low emissions could be maintained by conversion to Macaranga forest or Imperata grassland; these laboratory observations were corroborated by a literature synthesis of field N₂O measurements across tropical regions. Soil redox potential (Eh) and labile organic nitrogen (LON; amino acid mixture, arginine, and urea) mineralization were among the factors with greatest influence on N₂O emissions. In contrast to common understandings, the control of WFPS over N₂O emissions was largely indirect, and acted through Eh. The mineralization of LON, particularly arginine, potentially played multiple roles in N₂O production (e.g., bottlenecks of nitrifier-denitrification or simultaneous nitrification-denitrification versus substrate competition for co-denitrification). Structural equation models suggest that soil-environmental factors of different levels (from distal including land use, soil moisture, and pH to proximal such as LON mineralization) drive N₂O emissions through cascading interactions. Overall, we show that, despite identical initial soil conditions, land conversion can substantially alter the N₂O emission potential. Also, collectively considering soil-environmental regulators and their interactions associated with land conversion is crucial to predict and design mitigation strategies for N₂O emissions from land-use change.

Responses of soil greenhouse gas emissions to land use conversion and reversion-A global meta-analysis

Exploring the responses of greenhouse gas (GHG) emissions to land use conversion or reversion is significant for taking effective land use measures to alleviate global warming. A global meta-analysis was conducted to analyze the responses of carbon dioxide (CO₂ ), methane (CH₄ ), and nitrous oxide (N₂ O) emissions to land use conversion or reversion, and determine their temporal evolution, driving factors, and potential mechanisms. Our results showed that CH₄ and N₂ O responded positively to land use conversion while CO₂ responded negatively to the changes from natural herb and secondary forest to plantation. By comparison, CH₄ responded negatively to land use reversion and N₂ O also showed negative response to the reversion from agricultural land to forest. The conversion of land use weakened the function of natural forest and grassland as CH₄ sink and the artificial nitrogen (N) addition for plantation increased N source for N₂ O release from soil, while the reversion of land use could alleviate them to some degree. Besides, soil carbon would impact CO₂ emission for a long time after land use conversion, and secondary forest reached the CH₄ uptake level similar to that of primary forest after over 40 years. N₂ O responses had negative relationships with time interval under the conversions from forest to plantation, secondary forest, and pasture. In addition, meta-regression indicated that CH₄ had correlations with several environmental variables, and carbon-nitrogen ratio had contrary relationships with N₂ O emission responses to land use conversion and reversion. And the importance of driving factors displayed that CO₂ , CH₄ , and N₂ O response to land use conversion and reversion was easily affected by NH₄ ⁺ and soil moisture, mean annual temperature and NO₃ ^- , total nitrogen and mean annual temperature, respectively. This study would provide enlightenments for scientific land management and reduction of GHG emissions.

Different variations in soil CO2, CH4, and N2O fluxes and their responses to edaphic factors along a boreal secondary forest successional trajectory

Forest succession is an important process regulating the carbon and nitrogen budgets in forest ecosystems. However, little is known about how and extent by which vegetation succession predictably affects soil CO₂, CH₄, and N₂O fluxes, especially in boreal forest. Here, a field study was conducted along a secondary forest succession trajectory from Betula platyphylla forest (early stage), then Betula platyphylla-Larix gmelinii forest (intermediate stage), to Larix gmelinii forest (late stage) to explore the effects of forest succession on soil greenhouse gas fluxes and related soil environmental factors in Northeast China. The results showed significant differences in soil greenhouse gas fluxes during the forest succession. During the study period, the average soil CO₂ flux was greatest at mid-successional stage (444.72 mg m^-2 h^-1), followed by the late (341.81 mg m^-2 h^-1) and the early-successional (347.12 mg m^-2 h^-1) stages. The average soil CH₄ flux increased significantly during succession, ranging from -0.062 to -0.036 mg m^-2 h^-1. The average soil N₂O flux was measured as 17.95 μg m^-2 h^-1 at intermediate successional stage, significantly lower than that at late (20.71 μg m^-2 h^-1) and early-successional (20.85 μg m^-2 h^-1) stages. During forest succession, soil greenhouse gas fluxes showed significant correlations with soil and environmental factors at both seasonal and successional time scales. The seasonal variations of soil GHG fluxes were mainly influenced by soil temperature and water content. Meanwhile, soil MBN and soil NO₃^--N content were also important factors for soil N₂O fluxes. Structural equation modelling showed that forest succession affected soil CO₂ fluxes by changing soil temperature and microbial biomass carbon, affected soil CH₄ fluxes mainly by changing soil water content and soil pH value, and affected soil N₂O fluxes mainly by changing soil temperature, microbial biomass nitrogen, and soil NO₃^--N content. Our study suggests that forest succession mainly alters soil nutrient and soil environment/chemical properties affecting soil CO₂ and N₂O fluxes and soil CH₄ fluxes, respectively, in the secondary forest succession process.

Linking transcriptional dynamics of CH4-cycling grassland soil microbiomes to seasonal gas fluxes

Soil CH₄ fluxes are driven by CH₄-producing and -consuming microorganisms that determine whether soils are sources or sinks of this potent greenhouse gas. To date, a comprehensive understanding of underlying microbiome dynamics has rarely been obtained in situ. Using quantitative metatranscriptomics, we aimed to link CH₄-cycling microbiomes to net surface CH₄ fluxes throughout a year in two grassland soils. CH₄ fluxes were highly dynamic: both soils were net CH₄ sources in autumn and winter and sinks in spring and summer, respectively. Correspondingly, methanogen mRNA abundances per gram soil correlated well with CH₄ fluxes. Methanotroph to methanogen mRNA ratios were higher in spring and summer, when the soils acted as net CH₄ sinks. CH₄ uptake was associated with an increased proportion of USCα and γ pmoA and pmoA2 transcripts. We assume that methanogen transcript abundance may be useful to approximate changes in net surface CH₄ emissions from grassland soils. High methanotroph to methanogen ratios would indicate CH₄ sink properties. Our study links for the first time the seasonal transcriptional dynamics of CH₄-cycling soil microbiomes to gas fluxes in situ. It suggests mRNA transcript abundances as promising indicators of dynamic ecosystem-level processes.

Conversion of alpine pastureland to artificial grassland altered CO2 and N2O emissions by decreasing C and N in different soil aggregates

BACKGROUND

The impacts of land use on greenhouse gases (GHGs) emissions have been extensively studied. However, the underlying mechanisms on how soil aggregate structure, soil organic carbon (SOC) and total N (TN) distributions in different soil aggregate sizes influencing carbon dioxide (CO₂), and nitrous oxide (N₂O) emissions from alpine grassland ecosystems remain largely unexplored.

METHODS

A microcosm experiment was conducted to investigate the effect of land use change on CO₂and N₂O emissions from different soil aggregate fractions. Soil samples were collected from three land use types, i.e., non-grazing natural grassland (CK), grazing grassland (GG), and artificial grassland (GC) in the Bayinbuluk alpine pastureland. Soil aggregate fractionation was performed using a wet-sieving method. The variations of soil aggregate structure, SOC, and TN in different soil aggregates were measured. The fluxes of CO₂ and N₂O were measured by a gas chromatograph.

RESULTS

Compared to CK and GG, GC treatment significantly decreased SOC (by 24.9-45.2%) and TN (by 20.6-41.6%) across all soil aggregate sizes, and altered their distributions among soil aggregate fractions. The cumulative emissions of CO₂ and N₂O in soil aggregate fractions in the treatments of CK and GG were 39.5-76.1% and 92.7-96.7% higher than in the GC treatment, respectively. Moreover, cumulative CO₂emissions from different soil aggregate sizes in the treatments of CK and GG followed the order of small macroaggregates (2-0.25 mm) > large macroaggregates (> 2 mm) > micro aggregates (0.25-0.053 mm) > clay +silt (< 0.053 mm), whereas it decreased with aggregate sizes decreasing in the GC treatment. Additionally, soil CO₂ emissions were positively correlated with SOC and TN contents. The highest cumulative N₂O emission occurred in micro aggregates under the treatments of CK and GG, and N₂O emissions among different aggregate sizes almost no significant difference under the GC treatment.

CONCLUSIONS

Conversion of natural grassland to artificial grassland changed the pattern of CO₂ emissions from different soil aggregate fractions by deteriorating soil aggregate structure and altering soil SOC and TN distributions. Our findings will be helpful to develop a pragmatic management strategy for mitigating GHGs emissions from alpine grassland.

Land Use Change in the Cross-Boundary Regions of a Metropolitan Area: A Case Study of Tongzhou-Wuqing-Langfang

Since the 1980s, metropolitan areas have increased worldwide due to urbanization and regionalization. While the spatial integration of the labor and housing markets has benefitted the development of cities within metropolitan areas, they have also brought great challenges for land governance; this is particularly evident in cross-boundary regions due to the complex relations between the markets and the regulations and between governments at different levels. Extensive research has been conducted on the city-level analysis of socioeconomic integration, land use development, and urban governance within metropolitan areas; yet, it is insufficient for understanding the intricate interplay between the various forces in such regions. This study aims to reveal the dynamics of land use change from 1990–2020 and its driving forces in the recent decade in the Tongzhou-Wuqing-Langfang (TWL) region—a typical cross-boundary area between Beijing, Tianjin, and the Hebei Metropolitan Area—using Landsat imagery. We employed the land-use dynamic degree, kernel density analysis, principal component analysis, and multiple linear regression to explore the spatiotemporal patterns of land use change and its driving factors at the district/county level. The results show that the general land use changes from cultivated and forest land to urban and rural construction land across the region. The speed of the trend varies considerably over time between different areas as the land use policies and regulations of each local government change. The population growth and the tertiary and secondary industry growth are the main driving factors for the change in construction land across the whole TWL region, while the urbanization rate and fixed asset investment have different impacts across the cross-boundary region. The results suggest that expanding the integration of land use policies and regulations in the cross-boundary region is urgently required.

Disproportionate CH4 Sink Strength from an Endemic, Sub-Alpine Australian Soil Microbial Community

Soil-to-atmosphere methane (CH₄) fluxes are dependent on opposing microbial processes of production and consumption. Here we use a soil–vegetation gradient in an Australian sub-alpine ecosystem to examine links between composition of soil microbial communities, and the fluxes of greenhouse gases they regulate. For each soil/vegetation type (forest, grassland, and bog), we measured carbon dioxide (CO₂) and CH₄ fluxes and their production/consumption at 5 cm intervals to a depth of 30 cm. All soils were sources of CO₂, ranging from 49 to 93 mg CO₂ m⁻² h⁻¹. Forest soils were strong net sinks for CH₄, at rates of up to −413 µg CH₄ m⁻² h⁻¹. Grassland soils varied, with some soils acting as sources and some as sinks, but overall averaged −97 µg CH₄ m⁻² h⁻¹. Bog soils were net sources of CH₄ (+340 µg CH₄ m⁻² h⁻¹). Methanotrophs were dominated by USCα in forest and grassland soils, and Candidatus Methylomirabilis in the bog soils. Methylocystis were also detected at relatively low abundance in all soils. Our study suggests that there is a disproportionately large contribution of these ecosystems to the global soil CH₄ sink, which highlights our dependence on soil ecosystem services in remote locations driven by unique populations of soil microbes. It is paramount to explore and understand these remote, hard-to-reach ecosystems to better understand biogeochemical cycles that underpin global sustainability.

Divergent drivers of the microbial methane sink in temperate forest and grassland soils

Aerated topsoils are important sinks for atmospheric methane (CH₄ ) via oxidation by CH₄ -oxidizing bacteria (MOB). However, intensified management of grasslands and forests may reduce the CH₄ sink capacity of soils. We investigated the influence of grassland land-use intensity (150 sites) and forest management type (149 sites) on potential atmospheric CH₄ oxidation rates (PMORs) and the abundance and diversity of MOB (with qPCR) in topsoils of three temperate regions in Germany. PMORs measurements in microcosms under defined conditions yielded approximately twice as much CH₄ oxidation in forest than in grassland soils. High land-use intensity of grasslands had a negative effect on PMORs (-40%) in almost all regions and fertilization was the predominant factor of grassland land-use intensity leading to PMOR reduction by 20%. In contrast, forest management did not affect PMORs in forest soils. Upland soil cluster (USC)-α was the dominant group of MOBs in the forests. In contrast, USC-γ was absent in more than half of the forest soils but present in almost all grassland soils. USC-α abundance had a direct positive effect on PMOR in forest, while in grasslands USC-α and USC-γ abundance affected PMOR positively with a more pronounced contribution of USC-γ than USC-α. Soil bulk density negatively influenced PMOR in both forests and grasslands. We further found that the response of the PMORs to pH, soil texture, soil water holding capacity and organic carbon and nitrogen content differ between temperate forest and grassland soils. pH had no direct effects on PMOR, but indirect ones via the MOB abundances, showing a negative effect on USC-α, and a positive on USC-γ abundance. We conclude that reduction in grassland land-use intensity and afforestation has the potential to increase the CH₄ sink function of soils and that different parameters determine the microbial methane sink in forest and grassland soils.

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.

Drainage increases CO2 and N2 O emissions from tropical peat soils

Tropical peatlands are vital ecosystems that play an important role in global carbon storage and cycles. Current estimates of greenhouse gases from these peatlands are uncertain as emissions vary with environmental conditions. This study provides the first comprehensive analysis of managed and natural tropical peatland GHG fluxes: heterotrophic (i.e. soil respiration without roots), total CO₂ respiration rates, CH₄ and N₂ O fluxes. The study documents studies that measure GHG fluxes from the soil (n = 372) from various land uses, groundwater levels and environmental conditions. We found that total soil respiration was larger in managed peat ecosystems (median = 52.3 Mg CO₂  ha^-1  year^-1 ) than in natural forest (median = 35.9 Mg CO₂  ha^-1  year^-1 ). Groundwater level had a stronger effect on soil CO₂ emission than land use. Every 100 mm drop of groundwater level caused an increase of 5.1 and 3.7 Mg CO₂  ha^-1  year^-1 for plantation and cropping land use, respectively. Where groundwater is deep (≥0.5 m), heterotrophic respiration constituted 84% of the total emissions. N₂ O emissions were significantly larger at deeper groundwater levels, where every drop in 100 mm of groundwater level resulted in an exponential emission increase (exp(0.7) kg N ha^-1  year^-1 ). Deeper groundwater levels induced high N₂ O emissions, which constitute about 15% of total GHG emissions. CH₄ emissions were large where groundwater is shallow; however, they were substantially smaller than other GHG emissions. When compared to temperate and boreal peatland soils, tropical peatlands had, on average, double the CO₂ emissions. Surprisingly, the CO₂ emission rates in tropical peatlands were in the same magnitude as tropical mineral soils. This comprehensive analysis provides a great understanding of the GHG dynamics within tropical peat soils that can be used as a guide for policymakers to create suitable programmes to manage the sustainability of peatlands effectively.