Land cover changes and greenhouse gas emissions in two different soil covers in the Brazilian Caatinga

Understanding interactive processes: a review of CO2 flux, evapotranspiration, and energy partitioning under stressful conditions in dry forest and agricultural environments

Arid and semiarid environments are characterized by low water availability (e.g., in soil and atmosphere), high air temperature, and irregularity in the spatio-temporal distribution of rainfall. In addition to the economic and environmental consequences, drought also causes physiological damage to crops and compromises their survival in ecosystems. The removal of vegetation is responsible for altering the energy exchange of heat and water in natural ecosystems and agricultural areas. The fluxes of CO₂ are also changed, and environments with characteristics of sinks, which can be sources of CO₂ after anthropic disturbances. These changes can be measured through methods such as sap flow, eddy covariance, remote sensing, and energy balance. Despite the relevance of each method mentioned above, there are limitations in their applications that must be respected. Thus, this review aims to quantify the processes and changes of energy fluxes, CO₂, and their interactions with the surfaces of terrestrial ecosystems in dry environments. Studies report that the use of methods that integrate data from climate monitoring towers and remote sensing products helps to improve the accuracy of the determination of energy fluxes on a global scale, also helping to reduce the dissimilarity of results obtained individually. Through the collection of works in the literature, it is reported that several areas of the Brazilian Caatinga biome, which is a Seasonally Dry Tropical Forest have been suffering from changes in land use and land cover. Similar fluxes of sensible heat in areas with cacti and Caatinga can be observed in studies. On the other hand, one of the variables influenced mainly by air temperature is net radiation. In dry forest areas, woody species can store large amounts of carbon in their biomass above and belowground. The use of cacti can modify the local carbon budget when using tree crops together. Therefore, the study highlights the complexity and severity of land degradation and changes in CO₂, water, and energy fluxes in dry environments with areas of forest, grassland, and cacti. Vegetation energy balance is also a critical factor, as these simulations are helpful for use in forecasting weather or climate change. We also highlight the need for more studies that address environmental conservation techniques and cactus in the conservation of degraded areas.

Using Remote Sensing to Quantify the Joint Effects of Climate and Land Use/Land Cover Changes on the Caatinga Biome of Northeast Brazilian

Caatinga biome, located in the Brazilian semi-arid region, is the most populous semi-arid region in the world, causing intensification in land degradation and loss of biodiversity over time. The main objective of this paper is to determine and analyze the changes in land cover and use, over time, on the biophysical parameters in the Caatinga biome in the semi-arid region of Brazil using remote sensing. Landsat-8 images were used, along with the Surface Energy Balance Algorithm for Land (SEBAL) in the Google Earth Engine platform, from 2013 to 2019, through spatiotemporal modeling of vegetation indices, i.e., leaf area index (LAI) and vegetation cover (VC). Moreover, land surface temperature (LST) and actual evapotranspiration (ETa) in Petrolina, the semi-arid region of Brazil, was used. The principal component analysis was used to select descriptive variables and multiple regression analysis to predict ETa. The results indicated significant effects of land use and land cover changes on energy balances over time. In 2013, 70.2% of the study area was composed of Caatinga, while the lowest percentages were identified in 2015 (67.8%) and 2017 (68.7%). Rainfall records in 2013 ranged from 270 to 480 mm, with values higher than 410 mm in 46.5% of the study area, concentrated in the northern part of the municipality. On the other hand, in 2017 the lowest annual rainfall values (from 200 to 340 mm) occurred. Low vegetation cover rate was observed by LAI and VC values, with a range of 0 to 25% vegetation cover in 52.3% of the area, which exposes the effects of the dry season on vegetation. The highest LST was mainly found in urban areas and/or exposed soil. In 2013, 40.5% of the region’s area had LST between 48.0 and 52.0 °C, raising ETa rates (~4.7 mm day−1). Our model has shown good outcomes in terms of accuracy and concordance (coefficient of determination = 0.98, root mean square error = 0.498, and Lin’s concordance correlation coefficient = 0.907). The significant increase in agricultural areas has resulted in the progressive reduction of the Caatinga biome. Therefore, mitigation and sustainable planning is vital to decrease the impacts of anthropic actions.

Drought, desertification and poverty: A geospatial analysis of snakebite envenoming in the Caatinga biome of Brazil

Epidemiological data on snakebite in the Brazilian state of Ceará are scarce, as the only report on this subject was last published in 1997. However, according to the Brazilian system of recording disease incidents (Sistema de Informação de Agravos de Notificação [SINAN]), more than 13,000 snakebites have been registered since 2001 in the state of Ceará, making this disease an important public health issue. In the present study, we evaluate the influence of environmental changes, including drought and desertification, on the risk of snakebite envenoming in the Brazilian northeastern state of Ceará. We compare public data on snakebites from Brazilian Epidemiological Surveillance System (DATASUS), rainfall records, advanced desertification maps, pastures and socioeconomic information of the 184 municipals in Ceará between 2001 and 2017. During the period of investigation, 8,945 snakebites were recorded, the majority (93.8%) of which involved venomous snakes. Almost half of the municipals (48%) had 100 incidences or more per 100,000 inhabitants. Data collected also highlight month-to-month occurrences of snakebites, with trends to rise shortly after the onset of precipitation, peaking in July and then trending downward as rainfall decreases, reaching the lowest level in December. We deduce an inverse relationship between Human Development Index (HDI) and snakebites per area. Spearman correlation and principal component analysis support the hypothesis that water scarcity and desertification are linked to increased risk of snakebite envenoming. Our study indicates that besides poverty, dry and desertified areas represent risk factors associated with increased incidence of snakebite envenoming in the state of Ceará.

Seasonal variation in net ecosystem CO2 exchange of a Brazilian seasonally dry tropical forest

Forest ecosystems sequester large amounts of atmospheric CO₂, and the contribution from seasonally dry tropical forests is not negligible. Thus, the objective of this study was to quantify and evaluate the seasonal and annual patterns of CO₂ exchanges in the Caatinga biome, as well as to evaluate the ecosystem condition as carbon sink or source during years. In addition, we analyzed the climatic factors that control the seasonal variability of gross primary production (GPP), ecosystem respiration (R_eco) and net ecosystem CO₂ exchange (NEE). Results showed that the dynamics of the components of the CO₂ fluxes varied depending on the magnitude and distribution of rainfall and, as a consequence, on the variability of the vegetation state. Annual cumulative NEE was significantly higher (p < 0.01) in 2014 (−169.0 g C m⁻²) when compared to 2015 (−145.0 g C m⁻²) and annual NEP/GPP ratio was 0.41 in 2014 and 0.43 in 2015. Global radiation, air and soil temperature were the main factors associated with the diurnal variability of carbon fluxes. Even during the dry season, the NEE was at equilibrium and the Caatinga acted as an atmospheric carbon sink during the years 2014 and 2015.

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

Afforestation is of importance for terrestrial carbon sequestration as well as soil and water conservation in karst landscapes. However, few studies have evaluated the effects of afforestation on soil CH₄ and N₂O emissions in subtropical karst areas. Thus, a year-round field experiment was conducted to quantify the effects of afforestation on soil CH₄ and N₂O fluxes from a subtropical karst landscape in South China. In this study, soil CH₄ and N₂O fluxes were simultaneously monitored using static chamber-gas chromatography from three paired sites, including a cropland site (SC) and adjacent sites at two stages of afforestation, a shrubland (SD) and a woodland (AF). The results showed that annual soil CH₄ uptake for SC, SD, and AF sites were 1.53 ± 0.20 kg C ha^-1 yr^-1, 2.90 ± 0.20 kg C ha^-1 yr^-1, and 5.68 ± 0.18 kg C ha^-1 yr^-1, respectively. Afforestation (i.e., SD and AF sites) significantly increased soil CH₄ uptake compared with the adjacent cropland. Annual soil N₂O fluxes for SC, SD, and AF sites were 2.38 ± 0.17 kg N ha^-1 yr^-1, 0.94 ± 0.14 kg N ha^-1 yr^-1, and 0.47 ± 0.01 kg N ha^-1 yr^-1, respectively. Afforestation significantly decreased soil N₂O fluxes compared with the adjacent cropland. The effects of afforestation on soil CH₄ and N₂O fluxes in the present study were mainly attributed to changes in soil characteristics, such as temperature and moisture, as these were significantly correlated with soil CH₄ and N₂O fluxes across different experimental sites. The present study highlights that afforestation is an effective land use management practice to mitigate non-CO₂ greenhouse gas emissions from subtropical karst landscapes in South China.

An incubation study of temperature sensitivity of greenhouse gas fluxes in three land-cover types near Sydney, Australia

Greenhouse gas (GHG) fluxes play crucial roles in regulating the Earth surface temperature. However, our understanding of the effect of land-cover and soil depth on the potential GHG fluxes and their temperature sensitivities (Q₁₀) is limited, which consequently increases the uncertainty to predict GHG exchange between soils and the atmosphere. In the present study, we sampled soils with contrasting characteristics from three land-cover types (wetland, grassland, and forest) and soil depths (0-10, 10-20, and 20-30 cm) from the Cumberland Plain near Sydney, Australia, and incubated at optimal (60%) water holding capacity at three temperatures (15, 25, and 35 °C). Overall, GHG fluxes and Q₁₀ values differed significantly among land-cover types and soil depths. CO₂ and N₂O emissions were highest in wetland followed by grassland and forest soils, and they decreased with soil depth. In contrast, CH₄ uptake was highest in grassland followed by forest and wetland soils, and it increased with soil depth. Combining the three major GHGs, the global warming potential in soil from wetland was higher than that from grassland and forest. Moreover, Q₁₀ values of CO₂ and N₂O emissions were: wetland > grassland > forest, while Q₁₀ value of CH₄ uptake showed the opposite pattern. Q₁₀ values of CO₂ and N₂O emissions and CH₄ uptake all increased with soil depth, demonstrating that subsoil has a higher potential for CO₂ and N₂O emissions and CH₄ uptake in a warming climate. While these experiments were conducted under ideally controlled laboratory conditions, results suggest that the large carbon stocks in wetland soils are vulnerable to loss and thus may amplify climate warming; upland soils are crucial CH₄ sinks and thus potentially mitigate climate change. In addition, the greater temperature sensitivities of CO₂ and N₂O emissions and CH₄ uptake in subsoil should be accounted for in carbon and nitrogen cycling models.

Land-use and abandonment alters methane and nitrous oxide fluxes in mountain grasslands

Grasslands cover more than one fifth of total land area in Europe and contribute significantly to the total greenhouse gas budget. The impact of management and land use on the carbon cycle and carbon sequestration in grasslands has been well-studied, however effects on emissions of N₂O and CH₄ remain uncertain. Additionally, the majority of studies have focussed on management differences between intensively managed grasslands, with few results available for lightly managed grasslands and in particular grassland abandonment. We present N₂O and CH₄ flux measurements for an abandonment trajectory at low land-use intensity, comparing meadow (fertilized and cut), pasture (grazed) and abandoned (unmanaged since 1983) grassland sites located in the Austrian Alps. Mean growing season N₂O fluxes were 0.07, 0.07 and - 0.13 nmol m^-2 s^-1 and CH₄ fluxes were - 1.0, - 0.5 and - 1.6 nmol m^-2 s^-1 for the meadow, pasture and abandoned sites respectively. Variability for both gases at the abandoned site was dominated by 'hot moments', while 'hot spots' dominated at the managed meadow and pasture sites. Consideration of the diurnal cycle observed at the abandoned site, linear correlations within all data sets, and principal components analyses of the full data set revealed increased consumption of both N₂O and CH₄ with increasing temperature, but hardly any relationship between fluxes and soil moisture. Upscaled over a year, the observed fluxes correspond to enhanced non-CO₂ greenhouse gas uptake of 172 g CO₂-equiv. m^-2 y^-1 following abandonment. These results show that non-CO₂ greenhouse gases form an important part of the total climate impact of land use change and grassland abandonment, such that abandoned grassland is a net sink for both CH₄ and N₂O.