Peatlands are accumulations of partially decayed organic soil that cover approximately 3% of Earth’s surface and have been shown to serve essential environmental and ecological functions such as sequestering carbon, purifying water, and providing habitat for organisms. However, peatlands are threatened by pressures from agriculture, urban development, mining, and climate change. Geophysical methods have been used in peatlands to determine peat volume and carbon stocks (e.g., Comas et al., 2017), observe differences in humification and water content (e.g., Ulriksen, 1982), guide engineering projects (e.g., Jol and Smith, 1995), learn about subsurface greenhouse gas dynamics (Wright and Comas, 2016), observe seasonal variations in pore water salinity (Walter et al., 2018), and assess hydrological processes (Hare et al., 2017). Among various geophysical methods, ground penetrating radar (GPR) is arguably the most popular for studying peat properties given the method’s sensitivity to variations in water content and ability to resolve major structural properties within the peat at high spatial resolution. Though less widely applied, frequency-domain analysis of GPR may also yield useful information.
Tag: Soil organic carbon
Effect of elevation and aspect on carbon stock of bamboo stands (Bambusa nutans subsp. Cupulata) outside the forest area in Eastern Nepal
Bamboo has emerged as a promising option for climate change mitigation due to its rapid growth, versatility, and renewability. However, in Nepal, there exists a substantial knowledge gap on carbon (C) stock and the influence of aspect and elevation on C stock of bamboo species, particularly in areas outside forests where bamboo is dominant. Therefore, this research was conducted to quantify C stock and aspect-elevation influence on the C stock of Bambusa nutans subsp. cupulata outside the forest area. For this study, three elevation zones (0–400 m, 400–800 m, 800–1200 m) and two aspects (East and West) were considered. A total of 30 square plots having a 100 m2 area were established utilizing purposive sampling due to the scattered distribution of bamboo. Non-destructive methods were applied to measure bamboo culm diameters, while composite soil samples were systematically collected from 30 cm depth using soil augers and core samplers. Clump density (400 ha⁻¹), culm density (42,480 ha⁻¹) and culm diameter (6.82 ± 0.41 cm) were highest at middle elevations (p 0.05). The total mean C stock potential of B. nutans was 148.73 ± 3.43 Mg ha⁻¹. Our results indicated a significant difference in C stock among elevation zones, with middle elevation zones (161.77 ± 6.74 Mg ha-1) exhibiting notably higher C stock compared to both lower (150.26 ± 2.69 Mg ha-1) and higher (134.17 ± 4.26 Mg ha-1) elevation zones. Furthermore, East aspect was found to have significantly (p < 0.05) higher soil organic C stock (18.52 ± 1.32 Mg ha-1) compared to West aspect (11.4 ± 1.01 Mg ha-1). Further research is needed to explore other complex environmental interactions with C stock potential for better climate change strategies. Incorporating bamboo C into Nepal's REDD+ initiative can be crucial for optimizing opportunities to earn C credits.
Nitrogen and phosphorus supply controls soil organic carbon mineralization in tropical topsoil and subsoil
Nitrogen (N) deposition to soils is globally rising, but its effect on soil organic carbon (SOC) turnover is still uncertain. Moreover, common theories of stoichiometric decomposition and microbial N mining predict opposing effects of N supply on SOC turnover. We hypothesized that the effect of N deposition on SOC turnover depends on initial soil nutrient conditions. Thus, we sampled tropical forests and rubber gardens with pronounced gradients of nutrient availability from the topsoil to the deep subsoil (up to 400 cm) and measured substrate-induced respiration (SIR) for 30 days in four treatments (C, CN, CP, CNP additions). A natural 13C abundance approach was conducted to quantify priming effects (PE) of the added C on SOC mineralization. For this purpose we assessed the 13CO2 isotope composition after adding a C4 sugar to the C3 soil; to correct for isotopic fractionation a treatment with C3 sugar additions served as control. We found that nutrient additions to topsoil did neither alter cumulative CO2 release within 30 days (SIRacc) nor PE (PE = 1.6, i.e., sugar additions raised the release of SOC-derived CO2 by a factor of 1.6). In the upper subsoil (30-100 cm), however, both CN and CP additions increased SIRacc (by 239% and 92%, respectively) and the PE (PE = 5.2 and 3.3, respectively) relative to the treatments that received C only (PE = 1.7), while CNP additions revealed the largest increase of SIRacc (267%) and PE (PE = 6.0). In the deep subsoil (>130 cm depth), only the CNP addition consistently increased SIRacc (by 871%) and PE (PE = 5.2) relative to only C additions (PE = 2.0). We conclude that microbial activity was not limited by nutrients in the topsoil but was co-limited by both N and P in the subsoil. The results imply that microbes mine nutrients from previously unavailable pools under the conditions that 1) deficiency actually exists, 2) co-limitation is alleviated, and 3) nutrient reserves are present. Yet, as opposed to microbial nutrient mining theories, we showed that the subsoil PE is highest when nutrient supply matches microbial demand. As a result also N deposition might exert variable effects on SOC turnover in tropical soils: it might have no effect in nutrient-rich topsoils and in co-limited subsoils without P reserves but might increase SOC turnover in co-limited subsoils with potentially acquirable P reserves.
Towards spatially continuous mapping of soil organic carbon in croplands using multitemporal Sentinel-2 remote sensing
Intensified human activities can augment soil organic carbon (SOC) losses from the world’s croplands, making SOC a highly dynamic parameter both in space and time. Sentinel-2 spectral imagery is well placed to capture the spatiotemporal variability of SOC, but its capability has only been demonstrated for agricultural regions mostly located in Europe. Furthermore, most studies so far only used single-date images that resulted in spatially non-continuous SOC maps, hampering their ability to aid multiscale SOC assessments. Here, we aim to achieve spatially continuous mapping of SOC in croplands, by creating multitemporal bare soil composites that maximize cropland coverage in two regions of varying agroecosystems and landscape structure in the Northeast China Chernozem region and the Belgian Loam Belt. Bare soil pixels were extracted via spectral index thresholding that excluded contaminated pixels from external perturbance. Multitemporal soil composites were then obtained by averaging over multiple single-date bare soil images that were selected within pre-determined optimal time-windows, corresponding to the region-specific crop sowing periods when best possible surface conditions were expected. Results show that the optimal time-window filter ensured selective inclusion of single-date images that themselves yielded stable and robust SOC predictions across multiple years. Spectral-based models developed from multitemporal composites consistently produced better or similar prediction accuracies than single-date images for both study regions (R2: 0.52–0.62; RMSE: 0.17–0.21 g 100 g−1), while also achieved maximum cropland coverage (>82 %). Bootstrap modelling demonstrated that SOC mapping via multitemporal Sentinel-2 data was associated with small uncertainties. Investigations into the significant spectral bands that contributed to the prediction of SOC suggested that, regardless of the study regions, the physical relationship between spectral bands and SOC that predominantly exists for laboratory spectra is largely translated into Sentinel-2 platforms. This study highlights the widespread applicability of multitemporal Sentinel-2 remote sensing for effective and high-resolution SOC mapping, in order to detect localized soil degradation as well as to inform regional cropland management in diverse agroecosystems.
Grain legumes and dryland cereals contribute to carbon sequestration in the drylands of Africa and South Asia
Grain legumes and drylands cereals including chickpea (Cicer arietinum), common bean (Phaseolus vulgaris), cowpea (Vigna unguiculata), groundnut (Arachis hypogaea), lentil (Lens culinaris), pigeon pea (Cajanus cajan), soybean (Glycine max), finger millet (Eleusine coracana), pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor) are the leading sources of food grain in drylands of Africa and South Asia. These crops can help smallholder agriculture to become more resilient, productive, and profitable, but their quantitative impact on carbon sequestration is unknown. The aim of this review study was to quantify their contribution to carbon sequestration across the drylands of Africa and South Asia based on 437 publications with 1319 observations in studies conducted across 32 countries. Cropping systems with grain legumes showed the greatest increase in soil organic carbon (SOC) concentrations, while cereals (and pigeon pea) gave the largest amount of aboveground carbon stock (>2 Mg C ha−1). Estimated carbon stock in post-harvest residues of these crops was 1.51 ± 0.05 Mg C ha−1 in Africa and 2.29 ± 0.10 Mg C ha−1 in South Asia. These crops produced more aboveground carbon, and significantly increased SOC, when grown as intercrops. Soils with low initial SOC (32%) showed the greatest potential for carbon sequestration when cropped with grain legumes and dryland cereals. This study is the first of its kind to provide evidence that grain legumes and drylands cereals improve carbon sequestration across Africa and South Asia.
Agroforestry with contour planting of grass contributes to terrace formation and conservation of soil and nutrients on sloping land
In hilly areas, agroforestry can be a more sustainable way of producing food and other products and services than agriculture based on sole-cropping. However, research is needed to evaluate and quantify formation of natural terraces in agroforestry and their contribution to soil conservation. This study quantified natural terrace formation and examined its role in reducing soil and nutrient losses during early stages of agroforestry with fruit trees, contour grass strips and maize or coffee in agroforestry systems on sloping land in northwest Vietnam. Two agroforestry systems, comprising longan (Dimocarpus longan L.)-mango (Mangifera indica L.)-maize (Zea mays L.)-guinea grass (Panicum maximum Jacq.) (fruit-maize-AF) and son tra (Docynia indica (Wall.) Decne.)-coffee (Coffea arabica L.)-guinea grass (fruit-coffee-AF) were compared with sole-cropped maize (sole-maize) and sole-cropped coffee (sole-coffee), respectively. Terrace formation was evaluated over five years using erosion pins placed above grass strips and the volume of terrace formed was estimated. Soil and nutrient losses were quantified using soil traps. The results showed that terraces formed as the systems developed, through gradual deposition of soil sediment above the living grass strips and trees. Accumulated soil sedimentation above the grass strips during the five-year study period raised the soil surface by 4.0 cm in fruit-maize-AF and 4.2 cm in fruit-coffee-AF, and the volume of terraces generated by the grass strips was 0.26 and 0.43 m3/m respectively. The fruit-maize-AF and fruit-coffee-AF systems significantly reduced losses of soil, soil organic carbon (SOC) and associated nutrients (N, P, K) compared with sole-maize and sole-coffee already in the first two years, while the reductions were greater from year 3 onwards. On average across experiments and years, the agroforestry systems reduced soil, SOC, N, P and K losses by 27–76%, 21–78%, 20–82%, 24–82% and 22–84%, respectively. These findings show that agroforestry with fruit trees, grass strips and crops could be a useful management practice and viable option for sustainable agricultural systems on sloping land, by reducing soil (and carbon and nutrient) losses through terrace formation.
Modeling the Spatial Distribution of Soil Organic Carbon and Carbon Stocks in the Casanare Flooded Savannas of the Colombian Llanos
Flooded savannas are valuable and extensive ecosystems in South America, but not widely studied. In this study, we quantify the spatial distribution of soil organic carbon (SOC) content and stocks in the Casanare flooded savannas. We sampled 80 sites at two soil-depth intervals (0-10 and 10-30 cm), where SOC values ranged from 0.41% in the surface and 0.23% in the sub-surface of drier soils to over 14.50% and 7.51%, in soils that experienced seasonal flooding. Spatial predictions of SOC were done through two digital soil mapping (DSM) approaches: Expert-Knowledge (EK) and Random-Forest (RF). Although both approaches performed well, EK was slightly superior at predicting SOC. Covariates derived from vegetation cover, topography, and soil properties were identified as key drivers in controlling its distribution. Total SOC stocks were 55.07 Mt with a mean density of 83.1±24.3 t·ha-1 in the first 30 cm of soil, with 12.3% of this located in areas that experience long periods of flooding (semi-seasonal savannas) , which represented only 7.9% of the study area (664,752 ha). Although the study area represents only 15% of the total area of the Casanare department, the intensive pressure of human development could result in the reduction of its SOC stocks and the release of important amounts of greenhouse gases into the atmosphere. At regional level, the impact of a large-scale land use conversions of the flooded Llanos del Orinoco ecosystem area (15 Mha) could transform this area in a future source of important global emissions if correct decisions are not taken regarding the land management of the region.
Soils as Carbon Stores and Sinks: Expectations, Patterns, Processes, and Prospects of Transitions
The few percent of soil organic carbon (SOC) among mineral components form the interface of climate, plant growth, soil biological processes, physical transport infrastructure, and chemical transformations. We explore maps, models, myths, motivation, means of implementation, and modalities for transformation. Theories of place relate geographic variation in SOC to climate, soil types, land cover, and profile depth. Process-level theories of biophysical change and socioeconomic theories of induced change explain SOC transitions that follow from land use change when a declining curve is bent and recovery toward SOC saturation starts. While the desirability of recovering from SOC deficits has been mainstreamed into climate policy, the effectiveness of proposed measures taken remains contested. Process-level requirements for transitions at plot and landscape scales remain uncertain. Expectations of policy-induced SOC transitions have to align with national cross-sectoral C accounting and be managed realistically with land users (farmers) and commodity supply chains (private sector, consumers).
The Grain-for-Green project offsets warming-induced soil organic carbon loss and increases soil carbon stock in Chinese Loess Plateau
The dynamics of soil organic carbon (SOC) stock is a vital element affecting the climate, and ecological restoration is potentially an effective measure to mitigate climate change by enhancing vegetation and soil carbon stocks and thereby offsetting greenhouse gas emissions. The Grain-for-Green project (GFGP) implemented in Chinese Loess Plateau (LP) since 1999 is one of the largest ecological restoration projects in the world. However, the contributions of ecological restoration and climate change to ecosystem soil carbon sequestration are still unclear. In this study, we improved a soil carbon decomposition framework by optimizing the initial SOC stock based on full spatial simulation of SOC and incorporating the priming effect to investigate the SOC dynamics across the LP GFGP region from 1982 through 2017. Our results indicated that SOC stock in the GFGP region increased by 20.18 Tg C from 1982 through 2017. Most portion (15.83 Tg C) of the SOC increase was accumulated when the GFGP was initiated, with a SOC sink of 16.12 Tg C owing to revegetation restoration and a carbon loss of 0.29 Tg C due to warming during this period. The relationships between SOC and forest canopy height and investigations on the SOC dynamics after afforestation revealed that the accumulation rate of SOC could be as high as 24.68 g C m−2 yr−1 during the 70 years following afforestation, and that SOC could decline thereafter (−8.89 g C m−2 yr−1), which was mainly caused by warming. This study provides a new method for quantifying the contribution of ecological restoration to SOC changes, and also cautions the potential risk of LP SOC loss in the mature forest soil under future warming.
Modeling the spatial distribution of soil organic carbon and carbon stocks for the Casanare flooded Savannas, Colombia
Flooded savannas are extensive in South America and this study was conducted to assess two digital soil mapping (DSM) approaches to predict the spatial distribution of soil organic carbon (SOC) content and stocks in the Orinoco flooded savannas of Casanare department, located in the eastern plains of Colombia. SOC was estimated using a total of 80 sites sampled at two soil depth intervals (0-10 cm and 10-30 cm). SOC ranged from 0.41% at 0-10 cm and 0.23% at 10-30 cm in drier soils found in continental dunes with sandy textures and low vegetation cover (steppe) to over 14.50% and 7.51% in soils that experienced seasonal flooding located in depressions with loamy textures and flooded savanna vegetation. Predictions of the spatial distribution of SOC were done through Expert Knowledge (EK) and Random Forest (RF) approaches across the study area at 0-10 cm and 10-30 cm soil depth. Both DSM approaches were assessed through root mean square errors, mean absolute errors, and coefficients of determination. Although both DSM approaches performed very well, EK was considered slightly superior to predict SOC in the Casanare flooded savannas. Covariates derived from vegetation coverage, topography, and soil texture properties were identified as key drivers in controlling its distribution at the study area. We found total SOC stocks of 55.07 Mt with a mean density of 83.13 ± 24.32 t ha-1 stored in the first 30 cm soil depth, with 12.3% of this being located in the flooded parts of the savanna landscape, which represented only 7.9% of the study area (664,752 ha). This study provides the first effort to systematically quantify SOC stocks in the Casanare flooded savannas and shows the importance of conserving this ecosystem with the aim of avoiding SOC losses and consequent increased CO2 emissions to the atmosphere. We estimate that the department of Casanare would release an average of 2,42 Mton of CO2 emissions per year over 30 years if there were large scale conversion of the flooded savannas to intensive agriculture, which corresponds to 62% of the current emissions of the department. At regional level, the impact of a large-scale land use conversion of the flooded Llanos del Orinoco ecosystem area (15 Mha) would represent 1/3 of the current net Colombian CO2 emission (AFOLU), which makes this region a potentially important source of emissions if correct decisions are not taken regarding the land management.