Low Soil Organic Carbon Research Articles

Soil carbon stock and stability under Eucalyptus-based silvopasture and other land-use systems in the Cerrado biodiversity hotspot

During the past few decades, commercial silvopastoral systems (SPS) with exotic Eucalyptus (hybrid) trees have become popular in the Brazilian Cerrado (savanna). With the increasing awareness about the role of carbon (C) storage in soils as a climate-change mitigation strategy and the relationship between the nature of soil aggregates and the soil's carbon sequestration potential, it is important to understand the influence of such SPS systems on soil organic carbon (SOC) storage. We studied C content in three aggregate size classes in six land-use systems on Oxisols in Minas Gerais, Brazil. The systems were planted forest, native secondary forest, managed pasture, and three 8-year-old SPS, differing in their tree-planting configurations. Eucalyptus hybrid was the tree in SPS and planted forest treatments, and Urochloa decumbens was the grass in SPS and pasture treatments. From each treatment, replicated soil samples were collected from four depth-classes (0–10, 10–30, 30–60, and 60–100 cm), fractionated by wet sieving into the three aggregate-size classes, 2000 to 250 μm, 250 to 53 μm, and <53 μm size classes representing macroaggregates, microaggregates, and silt + clay, respectively, and their C contents determined. Down to 1 m, total SOC stock values ranged from 260 Mg ha−1 under pasture to 167 Mg ha−1 under native forest, with 174 Mg ha−1 for Eucalyptus plantation and about 195 Mg ha−1 for the three SPS. Compared to the degraded native forest, the pasture system had significantly higher SOC in the whole soil and the aggregate size fractions, especially in the lower soil-depth classes. The lower SOC stock of Eucalyptus hybrid SPS compared to open pasture differs from the general trend of SPS having higher stock. Given that the Cerrado biome is a biodiversity hotspot, the use of native nitrogen-fixing trees, of which there are several, is worth investigating. In addition, the conversion from Eucalyptus monocultures to SPS could be considered as a strategy to increase the SOC stock.

Linking soil carbon availability, microbial community composition and enzyme activities to organic carbon mineralization of a bamboo forest soil amended with pyrogenic and fresh organic matter

Despite fresh and pyrogenic organic matter have been widely used as amendments to improve soil organic carbon (SOC) storage, mineralization that links to C quality and soil temperature, microbial community composition and enzyme activity remain poorly understood. This study aims to explore the effects of amendments (bamboo leaves and its biochar) and incubation temperature on mineralization, and disentangle the relationships of SOC mineralization with chemical composition of SOC, labile organic C, microbial community composition, and activities of enzymes in a subtropical bamboo forest soil. Results showed that cumulative soil CO2 emissions ranked as bamboo leaf (Leaf) > bamboo leaf biochar (Biochar) > Control, regardless of the incubation temperature. Compared to the control, the Leaf treatment markedly increased, whereas the Biochar treatment decreased, the temperature sensitivity of SOC mineralization (P < 0.05). The cumulative soil CO2 emission was positively correlated (P < 0.05) with water-soluble organic C (WSOC), microbial biomass C (MBC), O-alkyl C and alkyl C contents, and activities of β-glucosidase and dehydrogenase, but negatively correlated (P < 0.01) with aromatic C content, regardless of the incubation temperature. This indicated that the lower SOC mineralization rate and lower temperature sensitivity in the Biochar (cf. Leaf) treatment were intimately associated with the lower WSOC, MBC, O-alkyl C content, and β-glucosidase and dehydrogenase activities, and higher aromatic C content in the Biochar. The high relative abundance of bacteria relating SOC mineralization included Rhizobiales, Sphingobacteriales and JG30-KF-AS9, whereas that of fungi included Eurotiales, Sordariales, Agaricales and Helotiales. Our results revealed that the application of pyrogenic organic matter, as compared to the application of fresh organic matter, can reduce SOC mineralization and its temperature sensitivity in a subtropical forest soil by limiting the availability of C and microbial activity, and thus has a great potential for maintaining soil carbon stock in subtropical forest ecosystems.

Forest land-use increases soil organic carbon quality but not its structural or thermal stability in a hedgerow system

How land-use and soil depth affect soil organic carbon (SOC) quality and stability, properties that are key to the long-term storage of C, is poorly understood in agroforestry systems. We examined the effects of land-use (forest vs. annual cropland) and soil depth (0–10 vs. 10–30 cm) on the quality (availability of C for decomposition), structural stability (composition of C functional groups), and thermal stability (energy yield and temperature characteristics during thermal oxidation) of SOC in a hedgerow system in central Alberta, using solid-state 13C nuclear magnetic resonance (NMR) spectroscopy and thermal analysis methods. The SOC quality (proportion of O-alkyl C) was higher, while the proportion of H- and C-substituted aromatic C and structural stability (hydrophobicity index (HB/HI), aromaticity index (ARM), and alkyl index (A/OA)) were all lower in the forest than in the cropland. The SOC also had lower thermal stability (higher energy density and lower temperature at which 50% of the mass was lost (TG-T50)) in the forest than in the cropland. Between the two soil depths, SOC quality (proportion of N-alkyl C, O-alkyl C and di-O-alkyl C) was higher, but the proportion of H- and C-substituted aromatic C and structural and thermal stabilities were lower, within the surficial 0–10 cm layer than in the 10–30 cm soil. We conclude that while the forest land-use within the hedgerow system had a higher SOC quality, it had lower SOC structural and thermal stabilities, suggesting that SOC under the forest land-use may be more susceptible to decline under climate change or other forms of anthropogenic disturbance.

Estimating soil organic carbon of sown biodiverse permanent pastures in Portugal using near infrared spectral data and artificial neural networks

Grasslands in Portugal are key managed ecosystems, supporting and providing a diverse number of ecosystem services. Here, we developed a procedure for rapid estimation of soil organic carbon (SOC) in soil samples of sown biodiverse permanent pastures rich in legumes (SBP) in Portugal. We combined laboratory NIR spectral data analysis with artificial neural networks (ANN) to estimate the SOC content of SBP soil samples. To train and test the ANN, we used more than 340 soil samples collected in the 0–20 cm topsoil layer from three farms in 2018 and 2019 and two other farms in 2019 only. The number of bands of the spectra (800–2778 nm) was reduced using two different approaches: (a) aggregation to Sentinel-2 (S2) bands using the average reflectance within each bandwidths; and (b) principal component analysis (PCA). For the S2 approach, we considered the six S2 bands that overlap with the spectral range of the instrument used. For the PCA approach, we considered the five first principal components. Additional covariates were used for prediction, including weather and terrain attributes, e.g. accumulated precipitation, average temperature, elevation, and slope. To test for transferability of the models to different farms, we used an eight-fold leave-one-out cross-validation approach to calculate estimation errors. Each fold is a unique combination of farm and year and is used to assess the model's performance calibrated from the seven other folds. The ANN was able to estimate both low and high SOC contents without systematic errors and with similar estimation errors for both full and reduced spectral data approaches. The average root mean squared error (RMSE) for the S2 approach was 1.95 g kg−1 (0.45 – 2.33 g kg−1 depending on the hold-out fold) and for the PCA approach was 1.81 g kg−1 (0.74 – 2.42 g kg−1) (compared to the average SOC content of 12 g kg−1). These RMSE values were similar to the RMSE obtained using the full spectra, suggesting that the original spectral resolution could be reduced without losing information. These results suggest the potential for using remotely sensed data to estimate the variation of SOC content for SBP. They are a first step towards developing algorithms that can alleviate the cost and time of soil sampling and chemical SOC laboratory analysis through indirect estimation.

Physicochemical properties as related to mineralogical composition of floodplain soils in humid tropical environment and the pedological significance.

Floodplains represent a huge but poorly understood and hence underutilised agricultural resource in the tropics. Insight into the pedogenesis of the soils could guide their exploitation. This study assessed the physicochemical and mineralogical properties of floodplain soils and explored the interrelationships among these properties for informed inferences on contemporary pedogenic processes. Surface (0-20cm) and subsurface (69-200cm) horizons of four pedons of River Benue floodplains (central Nigeria) on shale/alluvium were sampled and analysed. The physicochemical and mineralogical properties were examined for relationships whose pedological significance was discussed. Silt and clay contents were in the ranges of 117-614 and 50-500gkg-1, respectively, being generally higher in the surface and sub-surface horizons, respectively. The soils are young with one surface horizon being silt loam. The surface horizons had higher soil pH (5.9) but lower soil organic carbon (12.63gkg-1), total nitrogen (1.05gkg-1), effective cation exchange capacity (18.58 cmol kg-1), and available phosphorus (5.50mgkg-1) than the sub-surface horizons. The minerals followed the order quartz < illite < kaolinite. Quartz related inversely to the clay minerals (kaolinite and illite), but none of these minerals influenced the physicochemical properties. Instead, soil textural/acidity indices influenced those defining colloidal activity and fertility status, implying greater dependence on their mixed parent material than overall pedogenesis. It is inferred from the mineralogical relations that illitization prevails in these fast-weathering soils. The lesser influence of pedogenesis on their inherent fertility calls for effective management using the multifunctional sawah ecotechnology. The illitization may not cause environmental problems due to clay activity. Alluvial deposit-mediated increases in silt could promote carbon sequestration; however, off-site detrimental effects of nutrients associated with this erosion-susceptible silt would be expected.

Landscape-scale spatial variability of soil organic carbon content in a temperate grassland: Insights into the role of wind erosion

Understanding the spatial variability of soil organic carbon (SOC) and its main influencing factors is crucial for sustainable land use management and climate change mitigation. Existing studies mostly focus on the influence of environmental factors on the spatial variability of SOC, while little attention has been paid to the effects of wind erosion which has important influences in arid and semi-arid landscapes. In this study, spatial variability of SOC content at landscape scale and its main influencing factors were analysed at 0–1 cm and 1–6 cm in a typical temperate grassland. Results show that SOC content ranges from 0.45% to 4.11% and is higher at 0–1 cm. SOC presents strong and moderate spatial autocorrelation at 0–1 cm and 1–6 cm, respectively. SOC is high in part of the hilly area, and low in the west plain and southeast undulation. Elevation and vegetation coverage have significantly positive effects on SOC. SOC varies with land uses with the order that arable land (AL) < heavily grazed grassland (HG) < moderately grazed grassland (MG) < lightly grazed grassland (LG) < ungrazed grassland (UG) at 0–1 cm and AL < HG < MG < UG < LG at 1–6 cm. Under the same land use, SOC is generally the highest in the flatland at both layers and lowest in the leeward slope at 0–1 cm and the windward slope at 1–6 cm, respectively. Wind erosion and deposition can both alter SOC content and increase its spatial variability. SOC is high in areas without wind erosion and deposition and decreases with erosion and deposition rates due to the selective erosion and remote region sourced deposition materials which generally have low SOC. Such effects have important influence on land productivity, global carbon cycle and climate change, and it could not be neglected in carbon accounting.

Growth of Legume Cover Crops under Cassava and Its Effect on Soil Properties

Background: Low soil organic carbon is a constraint to cassava tuber formation. Some legume cover crops could be an alternative to provide organic matter on the cassava field as a source of soil organic carbon. The study was aimed to evaluate the growth of some legume cover crops under cassava and their effects on soil properties. Methods: During September 2017-July 2018 legume cover crops (Calopogonium mucunoides, Centrosema pubescens, Pueraria javanica and the mixed) were planted under cassava variety of Mangu and UJ-5. The land coverage by the legume cover crops was measured monthly from 2 to 10 months after planting. Cassava growth was observed weekly from 8 to 32 weeks after planting. Soil properties were analyzed before planting and at harvesting of cassava. Result: Pueraria javanica was tolerant toward cassava shading. The land coverage was linearly increased along with the growth of cassava. At the end of cassava growth, the land area coverage by this legume cover crop was 98.08%. It produced more organic matter and could maintain soil moisture content than other legume cover crops. P. javanica could consider being a suitable legume cover crop under cassava to improve soil quality.

Pruning residues incorporation and reduced tillage improve soil organic matter stabilization and structure of salt-affected soils in a semi-arid Citrus tree orchard

Soil salinization is an emerging problem worldwide as a result of unsustainable land management practices and climate change. However, salt-affected soils under agricultural use could act as a C sink if these negative effects can be offset by combination of sustainable land management practices (SLM). In this study, we assessed the effect of (i) intensive tillage along with flood irrigation (IT); (ii) combination of no-tillage with pruning residues (branches and leaves) as mulch, and drip-irrigation (NT + PM); and (iii) combination of reduced tillage with the incorporation of pruning residues and drip-irrigation (RT + PI), on physico-chemical soil parameters, aggregate stability, amount and quality of organic matter fractions and soil organic carbon (SOC) sequestration in a lemon tree orchards (Citrus limon var. Verna) under semi-arid climate conditions. The RT + PI management system showed a decrease in salinity and bulk density, and increased soil porosity, soil OC and N stocks, and percentage of OC-rich macroaggregates as compared to the IT system. The aggregate-occluded particulate organic matter fraction (oPOM) played a key role in macroaggregate stability. The NT + PM treatment also showed positive effects on the investigated soil properties, but this was limited to the upmost topsoil (0−5 cm). The IT management system revealed highest values of salinity and bulk density, and considerably lower SOC stocks. Moreover, a degradation of soil structure with a low percentage of macroaggregates depleted in SOC was observed. We conclude that the incorporation of pruning residues in combination with reduced tillage and drip-irrigation is an effective management system to improve soil structure and facilitate SOC sequestration. Therefore, conventional management systems based on intensive tillage and flood irrigation should be abandoned in salt-affected soils under semi-arid climate conditions in favour of systems with higher organic matter inputs incorporated into the soil combined with measures to reduce the salt content.

Assessing Lignin Decomposition and Soil Organic Carbon Contents Across a Tropical Savannah-Rainforest Boundary in Guyana

The soils beneath the rainforest of Guyana have the potential to hold, and release, large stores of carbon under land use and climate change. Little is known about soil carbon stocks or molecular dynamics in this region. This study therefore aims to elucidate differences in the molecular (lignin and tannin) and bulk soil organic carbon (SOC) stocks in different ‘sub-environments’ along a rainforest-savannah boundary, setting a framework for further investigation into the soil carbon dynamics of the region. Bulk SOC analysis shows that Gleysols have the highest stocks, particularly those under rainforest vegetation (swamp and island forests surrounded by savannah), whereas Plinthosols have significantly lower SOC stocks. Texture and soil water content analysis indicates that predominantly clay soils play a role in high SOC stocks, whilst predominantly sandy soils prevent SOC stocks from accumulating. Clay and sand are present in both Gleysols and Plinthosols, to different extents. Analysis of lignin and tannin in surface soils of the sub-environments reveals clear differences in molecular composition. Heavily degraded lignin signatures in rainforest Gleysols suggests a surrounding physio-chemical environment which promotes their degradation. Conversely, Plinthosols beneath woodland within the savannah have the greatest amount of lignin and tannin products. The presence of the clay mineral kaolinite and iron oxide strengite in these soils indicates a low ability for protection or complexing of organic matter. Therefore, water content and microbial activity may play a more important role in the degradation of lignin and tannin, as well as the SOC stock. With the potential for future deforestation due to land use or climate change, the high lignin degradation of Gleysols indicates a vulnerability to savannah encroachment. Forest Islands isolated from the main forest biome are the most vulnerable to change, and could lose a significant proportion of their SOC stock in a transition to savannah.

Long-term cultivation effects on soil properties variations in different landforms in an arid region of eastern Iran

The objective of this study was to investigate the long-term cultivation effects on soil properties changes in different geomorphic surfaces of an arid region in eastern Iran. Surface soil samples (0–30 cm) were collected at 160 locations from five geomorphic surfaces under pomegranate (Punica granatum L.), pistachio (Pistacia vera L.), and saffron (Crocus sativus L.) cultivations in 2004 and 2018. The results showed that dune field geomorphic surface (i.e., CK111) had significantly higher electrical conductivity (EC), soluble cations, including Caaq, Mgaq, Naaq, and sodium adsorption ratio (SAR) due to the high salinity of the irrigation water. The highest and lowest soil organic carbon (SOC), available phosphorus (Pav), and available potassium (Kav) values were in the soils under clay flats (i.e., PL111) and CK111, respectively, which positively correlated with clay content. Total nitrogen (TN) was higher in geomorphic surfaces, including pediment with recent and old alluvial deposits (Pi311 and Pi321) and CK111, which were under extensive pistachio orchards. On the other hand, due to the intensive agricultural practices and soil management, the amount of Pav decreased except in Pi321 through 14 years, while Kav has increased in most units. Overall, the results of this study revealed that during long-term cultivation, the size of the areas with high (12 to 16 dS m−1) and very high (16 to 32 dS m−1) salinity and alkalinity classes were increased in the studied regions, especially in pistachio cultivation during irrigation with inadequate water in playa margin lands. Moreover, increasing air temperature and rainfall reduction through recent years, water shortage and salinity, and over-extraction of groundwater are significant factors in increasing the concentration of EC and SAR of water and soil. Consequently, the planning for agricultural development in the region should be done considering the problems and limitations of unsustainable agricultural development on the edge of the desert.

Meanders as a scaling motif for understanding of floodplain soil microbiome and biogeochemical potential at the watershed scale

BackgroundBiogeochemical exports from watersheds are modulated by the activity of microorganisms that function over micron scales. Here, we tested the hypothesis that meander-bound regions share a core microbiome and exhibit patterns of metabolic potential that broadly predict biogeochemical processes in floodplain soils along a river corridor.ResultsWe intensively sampled the microbiomes of floodplain soils located in the upper, middle, and lower reaches of the East River, Colorado. Despite the very high microbial diversity and complexity of the soils, we reconstructed 248 quality draft genomes representative of subspecies. Approximately one third of these bacterial subspecies was detected across all three locations at similar abundance levels, and ~ 15% of species were detected in two consecutive years. Within the meander-bound floodplains, we did not detect systematic patterns of gene abundance based on sampling position relative to the river. However, across meanders, we identified a core floodplain microbiome that is enriched in capacities for aerobic respiration, aerobic CO oxidation, and thiosulfate oxidation with the formation of elemental sulfur. Given this, we conducted a transcriptomic analysis of the middle floodplain. In contrast to predictions made based on the prominence of gene inventories, the most highly transcribed genes were relatively rare amoCAB and nxrAB (for nitrification) genes, followed by genes involved in methanol and formate oxidation, and nitrogen and CO2 fixation. Within all three meanders, low soil organic carbon correlated with high activity of genes involved in methanol, formate, sulfide, hydrogen, and ammonia oxidation, nitrite oxidoreduction, and nitrate and nitrite reduction. Overall, the results emphasize the importance of sulfur, one-carbon and nitrogen compound metabolism in soils of the riparian corridor.ConclusionsThe disparity between the scale of a microbial cell and the scale of a watershed currently limits the development of genomically informed predictive models describing watershed biogeochemical function. Meander-bound floodplains appear to serve as scaling motifs that predict aggregate capacities for biogeochemical transformations, providing a foundation for incorporating riparian soil microbiomes in watershed models. Widely represented genetic capacities did not predict in situ activity at one time point, but rather they define a reservoir of biogeochemical potential available as conditions change.AU1kaDmL5ojSw8dBEKS2i-Video abstract

Effects of long-term (42 years) tillage sequence on soil chemical characteristics in a dryland farming system

No-tillage can improve soil quality but can also increase the stratification of soil chemical parameters. Nutrient uptake by crops might be limited when nutrients are stratified, especially in semi-arid or Mediterranean regions. To reduce stratification, infrequent tillage could be considered. However, there is a paucity of information on the effects of long-term infrequent tillage on the stratification of soil chemical parameters. This study aimed to assess the effects of long-term infrequent tillage on the stratification of selected soil chemical parameters to a depth of 300 mm. The research was conducted on a long-term (42 years) research site at Langgewens Research Farm in South Africa. Seven tillage treatments were investigated: continuous mouldboard ploughing to a depth of 200 mm, tine-tillage to 150 mm, shallow tine-tillage to 75 mm, no-tillage, shallow tine-tillage every second year in rotation with no-tillage, shallow tine-tillage every third year in rotation with no-tillage and shallow tine-tillage every fourth year in rotation with no-tillage. Tillage treatments had differential effects on the distribution of soil chemical parameters. The mouldboard plough prevented stratification of most soil chemical parameters, such as soil acidity, soil organic carbon (SOC), extractable P, exchangeable Ca and Mg and cation exchange capacity (CEC). However, mouldboard ploughing also led to significantly lower SOC stocks and extractable P stocks. The SOC stocks and extractable P stocks of the no-tillage treatment were not significantly different from those of the infrequent tillage treatments. Overall, the infrequent tillage treatments were no better (P > 0.05) than the no-tillage treatment as infrequent tillage could not effectively ameliorate the stratification of most soil chemical parameters and did not increase the stocks and stratification ratios of SOC and extractable P.

Improving soil respiration while maintaining soil C stocks in sunken plastic greenhouse vegetable production systems – Advantages of straw application and drip fertigation

The absence of fertile surface soil combined with high temperature and moisture in sunken plastic greenhouse vegetable production (SGVP) leads to low soil organic carbon (SOC) storage. This limits the rapid growth of vegetable crops in SGVP as low SOC goes along with a low capacity of soils for water and nutrient retention. It also results in low soil respiration, which leads to indoor CO2 limitations during the winter crop growing season. In our experiments we explored if drip fertigation might lead to an improvement of soil respiratory activity, specifically when maize straw is incorporated additionally into the soil. To address these questions, a two-factor field experiment with four treatments was conducted across four growing seasons. The treatments are (1) Conventional flood irrigation with over fertilization (CIF), (2) CIF + maize straw (CIF + S), (3) Drip irrigation with reduced fertilization (DIF), (4) DIF + maize straw (DIF + S). Compared to CIF, DIF significantly increased water and nitrogen use efficiency by 93% and 296%, respectively. Moreover, cumulated CO2 emission was significantly lower than assimilated carbon in plant for both CIF and DIF without straw incorporation, but the opposite result was obtained when straw was added (CIF + S and DIF + S). DIF results in reduced rates of soil respiration, mainly due to significantly reduced mineralization of autochthonous SOC stocks as compared CIF. Tracing the δ13C signature of added straw into the SOC indicated that DIF leads to a larger incorporation of straw-derived C into soil than CIF. Our results show that adding straw is a suitable measure for both to meet the CO2 demand of vegetable photosynthesis and to maintain SOC stocks associated soil functions. However, the tested and widely recommended straw application rate of 3.5 t C ha−1 appears to be still too low to reverse the trend of a net mineralization of SOC stocks. Thus, our results call for targeted experiments to optimize the amount of straw addition and to explore in field experiments if the application of mixed straw and biochar produced from vegetable residues, are a suitable way forward to increase soil health of SGVP systems.

Tree diversity and carbon stocks in agroforestry systems in northern Ethiopia

BackgroundAgroforestry (AF) is an ancient tradition in Ethiopian dryland farming system. Several studies have examined system design, soil fertility management and system interactions, but the biodiversity and mitigation aspects of climate change have received less focus. We assessed the diversity of woody species, biomass carbon (C), and soil organic carbon (SOC) stock associated with indigenous dryland AF practices. A total of 197 smallholder farmers representing four AF practices (home garden, parkland, boundary plantation, and woodlot) from lowland, midland, and highland areas were systematically selected. The inventory of woody species was carried out on each farm's randomly formed plot.ResultsWe identified a total of 59 woody species representing 48 genera and 32 families. Shannon diversity index (H') was highest in home garden and parkland AF, while woodlots had the highest mean total stock of biomass C (31 Mg C ha−1). C stocks for smallholding systems (total biomass C and SOC 0–60 cm) ranged from 77–135 Mg ha−1. Total biomass C stocks were significantly correlated with abundance (Spearman r = 0.333; p = 0.002) but biomass components were not significantly correlated with H'. SOC soil depth stock (0–60 cm) was positively and significantly associated with H' (Spearman r = 0.291 & 0.351; p < 0.01).ConclusionsWe report greater species richness in home garden and parkland AF systems than in woodlots. While parkland AF produce lower biomass and SOC stock relative to other AF systems. The strategic use of home gardens and boundary planting can improve tree diversity and carbon storage in Ethiopian dryland ecosystems.

Aligning agri-environmental subsidies and environmental needs: a comparative analysis between the US and EU

The global recognition of modern agricultural practices’ impact on the environment has fuelled policy responses to ameliorate environmental degradation in agricultural landscapes. In the US and the EU, agri-environmental subsidies (AES) promote widespread adoption of sustainable practices by compensating farmers who voluntarily implement them on working farmland. Previous studies, however, have suggested limitations of their spatial targeting, with funds not allocated towards areas of the greatest environmental need. We analysed AES in the US and EU—specifically through the Environmental Quality Incentives Program (EQIP) and selected measures of the European Agricultural Fund for Rural Development (EAFRD)—to identify if AES are going where they are most needed to achieve environmental goals, using a set of environmental need indicators, socio-economic variables moderating allocation patterns, and contextual variables describing agricultural systems. Using linear mixed models and linear models we explored the associations among AES allocation and these predictors at different scales. We found that higher AES spending was associated with areas of low soil organic carbon and high greenhouse gas emissions both in the US and EU, and nitrogen surplus in the EU. More so than successes, however, clear mismatches of funding and environmental need emerged—AES allocation did not successfully target areas of highest water stress, biodiversity loss, soil erosion, and nutrient runoff. Socio-economic and agricultural context variables may explain some of these mismatches; we show that AES were allocated to areas with higher proportions of female producers in the EU but not in the US, where funds were directed towards areas with less tenant farmers. Moreover, we suggest that the potential for AES to remediate environmental issues may be curtailed by limited participation in intensive agricultural landscapes. These findings can help inform refinements to EQIP and EAFRD allocation mechanisms and identify opportunities for improving future targeting of AES spending.

Soil organic carbon estimation using VNIR–SWIR spectroscopy: The effect of multiple sensors and scanning conditions

Visible–near infrared–shortwave infrared (VNIR–SWIR) spectroscopy is being increasingly used for soil organic carbon (SOC) assessment. Common practice consists of scanning soil samples using a single spectrometer. Considerations have rarely been documented of the effects of using multiple instruments and scanning conditions on SOC model calibration that occur when merging soil spectral libraries (SSLs), particularly in soils with low SOC concentration and using both field spectroradiometers and laboratory fixed spectrometers. To address this gap, we scanned 143 low-SOC-content soil samples using three spectrometers (ASD FieldSpec 3, ASD FieldSpec 4, and FOSS XDS) and four setup features - FOSS, contact probe (CP), dark-box (DB), and open laboratory (LAB) - at three laboratories. The application of an internal soil standard (ISS) to align one laboratory spectrum with another for spectral correction and spectral merging of various SSLs was examined. SOC models were developed using i) data from each single spectrometer – single laboratory separately and ii) merged data from multiple spectrometers – different laboratories, applying the 1st derivatives of spectra and random forest (RF) regression. The results indicate that the spectral shape and wavelength position of key features obtained from all spectrometers and setups did not show any noticeable differences, though spectra based on FOSS setup, particularly on low-SOC samples, demonstrated greater range in absolute derivative values regardless of ISS application. The derivative ISS-corrected spectra showed less variation among different spectrometers compared to their uncorrected raw reflectance spectra. All single spectrometer models predicted SOC reasonably well. However, the spectra acquired by the FOSS setup predicted SOC more accurately (R2 = 0.77, RPIQ = 3.30, RMSE = 0.22 %, and SD = 0.04) than the spectra acquired by the other setups. The models derived from merged uncorrected raw reflectance spectra yielded poor results (R2 = 0.48, RPIQ = 2.33, RMSE = 0.33 %, and SD = 0.10); nevertheless, assessment of SOC using the 1st derivative ISS-corrected merged SSLs considerably improved the prediction accuracy (R2 = 0.70, RPIQ = 3.10, RMSE = 0.25 %, and SD = 0.06). Hence, the derivative spectra coupled with the ISS correction improved the accuracy of SOC prediction models obtained from the merged soil spectra collected in different environments using different instruments. We therefore recommend application of the ISS spectral alignment method linked to the 1st derivative approach to enhance the compilation of SSLs at the regional and global scales for SOC assessment.

Grazing exclosures increase soil organic carbon stock at a rate greater than “4 per 1000” per year across agricultural landscapes in Northern Ethiopia

The establishment of grazing exclosures is widely practiced to restore degraded agricultural lands and forests. Here, we evaluated the potential of grazing exclosures to contribute to the “4 per 1000” initiative by analyzing the changes in soil organic carbon (SOC) stocks and sequestration (SCS) rates after their establishment on degraded communal grazing lands in Tigray region of Ethiopia. We selected grazing areas that were excluded from grazing for 5 to 24 years across the three agroecological zones of the region and used adjacent open grazing lands (OGLs) as control. Soil samples were collected from two depths (0–15 cm and 15–30 cm) and SOC and aboveground C stocks were quantified in both exclosures and OGLs. The mean SOC stock and SCS rate in exclosures (0–30 cm) were 31 Mg C ha−1 and 3 Mg C ha−1 year−1, which were respectively 166% and 12% higher than that in the OGLs, indicating a positive restoration effect of exclosures on SOC storage. With increasing exclosure age, SOC stock and SCS rate increased in the exclosures but decreased in the OGLs. Higher SOC stock and SCS rate were recorded in 0–15 cm than in 15–30 cm. The relative (i.e., to the SOC stock in OGLs) rates of increase in SOC stocks (70–189‰ year−1) were higher than the 4‰ year−1 and were initially high due to low initial SOC stock but declined over time after a maximum value of SOC stock is reached. Factors such as aboveground biomass, altitude, clay content and precipitation promoted SOC storage in exclosures. Our study highlights the high potential of exclosures for restoring SOC in the 0–30 cm soil depth at a rate greater than the 4‰ value. We argue that practices such as grazing exclosure can be promoted to achieve the climate change mitigation target of the “4‰” initiative.

Microbial community shifts reflect losses of native soil carbon with pyrogenic and fresh organic matter additions and are greatest in low-carbon soils.

Soil organic carbon (SOC) plays an important role in regulating global climate change, carbon and nutrient cycling in soils, and soil moisture. Organic matter (OM) additions to soils can affect the rate at which SOC is mineralized by microbes, with potentially important effects on SOC stocks. Understanding how pyrogenic organic matter (PyOM) affects the cycling of native SOC (nSOC) and the soil microbes responsible for these effects is important for fire-affected ecosystems as well as for biochar-amended systems. We used an incubation trial with five different soils from National Ecological Observatory Network sites across the US and 13C-labelled 350°C corn stover PyOM and fresh corn stover OM to trace nSOC-derived CO2 emissions with and without PyOM and OM amendments. We used high-throughput sequencing of rRNA genes to characterize bacterial, archaeal, and fungal communities and their response to PyOM and OM in soils that were previously stored at -80°C. We found that the effects of amendments on nSOC-derived CO2 reflected the unamended soil C status, where relative increases in C mineralization were greatest in low-C soils. OM additions produced much greater effects on nSOC-CO2 emissions than PyOM additions. Furthermore, the magnitude of microbial community composition change mirrored the magnitude of increases in nSOC-CO2, indicating a specific subset of microbes were likely responsible for the observed changes in nSOC mineralization. However, PyOM responders differed across soils and did not necessarily reflect a common "charosphere". Overall, this study suggests that soils that already have low SOC may be particularly vulnerable to short-term increases in SOC loss with OM or PyOM additions.Importance Soil organic matter (SOM) has an important role in global climate change, carbon and nutrient cycling in soils, and soil moisture dynamics. Understanding the processes that affect SOM stocks is important for managing these functions. Recently, understanding how fire-affected organic matter (or "pyrogenic" organic matter (PyOM)) affects existing SOM stocks has become increasingly important, both due to changing fire regimes, and to interest in "biochar" - pyrogenic organic matter that is produced intentionally for carbon management or as an agricultural soil amendment. We found that soils with less SOM were more prone to increased losses with PyOM (and fresh organic matter) additions, and that soil microbial communities changed more in soils that also had greater SOM losses with PyOM additions. This suggests that soils that already have low SOM content may be particularly vulnerable to short-term increases in SOM loss, and that a subset of the soil microbial community is likely responsible for these effects.

An assessment of urban horticultural soil quality in the United Kingdom and its contribution to carbon storage.

As participation in urban horticulture grows, understanding the quality of urban horticultural soils is of increasing importance. Until now, case studies of individual cities or gardens have limited the potential of such studies to draw generalised conclusions. Here, we present the first national scale assessment of soil quality in allotments, a dominant form of urban horticulture in the United Kingdom. We sampled soils in 200 allotments in 10 urban areas across Great Britain. We assessed a range of soil quality indicators (carbon and nitrogen concentration, C:N ratio, bulk density, carbon density, pH) comparing them to the quality of soils in rural arable and horticultural land. We present the first estimate of nationwide carbon storage on allotments. We found that allotment gardeners consistently employ management practices conducive to high soil quality. Allotment soil quality differed significantly between soil types but in general soils were of a high quality: low bulk density (0.92gcm-3) and high soil organic carbon concentration and density (58.2mgg-1 and 58.1mgcm-3 respectively). Allotment soil organic carbon concentration was 250% higher than in the surrounding arable and horticultural land. Covering only 0.0006% of Great Britain, allotments contribute a disproportionate 0.05-0.14% of nationwide total organic carbon stocks. This national-scale study provides compelling evidence that small-scale urban horticultural production, unlike conventional horticulture, does not degrade soil quality. Indeed, allotments hold a small but previously unaccounted for carbon stock nationally. Urban horticultural land is a vital part of the urban landscape with effectively functioning soils that should be protected. As public demand for urban horticultural land rises and policy-makers from local to trans-national levels of governance advocate for urban food production, our findings demonstrate that urban horticulture can protect or enhance the ecosystem services provided by soils in cities and towns where the majority of people live.

Evaluation of a microplate spectrophotometer for soil organic carbon determination in south‐central Idaho

AbstractDetermination of soil organic carbon (SOC) is highly desirable for assessing fertility and carbon sequestration; however, numerous methods of determination warrant study of method agreement. Recently, a novel method was developed following dichromate oxidation using a microplate spectrophotometer. This novel method was compared with (a) total C by dry combustion − soil inorganic carbon (DCw/o pretreatment–pressure calcimater [Pcal]); (b) traditional Walkley–Black titration (WBTIT) and (c) loss on ignition (LOI360°C) in calcareous soils of south‐central Idaho (n = 75) in conjunction with North American Proficiency Testing program soils (n = 10). A two‐way ANOVA was fit with soils as a blocking factor to identify any difference between methods; means were separated using Tukey's honestly significant difference test (α = .05). Additional comparisons were made for all soils (n = 85) and for soils in the lower 75th percentile of SOC determined by WBTIT (n = 56) using regression analysis. Only the WBTIT and LOI360°C methods were statistically equivalent nevertheless there was high agreement (Lin's concordance coefficients &gt; .90) between all methods (n = 85). Under low SOC soils (n = 56) the agreement between all methods decreased, but the Walkley–Black spectrophotometric method (WBSPEC) method fit other methods comparatively well r2 = .71, .74, and .78 for LOI360°C, DCw/o pretreatment–Pcal, and WBTIT respectively. The WBSPEC method provided estimates of SOC between the methods currently used in the region while reducing hazardous waste generation over traditional WBTIT and sample handling over LOI360°C and DCw/o pretreatment–Pcal methods, positioning it as a sensible option for SOC determination in low SOC calcareous soils of south‐central Idaho.