Soil Organic Carbon Stocks In Soils Research Articles

Grazing in a megagrazer‐dominated savanna does not reduce soil carbon stocks, even at high intensities

Recent studies suggest that wild animals can promote ecosystem carbon sinks through their impacts on vegetation and soils. However, livestock studies show that intense levels of grazing reduce soil organic carbon (SOC), leading to concerns that rewilding with large grazers may compromise ecosystem carbon storage. Furthermore, wild grazers can both limit and promote woody plant recruitment and survival on savanna grasslands, with both positive and negative impacts on SOC, depending on the rainfall and soil texture contexts. We used grazing lawns in one of the few African protected savannas that are still dominated by megagrazers (> 1000 kg), namely white rhinoceros Ceratotherium simum, as a model to study the impact of prolonged and intense wild grazing on SOC stocks. We contrasted SOC stocks between patches of varying grazing intensity and woody plant encroachment in sites across different rhino habitat types. We found no differences in SOC stocks between the most‐ and least grazed plots in any of the habitats. Intermediately grazed plots, however, had higher SOC stocks in the top 5 cm compared to most and least grazed plots, but only in the closed‐canopy woodland habitat and not in the open habitats. Importantly, we found no evidence to support the hypothesis that wild grazing reduces SOC, even at high grazing intensities by the world's largest megagrazer. Compared to the non‐encroached reference plots, woody encroached plots had higher SOC stocks in soils with low clay content and lower SOC stocks in soils with high clay content, although only in the top 5 cm. Accordingly, our study highlights that wild grazers may influence SOC indirectly through their impact on tree‐grass ratios in grassy ecosystems. Our study thus provides important insights for future natural climate solutions that focus on wild grazer conservation and restoration.Keywords: fire, grazing impact, rewilding, soil carbon, white rhinoceros, woody encroachment

Soil carbon stock assessment using depth and spatial models on afforested arable lands

The change from grasslands and natural shrubs to afforested arable land has a major impact on soil organic carbon (SOC) stocks. Grasslands are known to be SOC sinks as seen in the Chernozems of North America, Eurasia and South Africa. However, determining the SOC stocks of soils can be financially costly as each location must be sampled in depth increments. This study aimed to estimate the SOC stocks for the Mvoti catchment (30° 48′ E and 29° 18′ S) in Kwa-Zulu Natal, South Africa by developing depth functions on a limited number of soil samples and expanding these functions to known land-uses and soil types. The results showed that splines captured the short-term vertical distribution of SOC better than exponential decay functions, which has major implications on arable lands. Long-term forest plantations showed a positive correlation with SOC stocks (32.7 kg m−2), while annual crop cultivation (27.0 kg m−2) showed a negative correlation when compared to natural grasslands (28.8 kg m−2). The Cubist algorithm predicted the total SOC stock of the catchment area at between 12 248 and 17 624 Mg depending on the depth function used. Soils with yellow-brown subsoils tend to have higher SOC stocks and the lowest degree of uncertainty.

Crop intensification effects on soil quality and organic carbon stocks: a case study of Haramosh Valley in Central Karakorum, Pakistan

ABSTRACT Agricultural operations such as excessive tillage and intense cropping deplete soil organic carbon (SOC), making sustainable agriculture management critical for reducing greenhouse gas (GHG) emissions. This study evaluates the impact of crop intensification on soil quality and soil organic carbon stocks (SOCS) under double cropping (DC) and single cropping pattern (SC) in upper Haramosh of Gilgit, Pakistan. Soil samples were taken from cropping zones (DC and SC) under three depths (0–20, 20–40, and 40–60 cm). Standard methods were used to analyze selected soil quality parameters and SOC. Statistical analysis using ANOVA showed that soil temperature, moisture, pH, SOC, and SOCS highly significantly differed (p < 0.001) for different cropping patterns (DC and SC), whereas bulk density (BD), electrical conductivity (EC), and clay were not significantly different. The SC retained 4.4% more moisture and had lower BD than the DC, while BD increased with increasing depth. The texture of the soil was sandy loam at both cropping zones. The mean SOC and SOCS of SC were greater (by 12%) than in the DC zone. Pearson correlation showed a significant and positive correlation of SOC stock with SOC, moisture (p < 0.01), and EC (p < 0.05), but had a negative correlation with bulk density, pH (p < 0.01), and sand (p < 0.05). DC apparently degraded soil quality and organic carbon reserves, thus reducing the soil health in mountain agriculture.

Organic carbon stocks, quality and prediction in permafrost-affected forest soils in North Canada

High-latitude soils store a large amount of the global soil organic carbon (SOC). The SOC stocks in mineral soils under different permafrost conditions, however, are underrepresented in global carbon databases. We sampled mineral forest soils under continuous and discontinuous to sporadic permafrost conditions on the Canadian Boreal and Taiga Plain. We determined the SOC stocks in the upper 60 cm of 94 soil pits across eleven sites (5–9 pits per site) and SOC quality using 13C isotopic signatures, C:N ratios and composition of aliphatic/aromatic and cellulose/lignin-like compounds obtained from mid-infrared spectra analyses. Lastly, we evaluated the prediction of SOC stocks in these soils using mid-infrared spectra and partial least square regression modelling (PLSR). The SOC stocks were on average four times higher in soils under continuous permafrost conditions (93.7–203.8 Mg SOC ha−1 in 0–45 cm) compared to soils under discontinuous to sporadic permafrost conditions (26.7–60.2 Mg SOC ha−1 in 0–60 cm). In addition, the SOC stocks were larger at moist and wet locations compared to dryer locations and varied significantly between sites, stressing the importance of small-scale geomorphic differences in controlling SOC in boreal mineral forest soils. Continuous permafrost SOC had a lower degree of decomposition compared to soils under discontinuous and sporadic permafrost. This indicates a potentially large proportion of SOC in boreal mineral soils to be vulnerable to warming associate increases in decomposition. The combination of mid-infrared with PLSR was suitable to predict the SOC stocks (R2 > 0.8) with an average uncertainty of 14–23%, which was less than the observed spatial variability of the field replicates (29–41%). Mid-infrared spectroscopy can thus offer an alternative to fill SOC data gaps of high latitude mineral forest soils and reduce uncertainties originating from the limited number of currently available SOC observations of Canadian boreal mineral forest soils.

Soil organic carbon stock in arid and semi-arid steppe rangelands of North Africa

The assessment of soil organic carbon stock (SOCS) in degraded and rehabilitated soils in steppelands and rangelands of North Africa is crucial for the adoption of adequate management strategies that respond to sustainable development. This study aimed to estimate the SOCS in the surface layer of soils rehabilitated by prickly pear plantations (PPP) and analyze the effects of edaphic factors controlling the variation of SOCS in old PPP (OPPP), young PPP (YPPP) and control plots (unplanted plots). Under arid and semi-arid climates, three steppe sites were sampled in which three plots with OPPP (age = 10–20 years), YPPP (age < 5 years) and controls plots were considered. In each plot, five quadrats were delimited and within each quadrat, two pedological profiles were dug to collect soil samples. The determined parameters were soil organic carbon (SOC), bulk density (BD), pH, electrical conductivity (EC) and total calcium carbonate. The averages of SOCS varied between 14.51 ± 11.03 t/ha (min–max: 0.09–35.88 t/ha) under arid climate and 17.32 ± 9.05 t/ha (range: 0.26–41.84 t/ha) under semi-arid climate. The variations of SOCS were significant (P = 0.002) between the type of steppe managements but not significant (P > 0.05) among climate zones. Generally, the control plots and OPPP stored more carbon (14–19 t/ha) than steppes with YPPP (11–14 t/ha). SOCS varied significantly with changes in soil depths, it reached 27 t/ha at 15 cm and dropped to 10–18 t/ha at 20 cm. Regardless of the type of climate and rangeland management, SOCS evolved with BD and soil organic matter. This study estimated for the first time the SOCS in soils of degraded steppe rangelands in North Africa. Even if the SOCS remains low degraded steppes rangelands, it is necessary to avoid the loss of SOC in these ecosystems. The adoption of conservation agricultural practices, e.g. organic matter addition and reduced tillage techniques can increase SOC levels in these ecosystems.

Land Use and Soil Organic Carbon Stocks—Change Detection over Time Using Digital Soil Assessment: A Case Study from Kamyaran Region, Iran (1988–2018)

Land use change and soil organic carbon stock (SOCS) depletion over time is one of the predominant worldwide environmental problems related to global warming and the need to secure food production for an increasing world population. In our research, satellite images from 1988 and 2018 were analyzed for a 177.48 km2 region in Kurdistan Province, Iran. Across the study area. 186 disturbed and undisturbed soil samples were collected at two depths (0–20 cm and 20–50 cm). Bulk density (BD), soil organic carbon (SOC), rock fragments (RockF) and SOCS were measured. Random forest was used to model the spatial variability of SOCS. Land use was mapped with supervised classification and maximum likelihood approaches. The Kappa index and overall accuracy of the supervised classification and maximum likelihood land use maps varied between 83% and 88% and 78% and 85%, respectively. The area of forest and high-quality rangeland covered 5286 ha in 1988 and decreased by almost 30% by 2018. Most of the decrease was due to the establishment of cropland and orchards, and due to overgrazing of high-quality rangeland. As expected, the results of the analysis of variance showed that mean values of SOCS for the high-quality rangeland and forest were significantly higher compared to other land use classes. Thus, transformation of land with natural vegetation like forest and high-quality rangeland led to a loss of 15,494 Mg C in the topsoil, 15,475 Mg C in the subsoil and 15,489 Mg C−1 in total. We concluded that the predominant causes of natural vegetation degradation in the study area were mostly due to the increasing need for food, anthropogenic activities such as cultivation and over grazing, lack of government landuse legislation and the results of this study are useful for land use monitoring, decision making, natural vegetation planning and other areas of research and development in Kurdistan province.

Soil organic carbon assessment under different land uses in Cauvery delta zone of Tamil Nadu, India

Soil organic carbon (SOC) plays a vital role in soil fertility and is important for its contributions to mitigation and adaptation to climate change. The present study was undertaken to estimate the SOC stock in soils under different land uses of Cauvery Delta zone of Tamil Nadu. Four different land uses were selected for the study viz, Forests, Agriculture, Agro-forestry and Plantations. Soil samples were collected from Madukkur and Kalathur soil series of Cauvery Delta zone for soil carbon analysis. The soil samples were fractionated into three aggregate size classes viz., macro-aggregates (250-2000µm), micro-aggregates (53-250 µm) and silt and clay sized fraction (&lt;53 µm). At 0-30 cm depth, the forest land use stored the maximum SOC stock in the different size fractions viz. macro-sized fraction (73.0 Mg ha-1), a micro-sized fraction (76.0 Mg ha-1) and silt+clay sized fraction (77.0 Mg ha-1) in Madukkur series. Agriculture land use registered the lowest SOC stock. Among the different size fractions, silt+clay sized fraction (&lt; 53 µm) retained the maximum SOC in all the land uses. In Kalathur series also, maximum soil organic carbon stock was recorded in forest land use. The data generated in the study will be beneficial to the user groups viz., farmers in identifying the most suitable land use for enhancing the storage of soil organic carbon thereby improving yields of crops and trees.

Comparing deep soil organic carbon stocks under kiwifruit and pasture land uses in New Zealand

Soil organic carbon (SOC) is important natural capital for agricultural production, as it affects soil physical, chemical and biological functions and the provision of ecosystem services. Measures of land-use effects on SOC stocks generally focus on the top 0.3 m of soil, as the topsoil has the highest SOC concentration. However, while subsoil horizons have low SOC concentrations, they contain a greater absolute amount of SOC with longer mean residence times than topsoil layers. Perennial horticultural crops offer potential to store SOC deep in the soil profile because of their long-lived and deep rooting systems. To investigate the hypothesis that kiwifruit (Actinidia chinensis Planch.) can increase subsoil SOC stocks, we sampled soils from 19 paired kiwifruit and pasture sites in New Zealand in 2018. Pasture was selected for comparison as it was the antecedent land use before establishment of the kiwifruit orchards. Paired land uses were located within 100 m of each other on the same soil type. Kiwifruit vines were at least 15 years old and the pasture was not cultivated during that time. Total SOC and nitrogen (N), and labile soil SOC stocks were assessed to a depth of 2 m. Kiwifruit production resulted in a modest increase in SOC and N stocks at a depth of 1.5–2.0 m (1.6 Mg C ha−1 and 0.52 Mg N ha−1), averaging to increases of 0.06 Mg C ha−1 y−1 and 0.02 Mg N ha−1 y−1. However, cumulative SOC and N stocks to 2-m depth were not different between land uses. The labile water extractable pools of SOC were lower under kiwifruit in the topsoil (0–0.1 m) and corresponded with lower N stocks and a higher soil C:N ratio at this depth. Further work on the dynamics of subsoil SOC pools is needed to understand the contribution of perennial horticulture crops to subsoil SOC storage.

Concept of soil organic carbon stock

Soil organic carbon (SOC) controls ecosystem and agro-ecosystem function, influencing soil fertility, water holding capacity and many other functions. The total amount of C stored in the surface soil is higher than sub surface soil area. It is estimated that the amount of C in the atmospheric pool is about 766 Pg C and about 566 Pg C in living vegetation. It is also of global importance because of its role in the global carbon cycle and therefore, the part it plays in the mitigation of atmospheric levels of greenhouse gases (GHGs). Different factors such as topography, climate, and soil physico-chemical properties also effect SOC stock in soil. Past long-term experimental studies have shown that soil organic C is highly sensitive to changes in land use, with changes from native ecosystems such as forest or grassland to agricultural systems almost always resulting in a loss of SOC. Land use change in different part of the world has also been observed to influence SOC stocks in different depth of the soil. Proper management of land use and land management practices and application of fertilizers, organic compost and manures could leads to greater C-storage in the soil, improves soil fertility and crop yield.

Concept of soil organic carbon stock

Soil organic carbon (SOC) controls ecosystem and agro-ecosystem function, influencing soil fertility, water holding capacity and many other functions. The total amount of C stored in the surface soil is higher than sub surface soil area. It is estimated that the amount of C in the atmospheric pool is about 766 Pg C and about 566 Pg C in living vegetation. It is also of global importance because of its role in the global carbon cycle and therefore, the part it plays in the mitigation of atmospheric levels of greenhouse gases (GHGs). Different factors such as topography, climate, and soil physico-chemical properties also effect SOC stock in soil. Past long-term experimental studies have shown that soil organic C is highly sensitive to changes in land use, with changes from native ecosystems such as forest or grassland to agricultural systems almost always resulting in a loss of SOC. Land use change in different part of the world has also been observed to influence SOC stocks in different depth of the soil. Proper management of land use and land management practices and application of fertilizers, organic compost and manures could leads to greater C-storage in the soil, improves soil fertility and crop yield.

Fertilization effects on soil ecology strongly depend on the genotype in a willow (Salix spp.) plantation

Willows (Salix spp.) grown in short rotation coppice (SRC) for biomass production offer diverse soil ecological benefits, however fertilization might interfere these impacts. To specify the significance of demand-based fertilization (NPK 21:3:10) on genotype-specific soil ecological impacts, the upper topsoil (0–10 cm soil depth) under six willow genotypes was tested. Activities of dehydrogenases as indicator of soil microbial activity, the concentration of glomalin-related proteins (GRSP), as an indicator of arbuscular mycorrhizal fungal biomass, soil organic carbon (SOC) stock, content of plant-available phosphorus (P), and the soil aggregate stability were investigated in a Vertic Cambisol in central Sweden after a growth period of 17 years in 3-year cutting cycles. Significantly higher microbial activity in the soil after fertilization was restricted to two genotypes (‘Tora’ and ‘Tordis’) out of six. Generally, the abundance of arbuscular mycorrhizal fungi indicated by GRSP was increased after fertilization, however significantly only under the genotypes ‘Jorr’ and ‘Loden’. Non-fertilized genotype ‘Jorr’ was most effective to increase the soil SOC stock in 0–10 cm soil depth from initially 14 Mg ha−1 in 2001 to 31 Mg ha−1 in 2018. Under the genotype ‘Tora’ the highest and under ‘Jorr’ the lowest P plant-availability was revealed in the fertilized treatment. Site-specifically the aggregate-stability of soil was generally high without any significant effects of willow genotype or fertilization. We conclude that, from a soil ecological perspective, the genotype ‘Jorr’ is most promising at non-fertilized sites whilst ‘Tora’ and ‘Tordis’ are most promising for fertilized sites. In that context, the genotype ‘Gudrun’ was the least promising, independent of fertilization. Our results indicate large genotype-specific effects that might exceed the impact of demand-based fertilization on the soil ecological value of willow genotypes grown in SRC, suggesting that a genotype-specific response on fertilization should be considered.

Assessing soil organic carbon stocks under land-use change scenarios using random forest models

Identifying changes in soil organic carbon stocks (SOCS) is essential for determining appropriate ways to deal with land degradation, for understanding soil and crop management and for gathering useful information for a range of environmental studies. The aim of this study was to predict SOCS and compare under current and potential future land uses. Soil organic carbon (SOC) and bulk density were measured at 137 locations across the Marivan, Kurdistan Province, Iran, and soil SOCS was computed. Auxiliary data including, terrain attributes and Landsat 8 ETM+ data were acquired. Random forest (RF) models were used to relate the SOCS to the auxiliary data. Results suggested that the mean SOCS in the topsoil and subsurface in croplands were lower than in forestland and wetland, although not significantly so. In the area, approximately 18.48% of forestland and 17.39% of wetland has been brought into cultivation. The authors estimate that this has led to a loss of SOCS from forestland topsoil of 22,860 Mg C, and from subsurface of 15,685 Mg C. The SOCS loss from wetland topsoil and subsurface were not as great, at 4193 and 2680 Mg C, respectively, but this was due to the area not being as large.

Soil aggregation and soil fraction associated carbon under different vegetation types in a complex landscape

Land cover change has been shown to affect soil characteristics and soil organic carbon (SOC) storage. However, little is known about the driving factors associated with soil carbon (C) stabilisation in complex landscapes. This study was conducted to investigate the effects of both inherent soil characteristics and recent vegetation cover change on soil aggregation and soil fraction associated C in a complex landscape. The specific objectives were: (1) determine bulk soil properties and SOC saturation deficit; (2) quantify soil aggregation, soil size-density fractions, and soil fraction associated C; and (3) identify the factors that influence soil fractions and soil fraction associated C in two adjacent catchments differing in vegetation cover, Central Otago, New Zealand. Catchment GH1 (n = 17 plots) was dominated by tussock grassland and native shrubs. Catchment GH2 (n = 21 plots) was converted from tussock grassland into a pine forest in 1981. The catchments differed in soil texture (e.g. sand content GH1: 62.9%, GH2: 50.7%, P = 0.007), soil SOC stocks (GH1: 5.0 kg C m−2, GH2: 4.3 kg C m−2, P = 0.04), mean weight diameter (MWD) (GH1: 782.3 µm, GH2: 736.5 µm, P = 0.002), and proportion of the macroaggregate heavy fraction (macro_HF) (GH1: 72%, GH2: 55%, P = 0.01). No significant differences were found in SOC saturation deficit (GH1: 14.8 mg g−1, GH2: 13.1 mg g−1, P = 0.13). Dominant vegetation cover explained 21% of the variation in MWD in GH1, whereas silt+clay C content explained 31.6% of the MWD variation in GH2. The macro_HF fraction was negatively correlated with the proportion of silt+clay. Our findings illustrate that physical and chemical soil characteristics are important drivers in such a complex landscape and may have masked the effect of recent vegetation change on soil aggregation, and soil fraction associated C.

Soil organic carbon stocks in topsoil and subsoil controlled by parent material, carbon input in the rhizosphere, and microbial-derived compounds

Despite a large body of studies investigating soil organic carbon (SOC) stocks and potential influencing factors, the impact of contrasting parent material, particularly in the subsoil, has received little attention. To reveal potential effects varying parent materials exert on SOC stocks, we investigated chemical (14C content and overall chemical composition via 13C NMR spectroscopy) and plant/microbial related parameters (root mass, amino sugars) of bulk soil and soil organic matter fractions from topsoil, subsoil, and rhizosphere soil at three European beech stands (Fagus sylvatica L.) only differing in parent material (Tertiary sand, Quaternary loess, and Tertiary basalt).The results suggest that the clay fraction, its amount being largely dependent on the respective parent material, took a central role in shaping differences in SOC stocks among the investigated sites by affecting soil organic matter stabilization via organo-mineral association and aggregation. This fraction was particularly relevant in the subsoil, where it accounted for up to 80% of the bulk soil SOC stocks that decreased with decreasing amounts of the clay fraction (basalt > loess > sand site). Determining the soil's nutrient composition, parent material likely also indirectly affected SOC stocks by changing rhizosphere traits (such as fine root density or mortality) and by attracting root growth (and thus organic matter inputs) to subsoil with higher nutrient contents, where in situ root inputs in the form of rhizodeposits were likely the prime source of plant-derived SOC. However, root inputs also contributed in large part to topsoil SOC stocks and were associated with higher abundance of microbial compounds (amino sugars), whose relative importance increased with increasing soil depth.Independent of soil depth and site, amino sugars and the amount of the clay fraction, combined with parameters related to the input of organic matter (root mass and amount of the particulate organic matter fraction) explained more than 90% of the variability in SOC stocks, indicating a key role of these measures in impacting SOC stocks. Because parent material directly or indirectly influenced these parameters, we demonstrate the necessity to consider differences in parent material when estimating and predicting SOC stocks.

The increase of soil organic carbon as proposed by the “4/1000 initiative” is strongly limited by the status of soil development - A case study along a substrate age gradient in Central Europe

During COP 21 in Paris 2015, several states and organizations agreed on the “4/1000” initiative for food security and climate. This initiative aims to increase world's soil organic carbon (SOC) stocks by 4‰ annually. The influence of soil development status on SOC dynamics is very important but usually not considered in studies. We analyse SOC accumulation under forest, grassland and cropping systems along a soil age gradient (10–17,000years) to show the influence of soil development status on SOC increase.SOC stocks (0–40cm) and accumulation rates along a chronosequence in alluvial soils of the Danube River in the Marchfeld (eastern Austria) were analysed. The analysed Fluvisols and Chernozems have been used as forest, grassland and cropland for decades or hundreds of years.The results showed that there is a fast build-up of OC stocks (0–40cm) in young soils with accumulation of ~1.3tha−1a−1 OC in the first 100years and ~0.5tha−1a−1 OC between 100 and 350years almost independent of land use. Chernozems with a sediment deposition age older than 5.000years have an accumulation rate<0.01tOCha−1a−1 (0–40cm).Radiocarbon dating showed that the topsoil (0–10cm) consists mainly of “>modern” and “modern” carbon indicating a fast carbon cycling. Carbon in subsoil is less exposed to decomposition and OC can be stored at long-time scales in the subsoil (14C age of 3670±35 BP).In view of the ‘4/1000’ initiative, soils with constant carbon input (forest & grassland) fulfil the intended 4‰ growth rate of SOC stocks only in the first 60years of soil development. We proclaim that under the present climate in Central Europe, the increase of SOC stocks in soil is strongly affected by the state of soil development.

Matching policy and science: Rationale for the ‘4 per 1000 - soils for food security and climate’ initiative

At the 21st session of the United Nations Framework Convention on Climate Change (UNFCCC, COP21), a voluntary action plan, the ‘4 per 1000 Initiative: Soils for Food Security and Climate’ was proposed under the Agenda for Action. The Initiative underlines the role of soil organic matter (SOM) in addressing the three-fold challenge of food and nutritional security, adaptation to climate change and mitigation of human-induced greenhouse gases (GHGs) emissions. It sets an ambitious aspirational target of a 4 per 1000 (i.e. 0.4%) rate of annual increase in global soil organic carbon (SOC) stocks, with a focus on agricultural lands where farmers would ensure the carbon stewardship of soils, like they manage day-to-day multipurpose production systems in a changing environment. In this paper, the opportunities and challenges for the 4 per 1000 initiative are discussed. We show that the 4 per 1000 target, calculated relative to global top soil SOC stocks, is consistent with literature estimates of the technical potential for SOC sequestration, though the achievable potential is likely to be substantially lower given socio-economic constraints. We calculate that land-based negative emissions from additional SOC sequestration could significantly contribute to reducing the anthropogenic CO2 equivalent emission gap identified from Nationally Determined Contributions pledged by countries to stabilize global warming levels below 2 °C or even 1.5 °C under the Paris agreement on climate. The 4 per 1000 target could be implemented by taking into account differentiated SOC stock baselines, reversing the current trend of huge soil CO2 losses, e.g. from agriculture encroaching peatland soils. We further discuss the potential benefits of SOC stewardship for both degraded and healthy soils along contrasting spatial scales (field, farm, landscape and country) and temporal (year to century) horizons. Last, we present some of the implications relative to non-CO2 GHGs emissions, water and nutrients use as well as co-benefits for crop yields and climate change adaptation. We underline the considerable challenges associated with the non-permanence of SOC stocks and show how the rates of adoption and the duration of improved soil management practices could alter the global impacts of practices under the 4 per 1000 initiative. We conclude that the 4 per 1000 initiative has potential to support multiple sustainable development goals (SDGs) of the 2030 Agenda. It can be regarded as no-regret since increasing SOC in agricultural soils will contribute to food security benefits that will enhance resilience to climate change. However, social, economic and environmental safeguards will be needed to ensure an equitable and sustainable implementation of the 4 per 1000 target.

Changes in quality and quantity of soil organic matter stocks resulting from wastewater irrigation in formerly forested land

Land use changes often diminish soil carbon stocks and alter soil organic matter (SOM) quality affecting soil productivity. Therefore, intensively managed agricultural systems are intended to maintain and even increase SOM stocks. Here we compared soil organic carbon (SOC) stocks and their quality in fields under natural vegetation (mesquite forest and xerophytic shrubs), converted first to rain fed maize cultivation and then to irrigated alfalfa-maize rotation systems in a semiarid region of Central Mexico. Fields under the alfalfa-maize rotation have been irrigated for different lengths of time (15, 35, 51, 85 and 100years) with untreated wastewater from Mexico City. We tested if the SOC stocks of the irrigated land use system were similar or even larger than those found under natural vegetation, and investigated if the quality of the SOM changed under the different covers. We collected composite soil surface samples and soil profile samples from fields under the different land uses and irrigation lengths and determined their organic carbon concentrations; we also quantified root residues and analyzed their bromatological composition. Mid infrared spectra of the soil samples were recorded and analyzed by multivariate procedures to determine changes in SOM quality.We found the smallest SOC stocks in soils under rain fed maize (mean: 45Mgha−1 in 30cm depth). In the alfalfa-maize rotation SOC stocks increased 1.5 fold during the first 40years under wastewater irrigation, thereafter reaching an apparent new steady state, with similar C stocks to soils under xerophytic shrub cover (65Mgha−1). SOM in rain fed soils was more hydrophobic and more metabolized by microorganisms, while the SOM in irrigated soils had a more hydrophilic character. Lignin in surface horizons of long term irrigated soils has a lesser degree of condensation than in deeper soil horizons or in soils irrigated for shorter periods of time, indicating a less intensive microbial degradation. We conclude that wastewater irrigation increases SOM stocks due to the greater lignin rich root biomass produced by alfalfa in this land use system, which helps to recover the SOC loss of 17–34% of the rain fed agriculture system.

Spatial 3D distribution of soil organic carbon under different land use types.

Soil organic carbon (SOC) has been assessed in three dimension (3D) in several studies, but little is known about the combined effects of land use and soil depth on SOC stocks in semi-arid areas. This paper investigates the 3D distribution of SOC to a depth of 1m in a 4600-ha area in southeastern Iran with different land uses under the irrigated farming (IF), dry farming (DF), orchards (Or), range plants on the Gachsaran formation (RaG), and range plants on a quaternary formation (RaQ). Predictions were made using the artificial neural networks (ANNs), regression trees (RTs), and spline functions with auxiliary covariates derived from a digital elevation model (DEM), the Landsat 8 imagery, and land use types. Correlation analysis showed that the main predictors for SOC in the topsoil were covariates derived from the imagery; however, for the lower depths, covariates derived from both the DEM and imagery were important. ANNs showed more efficiency than did RTs in predicting SOC. The results showed that 3D distribution of SOC was significantly affected by land use types. SOC stocks of soils under Or and IF were significantly higher than those under DF, RaG, and RaQ. The SOC below 30cm accounted for about 59% of the total soil stock. Results showed that depth functions combined with digital soil mapping techniques provide a promising approach to evaluate 3D SOC distribution under different land uses in semi-arid regions and could be used to assess changes in time to determine appropriate management strategies.

Soil carbon storage and stratification under different tillage/residue-management practices in double rice cropping system

The importance of soil organic carbon (SOC) sequestration in agricultural soils as climate-change-mitigating strategy has become an area of focus by the scientific community in relation to soil management. This study was conducted to determine the temporal effect of different tillage systems and residue management on distribution, storage and stratification of SOC, and the yield of rice under double rice (Oryza sativa L.) cropping system in the southern China. A tillage experiment was conducted in the southern China during 2005–2011, including plow tillage with residue removed (PT0), plow tillage with residue retention (PT), rotary tillage with residue retention (RT), and no-till with residue retention on the surface (NT). The soil samples were obtained at the harvesting of late rice in October of 2005, 2007 and 2011. Multiple-year residue return application significantly increased rice yields for the two rice-cropping systems; yields of early and late rice were higher under RT than those under other tillage systems in both years in 2011. Compared with PT0, SOC stocks were increased in soil under NT at 0–5, 5–10, 10–20, and 20–30 cm depths by 33.8, 4.1, 6.6, and 53.3%, respectively, in 2011. SOC stocks under RT were higher than these under other tillage treatments at 0–30 cm depth. SOC stocks in soil under PT were higher than those under PT0 in the 0–5 and 20–30 cm soil layers. Therefore, crop residues played an important role in SOC management, and improvement of soil quality. In the 0–20 cm layer, the stratification ratio (SR) of SOC followed the order NT>RT>PT>PT0; when the 0–30 cm layer was considered, NT also had the highest SR of SOC, but the SR of SOC under PT was higher than that under RT with a multiple-year tillage practice. Therefore, the notion that conservation tillage lead to higher SOC stocks and soil quality than plowed systems requires cautious scrutiny. Nevertheless, some benefits associated with RT system present a greater potential for its adoption in view of the multiple-year environmental sustainability under double rice cropping system in the southern China.

Spatially explicit simulation of soil organic carbon dynamics in China's paddy soils

article i nfo Paddy soils support important croplands in many parts of the world, especially in Asia, so an accurate estima- tion of the magnitude and dynamics of paddy soil organic carbon (SOC) stocks is critical to support adoption of appropriate soil conservation measures and development of strategies for greenhouse gas mitigation. To reduce uncertainties from upscaling spatial processes of the DeNitrification-DeComposition (DNDC) model for improving the definition of China's paddy soils SOC stocks (0-30 cm) and their changes from 1980 to 2008, a soil polygon-based modeling method was developed by integrating DNDC with the latest 1:1,000,000 soil database, derived from 1490 paddy soil profiles collected during the Second National Soil Survey of China. The simulations demonstrated that Chinese paddy soils (0-30 cm), which cover 45.0 M ha, storaged 2.51 Pg in 1980 and 2.65 Pg in 2008 with an average sequestration rate of 5.0 Tg C year �1 . Approximately 83.3% of the paddy soils sequestrated C, while 16.1% lost C and only 0.6% were balanced over the past 28 years. There were, however, large regional differences in sequestration out- comes. Increases in paddy soil SOC stocks were found predominantly in South, Eastern and Southwest China, where topsoil SOC increased by 59.6, 56.4 and 28.3 Tg, respectively. SOC stock in Northwest and North China increased only 0.6 and 0.3 Tg, respectively, while that in Northeast China lost 0.5 Tg. Enhancement in the soil databases with higher resolution inputs should improve the C sequestration estimates of any area of interest. © 2011 Elsevier B.V. All rights reserved.