Higher Soil Organic Carbon Research Articles

Soil structural attributes in relation to land use in Shiwaliks region of northwest India

Intensive cultivation coupled with monoculture has deteriorated the physical as well as chemical fertility of the soils mainly due to depreciation of soil carbon. A study was undertaken in Shiwaliks region of northwest India to assess soil structural attributes under tree-based land uses viz., maize-wheat, agrohorticulture and agro-forestry (3 and 6-year plantation). The composite samples were collected from each cropping system at depth intervals of 0–15, 15–30, 30–60, 60–90 and 90–120 cm. The soils under agroforestry had significantly higher soil organic carbon (SOC) than in soils other land uses, (2.5 mg g−1) in 6year and (2.1 mg g−1) in 3-year plantation. The mean bulk density was highest (1.57 Mg m−3) in soils under maize-wheat and lowest in agro-forestry 3-year soils (1.41 Mg m−3). The total porosity was lowest in maize-wheat (40.6%) soils as compared to other land uses. The mean weight diameter and water stable macro aggregates (>0.25 mm) were highest in soils under agro-forestry 6-year plantation than in soils under maize-wheat. On the other hand, water stable aggregates (< 0.25 mm) were lowest in soils under agro-forestry 6-year plantation than in soils under maize-wheat.

The effects of ecological construction and topography on soil organic carbon and total nitrogen in the Loess Plateau of China

The construction of terraces and vegetation restoration in the hillslope are major soil and water conservation measures on the Chinese Loess Plateau and contributed to the distribution patterns of soil organic carbon (SOC) and soil total nitrogen (STN). Topography influences soil erosion and changes SOC and STN contents. However, little information is available regarding the effects of ecological construction and topography on SOC and STN. A study was undertaken in the Loess Plateau, to evaluate the effects of land use conversion and topographic factors on the topsoil SOC and STN content at three hillslope positions (upper, middle, and foot slopes) under four land uses types: artificial forest, grassland, terraced fields, and sloping cropland. The results showed that land use conversion from sloping cropland to artificial forest and grassland improved the SOC and STN content. Slope position was an important topographic factor governing the SOC and STN distribution at the slope scale in artificial forest, grassland, and sloping cropland, with the foot slope having the highest SOC and STN content, followed by the upper slope, while the middle slope had the lowest values. SOC and STN showed positive correlation with Caesium-137 (137Cs) content. Land use types, slope position, and soil erosion had significant relationships with SON and STN. The results suggested that vegetation restoration of sloping cropland will contribute to soil carbon (C) and nitrogen (N) sequestration in the loess hilly region. The quantitative estimation of land use change and topography effects on SOC and STN could improve the accuracy of SOC and STN predictions in the region with a complex topography.

Roles of soil organic carbon and iron oxides on aggregate formation and stability in two paddy soils

Knowledges of aggregate stability and formation depending on soil organic carbon (SOC) and iron oxides are essential to the conservation of soil structure. In this work, the effects of SOC and different forms of iron oxides on the stability of aggregates of paddy soils were determined by applying the Le Bissonnais (LB) and extractions method by water, oxalate (OA), dithionite–citrate–bicarbonate (DCB), or hydrogen peroxide (H2O2). Two topsoils (0–20 cm) were selected based on two levels of SOC and iron oxides, high SOC (22.1 g kg−1)–low iron oxides (S1) and low SOC (8.50 g kg−1)–high iron oxides (S2) paddy soils from subtropical China. For the fast-wetting (FW) test, the percentage of 2–0.25 mm (34.0%) and >2 mm (42.2%) aggregate fractions were highest in S1 and S2, respectively. Both the soils had a >2 mm fraction that was above 60% under the slow-wetting (SW) and stirring after prewetting (WS) test. The free Fe oxide (Fe(DCB)) could increase in aggregate stability when SOC contents were low in S2. All three chemical extractions considerably decreased the >2 mm fractions (35.5–87.5%), and consequently increased the <0.053 mm fractions. The S1 soil presented the lower mean weight diameter (MWD) after DCB extractions, whereas it was no significantly different after OA or DCB extraction in S2 soil, indicating that Fe(DCB) had a greater effect on the stability of aggregates than amorphous Fe oxide (Fe(OA)) in the S1 soil, but both had the same effect for S2. The H2O2 treatment destructed>2 mm and 2–0.25 mm aggregates intensively, indicating that SOC was the major binding agent for aggregates of these sizes. The SOC was a key agent influencing soil aggregation not only in macro-aggregates (>0.25 mm) but also in micro-aggregates (0.25–0.053 mm) in the S1. The formation of aggregates in S1 was attributed to its higher content of organo-mineral complexes because the contents of SOC (3.30–11.2 g kg−1) were higher in >0.053 mm aggregates compared with S2 after H2O2 extractions. Fe oxides were the key agents for <0.25 mm aggregates formation. The MWD of the FW treatment was found to have a significant positive correlations with Fe(DCB), and MWD of the WS treatment proved significant positive correlations with SOC and Fe(OA) oxide, indicated that SOC and Fe oxides improved aggregate stability against external force collapse. This implies that the differential roles of SOC and Fe oxides in mechanisms of aggregate formation and stability in two paddy soils.

Desertification control on soil inorganic and organic carbon accumulation in the topsoil of desert grassland in Ningxia, northwest China

Soil carbon is an essential part of the terrestrial carbon pool, which includes soil organic carbon (SOC) and soil inorganic carbon (SIC). Soils contain a huge amount of inorganic carbon in arid and semiarid regions, which may be a potential source of CO2 emitted to the atmosphere. Neglecting soil inorganic carbon could result in erroneous or misleading interpretations of the soil carbon pool of arid and semiarid ecosystems. Nevertheless, only a few studies have focused on variations of SIC, or have further used the soil stable carbon isotope to explore the inherent mechanism of SIC variations in semiarid regions. The study was designed to estimate the dynamic of SOC and SIC with the development of desertification in the topsoil. Here, four desertification stages were chosen, including potential desertification (PD), light desertification (LD), severe desertification (SD), and very severe desertification (VSD) stages, to evaluate the horizontal and vertical distributions of SOC and SIC, and the stable carbon isotope (δ13C) of SOC and SIC with the desertification development in the topsoil (0–30 cm soil layer). The SOC and SIC contents and storage changed significantly with desertification development. The PD stages had the highest SOC (21.01 mg/hm2) and SIC (11.60 mg/hm2) storage at 0–30 cm, while the contrary result for VSD stages, with the average of SOC and SIC, was 8.27 mg/hm2 and 6.15 mg/hm2, respectively. The vertical distribution of SIC and SOC has varied sharply with soil depth. The δ13C-SIC within the 0–30 cm soil profile in the PD stages were more negative than those in other desertification stages and gradually increased with desertification development. In contrast, the δ13C-SOC generally decreased with desertification progress. The entire data (108 soil samples) exhibited that δ13C-SIC were significantly and negatively correlated with δ13C-SOC and SIC content, while SIC storage was significantly and positively correlated with SOC storage. The results demonstrated that an increase of SOC might be related to enhancing accumulation of SIC. The results of this study provide information about the variation of SIC and SOC with the desertification development and contribute to our understanding of the important role of SIC in estimating the carbon sequestration capacity in the degraded grassland ecosystems in arid and semiarid regions.

Soil Organic Carbon and Isotope Composition Response to Topography and Erosion in Iowa

AbstractSoil redistribution (erosion and deposition) can greatly affect the fate of soil organic carbon (SOC) in agroecosystems. Landscape topography is one of the key factors controlling erosion processes and creating spatial variability in SOC. We combined carbon (C) isoscape (isotopic landscape) analysis, historic orthophoto interpretation, cesium (137Cs) inventory measurement, and digital terrain analysis to quantify SOC dynamics and soil redistribution relationship and their responses to landscape topography in an Iowa cropland field with soybean/maize (C3/C4) rotation. The historic orthophotos and 137Cs were used to reflect soil redistribution before and after the 1960s, respectively. Topography‐based models were developed to simulate 137Cs inventory, SOC density, and C isotopes using stepwise principal component regression. Spatial patterns of SOC were similar to soil erosion/deposition patterns with high SOC density in depositional areas and low SOC density in eroded areas. Soil redistribution, SOC density, and isotopic signature of SOC (δ13C) were highly correlated with topographic metrics, suggesting that topographic heterogeneity drove the spatial variability in erosion and SOC dynamics. Considering the isotopic composition of SOC, C3‐derived SOC density was strongly controlled by topographic metrics, but C4‐derived SOC density showed weaker expression of spatial pattern and poor correlation to topographic parameters. The resulting topography‐based stepwise principal component regression models captured more than 60% of the variability in SOC density, δ13C, and C3‐derived SOC density but could not reliably predict C4‐derived SOC density. Our results indicate that exploring C isotopes in response to soil erosion is important to understand the fate of eroded SOC within croplands under C3/C4 cultivation.

Changes in physico-chemical and biological properties of soil under conservation agriculture based pearl millet – mustard cropping system in rainfed semi-arid region

ABSTRACTPoor soil health and low soil water content during crop growing period are major factor for low productivity of pearl millet – mustard rotation under rainfed semi-arid regions. The authors evaluated five different tillage and residue management practices for improving physico–chemical and biological properties of soil. Results showed that conservation agriculture (CA) practice (zero tillage (ZT) with 4 t ha–1 residue retention) exhibited higher proportion of soil macro-aggregate. It also increased infiltration rate of about 15.2% over conventional tillage without residue but ZT increased soil penetration resistance in surface soil layer. In the residue applied plots, ~2–4% (w/w) higher soil water content was maintained throughout the season than the no-residue plots. CA practice had the highest soil organic carbon (4.96 g kg–1) and microbial biomass carbon (188.3 μg g–1 soil). Significant and positive correlation was also found between soil organic carbon with infiltration rate (r = 0.73**), mean weight diameter (r = 0.80**) and microbial biomass carbon (r = 0.86**). Thus, this study suggests that ZT with residue retention can be advocated in pearl millet – mustard rotations for improving, productivity, soil health and maintaining higher soil water content in rainfed semi-arid regions.

Smallholder preference and agroecosystem service trade-offs: A case study in Xinzheng County, China

The provision of agroecosystem services (ESs) is directly influenced by the production goals and activities preferred by smallholders. However, the relationship between smallholder preferences and ES provision has yet to be established. This study surveyed the preferences of smallholders for agricultural production goals and managements in the North China Plain, China. The DeNitrification–DeComposition model was utilised to derive ES indicators under annual winter wheat–maize, winter wheat–peanut and biennial mixed rotation (i.e. winter wheat–maize and winter wheat–peanut rotation) systems. The combination of socio-economic and bio-physical modelling data was used to establish a quantitative link between ES compositions and smallholder preferences. Results show that smallholder preferences follow the order of traditional cropping system > low labour input > disaster alleviation > high income generation > food provision > soil fertilisation > landscape beautification > crop diversity. Preferences for high income generation, low labour input, disaster alleviation and crop diversity remarkably affect the cropping system choices of smallholders and the composition of ES provision. The preference for disaster alleviation is associated with high agricultural income and soil organic carbon (SOC), whereas that for low labour input is associated with low agricultural income and SOC accumulation. The ES trade-offs occurring in the switch from the preferences for high income generation, disaster alleviation and crop diversity to that for low labour input decrease the agricultural income and SOC sequestration. The analysis of the effects of smallholder preferences on trade-offs in ESs should consider soil types because agroecosystems with loamy and sandy soil types exhibit different compositions of ES provision under the same preference.

Yield trends and changes in soil fertility in a long term pearl millet (Pennisetum glaucum)-wheat (Triticum aestivum) cropping system managed with organic, inorganic and integrated sources of nutrients

The present study was carried out to evaluate the yield trends and soil fertility under long term field experiment at soil research farm of CCS Haryana Agricultural University, Hisar, India. Results showed that average grain yield of wheat (Triticum aestivum L.) and pearl millet (Pennisetum glaucum (L.) R. Br) across the experimentation period was higher under integrated application of organic manures along with 150 kg N+30 kg P2O5/ha through chemical fertilizers as compared to application of recommended dose of nutrients through chemical fertilizers alone (150 kg N+60 kg P2O5/ha). Lowest grain yield of both crops was observed when organic manures applied alone. Positive yield trend of both the crops was observed under all the nutrient management practices. The highest soil organic carbon (SOC) (1.14%) was recorded with the application of farmyard manure (FYM) followed by pressmud and poultry manure. Build-up of organic carbon ranged from 0.016 to 0.037%/year with application of FYM, poultry manure, pressmud as a component of integrated nutrient management. Highest amount of humic acid-C (HA-C) and fulvicacid-C (FA-C) (0.40 and 0.21%, respectively) was observed with application of 7.5 Mg pressmud along with 150 kg N+30 kg P2O5 /ha. Integrated nutrient management (INM) showed superiority on better grain yield of pearl millet and wheat in these alluvial soils of Haryana.

Contrasting dynamics of polychlorinated biphenyl dissipation and fungal community composition in low and high organic carbon soils with biochar amendment.

Biochar amendment has been advocated as an effective method to remediate organic pollutant-contaminated soils through adsorption and stimulating microbial degradation. However, such effects can vary depending on soil properties and biochar physiochemical characteristics. The objective of this study was to compare the dynamic variations of polychlorinated biphenyls (PCBs) in low and high soil organic carbon (SOC) soils both amended with biochar and to investigate its linkage with fungal community composition. Two soils having the same texture and soil type but varying in SOC contents were contaminated with PCBs to a final concentration of 60mgkg-1 and amended with 2% bamboo biochar. Temporal changes of PCB remaining in soils and adsorbed on biochar particles were determined during a 2-month incubation. Diversity and composition of fungal communities in both low and high SOC soils were investigated with Illumina MiSeq sequencing. The results showed that the PCB concentrations in low SOC soil were significantly lower than those in high SOC soil during the incubation. In the low SOC soil, the biochar particle adsorbed higher amounts of PCB, tetra-, and penta-chlorobiphenyls (CBs) than those in high SOC soil, and stimulated the dissipation of di- and tri-CBs. The nonmetric multidimensional scaling profile showed significant (p < 0.05) differences in the fungal community composition between the low and high SOC soils. The relative abundances of Eurotiomycetes were gradually increased, whereas those of Sordariomycetes and Dothideomycetes were decreased with increasing incubation time in the low SOC soils. In contrast, the fungal communities in high SOC soils were relatively stable. The relative abundances of Eurotiomycetes and Sordariomycetes were positively correlated with PCB dissipation in low and high SOC content soils, respectively. Our results suggest that SOC content affects PCB dissipation and fungal community composition in biochar-amended soils, and biochars have a high remediation potential of PCB in soils with low SOC contents.

Bio-intensive Modulation of Sugarcane Ratoon Rhizosphere for Enhanced Soil Health and Sugarcane Productivity Under Tropical Indian Condition

Intensive cropping and exhaustive nature sugarcane crop in tropical India have led to depletion of inherent soil fertility resulting in serious threat to sustainable sugarcane production. Improving soil organic matter and soil fertility are important factors for sustainable sugarcane production. Microbial consortia comprising of Trichoderma viride, Humicola spp, Paecilomyces lilacinus, Gluconacetobater diazotropicus, Azospiriillum brasilense, and Bacillus subtilis have great potential to recycle crop residue and restore soil fertility which eventually promote sugarcane growth. Field experiments with in situ trash management in plant crop and bio-intensive modulation of sugarcane ratoon rhizosphere in ratoon crops were conducted during 2014, 2015 and 2016 at ICAR—Sugarcane Breeding Institute, Coimbatore, Tamil Nadu, India. The results revealed that green manuring with sunnhemp, in situ trash management and application of 280:62.5:120 kg NPK ha−1 in plant crop sustained the soil health by way of reducing soil bulk density (1.26 kg dm−3), lower soil penetration resistance in the ratoon rhizosphere (1.79, 1.82 and 1.75 MPa) and higher soil organic carbon (0.52%) which in turn increased ratoon cane yield 12.24% (99.02 t ha−1) and sugar yield by 8.64% (12.83 t ha−1) over the control. Further, bio-intensive modulation of ratoon rhizosphere by off barring, trash shredding plus soil incorporation with microbial consortia and application of 350:62.5:120 kg NPK ha−1 in ratoon crop recorded significantly higher cane yield (100.95 t ha−1) and sugar yield (13.19 t ha−1) over the control. The improvement in cane yield and sugar yield to the tune of 16.35 and 14.10% due to bio-intensive modulation of ratoon rhizosphere over control have clearly established that productivity of sugarcane ratoon can be significantly improved with balanced use of fertilizers, green manuring, crop residue recycling and microbial consortia application. Similarly, bio-intensive modulation of sugarcane ratoon rhizosphere (S3) with lowest soil bulk density (1.26 kg dm−3) and soil penetration resistance values (1.69, 1.81, and 1.75 MPa) was found effective in minimising the soil compaction and improving nutrient availability (NPK) with build-up of soil microbial population than control (Trash removal). Based on the results of 3-year field experiments, it is concluded that sunnhemp green manuring and in situ sugarcane trash management coupled with application of 280:62.5:120 kg NPK ha−1 in plant crop followed by bio-intensive modulation of ratoon rhizosphere including off barring, trash shredding and soil incorporation with microbial consortia and application of 350:62.5:120 kg NPK ha−1 in ratoon crop can be recommended for sustaining soil health and sugarcane productivity under wide-row sugarcane planting systems of tropical Indian condition.

Spatial distribution of soil organic carbon and total nitrogen stocks in a karst polje located in Bosnia and Herzegovina

Karst poljes in the Dinaric Mountains have a complex hydrological regime and high potential for crop production. Little information is available about soil organic carbon (SOC), total nitrogen (TN), carbon stocks (SOCS), and nitrogen stocks (TNS) in karst poljes located in the Dinaric area. The objective of this paper was to study the spatial distribution of SOC and TN in topsoil (ranged from 9 to 53 cm depth) and whole profile SOCS and TNS (ranged from 15 to 160 cm depth) in the Livno karst polje depression (Bosnia and Herzegovina) using kriging and co-kriging approaches. We used the following properties as co-variates: distance from hills (DFH), distance from the lake (DFL), sand, silt, and clay content, TN, SOC, SOCS, and TNS. We only used the properties that had a significant correlation with the estimated properties as co-variates. The results showed that soils in the study area had high average SOC (7.92%), TN (0.79%), SOCS (191.05 t ha−1), and TNS (17.91 t ha−1) values. Histosols had the highest SOCS and TNS and Arenosols the lowest. The experimental variogram of LogSOC and LogTN was best-fitted by the spherical model, while the exponential model was the most accurate for LogSOCS and LogTNS. The spatial dependence was moderate for all studied soil properties. The incorporation of auxiliary variables increased the precision of the estimations from 35.7% (SOC × TN) to 49.2% (TNS × SOCS).

Depth dependence of soil carbon temperature sensitivity across Tibetan permafrost regions

Permafrost regions with high soil organic carbon (SOC) storage are extremely vulnerable to global warming. However, our understanding of the temperature sensitivity of SOC decomposition in permafrost regions remains limited, leading to considerable uncertainties in predicting carbon-climate feedback magnitude and direction in these regions. Here, we investigate general patterns and underlying mechanisms of SOC decomposition rate and its temperature sensitivity (Q10) at different soil depths across Tibetan permafrost regions. Soils were collected at two depths (0–10 and 20–30 cm) from 91 sites across Tibetan permafrost regions. SOC decomposition rate and Q10 value were estimated using a continuous-flow incubation system. We found that the SOC decomposition rate in the upper layer (0–10 cm) was significantly greater than that in the lower layer (20–30 cm). The SOC content governed spatial variations in decomposition rates in both soil layers. However, the Q10 value in the upper layer was significantly lower than that in the lower layer. Soil pH and SOC decomposability had the greatest predictive power for spatial variations in Q10 value within the upper and lower layers, respectively. Owing to the greater temperature sensitivity in the lower layer, our results imply that subsurface soil carbon is at high risk of loss, and that soil carbon sequestration potential might decrease in these regions in a warming world.

Effect of multipurpose trees and shrubs on degraded mined‐out soil in a semi‐deciduous forest zone of West Africa

AbstractMultipurpose trees and shrubs (MPTS) establishment plays important role in accelerating natural restoration and productivity of degraded soils for ecosystem stability, biological diversity, and most importantly for agricultural uses. Multipurpose tree plantation with a mixed stands of Leucaena leucocephala, Albizia zygia, Faidherbia albida, Acacia auriculiformis, Senna siamea, Azadiractha indica, and Gliricidia sepium was established to reclaim degraded mined‐out Ferric acrisol in a semi‐deciduous forest zone of Ghana, West Africa. The main aim of the study was to evaluate the effects of the age of MPTS on some physical and chemical properties of degraded mined‐out soil of the Oweri Mines Company, Ltd., Konongo (6°37′56.8″N, 1°13′39.02″W). The study determined soil total nitrogen (tN), soil organic carbon (SOC), acidity (pH), aluminum (Al), arsenic (As), and mercuric (Hg) contents and the relationship between these soil parameters. Soils were sampled at two depths: 0–30 and 30–60 cm from four different age groups of MPTS plantation: non‐reclaimed (NR), 5‐year‐old MPTS plantation, 10‐year‐old MPTS plantation, and adjacent undisturbed natural forest (NF). The pH, SOC, tN, Al, As, and Hg contents and bulk density were significantly influenced by age of MPTS plantation. The 10‐year‐old MPTS soil had 53%, and the NF soil had 57%, higher SOC than NR soil. Soil tN in the top 0–30 cm depth for the 5‐year‐old was 33%, 10‐year‐old MPTS was 59%, and the NF was 69%, higher than tN in the NR soil. Whereas soil pH had positive significant relationship with soil Hg and As, the reverse was observed with soil Al content.

Simulation of Soil Organic Carbon Effects on Long-Term Winter Wheat (Triticum aestivum) Production Under Varying Fertilizer Inputs.

Soil organic carbon (SOC) has a vital role to enhance agricultural productivity and for mitigation of climate change. To quantify SOC effects on productivity, process models serve as a robust tool to keep track of multiple plant and soil factors and their interactions affecting SOC dynamics. We used soil-plant-atmospheric model viz. DAISY, to assess effects of SOC on nitrogen (N) supply and plant available water (PAW) under varying N fertilizer rates in winter wheat (Triticum aestivum) in Denmark. The study objective was assessment of SOC effects on winter wheat grain and aboveground biomass accumulation at three SOC levels (low: 0.7% SOC; reference: 1.3% SOC; and high: 2% SOC) with five nitrogen rates (0–200 kg N ha-1) and PAW at low, reference, and high SOC levels. The three SOC levels had significant effects on grain yields and aboveground biomass accumulation at only 0–100 kg N ha-1 and the SOC effects decreased with increasing N rates until no effects at 150–200 kg N ha-1. PAW had significant positive correlation with SOC content, with high SOC retaining higher PAW compared to low and reference SOC. The mean PAW and SOC correlation was given by PAW% = 1.0073 × SOC% + 15.641. For the 0.7–2% SOC range, the PAW increase was small with no significant effects on grain yields and aboveground biomass accumulation. The higher winter wheat grain and aboveground biomass was attributed to higher N supply in N deficient wheat production system. Our study suggested that building SOC enhances agronomic productivity at only 0–100 kg N ha-1. Maintenance of SOC stock will require regular replenishment of SOC, to compensate for the mineralization process degrading SOC over time. Hence, management can maximize realization of SOC benefits by building up SOC and maintaining N rates in the range 0–100 kg N ha-1, to reduce the off-farm N losses depending on the environmental zones, land use and the production system.

Comparing soil inventory with modelling: Carbon balance in central European forest soils varies among forest types

Forest soils represent a large carbon pool and already small changes in this pool may have an important effect on the global carbon cycle. To predict the future development of the soil organic carbon (SOC) pool, well-validated models are needed. We applied the litter and soil carbon model Yasso15 to 1838 plots of the German national forest soil inventory (NFSI) for the period between 1985 and 2014 to enables a direct comparison to the NFSI measurements. In addition, to provide data for the German Greenhouse Gas Inventory, we simulated the development of SOC with Yasso15 applying a climate projection based on the RCP8.5 scenario. The initial model-calculated SOC stocks were adjusted to the measured ones in the NFSI.On average, there were no significant differences between the simulated SOC changes (0.25 ± 0.10 Mg C ha−1 a−1) and the NFSI data (0.39 ± 0.11 Mg C ha−1 a−1). Comparing regional soil-unit-specific aggregates of the SOC changes, the correlation between both methods was significant (r2 = 0.49) although the NFSI values had a wider range and more negative values. In the majority of forest types, representing 75% of plots, both methods produced similar estimates of the SOC balance. Opposite trends were found in mountainous coniferous forests on acidic soils. These soils had lost carbon according to the NFSI (−0.89 ± 0.30 Mg C ha−1 a−1) whereas they had gained it according to Yasso15 (0.21 ± 0.10 Mg C ha−1 a−1). In oligotrophic pine forests, the NFSI indicated high SOC gains (1.36 ± 0.17 Mg C ha−1 a−1) and Yasso15 much smaller (0.29 ± 0.10 Mg C ha−1 a−1).According to our results, German forest soils are a large carbon sink. The application of the Yasso15 model supports the results of the NFSI. The sink strength differs between forest types possibly because of differences in organic matter stabilisation.

Soil organic carbon stocks controlled by lithology and soil depth in a Peruvian alpine grassland of the Andes

The soil is the largest carbon (C) pool in the terrestrial ecosystem, and soil organic carbon (SOC) stocks play an important role in global C dynamics. Alpine grasslands of the Andes are characterized by high SOC stocks. Quantifying SOC stocks and unraveling key factors controlling SOC stocks, is necessary to obtain a better understanding of the dynamics of the large C stocks in this environment. However, most studies on C dynamics of the Andes focus on volcanic-ash soils, whereas information about non-volcanic ash soils in this region is scarce. Our objectives were: (i) to estimate SOC stocks in an alpine grassland of the Peruvian Andes (7° 11′S, 78° 35′W) with parent materials other than volcanic ash, and (ii) to identify the underlying soil formation and environmental (SFE) factors and soil properties explaining observed patterns of SOC stocks. We sampled 69 plots up to the parent material to measure soil properties and to calculate SOC stocks, in relation to lithology, land use, grazing intensity, slope angle, slope position and altitude. We applied linear models to identify key factors controlling SOC stocks. Our results showed that total SOC stocks had a mean value of 215 ± 21 T ha−1, whereas SOC stocks of the upper 10 cm and 40 cm comprised 29.3% and 80.0% of total SOC stocks respectively. The variation of the total SOC stocks was mainly explained by soil depth and soil moisture. When soil depth and soil moisture were controlled as conditional variables, lithology became the key factor controlling the total SOC stocks. For the SOC stocks of the upper 10 cm, soil moisture explained a large part of the variation, whereas lithology, grazing intensity and altitude were also significant predictors. Our results also show that when soils are sampled with limited depths instead of the entire soil profile, SOC stocks can be underestimated, and the effects of the SFE factors on SOC stocks can be overestimated.

Quantifying soil organic carbon stocks in herbaceous biomass crops grown in Ontario, Canada

Nineteen farms growing herbaceous biomass crops, switchgrass (Panicum virgatum) and miscanthus (Miscanthus spp.), were sampled for soil organic carbon (SOC) across Ontario, Canada in 2016. Switchgrass and miscanthus fields were sampled in addition to nearby agricultural fields and woodlots to compare SOC in herbaceous biomass systems relative to alternative land-uses. The mean SOC concentration of the woodlots was 4.26 ± 0.29% and was significantly higher (p 0.05) when compared with the SOC stocks quantified for the woodlot. The study recorded numerically higher SOC concentrations and stocks in biomass fields compared to agricultural fields. Therefore, biomass systems contribute to higher SOC sequestration. However, challenges associated with this study such as accurate bulk density measures and lack of baseline data need to be resolved in order to improve quantification of SOC sequestration.

Potential of Wheat Straw, Spruce Sawdust, and Lignin as High Organic Carbon Soil Amendments to Improve Agricultural Nitrogen Retention Capacity: An Incubation Study.

Plants like winter wheat are known for their insufficient N uptake between sowing and the following growing season. Especially after N-rich crops like oilseed rape or field bean, nitrogen retention of the available soil N can be poor, and the risk of contamination of the hydrosphere with nitrate (NO3-) and the atmosphere with nitrous oxide (N2O) is high. Therefore, novel strategies are needed to preserve these unused N resources for subsequent agricultural production. High organic carbon soil amendments (HCA) like wheat straw promote microbial N immobilization by stimulating microbes to take up N from soil. In order to test the suitability of different HCA for immobilization of excess N, we conducted a laboratory incubation experiment with soil columns, each containing 8 kg of sandy loam of an agricultural Ap horizon. We created a scenario with high soil mineral N content by adding 150 kg NH4+-N ha-1 to soil that received either wheat straw, spruce sawdust or lignin at a rate of 4.5 t C ha-1, or no HCA as control. Wheat straw turned out to be suitable for fast immobilization of excess N in the form of microbial biomass N (up to 42 kg N ha-1), followed by sawdust. However, under the experimental conditions this effect weakened over a few weeks, finally ranging between 8 and 15 kg N ha-1 immobilized in microbial biomass in the spruce sawdust and wheat straw treatment, respectively. Pure lignin did not stimulate microbial N immobilization. We also revealed that N immobilization by the remaining straw and sawdust HCA material in the soil had a greater importance for storage of excess N (on average 24 kg N ha-1) than microbial N immobilization over the 4 months. N fertilization and HCA influenced the abundance of ammonia oxidizing bacteria and archaea as the key players for nitrification, as well as the abundance of denitrifiers. Soil with spruce sawdust emitted more N2O compared to soil with wheat straw, which in relation released more CO2, resulting in a comparable overall global warming potential. However, this was counterbalanced by advantages like N immobilization and mitigation of potential NO3- losses.

Does maize and legume crop residue mulch matter in soil organic carbon sequestration?

Soil management techniques, such as mulching, are used to enhance soil organic carbon sequestration. However, we demonstrate that the potential of crop residue mulching to increase soil organic carbon (SOC) sequestration varies by cropping system and soil type in Austrian agricultural soils. Effects of mulch (as harvested crop residues applied at 1.0 t C ha−1) on soil and microbial carbon (C) and nitrogen (N) and soil δ13C were measured in an Austrian Cambisol field experiment with sole maize or vetch or vetch-maize rotation cropping systems after five years with or without mulching to elucidate how SOC is affected. The direct role of mulch on SOC in different soil types was also investigated in a similar greenhouse mesocosm study with controlled moisture using the same Cambisols and an Austrian agricultural Chernozem. Only sole maize cropping in the field experiment resulted in higher SOC with mulching and when legumes were included in a legume-maize rotation SOC did not improve. Mulching in the field experiment only resulted in higher SOC in the top 0–5 cm of soils with sole maize cropping (by 22%) compared to soils without mulch. Although mulch did not increase SOC in vetch-maize rotation, the δ13C of SOC was less negative with mulch indicating larger C contribution from maize than vetch mulch. After four years of annual soybean-maize rotation in the mesocosm experiment, no significant differences in SOC were observed in Cambisols with or without mulch. Again, δ13C of both soil types was less negative with mulching indicating a larger C contribution from maize than soybean mulch. No relationships between microbial biomass C and N and SOC were observed in either experiment and only soil N concentration was positively correlated with SOC. Together these studies indicate that maize can increase SOC when crop residues are applied in Austrian Cambisols but that inclusion of legume production and legume mulch in rotation can mute these benefits.

Hot regions of labile and stable soil organic carbon in Germany – Spatial variability and driving factors

Abstract. Atmospheric carbon dioxide levels can be mitigated by sequestering carbon in the soil. Sequestration can be facilitated by agricultural management, but its influence is not the same on all soil carbon pools, as labile pools with a high turnover may be accumulated much faster but are also more vulnerable to losses. The aims of this study were to (1) assess how soil organic carbon (SOC) is distributed among SOC fractions on a national scale in Germany, (2) identify factors influencing this distribution and (3) identify regions with high vulnerability to SOC losses. The SOC content and proportion of two different SOC fractions were estimated for more than 2500 mineral topsoils (&lt; 87 g kg−1 SOC) covering Germany, using near-infrared reflectance spectroscopy. Drivers of the spatial variability in SOC fractions were determined using the machine learning algorithm cforest. The SOC content and proportions of fractions were predicted with good accuracy (SOC content: R2 = 0.87–0.90; SOC proportions: R2 = 0.83; ratio of performance to deviation (RPD): 2.4–3.2). The main explanatory variables for the distribution of SOC among the fractions were soil texture, bulk soil C ∕ N ratio, total SOC content and pH. For some regions, the drivers were linked to the land-use history of the sites. Arable topsoils in central and southern Germany were found to contain the highest proportions and contents of stable SOC fractions, and therefore have the lowest vulnerability to SOC losses. North-western Germany contains an area of sandy soils with unusually high SOC contents and high proportions of light SOC fractions, which are commonly regarded as representing a labile carbon pool. This is true for the former peat soils in this area, which have already lost and are at high risk of losing high proportions of their SOC stocks. Those “black sands” can, however, also contain high amounts of stable SOC due to former heathland vegetation and need to be treated and discussed separately from non-black sand agricultural soils. Overall, it was estimated that, in large areas all over Germany, over 30 % of SOC is stored in easily mineralisable forms. Thus, SOC-conserving management of arable soils in these regions is of great importance.