Soil Carbon Fractions Research Articles

Optimizing process-based models to predict current and future soil organic carbon stocks at high-resolution

From hillslope to small catchment scales (< 50 km2), soil carbon management and mitigation policies rely on estimates and projections of soil organic carbon (SOC) stocks. Here we apply a process-based modeling approach that parameterizes the MIcrobial-MIneral Carbon Stabilization (MIMICS) model with SOC measurements and remotely sensed environmental data from the Reynolds Creek Experimental Watershed in SW Idaho, USA. Calibrating model parameters reduced error between simulated and observed SOC stocks by 25%, relative to the initial parameter estimates and better captured local gradients in climate and productivity. The calibrated parameter ensemble was used to produce spatially continuous, high-resolution (10 m2) estimates of stocks and associated uncertainties of litter, microbial biomass, particulate, and protected SOC pools across the complex landscape. Subsequent projections of SOC response to idealized environmental disturbances illustrate the spatial complexity of potential SOC vulnerabilities across the watershed. Parametric uncertainty generated physicochemically protected soil C stocks that varied by a mean factor of 4.4 × across individual locations in the watershed and a − 14.9 to + 20.4% range in potential SOC stock response to idealized disturbances, illustrating the need for additional measurements of soil carbon fractions and their turnover time to improve confidence in the MIMICS simulations of SOC dynamics.

Responses of Labile Organic Carbon and Extractable Cadmium Fractions in an Agricultural Soil Following Long-Term Repeated Application of Pig Manure and Effective Microbes.

Long-term pig manure addition has been widely applied in red soil to improve soil fertility. However, the influence of combined utilization of pig manure and effective microbes (EM) on soil organic carbon (SOC) and Cd are not well understood. This study conducted a 23-year (1996-2019) long-term fertilization field trial to investigate the changes of different fractions of SOC and Cd under chemical fertilization (CF), pig manure (PM), and pig manure with effective microbes (PM + EM) treatments in an agricultural soil of Jiangxi Province, South China. The results showed that the pig manure addition significantly enhanced the contents of SOC and Cd in the soils compared with the CF treatments. Furthermore, with the increment of SOC, the PM + EM treatment significantly increased the contents of soil microbial biomass carbon, dissolved organic carbon and easily oxidizable carbon compared with the pig manure application alone. Meanwhile, compared with the CF treatments, the EM addition significantly enhanced the exchangeable and oxidizable fractions of Cd, thus the potential Cd environment risk due to pig manure application should be carefully assessed.

Impact of long-term nutrient management on sequestration and dynamics of soil organic carbon in a semi-arid tropical Alfisol of India

We aim to determine the effect of long term application of organic and inorganic nutrients on carbon sequestration and related biological attributes of a semi-arid tropical soil under an intensive maize-sunflower cropping system in India. The century-old experimental soil was assessed the impact of nutrient management on carbon sequestration, labile and passive pools of soil organic carbon, and soil biological attributes. The topsoil (0–25 cm) and subsoil (25–50 cm) samples from unfertilized (control), chemical fertilizer applied (IC), organic manure amended (OM), and integrated nutrient management adopted (INM) plots were analyzed for carbon sequestration and dynamics. The INM and OM treatments had a mean annual carbon sequestration rate of 0.18 Mg and 0.14 Mg carbon per ha, respectively. The continuous addition of organic amendment (12.5 t ha−1 year−1 of farmyard manure) with or without chemical fertilizer improved both topsoil and subsoil active carbon pools viz., microbial biomass carbon, labile carbon, water-soluble carbon, particulate carbon fraction, and passive pools like humic acid and fulvic acid significantly compared to synthetic fertilizer and unfertilized treatments. The Fourier-transformed infrared spectra of humic acids extracted from OM and INM treatment soils showed high humification, aromaticity, and redox with complex functional groups than IC and control treatment soils. OM and INM treatments significantly enhanced the abundance of carbon mineralizing gene (chiA) in topsoil and subsoil than IC and control treatments, while the carbon dioxide fixing gene (cbbL) had no apparent change due to nutrient management. Subsequently, the organics amended soils had better biological quality in terms of higher soil enzymes' activity, bacterial abundance, and respiration rate with lower metabolic quotient than chemical fertilized and control soils. The principal component analysis indicated that OM and INM treatment soils were distinctly positioned from IC and control treatment soils and the soil carbon fractions and soil enzymes were the significant factors influencing the nutrient management induced carbon sequestration. The active and passive carbon fractions, dehydrogenase, fluorescein diacetate hydrolysis, alkaline phosphatase, and substrate-induced respiration showed a strong and positive correlation with carbon sequestration of semi-arid tropical soil. Altogether, these findings indicated that the continuous addition of organic amendments would be a valuable practice to improve the soil carbon sequestration and carbon pools, and this practice is essential for the tropical soils with intensive farming to sustain the soil quality and agricultural productivity.

Soil dissolved carbon and nitrogen dynamics along a revegetation chronosequence of Caragana korshinskii plantations in the Loess hilly region of China

Widespread vegetation restoration is an effective measure used to control soil erosion and improve soil organic carbon (SOC) and total nitrogen (TN) storage; however, knowledge of soil dissolved carbon and nitrogen has not been well characterized. Caragana korshinskii plantations are valued for their critical role in soil and water conservation in the Loess Hilly Region. This study aimed to estimate the levels of soil carbon and nitrogen and their dissolved fractions in one farmland (Zea mays L.) and three C. korshinskii plantations at 15, 30 and 40 years; additionally, the distribution of soil carbon and nitrogen stocks among various soil depths were estimated. The results showed that afforestation with C. korshinskii resulted in a significant increase in the contents and stocks of SOC and TN and their dissolved fractions. At a depth of 0–10 cm, the 30-year-old C. korshinskii had the highest stocks of SOC and TN, but at depths of 40–100 and 100–200 cm, the highest SOC and TN stocks were found in the soil of 15-year-old C. korshinskii. The 15-year-old C. korshinskii had the highest stocks of SOC and TN at depths of 0–200 cm and the highest stocks of dissolved nitrogen across almost all soil layers. The soil at 100–200 cm contained 34%–54% of the total percentages of the cumulative stocks of SOC and TN and their fractions. The SOC stock was the most important factor affecting the variation in soil nitrogen fractions and contributed to more than 80% of the total variation; additionally, the nitrate (NO3-N) content and stock were the most important factors affecting the soil carbon fractions, contributing to 78% of the total variation. This study demonstrates that revegetation age and soil depth play important roles in determining carbon and nitrogen levels. The results also suggest that C. korshinskii plantations should be harvested and revegetated during the rapid growth period (∼15 years) during which they show the largest accumulations of soil carbon and nitrogen.

Dynamics of Soil Carbon Fractions and Carbon Stability in Relation to Grassland Degradation in Xinjiang, Northwest China

Grassland degradation usually results in significant shifts in vegetation species composition and plant biomass, thus altering the soil organic carbon (SOC) content and stability. Dynamics of labile carbon fractions after grassland degradation were well addressed; however, the changes in stable carbon fractions were poorly quantified. Soil samples at 0–10 cm and 10–20 cm depth were collected from a native grassland (NA), a lightly degraded grassland (LD), a moderately degraded grassland (MD), and a severely degraded grassland (SD) in northwest China to assess the influence of grassland degradation on the total SOC content, four SOC fractions (very labile carbon, CF1; labile carbon, CF2; less labile carbon, CF3; non-labile carbon, CF4), and SOC stability. Compared with the NA, the contents under LD, MD, and SD at 0–20 cm depth reduced by 20.58%, 29.22%, and 64.58% for total SOC, 21.38%, 23.00%, and 63.66% for CF1, 13.81%, 20.58%, and 62.26% for CF2, 24.30%, 35.05%, and 68.63% for CF3, and 22.17%, 38.80%, and 63.82% for CF4, respectively. The linear relationships between the total SOC and the four fractions of CF1, CF2, CF3, and CF4 were significant in this study. The lability index of SOC under the NA, LD, MD, and SD was 1.57, 1.59, 1.67, and 1.57, respectively, and no significant difference was found among the four grasslands. To conclude, grassland degradation changes the contents of total SOC and its labile and stable fractions but did not change the SOC stability in northwest China.

Effects of Byproduct Amendments and Nitrogen on Carbon Mineralization and Soil Enzyme Activities in Acidic Brown Soil from Jiaodong Peninsula of China

ABSTRACT It has been proposed strategy that acidic soil quality can be improved by addition of alkaline byproduct amendments containing silicon, calcium, magnesium and potassium. The effects of amendments and three different forms of nitrogen [(NH2)2CO, NaNO3 and NH4Cl] on quality of acidic soil were investigated in Jiaodong Peninsula of China. A 120-day incubation test was carried out to assess the effect of amendments and nitrogen on soil physicochemical properties, soil carbon (C) mineralization and enzyme activities. The results showed that the addition of alkaline amendments can increase soil pH from 5.93 to 6.99 at the beginning of cultivation, while when amendments combined with urea or NH4Cl, soil pH decreased with incubation time. The response of C mineralization was inversely proportional to soil pH value. The addition of amendments could improve soil C content via decreasing soil C-CO2 emissions, and the mineralization rate was reduced by 26%. Addition of amendments in combination with urea or NH4Cl, which could decrease soil pH, stimulated soil C mineralization and increased soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) fractions in acidic brown soil. However, when amendments were applied with NaNO3, the soil C mineralization could be inhibited. The addition of amendments or combination of amendments and nitrogen increased the soil invertase and phenol oxidase activities, and inhibited soil urease activity. Therefore, the chemical and biological properties of the acidic brown soil in Jiaodong Peninsula of China might be better improved by combination of amendments and NaNO3.

Temperature responsiveness of soil carbon fractions, microbes, extracellular enzymes and CO2 emission: mitigating role of texture.

The interaction of warming and soil texture on responsiveness of the key soil processes i.e. organic carbon (C) fractions, soil microbes, extracellular enzymes and CO2 emissions remains largely unknown. Global warming raises the relevant question of how different soil processes will respond in near future, and what will be the likely regulatory role of texture? To bridge this gap, this work applied the laboratory incubation method to investigate the effects of temperature changes (10–50 °C) on dynamics of labile, recalcitrant and stable C fractions, soil microbes, microbial biomass, activities of extracellular enzymes and CO2 emissions in sandy and clayey textured soils. The role of texture (sandy and clayey) in the mitigation of temperature effect was also investigated. The results revealed that the temperature sensitivity of C fractions and extracellular enzymes was in the order recalcitrant C fractions > stable C fractions > labile C fractions and oxidative enzymes > hydrolytic enzymes. While temperature sensitivity of soil microbes and biomass was in the order bacteria > actinomycetes > fungi ≈ microbial biomass C (MBC) > microbial biomass N (MBN) > microbial biomass N (MBP). Conversely, the temperature effect and sensitivity of all key soil processes including CO2 emissions were significantly (P < 0.05) higher in sandy than clayey textured soil. Results confirmed that under the scenario of global warming and climate change, soils which are sandy in nature are more susceptible to temperature increase and prone to become the CO2-C sources. It was revealed that clayey texture played an important role in mitigating and easing off the undue temperature influence, hence, the sensitivity of key soil processes.

Variations in soil aggregate distribution and associated organic carbon and nitrogen fractions in long-term continuous vegetable rotation soil by nitrogen fertilization and plastic film mulching

Small changes in soil aggregates-associated organic carbon (OC) can induce huge fluctuations in greenhouse gas emissions. However, there is a knowledge gap on the responses to nitrogen (N) fertilization under plastic film mulching, especially in long-term continuous rotation systems. This study assessed the impacts of plastic film mulching and N fertilization on the soil aggregate distribution and associated OC and N fractions in a 10-year continuous cucumber-cabbage rotation soil (0–40 cm). The impacts also were further quantified using the design of experiment (DOE) method. Plastic film mulching alleviated the impact of N fertilization on soil aggregate stability, which declined under higher N doses. Plastic film mulching coupled with N fertilization resulted in higher contents of soil OC and dissolved OC in macroaggregates but lower contents in silt+clay- aggregates. The total N and dissolved organic N (DON) contents in different aggregates varied significantly with N application doses, and the alternations were impacted by plastic film mulching, which improved the DON distribution in larger agglomerates, especially at medium and high N doses. Soil aggregate distribution and associated OC and N fractions did not show consistent trends in different soil depths, which was attributed to the contributions of plastic film mulching, N fertilization and their interactions. The study suggests that N fertilizer should be applied under plastic film mulches at appropriate levels to improve C assimilation and soil fertility and promote the sustainable development of long-term vegetable rotation systems.

Distributions and Influencing Factors of Soil Organic Carbon Fractions under Different Vegetation Restoration Conditions in a Subtropical Mountainous Area, SW China

Vegetation type is known to affect soil organic carbon (SOC) storage. However, the magnitudes and distributions of SOC sequestration and driving factors for different vegetation types are still largely unknown. Thus, we studied the changes in SOC fractions along soil profiles for different vegetation restoration types and their relationships with soil properties. We selected five vegetation types and collected soil samples from depth intervals of 0–10, 10–30, 30–60, and 60–90 cm. Five soil carbon fractions and the soil properties were tested to evaluate the soil carbon fraction distributions and influencing factors. Our results demonstrated that the concentrations of total organic carbon (TOC) and five carbon fractions were strongly affected by vegetation types and soil depths. The concentrations of all five soil carbon fractions in 0–10 cm depth were higher than those in the other three soil depths and generally increased with vegetation complexity. The Pearson correlations and redundancy analysis showed that the fractions of soil glomalin-related soil protein (GRSP) and Fe oxides as well as the soil bulk densities, were the most significant related to soil TOC levels and carbon fractions, which suggests that soil biochemical and physicochemical processes are among the most important mechanisms that contribute to SOC persistence. Considering the sensitive indices of the soil carbon variables and PCA results, soil permanganate oxidizable carbon (POXC) was considered to be the most sensitive index for differentiating the effects of vegetation types. These results provide important information regarding the distributions and driving factors of the carbon fractions that result from different vegetation restoration types and will help to improve our understanding of soil carbon sequestration during vegetation restoration processes.

Chemical composition of labile carbon fractions in Hungarian forest soils: Insight into biogeochemical coupling between DOM and POM

Understanding how the chemical composition of dissolved and particulate organic matter (DOM and POM) is affected by environment factors is critical because these labile pools of carbon are involved in an array of biological, chemical and physical processes. In this study, the chemical composition of DOM and POM was measured in 13 forest soils using UV–Vis spectroscopy, fluorescence spectroscopy with parallel factor analysis (PARAFAC modelling) and FT-IR spectroscopy.There were significant differences between the soils for the SUVA indexes, PARAFAC components and relative intensities of different IR bands. Redundancy analysis (RDA) revealed that soil parameters had a strong influence on the chemical composition of DOM and POM with high constrained variability (77.9 and 77.1 %, respectively). The pH of the soils proved to be an important controlling factor for both DOM and POM, regulating the concentration of the C3 PARAFAC component (low-molecular-weight compounds associated with biological activity) and the aromatic compounds of POM (aromaticity, rA1630 and rA1515). The silt content was the other main regulating factor controlling the chemical characteristics of the labile pool, having a strong negative correlation with the specific UV absorbance (SUVA) values of DOM due to the preferential adsorption of hydrophobic moieties. RDA analysis also revealed that there is a strong correlation between the chemical composition of POM and DOM: the relative amount of aromatic compounds (rA1515, rA1630 for aromatic rings and rA1270 for phenolic compounds) in the POM fraction was closely correlated with the SUVA values of the DOM. This clearly indicates a strong biogeochemical link between the two labile organic carbon fractions; the high molecular weight compounds of the DOM fraction may be derived from lignin-like structures of the POM fraction.

How soil carbon fractions relate to soil properties and crop yields in dryland cropping systems?

AbstractSoil labile C fractions have been proposed as promising soil health indicators, but they have not been related to extensive soil properties and crop yields. We evaluated the relationships among soil organic carbon (SOC), labile C fractions, 62 soil physical, chemical, biological, and biochemical properties, and mean crop yields in two long‐term (14‐ and 36‐yr‐old) sites (Froid and Sidney, eastern Montana) under dryland farming. Soil C fractions were SOC, soil inorganic carbon (SIC), water‐extractable carbon (WEC), KMnO4–extractable carbon (POXC), potential carbon mineralization (PCM), and microbially active carbon (MAC). Treatments were conventional till and no‐till spring wheat (Triticum aestivum L.)/barley (Hordeum vulgaris L.), pea (Pisum sativum L.), and fallow rotations without N fertilization in Froid and with and without N fertilization in Sidney. Carbon fractions were greater in no‐till continuous cropping than till crop–fallow, with no effect of N fertilization at both sites. The principal component analysis showed that PCM was strongly associated with most soil physical, chemical, biological, and biochemical properties, followed by SOC, POXC, WEC, MAC, and SIC at both sites. More C fractions were related to soil properties in longer than shorter duration of the experiment. All C fractions, except SIC, were also related to mean crop yields across years. Although SOC was related to soil properties and crop yields, PCM may be used as a promising soil health indicator because of its greater sensitivity to management practices and better relationship to soil properties and crop yields than other labile C fractions.

Responses in Soil Carbon and Nitrogen Fractionation after Prescribed Burning in the Montseny Biosphere Reserve (NE Iberian Peninsula)

Prescribed fire is one of the most widely-used management tools to recover encroached rangelands. Fire has been reported to cause changes in the soil physical and chemical properties. However, the legacy effects of former plant species on soil responses to fire remains unknown. The legacy effect of the former extant plant species on soil carbon (C) and nitrogen (N) fractionation distribution after prescribed burning in topsoil (0–5 cm and 5–10 cm) was investigated in Mediterranean shrublands in Montseny. We sampled soils under five vegetation patch types: Cytisus scoparius L., Calluna vulgaris L., Erica arborea L., Pteridium aquilinum L., and Cladonia biocrusts, pre- and post-burning. Multivariate analysis on soil C and N fractions showed that soils under the legume Cytisus and the biocrust were the most differentiated. Vegetation patch types tended to respond differently to burning, soils under Cytisus, Cladonia and Calluna showing the strongest response. Total C and N, and C and N in sand decreased after burning in the 0–5 cm soil layer. Conversely, C in silt, as well as N in clay and silt, increased with soil depth after burning. This study will be helpful for understanding ecological legacy effects and their possible consequences when planning prescribed burning.

Influence of Crops and Different Production Systems on Soil Carbon Fractions and Carbon Sequestration in Rainfed Areas of Semiarid Tropics in India

Organic agriculture’s economic benefits and widespread adoption are well documented, but its impact on soil C dynamics in rainfed regions of semiarid tropics is less understood. The use of organic amendments in organic farming not only supply nutrients but also have the potential to contribute to soil carbon sequestration. Carbon storage and various soil organic pools are affected differently by various crops and production systems. A study was conducted with three crops (sunflower, pigeonpea, and greengram) under three production systems (control, organic and integrated) to assess the effect on soil C stocks, carbon sequestration potential, and crop yield. After seven years of experiment, pigeonpea (Cajanus cajan L.) cultivation improved soil bulk density, porosity and water holding capacity compared to greengram [Vigna radiata (L) Wilczek] and sunflower (Helianthus annuus L.). Furthermore, plots under pigeonpea cultivation being on par with greengram had 15.6% higher total C (113.52 Mg C ha−1), 14% higher easily oxidizable organic C (17.5 Mg C ha−1) and C sequestration rate of 1.22 Mg C ha−1 yr−1 compared to sunflower. Among the three production systems, plots under organic management had significantly lower bulk density and higher water holding capacity and porosity at all of the profile depths compared to integrated production system and control. Similarly, organic production system being on par with integrated production system improved the easily oxidizable, oxidizable and weakly oxidizable organic C fractions at different soil depths compared to control. The C sequestration rate ranged from 0.21 to 0.85 Mg C ha−1 yr−1 in organic production systems compared to negligible rate (0.01–0.04 Mg ha−1 yr−1) in the plots under control. On average, integrated production system being on par with organic management recorded significantly higher pigeonpea equivalent seed yield (886 kg ha−1) compared to control (792 kg ha−1). These results suggest the potential of organic production system in improving soil properties, C sequestration, and crop yields in semiarid rainfed areas.

Soil labile carbon and nitrogen fractions after eleven years of manure and mineral fertilizer applications

ABSTRACT Manure nutrient management can affect soil carbon (C) and nitrogen (N) fractions. The objective of this study was to determine the effects of long-term manure and mineral fertilizer applications on C and N fractions. This study was conducted for 11 years under corn and soybean rotation. The study rates included low manure (LM (4,194 kg ha−1), based on the crop’s phosphorus (P) requirement), medium manure (MM (8,081 kg ha−1), based on the crop’s N requirement), high manure (HM (16,162 kg ha−1), two times the rate of MM), medium fertilizer (MF (204 kg N ha−1), recommended), high fertilizer (HF (224 kg N ha−1), high), and control. Soil samples were collected to measure C and N fractions. HM recorded higher particulate C (8%) at 0–10 cm and higher dissolved C (26%) at 10–20 cm compared to LM. All manure rates had higher permanganate oxidizable C compared to mineral fertilizer rates. Carbon management index was higher under MM compared to the HF (17%) and MF (33%). This study suggests that manure application (16,162 kg ha−1 rate and even 8,081 kg ha−1 and 4,194 kg ha−1 rates in some cases) can enhance C and N pools compared to mineral fertilizer application.

Biochar and compost amendments alter the structure of the soil fungal network in a karst mountainous area

AbstractThe karst ecosystem is prone to degradation and this accounts for a large proportion of land degradation in southwestern China. In order to improve the nutrient status and microbial community structure of karst soil, the amendment method of combining biochar, compost, and chemical fertilizer l was explored. With CK as the control, M as manure compost, B as wood biochar, and F as chemical fertilizer, field trials were set up with eight treatments. The results showed that the biochar treatments led to higher fungal diversity and soil nutrient content than no‐biochar treatments, and the compost treatments resulted in higher fungal diversity and soil nutrient content than no‐compost treatments. Cluster and linear discriminant analysis effect size (LEfSe) analyses indicated that the composition and structure of the fungal community varied under different treatments; both the compost fungal network and biochar fungal network were more complex than their respective control networks. Soil carbon and nutrient fractions had significant effects on soil fungal networks. In short, the addition of both biochar and manure compost enlarged the complexity of microbial‐related networks and enhanced the soil nutrient status, indicating that biochar‐based fertilizer was beneficial in karst‐degraded soil restoration.

Distribution of soil carbon fractions under different bamboo species in northwest Himalayan foothills, India.

Soil carbon and its fractions are important in understanding the mechanism of soil carbon sequestration. The present study evaluated the impact of seven commercial bamboo species, viz., Bambusa balcooa, B. bambos, B. vulgaris, B. nutans, Dendrocalamus hamiltonii, D. stocksii, and D. strictus, on labile and non-labile carbon fractions. In the 0-15-cm layer, B. nutans had the highest very labile C (7.65gkg-1) followed by B. vulgaris > B. balcooa > D. stocksii > D. hamiltonii > B. bambos > D. strictus > open. The active carbon pool was significantly low under the control plot (i.e. the open) indicating the positive influence of bamboo in soil C build-up in the top 0-15cm soil layer. Amongst the different species of bamboo evaluated in this study, D. strictus accumulated the highest active C pool in 0-30-cm soil layer followed by B. vulgaris. Of the total organic C in the 0-30cm soil depth, majority (55-60%) was contributed by the passive C pool comprising the less labile and the non-labile fraction of SOC. A high value of carbon stratification ratio (> 2) was observed for D. strictus, B. bambos, and D. hamiltonii which proves their potential for restoration of the degraded lands. The majority of bamboo species except for B. balcooa and D. stocksii showed a higher carbon management index than open systems, thereby indicating higher rates of soil C rehabilitation. Of the seven bamboo species, B. vulgaris, D. strictus, and B. nutans can be adopted for cultivation in the northwest Himalayas given their ability to positively impact the SOC and its fractions in both surface and sub-surface soil.

Latitude, Elevation, and Mean Annual Temperature Predict Peat Organic Matter Chemistry at a Global Scale

AbstractPeatlands contain a significant fraction of global soil carbon, but how these reservoirs will respond to the changing climate is still relatively unknown. A global picture of the variations in peat organic matter chemistry will aid our ability to gauge peatland soil response to climate. The goal of this research is to test the hypotheses that (a) peat carbohydrate content, an indicator of soil organic matter reactivity, will increase with latitude and decrease with mean annual temperatures, (b) while peat aromatic content, an indicator of recalcitrance, will vary inversely, and (c) elevation will have a similar effect to latitude. We used Fourier Transform Infrared Spectroscopy to examine variations in the organic matter functional groups of 1034 peat samples collected from 10 to 20, 30–40, and 60–70 cm depths at 165 individual sites across a latitudinal gradient of 79°N–65°S and from elevations of 0–4,773 m. Carbohydrate contents of high latitude peat were significantly greater than peat originating near the equator, while aromatic content showed the opposite trend. For peat from similar latitudes but different elevations, the carbohydrate content was greater and aromatic content was lower at higher elevations. Higher carbohydrate content at higher latitudes indicates a greater potential for mineralization, whereas the chemical composition of low latitude peat is consistent with their apparent relative stability in the face of warmer temperatures. The combination of low carbohydrates and high aromatics at warmer locations near the equator suggests the mineralization of high latitude peat until reaching recalcitrance under a new temperature regime.

Seven years of biochar amendment has a negligible effect on soil available P and a progressive effect on organic C in paddy soils

Little attention has been paid to how long-term application of crop straw and its biochar affects soil phosphorus (P) transformation and carbon (C) fractions. We conducted a 7-year field experiment including control treatment (chemical fertilizer only, CK), straw return (2.25 t ha−1), and different amounts of biochar addition (11.25 t ha−1 (0.5%BC) and 22.5 t ha−1 (1.0%BC), to investigate influence of these amendments on soil C structure, P fractions, and their interaction with microorganisms. The 13C nuclear magnetic resonance and soil P sequence fractionation were applied to capture changes of soil C compositions and P pool. Compared to CK, straw and biochar amendments decreased alkyl C/O-alkyl C, which is conducive to increased soil organic C. The 0.5%BC and 1.0%BC treatments enhanced recalcitrant aromatic C by 69.0% and 131%, respectively. Compared to CK (101.2 ± 33.32 mg kg−1), the 0.5%BC and 1.0%BC treatments had a negligible effect on soil available P, while negative effects were observed in straw treatment (59.79 ± 9.023 mg kg−1). Straw and biochar amendments increased primary P and occluded P, whereas had negligible effect on organic P. Redundancy analysis and correlation analysis indicated that C compositions and P pool correlated to microbial community composition and enzyme activities, and aromatic C was the most related factor. Moreover, structural equation modeling indicated available P was most related to phosphatase activity and C composition. Our findings reveal the changes of soil P and C response under long-term crop straw and its biochar amendment, and can contribute toward improving understanding of the effect of biochar and straw return in future agriculture management.

The Effect of Anaerobic Digestate on the Soil Organic Carbon and Humified Carbon Fractions in Different Land-Use Systems in Lithuania

The most important component of agricultural system are soils as the basis for the growth of plants, accumulation of water, plant nutrients and organic matter. The main task of our research was to ascertain changes in soil organic carbon (SOC) and mobile humified carbon fractions in digestate-treated soils. We have performed three field experiments using the same design on two soil types in 2019–2020. We studied the fertilization effects of different phases of digestate on Retisol and Fluvisol. Fertilization treatments: control; separated liquid digestate 85 kg ha−1 N; and 170 kg ha−1 170 N; separated solid digestate 85 kg ha−1 N; and 170 kg ha−1 N. We have found a greater positive effect on the increase in SOC because of the use of the maximum recommended fertilization rate of the solid digestate. The content of mobile humic substances (MHS) tended to increase in grassland and crop rotation field in digestate-treated soil. In our experiment, maximum concentration of SOC was found in 0–10 cm soil layer, while in the deeper layers the amount of SOC, MHS and mobile humic acids proportionally decreased. We concluded, that long-term factors as soil type and land use strongly affected the humification level expressed as HD (%) in the soil and the highest HD was determined in the grassland soil in Fluvisol.

Afforestation influences soil organic carbon and its fractions associated with aggregates in a karst region of Southwest China

Variations in soil organic carbon (SOC) and its fractions within soil aggregates in response to land-use change are important to understand the carbon cycles in terrestrial ecosystem. However, responses of total SOC, SOC fractions, and SOC stability in different soil aggregates to land-use change are less addressed, especially in karst regions with serious land degradation. Therefore, bulk soil samples were collected under four land uses with similar geographical characteristics and previous framing practices including farmland (FL), Bamboo forest (BA), landscape tree planting (LAT), and orange orchards (ORO) in a karst region of Southwest China. Contents of total SOC and three carbon fractions based on their degree of oxidizability (F1, very labile; F2, inert; F3, oxidizable resistant) in bulk soil and different soil aggregates (macro-aggregate, micro-aggregate, and silt+clay fraction) were measured. Afforestation significantly increased contents of total SOC and three carbon fractions in bulk soil and soil aggregates, and the influence was more obvious in macro-aggregate than the other aggregates. Contents of total SOC, F1, F2, and F3 under afforestation land increased by 41.73%, 58.19%, 33.91%, and 40.55%, respectively, in bulk soil, by 55.60%, 79.24%, 121.77%, and 43.30%, respectively, in macro-aggregate, by 52.80%, 33.57%, 20.14%, and 75.02%, respectively, in micro-aggregate, and by 26.21%, 35.60%, 29.26%, and 23.75%, respectively, in silt+clay fraction than those under FL. In bulk soil and soil aggregates, proportions of F1, F2, and F3 in total SOC ranged from 0.11 to 0.18, from 0.13 to 0.22, and from 0.60 to 0.73, respectively, suggesting that the stable carbon was the predominant carbon fraction in the study area. Afforestation decreased the values of stability of SOC in macro-aggregate and silt+clay fraction, while it increased the value in micro-aggregate. Although both BA and ORO had higher SOC content in bulk soil than the LAT, but the SOC stability in bulk soil under BA was significantly lower than that under ORO. In conclude, afforestation form FL improved SOC content and altered SOC stability in bulk soil and soil aggregates, and conversion of FL to ORO might be the best choice to increase SOC sequestration in the four land-use types compared in karst regions of Southwest China.