The Fraction of Carbon in Soil Organic Matter as a National‐Scale Soil Process Indicator

This study aimed to investigate the soil organic carbon (SOC) sequestration rate and soil organic matter (SOM) composition in conventional rotational cropping with mineral fertilization compared with organic cover cropping with and without composted manure addition during 2008–2018 to specify the SOM stabilization under different farming systems. The SOC proportion in particulate organic matter (POM) (63–2000 µm) and mineral-associated organic matter (MAOM) (<63 µm) fractions were estimated in different treatments, and the SOM composition in the fractions was characterized by FTIR spectroscopy. The SOC sequestration rate was treatment-dependent, with the higher SOC sequestration rate (1.26 Mg ha−1 y−1) in the organic treatment with cover crop and composted manure. Across all treatments, 57.3%–77.8% of the SOC stock was in the MAOM fraction. Mineral N fertilization increased POM-C concentration by 19%–52% compared with the unfertilized control. Under the organic treatments, the POM-C concentration was 83%–95% higher than the control. The MAOM-C concentration increased by 8%–20%. The mineral N fertilization and organic treatments (with and without cover crops and composted manure) increased the SOC stock proportion of POM. The highest proportion of SOC stock related to POM was in the cover cropping system, reducing the proportion of C related to the MAOM fraction, but the addition of composted manure with cover cropping also increased the proportion of C in MAOM. Compared with MAOM, the POM had a less resistant organic matter composition, and the POM resistance was higher in organic than conventional treatments. In general, the recalcitrance of SOM increased with SOC concentration. The POM fraction had higher aromaticity (or degree of decomposition) than the MAOM fraction. The aromaticity in POM and MAOM fractions was higher in the organic farming system and depended on mineral N fertilization and cover cropping, but the effect of manure was not significant. Although the SOC sequestration rate was higher under manure addition, resulting in the highest formation of both POM and MAOM in the soil, manure addition had little effect on overall SOM composition compared with cover crops.

Soil organic matter (SOM) contributes to a large number of ecosystem services and represents an important global carbon store. In grassland ecosystems, domestic grazing by large herbivores can alter organic carbon storage in soils greatly either directly (defoliation, trampling and defecation), or indirectly (change in plant species composition, quality of organic matter, nutrient cycling, soil temperature and moisture). Due to these changes and potential interactions, grazing may not only affect the amount of SOM but also its formation pathways, which both have implications for the distribution of particulate organic matter (POM) and mineral-associated organic matter (MAOM). Most studies conducted so far concern grasslands in temperate and continental regions of low to intermediate soil organic carbon (SOC) stocks, but less is known for cold and moist regions with intrinsic high SOC levels.Our study takes part in a unique long-term experiment in grasslands of the oceanic alpine region of Setesdal, Norway (elevation ~850-1050 m and annual precipitation 1170-1760 mm). The region has nutrient poor granitic parent material, deep, moist and acidic organic horizons and a long history of grazing. The distribution of SOC with respect to particulate - (POC) and mineral-associated organic carbon (MOC), stability mechanisms and radiocarbon dating will be analysed in grazed (44-88, sheep/km2), short term non-grazed (23 years exclusion) and long term non-grazed fields (more than 60 years exclusion). Recent studies show that grazing induced shifts in plant species composition have led to low quality litter with low decomposition rate, while long term grazing-excluded fields have more nutrient rich vegetation. We hypothesise that grazing will increase overall SOC and relative POC content compared to long-term excluded fields where there will be an overall faster turnover and higher relative MOC content.Here we present empirical results on the stocks and fractions of SOC that can inform how grazing impacts organic matter formation and stability in a cold, oceanic climate. More than 200 soil samples have been analysed for total C, N, texture, bulk density and pH, and a selection are currently being analysed for SOC fraction distribution (POC and MOC) and their 14C age. SOC stocks vary greatly (45 to 442 tonnes/ha) due to large variations in soil depth (9-37 cm) and soil type. Initial results suggest only small differences in total SOC stocks between the grazing treatments, and a lower MOC/POC ratio in the grazed areas. Preliminary results of 14C analysis indicate that although POM is considered a labile fraction of SOM, it can be preserved for hundreds of years due to climate-induced low decomposition rate. Investigating MAOM and POM in light of historic and current grazing pressure shows how land use and vegetation directs SOM formation pathways, and how this may affect carbon preservation in the long term.

Legume cover crops enhance soil organic carbon via microbial necromass in orchard alleyways

Abstract. The net loss of soil organic carbon (SOC) from terrestrial ecosystems is a likely consequence of global warming and may affect key soil functions. The strongest changes in temperature are expected to occur at high northern latitudes, with forest and tundra as prevailing land cover types. However, specific soil responses to warming in different ecosystems are currently understudied. In this study, we used a natural geothermal soil warming gradient (0–17.5 ∘C warming intensity) in an Icelandic spruce forest on Andosol to assess changes in the SOC content between 0 and 10 cm (topsoil) and between 20 and 30 cm (subsoil) after 10 years of soil warming. Five different SOC fractions were isolated, and their redistribution and the amount of stable aggregates were assessed to link SOC to changes in the soil structure. The results were compared to an adjacent, previously investigated warmed grassland. Soil warming depleted the SOC content in the forest soil by −2.7 g kg−1 ∘C−1 (−3.6 % ∘C−1) in the topsoil and −1.6 g kg−1 ∘C−1 (−4.5 % ∘C−1) in the subsoil. The distribution of SOC in different fractions was significantly altered, with particulate organic matter and SOC in sand and stable aggregates being relatively depleted and SOC attached to silt and clay being relatively enriched in warmed soils. The major reason for this shift was aggregate breakdown: the topsoil aggregate mass proportion was reduced from 60.7±2.2 % in the unwarmed reference to 28.9±4.6 % in the most warmed soil. Across both depths, the loss of one unit of SOC caused a depletion of 4.5 units of aggregated soil, which strongly affected the bulk density (an R2 value of 0.91 and p<0.001 when correlated with SOC, and an R2 value of 0.51 and p<0.001 when correlated with soil mass in stable aggregates). The proportion of water-extractable carbon increased with decreasing aggregation, which might indicate an indirect protective effect of aggregates larger than 63 µm on SOC. Topsoil changes in the total SOC content and fraction distribution were more pronounced in the forest than in the adjacent warmed grassland soils, due to higher and more labile initial SOC. However, no ecosystem effect was observed on the warming response of the subsoil SOC content and fraction distribution. Thus, whole profile differences across ecosystems might be small. Changes in the soil structure upon warming should be studied more deeply and taken into consideration when interpreting or modelling biotic responses to warming.

<p>Biochar has been described as relatively stable form of C with long mean residence time due to its predominantly aromatic structure. Addition of biochar can sequester C in the soil, albeit the effect of biochar on native soil organic C decomposition, whether it stimulates or reduces the decomposition of native soil organic matter, requires further understanding. The aim of this research was to study the long-term impact of biochar (BC) on the composition of soil organic matter (SOM) in Fragi-Stagnic Albeluvisol. The work was compiled on the basis of field experiment, set up on a production field in 2011. The experiment was drawn up of two treatments and four replicates, where on half of the replicates slow-pyrolysis hardwood BC (51.8% C, 0.43% N) produced at 500-600 °C was applied 50 Mg ha<sup>-1</sup>. The soil samples were collected from 0-10 cm soil layer in autumn 2020. The air-dried samples were sieved through a 2-mm sieve and divided into two fractions: the particulate organic matter (POM) fraction (soil particles larger than 0.063 mm) and the mineral-associated organic matter (MAOM) (<0.063 mm) by density fractionation method. The soil organic carbon (SOC) and total nitrogen (Ntot) concentrations of bulk soil and fractions were measured. The chemical composition of SOM was studied using <sup>13</sup>C nuclear magnetic resonance (NMR) spectroscopy. Bulk soil samples and fractions were pretreated with 10% HF solution before NMR spectroscopy analysis. Two indices were calculated: the ratio of alkyl C/O-alkyl C, which describes the degree of SOM decomposition and soil hydrophobicity (HI): (aromatic-C+alkyl-C)/O/N-Alkyl-C.</p><p>The addition of BC to the soil increased the SOC concentration but did not influence the Ntot concentration and the soil C/N ratio increased from 11.6 to 16.7. The distribution of POM and MAOM was not affected by the BC and POM proportion accounted for an average of 57–58%. The SOC concentrations of POM and MAOM fractions were higher in the BC variant. The BC increased the proportion of aromatic-C in the SOM, as the proportion of aromatic-C in initial BC was high (almost 92%). Initially the BC is inherently highly hydrophobic and increased the HI of bulk soil, POM, and MAOM fractions. The HI increased in line: MAOM<bulk<POM (1.51<1.67<1.97). An increase in HI inhibits the decomposition of SOM and it was also confirmed by a decreased ratio of alkyl-C/O-alkyl-C after the BC addition. The decomposition degree was lowest in POM fraction where SOC concentration was more than doubled due to BC. The suppressed decomposition was caused by the limitation of soil Ntot concentration and increased C/N ratio.</p><p>In conclusion, the effect of BC on the composition of SOM was still evident after 10 years of increasing SOC concentration and soil hydrophobicity and decreasing SOM decomposition degree promoting C sequestration to the soil.</p><p>This work was supported by the Estonian Research Council grant PSG147.</p>

Divergent changes in particulate and mineral-associated organic carbon under natural revegetation along a soil texture gradient in temperate grasslands of China

Soil carbon content and stability are primarily influenced by the stabilization of particulate organic matter (POM) and mineral-associated organic matter (MAOM). Despite extensive research on the stabilization processes of POM and MAOM carbon components under various land-use types, the investigation into stabilization processes of soil carbon remains limited in saline–alkali soils. Therefore, we collected soil samples from different positions of saline–alkali drainage ditches at four reclamation times (the first, seventh, fifteenth, and thirtieth year) to determine their carbon content and physicochemical properties. Moreover, POM and MAOM fractions were separated from soil samples, and Fourier transform infrared spectra (FTIR) were used to investigate changes in their chemical composition. The results showed that with increasing reclamation time, the soil total carbon and soil organic carbon (SOC) contents significantly increased from 14 to 15 and 2.9 to 5.5 g kg−1, respectively. In contrast, soil inorganic carbon content significantly decreased from 11 to 9.6 g kg−1. Notably, the changes in soil carbon components following the increasing reclamation time were primarily observed in the furrow sole at a depth of 20–40 cm. While the SOC content of the POM fraction (SOCPOM) decreased significantly, the SOC content of the MAOM fraction (SOCMAOM) increased significantly. These alterations were largely dominated by drainage processes after reclamation instead of a possible conversion from SOCPOM to SOCMAOM. FTIR results revealed that MAOM was greatly influenced by the reclamation time more than POM was, but the change in both POM and MAOM contributed to an increase in soil carbon stability. Our findings will deepen the comprehension of soil carbon stabilization processes in saline–alkali drainage ditches after reclamation and offer a research framework to investigate the stability processes of soil carbon components via alterations in POM and MAOM fractions.

Global climate change stress has greatly influenced agricultural crop production which leads to the global problems such as food security. To cope with global climate change, nature based solutions (NBS) are desirable because these lead to improve our environment. Environmental stresses such as drought and salinity are big soil problems and can be eradicated by increasing soil organic matter which is directly related to soil organic carbon (SOC). SOC is one of the key components of the worldwide carbon (C) cycle. Different types of land use patterns have shown significant impacts on SOC stocks. However, their effects on the various SOC fractions are not well-understood at the global level which make it difficult to predict how SOC changes over time. We aim to investigate changes in various SOC fractions, including mineral associated organic carbon (MAOC), mineral associated organic matter (MAOM), soil organic carbon (SOC), easily oxidized organic carbon (EOC), microbial biomass carbon (MBC) and particulate organic carbon (POC) under various types of land use patterns (NBS), including cropping pattern, residue management, conservation tillages such as no tillage (NT) and reduced tillage (RT) using data from 97 studies on a global scale. The results showed that NT overall increased MAOC, MAOM, SOC, MBC, EOC and POC by 16.2%, 26.8%, 24.1%, 16.2%, 27.9% and 33.2% (P < 0.05) compared to CT. No tillage with residue retention (NTR) increased MAOC, MAOM, SOC, MBC, EOC and POC by 38.0%, 29.9%, 47.5%, 33.1%, 35.7% and 49.0%, respectively, compared to CT (P < 0.05). RT overall increased MAOC, MAOM, SOC, MBC, EOC and POC by 36.8%, 14.1%, 25.8%, 25.9, 18.7% and 16.6% (P < 0.05) compared to CT. Reduced tillage with residue retention (RTR) increased MAOM, SOC and POC by 14.2%, 36.2% and 30.7%, respectively, compared to CT (P < 0.05). Multiple cropping increased MAOC, MBC and EOC by 14.1%, 39.8% and 21.5%, respectively, compared to mono cropping (P < 0.05). The response ratios of SOC fractions (MAOC, MAOM, SOC, MBC, EOC and POC) under NT and RT were mostly influenced by NBS such as residue management, cropping pattern along with soil depth, mean annual precipitation, mean annual temperature and soil texture. Our findings imply that when assessing the effects of conservation tillage methods on SOC sequestration, SOC fractions especially those taking part in driving soil biological activities, should be taken into account rather than total SOC. We conclude that conservation tillages under multiple cropping systems and with retention of crop residues enhance soil carbon sequestration as compared to CT in varying edaphic and climatic conditions of the world.

Soils are the largest carbon (C) reservoir in terrestrial ecosystems. There are still numerous uncertainties concerning the fate of soil organic carbon and its feedback on climate change. Radiocarbon is a useful approach to better understanding the carbon cycle. The nuclear weapon testing in the 1960s induced a peak in 14C atmospheric concentration &amp;#8211; a signal that can be used to trace the incorporation and turnover of C in soil. By separating the soil in different fractions and measuring the 14C in them, we can quantify how much C and for how long is stored in soils, and where soil organic carbon is stabilised.Our study aimed at identifying the impact of climate and mineralogy on soil organic matter turnover on a regional scale. We analysed C pools and 14C contents in the organic layer, mineral soil (0-20cm) and its fractions from 54 sites across Switzerland. These 54 sites are systematically spread across natural climatic and geological gradients and were repeatedly sampled in the 1990s and 2014. The mineral soil was incubated for 181 days and 14C was measured in the respired CO2. The mineral soil was fractionated according to density into particulate organic matter (POM) and mineral-associated organic matter (MAOM). We then oxidised the mineral-associated organic matter with hydrogen peroxide to remove its labile fraction of carbon. Our 14C dataset was analysed together with ancillary data comprising soil properties and climatic variables from the studied sites.Our radiocarbon dataset showed that the carbon that was respired from the mineral soil originated predominantly from particulate organic matter. The bomb spike signal was incorporated in the organic layer and in the particulate organic matter, while the mineral-associated organic matter had turnover times on centennial to millennial time scales (from 94 to 3060 years). Further chemical oxidation of MAOM using hydrogen peroxide revealed a stronger depletion in radiocarbon of the residual fraction with &amp;#916;14C values ranging between -173 &amp;#8240; and -47 &amp;#8240;. This indicates that the MAOM is a mixture of 14C-enriched organic matter and very old material.With respect to the controlling factors of soil organic matter turnover time, the radiocarbon signature of the POM is most strongly affected by climatic variables such as mean annual temperatures. In contrast to POM, the mineral-associated organic matter, comprising the greatest pool of soil organic carbon is driven by chemical soil properties. For instance, older 14C ages are found in acidic soils with low pH values ranging between 3 and 4. In these soils, Al and Fe oxides concentrations are high. We showed that the concentrations of pedogenic oxides in the soil correlate with soil organic carbon concentrations in the mineral-associated organic matter. In soils with higher pH (&gt;7), we can also find old 14C ages. In these soils, C is stabilised by interactions with calcium ions and carbonates.Overall, our regional scale dataset shows that the net accumulation of labile soil organic matter seems to be climate sensitive, while mineralogy and weathering contribute most significantly to the stabilisation of organic carbon in the soil.&amp;#160;&amp;#160;

Recent meta-analyses suggest a global potential of cover crops to increase soil organic carbon (SOC), thus contributing to climate change mitigation. However, some studies also found that cover cropping did not affect or even reduced SOC, thus it is uncertain how this effect is controlled. Here we aimed at comprehensively evaluating the potential and mechanisms of carbon (C) sequestration from cover crops in a Danish long-term crop rotation field experiment (LTE) initiated in 1997. We quantified SOC to 1-m depth, and also operationally divided soil organic matter into fractions of particulate organic matter (POM) and mineral associated organic matter (MAOM) to investigate the C saturation status of soils. Moreover, we performed a mescosm study with topsoils where the fate of varying doses (0.1-1.6 mg C g-1 soil) of 14C-labeled cover crop residues (fodder radish, FR; Raphanus sativus L.) and SOC priming were traced in two texturally similar soils having the same long-term management, but different SOC contents (2.0 vs. 2.6% SOC). Our LTE results showed that cover cropping for up two decades had negligible effect on SOC contents in POM and MAOM fractions in the topsoil and in the subsoil. However, the mesocosm study showed considerable net C increases (20-25% of added) when the cover crop C input exceeded 0.3 and 0.6 mg C g-1 in soils with 2.0 and 2.6% SOC, respectively. This was due a combination of new SOC formation and priming effects shifting from positive to negative. Collectively the LTE and mesocosm study suggests that buildup of SOC stock was not essentially constrained by soil C saturation, but rather by the low productivity and C input from cover crops. Our study suggests that agricultural management practices should be adopted (e.g., species choice and sowing time) to achieve a cover crop C input that exceeds a certain threshold to ensure effective C sequestration.

Organic matter accumulation in soil is understood as the result of the dynamics between mineral‐associated (more decomposed, microbial derived) organic matter and free particulate (less decomposed, plant derived) organic matter. However, from regional to global scales, patterns and drivers behind main soil organic carbon (SOC) fractions are not well understood and remain poorly linked to the pedogenetic variation across soil types. Here, we separated SOC associated with silt‐ and clay‐sized particles (S + C), stable aggregates (&gt;63 μm, SA) and particulate organic matter (POM) from a diverse range of grassland topsoils sampled along a geoclimatic gradient. The relative contribution of the two mineral‐associated fractions (S + C &amp; SA) to SOC differed significantly across the gradient, while POM was never the dominant SOC fraction. Stable aggregates (&gt;63 μm) emerged as the major SOC fraction in carbon‐rich soils. The degree of decomposition of carbon in stable aggregates (&gt;63 μm) was consistently between that of the S + C and POM fractions and did not change along the investigated gradient. In contrast, carbon associated with the S + C fraction was less microbially decomposed in carbon‐rich soils than in carbon‐poor soils. The amount of SOC in the S + C fraction was positively correlated to pedogenic oxide contents and texture, whereas the amount of SOC associated with stable aggregates (&gt;63 μm) was positively correlated to pedogenic oxide contents and negatively to temperature. We present a conceptual summary of our findings, which integrates the role of stable aggregates (&gt;63 μm) with other major SOC fractions and illustrates their changing importance across (soil‐)environmental gradients.

Global warming is accelerating the decomposition of soil organic matter (SOM). When predicting the net SOM dynamics in response to warming, there are considerable uncertainties owing to experimental limitations. Long-term in situ whole-profile soil warming studies are particularly rare. This study used a long-term, naturally occurring geothermal gradient in Yukon, Canada, to investigate the warming effects on SOM in a forest ecosystem. Soils were sampled along this thermosequence which exhibited warming of up to 7.7℃; samples were collected to a depth of 80cm and analysed for soil organic carbon (SOC) and nitrogen (N) content, and estimates made of SOC stock and fractions. Potential litter decomposition rates as a function of soil temperature and depth were observed for a 1-year period using buried teabags and temperature loggers. The SOC in the topsoil (0-20cm) and subsoil (20-80cm) responded similar to warming. A negative relationship was found between soil temperature and whole-profile SOC stocks, with a total loss of 27% between the warmest and reference plots, and a relative loss of 3%℃-1 . SOC losses were restricted to the particulate organic matter (POM) and dissolved organic carbon (DOC) fractions with net whole-profile depletions. Losses in POM-C accounted for the largest share of the total SOC losses. In contrast to SOC, N was not lost from the soil as a result of warming, but was redistributed with a relatively large accumulation in the silt and clay fraction (+40%). This suggests an immobilization of N by microbes building up in mineral-associated organic matter. These results confirm that soil warming accelerates SOC turnover throughout the profile and C is lost in both the topsoil and subsoil. Since N stocks remained constant with warming, SOM stoichiometry changed considerably and this in turn could affect C cycling through changes in microbial metabolism.

Winter wheat cover crop increased subsoil organic carbon in a long-term cotton cropping system in Tennessee

Changes in Soil Organic Carbon Dynamics in a Native C4 Plant-Dominated Tidal Marsh Following Spartina alterniflora Invasion

Effective land-based solutions to climate change mitigation require actions that maximize soil carbon storage without generating surplus nitrogen. Land management for carbon sequestration is most often informed by bulk soil carbon inventories, without considering the form in which carbon is stored, its capacity, persistency and nitrogen demand. Here, we present coupling of European-wide databases with soil organic matter physical fractionation to determine continental-scale forest and grassland topsoil carbon and nitrogen stocks and their distribution between mineral-associated and particulate organic matter pools. Grasslands and arbuscular mycorrhizal forests store more soil carbon in mineral-associated organic carbon, which is more persistent but has a higher nitrogen demand and saturates. Ectomycorrhizal forests store more carbon in particulate organic matter, which is more vulnerable to disturbance but has a lower nitrogen demand and can potentially accumulate indefinitely. The share of carbon between mineral-associated and particulate organic matter and the ratio between carbon and nitrogen affect soil carbon stocks and mediate the effects of other variables on soil carbon stocks. Understanding the physical distribution of organic matter in pools of mineral-associated versus particulate organic matter can inform land management for nitrogen-efficient carbon sequestration, which should be driven by the inherent soil carbon capacity and nitrogen availability in ecosystems. Land management strategies for enhancing soil carbon sequestration need to be tailored to different soil types, depending on how much organic matter is stored in pools of mineral-associated and particulate organic matter, suggests an analysis of soil organic matter across Europe.