User:Alandmanson/soil organic matter

What is Soil Organic Matter?
Soil organic matter (SOM) is defined as “the organic matter component of soil, consisting of plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by soil organisms” (Brady et al., 1999). It is made up of living biological biomass, fresh and partially decomposed residues, as well as well-decomposed organic material, sometimes known as humus, or humic supramolecular substances (Chen et al., 2004, cited by Manson, 2017). The living biomass includes microorganisms (archaea, bacteria, fungi, algae and protozoa), mesofauna (earthworms, termites, insects and mites) as well as plant roots. The fresh and partially decomposed residues are the decaying plant litter, roots and remains of other organisms in the soil. Carbohydrates, proteins, amino acids, enzymes, phenols, waste products such as faecal matter (manure), urine, root exudates, and microbial exudates also form part of the fresh and partially decomposed SOM. The well-decomposed SOM is understood to occur as a wide range of organic (or humic) substances. These substances may form complexes with positively charged substances such as enzymes or Al3+ in the soil matrix, and may also be adsorbed on the surfaces of clay minerals. A particularly stable fraction of SOM is biochar (charcoal), the result of pyrolysis (the incomplete oxidation of organic matter) in fires. (Jien et al., 2015, Fahramand et al., 2014, Pettit, 2004). Moreover, as SOM decomposes, the products eg. sugars and lignin decomposition products, tend to form supramolecular associations with humic substances, resulting in a form of organic matter that cannot be classified, as yet, as fresh, partially decomposed or well-decomposed SOM (Piccolo, 2005). Although SOM generally makes up 2 to 8% of the total topsoil mass, the amount of SOM in the soil depends on a variety of factors, which include land use, environmental factors and climatic conditions of that specific area. Topsoils or upper horizons in deserts normally contain less than 1% organic matter, while the SOM content of soils in low-lying, wet areas can be as high as 90% (peat). Soils containing greater than 12% SOC (20% SOM) are generally classified as organic soils (Troeh and Thompson, 2005).

Functions of SOM
Soil organic matter provides a source of nutrients for growth and development of plants and microbial populations living in the soil. Upon decomposition of fresh organic matter, nutrients such as nitrogen (N), phosphorus, (P), potassium, (K) and sulphur (S) are released as soluble, inorganic ions into the soil matrix, thereby becoming available for plant uptake. These nutrients are taken up through the plant-root system or microbial membrane for use within the cells and tissues. Soil organic matter provides a source of food for decomposers which may supress plant pathogens of (soil-borne diseases). Micronutrients such as magnesium (Mg) and calcium (Ca) can also be provided by SOM, depending on the quality of the organic matter. Soil organic matter is classified as of good quality when it is still fairly fresh, its C:N ration is below 30:1, and can provide (when mineralised) a wide range of nutrients at reasonable concentrations. Generally, with low quality SOM, the C:N ration is more than 30:1, and the mineralised N is quickly immobilised by microorganisms for metabolic purposes, resulting in the unavailability of this nutrient for plant uptake. Some fractions of SOM can form complexes (chelates) with micronutrients such as iron (Fe), copper (Cu), manganese (Mn), and zinc (Zn), preventing them from leaching, increasing their availability for biological uptake as well as reducing their toxicity as cations. Soil organic matter also serves as a source of cation exchange capacity. Humic substances can also stimulate root growth, proliferation and development, and shoot development. (Atiye, 2002, Pettit, 2004). Soil organic matter bonds soil particles together, forming stable aggregates. Soil organic matter increases water holding capacity this is particularly important in sandy soils.

Effect of land management practices on SOM
Land management practices can either increase or decrease organic matter in the soils. The decrease of SOM may result in decreased soil fertility, with loss of nutrient supplying capacity (especially N), pH buffering capacity (resistance to soil pH change), and cation storage capacity (ability to hold the exchangeable cations K+, NH4+, Ca2+, Mg2+). Loss of SOM also frequently results in lower aggregate stability, soil structure and tilth. However, increases in SOM content may reverse these losses, rehabilitating and conditioning the soil for sustainable production. Different land management practices have different impact on SOM.

Tillage/cropping systems
When land under natural vegetation is tilled, SOM declines. Its decline is rapid in the first 10 years, slows down with time, and comes to equilibrium after 20-60 years. Often a loss of 25-50% can occur over 30 years (Du Toit, 1994). This decline in SOM occurs because with conventional tillage systems, much of the produced biomass is removed, e.g. when maize is planted for silage or residue is used to supplement winter feed for livestock. This results in less plant residue being added to the soil as fresh SOM inputs, compared to the amount of biomass returned to soils in natural vegetation systems. Moreover, if cover crops or crop rotation practices are absent in these systems (annual crop lands), the land is usually fallow outside the growing season, hence there is no biomass being returned to the soil as SOM.

During the fallow period, plant debris on the soil surface is usually susceptible to wind and water erosion. As the topsoil is eroded together with residues and SOM contained in the topsoil, SOM which is lighter than soil particles in mass, is preferentially eroded by wind and surface runoff. With the topsoil not being covered by residue, the velocity at which the rain-drops hit the soil surface is usually strong enough to break open soil aggregates, exposing SOM which was protected within the aggregates. The exposed organic matter is usually lost through erosion or decomposition by microorganisms. Cultivation however, results in many more aggregates being broken, exposing much of the protected SOM to erosion and decomposition, and CO2 emissions and the bioavailability of mineral nutrients may spike. In zero tillage or direct drilling tillage systems, however, seed is placed in a narrow slot in otherwise undisturbed soil, minimising the effect of cultivation on SOM. The surface crop residues or plant litter in these systems is not only a source of fresh SOM, it also minimises the impact of rain drops on aggregates, as well as protects soils from drying out, keeping life near the soil more active. However, it is worth noting that although minimum tillage systems may seem the ideal practice for increasing organic matter in soils, this SOM is only accumulated near the surface, rather than mixed with the whole plough layer. Near the surface, this SOM is still prone to be lost through sheet erosion, especially on steep slopes.

Pastures
Other land use systems such as perennial cultivated pastures also increase SOM content, especially if previously cultivated. This increase is due to high production of organic material, particularly when fertilizers have been used effectively, with adequate N inputs in the system. These N inputs may be supplied by mineral fertilizer or fixed by legumes incorporated into the system. Root biomass plays a major role in increasing SOM in these systems; much of the above-ground biomass is utilised by livestock, but grazing stimulates root production and turnover, thereby increasing organic-matter inputs. Grazing also recycles organic matter as dung.

Other beneficial practices
In all land use systems, appropriate fertilizer application (N, P, K and micronutrients) generally increases plant growth and dry matter production, therefore increasing return of organic residues to the soil (including roots). Higher row-crop yields have been observed to result in slower decline in SOM levels. Additions of organic material such as animal manures, composts, mulch of plant material, cover crops and green manures have also been observed to be beneficial in increasing SOM. Composted material has been observed to increase SOM content, while fresh organic matter tends to boosts microbial growth and aggregate stability, especially in cropping systems.

Strategies
To increase SOM in land management systems, the following strategies may be used: Elimination of fallows by practicing crop rotation and the use of cover crops and green manures may increase SOM inputs. The use of organic sources of plant nutrients as well as including a long-term (more than two years) pasture in the rotation may contribute positively in increasing SOM, among other benefits.  Reducing SOM losses by minimising erosion, tillage and retention of residue in cropping systems also has a significant impact in maintaining the systems SOM.

Microbial necromass contribution to soil organic matter
Angst, G., Mueller, C.W., Prater, I., Angst, Š., Frouz, J., Jílková, V., Peterse, F. and Nierop, K.G., 2019. Earthworms act as biochemical reactors to convert labile plant compounds into stabilized soil microbial necromass. Communications biology, 2(1), pp.1-7. DOI Abstract: Earthworms co-determine whether soil, as the largest terrestrial carbon reservoir, acts as source or sink for photosynthetically fixed CO2. However, conclusive evidence for their role in stabilising or destabilising soil carbon has not been fully established. Here, we demonstrate that earthworms function like biochemical reactors by converting labile plant compounds into microbial necromass in stabilised carbon pools without altering bulk measures, such as the total carbon content. We show that much of this microbial carbon is not associated with mineral surfaces and emphasise the functional importance of particulate organic matter for long-term carbon sequestration. Our findings suggest that while earthworms do not necessarily affect soil organic carbon stocks, they do increase the resilience of soil carbon to natural and anthropogenic disturbances. Our results have implications for climate change mitigation and challenge the assumption that mineral-associated organic matter is the only relevant pool for soil carbon sequestration.

Angst, G., Mueller, K.E., Nierop, K.G. and Simpson, M.J., 2021. '''Plant-or microbial-derived? A review on the molecular composition of stabilized soil organic matter.''' Soil Biology and Biochemistry, p.108189. DOI PDF Highlights:
 * Microbial necromass accounts for ~50% or less of the SOM in aggregates and mineral-associated organic matter (MAOM).
 * Plant compounds preferentially stabilized in aggregates & MAOM in some ecosystems.
 * Soil type, land use/cover, substrate quality explain variability in biomarker content.
 * Specifically, the effects of climate, soil fauna, soil depth require further study.

Buckeridge, K.M., La Rosa, A.F., Mason, K.E., Jeanette, W., McNamara, N.P., Grant, H.K. and Nick, O., 2020. Sticky dead microbes: Rapid abiotic retention of microbial necromass in soil. Soil Biology and Biochemistry, p.107929. DOI PDF Abstract: Microbial necromass dominates soil organic matter. Recent research on necromass and soil carbon storage has focused on necromass production and stabilization mechanisms but not on the mechanisms of necromass retention. We present evidence from soil incubations with stable-isotope labeled necromass that abiotic adsorption may be more important than biotic immobilization for short-term necromass retention. We demonstrate that necromass adsorbs not only to mineral surfaces, but may also interact with other necromass. Furthermore, necromass cell chemistry alters necromass-necromass interaction, with more bacterial tracer retained when there is yeast necromass present. These findings suggest that the adsorption and abiotic interaction of microbial necromass and its functional properties, beyond chemical stability, deserve further investigation in the context of soil carbon sequestration.

Creamer, C.A., Foster, A.L., Lawrence, C., McFarland, J., Schulz, M. and Waldrop, M.P., 2019. Mineralogy dictates the initial mechanism of microbial necromass association. Geochimica et Cosmochimica Acta, 260, pp.161-176. [10.1016/j.gca.2019.06.028 DOI] Abstract: Soil organic matter (SOM) improves soil fertility and mitigates disturbance related to climate and land use change. Microbial necromass (the accumulated cellular residues of microorganisms) comprises the majority of soil C, yet the formation and persistence of necromass in relation to mineralogy is poorly understood. We tested whether soil minerals had different microbial necromass association mechanisms. Specifically, we tested whether microbial necromass directly sorbed to mineral surfaces or was consumed by live microorganisms prior to mineral association. Applying Raman microspectroscopy with 13C enriched microbial necromass to quantify microbe-mineral interactions, we show that mineralogy alters the initial mechanism of microbial necromass association. In the presence of K-feldspar (lower abiotic C preservation potential), microbial necromass required assimilation by live microorganisms for mineral retention. In contrast, with amorphous aluminum hydroxide (higher abiotic C preservation potential) microbial necromass was retained predominately through abiotic sorption, and was subsequently protected from microbial decomposition. Despite different mechanisms, both minerals retained similar quantities of microbial necromass under biotic conditions. Mineralogy determined not only the quantity of mineral-associated C, but the distinct pathway of microbial necromass association. These findings show the utility of Raman microspectroscopy as a technique to study microbe-mineral interactions, and imply that heterogeneity in mineral-organic interactions could result in gradients of organic matter stability.

Dwivedi, D., Tang, J., Bouskill, N., Georgiou, K., Chacon, S.S. and Riley, W.J., 2019. Abiotic and biotic controls on soil organo–mineral interactions: developing model structures to analyze why soil organic matter persists. Reviews in Mineralogy and Geochemistry, 85(1), pp.329-348. DOI PDF Summary: Soils store vast amounts of terrestrial organic carbon, more than the atmosphere and terrestrial vegetation combined. This carbon is vulnerable to release to the atmosphere under a changing climate. Although the mechanisms leading to the decomposition of SOM are well documented, uncertainties persist regarding the stability of SOM. To address this critical gap, a robust predictive understanding and modeling of SOM dynamics is essential for examining short- and long-term changes in soil carbon storage and feedbacks with climate. In this chapter, we reviewed recent research that improves emergent understanding of the important factors contributing to SOM stability. While there currently exists a suite of models representing SOM dynamics that span a range of complexity, some recent mechanistic models are more consistent with this emergent understanding of SOM persistence. Yet even those recent models do not represent several processes that can be important for SOM dynamics. We conclude that the next-generation models need to represent the full spectrum of quantitatively important mechanisms for determining SOM persistence—including rate-limited and equilibrium-based sorption, the formation of soil aggregates, representative soil minerals, microbial community dynamics, and vegetation interactions—to accurately predict short- and long-term SOM dynamics. Because this recommendation is obviously challenging, we have assembled an open-source, reactive-transport based SOM model that can be used to robustly integrate many of these processes (called BeTR-S; https://github.com/BeTR-biogeochemistry-modeling/sbetr) and invite the community to experiment with it (Riley et al. 2019). Overall, it is important to understand SOM dynamics because SOM decomposition gives rise to potential greenhouse gases. Microbial dynamics, MAOM, and the molecular structure of SOM compounds have implications for understanding and modeling the short- and long-term responses of soil carbon stocks under local and regional climatic perturbations. Finally, this chapter illustrates the need to evaluate SOM dynamics in a reactive transport modeling framework that includes ecosystem properties.

Gerke, J., 2018. Concepts and misconceptions of humic substances as the stable part of soil organic matter: A review. Agronomy, 8(5), p.76. Abstract: In the last three decades, the concept of soil humic substances has been questioned in two main directions. Misinterpretations of CP MAS13C NMR spectroscopy led to the conclusion that soil organic matter is mainly aliphatic, questioning the theory of polymerization of humic substances from phenolic molecules. Conversely, some critics of humic substances assume that a great proportion of aromatic soil organic carbon originates from fire-affected carbon, often termed as black carbon (BC). However, the determination of BC in soil by two widely applied methods, the benzene polycarboxylic acid marker method and the UV method, is not reliable and seems to strongly overestimate the BC content of soils. The concept of humic substances continues to be relevant today. The polymerization of phenolic molecules that originate from the degradation of lignin or synthesis by microorganisms may lead to humic substances which can incorporate a variety of organic and inorganic molecules and elements. The incorporation, e.g., of triazines or surfactants into the humic matrix, leading to bound residues, illustrates that humic substances are important to explain central reactions in soil. Humic substances are also important to understand the availability of plant nutrients in soil, including P, Fe, and Cu, and they may have a direct effect on the growth of higher plants in soil. Therefore, there are good reasons to reformulate or to further develop the concepts and models of humic substances introduced and developed by M. Schnitzer, W. Flaig, W. Ziechmann, and F.J. Stevenson.

Kallenbach, C.M., Frey, S.D. and Grandy, A.S., 2016. Direct evidence for microbial-derived soil organic matter formation and its ecophysiological controls. Nature communications, 7, p.13630. Abstract: Soil organic matter (SOM) and the carbon and nutrients therein drive fundamental submicron- to global-scale biogeochemical processes and influence carbon-climate feedbacks. Consensus is emerging that microbial materials are an important constituent of stable SOM, and new conceptual and quantitative SOM models are rapidly incorporating this view. However, direct evidence demonstrating that microbial residues account for the chemistry, stability and abundance of SOM is still lacking. Further, emerging models emphasize the stabilization of microbial-derived SOM by abiotic mechanisms, while the effects of microbial physiology on microbial residue production remain unclear. Here we provide the first direct evidence that soil microbes produce chemically diverse, stable SOM. We show that SOM accumulation is driven by distinct microbial communities more so than clay mineralogy, where microbial-derived SOM accumulation is greatest in soils with higher fungal abundances and more efficient microbial biomass production.

Liang, C., Amelung, W., Lehmann, J. and Kästner, M., 2019. Quantitative assessment of microbial necromass contribution to soil organic matter. Global change biology, 25(11), pp.3578-3590. DOI PDF Abstract: Soil carbon transformation and sequestration have received significant interest in recent years due to a growing need for quantitating its role in mitigating climate change. Even though our understanding of the nature of soil organic matter has recently been substantially revised, fundamental uncertainty remains about the quantitative importance of microbial necromass as part of persistent organic matter. Addressing this uncertainty has been hampered by the absence of quantitative assessments whether microbial matter makes up the majority of the persistent carbon in soil. Direct quantitation of microbial necromass in soil is very challenging because of an overlapping molecular signature with nonmicrobial organic carbon. Here, we use a comprehensive analysis of existing biomarker amino sugar data published between 1996 and 2018, combined with novel appropriation using an ecological systems approach, elemental carbon–nitrogen stoichiometry, and biomarker scaling, to demonstrate a suit of strategies for quantitating the contribution of microbe‐derived carbon to the topsoil organic carbon reservoir in global temperate agricultural, grassland, and forest ecosystems. We show that microbial necromass can make up more than half of soil organic carbon. Hence, we suggest that next‐generation field management requires promoting microbial biomass formation and necromass preservation to maintain healthy soils, ecosystems, and climate. Our analyses have important implications for improving current climate and carbon models, and helping develop management practices and policies.

Ritz, K., 2020. Special issue editorial: Microbial necromass on the rise in SOM: advances, challenges, and perspectives. This special issue aims to bring together a number of studies that provide knowledge and perspectives, mechanistically and quantitatively, into microbial necromass and its contribution to soil organic matter (SOM) formation, transformation, and storage. Recent studies and conceptual models have persistently suggested that the incorporation of microbial biomass components into soils via microbial remnants is likely to be disproportionately large. This means that microbial inputs and microbial energetic processes play a far greater role in the sequestration of carbon and nitrogen into soils than traditionally considered, particularly when a significant portion of those inputs are more likely to be stabilized, rather than plant inputs. However, these inputs have rarely been reliably be determined, and thus the microbial anabolic controls of biomass formation, energy fluxes, and related SOM processes, as well as their interactions with other factors remains largely unknown. This special issue focusing on microbial necromass in soil aims to narrow-down the role of microbial anabolism in the creation and storage of SOM, as well as its resilience to disturbance or changing environmental conditions. We invite submissions on recent findings, methodological breakthroughs and challenges, and innovative concepts for inspiring discussions on microbial necromass in soils. We particularly call for the interactions of studies on soil microbial necromass from different disciplinary domains and the integrated system-related research.

Stoichiometry of soil organic matter
Cui, J., Zhu, Z., Xu, X., Liu, S., Jones, D.L., Kuzyakov, Y., Shibistova, O., Wu, J. and Ge, T., 2020. Carbon and nitrogen recycling from microbial necromass to cope with C: N stoichiometric imbalance by priming. Soil Biology and Biochemistry, 142, p.107720. DOI PDF Abstract: The impact of increasing amounts of labile C input on priming effects (PE) on soil organic matter (SOM) mineralization remains unclear, particularly under anoxic conditions and under high C input common in microbial hotspots. PE and their mechanisms were investigated by a 60-day incubation of three flooded paddy soils amended with13C-labeled glucose equivalent to 50–500% of microbial biomass C (MBC). PE (14–55% of unamended soil) peaked at moderate glucose addition rates (i.e., 50–300% of MBC). Glucose addition above 300% of MBC suppressed SOM mineralization but intensified microbial N acquisition, which contradicted the common PE mechanism of accelerating SOM decomposition for N-supply (frequently termed as “N mining”). Particularly at glucose input rate higher than 3 g kg−1 (i.e., 300–500% of MBC), mineral N content dropped on day 2 close to zero (1.1–2.5 mg N kg−1) because of microbial N immobilization. To cope with the N limitation, microorganisms greatly increased N-acetyl glucosaminidase and leucine aminopeptidase activities, while SOM decomposition decreased. Several discrete peaks of glucose-derived CO2 (contributing >80% to total CO2) were observed between days 13–30 under high glucose input (300–500% of MBC), concurrently with CH4 peaks. Such CO2 dynamics was distinct from the common exponential decay pattern, implicating the recycling and mineralization of 13C-enriched microbial necromass driven by glucose addition. Therefore, N recycling from necromass was hypothesized as a major mechanism to alleviate microbial N deficiency without SOM priming under excess labile C input. Compound-specific 13C-PLFA confirmed the redistribution of glucose-derived C among microbial groups, i.e., necromass recycling. Following glucose input, more than 4/5 of total 13C-PLFA was in the gram-negative and some non-specific bacteria, suggesting these microorganisms as r-strategists capable of rapidly utilizing the most labile C. However, their 13C-PLFA content decreased by 70% after 60 days, probably as a result of death of these r-strategists. On the contrary, the 13C-PLFA in gram-positive bacteria, actinomycetes and fungi (K-strategists) was initially minimal but increased by 0.5–5 folds between days 2 and 60. Consequently, the necromass of dead r-strategists provided a high-quality C–N source to the K-strategists. We conclude that under severe C excess, N recycling from necromass is a much more efficient microbial strategy to cover the acute N demand than N acquisition from the recalcitrant SOM.

Kirkby, C.A., Richardson, A.E., Wade, L.J., Batten, G.D., Blanchard, C. and Kirkegaard, J.A., 2013. Carbon-nutrient stoichiometry to increase soil carbon sequestration. Soil Biology and Biochemistry, 60, pp.77-86. DOI PDF Abstract: The more stable fine fraction pool of soil organic matter (FF-SOM; <0.4 mm) has more nitrogen, phosphorus and sulphur (N, P, S) per unit of carbon (C) than the plant material from which it originates and has near constant ratios of C:N:P:S. Consequently, we hypothesised that the sequestration of C-rich crop residue material into the FF-SOM pool could be improved by adding supplementary nutrients to the residues based on these ratios. Here we report on the effect of N, P and S availability on the net humification efficiency (NHE), the change in the size of the FF-SOM pool (as estimated by fine fraction C (FF-C)), following incubation of soil with wheaten straw. Four diverse soils were subjected to seven consecutive incubation cycles, with wheaten straw (10 t ha−1 equivalent) added at the beginning of each cycle, with and without inorganic N, P, S addition (5 kg N, 2 kg P and 1.3 kg S per tonne of straw). This nutrient addition doubled the mean NHE in all soils (from 7% to 15%) and when applied at twice the rate increased NHE further (up to 29%) for the two soils that received this treatment. The FF-N, -P and -S levels increased in concert with FF-C levels in all soils in close agreement with published stoichiometric ratios (C:N:P:S = 10,000:833:200:143). Microbial biomass-C (MB-C) levels were estimated during one incubation cycle and found to increase in parallel with FF-C from 448 μg MB-C g−1 soil (no nutrient addition) to 727 μg MB-C g−1 soil (plus nutrients) and 947 μg MB-C g−1 soil (plus 2× nutrients). There was a significant relationship between MB-C and the change in FF-C during that incubation cycle, providing evidence of a close relationship between the microbial biomass and FF-SOM formation. The two to four-fold increases in NHE achieved with nutrient addition demonstrated that inorganic nutrient availability is critical to sequester C into the more stable FF-SOM pool irrespective of soil type and C input. This has important implications for strategies to build soil fertility or mitigate climate change via increased soil organic C, as the availability and value of these nutrients must be considered.

Liang, C., Amelung, W., Lehmann, J. and Kästner, M., 2019. Quantitative assessment of microbial necromass contribution to soil organic matter. Global change biology, 25(11), pp.3578-3590. DOI PDF Abstract: Soil carbon transformation and sequestration have received significant interest in recent years due to a growing need for quantitating its role in mitigating climate change. Even though our understanding of the nature of soil organic matter has recently been substantially revised, fundamental uncertainty remains about the quantitative importance of microbial necromass as part of persistent organic matter. Addressing this uncertainty has been hampered by the absence of quantitative assessments whether microbial matter makes up the majority of the persistent carbon in soil. Direct quantitation of microbial necromass in soil is very challenging because of an overlapping molecular signature with nonmicrobial organic carbon. Here, we use a comprehensive analysis of existing biomarker amino sugar data published between 1996 and 2018, combined with novel appropriation using an ecological systems approach, elemental carbon–nitrogen stoichiometry, and biomarker scaling, to demonstrate a suit of strategies for quantitating the contribution of microbe‐derived carbon to the topsoil organic carbon reservoir in global temperate agricultural, grassland, and forest ecosystems. We show that microbial necromass can make up more than half of soil organic carbon. Hence, we suggest that next‐generation field management requires promoting microbial biomass formation and necromass preservation to maintain healthy soils, ecosystems, and climate. Our analyses have important implications for improving current climate and carbon models, and helping develop management practices and policies.

Tipping, E., Somerville, C.J. and Luster, J., 2016. The C:N:P:S stoichiometry of soil organic matter. Biogeochemistry, 130(1-2), pp.117-131. PDF Abstract: The formation and turnover of soil organic matter (SOM) includes the biogeochemical processing of the macronutrient elements nitrogen (N), phosphorus (P) and sulphur (S), which alters their stoichiometric relationships to carbon (C) and to each other. We sought patterns among soil organic C, N, P and S in data for c. 2000 globally distributed soil samples, covering all soil horizons. For non-peat soils, strong negative correlations (p < 0.001) were found between N:C, P:C and S:C ratios and % organic carbon (OC), showing that SOM of soils with low OC concentrations (high in mineral matter) is rich in N, P and S. The results can be described approximately with a simple mixing model in which nutrient-poor SOM (NPSOM) has N:C, P:C and S:C ratios of 0.039, 0.0011 and 0.0054, while nutrient-rich SOM (NRSOM) has corresponding ratios of 0.12, 0.016 and 0.016, so that P is especially enriched in NRSOM compared to NPSOM. The trends hold across a range of ecosystems, for topsoils, including O horizons, and subsoils, and across different soil classes. The major exception is that tropical soils tend to have low P:C ratios especially at low N:C. We suggest that NRSOM comprises compounds selected by their strong adsorption to mineral matter. The stoichiometric patterns established here offer a new quantitative framework for SOM classification and characterisation, and provide important constraints to dynamic soil and ecosystem models of carbon turnover and nutrient dynamics.

Urbina, I., Grau, O., Sardans, J., Ninot, J.M. and Peñuelas, J., 2020. Encroachment of shrubs into subalpine grasslands in the Pyrenees changes the plant-soil stoichiometry spectrum. Plant and Soil, pp.1-17. DOI Abstract - Aims: Shrub encroachment has been reported over a large proportion of the subalpine grasslands across Europe and is expected to have an important impact on the biogeochemical cycle of these ecosystems. We investigated the stoichiometric changes in the plant-soil system along the succession (e.g. increase in encroachment from unencroached grassland to mature shrubland) at two contrasting sites in the Pyrenees. Methods: We analyzed the chemical composition (C, N,15N, P, K, Ca, Mg and Fe) in the soil and in the aboveground plant compartments (leaves, leaf-litter and stems) of the main herbaceous species and shrubs at three contrasting stages of the succession: unencroached grassland, young shrubland and mature shrubland. Results: The plant-soil stoichiometry spectrum differed between the successional stages. Shrub encroachment generally increased the concentration of C and Ca and the C:N ratio and often reduced to concentrations of N, P and K in the leaves and leaf-litter, while several soil nutrient concentrations (N, P, K Ca and Mg) decreased. The stocks of C, N, P, Ca, and Mg in the total aboveground biomass increased with encroachment.

Fractions of soil organic matter
Lavallee, J.M., Soong, J.L. and Cotrufo, M.F., 2020. Conceptualizing soil organic matter into particulate and mineral‐associated forms to address global change in the 21st century. Global Change Biology, 26(1), pp.261-273. PDF Abstract: Managing soil organic matter (SOM) stocks to address global change challenges requires well‐substantiated knowledge of SOM behavior that can be clearly communicated between scientists, management practitioners, and policy makers. However, SOM is incredibly complex and requires separation into multiple components with contrasting behavior in order to study and predict its ynamics. Numerous diverse SOM separation schemes are currently used, making cross‐study comparisons difficult and hindering broad‐scale generalizations. Here, we recommend separating SOM into particulate (POM) and mineral‐associated (MAOM) forms, two SOM components that are fundamentally different in terms of their formation, persistence, and functioning. We provide evidence of their highly contrasting physical and chemical properties, mean residence times in soil, and responses to land use change, plant litter inputs, warming, CO2 enrichment, and N fertilization. Conceptualizing SOM into POM versus MAOM is a feasible, well‐supported, and useful framework that will allow scientists to move beyond studies of bulk SOM, but also use a consistent separation scheme across studies. Ultimately, we propose the POM versus MAOM framework as the best way forward to understand and predict broad‐scale SOM dynamics in the context of global change challenges and provide necessary recommendations to managers and policy makers.

Olk, D.C., Bloom, P.R., Perdue, E.M., McKnight, D.M., Chen, Y., Farenhorst, A., Senesi, N., Chin, Y.P., Schmitt-Kopplin, P., Hertkorn, N. and Harir, M., 2019. Environmental and agricultural relevance of humic fractions extracted by alkali from soils and natural waters. Journal of Environmental Quality, 48(2), pp.217-232. DOI PDF Abstract: To study the structure and function of soil organic matter, soil scientists have performed alkali extractions for soil humic acid (HA) and fulvic acid (FA) fractions for more than 200 years. Over the last few decades aquatic scientists have used similar fractions of dissolved organic matter, extracted by resin adsorption followed by alkali desorption. Critics have claimed that alkaliextractable fractions are laboratory artifacts, hence unsuitable for studying natural organic matter structure and function in field conditions. In response, this review first addresses specific conceptual concerns about humic fractions. Then we discuss several case studies in which HA and FA were extracted from soils, waters, and organic materials to address meaningful problems across diverse research settings. Specifically, one case study demonstrated the importance of humic substances for understanding transport and bioavailability of persistent organic pollutants. An understanding of metal binding sites in FA and HA proved essential to accurately model metal ion behavior in soil and water. In landscape-based studies, pesticides were preferentially bound to HA, reducing their mobility. Compost maturity and acceptability of other organic waste for land application were well evaluated by properties of HA extracted from these materials. A young humic fraction helped understand N cycling in paddy rice (Oryza sativa L.) soils, leading to improved rice management. The HA and FA fractions accurately represent natural organic matter across multiple environments, source materials, and research objectives. Studying them can help resolve important scientific and practical issues. Core Ideas:
 * Humic substances have long been extracted from soils, waters, and organic materials.
 * Their chemical composition has well represented that of natural organic matter.
 * Compost maturation and composition are well represented by their properties.
 * Soil humic studies elucidated metal and organic xenobiotic binding and nutrient cycling.
 * Their quantities and composition in soil respond to field treatments.

Carbon sequestration and soil organic matter
Alemu, A.W., Kröbel, R., McConkey, B.G. and Iwaasa, A.D., 2019. Effect of increasing species diversity and grazing management on pasture productivity, animal performance, and soil carbon sequestration of re-established pasture in Canadian Prairie. Animals, 9(4), p.127. DOI PDF Simple Summary Canadian grasslands are recognized for providing high quality forage for grazing livestock and wildlife. The study was conducted on a re-established pasture in aWestern Canadian semi-arid climate to investigate the effect of pasture species mixture and grazing management on pasture productivity, animal performance, and soil carbon sequestration. Pasture productivity and animal response were independent of pasture mixture but affected by grazing management. Average pasture dry matter productivity was greater with deferred-rotational grazing while pasture quality and animal gain were higher with continuous grazing. Soil carbon change varied with pasture seed mixture and grazing management interaction where pasture with 7-species mixture under continuous grazing had the lowest soil carbon gain.

Yang, Y., Tilman, D., Furey, G. and Lehman, C., 2019. Soil carbon sequestration accelerated by restoration of grassland biodiversity. Nature communications, 10(1), pp.1-7. DOI PDF Summary Agriculturally degraded and abandoned lands can remove atmospheric CO2 and sequester it as soil organic matter during natural succession. However, this process may be slow, requiring a century or longer to re-attain pre-agricultural soil carbon levels. Here, we find that restoration of late-successional grassland plant diversity leads to accelerating annual carbon storage rates that, by the second period (years 13–22), are 200% greater in our highest diversity treatment than during succession at this site, and 70% greater than in monocultures. The higher soil carbon storage rates of the second period (years 13–22) are associated with the greater aboveground production and root biomass of this period, and with the presence of multiple species, especially C4 grasses and legumes. Our results suggest that restoration of high plant diversity may greatly increase carbon capture and storage rates on degraded and abandoned agricultural lands.

Oldfield, E.E., Bradford, M.A. and Wood, S.A., 2019. Global meta-analysis of the relationship between soil organic matter and crop yields. Soil, 5(1), pp.15-32. DOI PDF Abstract Resilient, productive soils are necessary to sustainably intensify agriculture to increase yields while minimizing environmental harm. To conserve and regenerate productive soils, the need to maintain and build soil organic matter (SOM) has received considerable attention. Although SOM is considered key to soil health, its relationship with yield is contested because of local-scale differences in soils, climate, and farming systems. There is a need to quantify this relationship to set a general framework for how soil management could potentially contribute to the goals of sustainable intensification. We developed a quantitative model exploring how SOM relates to crop yield potential of maize and wheat in light of co-varying factors of management, soil type, and climate. We found that yields of these two crops are on average greater with higher concentrations of SOC (soil organic carbon). However, yield increases level off at ∼ 2 % SOC. Nevertheless, approximately two-thirds of the world’s cultivated maize and wheat lands currently have SOC contents of less than 2 %. Using this regression relationship developed from published empirical data, we then estimated how an increase in SOC concentrations up to regionally specific targets could potentially help reduce reliance on nitrogen (N) fertilizer and help close global yield gaps. Potential N fertilizer reductions associated with increasing SOC amount to 7 % and 5 % of global N fertilizer inputs across maize and wheat fields, respectively. Potential yield increases of 10 ± 11 % (mean ± SD) for maize and 23 ± 37 % for wheat amount to 32 % of the projected yield gap for maize and 60 % of that for wheat. Our analysis provides a global-level prediction for relating SOC to crop yields. Further work employing similar approaches to regional and local data, coupled with experimental work to disentangle causative effects of SOC on yield and vice versa, is needed to provide practical prescriptions to incentivize soil management for sustainable intensification.

Rasmussen, C., Heckman, K., Wieder, W.R., Keiluweit, M., Lawrence, C.R., Berhe, A.A., Blankinship, J.C., Crow, S.E., Druhan, J.L., Pries, C.E.H. and Marin-Spiotta, E., 2018. Beyond clay: towards an improved set of variables for predicting soil organic matter content. Biogeochemistry, 137(3), pp.297-306. DOI PDF Abstract Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.

Sanderman, J., Hengl, T. and Fiske, G.J., 2017. Soil carbon debt of 12,000 years of human land use. Proceedings of the National Academy of Sciences, 114(36), pp.9575-9580. DOI PDF Significance "Land use and land cover change has resulted in substantial losses of carbon from soils globally, but credible estimates of how much soil carbon has been lost have been difficult to generate. Using a data-driven statistical model and the History Database of the Global Environment v3.2 historic land-use dataset, we estimated that agricultural land uses have resulted in the loss of 133 Pg C from the soil. Importantly, our maps indicate hotspots of soil carbon loss, often associated with major cropping regions and degraded grazing lands, suggesting that there are identifiable regions that should be targets for soil carbon restoration efforts." Abstract "Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes “grazing” and “cropland” contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts."

Woolf, D. and Lehmann, J., 2019. Microbial models with minimal mineral protection can explain long-term soil organic carbon persistence. Scientific reports, 9(1), pp.1-8. DOI PDF Abstract Soil organic carbon (SOC) models currently in widespread use omit known microbial processes, and assume the existence of a SOC pool whose intrinsic properties confer persistence for centuries to millennia, despite evidence from priming and aggregate turnover that cast doubt on the existence of SOC with profound intrinsic stability. Here we show that by including microbial interactions in a SOC model, persistence can be explained as a feedback between substrate availability, mineral protection and microbial population size, without invoking an unproven pool that is intrinsically stable for centuries. The microbial SOC model based on this concept reproduces long-term data (r2 = 0.92; n = 90), global SOC distribution (rmse = 4.7 +/− 0.6 kg C m−2), and total global SOC in the top 0.3 m (822 Pg C) accurately. SOC dynamics based on a microbial feedback without stable pools are thus consistent with global SOC distribution. This has important implications for carbon management, suggesting that relatively fast cycling, rather than recalcitrant, SOC must form the primary target of efforts to build SOC stocks.

Soil organic matter and agriculture
Obour, P.B., Jensen, J.L., Lamandé, M., Watts, C.W. and Munkholm, L.J., 2018. Soil organic matter widens the range of water contents for tillage. Soil and Tillage Research, 182, pp.57-65. DOI PDF Abstract: The effects of soil organic matter on the water contents for tillage were investigated by sampling soils with a uniform texture, but a range of soil organic carbon (SOC) from two long-term field experiments at Highfield in Rothamsted Research, UK and Askov Experimental Station, Denmark. The treatments studied in Highfield were Bare fallow (BF), Continuous arable rotation (A), Ley-arable (LA) and Grass (G); and in Askov: unfertilized (UNF), ½ mineral fertilizer (½ NPK), 1 mineral fertilizer (1NPK), and 1½ animal manure (1½AM). Minimally disturbed soil cores (100 cm3) were sampled per plot in both locations from 6 to 10 cm depth to generate water retention data. Soil blocks were also sampled at 6–15 cm depth to determine basic soil properties and to measure soil aggregate strength parameters. The range of soil water contents appropriate for tillage were determined using the water retention and the consistency approaches. SOC content in Highfield was in the order: G > LA = A > BF, and in Askov: 1½ AM > 1NPK = ½NPK > UNF. Results showed that different long-term management of the silt loam Highfield soil, and fertilization of the sandy loam Askov soil affected the mechanical properties of the soils— for Highfield soil, aggregates from the G treatment were stronger in terms of rupture energy when wet (−100 hPa matric potential) than the BF treatment. As the soil dried (−300 and −1000 hPa matric potentials), soil aggregates from the G treatment were relatively weaker and more elastic than the BF soil. Our study showed, for both Highfield and Askov soils, a strong positive linear increase in the range of water contents for tillage with increasing contents of SOC. This suggests that management practices leading to increased SOC can improve soil workability by increasing the range of water contents for tillage. We recommended using the consistency approach over the water retention approach for determining the range of water contents for tillage because it seems to give realistic estimates of the water contents for tillage.