Mineral-associated Organic Matter Research Articles

Adaptive multi-paddock grazing enhances soil carbon and nitrogen stocks and stabilization through mineral association in southeastern U.S. grazing lands

Grassland soils are a large reservoir of soil carbon (C) at risk of loss due to overgrazing in conventional grazing systems. By promoting regenerative grazing management practices that aim to increase soil C storage and soil health, grasslands have the potential to help alleviate rising atmospheric CO2 as well as sustain grass productivity across a vast area of land. Previous research has shown that rotational grazing, specifically adaptive multi-paddock (AMP) grazing that utilizes short-duration rotational grazing at high stocking densities, can increase soil C stocks in grassland ecosystems, but the extent and mechanisms are unknown. We conducted a large-scale on-farm study on five “across the fence” pairs of AMP and conventional grazing (CG) grasslands covering a spectrum of southeast United States grazing lands. We quantified soil C and nitrogen (N) stocks, their isotopic and Fourier-transform infrared spectroscopy signatures as well as their distribution among soil organic matter (SOM) physical fractions characterized by contrasting mechanisms of formation and persistence in soils. Our findings show that the AMP grazing sites had on average 13% (i.e., 9 Mg C ha−1) more soil C and 9% (i.e., 1 Mg N ha−1) more soil N compared to the CG sites over a 1 m depth. Additionally, the stocks’ difference was mostly in the mineral-associated organic matter fraction in the A-horizon, suggesting long-term persistence of soil C in AMP grazing farms. The higher N stocks and lower 15N abundance of AMP soils also point to higher N retention in these systems. These findings provide evidence that AMP grazing is a management strategy to sequester C in the soil and retain N in the system, thus contributing to climate change mitigation.

Contribution of Particulate and Mineral-Associated Organic Matter to Potential Denitrification of Agricultural Soils

Water-extractable organic carbon (WEOC) is considered as the most important carbon (C) source for denitrifying organisms, but the contribution of individual organic matter (OM) fractions (i.e., particulate (POM) and mineral-associated (MOM)) to its release and, thus, to denitrification remains unresolved. Here we tested short-time effects of POM and MOM on potential denitrification and estimated the contribution of POM- and MOM-derived WEOC to denitrification and CO2 production of three agricultural topsoils. Suspensions of bulk soils with and without addition of soil-derived POM or MOM were incubated for 24 h under anoxic conditions. Acetylene inhibition was used to determine the potential denitrification and respective product ratio at constant nitrate supply. Normalized to added OC, effects of POM on CO2 production, total denitrification, and its product ratios were much stronger than those of MOM. While the addition of OM generally increased the (N2O + N2)-N/CO2-C ratio, the N2O/(N2O + N2) ratio changed differently depending on the soil. Gas emissions and the respective shares of initial WEOC were then used to estimate the contribution of POM and MOM-derived WEOC to total CO2, N2O, and N2O + N2 production. Water-extractable OC derived from POM accounted for 53–85% of total denitrification and WEOC released from MOM accounted for 15–47%. Total gas emissions from bulk soils were partly over- or underestimated, mainly due to nonproportional responses of denitrification to the addition of individual OM fractions. Our findings show that MOM plays a role in providing organic substrates during denitrification but is generally less dominant than POM. We conclude that the denitrification potential of soils is not predictable based on the C distribution over POM and MOM alone. Instead, the source strength of POM and MOM for WEOC plus the WEOC’s quality turned out as the most decisive determinants of potential denitrification.

Erosion effects on soil carbon and nitrogen dynamics on cultivated slopes: A meta-analysis

Soil carbon (C) redistribution within cultivated landscapes is strongly controlled by soil erosion and sedimentation and it is widely appreciated that C is preferentially transported during erosion. In contrast, it remains elusive whether erosion induced transport of C and N is coupled although changes in the balance between C and N strongly affect soil C and N dynamics. We therefore reviewed the literature on carbon and nitrogen redistribution by erosion.Twenty-nine studies reported results on C and N enrichment of freshly eroded sediments after erosion events. Thirty-nine studies reported results on C and N contents and stocks along eroded slopes.Eroded sediments were enriched in C and N by 51.3% and 50.6% indicating that both elements are stored in soil fractions that are preferentially eroded. Slope gradient and soil texture strongly affected C and N enrichment. Decreasing C and N enrichment in fine-textured soils was counterbalanced by increasing erosion rates in these soils. This suggests similar SOM losses independent of textural class. The C/N ratio increased by 9.9%, pointing to preferential movement of C-rich particulate organic matter (POM) compared to N-rich mineral associated organic matter (MAOM). Breakdown of aggregates by rainfall energy possibly released POM which is then preferentially eroded. Soil C and N contents and total stocks showed similar percentage increases from upslope to depositional sites, indicating that downslope C and N stocks were largely driven by enrichment, rather by than increases in depth or bulk density.Altogether, our findings confirm that quantification of soil loss alone is not sufficient to estimate erosion-induced changes in soil fertility, because soil organic matter and plant nutrients are selectively moved during erosion. This leads to a shift in C and N dynamics in different slope positions and thus to an increase in the spatial variability of the C and N along the slope.

The advancing mycelial frontier of ectomycorrhizal fungi.

This article is a Commentary on Hagenbo et al. (2021), 230: 1609–1622.

Iron oxides and aluminous clays selectively control soil carbon storage and stability in the humid tropics

Clay minerals and pedogenic metal (oxyhydr)oxides are the most reactive soil mineral constituents controlling the long-term persistence of organic carbon (OC) in terrestrial ecosystems. However, their co-occurrence in most soils complicates direct assessment of their individual contribution to OC persistence. Making use of unique mineralogical combinations in soils located in the East Usambara Mountains of Tanzania, we disentangled the contribution of clay-sized aluminous minerals (kaolinite, gibbsite) and pedogenic Fe (oxyhydr)oxides (predominant goethite and hematite) on OC storage and stabilization under natural forests and croplands. Topsoil samples, varying in contents but not types of aluminous clays and pedogenic Fe (oxyhydr)oxides, were identified by selective extractions, X-ray diffraction, and Mössbauer spectroscopy. Associated abundance of particulate and mineral-associated organic matter (OM) was quantified by density fractionation and their changes during land-use conversion were determined as a measure of OC persistence. Additionally, we assessed the resistance of OC to chemical oxidation as well as microbial decomposition in a 50-day laboratory incubation. We found that the ratio of pedogenic Fe to aluminous clay is more consequential for OC storage and stabilization than their individual contents, despite the fact that Fe (oxyhydr)oxides generally exert a stronger impact on OC than aluminous clays. Conjunction of large amounts of Fe (oxyhydr)oxides with low aluminous clay contents caused the strongest accumulation of mineral-associated OC, a low soil respiration, high OC stability against chemical oxidation, and high OC persistence during land-use change. Our study suggests that certain mineralogical combinations in the humid tropics alleviate OM losses during land conversion because of the strong and selective mineral control on OC stabilization, particular if the weight ratio of pedogenic Fe to aluminous clay exceeds the threshold range of 0.44‒0.56.

Retaining eucalyptus harvest residues promotes different pathways for particulate and mineral‐associated organic matter

AbstractEucalyptus plantations have replaced other (agro)ecosystems over 5.6 Mha in Brazil. While these plantations rapidly accumulate carbon (C) in their biomass, the C storage in living forest biomass is transient, and thus, longer‐term sustainability relies on sustaining soil organic matter (SOM) stocks. A significant amount of harvest residues (HR) is generated every rotation and can yield SOM if retained in the field. Yet, there is little information on how managing eucalyptus HR changes SOM dynamics. We used isotopic and molecular approaches in a 3‐yr field decomposition experiment where a native grassland has been replaced by eucalyptus plantations to assess how HR management practices influence content and chemistry of two distinct SOM fractions [particulate (POM) and mineral‐associated organic matter (MAOM)] at two soil depths (0–1 and 1–5 cm). The management practices investigated were HR removal (−R), only bark removal (−B), and retention of all HR (including bark, +B), combined with two levels of nitrogen (N) fertilization [0 (−N) and 200 (+N) kg/ha]. N fertilization inhibited HR decomposition (P = 0.0409), while bark retention had little effect (P = 0.1164). Retaining HR, especially with bark, increased POM‐C and MAOM‐C content (2.1‐ and 1.2‐fold, respectively), decreased POM‐δ13C (1.2‐fold), and increased inorganic N retention (1.7‐fold) compared with plots where HR had been removed. Inorganic N applications, however, diminished the positive impacts of bark retention. Although the influence of HR management was most pronounced in POM, retaining HR reduced potential soil C mineralization by up to 20%. POM and MAOM chemistry shifted over time and revealed distinct influence of HR on the formation of these fractions. We demonstrate that HR management alters SOM dynamics and that retaining HR, particularly including bark, enhances SOM retention. With continuing conversion of native grassland ecosystems to eucalyptus, long‐term sustainability will require careful HR and fertilizer management to balance total biomass harvest with sustaining belowground SOM concentrations.

A combined microbial and ecosystem metric of carbon retention efficiency explains land cover-dependent soil microbial biodiversity–ecosystem function relationships

While soil organic carbon (C) is the foundation of productive and healthy ecosystems, the impact of the ecology of microorganisms on C-cycling remains unknown. We manipulated the diversity, applied here as species richness, of the microbial community present in similar soils on two contrasting land-covers—an adjacent pasture and forest—and observed the transformations of plant detritus and soil organic matter (SOM) using stable isotope (13C) tracing coupled with a novel nuclear magnetic resonance (NMR) experiment. The amount of detritus-C degraded was not affected by the microbial diversity (p > 0.05), however the fate of detritus- and SOM-C across the diversity gradient was complex and land cover-dependent. For example, in the pasture soil, higher diversity led to lower CO2 production (p = 0.001), a trend driven solely by SOM-C mineralization. There was no relationship between diversity and detritus-C mineralization or production of new mineral-associations after one year (p > 0.05). In contrast, in the forest soil higher diversity resulted in increased detritus-C (p = 0.01) and SOM-C (p = 0.0008) mineralization and decreased mineral-associated organic matter formation (p = 0.02). In both land cover types, retention efficiency—a measure that integrates both microbial physiology and the ability of the ecosystem to retain C—explained C loss and transformation trends. Overall, this demonstrates that the trajectory of C gained and lost is altered by land management-induced changes to microbial communities, soil structure, and chemical characteristics underlying SOM persistence.

Simple Plant and Microbial Exudates Destabilize Mineral-Associated Organic Matter via Multiple Pathways

Most mineral-associated organic matter (MAOM) is protected against microbial attack, thereby contributing to long-term carbon storage in soils. However, the extent to which reactive compounds released by plants and microbes may destabilize MAOM and so enhance microbial access, as well as the underlying mechanisms, remain unclear. Here, we tested the ability of functionally distinct model exudates-ligands, reductants, and simple sugars-to promote microbial utilization of monomeric MAOM, bound via outer-sphere complexes to common iron and aluminum (hydr)oxide minerals. The strong ligand oxalic acid induced rapid MAOM mineralization, coinciding with greater sorption to and dissolution of minerals, suggestive of direct MAOM mobilization mechanisms. In contrast, the simple sugar glucose caused slower MAOM mineralization, but stimulated microbial activity and metabolite production, indicating an indirect microbially-mediated mechanism. Catechol, acting as reductant, promoted both mechanisms. While MAOM on ferrihydrite showed the greatest vulnerability to both direct and indirect mechanisms, MAOM on other (hydr)oxides was more susceptible to direct mechanisms. These findings suggest that MAOM persistence, and thus long-term carbon storage within a given soil, is not just a function of mineral reactivity but also depends on the capacity of plant roots and associated microbes to produce reactive compounds capable of triggering specific destabilization mechanisms.

Enhanced and selective phototransformation of chlorophene on aluminum hydroxide-humic complexes

Mineral-humic complexes, known as mineral-associated organic matter (MAOM), are ubiquitous in natural waters. However, the interaction between organic pollutants and MAOM remains elusive, which may affect their degradation process. In this study, photochemical transformation of chlorophene (CP) in the presence of MAOM, prepared by coating aluminum hydroxide with humic acid (HA-HAO), was investigated. Our results showed that the degradation of CP was significantly enhanced in the presence of HA-HAO, and the degradation rate constant was ~5 times as that with HA only. It was because the adsorption of CP to HA-HAO particles was greatly enhanced, and concentration of reactive oxygen species (ROS) was increased on HA-HAO surfaces, which further promoted the reactions between CP and ROS. The quenching experiments combined with EPR technology confirmed that superoxide anion (O2·–) was the primary reactive radical on CP photodegradation. More importantly, the degradation of CP with HA-HAO followed a hydroxylation process, rather than the oligomerization reaction with HA only. Spectroscopic analysis provided direct evidence for the formation of hydrogen bonding between CP phenolic hydroxyl group and surface oxygen of HAO, which would suppress the reactivity of phenolic hydroxyl group, consequently the ortho- and meta-positions of CP became more facile for the hydroxylation reaction. This study shows the importance of MAOM in altering the photochemical behavior and transformation pathway of organic contaminants.

Particles under stress: ultrasonication causes size and recovery rate artifacts with soil-derived POM but not with microplastics

Abstract. The breakdown of soil aggregates and the extraction of particulate organic matter (POM) by ultrasonication and density fractionation is a method widely used in soil organic matter (SOM) analyses. It has recently also been used for the extraction of microplastic from soil samples. However, the investigation of POM physiochemical properties and ecological functions might be biased if particles are comminuted during the treatment. In this work, different types of POM, which are representative of different terrestrial ecosystems and anthropogenic influences, were tested for their structural stability in the face of ultrasonication in the range of 0 to 500 J mL−1. The occluded particulate organic matter (oPOM) of an agricultural and forest soil as well as pyrochar showed a significant reduction of particle size at ≥50 J mL−1 by an average factor of 1.37±0.16 and a concurrent reduction of recovery rates by an average of 21.7±10.7 % when being extracted. Our results imply that increasing ultrasonication causes increasing retention of POM within the sedimenting phase, leading to a misinterpretation of certain POM fractions as more strongly bound oPOM or part of the mineral-associated organic matter (MOM). This could, for example, lead to a false estimation of physical stabilization. In contrast, neither fresh nor weathered polyethylene (PE), polyethylene terephthalate (PET) and polybutylene adipate terephthalate (PBAT) microplastics showed a reduction of particle size or the recovery rate after application of ultrasound. We conclude that ultrasonication applied to soils has no impact on microplastic size distribution and thus provides a valuable tool for the assessment of microplastics in soils and soil aggregates.

Altered litter inputs modify carbon and nitrogen storage in soil organic matter in a lowland tropical forest

Soil organic matter (SOM) in tropical forests is an important store of carbon (C) and nutrients. Although SOM storage could be affected by global changes via altered plant productivity, we know relatively little about SOM stabilisation and turnover in tropical forests compared to temperate systems. Here, we investigated changes in soil C and N within particle size fractions representing particulate organic matter (POM) and mineral-associated organic matter (MAOM) after 13 years of experimental litter removal (L−) and litter addition (L+) treatments in a lowland tropical forest. We hypothesized that reduced nitrogen (N) availability in L− plots would result in N-mining of MAOM, whereas long-term litter addition would increase POM, without altering the C:N ratio of SOM fractions. Overall, SOM-N declined more than SOM-C with litter removal, providing evidence of N-mining in the L− plots, which increased the soil C:N ratio. However, contrary to expectations, the C:N ratio increased most in the largest POM fraction, whereas the C:N ratio of MAOM remained unchanged. We did not observe the expected increases in POM with litter addition, which we attribute to rapid turnover of unprotected SOM. Measurements of ion exchange rates to assess changes in N availability and soil chemistry revealed that litter removal increased the mobility of ammonium-N and aluminium, whereas litter addition increased the mobility of nitrate-N and iron, which could indicate SOM priming in both treatments. Our study suggests that altered litter inputs affect multiple processes contributing to SOM storage and we propose potential mechanisms to inform future work.

315 Formation, persistence, and function of soil organic matter: how recent scientific advances apply in agroecosystems

Abstract Soil organic matter is fundamental to healthy and productive soils and building it is an effective means by which to draw down atmospheric greenhouse gas concentrations with added co-benefits. Scientific understanding of soil organic matter dynamics is constantly evolving, and the past decade has seen major advances and paradigm shifts. Soil organic matter creation from decaying plant litter is now thought to occur under two separate pathways, yielding two functionally different types: predominantly plant-derived, unprotected particulate organic matter (POM) and predominantly microbially-derived, mineral-associated organic matter (MAOM). The idea of naturally-occurring humic substances in soils has been largely abandoned, and long-term soil organic matter persistence is now understood to be driven mainly by mineral association. We will present the research behind these paradigm shifts, and show how considering POM and MAOM separately is key to understanding the mechanisms driving carbon accrual and persistence in soil, and therefore to guiding policy and management for soil carbon sequestration. We will present drivers of POM and MAOM contents, from individual fields to continents, including their capacity for sequestration and saturation in agricultural soils of the USA, and their responses to common management practices in agroecosystems.

Less efficient residue‐derived soil organic carbon formation under no‐till irrigated corn

AbstractSoil organic carbon (SOC) stores a large portion of terrestrial C, yet the mechanisms that affect its formation efficiency under different residue management in irrigated systems are still not well understood. No‐till (NT) increases SOC concentrations in topsoil compared with conventional tillage (CT) but there is uncertainty surrounding the stability of these gains in irrigated systems. We investigated the effect of NT and CT residue management on CO2 loss and SOC formation by applying isotopically (13C) labeled residues on the surface or incorporated in the mineral soil of disturbed (i.e., CT) or undisturbed (i.e., NT) soil in a semiarid, NT, sprinkler‐irrigated, continuous corn (Zea mays L.) field. We measured residue‐derived C in CO2, bulk soil, dissolved organic C, particulate organic matter (POM), and silt‐ and clay‐sized mineral‐associated organic matter (MAOM) fractions. Twelve months after residue addition, soils with surface‐applied residue produced 19% more CO2 and formed 41% less SOC than residue‐incorporated soils. Across all treatments, 62% of residue‐derived C was recovered in POM, and 38% in the MAOM fractions. Residue‐derived bulk soil C formation was more efficient when the residue was incorporated (0.60) rather than surface‐applied (0.42), suggesting that mixing and burial of residue within the soil matrix is a key pathway of SOC formation. During the growing season, surface residue under NT management in irrigated systems is particularly vulnerable to decomposition and will require additional conservation strategies to generate long‐term C sequestration.

Sources of soil carbon loss during soil density fractionation: Laboratory loss or seasonally variable soluble pools?

To model global C budgets more accurately, we must better understand the dynamics and turnover of the many functional C pools that exist in soil. While soil density fractionation (SDF) is widely used to separate soil C pools based on the degree of stabilization by soil minerals, several studies have noted substantial losses of soil mass and soil C following SDF. As the source of these losses is unknown, they may lead to erroneous conclusions about SOM dynamics and inaccuracies in models of soil processes and global C cycling. For example, lab handling techniques such as air-drying soil could have an impact on soluble C losses. Alternatively, the observed C losses could represent pools of potentially soluble C that can vary by ecosystem and season, and thus might represent ephemeral, not well stabilized, and generally overlooked pools of soil C that are missing from SOM budgets and models. For example, in summer-dry climates such as the Pacific Northwest, soluble organic products of decomposition could accumulate over the summer when temperatures and microbial activities are high and precipitation is low, and subsequently leach during the fall and winter with precipitation. To address these divergent possibilities, soils were collected seasonally from a summer-dry forest in Oregon, and subsamples were subjected to 4 different laboratory handling procedures prior to fractionation: 1) air-drying and 2) oven-drying to simulate common laboratory drying techniques; 3) leaching to evaluate if potentially soluble C represented a pool that could be removed with simulated precipitation; and 4) immediate fractionation of fresh soil to determine if drying of soil caused artifacts in pools of potentially soluble soil C. Contrary to initial hypotheses, there were no seasonal trends to soluble DOC pools or total C loss during fractionation. Average mass loss during fractionation was 6% of initial dry weight and total C loss was 9% of total soil C. Soluble losses represented only 9% of total soil C loss. Particulate, or non-soluble mass loss was dominated by the high C:N free particulate organic matter, or LF. Thus, most C loss in this system was due to laboratory losses and that loss was biased towards an underestimation of LF matter in soil. DOC was a small enough component of loss that soil preparation prior to SDF was not significant. These results imply that in temperate soils, even in seasonally extreme ecosystems, relative proportions of mineral-associated organic matter and free particulate organic matter are seasonally robust, air drying of soil does not introduce error, and that immediate fractionation after field collection is not critical.

Reductive release of Fe mineral-associated organic matter accelerated by oxalic acid

The properties and composition of soil dissolved organic matter (DOM) are highly affected by the adsorption and desorption of organic matter (OM) on soil minerals and heterotrophic microbial respiration. Organic acids (e.g., oxalic acid), components of root exudates, have been revealed to liberate organic matter (OM) by the dissolution of protective mineral phases and stimulate microbial degradation of OM. However, the effects of organic acids on the properties and composition of soil DOM molecules and the related mechanisms are still poorly understood. In this study, we conducted microcosm incubation experiments with and without oxalic acid addition, and aimed to elucidate the variations of DOM properties and composition, employing a combination of Fourier transform ion cyclotron resonance mass spectrometry, optical spectroscopy, and bacterial community composition analysis. Our results indicated that the released OM from the direct dissolution of protective mineral phases by oxalic acid further stimulated the microbial reductive release of Fe mineral-associated OM under anoxic conditions. Furthermore, the addition of oxalic acid enhanced the degradation of aliphatic compounds and lignins with low O/C ratios, and increased the accumulation of lignins with high O/C ratios, tannins, and condensed aromatics. Linking the bacterial community composition to DOM molecular properties and composition further suggested that the enhanced reductive release of Fe mineral-associated OM was highly related to the increased abundances of Proteobacteria and Actinobacteria. Overall, oxalic acid induced long-lasting impacts on soil DOM properties and composition under anoxic soil conditions in our study. We expect that our results will contribute to understanding the dynamics of soil DOM molecules in the environment.

Summer irrigation of pasture enhances the transfer and short-term storage of soil organic carbon in the particulate and mineral-associated organic matter fractions

Soil organic carbon (SOC) is both a source and sink of atmospheric carbon dioxide (CO2), with important implications for global climate change. Irrigation of grazed pastures has reportedly increased, reduced or made no difference to SOC stocks relative to dryland management. This study examined, over an annual plant growth cycle, the persistence of photosynthate-derived carbon (C) previously allocated to the plant–soil system during summer, under irrigated or dryland conditions. A continuous 13CO2 pulse labelling method was used to label ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) mesocosms under simulated dryland or irrigated conditions. Plant and soil 13C was traced over 349 days using destructive sampling on days 1, 12, 125, 237 and 349 (T1–T5 respectively). After the cessation of labelling and summer irrigation, the mesocosms were maintained under the same seasonal soil moisture conditions. The persistence of 13C in the aboveground plant C pools was lower than in the root C pool. Approximately 50% of the initial mass of 13C recovered in roots at T1 remained by T5. There was no difference between the summer irrigated and dryland treatments in terms of the 13C recovered from the soil over the subsequent annual growing season. There was also no significant change in the 13C recovered in the soil between T1 and T5. However, summer irrigation did affect the spatial and temporal distribution of the photosynthate-derived C within the soil size fractions relative to summer dryland conditions. Summer irrigation promoted the transfer and storage of 13C in the fine particulate organic matter and clay size fractions.

Volatile organic compounds from leaf litter decomposition alter soil microbial communities and carbon dynamics.

Investigations into the transfer of carbon from plant litter to underlying soil horizons have primarily focused on the leaching of soluble carbon from litter belowground or the mixing of litter directly into soil. However, previous work has largely ignored the role of volatile organic compounds (VOCs) released during litter decomposition. Unlike most leaf carbon, these litter-derived VOCs are able to diffuse directly into the soil matrix. Here, we used a 99-d microcosm experiment to track VOCs produced during microbial decomposition of 13 C-labeled leaf litter into soil carbon fractions where the decomposing litters were only sharing headspace with the soil samples, thus preventing direct contact and aqueous movement of litter carbon. We also determined the effects of these litter-derived VOCs on soil microbial community structure. We demonstrated that the litter VOCs contributed to all measured soil carbon pools. Specifically, VOC-derived carbon accounted for 2.0, 0.61, 0.18, and 0.08% of carbon in the microbial biomass, dissolved organic matter, mineral-associated organic matter, and particulate organic matter pools, respectively. We also show that litter-derived VOCs can affect soil bacterial and fungal community diversity and composition. These findings highlight the importance of an underappreciated pathway where VOCs alter soil microbial communities and carbon dynamics.

Eucalypt harvest residue management influences microbial community structure and soil organic matter fractions in an afforested grassland

Retaining harvest residues (HR) in the field is considered an option to recycle carbon (C) and nutrients in short-rotation plantations, but the fate of HR in the soil remains unclear. The effects of HR management and nitrogen (N) availability on soil organic matter (SOM) pools and microbial community structure of forest plantations represent a major knowledge gap. To fill this gap, we explored how management scenarios that differed in the amount and type of HR [removal of all residues (-R), residue retention without (+R/-B), or with bark (+R/+B)] and N availability [0 (-N) or 200 kg N ha-1 (+N)] influenced microbial activity and structure and SOM fractions in a recently afforested grassland. Specifically, we measured HR decomposition, carbon dioxide efflux (respired CO2), phospholipid fatty acid (PLFA), and determined changes in particulate (POM) and mineral-associated organic matter (MAOM) over 12 months using differences in δ13C natural abundance between Eucalyptus HR and grassland soils (δ13C ∼-28 ‰ and -13 ‰, respectively). Microbial respiration was stimulated by HR retention (+R). Bark retention reduced HR half-life by ∼70 days, on average, while N had little influence. Bacterial groups (Gram-positive and Gram-negative) were the primary decomposer of eucalypt HR, while Actinobacteria used more of the former soil organic carbon (SOC). +R/+B increased fungal biomarker PLFA concentration and fungal:bacterial ratio, suggesting a key role of Fungi in the fragmentation of woody HR. N influence on microbial community structure and SOM pools was dependent upon HR management. Retaining HR increased SOC concentrations, most significantly in the 0–1 cm soil depth and POM fraction, and when bark was included. +R/+B resulted in higher POM-C concentrations (up to +37%) and more depleted δ13C-POM compared with -R and +R/-B, respectively. Collectively, our results suggest it is feasible to increase POM-C and fungal abundance through HR management practices in the early stages of decomposition, which may potentially contribute to SOC stabilization in the long-term. Yet our findings remain to be tested in long-term studies, we provide quantitative evidence of the potential of a more conservative HR management to contribute to the sustainability of eucalypt plantations.

Chemical fractionation of organic matter and organic phosphorus extractions from freshwater lake sediment

Lake sediment organic matter (OM) is composed of a variety of organic compounds differing in their biolability and origin. Sources of sediment OM can include terrestrial input from the watershed and algal/microbial metabolic byproducts residing in the water column or sediment. Dissolved organic phosphorus (DOP) is a critical component of OM in freshwater eutrophic lakes, often acting as a source for bioavailable phosphorus that fuels harmful algal and/or cyanobacterial blooms. Parallel extractions of lake sediment collected from Missisquoi Bay, a eutrophic bay in Lake Champlain, were conducted with the goal of identifying OM and organic P sediment constituents using ultrahigh-resolution mass spectrometry from various extractants. Extractants converged into two groups based on the characteristics of their extracted OM; “stronger extractants” were composed of highly acidic and alkali media, while “milder extractants” represented weaker acids and bases. Sediment treated with the strong extractants afforded highly oxygenated and unsaturated OM thought to be stable with mostly lower heteroatomic content. In contrast, milder extractants yielded highly aliphatic and saturated compounds with lower masses and greater heteroatom functionally, sharing characteristics with labile molecules. Extracted organic P molecules mirrored the bulk OM in terms of lability, mass, and oxygenation within their corresponding extractants. Milder extractants resulted in greater organic P formulae assignments than the stronger extractants, with NaHCO3 resulting in the most aliphatic organic P formulae. We recommend the use of acetic acid to probe lake sediment for overall molecular characterization, spanning the greatest ranges of O/C and H/C ratios and representing both labile and mineral-associated OM. Other extractants should be implemented for a more targeted analysis. For instance, the use of NaHCO3 for organic P characterization, while using NaOH when interested in sediment geochemistry; both of which are critical for understanding the factors contributing to internal P loading.

Coarse mineral-associated organic matter is a pivotal fraction for SOM formation and is sensitive to the quality of organic inputs

Recent models of soil organic matter (SOM) formation propose that the coarse mineral-associated fraction (coarse-MAOM) could be a pivotal fraction bridging the gap between fresh organic matter inputs and stabilized, mineral-associated SOM. However, little is known about the effect of organic input chemistry and soil texture on the carbon (C) and nitrogen (N) dynamics of this fraction. We studied SOM dynamics in a factorial field experiment comparing 10 combinations of crop residue management (retained or exported) with different fertilizer sources, including solid poultry, liquid dairy and liquid swine manure, a mineral fertilizer (NPK), and a control without N (PK) in two soils with contrasted texture. After 9 y of management, we compared the distribution of C and N within the light fraction (LF), the coarse-MAOM, and the fine mineral-associated organic matter fraction (fine-MAOM) of surface (0–5 cm) soil. We found that retention of mature crop residues increased soil C content through an increase in LF and coarse-MAOM but did not influence N content in any fractions. Conversely, the fertilizer source influenced C and N accumulation in the coarse- and fine-MAOM in the silty clay soil where greater values were generally found with poultry and liquid dairy manures. Similar trends were found in the sandy loam soil, but statistically significant differences were only found for C in the coarse-MAOM fraction. The coarse-MAOM fraction had a much lower C:N ratio (15–16) than LF (26–36) and appeared generally responsive to the different organic inputs. Despite its limited size (∼10% of total soil organic carbon), coarse-MAOM appeared to represent a central location for the formation of new SOM and for the accumulation of C and N following fresh organic inputs. We therefore propose that it is considered in models as a pivotal fraction of SOM formation in arable soils.