Mineral-associated Organic Matter Research Articles

Unearthing the legacy of wildfires: post fire pyrogenic carbon and soil carbon persistence across complex Pacific Northwest watersheds

AbstractWildfires have the potential to dramatically alter the carbon (C) storage potential, ecological function, and the fundamental mechanisms that control the C balance of Pacific Northwest (PNW) forested ecosystems. In this study, we explored how wildfire influences processes that control soil C stabilization and the consequent soil C persistence, and the role of previous fire history in determining soil C fire response dynamics. We collected mineral soils at four depth increments from burned (low, moderate, and high soil burn severity classes) and unburned areas and surveyed coarse woody debris (CWD) in sites within the footprint of the 2020 Holiday Farm Fire and in surrounding Willamette National Forest and the H.J. Andrews Experimental Forest. We found few changes in overall soil C pools as a function of fire severity; we instead found that unburned sites contained high levels of pyrogenic C (PyC) that were commensurate with PyC concentrations in the high severity burn sites—pointing to the high background rate of fire in these ecosystems. An analysis of historical fire events lends additional support, where increasing fire count is loosely correlated with increasing PyC concentration. An unexpected finding was that PyC concentration was lower in low soil burn severity sites than in control sites, which we attribute to fundamental ecological differences in regions that repeatedly burn at high severity compared with those that burn at low severity. Our CWD analysis showed that high mean fire return interval (decades between fire events) was strongly correlated with low annual CWD accumulation rate; whereas areas that burn frequently had a high annual CWD accumulation rate. Within the first year postfire, trends in soil density fractions demonstrated no significant response to fire for the mineral-associated organic matter pool but slight increases in the particulate pool with increasing soil burn severity—likely a function of increased charcoal additions. Overall, our results suggest that these PNW forest soils display complex responses to wildfire with feedbacks between CWD pools that provide varying fuel loads and a mosaic fire regime across the landscape. Microclimate and historic fire events are likely important determinants of soil C persistence in these systems.

Impacts of conservation agriculture on soil C and N stocks and organic matter fractions: comparing commercial producer fields with a long-term small-plot experiment in Brown Chernozems of Saskatchewan

Conservation agriculture (CA) is increasingly promoted to build soil organic matter (SOM) based on findings from predominantly small-plot long-term agroecosystem experiments (LTAEs), with minimal on-farm data. Using commercial producer fields (n = 20) in the Brown Chernozemic soil zones of Saskatchewan, Canada, which were sampled before (1996) and after (2018) adopting direct-seeding and continuous cropping (1997), we examined changes in soil organic carbon (SOC) and total nitrogen (STN) stocks, along with C and N stocks in particulate (POM) and mineral-associated organic matter (MAOM), and compared them to a LTAE in the same soil zone. After 21 years, SOC and STN stocks (0-30 cm depth) increased by 13% and 21%, respectively, in commercial producer fields, and were more pronounced in finer- than coarser-textured soils. Conversely, there were no significant changes (0-30 cm depth) after 18 years (1998-2016) with CA [continuous wheat (CW-NT) and pulse-wheat under no-tillage (PW-NT)] in the LTAE, except that STN stock for PW-NT decreased by 7.7%. The estimated rate of change to 30 cm depth was similar between the commercial fields and LTAE for SOC (0.28 and 0.16 Mg C ha-1 yr-1, respectively), but not STN (0.04 and -0.03 Mg N ha-1 yr-1, respectively). Changes were more evident in the MAOM than POM fraction in both cases. Although the impact of CA may be similar, as observed for SOC, actual on-farm changes will depend on site-specific factors, and specific CA practice. Therefore, on-farm monitoring studies are needed for more accurate assessments of SOM changes and C sequestration potentials.

Shifts in controls and abundance of particulate and mineral-associated organic matter fractions among subfield yield stability zones

Abstract. Spatiotemporal yield heterogeneity presents a significant challenge to agricultural sustainability efforts and can strain the economic viability of farming operations. Increasing soil organic matter (SOM) has been associated with increased crop productivity, as well as the mitigation of yield variability across time and space. Observations at the regional scale have indicated decreases in yield variability with increasing SOM. However, the mechanisms by which this variability is reduced remain poorly understood, especially at the farm scale. To better understand the relationship between SOM and yield heterogeneity, we examined its distribution between particulate organic matter (POM) and mineral-associated organic matter (MAOM) at the subfield scale within nine farms located in the central United States. We expected that the highest SOM concentrations would be found in stable, high-yielding zones and that the SOM pool in these areas would have a higher proportion of POM relative to other areas in the field. In contrast to our predictions, we found that unstable yield areas had significantly higher SOM than stable yield areas and that there was no significant difference in the relative contribution of POM to total SOM across different yield stability zones. Our results further indicate that MAOM abundance was primarily explained by interactions between crop productivity and edaphic properties such as texture, which varied amongst stability zones. However, we were unable to link POM abundance to soil properties or cropping system characteristics. Instead, we posit that POM dynamics in these systems may be controlled by differences in decomposition patterns between stable and unstable yield zones. Our results show that, at the subfield scale, increasing SOM may not directly confer increased yield stability. Instead, in fields with high spatiotemporal yield heterogeneity, SOM stocks may be determined by interactive effects of topography, weather, and soil characteristics on crop productivity and SOM decomposition. These findings suggest that POM has the potential to be a useful indicator of yield stability, with higher POM stocks in unstable zones, and highlights the need to consider these factors during soil sampling campaigns, especially when attempting to quantify farm-scale soil C stocks.

Upper limit of mineral-associated organic carbon in temperate and sub-tropical soils: How far is it?

Soil organic carbon (SOC) is the main terrestrial C pool and is roughly divided into stable mineral-associated organic carbon (MAOC) and particulate organic matter fractions. However, the sequestration capacity of MAOC remains unquantified. Currently, soils are considered to have a limited capacity to accumulate MAOC, primarily due to the presence of fine fractions, i.e. silt and clay particles. It is unlikely that saturation will occur in all soils due to land use, thus necessitating the investigation of whether soils are from different latitudes and whether SOC affects the saturation of the silt and clay fraction comprising MAOC. We selected 71 soil samples that were physically fractionated from a Mexican inventory of vegetation with a wide range of SOC (4–186 g kg−1 soil) and clay (126–777 g kg−1 soil) contents. Soils were grouped into non-volcanic soils and allophanic Andosols. In the first group, we found a positive and highly significant linear relationship between SOC and MAOC contents, but no upper limit was detected. We found a common slope of 0.86 (R2 = 0.97, p < 0.01), i.e. 86% of the SOC was contained in MAOC irrespective of soil types and management, a value very similar to that in other studies. Conversely, for allophanic Andosols, which had SOC content twice that of non-volcanic soils, a second-order polynomial function (R2 = 0.98, p < 0.01) was established, the upper limit of which exceeded 186 g SOC kg−1 for an MAOC of 134 g kg−1. This was also supported by an increasing polynomial function of particulate organic carbon as the SOC increased. In both types of soils, maximum MAOCs were found between 700 and 800 g kg−1 of silt and clay in medium-textured soils. The amount of silt and clay particles was limited by SOC accumulation in the silt fraction as clay became saturated. These results are influenced by the mass proportion of silt and clay that follows a unique pattern in allophanic Andosols and non-volcanic soils. An investigation of a comprehensive database comprising 21,315 soil profiles sampled 0–30 cm across Mexico showed that most soils are below 186 g SOC kg−1 the bending point above which the upper limit does exist.

Does microbial carbon use efficiency differ between particulate and mineral‐associated organic matter?

Abstract Microbial carbon use efficiency (CUE), a key parameter to characterize microbial carbon conversion efficiency, is assumed to be similar in soil models for different soil functional pools with varied organic matter composition and nutrient availability, that is, particulate organic matter (POM) and mineral‐associated organic matter (MAOM). However, empirical studies comparing microbial CUE in POM versus MAOM are largely lacking. It is not known whether microbial CUE variance may underpin the variant behaviour (i.e. turnover and composition) of different soil functional pools. Here we collected surface soils from 25 natural forests and grasslands with divergent edaphic properties, and compared microbial CUE in their POM versus MAOM using soil fractionation in combination with incubation using 18O‐labelled water. We also quantified edaphic properties, organic matter composition, microbial community structures and the stoichiometric imbalance of nitrogen (N) and phosphorus (P) relative to carbon based on the dissolved pool relative to microbial biomass (ImN and ImP) to investigate variables regulating microbial CUE and its variation in POM relative to MAOM (CUEPOM/CUEMAOM). In contrast to our expectation, microbial CUE did not consistently differ between POM and MAOM across sites, albeit with large inter‐sample variations in CUEPOM/CUEMAOM (from 0.3 to 4.4). Although MAOM had higher substrate quality, indicated by lower ratios of soil organic carbon to total nitrogen (C/NOM) and higher N‐compounds/aromatic ratios, and higher proportions of r‐strategists and fast‐growing bacteria in the microbial community than POM, POM and MAOM had similar degrees of N limitation (i.e. ImN), which was the best predictor of microbial CUE across all samples. Therefore, although MAOM harboured more N‐containing compounds than POM, N limitation was not necessarily lower, leading to an overall similar microbial CUE in POM and MAOM. Nevertheless, CUEPOM/CUEMAOM decreased with increasing P limitation in POM relative to MAOM. Overall, our paper presents a comprehensive, empirical study comparing microbial CUE in POM and MAOM of diverse soils, and our results support the use of similar microbial CUE for POM and MAOM in soil models, but also highlight potential contrasts in microbial CUE between soil pools under strong P limitation. Such inferences deserve attention under potentially increasing P limitation induced by N deposition. This study advances our mechanistic understanding of ecological patterns and processes from the organismic to the ecosystem scale based on soil microbial physiology and different soil functional pools. Read the free Plain Language Summary for this article on the Journal blog.

Shifting Mountain Tree Line Increases Soil Organic Carbon Stability Regardless of Land Use.

Climate and land use changes are causing trees line to shift up into mountain meadows. The effect of this vegetation change on the partitioning of soil carbon (C) between the labile particulate organic matter (POM-C) and stable mineral-associated organic matter (MAOM-C) pools is poorly understood. Therefore, we assessed these C pools in a 10 cm topsoil layer along forest-meadow ecotones with different land uses (reserve and pasture) in the Northwest Caucasus of Russia using the size fractionation technique (POM 0.053-2.00 mm, MAOM < 0.053 mm). Potential drivers included the amount of C input from aboveground grass biomass (AGB) and forest litter (litter quantity) and their C/N ratios, aromatic compound content (litter quality), and soil texture. For both land uses, the POM-C pool showed no clear patterns of change along forest-meadow ecotones, while the MAOM-C pool increased steadily from meadow to forest. Regardless of land use, the POM-C/MAOM-C ratio decreased threefold from meadow to forest in line with decreasing grass AGB (R2 = 0.75 and 0.29 for reserve and pasture) and increasing clay content (R2 = 0.63 and 0.36 for reserve and pasture). In pastures, an additional negative relationship was found with respect to plant litter aromaticity (R2 = 0.48). Therefore, shifting the mountain tree line in temperate climates could have a positive effect on conserving soil C stocks by increasing the proportion of stable C pools.

Enhanced Soil Carbon Stability through Alterations in Components of Particulate and Mineral-Associated Organic Matter in Reclaimed Saline–Alkali Drainage Ditches

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.

Soil recalcitrant but not labile organic nitrogen mineralization contributes to microbial nitrogen immobilization and plant nitrogen uptake.

Soil organic nitrogen (N) mineralization not only supports ecosystem productivity but also weakens carbon and N accumulation in soils. Recalcitrant (mainly mineral-associated organic matter) and labile (mainly particulate organic matter) organic materials differ dramatically in nature. Yet, the patterns and drivers of recalcitrant (MNrec) and labile (MNlab) organic N mineralization rates and their consequences on ecosystem N retention are still unclear. By collecting MNrec (299 observations) and MNlab (299 observations) from 57 15N tracing studies, we found that soil pH and total N were the master factors controlling MNrec and MNlab, respectively. This was consistent with the significantly higher rates of MNrec in alkaline soils and of MNlab in natural ecosystems. Interestingly, our analysis revealed that MNrec directly stimulated microbial N immobilization and plant N uptake, while MNlab stimulated the soil gross autotrophic nitrification which discouraged ammonium immobilization and accelerated nitrate production. We also noted that MNrec was more efficient at lower precipitation and higher temperatures due to increased soil pH. In contrast, MNlab was more efficient at higher precipitation and lower temperatures due to increased soil total N. Overall, we suggest that increasing MNrec may lead to a conservative N cycle, improving the ecosystem services and functions, while increasing MNlab may stimulate the potential risk of soil N loss.

Losses of native mineral-associated organic nitrogen through microbial mineralization and gaseous emissions induced by ammonium and nitrate addition

Atmospheric nitrogen (N) deposition has a significant impact on terrestrial N cycling. However, it has not been well understood how external inorganic N addition affects the dynamics of soil organic N (SON) that supports long-term plant productivity and carbon (C) sequestration. Here, we added six levels of ammonium (NH4+-N) or nitrate (NO3−-N) to a temperate grassland soil to reveal the mechanisms of SON responses to inorganic N addition. Results showed that the SON content decreased by 7% ± 2%–15% ± 1%, which corresponded with the increase in the abundance of functional gene related to SON mineralization (i.e., chiA) by both NH4+-N and NO3−-N. The inorganic N addition stimulated gaseous losses from the native soil N, leading to at least 6% ± 2%–12% ± 1% of the initial SON lost at different N addition levels (i.e., N-priming). However, the responses of nitrification- and denitrification-related functional genes to NH4+-N and NO3−-N treatments varied remarkably. The positive responses of nitrification- and denitrification-related functional genes (i.e., hao, nirS, and narG) to NO3−-N addition contributed to gaseous N losses. The negative responses of some denitrification-related functional genes (i.e., nirK, narG, and napA) to NH4+-N addition resulted in less gaseous N losses by NH4+-N compared with that by NO3−-N addition. Additionally, the release of H+ by NH4+-N addition may liberate organic matter from associated minerals, showing a decrease in mineral-associated organic matter (MAOM) but an increase in extractable Fe. These findings provide direct evidence that the addition of inorganic N results in a net native SON loss through microbe-mediated mineralization and gaseous emissions. The study also emphasizes the different mechanisms of reduced and oxidized N additions in affecting SON mineralization and gaseous N losses.

Organic carbon distribution between structural and process pools in the gray forest soil of different land use

The summarized data on the content of organic carbon (Corg) in the subtypes of gray forest soils occurring on the territory of Russia was presented. It was shown that the humus horizons of virgin light-gray, typical-gray, and dark-gray forest soils contain, on average, 2.16 ± 0.67, 2.42 ± 0.61, and 3.58 ± 0.95% Сorg, respectively, while the plowing layers of arable soils contain 1.36 ± 0.40, 1.71 ± 0.40, and 2.84 ± 0.86%, respectively. Structural (particulate organic matter 0.05–2 mm in size, CPOM, and mineral-associated organic matter &lt;0.05 mm in size, CMAOM) and process (potentially mineralizable organic matter, C0, and microbial biomass, Cmic) pools were isolated in the organic matter of samples from different horizons of gray forest soils (Luvic Retic Greyzemic Phaeozems (Loamic)) under small-leaved forest and barley crop. The CPOM/CMAOM ratio in the upper soil horizons under forest and arable land was 0.60 and 0.26, respectively, and this ratio decreased with depth to 0.05 under both land uses. The sizes of the CMAOM, CPOM, C0, and Cmic pools correlated with each other and depended on the depth of the soil horizon, while the effect of land use on the pool ratios was found only for the surface horizons. The contribution of CPOM and CMAOM to the potentially mineralizable pool of organic matter in gray forest soil was 20–41 and 71–87%, respectively. According to the obtained data, the size of the C0 pool was almost equal to the annual amount of the heterotrophic CO2 emission from the soil. It was emphasized that determining of the sizes and ratios of structural and process soil organic matter pools should be important in the programs of carbon monitoring and recarbonization of agroecosystems.

The occurrence and distribution characteristics of microbial necromass carbon in lake sediments

As a critical component of SOC, microbial necromass carbon (MNC) has been well studied in forest, farmland, and grassland ecosystems. However, the distribution, forms, and storage of MNC, which bound to the sediments in lakes, remain unclear. Herein, the lake sediments in the West Dongting Lake have been collected to investigate the distributions of MNC in sediments, particulated organic matter (POM), and mineral-associated organic matter (MAOM). The contribution of MNC to the sedimentary organic carbon was 20–30 %, which was lower than the average value of 50 % in other ecosystems. The reasons might be related to the selective migration of soil particles with uneven distribution of MNC and the accelerated mineralization of MNC prior to sedimentation. The results revealed that MNCmaom was the predominant form of MNC, accounting for 61–73 % of the total MNC. In addition, the MNC content in the sediments ranged from 2.03 g kg−1 to 4.33 g kg−1, showing an increasing trend along the direction of the water flow from the lake shore to the center. The redundancy analyses and the partial least square path model analyses indicated that sediment organic carbon, total nitrogen, silt and MAOM had a substantial impact on the MNC. Silt played an essential role in regulating the distributions of MNC in lake sediments by promoting MNC sedimentation and providing protection from degradation. These findings revealed that the accumulation of MNC in lake sedimentwas mainly affected by sediment particles, which differed from other non-lake ecosystems. The presented findings provided essential insights into the accumulation of MNC in lake sediments, highlighting the importance of the MNC in lake sediments and a more profound understanding of the role of MNC in lake carbon cycling.

Bacterial necromass determines the response of mineral-associated organic matter to elevated CO2

Bacterial necromass determines the response of mineral-associated organic matter to elevated CO2

Ruminating on soil carbon: Applying current understanding to inform grazing management.

Among options for atmospheric CO2 removal, sequestering soil organic carbon (SOC) via improved grazing management is a rare opportunity because it is scalable across millions of globally grazed acres, low cost, and has high technical potential. Decades of scientific research on grazing and SOC has failed to form a cohesive understanding of how grazing management affects SOC stocks and their distribution between particulate (POM) and mineral-associated organic matter (MAOM)-characterized by different formation and stabilization pathways-across different climatic contexts. As we increasingly look to grazing management for SOC sequestration on grazinglands to bolster our climate change mitigation efforts, we need a clear and collective understanding of grazing management's impact on pathways of SOC change to inform on-the-ground management decisions. We set out to review the effects of grazing management on SOC through a unified plant ecophysiology and soil biogeochemistry conceptual framework, where elements such as productivity, input quality, soil mineral capacity, and climate variables such as aridity co-govern SOC accumulation and distribution into POM and MAOM. To maximize applicability to grazingland managers, we discuss how common management levers that drive overall grazing pattern, including timing, intensity, duration, and frequency can be used to optimize mechanistic pathways of SOC sequestration. We discuss important research needs and measurement challenges, and highlight how our conceptual framework can inform more robust research with greater applicability for maximizing the use of grazing management to sequester SOC.

Arbuscular mycorrhiza convey significant plant carbon to a diverse hyphosphere microbial food web and mineral-associated organic matter.

Arbuscular mycorrhizal fungi (AMF) transport substantial plant carbon (C) that serves as a substrate for soil organisms, a precursor of soil organic matter (SOM), and a driver of soil microbial dynamics. Using two-chamber microcosms where an air gap isolated AMF from roots, we 13CO2-labeled Avena barbata for 6 wk and measured the C Rhizophagus intraradices transferred to SOM and hyphosphere microorganisms. NanoSIMS imaging revealed hyphae and roots had similar 13C enrichment. SOM density fractionation, 13C NMR, and IRMS showed AMF transferred 0.77 mg C g-1 of soil (increasing total C by 2% relative to non-mycorrhizal controls); 33% was found in occluded or mineral-associated pools. In the AMF hyphosphere, there was no overall change in community diversity but 36 bacterial ASVs significantly changed in relative abundance. With stable isotope probing (SIP)-enabled shotgun sequencing, we found taxa from the Solibacterales, Sphingobacteriales, Myxococcales, and Nitrososphaerales (ammonium oxidizing archaea) were highly enriched in AMF-imported 13C (> 20 atom%). Mapping sequences from 13C-SIP metagenomes to total ASVs showed at least 92 bacteria and archaea were significantly 13C-enriched. Our results illustrate the quantitative and ecological impact of hyphal C transport on the formation of potentially protective SOM pools and microbial roles in the AMF hyphosphere soil food web.

Soil carbon and nitrogen after eight years of rotational grazing in the Nebraska Sandhills meadows

Grassland provides many ecosystem services; therefore, sustainable management practices of grassland are crucial for maintaining and enhancing its ecosystem health and resilience. Rotational grazing at a high stocking density (a.k.a., ultrahigh stocking density) is purported to sequester greater amounts of carbon (C) in grassland soils than rotational grazing at low stocking densities. This study was conducted in the Nebraska Sandhills meadows for eight years to evaluate how rotational grazing with different stocking densities can affect soil C and total nitrogen (TN) in bulk soils, soil organic matter fractions, and sequestration rate. The grazing management included a high stocking density with one grazing cycle (MOB), a 4-pasture rotation system with one grazing cycle at a low stocking density (4PR1), and no grazing (CNT). Results showed that soil C and N contents were higher in the total particulate organic matter fraction than in the soil mineral-associated organic fraction at the 0–10 cm depth and visa verse at the 10–20 cm depth. Grazing management did not affect C and N contents in the bulk soils (SOC averaged 28.19 g C kg−1 and TN averaged 2.67 g N kg−1 at the 0–10 cm depth). Both MOB and 4PR1 grazing treatments recorded similar C and N contents in all SOM fractions. However, the MOB grazing treatment increased C and N contents in the macro-aggregate occluded particulate and dissolved organic matter fractions compared to the CNT. The 4PR1 grazing treatment increased soil C and N contents in the dissolved organic matter and mineral-associated organic matter fractions and increased SOC sequestration rate compared to the CNT. Rotational grazing at a low stocking density for one cycle appears to enhance long-term SOC accumulation in these Sandhills meadows soil.

Stabilized microbial necromass in soil is more strongly coupled with microbial diversity than the bioavailability of plant inputs

Microbial necromass carbon (C) can substantially contribute to stabilized soil organic matter (SOM), and effective management of this C may help mitigate climate change. However, factors important to the formation of microbial necromass are only partly understood. While bioavailable plant inputs may induce necromass formation by boosting microbial growth and C use efficiency, other microbial traits, such as those related to secretion systems or adhesion and motility, may also be relevant. These traits may be independent of the bioavailability of plant inputs and modulated by environmental factors such as soil depth or site age. Such links, however, have hardly been studied.Here, we used replicated plots of European alder (more bioavailable inputs) and Scots pine (less bioavailable inputs) to investigate links among plant inputs, soil depth, site age, microbial community composition, and microbial necromass C in stabilized SOM, i.e., particulate organic matter occluded within aggregates (oPOM) and mineral-associated organic matter (MAOM).We did not find evidence that bioavailable plant inputs, nor soil depth and site age, were major drivers of microbial necromass formation. Instead, certain microbial taxa, and microbial diversity in particular, were most tightly related to microbial necromass C in MAOM. Microbial necromass C also substantially contributed to oPOM (up to ∼57% of the C stored in that fraction), a C pool considered to largely derive from plant biomolecules. Combined, however, microbial necromass C in oPOM and MAOM only accounted for ∼23% of bulk C contents. Our results imply that effective C-focused research and management have to consider constraints on microbial community composition and diversity, microbial necromass in pools other than MAOM, and formation of plant-derived SOM.

Soil organic matter and aggregate stability dynamics under major no-till crop rotations on the Canadian prairies

There is a need to develop and adopt novel, more resilient, or “climate-smart” cropping strategies on the Canadian prairies, which are crucial for sustainable agroecosystem management. To examine how crop rotation influences soil organic matter and aggregate stability (AS) dynamics, six crop rotations: (i) conventional (control), (ii) pulse/oilseed intensified, (iii) diversified, (iv) market-driven, (v) high-risk and high-reward, and (vi) soil health-enhanced were established in 2018 at three field sites: Lethbridge, Alberta and Swift Current and Scott, Saskatchewan under no-till management. After 4-year, soil organic carbon (SOC), total nitrogen (TN), particulate organic matter carbon (POM-C) and nitrogen (POM-N), or mineral-associated organic matter C (MAOM-C) and N (MAOM-N) concentrations were not significantly affected by crop rotation in the 0–7.5 cm soil depth. However, crop rotation significantly altered AS at two of three sites, with both soil health-enhanced and high-risk and high-reward rotations having the highest AS at Lethbridge and Swift Current. Across all three sites, strong positive correlations were found among SOC, TN, MAOM, and AS. Moreover, MAOM-C and MAOM-N showed stronger relationships with AS than POM, perhaps suggesting a positive feedback loop on the stability of SOC and aggregation. Overall, the inclusion of pulses in rotations showed the potential to sustain soil quality, likely by offsetting low residue quantity with better residue quality and diversity, thereby supporting SOC accrual and AS similar to or greater than conventional cereal- or oilseed-dominated rotations.

Subtropical forest macro-decomposers rapidly transfer litter carbon and nitrogen into soil mineral-associated organic matter

BackgroundForest soils in tropical and subtropical areas store a significant amount of carbon. Recent frameworks to assess soil organic matter (SOM) dynamics under evolving global conditions suggest that dividing bulk SOM into particulate and mineral-associated organic matter (POM vs. MAOM) is a promising method for identifying how SOM contributes to reducing global warming. Soil macrofauna, earthworms, and millipedes have been found to play an important role in facilitating SOM processes. However, how these two co-existing macrofaunae impact the litter decomposition process and directly impact the formation of POM and MAOM remains unclear. MethodsHere, we set up a microcosm experiment, which consisted of 20 microcosms with four treatments: earthworm and litter addition (E), millipedes and litter addition (M), earthworm, millipedes, and litter addition (E+M), and control (only litter addition) in five replicates. The soil and litter were sterilized prior to beginning the incubation experiment to remove any existing microbes. After incubating the samples for 42 days, the litter properties (mass, C, and N contents), soil physicochemical properties, as well as the C and N contents, and POM and MAOM 13C abundance in the 0–5 and 5–10 ​cm soil layers were measured. Finally, the relative influences of soil physicochemical and microbial properties on the distribution of C and N in the soil fractions were analyzed. ResultsThe litter mass, C, and N associated with all four treatments significantly decreased after incubation, especially under treatment E+M (litter mass: −58.8%, litter C: −57.0%, litter N: −75.1%, respectively), while earthworm biomass significantly decreased under treatment E. Earthworm or millipede addition alone showed no significant effects on the organic carbon (OC) and total nitrogen (TN) content in the POM fraction, but joint addition of both significantly increased OC and TN regardless of soil depth. Importantly, all three macrofauna treatments increased the OC and TN content and decreased the 13C abundance in the MAOM fraction. More than 65% of the total variations in the distribution of OC and TN throughout the two fractions can be explained by a combination of soil physicochemical and microbial properties. Changes in the OC distribution in the 0–5 ​cm soil layer are likely due to a decrease in soil pH and an increase in arbuscular mycorrhizal fungi (AMF), while those in the 5–10 ​cm layer are probably caused by increases in soil exchangeable Ca and Mg, in addition to fungi and gram-negative (GN) bacteria. The observed TN distribution changes in the 0–5 ​cm soil likely resulted from a decrease in soil pH and increases in AMF, GN, and gram-negative (GP) bacteria, while TN distribution changes in the 5–10 ​cm soil could be explained by increases in exchangeable Mg and GN bacteria. ConclusionsThe results indicate that the coexistence of earthworms and millipedes can accelerate the litter decomposition process and store more C in the MAOM fractions. This novel finding helps to unlock the processes by which complex SOM systems serve as C sinks in tropical forests and addresses the importance of soil macrofauna in maintaining C-neutral atmospheric conditions under global climate change.

Land use determines the composition and stability of organic carbon in earthworm casts under tropical conditions

Environmental conditions play an important role in controlling the fate of organic carbon (OC) in earthworm casts. However, the mechanisms that lead to the destabilization of OC in casts deposited in different land use systems are poorly understood. In this study, we investigated the impact of land use on the fate and composition of particulate organic matter (POM) and mineral-associated organic matter (MAOM) in earthworm casts under tropical conditions. We conducted a 400-day field exposure experiment in a woodland and a meadow in northern Vietnam with earthworm casts and soil aggregates without trace of earthworm activity (reference soil) under natural rainfall conditions. We analyzed the element content, stable carbon isotope composition and mid-infrared signatures of the two types of materials after 9 and 400 days of field exposure. The results showed that the casts were initially enriched in OC as compared to the reference soils, with the MAOM fraction accounting for over 90% of the earthworm-induced OC accumulation. The POM fraction occluded in casts consisted of fresh plant material and disappeared during the 400 days of field exposure. Enrichment and composition of OC differed between woodland and meadow casts. POM and MAOM showed contrasting persistence in the two land use systems. While woodland casts showed the highest potential to stabilize OC, the actual amount of cast OC stabilized for more than 400 days was more important under meadow. The processes affecting cast OC dynamics were contrasting in both systems. Under woodland, cast OC destabilization was most probably related to microbial degradation, while under meadow, OC accumulation was most probably related to root activity. Our study highlights that the impact of earthworms on the origin, composition, and fate of cast OC in tropical environments is strongly influenced by land use.

Dynamic changes in soil organic carbon induced by long-term compost application under a wheat-maize double cropping system in North China

Soil organic carbon (SOC) plays a vital role in improving soil quality and alleviating global warming. Understanding the dynamic changes in SOC is crucial for its accumulation induced by compost application in agroecosystem. In this study, soil samples were collected from three treatments: high-rate bio-compost (BioMh), low-rate bio-compost (BioMl), and control (CK, no fertilization) during 2002–2020 in a wheat-maize double cropping system in North China. The soils were separated into three functional fractions, i.e., coarse particle organic matter (cPOM, >250 μm), microaggregates (μAgg, 53–250 μm) and mineral-associated organic matter (MAOM, < 53 μm), and the associated SOC contents were determined. During 1993–2002, SOC contents in bulk soil significantly increased with the duration in the BioMh and BioMl plots. However, there was no significant correlation between SOC content and duration during 2002–2020. These results suggested that compost application positively improved SOC sequestration, while the duration of SOC sequestration (i.e., the longevity of increased SOC with time) under compost inputs maintained only 9 years. Moreover, there was a significant increase in mean annual SOC contents in bulk soil with compost application rate during 2002–2020, indicating that carbon saturation did not occur. Additionally, the SOC contents in the cPOM fraction increased with time (p < 0.01), but the corresponding μAgg and MAOM associated SOC was insignificant (p > 0.05). The MAOM fraction exhibited no additional carbon accumulation with expanding compost application, confirming a hierarchical carbon saturation in these fractions. We concluded that soils under wheat-maize double cropping system in North China have greater potential to sequester C through additional compost inputs, despite showing hierarchical saturation behavior in the non-protected coarse particulate fraction.