Mineral-associated Organic Matter Research Articles

Different Molecular Characterization of Soil Particulate Fractions under N Deposition in a Subtropical Forest

Key Findings: Combining physical fractionation and pyrolysis–gas chromatography/mass spectrometry (py-GC/MS) technique can help better understand the dynamics of soil organic matter (SOM). Background and Objectives: SOM plays a critical role in the global carbon (C) cycle. However, its complexity remains a challenge in characterizing chemical molecular composition within SOM and under nitrogen (N) deposition. Materials and Methods: Three particulate organic matter (POM) fractions within SOM and under N treatments were studied from perspectives of distributions, C contents and chemical signatures in a subtropical forest. N addition experiment was conducted with two inorganic N forms (NH4Cl and NaNO3) applied at three rates of 0, 40, 120 kg N ha−1 yr−1. Three particle-size fractions (>250 μm, 53–250 μm and <53 μm) were separated by a wet-sieving method. Py-GC/MS technique was used to differentiate between chemical composition. Results: A progressive proportion transfer of mineral-associated organic matter (MAOM) to fine POM under N treatment was found. Only C content in fine POM was sensitive to N addition. Principal component analyses (PCA) showed that the coarse POM had the largest plant-derived markers (lignins, phenols, long-chain n-alkanes, and n-alkenes). Short-chain n-alkanes and n-alkenes, benzofurans, aromatics and polycyclic aromatic hydrocarbons mainly from black carbon prevailed in the fine POM. N compounds and polysaccharides from microbial products dominated in the MAOM. Factor analysis revealed that the degradation extent of three fractions was largely distinct. The difference in chemical structure among three particulate fractions within SOM was larger than treatments between control and N addition. In terms of N treatment impact, the MAOM fraction had fewer benzofurans compounds and was enriched in polysaccharides, indicating comparatively weaker mineralization and stronger stabilization of these substances. Conclusions: Our findings highlight the importance of chemical structure in SOM pools and help to understand the influence of N deposition on SOM transformation.

Hummocks in alpine tundra, northern British Columbia, Canada: distribution, morphology and organic carbon composition

Hummocks develop by cryoturbation in fine-grained frost-susceptible soils and their stage of maturity may affect the translocation of organics in Cryosols. This study examines the distribution and morphology of hummocks in the Chuck Creek Trail Valley (northern British Columbia) and determines the quantity, distribution, and composition of organic matter in their soils. Hummocks occupy about 5%–20% of the valley and their morphology is largely affected by their silt content. Cryoturbated intrusions, radiocarbon dated to 2814 and 1648 cal year B.P., suggest that hummock development was initiated during the cooler late Holocene. Hummocks have an average soil organic carbon density of 16.3 kg m−2 in the uppermost 1 m, with 62% stored in the top 25 cm. Organics are mainly present as particulate organic matter in the O-horizon (25%–80%), characterized by degradable alkyl C and O/N-alkyl groups, but occur as mineral-associated organic matter (96%–98%) composed of recalcitrant aromatic and aliphatic C groups in the underlying B and C horizons. Minor differences in organic content and composition occur between hummock tops and troughs, and between hummocks showing different stages of maturity. In the absence of an observed frost table, contemporary hummock activity is attributed to seasonal freezing and thawing.

Particulate organic carbon is more vulnerable to nitrogen addition than mineral-associated organic carbon in soil of an alpine meadow

Long-term nitrogen (N) addition can affect soil organic carbon (SOC) pool within different soil fractions with different turnover rates. However, the mechanisms of these effects, particularly in alpine grassland ecosystems, are not clear. We studied the responses of SOC content in different soil fractions to N addition based on a six-year N addition field experiment in an alpine meadow ecosystem on the Tibetan Plateau. We measured soil chemical and microbial properties, and SOC content in bulk soil, particular organic matter (POM) and mineral-associated organic matter (MAOM) fractions in response to N addition. N addition increased soil N availability, decreased soil pH and microbial biomass, but had minimal effect on plant biomass, soil enzyme activity, and SOC content in bulk soil. With increasing levels of N addition, SOC in the POM fraction (POC) showed a significant negative trend, while SOC in the MAOM fraction (MAOC) did not change significantly. As plant biomass input and soil enzyme activity were not significantly altered with N addition, the decline in POC was likely caused by changes in microbial physiology (carbon use efficiency), while the insignificant change in MAOC may be determined by the balance between input (from microbial necromass) and output (from microbial decomposition). Taken together, our study showed that the less-protected POC fraction is more vulnerable to N addition than the more-protected MAOC fraction in the alpine grassland. This finding may improve the prediction of soil C dynamics in response to N deposition in alpine grassland ecosystems on the Tibetan Plateau.

Mineral vs. organic matter supply as a limiting factor for the formation of mineral-associated organic matter in forest and agricultural soils

Physical and chemical interactions between soil organic matter (OM) and minerals is one of the primary mechanisms for stabilizing OM in terrestrial ecosystems. Focusing on OM association with mineral surfaces, this study sought to examine mineral-associated OM from the perspectives of both mineral surface characteristics and organic matter chemistry. The research was conducted at paired-sites under North American Mid-Atlantic Coastal forest and crop production with shared environmental factors. Using carbon (C) and nitrogen (N) 1s micro- X-ray absorption near-edge fine structure (XANES) spectroscopy, we investigated the amounts and types of mineral-associated OM. Mineral specific surface area (SSA) of bulk soil was determined for three conditions: untreated, post OM removal and post iron (Fe) (oxyhydr)oxides removal. Amounts of mineral-associated OM were smaller in the agricultural soil, where greater SSA sourced from clay-sized phyllosilicates and Fe (oxyhydr)oxide minerals did not result in greater OM coverage of the mineral surface area. Although agricultural surface soil showed less abundance of phenolic C, speciation of mineral-associated OM did not differ between comparable horizons. Our results suggest that despite the plow-derived mixing of soil, which increased SSA and secondary minerals available to interact physically and chemically with OM in the plowed layer, the formation of mineral-associated OM in agricultural soil is ultimately limited by available OM.

Microbial and abiotic controls on mineral-associated organic matter in soil profiles along an ecosystem gradient

Formation of mineral-organic associations is a key process in the global carbon cycle. Recent concepts propose litter quality-controlled microbial assimilation and direct sorption processes as main factors in transferring carbon from plant litter into mineral-organic associations. We explored the pathways of the formation of mineral-associated organic matter (MOM) in soil profiles along a 120-ky ecosystem gradient that developed under humid climate from the retreating Franz Josef Glacier in New Zealand. We determined the stocks of particulate and mineral-associated carbon, the isotope signature and microbial decomposability of organic matter, and plant and microbial biomarkers (lignin phenols, amino sugars and acids) in MOM. Results revealed that litter quality had little effect on the accumulation of mineral-associated carbon and that plant-derived carbon bypassed microbial assimilation at all soil depths. Seemingly, MOM forms by sorption of microbial as well as plant-derived compounds to minerals. The MOM in carbon-saturated topsoil was characterized by the steady exchange of older for recent carbon, while subsoil MOM arises from retention of organic matter transported with percolating water. Overall, MOM formation is not monocausal but involves various mechanisms and processes, with reactive minerals being effective filters capable of erasing chemical differences in organic matter inputs.

Similar temperature sensitivity of soil mineral-associated organic carbon regardless of age

Most of the carbon (C) stored in temperate arable soils is present in organic matter (OM) intimately associated with soil minerals and with slow turnover rates. The sensitivity of mineral-associated OM to changes in temperature is crucial for reliable predictions of the response of soil C turnover to global warming and the associated flux of carbon dioxide (CO2) from the soil to the atmosphere. We studied the temperature sensitivity of C in <63 μm fractions rich in mineral-associated organic matter (MOM) and of C in >63 μm fractions rich in particulate organic matter (POM). The fractions were isolated by physical separation of two light-textured arable soils where the C4-plant silage maize had replaced C3-crops 25 years ago. Differences in 13C abundance allowed for calculation of the age of C in the soil-size fractions (old C, C3–C > 25 years; recent C, C4–C < 25 years). We incubated bulk soils (<2 mm) and size fractions sequentially at 6, 18, 26 and 34 °C (ramping up and down the temperature scale) and calculated the temperature sensitivity of old and recent C from 12CO2 and 13CO2 evolution rates. The temperature sensitivity was similar or slightly higher for POM than for MOM. Within the POM fraction, old C3–C was more sensitive to changes in temperature than recent C4–C. For the MOM fraction, the temperature sensitivity was unrelated to the age of C. Quantitative PCR analysis indicated that the proportions of bacteria, archaea and fungi did not change during incubation. Our results suggest that while OM stabilizing mechanisms affect the temperature sensitivity of soil C, temperature sensitivity appears unrelated to the age of mineral-associated OM.

Impacts of an invasive grass on soil organic matter pools vary across a tree-mycorrhizal gradient

Increases in carbon (C) inputs can augment soil organic matter (SOM), or reduce SOM by accelerating decomposition. Thus, there is a need to understand how and why ecosystems differ in their sensitivity to C inputs. Invasive plants that invade wide-ranging habitats, accumulate biomass rapidly, and contribute copious amounts of C to soil can be ideal for addressing this gap. We quantified the effects of the invasive C4 grass, Microstegium vimineum, on SOM in three temperate forests across plots varying in their relative abundance of arbuscular mycorrhizal (AM) versus ectomycorrhizal (ECM) trees. We hypothesized that invasion would differentially affect SOM along the mycorrhizal gradient owing to recognized patterns in nitrogen availability (AM > ECM) and the proportion of unprotected SOM (ECM > AM). Across all sites, M. vimineum was associated with lower particulate organic matter (POM) in ECM-dominated plots, consistent with our hypothesis that invader-derived C inputs should stimulate decomposers to acquire nitrogen from unprotected SOM in soils with low nitrogen availability. However, the pattern of lower POM in the ECM-dominated soils was offset by greater mineral-associated organic matter (MAOM)—and isotopic data suggest this was largely driven by native- rather than invader-derived SOM—implying an invasion-associated transfer of native-derived POM into MAOM. Our results demonstrate a context-dependent shift in the form of SOM in a system with presumably enhanced C inputs. This finding suggests a need to look beyond changes in total SOM stocks, as intrinsic SOM changes could lead to important long-term feedbacks on invasion or priming effects.

Soil aggregate and intra-aggregate carbon fractions associated with vegetation succession in an alpine wetland of Northwest China

Alpine wetlands can function as carbon sinks because of their high soil organic content and low decomposition rates. However, the effects of alpine wetland succession on soil structure and soil organic carbon (SOC) stability have rarely been reported. The objective of this study was to investigate the effects of vegetation succession on soil aggregate distribution, bulk soil carbon, aggregate-associated C, and intra-aggregate C fraction in the Bayinbuluk alpine wetland. Research sites were selected in adjacent swamp (S), swampy meadow (SM), and meadow (M) locations. Five soil aggregate fractions (>5000, 2000–5000, 250–2000, 53–250 and <53 μm) were separated using a modified Yoder method. The light fraction organic carbon (LFOC), coarse/fine intra-aggregate particulate organic matter carbon (iPOM-C), and mineral-associated organic matter carbon (mSOM-C) were isolated from soil aggregates. In the wetland succession sequence, the mean weight diameter of soil aggregates increased (from 4.16 to 4.63 mm) from S to SM, and then decreased (from 4.63 to 3.23 mm) from SM to M. The bulk soil carbon content initially increased (from 93 to 129 mg kg−1) from S to SM, and then decreased (from 129 to 54 mg kg−1) from SM to M. The same trends were found for the aggregate-associated C in different size aggregates. The coarse iPOM-C content (88–133 mg kg−1) was higher than the fine iPOM-C content (77–118 mg kg−1) in different size aggregates in S samples. In SM and M samples, however, the coarse iPOM-C contents (98–124 mg kg−1 (SM); 60–82 mg kg−1 (M)) were lower than the fine iPOM-C contents (129–151 mg kg−1 (SM); 71–103 mg kg−1 (M)). The ratio of coarse iPOM-C to fine iPOM-C content (ROC/F) exceeded 1 in S but was lower than 1 in SM and M. These results indicate that soil aggregation stability and SOC content increased and then decreased across the wetland succession sequence, and that ROC/F can be used as an indicator of changes in SOC content caused by wetland succession.

Enrichment of Lignin-Derived Carbon in Mineral-Associated Soil Organic Matter.

A modern paradigm of soil organic matter proposes that persistent carbon (C) derives primarily from microbial residues interacting with minerals, challenging older ideas that lignin moieties contribute to soil C because of inherent recalcitrance. We proposed that aspects of these old and new paradigms can be partially reconciled by considering interactions between lignin decomposition products and redox-sensitive iron (Fe) minerals. An Fe-rich tropical soil (with C4 litter and either 13C-labeled or unlabeled lignin) was pretreated with different durations of anaerobiosis (0-12 days) and incubated aerobically for 317 days. Only 5.7 ± 0.2% of lignin 13C was mineralized to CO2 versus 51.2 ± 0.4% of litter C. More added lignin-derived C (48.2 ± 0.9%) than bulk litter-derived C (30.6 ± 0.7%) was retained in mineral-associated organic matter (MAOM; density >1.8 g cm-3), and 12.2 ± 0.3% of lignin-derived C vs 6.4 ± 0.1% of litter C accrued in clay-sized (<2 μm) MAOM. Longer anaerobic pretreatments increased added lignin-derived C associated with Fe, according to extractions and nanoscale secondary ion mass spectrometry (NanoSIMS). Microbial residues are important, but lignin-derived C may also contribute disproportionately to MAOM relative to bulk litter-derived C, especially following redox-sensitive biogeochemical interactions.

Soil organic matter pools under management intensification of loblolly pine plantations

Early thinning of loblolly pine plantations can potentially deliver sustainable feedstocks for biofuel/bioenergy. However, the management intensification for increased productivity and the removal of additional biomass from these plantations could reduce carbon (C) inputs belowground and therefore reduce overall ecosystem C storage. Increased fertilization could also affect C stocks, and their relative distribution between soil organic matter (SOM) fractions. We analyzed soil C stocks as a function of soil type and different pine plantation management systems across the Western Gulf region of the United States. Additionally, we analyzed SOM fractions with inherently different stabilization mechanisms and potential C persistence. We found no significant differences in bulk soil C stocks across management intensities or soil types. The early thinning treatment had no effect on the C distribution across each soil organic matter fraction. However, proportionally more C was found in mineral-associated organic matter and less in particulate organic matter in the more intensive management regime treatment, possibly due to higher below ground nutrient inputs and enhanced microbial activity. Our results suggest that management intensification to support biofuel production from loblolly pine plantations will not affect soil C stocks, but may increase their persistence. This study demonstrates that, from a soil C perspective, early thinning of intensively managed loblolly pine plantations has potential as a sustainable biofuel feedstock.

Changes in soil carbon pools and components induced by replacing secondary evergreen broadleaf forest with Moso bamboo plantations in subtropical China

Moso bamboo (Phyllostachys edulis) is widely distributed in southern China, and is one of the fastest growing plants worldwide; however, information remains limited on the impact of converting secondary broad-leaved evergreen forests to Moso bamboo plantations, and how the soil organic carbon (SOC) pool and its chemical composition should be managed subsequently. To elucidate these effects, three representative areas were chosen, all with very similar site conditions. In each area, four comparable stands were selected; namely, undisturbed (M0), extensively managed (M1), and intensively managed (M2) stands in each Moso bamboo plantation, and a secondary broad-leaved evergreen forest (CK). Soil samples were collected and examined from depths of 0–20 and 20–40 cm in all 12 stands. The results showed that, SOC and mineral-associated organic matter C (MOM-C) stocks in 0–40 cm soil depths were significantly higher in M0 and M1 than in CK; however, these two parameters were significantly lower in M2. M0 and M1 showed a significant decline in the ratio of microbial biomass C (MBC) to total organic C (TOC), hot-water-extractable organic C (DOC) to TOC, and the C mineralization rate. However, M2 showed a significant increase compared to CK for all of these parameters. Fourier-transform infrared spectroscopy (FTIR) showed that land-use conversion also changed SOC chemical composition. Compared with CK and M2, M0 and M1 showed lower relative content of polysaccharides and higher content of recalcitrant compounds and soil hydrophobicity. Aliphatic and aromatic compounds were positively correlated with accumulated C sequestration in all fractions but negatively correlated with microbial activity in both soil layers; thus, chemical protection mechanism was important for stabilizing the soil in M0 and M1. Overall, Moso bamboo plantations with management strategies M0 and M1 could stabilize more C through promoting the formation of stable organic-mineral complexes and the accumulation of resistant organic components, showing much higher potential in terms of soil C sequestration than M2.

Substitution of mineral fertilizers with biogas digestate plus biochar increases physically stabilized soil carbon but not crop biomass in a field trial

Various organic amendments are scrutinized as potential agricultural management strategies to ensure soil productivity while mitigating climate change due to the accumulation of soil organic matter (OM). The objectives of this experiment were to study the effects of biochar and biogas digestate versus mineral fertilizer on crop aboveground biomass as well as fractions and mineralization of soil organic carbon (SOC). Samples of a sandy Cambisol were taken 14 months after establishment of a field experiment in Germany. Treatments included application of equal nitrogen in the form of mineral fertilizer or liquid biogas digestate without biochar (B0), with 1 Mg biochar ha−1season−1 for two growing seasons (B2), or with 40 Mg biochar ha−1 application (B40). Soil fractionation in water separated water-extractable and free particulate (fPOM) OM, followed by sonification and sieving to isolate occluded particulate (oPOM) and < 20 μm aggregate-occluded and mineral-associated OM. CO2 emissions were measured during 92-day laboratory incubations at 10 and 20 °C. Analysis of variance found digestate lowered (p < 0.05) rye aboveground biomass compared to mineral fertilizer (9.3 vs. 10.6 Mg ha−1), while biochar had no effect. B40 treatments increased C mineralization during incubation by 16% and contained 3.8 times more SOC than B0 treatments. This additional SOC was allocated to fPOM (52%), oPOM (22%), and the <20 μm fraction (26%). Digestate application increased SOC content of oPOM by 11% compared to mineral fertilizer. Furthermore, combined application of 40 Mg biochar ha−1 with digestate resulted in 20% more SOC in the <20 μm fraction than biochar with mineral fertilizer. The lack of a significant fertilizer or biochar-fertilizer interaction effect on C mineralization during incubation demonstrates the stability of SOC from digestate alone or in combination with biochar. The absence of significant differences in SOC content between B0 and B2 treatments demonstrates the difficulty of documenting SOC sequestration in the field at low biochar application rates.

Particulate Soil Organic Matter in Bahiagrass–Rhizoma Peanut Mixtures and Their Monocultures

Core Ideas Bahiagrass–rhizoma peanut mixtures resulted in similar whole soil C stocks as N‐fertilized bahiagrass monocultures. Bahiagrass–rhizoma peanut mixtures had greater total and particulate organic matter C than rhizoma peanut monocultures. Particulate organic matter C/N ratio of rhizoma peanut and N‐fertilized bahiagrass monocultures were lower than C/N ratio in bahiagrass–rhizoma peanut mixtures. The use of δ13C to verify changes in the soil organic matter caused by the plant component was a better predictor than the use of δ15N. Soil particulate organic matter (POM) represents a labile soil organic matter (SOM) fraction and is an important source of nutrients for soil microorganisms and plants. This study assessed C and N stocks in POM and soil mineral‐associated fractions for forage mixtures of rhizoma peanut (RP; Arachis glabrata Benth.) and bahiagrass (Paspalum notatum Flüggé) in contrast with their monocultures. Treatments included two bahiagrass entries receiving 90 kg N ha–1 harvest–1, two RP entries, and the four bahiagrass–RP combinations. Samples were collected from the 0‐ to 15‐cm soil depth in July 2015 and in September 2016, after seven harvests. Soil samples were fractionated into POM and mineral‐associated organic matter (MAOM) by dispersion with sodium hexametaphosphate and sieving (53 μm). Samples were analyzed for δ13C, δ15N, and total C and N. Groups of bahiagrass, bahiagrass–RP, and RP were contrasted. There were no differences in N stocks among forage treatments. Bahiagrass–RP mixtures had greater soil POM‐C stock (5.1 Mg C ha–1) than RP monocultures (4.0 Mg C ha–1). Soil POM C/N ratio was lower for bahiagrass (18.2) and RP monocultures (16.2) than bahiagrass–RP (20.6). Bahiagrass–RP and bahiagrass had similar soil δ13C and were less depleted than RP (–23‰ vs. –26‰ and –26‰ vs. –28‰, for POM and MAOM, respectively). Soil POM‐C stock under forage mixtures was not different from N‐fertilized bahiagrass, and it was greater than RP. Hence, mixing RP and bahiagrass can be an option to increase POM‐C without the need of N fertilizer.

Earthworm Cast Formation and Development: A Shift From Plant Litter to Mineral Associated Organic Matter

Earthworms play a major role in litter decomposition, in processing soil organic matter and driving soil structure formation. Earthworm casts represent hot spots for carbon turnover and formation of biogeochemical interfaces in soils. Due to the complex microscale architecture of casts, understanding the mechanisms of cast formation and development at a process relevant scale, i.e. within microaggregates and at the interface between plant residues, microorganisms and mineral particles, remains challenging. We used stable isotope enrichment to trace the fate of shoot and root litter in intact earthworm cast samples. Surface casts produced by epi-anecic earthworms (Lumbricus terrestris) were collected after 8 and 54 weeks of soil incubation in mesocosms, in the presence of 13C-labeled Ryegrass shoot or root litter deposited onto the soil surface. To study the alteration in the chemical composition from initial litter to particulate organic matter (POM) and mineral-associated organic matter (MOM) in cast samples, we used solid-state 13C Nuclear Magnetic Resonance spectroscopy (13C-CPMAS-NMR) and isotopic ratio mass spectrometry (EA-IRMS). We used spectromicroscopic approach to identify plant tissues and microorganisms involved in plant decomposition within casts. A combination of transmission electron microscopy (TEM) and nano-scale secondary ion mass spectrometry (NanoSIMS) was used to obtain the distribution of organic carbon and δ13C within intact cast sample structures. We clearly demonstrate a different fate of shoot- and root-derived organic carbon in earthworm casts, with a higher abundance of less degraded root residues recovered as particulate organic matter on the short-term (8 weeks) (73 mg.g-1 in Cast-Root vs 44 mg.g-1 in Cast-Shoot). At the early stages of litter decomposition, the chemical composition of the initial litter was the main factor controlling the composition and distribution of soil organic matter within casts. At later stages, we can demonstrate a clear reduction of structural and chemical differences in root and shoot-derived organic products. After one year, MOM clearly dominated the casts (more than 85 % of the total OC in the MOM fraction). We were able to highlight the shift from a system dominated by free plant residues to a system dominated by MOM during cast formation and development.

Assessing the effects of 17 years of grazing exclusion in degraded semi-arid soils: Evaluation of soil fertility, nutrients pools and stoichiometry

Grazing exclusion (GE) has been used as a strategy to restore degraded pastures worldwide. In this study, the effects of 17 years of GE on a) soil fertility, b) stoichiometric ratios between soil nutrients and c) carbon (C) and nitrogen (N) storage in particulate and mineral-associated organic matter soil fractions were assessed in an area degraded by overgrazing (OG) in a semi-arid region in Brazil. For this purpose, three sites with two treatments (exclusion and overgrazing) were studied at three sampling depths (0–10, 10–20 and 20–40 cm). Our results showed that pH, total organic carbon, particulate organic carbon, total N, particulate organic nitrogen, Ca2+ and K+ increased, whereas the Al3+ content decreased with GE. Conversely, the available P content, Mg2+ and Na+ showed no significant variations as a function of GE. A relatively constant C:N:P ratio (30:4:1) was observed, with little variation as a function of GE or OG, indicating a high total P content in the soil. Labile fractions (particulate) of C and N increased with GE. According to our results, GE can be used as a strategy to restore degraded natural pastures in arid and semi-arid ecosystems.

Pathways of soil organic matter formation from above and belowground inputs in a Sorghum bicolor bioenergy crop

AbstractWhen aboveground materials are harvested for fuel production, such as with Sorghum bicolor, the sustainability of annual bioenergy feedstocks is influenced by the ability of root inputs to contribute to the formation and persistence of soil organic matter (SOM), and to soil fertility through nutrient recycling. Using 13C and 15N labeling, we traced sorghum root and leaf litter‐derived C and N for 19 months in the field as they were mineralized or formed SOM. Our in situ litter incubation experiment confirms that sorghum roots and leaves significantly differ in their inherent chemical recalcitrance. This resulted in different contributions to C and N storage and recycling. Overall root residues had higher biochemical recalcitrance which led to more C retention in soil (27%) than leaf residues (19%). However, sorghum root residues resulted in higher particulate organic matter (POM) and lower mineral associated organic matter (MAOM), deemed to be the most persistent fraction in soil, than leaf residues. Additionally, the overall higher root‐derived C retention in soil led to higher N retention, reducing the immediate recycling of fertility from root as compared to leaf decomposition. Our study, conducted in a highly aggregated clay‐loam soil, emphasized the important role of aggregates in new SOM formation, particularly the efficient formation of MAOM in microaggregate structures occluded within macroaggregates. Given the known role of roots in promoting aggregation, efficient formation of MAOM within aggregates can be a major mechanism to increase persistent SOM storage belowground when aboveground residues are removed. We conclude that promoting root inputs in S. bicolor bioenergy production systems through plant breeding efforts may be an effective means to counterbalance the aboveground residue removal. However, management strategies need to consider the quantity of inputs involved and may need to support SOM storage and fertility with additional organic matter additions.

Tracing organic carbon and microbial community structure in mineralogically different soils exposed to redox fluctuations

Submerged rice cultivation is characterized by redox fluctuations and results in the formation of paddy soils, often accompanied by soil organic carbon (SOC) accumulation. The impact of redox fluctuations and the underlying soil type on the fate of organic carbon (OC) in paddy soils are unknown. Hence, we mimicked paddy soil development in the laboratory by exposing two soil types with contrasting mineral assemblages (Alisol and Andosol) to eight anoxic–oxic cycles over 1 year. Soils regularly received 13C-labeled rice straw. As control we used a second set of samples without straw addition as well as samples under static oxic conditions with and without straw. Headspaces were analyzed for carbon dioxide and methane as well as their δ13C signatures, whereas soil solutions were analyzed for redox potential, pH, dissolved iron, and dissolved organic carbon (DOC and DO13C). At the end of the experiment, when eight redox cycles were completed, mineral-associated organic matter (MOM) was isolated by density fractionation and characterized for δ13C, non-cellulosic carbohydrates, and lignin-derived phenols. Moreover, changes in the soil’s microbial community structure were measured. For both soil types, headspace data confirmed less respiration in straw-amended soils with redox fluctuation than in those under static oxic conditions. The δ13C data revealed that, irrespective of soil type, straw carbon allocation into MOM was larger in soils with redox fluctuation than in those with static oxic conditions. A net increase in MOM after the one-year incubation, however, was only observed in the respective Andosol, probably due to abundant reactive minerals capable of OC uptake. In the Alisol, straw OC most likely exchanged initial MOM. A potential for lignin accumulation in the MOM of soils incubated with straw and redox fluctuation was observed for both soil types. Lignin and carbohydrates suggest a plant origin of MOM formed under redox fluctuation. The initially similar bacterial community composition of the Alisol and Andosol changed differently under redox fluctuation. The stronger change in the Alisol indicates less protective microbial habitats. In summary, the overall turnover of straw OC in soils under redox fluctuation seems to be independent of soil type, while net accumulation of SOC as well as the evolution of the bacterial community structure may in part depend on soil type, suggesting an impact of the soil’s mineral composition.

Crop-weed competition changes the decomposition of soil organic matter fractions in the rhizosphere

ABSTRACTCertain plant combinations can stimulate the mineralization of soil organic matter (SOM), thereby changing the storage of soil organic carbon and affecting the physical and chemical soil properties. The aim of this study was to evaluate whether competition between weeds and maize can stimulate SOM decomposition. Eight treatments were performed: monoculture of maize and weeds (Amaranthus viridis, Bidens pilosa, and Ipomoea grandifolia), maize in competition with weeds, and non-cultivated soil. During cultivation, the rhizosphere priming effect (RPE) on SOM was estimated. Additionally, at 60 days after planting, soil samples were collected to measure the C contents of particulate (POM) and mineral-associated (MAOM) organic matter. From the 43rd day of cultivation onwards, the coexistence between maize and B. pilosa led to the highest RPE values, while maize vs. A. viridis showed negative RPE. Maize vs. A. viridis and maize vs. I. grandifolia caused increase in MAOM-C and decreases in POM-C. Ipomoea grandifolia monoculture and maize vs. B. pilosa led to the highest MAOM-C losses and reduced POM-C compared to the non-cultivated soils. Here it is demonstrated that competition between maize and B. pilosa increases SOM mineralization, while competition between maize and A. viridis or I. grandifolia retards this process.

Opposite influences of mineral-associated and dissolved organic matter on the transport of hydroxyapatite nanoparticles through soil and aggregates

The mechanism by which soil organic matter (SOM) controls nanoparticle transport through natural soils is unclear. In this study, we distinguished the specific effects of two primary SOM fractions, mineral-associated organic matter (MOM) and dissolved organic matter (DOM), on the transport of hydroxyapatite nanoparticles (nHAP) through a loamy soil under the conditions of saturated steady flow and environmentally relevant solution chemistry (1 mM NaCl at pH 7). The results showed that MOM could inhibit the transport of nHAP by decreasing electrostatic repulsion and increasing mechanical straining and hydrophobic interactions. Specifically, the presence of MOM reduced the mobility of nHAP in the bulk soil and its macroaggregates by ~4 fold and ~6 fold, respectively, and this hindered effect became further conspicuous in microaggregates (~36 fold decrease). An analysis of extended Derjaguin-Landau-Vervey-Overbeek (abbreviated as XDLVO) interactions indicated that MOM could decrease the primary energy barrier (Φmax1), primary minimum (Φmin1), and secondary minimum (Φmin2) to promote nHAP attachment. Conversely, DOM (10–50 mg L−1) favored nHAP mobility due to an increase in electrostatic repulsion among nHAP particles and between nHAP and soil surfaces. Pre-flushing soil with DOM (causing DOM sorption on soil) increased nHAP mobility by ~2 fold in the bulk soil and its macroaggregates, and this facilitated effect was furthered in microaggregates (~11 fold increase). The results of XDLVO interactions showed that DOM increased Φmax1, Φmin1, and Φmin2, producing an unfavorable effect on nHAP attachment. Mass recovery data revealed that the MOM-hindered effect was stronger than the DOM-facilitated effect on nHAP transport. This study suggested that changing SOM fractions could control the mobility of nanoparticles in the subsurface considerably.

Controls on mineral-associated organic matter formation in a degraded Oxisol

Oxisols are the dominant soil type in humid tropical and subtropical regions and are subjected to both drying–rewetting (DRW) cycles and fluctuating oxygen (O2) availability driven by warm temperatures and abundant rainfall in surface layers. Drying-rewetting cycles and O2 fluctuations may critically affect the microbial transformation of plant litter and subsequent stabilization as mineral-associated organic carbon (MAOC), but experimental data are still limited. We examined the impacts of DRW cycles, and variable O2 regimes with constant moisture, on carbon (C) and iron (Fe) dynamics in a degraded Oxisol (under long-term fallow) with added plant residues. In laboratory incubations (>3 months), both DRW cycling and fluctuating O2 availability induced a flush of respiration and a temporary increase in microbial biomass C (MBC) following soil rewetting or O2 exposure, although MBC was consistently suppressed in these treatments relative to the control (60% water holding capacity under constantly aerobic condition). Consequently, DRW cycles significantly increased but O2 fluctuations significantly decreased cumulative C mineralization relative to the control. Concentrations of short-range-ordered Fe oxides peaked immediately after litter addition and decreased five-fold during the remainder of the experiment. Mineral-associated C (defined as the chemically dispersed <53 μm soil fraction) increased 42–64% relative to initial values but was significantly lower under DRW cycling and fluctuating O2 relative to the control. Correspondingly, these treatments had greater fine particulate organic C (53–250 μm), despite increased CO2 production under DRW cycling. Our data indicate the potential for rapid and significant accrual of MAOC in a degraded Oxisol, but environmental factors such as DRW cycling and fluctuating O2 can inhibit the conversion of plant litter to MAOC—possibly by suppressing microbial biomass formation and/or microbial transformations of organic matter.