Source Of Soil Organic Matter Research Articles

Contribution of Litter and Root to Soil Nutrients in Different Rocky Desertification Grasslands in a Karst Area.

Litter and root decomposition is an important source of soil organic matter and nutrients. To ascertain the contribution of litter and root to natural grassland nutrients in rocky desertification areas, from March 2017 to January 2018, the continuous soil column method, collector method, and litter decomposition method were used to study the soil nutrients, litter and root biomass, decomposition, and nutrient release of potential, moderate, and severe rocky desertification grasslands, as well as their responses to rocky desertification. The results showed that the litter and root decomposition rate showed a trend of being first fast and then slow, and the decomposition rate of litter and root was greater than 50% after 300 days. The annual litter decomposition rates of potential, moderate, and severe rocky desertification grasslands were 69.98%, 62.14%, and 49.79%, respectively, and the annual decomposition rates of root were 73.64%, 67.61%, and 64.09%, respectively. With a deepening degree of rocky desertification, the litter and root decomposition rate decreased. The decomposition coefficients, k, of litter in potential, moderate, and severe rocky desertification grasslands were 1.128, 0.896, and 0.668, respectively, and the decomposition coefficients, k, of root were 1.152, 1.018, and 0.987, respectively. The nutrient release processes of litter and root were different, and the release mode ultimately manifests as "release". In rocky desertification grasslands, the organic carbon (OC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) released by litter and root decomposition were 18.93-263.03 g·m-2·yr-1, 1.79-5.59 g·m-2·yr-1, 0.18-0.47 g·m-2·yr-1, and 0.66-3.70 g·m-2·yr-1, respectively. The contribution of root to soil nutrients was greater than that of litter. The degree of rocky desertification was negatively correlated with the biomass, decomposition rate, and nutrient return amount of litter and root. The results of this study provide direct field evidence and illustrate the contribution of litter and root decomposition in rocky desertification grasslands to soil nutrients.

Lipid biomarkers and stable isotopes uncover paleovegetation changes in extremely species-rich forest-steppe ecosystems, Central Europe

The historical development of the vegetation of semi-dry grasslands in Central Europe is not satisfactorily understood. Long-term continuity of open vegetation or, conversely, deep-past forest phases are considered possible sources of the current extreme species diversity of these ecosystems. We aimed to reveal the trajectory of paleovegetation development in these ecosystems through detailed analysis of terrestrial in-situ soil geoarchives. We measured the bulk soil carbon and nitrogen contents, lipid molecular distribution, and compound-specific stable carbon and hydrogen isotopic signatures of mid- and long-chain n-alkanes extracted from soil and modern plant material tissues (i.e., deciduous and Pinus leaves and grass/herbaceous species).The C23-C33 n-alkane homologues were identified in soils with different abundances. Normally, C27 and C29 n-alkanes were the most abundant homologues in tree-leaf samples, while grass-derived n-alkanes were mostly C31 and C33 homologues. Soils were largely dominated by C29 and C31 n-alkanes. Odd-numbered C27-C33 soil n-alkane δ13C values ranged from -36.2‰ to -23.2‰, whereas their δ2H values showed a wider range of variability that fluctuated from -224‰ to -172‰. Molecular distribution in combination with radiocarbon analysis of soil organic matter (SOM) and δ13C and δ2H values of n-alkanes revealed a large contribution of C3 trees (both deciduous and coniferous trees/pine trees) as the main source of n-alkanes between the late Pleistocene and early Holocene (ca 15,000–8,200 calibrated year before present/cal year BP). A clear shift toward more grassy/herbaceous vegetation was observed from the early Holocene (ca 11,700–8,200 cal year BP) onwards. Distribution patterns of lipids and soil geochemical parameters showed that plants are the main source of SOM and that biodegradation and kinetic isotope fractionation are not the main reasons for 13C enrichment in soil profiles. Past C3 vegetation shifts as well as paleoclimate changes (i.e., aridity) can have played a role in the observed 13C depth profiles.

Сроки внесения аммиачной селитры при возделывании горчицы сарептской в качестве сидерата

In the southern regions of the Russian Federation, there has been a sharp decline in livestock production due to its low profitability. As a result, many farmers lack organic fertilizers, which are difficult to replace fully with mineral fertilizers. In addition, mineral salts cause negative processes (peptization, coagulation) in the soil profile, which worsen the physical and physical and chemical properties of the soil and its fertility in general. However, a good source of soil organic matter is not only livestock waste, but also plant residues or so-called green fertilizers. In this regard, in 2021–2022, we studied the effect of ammonium nitrate on the yield of green mass of brown mustard (Brassica juncea L.) variety Yunona on leached black soil of the Western Ciscaucasia on the fields of V.S. Pustovoit All-Russian Research Institute of Oil Crops (Krasnodar, vil. Oktyabrsky). The purpose of the research was to assess fertilizing value of ammonium nitrate (ability to return the elements of mineral nutrition at full green manuring of plants). It was found that the highest efficiency of the nitrogen fertilizer application is achieved with a single application of N30 at the seedling stage of mustard. With this specific method of N30 application, more green mass and dry biomass are incorporated into the soil, by 10.6 t/ha and 3.34 t/ha, respectively, compared to the control. At the same time, the yield of macroelements is also higher: nitrogen – by 64.3 kg/ha, phosphorus – by 14.3 kg/ha, potassium – by 155.3 kg/ha, and the estimated amount of newly formed humus increases by 0.7 t/ha comparing to control.

Developing an ecological stoichiometry-based framework for tracing the sources of soil organic matter.

Developing an ecological stoichiometry-based framework for tracing the sources of soil organic matter.

Land use changes and edaphic properties control contents and isotopic compositions of soil organic carbon and nitrogen in wetlands

Land use change in wetlands leads to significant losses of soil organic matter (SOM). Stable carbon (C) and nitrogen (N) isotopes offer insights into changes in C3/C4 vegetation, SOM sources, and decomposition processes. Yet, predicting the spatial–temporal dynamics of SOM contents and isotopes under land use changes remains challenging. This study delves into the effects of land use changes on soil organic carbon (SOC), total nitrogen (TN), δ13C and δ15N values, and soil physico-chemical properties and lignin phenols. Our results highlight the significance of soil water content (SWC) in determining the outcomes of land use changes. The conversion of wetland to cropland, forestland and construction land, led to notable reductions in SOC contents (8.71–56.33 %), and TN contents (7.87–37.12 %). Wetland conversion resulted in an enrichment of 13C and 15N abundance, with wetlands exhibiting the lowest δ13C (−25.57 to –22.89 ‰) and δ15N (2.66 to 6.67 ‰) values. A significant correlation occurred between δ13C and δ15N values in wetlands, but underwent considerable changes after wetland conversion. Key parameters, including bulk density (BD), C:N, the acid-to-aldehyde of vanillyl ((Ad/Al)v), lignin content (Λ8), and total phosphorus (TP), were identified as influencing factors for both SOC and TN contents. When evaluating δ13C values, the most influential factors included silt, C:N, SOC, sand, and BD. These indicate the importance of soil chemical group (from 41 % to 21 %) in elucidating δ13C values declined, while lignin group’s (from 9 % to 28 %) importance increased from topsoil to subsoil. The acid-to-aldehyde of syringyl ((Ad/Al)s), Λ8, C:N, BD and the cinnamyl-to-vanillyl ratio (C/V) were identified as the primary factors influencing δ15N values, with chemical group accounting for 36 % and lignin group for 48 % in topsoil, while physical group dominated 42 % in subsoil. Our findings underscore the shifts in SOM sources and distinct mechanisms of degradation/preservation of SOM following land use changes.

Coupling use of stable isotopes and functional genes as indicators for the impacts of artificial restoration on the carbon storage of a coastal wetland invaded by Spartina alterniflora, southeastern China

Coastal wetlands are characterized by high production and thus play an important role in global climate change. In past decades, the invasion of Spartina alterniflora has caused many problems of coastal wetlands in southeastern China, and the restoration of such areas was mainly conducted by replacing Spartina alterniflora with mangrove plants. This may impact the carbon storage dynamics in such areas. In this study, stable isotopes (δ13C and δ15N) and molecular analysis were used to reveal the impact of artificial restoration on the carbon storage of Quanzhou Bay Estuary Wetland Natural Reserve. The major results are as follows: (1) the change in dominant plants results in a changing major source of soil organic matter, from external sources to mangrove plants; (2) the decrease in soil organic matter following the removal of Spartina alterniflora may be primarily caused by the loss of external organic matter, while the production of mangroves may offset such loss and enhance the content and stability of carbon storage over the long term; (3) microbial CO2 assimilation may serve as an alternative source of bioavailable carbon and thus support the activity of benthic community. Our results revealed the long-term benefits of such restoration on the carbon storage function of wetlands invaded by Spartina alterniflora. Furthermore, the integrating of isotopic tracers and molecular technology may provide new insights in understanding the response of the carbon storage in coastal areas to human activity.

Litter and root sources of soil organic matter in a temperate forest: Thirty years in the DIRT

AbstractManagers increasingly seek to increase forest soil carbon but long‐term controls on soil organic matter (SOM) sources and stability are weakly understood. We used a 30‐year detrital input/removal treatment experiment in a deciduous forest to evaluate the importance of root and leaf litter to SOM. Inputs were assessed by excluding roots and leaves (litter) or by doubling litter inputs. In mineral soil, %SOM differed only at 0–10 cm (p = 0.11), with concentrations in the no litter and no roots treatments of 26% and 9%, respectively, lower than the controls. Cessation of litter inputs had a stronger effect on SOM than cessation of root inputs, but root litter inputs may contribute stable SOM. Doubled litter increased mineral SOM.

Fire Impacts on the Soil Metabolome and Organic Matter Biodegradability.

Global wildfire activity has increased since the 1970s and is projected to intensify throughout the 21st century. Wildfires change the composition and biodegradability of soil organic matter (SOM) which contains nutrients that fuel microbial metabolism. Though persistent forms of SOM often increase postfire, the response of more biodegradable SOM remains unclear. Here we simulated severe wildfires through a controlled "pyrocosm" approach to identify biodegradable sources of SOM and characterize the soil metabolome immediately postfire. Using microbial amplicon (16S/ITS) sequencing and gas chromatography-mass spectrometry, heterotrophic microbes (Actinobacteria, Firmicutes, and Protobacteria) and specific metabolites (glycine, protocatechuate, citric cycle intermediates) were enriched in burned soils, indicating that burned soils contain a variety of substrates that support microbial metabolism. Molecular formulas assigned by 21 T Fourier transform ion cyclotron resonance mass spectrometry showed that SOM in burned soil was lower in molecular weight and featured 20 to 43% more nitrogen-containing molecular formulas than unburned soil. We also measured higher water extractable organic carbon concentrations and higher CO2 efflux in burned soils. The observed enrichment of biodegradable SOM and microbial heterotrophs demonstrates the resilience of these soils to severe burning, providing important implications for postfire soil microbial and plant recolonization and ecosystem recovery.

Effects of citric acid on arsenic transformation and microbial communities in different paddy soils

Root exudate is a major source of soil organic matter and can significantly affect arsenic (As) migration and transformation in paddy soils. Citric acid is the main component of rice root exudate, however, the impacts and rules of citric acid on As bioavailability and rhizobacteria in different soils remains unclear. This study investigated the effects of citric acid on As transformation and microbial community in ten different paddy soils by flooded soil culture experiments. The results showed that citric acid addition increased total As and arsenate (As(V)) in the soil porewater by up to 41-fold and 65-fold, respectively, after 2-h incubation. As(V) was the main As species in soil porewater within 10 days with the addition of citric acid. Non-specifically sorbed As of soils, total Fe and total As were the main environmental factors affecting the soil microbial communities. High-throughput sequencing analysis demonstrated that citric acid addition significantly altered the soil microbial community structure, shifting the Proteobacteria-related reducing bacteria to Firmicutes-related reducing bacteria in different paddy soils. The relative abundance of Firmicutes was promoted by 174–196%. Clostridium-related bacteria belonging to Firmicutes became the dominant genera, which is believed to regulate As release through the reductive dissolution of iron oxides or the direct reduction of As(V) to arsenite (As(III)). However, citric acid addition significantly decreased the relative abundance of Geobacter and Anaeromyxobacter, which are also typical active As(V)- and ferric-reducing bacteria. Real-time quantitative polymerase chain reaction (qPCR) also revealed that the addition of citric acid significantly decreased the relative abundances of Geobacter in the different soils by 8–28 times while the relative abundances of Clostridium increased by 2–5 times. These results provide significant insight on As transformation in different types of rice rhizospheric soils and guidance for the application of rice varieties with low citric acid exuding to restrict As accumulation.

Source or decomposition of soil organic matter: what is more important with increasing forest age in a subalpine setting?

Afforestation has been the dominant land-use change in the Swiss Alps during the last decades which has not only the potential to increase soil organic carbon sequestration, but it has also the potential to alter soil organic matter (SOM) dynamics through the vegetation shift and change in organic matter (OM) input into soils. The effects of afforestation on SOM dynamics, however, are still not fully understood as specific sources of OM and modifications of soil processes influencing decomposition and preservation remain largely unknown on alpine to subalpine slopes. Within this study we aimed to identify the potential sources and the decomposition of OM in a subalpine afforestation chrono-sequence (0–130 years) with Norway spruce (Picea abies L.) on a former pasture by using a multi-proxy molecular marker approach. We observed that leaf-derived OM plays an essential role in the pasture areas, while root-derived OM only plays a minor role in pasture and forest areas. Needle-derived OM represents the dominant source of SOM with increasing forest age, while understory shrubs and moss also contribute to the OM input in younger forest stand ages. However, needle litter and buildup of organic layers and subsequently less input of fresh OM from organic horizons to mineral soil can result in increased OM decomposition in mineral soils rather than contributing to additional SOM stabilization in mineral soils. This was most pronounced in the oldest forest stand (130-year-old) in the investigated afforestation sequence, particularly in deeper soil horizons (10–45 cm). Thereby, our study provides new insights into SOM dynamics following afforestation, especially with respect to the long-term SOM sequestration potential of afforestation of subalpine pasture soils.

Divergence in soil particulate and mineral-associated organic carbon reshapes carbon stabilization along an elevational gradient

Variations in climates and vegetation types along elevations in mountain ecosystems strongly reshape soil organic matter (SOM) stabilization. However, soil carbon (C) stabilization pathways from litter to SOM and potential mechanisms along elevations in the subalpine area remain unclear. Here, we investigated the C stabilization pathway from litter to particulate and mineral-associated organic matter (POM and MAOM) and associated drivers along elevational gradients in the Hengduan Mountains, China, based on δ13C signatures. Our results showed C stabilization pathways from litter to POM to MAOM, irrespective of elevation. Unexpectedly, the δ13C values of SOM and its fractions (POM and MAOM) were not significantly related to litter δ13C across elevations although litter was the main source of SOM, suggesting that other potential drivers could weaken the coordination of δ13C among litter and SOM fractions. We observed that the△13CLitter→POM (△13C between litter and POM) was more dependent on the direct effect of soil texture (i.e., sand, silt and clay). However, the △13CPOM→MAOM (△13C between POM and MAOM) was more related to microbial attributes (i.e., microbial community and enzyme activities).These results emphasizing the different stabilization procedures between POM and MAOM. Particularly, soil temperature dominantly determined △13CLitter→POM, but soil pH was the primary factor regulating the △13CPOM→MAOM. Overall, our results suggested that POM and MAOM formation occurred with different regulation processes, the POM formation was primarily via more physical pathways and MAOM formation mainly via more microbial pathways, thus providing novel insights into the better understanding of SOM formation and stabilization mechanisms in mountain ecosystems.

Soluble phenols in litter are reduced during passage through the soil macrofauna gut due to the formation of insoluble complexes with proteins: A case study with isopods and Diptera larvae

Plant litter represents a major source of soil organic matter and hence understanding the pathway of its decomposition and stabilisation is crucial for understanding the soil carbon dynamic. Importantly, more than half of annual litter production is consumed by soil fauna globally. Here we test the hypothesis that soluble phenols can be removed from the litter during gut passage and bound on nitrogen rich compounds, which results in formation of insoluble complexes in the faeces produced by soil fauna. Two invertebrate species, Diptera larvae Bibio marci and terrestrial isopod Armadilidium vulgare, and litter of two tree species, Quercus robur and Alnus glutinosa, were used. Both species of invertebrates were fed by both litter types in full factorial manner. The litter contained significantly more free phenols than the faeces. In opposite, the content of bound phenols was significantly higher in faeces than in the litter. The loss of total nitrogen associated with removal of bound phenolic compounds from faeces was higher than that of the litter. Thus, soil fauna contributes to the stabilisation of soil organic matter by the conversion of soluble phenols to insoluble ones together with nitrogen immobilization. In conclusion, this process of stabilisation likely plays an important role in mature soils where mineral surfaces have reached carbon saturation or in soils with higher organic horizons.

Drivers of microbially and plant-derived carbon in topsoil and subsoil.

Plant- and microbially derived carbon (C) are the two major sources of soil organic matter (SOM), and their ratio impacts SOM composition, accumulation, stability, and turnover. The contributions of and the key factors defining the plant and microbial C in SOM along the soil profile are not well known. By leveraging nuclear magnetic resonance spectroscopy and biomarker analysis, we analyzed the plant and microbial C in three soil types using regional-scale sampling and combined these results with a meta-analysis. Topsoil (0-40 cm) was rich in carbohydrates and lignin (38%-50%), whereas subsoil (40-100 cm) contained more proteins and lipids (26%-60%). The proportion of plant C increases, while microbial C decreases with SOM content. The decrease rate of the ratio of the microbially derived C to plant-derived C (CM:P ) with SOM content was 23%-30% faster in the topsoil than in the subsoil in the regional study and meta-analysis. The topsoil had high potential to stabilize plant-derived C through intensive microbial transformations and microbial necromass formation. Plant C input and mean annual soil temperature were the main factors defining CM:P in topsoil, whereas the fungi-to-bacteria ratio and clay content were the main factors influencing subsoil CM:P . Combining a regional study and meta-analysis, we highlighted the contribution of plant litter to microbial necromass to organic matter up to 1-m soil depth and elucidated the main factors regulating their long-term preservation.

A novel framework for improving soil organic matter prediction accuracy in cropland by integrating soil, vegetation and human activity information

Remote sensing is an important tool for monitoring soil information. However, accurate spatial modeling of soil organic matter (SOM) in areas with high vegetation coverage, typically represented by agroecosystems, remains a challenge for field-scale estimation using remote sensing. To date, studies have focused on using single-period or multi-temporal vegetation information to characterize SOM. Thus, the relationship between SOM content and time-series vegetation biomass has not yet been fully explored. In addition, most studies have ignored the effects of critical soil properties and human activities (e.g., soil salinization, soil particle size fractions, history of land-use changes) on SOM. By integrating information on vegetation, soil, and human activities, we propose a novel framework for assessing SOM in cotton fields of artificial oases in northwest China, where returned straw is one of the primary sources of SOM coming from vegetation. We developed an Annual Maximum Biomass Accumulation Index (AMBAI) using time-series Landsat images from 1990 to 2019. Subsequently, we quantified the information of the planting years (PY) of cropland using spectral index threshold and incorporated proximal sensing data (soil hyperspectral and apparent conductivity data) and soil particle size fractions to establish a predictive model of SOM using partial least squares regression (PLSR), random forest (RF), and convolutional neural network (CNN). The results revealed that AMBAI had the highest correlation coefficient (r) with SOM (0.76, P < 0.01). AMBAI, soil hyperspectral data, and PY were the most relevant predictors for estimating SOM. The CNN model integrating vegetation, soil, and human activity information performed best, with coefficient of determination (R2), relative analysis error (RPD), and root mean square error (RMSE) of 0.83, 2.38 and 1.38 g kg−1, respectively. This study confirmed that AMBAI and PY had great potential for characterizing SOM in arid and semi-arid regions, providing a reference for other relevant studies.

Tracing sources and turnover of soil organic matter in a long-term irrigated dry forest using a novel hydrogen isotope approach

The contribution from foliage, roots, and fungi to soil organic matter (SOM) represents a key unknown in SOM dynamics. Since stable hydrogen isotope ratios (δ2Hn) can greatly differ among these sources, we explored δ2Hn to elucidate sources contribution to SOM. Moreover, we assessed whether addition of 2H-depleted water – providing 2H-signal that gets incorporated into organic matter – allows tracing of new organic H and thus assessing SOM turnover.In a 17-year-long irrigation experiment in a dry pine forest, we measured δ2Hn and stable carbon and nitrogen isotope ratios (δ13C, δ15N) in SOM sources, bulk SOM and particle-size fractions. Relative source contribution to SOM was estimated by Bayesian mixing models including all three isotopes.The δ2Hn increased from foliar litter (−153‰) to fine roots (−118‰) and fungal mycelia (−81‰). Mixing models indicated that foliar litter dominated organic layers (69 ± 15%) and coarse particulate organic matter (POM) at 0–5 cm (65 ± 12%). In contrast, roots dominated fine POM at 2–5 cm (58 ± 12%). Fungal mycelia contributed only to 1–5% of coarse and fine POM, but to 5–25% of mineral associated organic matter (MOM) at 0–5 cm.The soil water 2H-depletion with irrigation (−76‰ vs −68‰ at 0–10 cm) was paralleled by lower δ2Hn in coarse POM of irrigated vs dry plots (−122‰ vs −111‰ at 0–5 cm), indicating organic H turnover of less than 17 years. In contrast, δ2Hn in fine POM and MOM decreased only by 3‰ with irrigation, implying mean residence times of approximately 30 years.While δ13C and δ15N differed by a maximum of 1‰ between plant sources, δ2Hn were 30–39‰ higher in roots than foliar litter, thus improving SOM sources differentiation. Long-term 2H-labelling by irrigation indicated higher organic H turnover in coarse vs fine soil fractions, offering a novel tool to identify SOM cycling and microbial H incorporation into SOM pools.

The relationships between heavy metals and bacterial communities in a coal gangue site

Coal is the main source of energy for China's economic development, but coal gangue dumps are a major source of heavy metal pollution. Bacterial communities have a major effect on the bioremediation of heavy metals in coal gangue dumps. The effects of different concentrations of heavy metals on the composition of bacterial communities in coal gangue sites remain unclear. Soil bacterial communities from four gangue sites that vary in natural heavy metal concentrations were investigated using high-throughput sequencing in this study. Correlations among bacterial communities, heavy metal concentrations, physicochemical properties of the soil, and the composition of dissolved organic matter of soil in coal gangue dumps were also analyzed. Our results indicated that Actinobacteriota, Proteobacteria, Chloroflexi, Acidobacteriota, and Gemmatimonadota were the bacterial taxa most resistant to heavy metal stress at gangue sites. Heavy metal contamination may be the main cause of changes in bacterial communities. Heavy metal pollution can foster mutually beneficial symbioses between microbial species. Microbial-derived organic matter was the main source of soil organic matter in unvegetated mining areas, and this could affect the toxicity and transport of heavy metals in soil. Polar functional groups such as hydroxyl and ester groups (A226-400) play an important role in the reaction of cadmium (Cd) and lead (Pb), and organic matter with low molecular weight (SR) tends to bind more to mercury (Hg). In addition to heavy metals, the content of nitrogen (N), phosphorus (P), and total organic carbon (TOC) also affected the composition of the bacterial communities; TOC had the strongest effect, followed by N, SOM, and P. Our findings have implications for the microbial remediation of heavy metal-contaminated soils in coal gangue sites and sustainable development.

Complementation between microbial necromass and plant debris governs the long-term build-up of the soil organic carbon pool in conservation agriculture

Plant residue is ultimately the source of soil organic matter but occurs as a continuum from relatively fresh plant tissue to highly evolved components. Nevertheless, the extent to which crop residue accumulates as plant tissue versus microbial necromass in conservation agricultural systems is still uncertain. A 12-year conservation tillage experiment with maize additions of zero, half and full harvest was conducted in Northeast China. Soil samples in the 0–10 cm depth were collected at 1–4 year intervals. The microbial residue biomarker amino sugar and plant component biomarker lignin phenol were quantified to trace the dynamics of microbial necromass and plant debris. Compared with the initial value, the content of amino sugar increased exponentially to 21–45% and approached steady-state equilibrium after 12 years of maize straw mulching, indicating the existence of “microbial carrying capacity” under repeated plant residue addition. With prolonged duration of conservation tillage and increasing rate of maize straw return, the ratio of amino sugar to lignin phenol decreased from 6.2 to 1.7, accompanied by a decreased degree of lignin side-chain oxidation, indicating that organic input enhanced the selective retention of fresh plant lignin to a larger extent than microbial necromass. The build-up of the SOC pool under conservation tillage was critically controlled by the functional complementation of microbial- and plant-derived components. Maize straw input was able to enhance the contribution of microbial necromass to SOC stabilization by increasing fungal necromass accumulation, while the retention of lignin-like plant debris enlarged the SOC pool capacity and potentially maintained the decomposability of SOC in the long term.

Biochemical composition of soil organic matter physical fractions under 32-year fertilization in Ferralic Cambisol

Biochemical properties of soil organic matter (SOM) are fundamental for soil fertility and health. However, it is unclear how fertilization regime influences the biochemical compositions and oxidation states of SOM and physical fractions. In this study, this issue was studied under four 32-year amendment regimes: unfertilized control, urea (N), N + calcium dihydrogen phosphate + potassium chloride (NPK), and NPK plus manure (NPKM). Three physical fractions: coarse particulate (> 250 μm, cPOM), fine particulate (53–250 μm, fPOM) and mineral-associated OM (< 53 μm, MAOM) were separated and measured by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). Compared with the background in 1986, the SOM increased by 10.6%, 14.2%, 23% and 52% in unfertilized control, N, NPK, and NPKM, respectively. The red soil here had not reached carbon saturation, because of the low conversion efficiency (6.8%) from input-carbon to soil organic carbon (SOC). Physical size but not amendment type primarily regulated the SOM molecular composition, with relative selective retention of aromatics and lignin in both the cPOM and fPOM, whereas N-containing compounds (particularly amino-N) were enriched in MAOM due to their high abilities to adsorb soil minerals. The C oxidation state was also mainly dependent on physical size, with the highest value in fPOM. The sources of SOM and its fractions, dominated by microbial-derived compounds (60–90%), were independent of physical size and fertilization. In conclusion, physical size arrangement (proxy of microbial decomposition degree) played a more important role in regulating the SOM biochemical features than initial quality of various amendments.Graphical

Coupling of δ13C and δ15N to understand soil organic matter sources and C and N cycling under different land-uses and management: a review and data analysis

Coupling of δ13C and δ15N to understand soil organic matter sources and C and N cycling under different land-uses and management: a review and data analysis

Soil Carbon within the Mangrove Landscape in Rufiji River Delta, Tanzania

Mangroves are among the most carbon-rich terrestrial ecosystems, primarily attributable to the soil pool. There are substantial differences in soil carbon (C) and nitrogen (N) due to the disparities in geomorphic settings and ecological drivers, but this insight is drawn primarily from observational studies. An objective inventory of carbon stocks in mangroves of the Rufiji River Delta, Tanzania was conducted. Seventy-five soil cores were collected within a 12,164 ha inventory area, comprising the northern portion of the delta. Cores were collected from intact and dwarf mangroves, agricultural fields, and mudflats. The spatial mean soil organic carbon (SOC) density in mangroves was 16.35 ± 6.25 mg C cm−3. Mean SOC density in non-vegetated mudflats was 12.16 ± 4.57 mg C cm−3, demonstrating that mangroves develop on soils with a substantial soil C stock. However, long-established mangroves had had a higher C density (17.27 ± 5.87 mg C cm−3). Using a δ13C mixing model, the source of soil organic matter in mudflats was primarily marine, while long-established mangroves was predominantly mangrove. There were small differences in SOC among long-established mangrove sites in different geomorphic settings. The proportion of marine-sourced SOC increased with soil depth in mangroves. The SOC and nitrogen of agricultural sites resemble those of mudflats, suggesting those sites are developed from relatively young forests. The SOC and nitrogen density in dwarf mangrove sites were lower than others, perhaps reflecting past disturbances.