Soil Organic Matter Stabilization Research Articles

Differences in soil carbon and nitrogen pools between afforested pine forests and natural shrublands in a Mediterranean area

Plant cover and microclimatic conditions can profoundly alter the balance between productivity and decay, with relevant effects on soil carbon (C) and nitrogen (N) pools. Despite their importance, the combined effects of plant cover and microclimatic conditions on soil C and N pools have been poorly assessed, especially in the Mediterranean environment. Therefore, the aims of the present study were to assess how, in Mediterranean areas, C and N pools differ between afforested pine forests and natural shrublands and to highlight the different impacts of plant cover on C and N sequestration at low and high elevations Along a forest floor-mineral soil continuum, plant cover and elevation effects were evaluated through measurements of C and N pools and water-soluble fractions, molecular characterization by 13C and 1H NMR, and microbial and fungal amounts and activities. Our data show that C accumulated more in afforested pine forest soils than in shrubland soils, especially at low elevations. In pine forests, the higher content of aromatic and O alkyl compounds in the upper organic layers and the abundance of aromatic and carboxylic components in the soluble fraction suggest a greater stability of soil organic matter than in shrublands. Additionally, the high concentration of N in the upper organic layer and its reduction in the fermentative layer stimulate soil C accumulation mainly in pine forests at low elevations. The abundant organic mass at high elevations in pine forests improves microbial growth, whereas the greater recalcitrance of organic residues at low elevations in pine forests leads to a decrease in the bacterial component compared with the fungal component.In conclusion, the plant cover effect appears to be strongly conditioned by elevation, and afforestation with pine at low elevations could favour long-term soil C storage.

Arbuscular mycorrhizal symbiosis enhances water stable aggregate formation and organic matter stabilization in Fe ore tailings

Organo-mineral association and water-stable aggregation in finely textured tailings are critically important to the eco-engineered soil formation from alkaline Fe ore tailings for sustainable mine site rehabilitation. Arbuscular mycorrhizal (AM) symbiosis plays important roles in soil aggregate formation and organic matter (OM) stabilization. However, it is unknown if AM symbiosis could enhance aggregate formation and OM stabilization in alkaline Fe ore tailings. The present study aimed to investigate the establishment of AM symbiosis and their role in tailing aggregate formation coupled with OM stabilization, as well as the underlying mechanisms. After initial eco-engineering (OM amendment and pioneer plant cultivation) to improve physicochemical conditions for plant survival, Sorghum spp. Hybrid cv. Silk inoculated with/without AM fungi (Glomus spp.) were cultivated in the tailings under glasshouse conditions for 14 weeks. The results indicated that AM fungi formed symbiotic association with Sorghum spp. plants, improved mineral nutrient (e.g., P) acquisition and root growth in the eco-engineered tailings. The AM symbiosis significantly improved aggregate formation. The association of organic carbon and nitrogen with tailing minerals of the aggregates was enhanced by the AM symbiosis. As revealed by synchrotron-based C 1 s near edge X-ray absorption fine structure (C 1 s NEXAFS) and Fe K edge X-ray absorption fine structure (Fe K edge XAFS) spectroscopy, the AM symbiosis favoured carboxyl and aromatic C association with secondary Fe-Si minerals, which may have been formed from AM driven mineral weathering. Overall, the study revealed that the AM symbiosis could not only improve the growth of pioneer plant species in the early eco-engineered tailings, but also advance soil formation through enhancing organic C and N sequestration and physical structure development via water-stable aggregation. These findings help to advance our understanding of the importance of AM symbiosis in the eco-engineering of tailings into functional soil (or technosols) for sustainable rehabilitation of Fe-ore tailings.

Long‐term biochar application governs the molecular compositions and decomposition of organic matter in paddy soil

AbstractBiochar addition can enhance soil quality and sequester carbon. However, changes in soil organic matter (SOM) molecular compositions in response to long‐term biochar addition have rarely been studied. Therefore, we quantified soil organic carbon fractions, carbon‐cycling enzyme activities, and a range of organic compounds and lignin‐derived phenols in the rhizosphere and bulk soils in different rice growth stages in the 8‐year field trial with biochar application rates of 0 (BC‐0), 4.5 (BC‐L), and 13.5 t ha−1 year−1 (BC‐H). We found that higher amounts of biochar addition (BC‐H) increased labile organic carbon (LOC), dissolved organic carbon (DOC) and particulate organic carbon (POC), and promoted activities of α‐1,4‐glucosidase, β‐D‐cellobiohydrolase and β‐1,4‐xylosidase; in contrast, BC‐L treatment reduced activities of these enzymes. The concentrations of dichloromethane/methanol‐extractable plant‐ and microbial‐derived organic compounds in the rhizosphere and bulk soils at tillering decreased significantly in the treatment with low amount of biochar addition. BC‐L also significantly altered the concentrations of extracted compounds in the rhizosphere soil at tillering and harvest. Concentration of lignin in the bulk soil was significantly reduced in BC‐L at tillering (by 19%) and harvest (by 28%). The concentrations of extracted compounds (e.g., n‐alkanols, n‐alkanoic acids, steroids, and carbohydrates) and lignin were generally significantly higher in the bulk than the rhizosphere soil at tillering and harvest. Long‐term biochar application (BC‐H) promoted lignin decomposition in the bulk soil (at tillering) and the rhizosphere soil (at harvest). Hence, biochar decreased stability of lignin in paddy soil. Our study provided evidence that long‐term biochar application changed the molecular composition and dynamics of degradation of SOM. These results deepen our understanding of the mechanisms governing SOM stability in agricultural ecosystems.

Contribution of Tamarix aphylla to soil organic matter evolution in a natural semi-desert area in Tunisia

A soil, classified as Arenosol (Eutri-Aridic Arenosol (Calcaric)), located in Neffatia (Tunisia) and populated by the shrub tamarisk (Tamarix aphylla), was studied to assess how the litter deriving from tamarisk can affect its characteristics. Several parameters were considered: particle size distributions (PSD), pH, cation exchange capacity (CEC), total CaCO3 content, total nitrogen (TKN), phosphorus and total organic carbon (TOC). Down the soil profile the pH increased from 7.83 to 8.32 as a probable consequence of salts accumulation deriving from the mineralization of the organic matter and the limited leaching due to low rainfall. As expected, TOC and TKN decreased, from the top downwards, and the two parameters were well correlated (TOC vs TKN: R2 = 0.98; p < 0.05; n = 3). CEC assumed progressively lower values reflecting the decreasing organic matter content (CEC vs TOC: R2 = 0.93; p < 0.05; n = 3). PSD showed that the presence of roots influenced the quantity of fine particles down the profile and the PSD cumulative curves were indicative of an aeolian origin for the soil parent material, confirming the hypothesis that tamarisk interacts with the environment, trapping sediment and forming the so-called phytogenic dunes. By chemical and spectroscopic analyses, it was possible to assess that tamarisk plant residues directly contributed to the soil organic matter (SOM) accumulation and characteristics. Stable SOM (ligno-humic fraction) closely resembles that of the plant (leaves and stems) and is chemically lacking in the more easily degradable organic components such as fats, hemicellulose, cellulose and proteins. 13C CPMAS NMR spectroscopy showed that the so-called soil ligno-humic fraction consists of aromatic molecules such as tannins, and aliphatic carbon (i.e. cutins and suberins) already present in the plant and preserved by mineralization processes because they are the most resistant to biological degradation.

Soil organic carbon content and mineralization controlled by the composition, origin and molecular diversity of organic matter: A study in tropical alpine grasslands

The consensus for mechanisms controlling soil organic matter (SOM) persistence has shifted from traditional views based on SOM recalcitrance to a new paradigm based on SOM stabilization controlled by soil minerals and aggregates. Recent studies indicate that the origin, composition and molecular diversity of SOM are crucial to the decomposition and stabilization of SOM. However, it is not fully understood how the decomposition and stabilization of SOM are controlled at the molecular level. The objectives of this study were to investigate whether soil organic carbon (SOC) contents and mineralization are controlled by the composition, origin and molecular diversity of SOM. Soil samples were collected from contrasting bedrocks with different precipitation levels at tropical alpine grasslands of the Peruvian Andes. We applied a combination of a 76-day soil incubation experiment and pyrolysis-GC/MS assisted by thermochemolysis to investigate SOM decomposition and stabilization at the molecular level. The results indicated that soil samples with high SOC contents (92.6 ± 7.6 g kg−1 soil) and low SOC mineralization had abundant derivates of lignin, polysaccharides and n-alkanes. After the incubation, we observed neither a selective decomposition of any compound groups nor a decline of molecular diversity. In contrast, soil samples with low SOC contents (30.7 ± 2.8 g kg−1 soil) and higher SOC mineralization showed a depletion of plant-derived compounds, an accumulation of microbial-derived compounds and declined molecular diversity after the incubation. Furthermore, the SOC mineralization of these samples was positively correlated to the depletion of unsaturated fatty acids and the decrease in molecular diversity after the incubation. Therefore, we proposed that SOC contents and mineralization in our soils are (1) controlled by selective preservation of SOM molecular groups (e.g. plant-derived compounds), and (2) associated with changes in molecular diversity of SOM during microbial decomposition. Due to the selective preservation of organic compounds under different environmental conditions, we propose that environmental factors should be considered for the management of ecosystem services such as SOC sequestration in the studied region.

Antioxidant properties of soils and associated vegetation in the polar urals

The increase in global air temperatures and climate aridization, associated with climate change, contribute to the biodegradation of soil organic matter (SOM) which is crucial for the ecosystems in cold and humid environments. SOM of the high-latitude mountain systems might be more vulnerable to temperature growth compared to that of plain landscapes due to higher soil drainage and solar irradiation. Thus, it is important to study the biochemical processes underlying the conservation of biomolecules of organic residues in these environments. Even though antioxidants might in principle contribute to stabilization of SOM, at the moment there is no established theory about the sources of soil antioxidant activity (AOA) and the mechanisms of its realization. In particular, in the relevant literature there is no data on the AOA of cryogenic soils. In this study we assess the antioxidant activity (radical scavenging activity and the ability to inhibit the initiated lipid peroxidation) of soil extracts from mountain tundra and meadows in Polar Ural (Russia) in model biological substrate. We show that SOM from sites with closed vegetation is less susceptible to oxidative degradation due to the high content of amino acids, organic forms of C and N including their water-soluble fractions, and low C/N ratio. On sites with scarce vegetation, soils exhibit low AOA and are vulnerable to the increase in temperature and aeration. Decreased AOA, low productivity of tundra plant communities, and the occurrence of large bare spots (up to 70% of the habitat) make these ecosystems most sensitive to climate change.

Interactions between microbial diversity and substrate chemistry determine the fate of carbon in soil

Microbial decomposition drives the transformation of plant-derived substrates into microbial products that form stable soil organic matter (SOM). Recent theories have posited that decomposition depends on an interaction between SOM chemistry with microbial diversity and resulting function (e.g., enzymatic capabilities, growth rates). Here, we explicitly test these theories by coupling quantitative stable isotope probing and metabolomics to track the fate of 13C enriched substrates that vary in chemical composition as they are assimilated by microbes and transformed into new metabolic products in soil. We found that differences in forest nutrient economies (e.g., nutrient cycling, microbial competition) led to arbuscular mycorrhizal (AM) soils harboring greater diversity of fungi and bacteria than ectomycorrhizal (ECM) soils. When incubated with 13C enriched substrates, substrate type drove shifts in which species were active decomposers and the abundance of metabolic products that were reduced or saturated in the highly diverse AM soils. The decomposition pathways were more static in the less diverse, ECM soil. Importantly, the majority of these shifts were driven by taxa only present in the AM soil suggesting a strong link between microbial identity and their ability to decompose and assimilate substrates. Collectively, these results highlight an important interaction between ecosystem-level processes and microbial diversity; whereby the identity and function of active decomposers impacts the composition of decomposition products that can form stable SOM.

Soil Particulate and Mineral-Associated Organic Matter Increases in Organic Farming under Cover Cropping and Manure Addition

This study aimed to investigate the soil organic carbon (SOC) sequestration rate and soil organic matter (SOM) composition in conventional rotational cropping with mineral fertilization compared with organic cover cropping with and without composted manure addition during 2008–2018 to specify the SOM stabilization under different farming systems. The SOC proportion in particulate organic matter (POM) (63–2000 µm) and mineral-associated organic matter (MAOM) (&lt;63 µm) fractions were estimated in different treatments, and the SOM composition in the fractions was characterized by FTIR spectroscopy. The SOC sequestration rate was treatment-dependent, with the higher SOC sequestration rate (1.26 Mg ha−1 y−1) in the organic treatment with cover crop and composted manure. Across all treatments, 57.3%–77.8% of the SOC stock was in the MAOM fraction. Mineral N fertilization increased POM-C concentration by 19%–52% compared with the unfertilized control. Under the organic treatments, the POM-C concentration was 83%–95% higher than the control. The MAOM-C concentration increased by 8%–20%. The mineral N fertilization and organic treatments (with and without cover crops and composted manure) increased the SOC stock proportion of POM. The highest proportion of SOC stock related to POM was in the cover cropping system, reducing the proportion of C related to the MAOM fraction, but the addition of composted manure with cover cropping also increased the proportion of C in MAOM. Compared with MAOM, the POM had a less resistant organic matter composition, and the POM resistance was higher in organic than conventional treatments. In general, the recalcitrance of SOM increased with SOC concentration. The POM fraction had higher aromaticity (or degree of decomposition) than the MAOM fraction. The aromaticity in POM and MAOM fractions was higher in the organic farming system and depended on mineral N fertilization and cover cropping, but the effect of manure was not significant. Although the SOC sequestration rate was higher under manure addition, resulting in the highest formation of both POM and MAOM in the soil, manure addition had little effect on overall SOM composition compared with cover crops.

Predicting spatial distribution of soil organic matter using regression approaches at the regional scale (Eastern Azerbaijan, Iran).

Soil organic matter (SOM) is one of the important factors in arid and semiarid areas, which describes the soil quality. Spatial estimation of SOM is important to understand the SOM storage and the emphasis of the SOM in the global carbon cycle and environmental issues. Mapping of SOM content can have significant uses in environmental modeling. In the current study, various methods have been evaluated for estimating the SOM content through soil samples and using auxiliary variables in the west of Eastern Azerbaijan province, Iran. In this study, support vector machine (SVM), multi-factor regression (MFR), and multi-factor weighted regression model (MWRM) approaches have been suggested for predicting and investigating the spatial distribution of SOM. In total, 155 surface soil samples (from the depth of 0 to 30cm) were obtained. These soil samples were randomly divided into training data set (105 soil samples) and testing data set (50 samples). According to the results, SOM is affected by soil properties as well as environmental factors (normalized difference vegetation index (NDVI)). In this study, clay, silt/sand, NDVI, and soil moisture were used as auxiliary variables in the estimation of SOM. Three methods were compared to determine a suitable method for spatial estimation of SOM, and results showed that SVM has the lowest estimation error (RMSE = 0.100, MAE = 0.07, and MRE = 3.32) and highest regression coefficient (R2 = 0.719) during SOM estimation. The present results show the indirect effect of elevation by controlling auxiliary variables and confirm the importance of auxiliary variables in spatial distribution patterns of SOM.

Impact of siltation on the stability of organic matter in drained peatlands

Peatlands are unique ecosystems but when they are drained for agriculture or peat mining, stored carbon is released to the atmosphere as CO2. The aim of the study was to assess the influence of siltation on secondary humification basing on humification indicators (C/N ratio, humification degree, humification index, E4/E6 ratio). It is essential to examine to what extent siltation hampers soil organic matter (SOM) oxidation. The study revealed that after drainage the amount of humus fractions, both acid and alkali soluble, were increasing at the expense of non-soluble carbon pool (residuum). Siltation hampered the transformation of organic matter and the quantities of OC and N in humus fractions were lower in silted soil samples than in not silted ones. The carbon extracted in acid-soluble fraction was not related to the state of siltation and was associated with the depth. The nitrogen content in this fraction did not change after drainage or siltation. The siltation did not affect C/N ratios. In unaltered peats these ratios were typical for fen soils (lowland peats) but in other studied soil samples the values of the C/N ratios suggested secondary transformation. The humification indicators (HI, HD and E4/E6) also revealed that secondary humification was more advanced in strongly silted soils, i.e. the stability of SOM in these soils is lower and the values of these indicators were dependent on the state of siltation. The study revealed that both drainage and siltation of soils changed the relations in SOM fractions. Drainage of peatlands resulted in initiation of the secondary humification of SOM and increase of chemically extractable humus fractions as well as increase of the humification degree. Stable SOM, which can have a mean residence time of centuries, became a potential source of CO2. However, siltation which led to a 50% decrease in SOM, hampered oxidation of SOM and had a stabilizing effect on SOM quality. These findings provide new insights into possibilities of mitigation of C losses from agricultural areas and may serve as a guidance for soil C management.

Sorption and desorption of organic matter in soils as affected by phosphate

The contribution of different adsorption processes to soil organic matter (SOM) stabilization and the consequences of the intensification of land use on the adsorption of SOM are not yet fully understood. Therefore, this study aimed to explore the adsorption of dissolved organic carbon (DOC) in soils as well as desorption of organic carbon (OC) caused by phosphate addition. We conducted desorption and sorption experiments with DOC, phosphate (Na2HPO4), and chloride (KCl) in topsoil and subsoil samples from a Ferralsol, an Andosol, and a Podzol. Furthermore, we quantified the size of DOC by size-exclusion chromatography.We found that phosphate addition caused a strong increase in the DOC concentration in all soils. The DOC concentration was elevated by up to a factor of 4.5 compared to a control (water only) within five minutes after phosphate addition and kept increasing further with time, particularly in the Ferralsol. After 10 days, the DOC concentrations in the P addition treatment were between 5.5 and 18.0 times higher than in the control treatment. In contrast, chloride addition did not lead to increased DOC concentrations compared to the control (water only). Phosphate addition led mostly to desorption of organic matter of medium molecular size (10–100 and 100–1000 kDa) in the Ferralsol and the Andosol and of large molecular size (>1000 kDa) in the Podzol. In contrast, potassium chloride addition shifted the size distribution of DOC in the soil solution towards small compounds (<10 kDa), likely because KCl addition affected the aggregation of DOC compounds, in contrast to Na2HPO4 addition. Furthermore, phosphate addition also decreased subsequent sorption of DOC in the Ferralsol and the Andosol by a factor of up to 2.9 and 2.1, respectively.Our results have far-reaching implications because they show that phosphate addition can lead to desorption, and thus to destabilization of SOM, particularly in Ferralsols, and can also prevent further sorption of organic matter in soils. Our study provides a mechanistic explanation of why phosphate addition can decrease soil organic carbon sequestration and stabilization. This mechanism should be considered in the analysis of land-use and tillage effects on SOM sequestration in the future. Furthermore, our study indicates that also medium and large molecular size organic matter is stabilized through adsorption in soils.

Limited potential of biosolids application for long-term soil carbon stabilization in coastal dune forests

Whether or not biosolids application facilitates long-term soil organic matter (SOM) stability in coastal dune forests remains unsolved. We conducted a long-term experiment in a coastal radiata pine (Pinus radiata D. Don) forest growing on sandy soils in New Zealand, where biosolids have been applied at moderate (3 t ha−1) and high (6 t ha−1) rates every three years for two decades. We determined total carbon (C) and nitrogen (N) contents, the amounts of iron and aluminum oxides, labile and recalcitrant C and N fractions and the chemical composition of SOM in the topsoil, and explore their relations to plant, microbial and geochemical controls. Biosolids application had increased total, labile and recalcitrant C by 6%–94%, 6%–56% and 0%–55%, respectively; increased total, labile and recalcitrant N by 14%–124%, 5%–214% and 0%–173%, respectively. Biosolids application had no significant effects on free and amorphous iron oxides, but significantly increased aluminum oxides with enhanced crystallization of amorphous aluminum. The metal/recalcitrant-C ratios, the recalcitrant/total-C ratios and the alkyl/O-alkyl-C ratios remained unchanged, indicating both the potential of mineral protection and the chemical recalcitrance of SOM were not reinforced. Decreased ratios of microbial lipid biomass to recalcitrant C further indicated biosolids application may limit microbial contribution to SOM stabilization. These limitations to confer higher stability to SOM under biosolids application is likely due to reduced belowground C allocation (root biomass and mycorrhizal associations) and microbial C limitation (higher C- versus nutrient-acquiring enzyme activities) and stress response (higher ratio of cyclopropyl-to-precursor fatty acids). Given unchanged mineral protection to SOM, decreased mycorrhizal associations coupled with increases in Gram-positive bacteria and microbial metabolic quotient may facilitate the decomposition of recalcitrant SOM. Hence, although biosolids application usually promoted immediate C accumulation, weak mineral protection and altered plant-microbial interactions may undermine its potential for long-term soil C sequestration in coastal dune forests.

Tillage-residues affect mineral-associated organic matter on Vertisols in northern Mexico

Managing croplands for increased storage of soil organic matter (SOM) contributes to the development of resilient farming systems in a changing climate. We examined SOM dynamics in a wheat (Triticum durum L.) – maize (Zea mays L.) irrigated bed planting system established near Ciudad Obregón, Sonora, Mexico. Soil samples (0–15 cm) were collected from conventionally tilled raised beds (CTB) with all crop residues incorporated (CTB-I) and permanent raised beds (PB) with crop residues burned (PB-B), removed (PB-R), partly retained (PB-P) or fully retained (PB-K) receiving 0, 150 or 300 kg N ha−1, and analyzed for organic C (OC), total N (TN) and δ13C in whole-soil, light fraction (LF) and coarse- (sand) and fine- (silt and clay) mineral-associated organic matter (MAOM). Results indicated that PB-K and PB-B increased soil OC (p <0.05) in whole-soil relative to CTB-I, mainly through increases in sand- and silt-size MAOM, respectively. Similarly, N-fertilization increased soil OC and TN contents in whole-soil, coarse-MAOM and fine-MAOM, but not in the LF pool. Soil δ13C was higher (p <0.05) in PB-K (−20.18‰) relative to PB-B (−20.67‰), possibly due to the stabilization of partly decomposed maize-C in silt- and clay-size MAOM. Composition of SOM surveyed by CPMAS 13C NMR was not affected by tillage-residue management, and roughly consisted of 35% O-alkyl-C, 31% alkyl-C, 24% aromatic-C and 10% carboxyl-C. Our results indicate that long-term PB-K and PB-B adoption increased surface soil OC contents relative to CTB-I, even though pathways of SOM stabilization differed between systems. Under PB-K, accumulation of fine-MAOM was mostly related to straw-C inputs, whereas in PB-B it was closely associated to black-C precursors. Fine-MAOM appeared responsive to crop residue management, and should be therefore considered when analyzing mechanisms of SOM stabilization in irrigated croplands.

Bioenergy production effects on SOM with depth of loblolly pine forests on Paleaquults in southeastern USA

Managed loblolly pine (P. taeda L.) forests comprise a major land-use across the “wood basket” of the southeastern US. Lower coastal plain loblolly pine forests can enhance carbon (C) storage in fast-growing vegetation and soils, representing a significant opportunity to manage these forests for improved soil health through an understanding of soil C and nitrogen (N) storage. Furthermore, these forests have unrealized potential to produce biomass for emerging bioenergy markets. Despite this, very little is known about how intensified management (herbaceous bioenergy production) of these forests influences short-term soil organic C (SOC) and soil organic N (SON) pools in surface and subsurface soil horizons. The field site was located in the Lower Coastal Plain of North Carolina, USA in which trees were planted in bedded (e.g., raised) rows (bed) and intercropped with or without switchgrass (P. virgatum L.) between the bedded rows of trees (interbed). Soils were collected from four depths (0–5, 5–15, 15–30, and 30–45 cm) and two locations (bed and interbed regions) and fractionated into light and heavy mineral-associated SOC and SON pools using a sequential density fractionation technique. Bulk soil SOC and SON concentration as well as C:N ratio decreased with depth for both treatments, while 15N became enriched. Free and occluded lighter fractions (< 1.65 g cm−3) had higher SOC and SON concentration (~45% C and ~ 1.15% N) compared to medium (1.65–2.00 g cm−3) and heavy (> 2.00 g cm−3) fractions. At the 0–5 cm depth in interbeds, SOC and SON were concentrated in the free light fraction (~45%), but transitioned to being concentrated in the heaviest mineral-associated fraction at the 15–30 (~55%) and 30–45 (~70%) cm depth. The 15N analysis of density fractions indicated progressive enrichment with increasing density, suggesting increasing incorporation of microbially-derived products into stable mineral-associated soil organic matter (SOM) pools. In the pine-switchgrass treatment, SOC and SON concentrations were higher in the light and medium fractions in beds and interbeds compared to the pine only treatment, particularly at the 30–45 cm depth. The C:N ratio in the pine-switchgrass treatment was higher compared to the pine only treatment indicating inputs of new plant-derived organic matter. Furthermore, the δ13C stable isotope signature was enriched in both the occluded light fraction (1.65 g cm−3) and the mineral-associated fraction (1.65–1.85 g cm−3). Our results suggest that switchgrass intercropping is contributing to the early buildup of SOM in particulate and mineral-associated SOM pools possibly through additions of new root-derived organic matter and the positive effect on soil microbial activity through priming of microbes.

Effects of tillage managements on soil microbial community structure in soil aggregates of fluvo-aquic soil

In order to explore the impacts of different tillage managements on the structure and diversity of microbial community in fluvo-aquic soil, the phospholipid fatty acid (PLFA) method was used to determine microbial community composition in soil aggregates. Four tillage treatments were set up in Qihe County, Shandong Province, including rotary tillage with straw return (RT), deep ploughing with straw return (DP), subsoiling with straw return (SS) and no-tillage with straw return (NT). Our results showed that DP treatment significantly increased the amount of fungal PLFAs and fungi/bacteria ratio in >5 mm soil aggregates compared with RT. DP could provide favorable conditions for fungi reproduction, facilitate soil organic matter storage and soil buffering capacity. DP increased the amount of PLFAs in 5-2 mm soil aggregates, reduced the gram-positive (G+) /gram-negative (G-) bacteria ratio in the soil, and improved soil nutritional status. In addition, DP improved the microbial abundance index in <0.25 mm soil aggregates. In general, DP could not only increase the abundance of bacteria and fungi in soil aggregates, but also improve the microbial community structure of soil aggregate, which help increase soil carbon sequestration capacity and keep soil microbial diversity to a certain extent. Results of the redundancy analysis showed that the total PLFAs, PLFAs of bacteria, G- bacteria and actinomycetes in soil aggregates are closely correlated with soil organic carbon, while PLFAs of G+ bacteria had a strong correlation with soil total nitrogen concentration. In each treatment, microbial communities in larger sizes of soil aggregates were mainly affected by the ratio of organic carbon to total nitrogen, soil moisture, pH, and mass fractions of soil aggregates, while the microbial communities in smaller sizes of soil aggregates were affected by the concentrations of organic carbon and total nitrogen.

Effects of Biochar Feedstock and Pyrolysis Temperature on Soil Organic Matter Mineralization and Microbial Community Structures of Forest Soils

Biochar has received much attention as a strategy to enhance soil carbon (C) sequestration and mitigate climate change. Previous studies found that the feedstock and pyrolysis temperature can largely determine biochar properties, which in turn, impact the stability of native soil organic matter (SOM) and soil microorganisms. The Schima superba and Cunninghamia lanceolata are two tree species widely distributed in the subtropical region of southern China, but how the biochars from these two species influence the soil C sequestration and microbial communities of plantation remain poorly understood. In this study, we produced biochars from these two different feedstocks (13C-labeled S. superba and C. lanceolata litters) at three pyrolysis temperatures (350°C, 550°C, 750°C), then added them to the soils from C. lanceolata plantation, and maintained the experiments at 25°C for 112 days. We found both C mineralization and soil microbial community structures were strongly, but inconsistent, affected by biochar feedstock and pyrolysis temperature. The C. lanceolata biochar triggered the negative priming effect faster and greater compared with the S. superba biochar amendment. Biochars produced at 550°C showed the most significant negative priming effect during the whole incubation period, regardless of the different feedstocks. The cumulative amount of CO2 derived from biochars was significantly decreased with pyrolysis temperature (p &amp;lt; 0.05), indicating that biochars prepared at higher temperatures were more stable in the soil. Further, the soil microbial community structure was only affected by biochar pyrolysis temperature rather than biochar feedstock and their interaction. Together, our results reveal that biochar feedstock and pyrolysis temperature may play more important roles in dictating the priming effect than the structure of microbial community for C. lanceolata plantation. Overall, we concluded that the biochars prepared at 550°C could rapidly decrease the turnover of native SOM in a short term and biochar amendment has the potential to be a management practice for soil C sequestration in the C. lanceolata plantation.

Effects of horticulture on soil organic matter properties in highly weathered tropical soils

Soil organic matter (SOM) plays an important role not only for agriculture but also in the global carbon (C) balance with a potential to mitigate climate change. Land-use changes from native vegetation to intensive agricultural use enhance mineralization of SOM and thereby cause decreases in the amount of soil C. Association of SOM on soil minerals is highlighted as key stabilization mechanism of SOM. To examine the contribution of land-use change and soil mineralogy on soil C losses, soils with different soil mineralogy assemblage were studied from four areas in Brazil, all having sites with agriculture and nearby soils under native vegetation. We assessed SOM stability by thermogravimetric (TGA) and differential scanning calorimetry (DSC) and the molecular composition of functional soil C pools by pyrolysis gas chromatography/mass spectrometry (Py–GC/MS). On average, 33 % of soil C was lost in three sites after land-use change. Py-GC/MS indicates a dominance of reworked and/or microbial material (58–99 % N-containing products and carbohydrates), and low contribution from plant-derived products such as lignin phenols (from 0.1 to 7.7 %), and striking similarities in molecular composition between native vegetation and corresponding sites under agriculture. Thermal data indicate lower energy content in soils under native vegetation than agriculture, confirming microbial-derived SOM. The gibbsite content was positively correlated to N-containing products in both SOM extracted by alkaline solution (r2 = 0.60) and SOM residual after alkaline extraction (r2 = 0.82), while the kaolinite content showed a negative correlation with N-containing compounds (r2 = 0.72) and a positive one with carbohydrates (r2 = 0.48). Thus although SOM content highly decreased upon horticulture, both Py-GC/MS and thermal analyses indicate that SOM composition is mainly controlled by site instead of land-use, and the strong correlations with clay minerals suggest a mineralogical control on SOM composition.

Nonlinear turnover rates of soil carbon following cultivation of native grasslands and subsequent afforestation of croplands

Abstract. Land use conversions can strongly impact soil organic matter (SOM) storage, which creates paramount opportunities for sequestering atmospheric carbon into the soil. It is known that land uses such as annual cropping and afforestation can decrease and increase SOM, respectively; however, the rates of these changes over time remain elusive. This study focused on extracting the kinetics (k) of turnover rates that describe these long-term changes in soil C storage and also quantifying the sources of soil C. We used topsoil organic carbon density and δ13C isotopic composition data from multiple chronosequences and paired sites in Russia and United States. Reconstruction of soil C storage trajectory over 250 years following conversion from native grassland to continual annual cropland revealed a C depletion rate of 0.010 yr−1 (first-order k rate constant), which translates into a mean residence time (MRT) of 100 years (R2≥0.90). Conversely, soil C accretion was observed over 70 years following afforestation of annual croplands at a much faster k rate of 0.055 yr−1. The corresponding MRT was only 18 years (R2=0.997) after a lag phase of 5 years. Over these 23 years of afforestation, trees contributed 14 Mg C ha−1 to soil C accrual in the 0 to 15 cm depth increment. This tree-C contribution reached 22 Mg C ha−1 at 70 years after tree planting. Over these 70 years of afforestation, the proportion of tree C to whole-soil C increased to reach a sizable 79 %. Furthermore, assuming steady state of soil C in the adjacent croplands, we also estimated that 45 % of the prairie C existent at the time of tree planting was still present in the afforested soils 70 years later. As an intrinsic property of k modeling, the derived turnover rates that represent soil C changes over time are nonlinear. Soil C changes were much more dynamic during the first decades following a land use conversion than afterwards when the new land use system approached equilibrium. Collectively, results substantiated that C sequestration in afforested lands is a suitable means to proactively mitigate escalating climate change within a typical person's lifetime, as indicated by MRTs of a few decades.

Pruning residues incorporation and reduced tillage improve soil organic matter stabilization and structure of salt-affected soils in a semi-arid Citrus tree orchard

Soil salinization is an emerging problem worldwide as a result of unsustainable land management practices and climate change. However, salt-affected soils under agricultural use could act as a C sink if these negative effects can be offset by combination of sustainable land management practices (SLM). In this study, we assessed the effect of (i) intensive tillage along with flood irrigation (IT); (ii) combination of no-tillage with pruning residues (branches and leaves) as mulch, and drip-irrigation (NT + PM); and (iii) combination of reduced tillage with the incorporation of pruning residues and drip-irrigation (RT + PI), on physico-chemical soil parameters, aggregate stability, amount and quality of organic matter fractions and soil organic carbon (SOC) sequestration in a lemon tree orchards (Citrus limon var. Verna) under semi-arid climate conditions. The RT + PI management system showed a decrease in salinity and bulk density, and increased soil porosity, soil OC and N stocks, and percentage of OC-rich macroaggregates as compared to the IT system. The aggregate-occluded particulate organic matter fraction (oPOM) played a key role in macroaggregate stability. The NT + PM treatment also showed positive effects on the investigated soil properties, but this was limited to the upmost topsoil (0−5 cm). The IT management system revealed highest values of salinity and bulk density, and considerably lower SOC stocks. Moreover, a degradation of soil structure with a low percentage of macroaggregates depleted in SOC was observed. We conclude that the incorporation of pruning residues in combination with reduced tillage and drip-irrigation is an effective management system to improve soil structure and facilitate SOC sequestration. Therefore, conventional management systems based on intensive tillage and flood irrigation should be abandoned in salt-affected soils under semi-arid climate conditions in favour of systems with higher organic matter inputs incorporated into the soil combined with measures to reduce the salt content.

Erratum to: A Study of Soil Organic Matter Stability Using Derivatography and Long-Term Incubation Methods

Erratum to: A Study of Soil Organic Matter Stability Using Derivatography and Long-Term Incubation Methods