Chemical Composition Of Soil Organic Matter Research Articles

Effect of Land Use Change on Molecular Composition and Concentration of Organic Matter in an Oxisol.

Land use change from native vegetation to cropping can significantly affect the quantity and quality of soil organic matter (SOM). However, it remains unclear how the chemical composition of SOM is affected by such changes. This study employed a sequential chemical extraction to partition SOM from an Oxisol into several distinct fractions: water-soluble fractions (ultrapure water (W)), organometal complexes (sodium pyrophosphate (PP)), short-range ordered (SRO) oxides (hydroxylamine-HCl (HH)), and well-crystalline oxides (dithionite-HCl (DH)). Coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), the impact of land use change on the molecular composition of different OM fractions was investigated. Greater amounts of OM were observed in the PP and HH fractions compared to other fractions, highlighting their importance in SOM stabilization. The composition of different OM fractions varied based on extracted phases, with lignin-like and tannin-like compounds being prevalent in the PP and HH fractions, while aliphatic-like compounds dominated in the DH fraction. Despite changes in the concentration of each OM fraction from native vegetation to cropping, there was little influence of land use change on the molecular composition of OM associated with different mineral phases. No significant selective loss or preservation of organic carbon compounds was observed, indicating the composition of SOM remained unchanged.

Grazing intensity and nitrogen fertilization timing to increase soil organic carbon stock and nitrogen in integrated crop-livestock systems

ABSTRACT Integrated crop-livestock systems (ICLS) foster synergistic relationships to increase nitrogen (N) cycling and soil organic carbon (SOC) accrual in agricultural setups. This study evaluated how the grazing intensity and N fertilization (rates and timing) affect both SOC and N fractions, and soil organic matter chemical composition in an ICLS managed under no-tillage in an Oxisol, six years after initiation. The ICLS was compared to a nearby pasture (PA) and a native forest (NF). The treatments consisted of two grazing intensities: Low Sward Height (LH) and High Sward Height (HH) were maintained with high and low stocking rates, respectively. The HH varied between 0.20 and 0.60 m, and LH between 0.10 and 0.30 m according to the plant forage species throughout the experiment. Fertilization using 200 kg ha -1 N-urea, not splitting up, was conducted at two timings, either at the winter pasture establishment (autumn), about 35 days after sowing or during the summer cash crop cycle (spring). Total N amount per year, including both phases, pasture and cash crop was the same for all treatments. The SOC and N contents were assessed in soil and particulate organic matter (POM), while carbon (C) and N stocks were specifically determined in the soil. Soil organic matter composition was characterized by FTIR. The combination of HH and N fertilization during the pasture phase increased the content of C from 36.1 to 39.9 ± 0.7 g C kg -1 and of N from 2.7 to 3.2 ± 0.1 g N kg -1 . The SOC stocks varied from 37.3 to 41.1 ± 0.7 Mg C ha -1 , and the N stocks from 2.1 to 3.3 ± 0.1 Mg N ha -1 at 0.0-0.10 m soil layer. The SOC content of the POM and the soil organic matter chemical composition determined by FTIR were mainly affected by the grazing intensity. The HH led to an increased in C content within the POM fraction, reaching values of 51.6 ± 1 and 49.2 g C kg -1 , respectively to N crop fertilization and N pasture fertilization. Land-use changed how organic functional groups were stored in soil organic matter fractions. The NF had a greater abundance of aliphatic and phenol in the MAOM, while pasture and ICLS systems had greater aliphatic in the POM fraction. In ICLS, SOC accrual was positively associated with more recalcitrant organic functional groups of phenol, aromatic, and carbonyl C-O. The HH increases SOC accrual, while N-fertilization on pasture ensures adequate nutrition of plants and animals during the winter ICLS phase, at the same time as providing greater residual N for subsequent cash crops through enhanced catalyzed by ruminants. Therefore, grazing and fertilization management strategies should be considered to promote sustainable agriculture intensification with ICLS.

How does soil organic matter stabilize with soil and environmental variables along a black soil belt in Northeast China? An explanation using FTIR spectroscopy data

Chemical composition of soil organic matter (SOM) is an important determinant of SOM stabilization and soil function. Better understanding of the chemical composition of SOM is vital for assessing soil formation processes and its controlling factors. Fourier transform infrared (FTIR) spectroscopy was applied to characterize the chemical compositions of SOM for 234 surface soils collected from a representative black soil belt in Northeastern China. The relationships of SOM chemical composition with soil and environmental variables were assessed. Soil samples were divided into eight regional groups and four soil organic carbon (SOC) range groups according to the administrative division and SOC contents, respectively. Generally, SOC contents varied from 8.2 to 34.5 g kg−1, together with the relative abundances of aliphatic-C increased from the south to the north along the black soil belt, while the relative abundance of aromatic-C decreased. Total nitrogen (TN), relative abundance of polysaccharide-C, and minerals showed no obvious zonal changes. In four SOC range groups, averaged SOC, TN, and aliphatic-C increased significantly with increasing SOC contents, while the aromatic-C decreased (P < 0.05). The polysaccharide-C and minerals also increased. Correlation analysis showed that SOC was positively related with aliphatic-C, and negatively related with aromatic-C regardless of regions and SOC contents (P < 0.05). Principal component analysis demonstrated obvious separations among soils divided into four SOC ranges compared to those divided into eight regions. SOC, TN, and aliphatic-C pointing to SOC > 20.0 g kg−1 groups, and aromatic-C pointing to SOC < 15.0 g kg−1 groups were the main driving factors via PC1. The aromatic-C/aliphatic-C ratio representing SOM decomposition degree, mean air temperature, and precipitation pointing to SOC < 10.0 g kg−1 groups, and minerals were the main driving factors via PC2. Our results contribute to a further understanding of spatial variations in SOM chemical compositions regarding soil formation and SOC stabilization caused by environmental conditions and agricultural managements.

Fungal necromass increases soil aggregation and organic matter chemical stability under improved cropland management and natural restoration

The formation and stability of soil organic matter (SOM) is crucial for food security, soil health, and climate change mitigation. Although various SOM stabilization mechanisms have been proposed and investigated, the contribution of plant- and microbial-derived carbon into physical and chemical stabilization processes remain unclear. Therefore, this study investigates lignin phenols, microbial necromass, soil aggregation and SOM chemical composition under three cropland management and two natural restoration strategies: NPK, NPK + manure (NPK + M) and NPK + peat vermiculite (NPK + PV) after 5 years, and natural restoration for 10 and 40 years (NR10 and NR40, respectively). Addition of manure or peat vermiculite and NR40 increased soil organic carbon (SOC) by 86–122 % and 16 %, respectively, compared to the NPK fertilization. Lignin phenols and bacterial necromass-C were the highest under NPK + M, and lignin phenols increased by 0.07 g and microbial necromass-C by 0.44 g with each additional 1 g of SOC. Fungal necromass-C in NPK + PV was 0.14–1.1 times higher than in other treatments. The mean weight diameter of aggregates was the highest, while macroaggregate turnover was the slowest under NPK + PV, indicating increased soil aggregation and physical stability. Natural restoration reduced lignin phenols by 33–40 % and labile O-alkyl C by 4–9 %, but increased resistant alkyl C by 9–15 % compared with other treatments, reflecting the highest chemical stability. High fungal necromass was beneficial to the accumulation of particulate and mineral-associated C and aggregate stability, and decelerated macroaggregate turnover. Aromatic C increased but aliphatic-C/aromatic-C decreased with increasing fungal necromass-C. Consequently, fungal necromass C increases SOM physical stability by slowing aggregate turnover and enhances the chemical stability through the accumulation of recalcitrant C under improved cropland management and natural restoration.

Particle size primarily shifts chemical composition of organic matter under long‐term fertilization in paddy soil

AbstractThe chemistry of soil organic matter (SOM) is critical for soil functions. However, the chemical composition and oxidation state of physical SOM fractions under long‐term mineral and organic amendments in paddy soils are still unclear. Here, we investigated this issue using three particle size classes of SOM sampled from five long‐term (37‐years) fertilisation regimes in a subtropical paddy soil which were analysed using pyrolysis–gas chromatography–mass spectrometry (Py–GC/MS). Coarse particulate organic matter (cPOM, &gt;250 μm), fine particulate organic matter (fPOM, 53–250 μm) and mineral‐associated organic matter (MOM, &lt;53 μm) were isolated. The five fertilisation regimes were (1) unfertilised (control), (2) nitrogen (N, urea), (3) nitrogen + phosphorus + potassium (NPK, urea + Ca(H2PO4)2 + KCl), (4) NPK + straw (NPKS) and (5) NPK + Astragalus sinicus L. + pig manure (NPKM). After 37‐years, the SOM content of the control, N, NPK, NPKS and NPKM treatments had increased by 16.3%, 12.3%, 31.6%, 41.7% and 59.8% compared with the initial level, respectively. The pools of soil organic carbon (SOC) and total N (TN), as well as the C/N ratio in the soil matrix, were localised by particle size, with the majority in the mineral‐associated organic matter (MOM). Particle size, not fertilisation regime, primarily associated with shifts in the chemical composition of SOM, with selective preservation of lignin and polysaccharides in the particulate organic matter (POM) and enrichments of lipids and N‐containing compounds in the MOM. The origin (plant‐ and microbial‐derived) and oxidation state of SOM were also regulated by particle size (especially the coarse particles) but not fertilisation treatment. Despite long‐term persistence, the MOM had a lower C oxidation state (implying higher vulnerability to microbial utilisation) than the coarse POM (cPOM) and fine POM (fPOM). In conclusion, fertilisation regime regulated the bulk SOM (SOC and TN) pool, whereas physical particle size generated a divergence trend in the chemical composition and oxidation state of SOM in the subropical paddy soil, indicating the greater role of physical arrangement in SOM chemistry than the quality of amendments.Highlights Particle size, not fertilisation regime, regulated chemical composition and oxidation state of SOM. POM is enriched in lignin and polysaccharides while MOM is enriched in lipids and N‐containing compounds. Physical arrangement plays a greater role in SOM chemistry than the quality of amendments.

Dynamics and composition of soil organic carbon in response to 15 years of straw return in a Mollisol

Crop straw return is being widely applied as a sustainable soil management practice to improve soil organic carbon (SOC) storage in intensive agricultural ecosystems. However, the dynamics and chemical composition of SOC under long-term straw return are not fully understood. Based on a 15-year field experiment with soybean (Glycine max (L.) Merrill.)–maize (Zea mays L.) cropping system, we studied SOC temporal dynamics under no fertilization (NF), mineral fertilizers (NPK) and mineral fertilizers with straw return (NPKS). Meanwhile, we determined five labile carbon (C) pools, i.e., microbial biomass C (MBC), water-soluble organic C (WSOC), light fraction C (LFC), readily oxidizable organic C (ROC), and particulate organic C (POC), as well as δ13C values and organic functional groups of soil organic matter (SOM). After 15-year application of continuous straw return with mineral fertilizer application, SOC content increased by 14.2%. The δ13C values of SOC increased with time under all treatments, and its positive relationship with cumulative maize C input indicated a larger C contribution to SOC from maize than soybean residues. The NPKS treatment significantly increased the contents of MBC, WSOC, LFC, ROC, and POC and their proportion in SOC, compared with the NF treatment. The aliphatic and aromatic relative peak areas were significantly affected by straw return, with an increasing relative peak area at 2930 cm−1 and decreasing at 1620 cm−1, respectively. Our results demonstrate that straw return could continuously increase SOC in an inter-annual rotation of soybean and maize cropping system in Mollisol. Meanwhile, long-term fertilization can alter SOM chemical composition, with lower humification degree under the combined application of straw and mineral NPK fertilizers.

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.

Soil organic matter formation is controlled by the chemistry and bioavailability of organic carbon inputs across different land uses

Soil organic matter (SOM) formation involves microbial transformation of plant materials of various quality with physico-chemical stabilisation via soil aggregation. Land use and vegetation type can affect the litter chemistry and bioavailability of organic carbon (OC), and consequently influence the processing and stabilisation of OC into SOM. We used 13C nuclear magnetic resonance (13C NMR) and hot-water extraction to assess the changes in chemical composition and labile OC fractions during the transformation processes from leaf to litter to SOM depending on land use and vegetation type. The hot-water-extractable OC (HWEOC) decreased from leaf (43–65 g kg−1) to litter (19–23 g kg−1) to SOM (8–16 g kg−1) similar in four land use types: grassland, sugarcane, forest and banana. These trends demonstrated the uniform converging pathways of OC transformation and increasing stability by SOM formation. The preferential decomposition and decrease of labile OC fractions (∑% di-O-alkyl, O-alkyl and methoxyl) from leaf (54–69%) to SOM (41–43%) confirmed the increasing stability of the remaining compounds. Despite differences in the biochemical composition of the leaf tissues among the vegetation types, the proportions of labile OC fractions in SOM were similar across land uses. The OC content of soil was higher in forest (7.9%) and grassland (5.2%) compared to sugarcane (2.3%) and banana (3.0%). Consequently, the HWEOC per unit of soil weight was higher in forest and grassland (2.0 and 1.2 g kg−1 soil, respectively) compared to sugarcane and banana (0.3 and 0.4 g kg soil−1, respectively). The availability of labile SOM is dependent on the quantity of SOM not the chemical composition of SOM. In conclusion, labile OC fractions in SOM, as identified by 13C NMR, were similar across land use regardless of vegetation type and consequently, SOM formation leads to convergence of chemical composition despite diversity of OC sources.

Organic amendments change soil organic C structure and microbial community but not total organic matter on sub-decadal scales

Organic C has many benefits for soil, but it is depleted by tillage and crop harvest, and especially so for biofuel crops. Accordingly, strategies such as partially retaining stover or planting a cover crop can help ameliorate the negative effect of C removal. We used a long-term field experiment to study the impacts of stover retention and planting a cover crop on soil organic matter (SOM), its extractable components, and the soil microbial community. SOM chemical composition characterization was determined by electrospray ionization (ESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in sequential water, methanol (MeOH), and chloroform (CHCl3) extracts. The characteristics of the soil bacterial community were measured by phospholipid fatty acid (PLFA), real-time quantitative PCR, and 16S rRNA gene sequence. The variations in total SOM content, total microbial biomass, and bacterial population were slight among treatments, but SOM chemical compounds, arbuscular mycorrhizal fungi (AMF) biomass, and bacterial structure changed significantly, and especially so in the coupled application of stover retention and cover crop. Specifically, stover retention enriched more lignin-like compounds in soil, whereas cover crop enriched more condensed hydrocarbons, and had more compounds with an aromaticity index (AI) > 0.5. The bacterial community was not altered by the cover crop, but the corn stover retention increased the relative abundances of Myxococcales (Deltaproteobacteria) and decreased that of Actinobacteria. Redundancy analysis (RDA) further revealed that the bacterial community in the stover treatments had a significant positive association with CHCl3-extracted chemical classes, i.e. unsaturated hydrocarbons and lipids, with the coupled application (stover and cover crop), and lignin and proteins with the corn stover only treatment. Taken together, our study shows how different C addition practices influence the molecular composition of SOM and the structure of soil microbial communities.

Spectroscopic and chromatographic investigation of soil organic matter composition for different agrosystems from arid saline soils from Southeastern Tunisia

In Tunisia, several studies estimated the soil organic matter (SOM) stocks. However, little is known about SOM composition and dynamics especially in arid regions. Globally, arid soils of Tunisia are saline. This study aims to investigate the composition and stability of SOM in salt-affected soils under different vegetation covers in Southeastern Tunisia. Two depths are sampled in ten profiles from different localities. SOM composition was assessed by FTIR spectroscopy and gas chromatography-mass spectrometry (GC/MS). The results showed that, despite the differences of studied soils, SOM contents are low (< 20 g/kg) and decrease with soil depth. The lowest content of SOM is attributed to native soils which suggest that SOM in arid saline soils is mainly controlled by available litter input. FTIR spectroscopy revealed that SOM is predominated by polysaccharides, aliphatic C, and aromatic C. Polysaccharides and aliphatic C increased with soil depth for agricultural and native soils. However, aromatic C increased with soil depth for agricultural soils, while it decreased for native soils. Agricultural soils are enriched in aliphatic C and polysaccharides compared with native soils that had higher content of aromatic C. Thus, the chemical composition of SOM in arid saline soils is strongly influenced by litter input quality and decomposition rate of SOM. GC/MS analyses which showed the predominance of mid-chain n-alkanes confirm those finding. Because of their relative recalcitrance and/or their protection by association with soil minerals, polysaccharides, mid-chain n-alkanes, aliphatic C, and aromatic C may have crucial effect on SOM stabilization in arid saline soils.

Selective stabilization of soil fatty acids related to their carbon chain length and presence of double bonds in the Peruvian Andes

Recent studies show increasing evidence for perseveration of soil organic matter (SOM) controlled by interactions with the soil matrix (i.e. mineral surfaces and aggregates) rather than chemical recalcitrance of the SOM. However, a consensus is still absent for potential controls of SOM chemical composition on SOM stabilization and persistence. Soil fatty acids (FAs), which form an important SOM component, can be used to investigate the effects of chemical properties on SOM stabilization because they are easily degraded by microorganisms but can be stabilized by the soil matrix against decomposition. Here we investigated whether inherent molecular properties of FAs control their stability in soils and their interactions with the soil matrix. Soil samples were collected from alpine grasslands of the Peruvian Andes (Andosols, Umbrisols and Phaeozems), as they are characterized by high carbon stocks and abundant aliphatics. We applied pyrolysis - gas chromatography/mass spectrometry analyses assisted by tetramethylammonium hydroxide (TMAH-pyrolysis-GC/MS) to determine the chemical composition of bulk SOM and FAs before and after a 76-day incubation experiment, comparing a situation with intact versus crushed soil aggregates. The results showed that the TMAH-pyrolysis-GC/MS yielded a large proportion of FAs (>60% relative abundance of identified compounds), with a major contribution of free FAs. FA stability was controlled by the presence of double bonds (unsaturated vs. saturated FAs) and carbon chain length. Unsaturated FAs significantly (P < 0.05) predicted soil organic carbon mineralization rates and were more depleted after the incubation compared to saturated FAs. The depletion of unsaturated FAs is likely explained by their easier degradation compared to saturated FAs. The easier degradation might be explained by the smaller extent of stabilization through association with mineral surfaces and/or chemical properties rather than stabilization through occlusion in aggregates. In terms of carbon chain length, FA stability decreased from short-chain to long-chain FAs. A possible explanation for this is that short-chain FAs received more protection by occlusion in aggregates compared to long-chain FAs or that short-chain FAs were produced during the incubation as a result of microbial transformation of FAs. Such microbial transformation has limited effects on the prediction of FA stability using double bonds and carbon chain length. However, we observed that soil types and horizons did influence the controls of double bonds and carbon chain length on FA stability. Our results corroborate the hypothesis that the inherent properties of soil FAs control their interactions with the soil matrix and indirectly govern their stabilization and persistence in the Peruvian Andean soils under study.

Labile and recalcitrant components of organic matter of a Mollisol changed with land use and plant litter management: An advanced 13C NMR study

Soil organic matter (SOM) changes with land use and soil management, yet the controlling factors over the chemical composition of SOM are not fully understood. We applied quantitative 13C nuclear magnetic resonance and spectral editing techniques to measure chemical structures of SOM from different land use types. The land use types included a native grassland (nGL), a crop land with straw burning in the field (bCL), a restored grassland (rGL) and a cropland with straw removed out of the field (rCL) for 28years. The abundances of OCH groups from carbohydrates were higher in the SOMs of the nGL and rGL than in those of the rCL and bCL, while the abundances of OCH3 and aromatic CO groups from lignin were higher in the SOMs of the three-ever cultivated lands (rGL, rCL and bCL) than in that of the nGL. Although aromatic CC groups were most dominant in the Mollisols, they did not consistently decrease after the burnings of straw were ceased in the fields of the rCL and rGL compared to the bCL with continuous burning. In addition, the COO groups were bound with the aromatic CC groups in all the land use types, and the sizes of the aromatic clusters were affected by the land use types. The labile and recalcitrant components were correlated with SOC contents the mineral-associated and particular SOM in a contrasting way. Our results suggested that the chemical composition of SOM in the Mollisol depended on land use types, and that labile and recalcitrant components might be protected through mineral associations and aggregation, respectively. The most abundant aromatics in the Mollisols might not just be pyrogenic and could be oxidized to different extents, depending on field drainage conditions.

Aggregation but Not Organo‐Metal Complexes Contributed to C Storage in Tidal Freshwater Wetland Soils

Core Ideas Water‐stable aggregates may assist in the preservation of organic carbon in wetland soils. Soil C was more evenly distributed among the aggregate‐size classes in the restored but not the natural site. Macroaggregate‐C more closely resembled plan detritus whereas mineral‐C was enriched with N‐containing compounds. One of the many goals of wetland restoration is to promote the long‐term storage of carbon (C) in the terrestrial biosphere. Unfortunately, soil C reservoirs in restored wetlands are slow to accumulate even after hydrology and plant communities are reestablished. Oftentimes wetland restoration changes the soil matrix and thus can dramatically alter how soil C is stored and processed. Our research investigated whether soil organic matter (SOM) preservation theories derived from studies in non‐wetland soil systems can be extended to wetland soils. We examined C associated with water‐stable soil aggregates, minerals, and metal oxides within habitats of one natural and one restored tidal freshwater wetland. This study revealed that a majority of the soil C in the natural site was associated with large macroaggregates (&gt;2000 μm), and soils from the restored site stored more C in small macroaggregates (≥250 to &lt;2000 μm). Despite these different associations, the chemical composition of SOM followed similar patterns across each aggregate‐size class. Results from the sequential extraction procedure suggest organo‐metal oxide complexes do not contribute to C stabilization in these habitats. This research is one of the few studies that have examined C stabilization related to soil structure in wetland soils. Our results suggest soil aggregate formation may be an important mechanism driving C stabilization, and that disruption to macroaggregates may limit C accumulation in restored wetlands. Additional empirical research and long‐term field monitoring are needed to confirm linkages between aggregate‐C stabilization and accumulation in wetland soils.

Quantifying biochar content in a field soil with varying organic matter content using a two-temperature loss on ignition method

While the use of biochar as a soil amendment for agronomic and environmental management is gaining popularity, quantification of biochar in soil is still challenging. The objective of this work was to develop a fast, simple and inexpensive method to quantify biochar content in field soil with varying organic matter content – the two-temperature loss on ignition (LOI) method. In this approach, biochar mass fraction in a biochar-amended soil is computed by measuring the dry mass of biochar/soil mixture after heating sequentially at two temperatures: low temperature (LT), and high temperature (HT). This method requires the LOI profile for pure soil and pure biochar that are representative of soil and biochar in the field. Although the soil LOI profile may vary due to spatial variation in soil organic matter (SOM) content, the method only requires that the relative soil LOI at LT with respect to LOI at HT is uniform because of similarity in SOM chemical composition. In this method, LT and HT are selected such that the maximum difference in LOI exists at these temperatures between pure soil and biochar. The method was tested by quantifying the biochar content in roadway filter strips with and without a wood biochar pyrolyzed at high temperature (550 °C). The estimates of biochar content from the method matched independent measurements for soils with low (−0.23 ± 0.09 CI%, CI = 95% confidence interval, versus actual 0%) and high (3.9 ± 0.3 CI% versus actual 4.0 ± 1.1 CI%) biochar mass fraction. The method is applicable when SOM content is low to moderate (e.g. <15%) and mostly composed of labile organic compounds, and when biochars are pyrolyzed at moderate to high temperatures (i.e. >400 °C) and composed of relatively low ash content (e.g. <30%).

Distinct changes in composition of soil organic matter with length of cropping time in subsoils of a Phaeozem and Chernozem

SummaryDeeper soil horizons might provide an opportunity to enhance C sequestration because soil organic matter (SOM) at depth is assumed to be stable. However, it is unknown whether the stable composition of organic carbon in subsoils changes with the length of cropping time and the type of soil. The objectives of this study were to determine the effects on the chemical structures of SOM of cropping time after conversion from grassland to arable land under a Phaeozem and Chernozem in northeast China. Near‐quantitative multiple cross‐polarization (multiCP) 13C nuclear magnetic resonance (NMR) spectroscopy was applied, and 11 types of carbon (C) functional groups were identified. Principal component analysis of these functional groups showed that the chemical composition of SOM was differentiated by soil type and depth. The Phaeozem and Chernozem profiles differed mainly in their relative proportions of aromatic C–C and (CH2)n groups: the Phaeozem contained relatively more aromatic C–C, whereas the Chernozem contained relatively more (CH2)n groups. The fused‐ring aromatic C–C carbon was probably derived from char‐like organic matter generated by burning of plant litter or from SOM humification, whereas the (CH2)n groups were likely to be from plant‐ or microbially‐derived residues. The main differences between top‐ and sub‐soils were the occurrence of more protonated C in the topsoils and more non‐protonated C in the subsoils. With increasing length of cropping time, aromatic C–C and C–O groups and COO/N–C=O groups increased, but (CH2)n groups decreased in the Phaeozem subsoils and increased in the Chernozem subsoils. Our findings suggested that leaching and soil moisture might influence the origin, redistribution and transformation of the recalcitrant components of SOM in the soil profile, resulting in changes in SOM composition under different climates and soil types.Highlights Characterization of chemical structures of soil organic matter (SOM) in soil profiles by solid‐state 13C NMR SOM composition varied between a Phaeozem and Chernozem and to a larger extent in the subsoils Fused‐ring aromatics and (CH2)n groups accumulated in subsoils of the Phaeozem and Chernozem, respectively SOM composition of subsoil changed probably with precipitation‐controlled translocation or transformation

Distinct bioenergetic signatures in particulate versus mineral-associated soil organic matter

Physical and chemical stabilization, environmental conditions, and organic matter composition all play vital roles in determining the persistence of soil organic matter (SOM). Fundamentally, SOM stability depends on the balance of microbial bioenergetics between the input of energy needed to decompose it (i.e., activation energy; Ea) and the net energy gained (i.e., energy density; ED) from its decomposition. This relationship is complicated in soils by chemical and physical protection mechanisms, which require additional energies to overcome for decomposition to occur. In this study, we analyze the bioenergetics of soil density fractions, which vary in their degrees of organic matter-mineral association, and show that the relationship of ED and Ea has the ability to provide information about relative differences in SOM chemical composition and stability. Our results demonstrate distinct bioenergetic signatures between particulate, light (free and occluded) fractions versus mineral-associated, heavy fractions isolated from soil samples collected at two depths from a climosequence along an elevation gradient in the Sierra Nevada, California. While there were no significant differences in ED and Ea within each fraction across climates, the light fractions (LF) were characterized by larger ED and Ea values, whereas the heavy fractions (HF) were characterized by smaller ED and Ea values. Combined with CHN analyses, we conclude that SOM in HF pools is likely comprised of relatively simple organic compounds that have long turnover rates because of chemical association with soil minerals, whereas the LF pools are comprised of more chemically complex molecules with low chemical reactivity and high Ea.

Effects of long-term mowing on the fractions and chemical composition of soil organic matter in a semiarid grassland

Abstract. The grassland ecosystem is a significantly important terrestrial carbon pool. Intensive mowing is common to meet the need of increased livestock. However, little information on the quality and quantity of soil organic matter (SOM) under different mowing managements has been documented. In this work, in order to evaluate the impacts of different mowing managements on the quality and quantity of SOM, the fractions and chemical composition of SOM under different mowing managements were determined using traditional fractionation methods and spectroscopy technologies, including advanced nuclear magnetic resonance (NMR) (e.g. cross-polarization magic angle spinning 13C-NMR, CPMAS 13C-NMR) and Fourier-transform infrared (FTIR) based on a 13-year field mowing trial with four treatments: unmown (M0), mowing once every second year (M1/2), mowing once a year (M1) and mowing twice a year (M2). The results showed that compared with M0, M1/2 and M1 significantly enhanced the SOM accumulation and increased the stability of SOM by enhancing humification, while M2 limited SOM accumulation and microbial biomass. Substituted alkyl carbon (C) was the major organic C type in the grassland ecosystem, and it made up over 40 % of the total C. M1/2 and M1 significantly increased stable C functional groups (alkyl C and aromatic C) by degrading labile C functional groups (O-alkyl and carbonyl C) and forming recalcitrant humus, while M2 had opposite effects. The consistent increase in the values of NMR indices reflecting the degradation degree, hydrophobicity and aromaticity of SOM in M1 reflected the fact that M1 had the largest contribution to increasing the stability of SOM, while these values in M2 were similar to those in M0. Significant correlations between different SOM fractions and nitrogen (N) mineralization, and between the contents of different C functional groups and net soil organic nitrogen mineralization or microbial biomass C, indicated that the shifts in SOM fractions and chemical composition were closely related to soil microbial biomass and activity. Therefore, in view of the quality and quantity of SOM and the sustainable development of grassland ecosystems, M1 was the optimal mowing management, while M2 should be avoided in the semiarid grassland.

Altitudinal, seasonal and interannual shifts in microbial communities and chemical composition of soil organic matter in Alpine forest soils

The study of soil microbial communities and chemical properties of soil organic matter (SOM) over altitudinal mountain gradients have been regarded as especially useful to better predict and help mitigate the effects of climate change. However, there is a lack of surveys considering altitudinal, seasonal and interannual variations at the same time. Here, we investigated four forest sites, along an altitude gradient (545–2000 m a.s.l.), in spring and autumn during two consecutive years (2014 and 2015) regarding i) soil temperature and physicochemical properties; ii) SOM chemical composition using pyrolysis; iii) soil microbial activity (basal respiration, potential enzyme activities, community level physiological profiles (CLPP)); iv) archaeal, bacterial and fungal abundance (qPCR, phospholipid fatty acid (PLFA) analysis, numbers of culturable heterotrophic bacteria); and v) microbial community structure (PLFA analysis). Sites at high altitudes (alpine, subalpine) showed lower mean, maximum and minimum soil temperatures respect to those at lower altitudes (montane, submontane) and were characterized by increased levels of SOM and nutrients, higher archaeal, bacterial and fungal abundance as well as higher microbial activities. Soils at the sites at higher altitudes presented a higher humification degree of SOM, as demonstrated by an enhanced content of aromatic compounds and fatty acids. The seasonal effect determined a rise in the content of SOM, some soil nutrients, basal respiration and microbial abundance in autumn respect to spring over the altitudinal gradient as well as a change in the structure of microbial community and SOM molecular composition. Interannuality had a significant effect on the relative abundance of various chemical groups of SOM compounds and on the ratios of microbial groups, and produced an increase in microbial abundance in the second study year, as a consequence of the increased soil maximum temperatures recorded in this year. CLPP showed to be site-specific with limited seasonal and interannual variations. Altitudinal, seasonal and interannual changes in chemical composition of SOM were explained by variations in PLFA-based microbial community structure as well as in the abundance of bacteria and fungi, which, in turn, were affected by climatic conditions.

Spectroscopic investigation of soil organic matter composition for shelterbelt agroforestry systems

While the role of agroforestry systems in increasing soil organic matter (SOM) storage has been studied, insufficient information is available on their effect on the chemical composition of SOM. The objective of this study was to determine the carbon (C) functional group chemistry of SOM for shelterbelts and compare it to the adjacent agricultural fields by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Carbon K-edge X-ray absorption near edge structure (XANES) spectroscopies. ATR-FTIR spectral analysis indicated larger proportions of conjugated carboxylic and aromatic C groups for hybrid poplar, white spruce and caragana shelterbelts, phenolic C for hybrid poplar and Manitoba maple shelterbelts and aliphatic and aromatic C for Manitoba maple shelterbelts compared to the adjacent agricultural fields. Polysaccharide, ether and alcoholic C functional groups were generally lower for shelterbelts compared to agricultural fields, with the exception of hybrid poplar species. Analysis by C K-edge XANES spectroscopy on a subset of soils showed the accumulation of aromatic C, ketones and carbohydrates in the surface soil layer (0–5cm) for the shelterbelts compared to agricultural fields. Pearson correlation analysis indicated that the majority of SOM added under the shelterbelts was in the form of plant-derived aromatic, phenolic and carboxylic C groups. The results of this study suggested that the initial composition of litter and its decomposition rate had a strong influence on the composition of SOM under the shelterbelts. The higher proportion of processed forms of SOM, such as ketones, indicated that the SOM for shelterbelts was at a more advanced stage of decomposition compared to agricultural fields; likely due to the surface deposition of litter under shelterbelts.

Chemical Composition of Soil Organic Matter in a Subarctic Peatland: Influence of Shifting Vegetation Communities

Core Ideas Soil organic matter chemical composition highlighted the inputs of varied vegetation communities in the past. Soil N is an indicator of peat decomposition based on relationship of functional group C and total N. Future surveys of soil organic matter biogeochemical parameters and C chemical composition should be pursued. Climate change in the subarctic region has increased the rate of inundation of peatlands due to increased temperatures, precipitation, and permafrost thaw. Increased inundation may result in vegetation community shifts, as documented in a subarctic mire near Abisko, Sweden. The wet fen communities have established in former sphagnum areas, and sphagnum colonized in degraded palsa hummocks. At Stordalen mire, we studied the influence of vegetation community on chemical composition of peat soil organic matter (SOM). Vegetation and soil samples were obtained along a hydrologic gradient with representative communities: palsa, sphagnum, and fen. Soil organic matter chemical composition indicated shifts in vegetative communities. Total N and N isotope signatures in fen soils showed characteristics of sphagnum and palsa communities at &gt;6‐cm depth, and sphagnum soil profile signatures shifted from sphagnum to palsa properties at a 20‐cm depth. Soil chemical composition measured by Fourier Transform Infrared (FTIR) spectroscopy and 13C Nuclear Magnetic Resonance (NMR) spectroscopy showed increasing recalcitrant C (alkyl and aromatic) in palsa soil. Sphagnum soil profiles sustained labile organic C (O‐alkyl) until 15 cm then shifted to humified soil, and fen soil profiles showed areas of sphagnum and palsa signatures. Furthermore, the strong relationship between functional group C (O‐alkyl and alkyl) and total N demonstrated that soil N is an effective indicator of peat decomposition. Our results identified change points in soil chemical composition in regards to N content and C functional group which highlights the importance of historic vegetation community on chemical composition of peat soils.