Cultivation effects on the nature of organic matter in soils and water extracts using CP/MAS 13C NMR spectroscopy

The objective of this study was to examine the chemical structure of the organic matter (SOM) of Oxisols soils in slash and burn agriculture, in relation to its biological properties and soil fertility. The CP/MAS ¹³C technique was used to identify the main structural groups in litter and fine roots as SOM precursors; to identify the changes on the nature of the SOM upon cultivation and the proportion of labile and stable components; and to identify the nature of the organics present in water extracts (DOC). Carbohydrates were the main structural components in litter whereas components such as carbonyl C, carboxyl C,O-alkyl C and alkyl C were more common in SOM. Phenolic C and the degree of aromaticity were similar in litter and SOM. Cultivation resulted in a small decrease in the relative proportion of carbohydrates in SOM, little change in the levels of O-alkyl C and carbonyl C, but an increase in carboxyl C, phenolic C and aromaticity of the SOM. The level of alkyl C in soil was higher than the level of O-alkyl C, indicating the importance of long-chain aliphatics along with lignins in the stabilization of the SOM in Oxisols. The SOM of Mollisols from the Canadian Prairies differed from the Oxisol, with a generally stronger expression of aromatic structures, particularly in a cultivated soil in relation to a native equivalent. Carbohydrate components were the predominant structures in the DOC, indicating their importance in nutrient cycling and vertical translocations in the Oxisol.     

有机碳质量对黄河三角洲芦苇凋落物分解及其温度敏感性的影响

凋落物分解速率及其温度敏感性Q_(10)能够影响凋落物对土壤的碳归还及其对全球变暖的响应。然而,凋落物有机碳质量对凋落物分解及其温度敏感性的影响研究仍不充分。以黄河三角洲芦苇(Phragmites australi)为例,通过凋落物袋法、室内模拟实验及固态~(13)C核磁共振技术,研究有机碳质量对凋落物分解及其温度敏感性的影响,探讨预测凋落物分解及其温度敏感性的指标。结果表明:(1)随着凋落物分解,易分解碳组分(烷氧碳、双烷氧碳)相对含量逐渐降低,而难分解碳组分(芳香碳)相对含量显著增加,疏水碳/亲水碳、芳香碳/烷氧碳比值逐渐增大,凋落物有机碳更加稳定,凋落物呼吸速率及失重率呈下降趋势。(2)凋落物失重主要受烷基碳、烷氧碳相对含量及C/N的影响,凋落物CO_2累积释放量主要受烷氧碳及双烷氧碳相对含量的影响。羰基碳相对含量可以用来解释Q_(10)的变异。因此,相对于生态化学计量比,烷基碳、烷氧碳、双烷氧碳、羰基碳相对含量是预测凋落物分解及其温度敏感性的敏感性指标。     

The heterogeneous nature of microbial products as shown by solid-state13C CP/MAS NMR spectroscopy

Homoionic Na-, Ca-, and Al-clays were prepared from the <2 m fractions of Georgia kaolinite and Wyoming bentonite and mixed with sand to give artificial soils with 5, and 25% clay. The artificial soils were inoculated with microbes from a natural soil before incubation. Unlabelled and uniformly¹³C-labelled (99.9% atom) glucose were incorporated into the artificial soils to study the effects of clay types, exchangeable cations and clay contents on the mineralization of glucose-carbon and glucose-derived organic materials. Chemical transformation of glucose-carbon upon incorporation into microbial products and metabolites, was followed using solid-state¹³C CP/MAS NMR spectroscopy.There was a significant influence of exchangeable cations on the mineralization of glucose-carbon over a period of 33 days. At 25% clay content, mineralization of glucose-carbon was highest in Ca-soils and lowest in Al-soils. The influence of exchangeable cations on mineralization of glucose-carbon was more pronounced in soils with bentonite clay than those with kaolinite clay. Statistical analysis of data showed no overall effect of clay type on mineralization of glucose-carbon. However, the interactions of clay type with clay content and clay type with clay content and exchangeable cations were highly significant. At 25% clay content, the mineralization of glucose-carbon was significantly lower in Na- and Al-soils with Wyoming bentonite compared with Na- and Al-soils with Georgia kaolinite. For Ca-soils this difference was not significant. Due to the increased osmotic tension induced by the added glucose, mineralization of glucose-carbon was slower in soils with 5% clay than soils with 25% clay.Despite the differences in the chemical and physical characteristics of soils with Ca-, Na- and Al-clays, the chemical composition of organic materials synthesised in these soils were similar in nature. Assuming CP/MAS is quantitative, incorporation of uniformly¹³C-labelled glucose (99.9% atom) in these soils resulted in distribution of carbon in alkyl (24–25%), O-alkyl (56–63%), carbonyl (11–15%) and small amounts of aromatic and olefinic carbon (2–4%). However, as decomposition proceeded, the chemistry of synthesised material showed some changes with time. In the Ca- and Na-soils, the proportions of alkyl and carbonyl carbon decreased and that of O-alkyl carbon increased with time of incubation. However, the opposite trend was found for the Al-soil.Proton-spin relaxation editing (PSRE) subspectra clearly showed heterogeneity within the microbial products. Subspectra of the slowly-relaxing (long T₁(H)) domains were dominated by alkyl carbon in long- and short-chain structures. The signals due to N-alkyl (55 ppm) and carbonyl carbon were also strong in these subspectra. These subspectra were very similar to those obtained for microbial and fungal materials and were probably microbial tissues attached to clay surfaces by polysaccharide extracellular mucilage. Subspectra of fast-relaxing (short T₁(H)) domains comprised mostly O-alkyl and carbonyl carbon and were probably microbial metabolites released as neutral and acidic sugars into the extracellular environment, and strongly sorbed by clay surfaces.     

Characterization of solid and dissolved carbon in a spruce-fir Spodosol

Organic substances are an integral part of the biogeochemistry of many elements in forest ecosystems. However, our understanding of the composition, chemistry, and reactions of these materials are incomplete and sometimes inconsistent. Therefore, we examined in detail dissolved organic carbon (DOC) in forest floor leachates over a two-year period (1992–1993), soil C, and DOC adsorption by a mineral soil to determine the relationship between soil solid and solution C characteristics in a spruce-fir ecosystem. The structural composition of DOC, DOC fractions (hydrophobic and hydrophilic acids, hydrophilic neutrals), and soil samples from the organic and mineral horizons were also analyzed using¹³C nuclear magnetic resonance (NMR) spectroscopy.Total DOC in forest floor leachates ranged from 7.8 to 13.8 mmol L^–1 with an average of 8.6 mmol L^–1. Concentrations were highest in September of both 1992 and 1993. Fractionation of the forest floor DOC indicated these solutions contained high organic acid contents that averaged 92% of the total DOC. Hydrophobic acids were also preferentially adsorbed by the B horizon. The¹³C NMR data suggested alkyl, carbohydrate, aromatic, and carboxylic C were the primary constituents for organic and mineral soils, DOC, and DOC fractions. Compositional changes of C were observed as aromatic and carbohydrate decreased, whereas alkyl, methoxy, and carbonyl moieties increased with depth. However, C composition changed little among the three organic layers based on the similarity of alkyl/carbohydrates ratios as determined from NMR area integration, suggesting that in this acid soil, decomposition proceeds rather slowly. Hydrophobic acids contained high contents of aromatic C, whereas hydrophilic acids were comprised primarily of carboxylic C. Hydrophilic neutrals were rich in carbohydrate C. Results indicated that these DOC fractions were unaltered during the isolation process. Carboxylic C groups appeared to dissolve easily and were probably the primary contributor to organic acidity in our organic dominate leachates. Results also suggested that DOC materials adsorbed on the B horizon underwent further biodegradation. Several seasonal patterns of C composition were observed in the forest floor leachates and DOC fractions collected between 1992 and 1993.Overall, the evidence from this study suggested that (i) DOC levels were mainly controlled by biological activity, (ii) forest floor DOC was comprised primarily of organic acids, (iii) contact of soil leachates with B horizon material affected DOC quantitatively and qualitatively, (iv) phenolic, carboxylic, and carbonyl C appeared to dissolve readily in the forest Oa horizon, (v) DOC materials adsorbed on the B horizon selectively underwent further decomposition, and (vi) C composition is a function of the extent of decomposition and DOC fractions.     

亚热带不同林分土壤表层有机碳组成及其稳定性

在浙江临安玲珑山选取了常绿阔叶林、马尾松林、板栗林和雷竹林4种林分,采用传统的化学方法与固态13C核磁共振(NMR)技术研究其土壤有机碳在不同粒径土壤颗粒中的分布规律和结构特征,探讨林分类别和管理措施对土壤有机碳含量及其结构的影响,为亚热带地区森林固碳和土壤碳库管理提供科学依据。结果显示:(1)土壤表层(0—20 cm)有机碳含量按以下次序递减:雷竹林>常绿阔叶林>马尾松林>板栗林,且板栗林以粉黏粒结合态碳为主,其他林分土壤则以粗砂结合态碳为主;(2)13C NMR结果表明,阔叶林和马尾松林土壤有机碳中烷基碳所占比例最大,而雷竹林和板栗林则是烷氧碳比例最大,表明人工经营措施改变了土壤有机碳的成分组成;(3)随着土壤颗粒变细,有机碳中烷基碳比例增加,烷氧碳比例减少,A/O-A值和疏水碳/亲水碳值逐渐增大,表明颗粒越细,其结合的有机碳结构稳定性越高。     

Irrigation,fertilization and initial substrate quality effects on decomposing Loblolly pine litter chemistry

Changes in carbon chemistry (i.e., carbon compound classes such as aromatics, phenolics, etc.) of loblolly pine (Pinus taeda L.) litter were examined during three years of decomposition under factorial combinations of irrigation and fertilization treatments. Cross polarization magic angle spinning ¹³C nuclear magnetic resonance revealed that total carbon and nutrient concentrations correlated strongly with carbohydrate and O-alkyl carbon concentrations but did not relate well with concentrations of lignin, aromatic and phenolic carbon, or with lignin-related decomposition indices. The best correlations to carbon and nutrient concentrations occurred with the C/N (R²=0.86, P > 0.0001) and alkyl/O-alkyl (R²=0.75, P > 0.0001) decomposition indices. In all situations, the carbon chemistry of the decomposing litter followed the general pattern of accumulation of alkyl and carbonyl carbon with a loss of O-alkyl and methoxy carbon. Only small variations in the aromatic and phenolic carbon concentrations were detected. Since lignin is composed primarily of aromatic and phenolic carbons, the observation that there were only small changes in the aromatic and phenolic carbons of the litter is consistent with the general stability of lignin in these ecosystems. Trends in carbon chemistry during decomposition suggested that fertilization accelerated the decomposition process by about 100% as compared with the control plots. Irrigation also accelerated the decomposition process but to a lower extent (about 62% greater than control plots). Initial litter quality, as defined by the litter C/N, did not have a significant effect on the carbon chemistry of the decomposing litter. This study demonstrated that the decomposition mechanisms were not altered by the treatments but there were important changes in the relative chemistry of the decomposing litter which impacted the rate of decomposition.     

Linkages between phosphorus transformations and carbon decomposition in a forest soil

Phosphorus mineralization is chemically coupled with organic matter (OM) decomposition in surface horizons of a mixed-conifer forest soil from the Sierra Nevada, California, and is also affected by the disturbance caused by forest harvesting. Solution¹³C nuclear magnetic resonance (NMR) spectroscopy of NaOH extracts revealed a decrease of O-alkyl and alkyl-C fractions with increasing degree of decomposition and depth in the soil profile, while carbonyl and aromatic C increased. Solid-state¹³C-NMR analysis of whole soil samples showed similar trends, except that alkyl C increased with depth. Solution³¹P-NMR indicated that inorganic P (P₁) increased with increasing depth, while organic-P (P_o) fractions decreased. Close relationships between P mineralization and litter decomposition were suggested by correlations between P₁ and C fractions (r = 0.82, 0.81, –0.87, and –0.76 for carbonyl, aromatic, alkyl and O-alkyl fractions, respectively). Correlations for diester-P and pyrophosphate with O-alkyl (r = 0.63 and 0.84) and inverse correlations with aromatics (r = –0.74 and –0.72) suggest that mineralization of these P fractions coincides with availability of C substrate. A correlation between monoester P and alkyl C (r = 0.63) suggests mineralization is linked to breakdown of structural components of the plant litter. NMR analyses, combined with Hedley-P fractionation, suggest that post-harvest buildup of labile P in decomposed litter increases the potential for leaching of P during the first post-harvest season, but also indicates reduced biological activity that transports P from litter to the mineral soil. Thus, P is temporarily stored in decomposed litter, preventing its fixation by mineral oxides. In the mineral horizons,³¹P-NMR provides evidence of decline in biologically-available P during the first post-harvest season.     

O/N-alkyl and alkyl C are stabilised in fine particle size fractions of forest soils

Carbon stocks and organic matter composition in bulk soils and particle size fractions of Ah horizons from Luvisols, Leptosols and Phaeozems under European beech (Fagus silvatica L.) forest were investigated by elemental analysis, solid state ¹³C nuclear magnetic resonance (¹³C CPMAS NMR) spectroscopy and lignin analysis (CuO-oxidation). Radiocarbon age was used as an indicator for C turnover. The SOM of bulk soils and particle size fractions is dominated by O/N-alkyl C and alkyl C. Compared to sand and silt fractions, clay fractions had lower C/N ratios and ¹⁴C abundances. Aryl C and more specifically phenolic components (O-aryl C) decreased from sand to clay fractions. The concomitant decrease of lignin, determined by CuO oxidation, suggests that a major proportion of O-aryl C can be attributed to lignin. Positive nonlinear relations between the O-aryl C and the C/N ratio reveal the trend of decreasing O-aryl C proportions with increasing decomposition. Although lignin is believed to be highly recalcitrant, only low amounts of lignin are found in the stable clay fractions. In contrast to O-aryl C, the O/N-alkyl C contribution decreased from sand to silt fractions, but increased again in the clay fractions, whereas alkyl C contents exhibited lowest values in the sand fractions. These results are indicative of stabilisation processes operating specifically on polysaccharides and alkyl C, but not on aryl C, through association with the clay fraction.     

Distribution of soil organic matter between fractions and aggregate size classes in grazed semiarid steppe soil profiles

Grazed steppe ecosystems are discussed as one of the big global carbon sinks that may have the potential to sequester large amounts of atmospheric CO₂ and mitigate the effects of global change if grazing is abandoned or management improved. But until today, little is known about sequestration potentials and stabilisation mechanisms in complete soil profiles of semiarid grasslands and how these systems react to grazing cessation. We applied a combined aggregate size, density and particle size fractionation procedure to sandy steppe soils under different grazing intensities (continuously grazed = Cg, winter grazing = Wg, ungrazed since 1999 = Ug99, ungrazed since 1979 = Ug79). Higher inputs of organic matter in ungrazed treatments led to higher amounts of OC in coarse aggregate size classes (ASC) and especially in particulate organic matter (POM) fractions across all depth. These processes started in the topsoil and took more than 5 years to reach deeper soil horizons (>10 cm). After 25 years of grazing cessation, subsoils showed clearly higher POM amounts. We found no grazing-induced changes of soil organic matter (SOM) quantity in fine ASC and particle size fractions. Current C-loading of fine particle size fractions was similar between differently grazed plots and decreased with depth, pointing towards free sequestration capacities in deeper horizons. Despite these free capacities, we found no increase in current C-loading on fine mineral soil fractions after 25 years of grazing exclusion. Silt and clay fractions appeared to be saturated. We suppose empirical estimations to overestimate sequestration potentials of particle size fractions or climatic conditions to delay the decomposition and incorporation of OM into these particle size fractions. POM quality was analysed using solid-state ¹³C NMR spectroscopy to clarify if grazing cessation changed chemical composition of POM in different ASC and soil depths via changing litter quality or changing decomposition dynamics. We found comparable POM compositions between different grazing intensities. POM is decomposed hierarchically from coarse to fine particles in all soil depths and grazing cessation has not affected the OM decomposition processes. The surplus of OM due to grazing cessation was predominately sequestered in readily decomposable POM fractions across all affected horizons. We question the long-term stabilisation of OM in these steppe soils during the first 25 years after grazing cessation and request more studies in the field of long-term OM stabilisation processes and assessment of carbon sequestration capacities to consider deeper soil horizons.     

天然常绿阔叶林改造为板栗林对土壤有机碳库的影响

天然林改造为人工林后,由于植被覆盖类型和经营管理措施发生改变,从而显著影响土壤有机碳库的特征.测定浙江省临安市相邻的天然常绿阔叶林和板栗林(板栗林由常绿阔叶林改造而来,集约经营10年)表层(0～20 cm)和亚表层(20～40 cm)土壤有机碳储量和不同形态活性有机碳库,用固态核磁共振方法分析土壤有机碳的化学结构特征,研究天然常绿阔叶林改造为板栗林对土壤有机碳库的影响.结果表明: 常绿阔叶林改造为板栗林后,土壤表层有机碳储量、水溶性有机碳、热水溶性有机碳、微生物生物量碳和易氧化碳含量分别下降19.7%、34.4%、25.8%、30.4%和25.2%,土壤亚表层的各指标分别下降13.5%、38.4%、19.8%、34.1%和22.2%.土壤表层烷氧碳含量、芳香碳含量以及芳香度显著降低,而烷基碳含量、羰基碳含量以及A/O-A值均显著增加;土壤亚表层烷氧碳含量显著降低,而烷基碳含量和A/O-A值显著增加,而芳香碳含量、羰基碳含量以及芳香度无显著变化.天然常绿阔叶林改造为板栗林并长期集约经营后,土壤有机碳储量和活性有机碳库均显著下降,有机碳的化学结构发生显著变化.     

Old and stable soil organic matter is not necessarily chemically recalcitrant: implications for modeling concepts and temperature sensitivity

Soil carbon turnover models generally divide soil carbon into pools with varying intrinsic decomposition rates. Although these decomposition rates are modified by factors such as temperature, texture, and moisture, they are rationalized by assuming chemical structure is a primary controller of decomposition. In the current work, we use near edge X‐ray absorption fine structure (NEXAFS) spectroscopy in combination with differential scanning calorimetry (DSC) and alkaline cupric oxide (CuO) oxidation to explore this assumption. Specifically, we examined material from the 2.3–2.6 kg L^?1 density fraction of three soils of different type (Oxisol, Alfisol, Inceptisol). The density fraction with the youngest ¹⁴C age (Oxisol, 107 years) showed the highest relative abundance of aromatic groups and the lowest O‐alkyl C/aromatic C ratio as determined by NEXAFS. Conversely, the fraction with the oldest C (Inceptisol, 680 years) had the lowest relative abundance of aromatic groups and highest O‐alkyl C/aromatic C ratio. This sample also had the highest proportion of thermally labile materials as measured by DSC, and the highest ratio of substituted fatty acids to lignin phenols as indicated by CuO oxidation. Therefore, the organic matter of the Inceptisol sample, with a ¹⁴C age associated with ‘passive’ pools of carbon (680 years), had the largest proportion of easily metabolizable organic molecules with low thermodynamic stability, whereas the organic matter of the much younger Oxisol sample (107 years) had the highest proportion of supposedly stable organic structures considered more difficult to metabolize. Our results demonstrate that C age is not necessarily related to molecular structure or thermodynamic stability, and we suggest that soil carbon models would benefit from viewing turnover rate as codetermined by the interaction between substrates, microbial actors, and abiotic driving variables. Furthermore, assuming that old carbon is composed of complex or ‘recalcitrant’ compounds will erroneously attribute a greater temperature sensitivity to those materials than they may actually possess.     

Phosphorus and carbon in soil particle size fractions: A synthesis

Despite the importance of phosphorus (P) as a macronutrient, the factors controlling the pool sizes of organic and inorganic P (OP and IP) in soils are not yet well understood. Therefore, the aim of this study was to gain insights into the pools sizes of OP, IP and organic carbon (OC) in soils and soil particle size fractions. For this purpose, I analyzed the distribution of OP, IP, and OC among particle size fractions depending on geographical location, climate, soil depth, and land use, based on published data. The clay size fraction contained on average 8.8 times more OP than the sand size fraction and 3.9 and 3.2 times more IP and OC, respectively. The OP concentrations of the silt and clay size fraction were both negatively correlated with mean annual temperature (R² = 0.30 and 0.31, respectively, p < 0.001). The OC:OP ratios of the silt and clay size fraction were negatively correlated with latitude (R² = 0.49 and 0.34, respectively, p < 0.001). Yet, the OC:OP ratio of the clay size fraction changed less markedly with latitude than the OC:OP ratio of the silt and the sand size fraction. The OC concentrations of all three particle size fractions were significantly (p < 0.05) lower in soils converted to cropland than in adjacent soils under natural vegetation. In contrast, the OP concentration was only significantly (p < 0.05) decreased in the sand size fraction but not in the other two particle size fractions due to land-use change. Thus, the findings suggest that OP is more persistent in soil than OC, which is most likely due to strong sorptive stabilization of OP compounds to mineral surfaces.

     

天然灌木林改造成板栗林对土壤碳库和氮库的影响

在浙江省安吉县采集了相邻的天然灌木林和板栗林土壤,分析土壤水溶性碳(WSOC)、微生物生物量碳(MBC)、易氧化碳(ROC)、水溶性有机氮(WSON)和微生物生物量氮(MBN),并利用核磁共振方法分析土壤总有机碳的波谱特征,研究天然灌木林改造成板栗林对土壤碳库和氮库的影响.结果表明： 天然灌木林改造成板栗林后,土壤中的碱解氮、有效磷和速效钾显著增加,而WSOC、MBC、ROC、WSON和MBN显著下降.天然灌木林和板栗林土壤有机碳以烷基碳和烷氧碳为主.天然灌木林改造成板栗林后,土壤有机碳中的烷氧碳和羰基碳比例显著下降,而烷基碳和芳香碳比例以及A/O-A值和芳香度均显著增加.天然灌木林改造成板栗林并长期集约经营后,土壤活性碳库和氮库含量均显著下降,而土壤碳库的稳定性显著增加.     

Nitrogen deposition promotes the production of new fungal residues but retards the decomposition of old residues in forest soil fractions

Atmospheric nitrogen (N) deposition has frequently been observed to increase soil carbon (C) storage in forests, but the underlying mechanisms still remain unclear. Changes in microbial community composition and substrate use are hypothesized to be one of the key mechanisms affected by N inputs. Here, we investigated the effects of N deposition on amino sugars, which are used as biomarkers for fungal‐ and bacterial‐derived microbial residues in soil. We made use of a 4‐year combined CO₂ enrichment and N deposition experiment in model forest ecosystems, providing a distinct ¹³C signal for ‘new’ and ‘old’ C in soil organic matter and microbial residues measured in density and particle‐size fractions of soils. Our hypothesis was that N deposition decreases the amount of fungal residues in soils, with the new microbial residues being more strongly affected than old residues. The soil fractionation showed that organic matter and microbial residues are mainly stabilized by association with soil minerals in the heavy and fine fractions. Moreover, the bacterial residues are relatively enriched at mineral surfaces compared to fungal residues. The ¹³C tracing indicated a greater formation of fungal residues compared to bacterial residues after 4 years of experiment. In contradiction to our hypotheses, N deposition significantly increased the amount of new fungal residues in bulk soil and decreased the decomposition of old microbial residues associated with soil minerals. The preservation of old microbial residues could be due to decreased N limitation of microorganisms and therefore a reduced dependence on organic N sources. This mechanism might be especially important in fine heavy fractions with low C/N ratios, where microbial residues are effectively protected from decomposition by association with soil minerals.     

Evolution of non-dissolved particulate organic matter during composting of sludge with straw

Long term composting induces loss of C and organic matter stabilisation. These two processes may have opposite effects on long term carbon storage in soils. To check whether raw materials should be composted or not before being spread on the soil, changes in particle size fractions were quantified during composting of 9 tons of sewage sludge and straw. Both the mass of the fine fraction (<2 microm) and the amount of carbon contained in it increased after seven months, respectively, +37% and +43%. The fine fraction contributes to carbon sequestration. A literature review supported the assumption that composting should increase long term C storage. Nevertheless, soil texture or agricultural practices modify the behaviour of this fraction. Thus, the fractionation method used for soils is relevant to predict the effect of composting as a mitigation option in greenhouse gas reduction strategies, but is not sufficient in itself.     

秸秆还田条件下盐渍土团聚体中有机碳化学结构特征

为揭示秸秆还田后盐渍土团聚体中有机碳分布规律和化学结构特征,本研究以苏打碱化潮土为对象,于2020年设置0(CK)、2100(ST₁)、4200(ST₂)、6300(ST₃)、8400(ST₄)和10500 kg·hm^-2(全量还田,ST₅)6个不同秸秆还田用量处理,通过物理分组方法并结合红外光谱技术,测定土壤各粒级团聚体及内部不同组分有机碳含量和红外光谱特征。结果表明: 1)随着秸秆还田量增加,土壤及各级团聚体有机碳含量均呈增加趋势。2)不同秸秆还田处理均显著提高了53～250 μm团聚体轻组有机碳(LOC)含量;与CK相比,ST₃、ST₄处理显著提高250～2000 μm团聚体中矿物结合有机碳(MOC)和53～250 μm团聚体中细颗粒有机碳(fPOC)含量;团聚体内部不同组分有机碳含量由高到低均表现为: LOC>MOC>POC;250～2000 μm团聚体内部fPOC含量高于粗颗粒有机碳(cPOC)含量。3)主成分分析结果显示,秸秆还田对团聚体不同组分有机碳化学结构的影响较小,有机碳化学结构差异主要受粒级的影响。4)>250 μm粒级团聚体有机碳主要来源于芳香碳和多糖,53～250 μm粒级团聚体有机碳主要来源于单糖和多糖等碳水化合物,<53 μm黏粒有机碳主要来源于脂肪碳、烷基碳、芳香碳和酚醇化合物;不同粒级团聚体内部LOC主要来源于脂肪碳、芳香碳、酚醇化合物,颗粒有机碳(POC)主要来源于碳水化合物,MOC主要来源于烷基碳。综上,秸秆还田短期内能够提高土壤团聚体有机碳含量,但对团聚体有机碳化学结构无显著影响,且不同团聚体内部相同粒级组分有机碳化学结构相似,随着粒径减小,有机碳含量增加,化学结构趋于稳定。因此,秸秆还田短期内可促进盐渍土团聚体对有机碳的固定,但不改变有机碳化学结构特征,同时,土壤有机碳在团聚体中的存在位置和受保护程度是影响有机碳化学结构的主要因素。     

长期施肥下灰漠土矿物颗粒结合有机碳的含量及其演变特征

采用物理分组方法分析了长期（1990-2007年）不同施肥条件下灰漠土各粒径矿物颗粒结合有机碳含量和分布差异及其随施肥时间的演变特征.结果表明：与不施肥相比,配施有机肥对增加各有机碳组分的效果最显著,并以砂粒有机碳含量的增速（0.34 g·kg^-1·a^-1）最高,对施肥最敏感;撂荒地可以显著增加不同黏粉粒结合有机碳含量;秸秆还田仅能维持各级矿物颗粒结合有机碳的含量;长期施用化肥不利于各级颗粒结合有机碳含量的增加.从分配比例来看,以粗粉粒(27.9%)和粗黏粒(27.1%)有机碳所占比例最高,是固持有机碳的重要组分;配施有机肥使砂粒有机碳比例显著提高119.4%,细粉粒和粗黏粒有机碳比例却分别降低了40.3%和37.9%,从而提高了颗粒有机碳含量（W_POC）与矿物结合有机碳含量（W_MOC）的比值,改良了土壤有机碳性质.长期配施有机肥是增加灰漠土各级矿物颗粒结合有机碳积累和提升灰漠土肥力的最佳方式.     

Topographic controls on black carbon accumulation in Alaskan black spruce forest soils: implications for organic matter dynamics

There is still much uncertainty as to how wildfire affects the accumulation of burn residues (such as black carbon (BC)) in the soil, and the corresponding changes in soil organic carbon (SOC) composition in boreal forests. We investigated SOC and BC composition in black spruce forests on different landscape positions in Alaska, USA. Mean BC stocks in surface mineral soils (0.34 ± 0.09 kg C m^?2) were higher than in organic soils (0.17 ± 0.07 kg C m^?2), as determined at four sites by three different ¹³C Nuclear Magnetic Resonance Spectroscopy-based techniques. Aromatic carbon, protein, BC, and the alkyl:O-alkyl carbon ratio were higher in mineral soil than in organic soil horizons. There was no trend between mineral soil BC stocks and fire frequencies estimated from lake sediment records at four sites, and soil BC was relatively modern (<54–400 years, based on mean Δ¹⁴C ranging from 95.1 to ?54.7‰). A more extensive analysis (90 soil profiles) of mineral soil BC revealed that interactions among landscape position, organic layer depth, and bulk density explained most of the variance in soil BC across sites, with less soil BC occurring in relatively cold forests with deeper organic layers. We suggest that shallower organic layer depths and higher bulk densities found in warmer boreal forests are more favorable for BC production in wildfire, and more BC is integrated with mineral soil than organic horizons. Soil BC content likely reflected more recent burning conditions influenced by topography, and implications of this for SOC composition (e.g., aromaticity and protein content) are discussed.     

Temperature response of litter and soil organic matter decomposition is determined by chemical composition of organic material

The global soil carbon pool is approximately three times larger than the contemporary atmospheric pool, therefore even minor changes to its integrity may have major implications for atmospheric CO₂ concentrations. While theory predicts that the chemical composition of organic matter should constitute a master control on the temperature response of its decomposition, this relationship has not yet been fully demonstrated. We used laboratory incubations of forest soil organic matter (SOM) and fresh litter material together with NMR spectroscopy to make this connection between organic chemical composition and temperature sensitivity of decomposition. Temperature response of decomposition in both fresh litter and SOM was directly related to the chemical composition of the constituent organic matter, explaining 90% and 70% of the variance in Q₁₀ in litter and SOM, respectively. The Q₁₀ of litter decreased with increasing proportions of aromatic and O‐aromatic compounds, and increased with increased contents of alkyl‐ and O‐alkyl carbons. In contrast, in SOM, decomposition was affected only by carbonyl compounds. To reveal why a certain group of organic chemical compounds affected the temperature sensitivity of organic matter decomposition in litter and SOM, a more detailed characterization of the ¹³C aromatic region using Heteronuclear Single Quantum Coherence (HSQC) was conducted. The results revealed considerable differences in the aromatic region between litter and SOM. This suggests that the correlation between chemical composition of organic matter and the temperature response of decomposition differed between litter and SOM. The temperature response of soil decomposition processes can thus be described by the chemical composition of its constituent organic matter, this paves the way for improved ecosystem modeling of biosphere feedbacks under a changing climate.