Soil organic C stabilization and thresholds in C saturation

When building soil organic matter (SOM) contents in agricultural production systems, stabilization of both pre-existing as well as added C is important. A laboratory mineralization experiment was conducted over 374 days to evaluate the effect of pre-existing SOM on soil C mineralization after addition of organic matter (OM) using sugar cane. The SOM gradient used here stretched from 21 to 106 g C kg⁻¹ soil and was a result of different periods of continuous cultivation of 5, 20, 35 and 105 years in comparison to a forest soil. The rate of organic C mineralization was found to be dependent on the status of pre-existing soil organic C (SOC). Highly degraded soil which had been under continuous cultivation for 35 years and more showed the highest rate of C mineralization per unit SOC (117.9 mg C g⁻¹ C) while forest soil had the lowest amount of C mineralized per unit SOC (73.5 mg C g⁻¹ C). Forest soil had the highest amount of increased C mineralization as a result of organic matter (OM) additions (8.0 mg C g⁻¹ soil) followed by the highly degraded soil that had been under cultivation for 105 years (5.5 mg C g⁻¹ soil). Additional mineralized C as a function of time after forest conversion declined progressively within the first 20 years of continuous soil use. Soil which had been under continuous cultivation for 20 years had the lowest amount of additional mineralized C (4.0 mg C g⁻¹ soil). SOM stabilization efficiency in the studied soils appears to be highest with intermediate cultivation history of about 20 years. These soils that have been recently converted to cultivation also appear to have a greater ability to stabilize added OM than the most degraded soils investigated in this study. It is thus advisable to provide intervention strategies to reverse SOM decline for farming communities at an intermediate stage before the soils are highly depleted of SOC.     

Species-dependent partitioning of C and N stable isotopes between arbuscular mycorrhizal fungi and their C3 and C4 hosts

Natural ¹³C and ¹⁵N abundances of mycorrhizal fungi are increasingly used in ecology but reference data on arbuscular mycorrhizal fungi (AMF) are scarce. In experiments with nine phylogenetically dispersed AMF strains inoculated on leek (C3 plant) and sorghum (C4) in pot cultures, we measured the ¹³C/¹²C and ¹⁵N/¹⁴N ratios in shoots, roots, AMF spores as well as carbon isotope signature of the C16:1ω5 fatty acid (FA), which is diagnostic for AMF. Spore δ¹³C values varied among AMF strains on any given host. They were significantly lower than shoot δ¹³C for sorghum (−2.5‰ on average) while for leek, no clear C isotope partitioning between spores and host shoots was observed. The FA C16:1ω5 fatty acids were more ¹³C-depleted than spores, without correlation with spore δ¹³C values. For both, sorghum and leek, spore δ¹⁵N was higher (+1–2‰ on average) than for shoots. We found no evidence that isotopic partitioning between the partners drives ¹³C and ¹⁵N abundances in plant–AMF symbiosis. Mycorrhizal roots displayed relatively high δ¹³C typical for heterotrophic organs, and not a mix between AMF and plant signatures. Interestingly, inoculation slightly decreased shoot δ¹³C on leek but not on sorghum, as compared with non-mycorrhizal plants, suggesting that AMF improved the plant's water status, a parameter affecting the δ¹³C of C3 but not C4 plants. Phylogenetically closer AMF displayed more similar spore δ¹³C and induced similar ¹³C and ¹⁵N abundances on leek shoots, but this observation was not confirmed for sorghum. Plant and AMF isotopic abundances hardly correlated with other parameters related to plant development, mineral nutrition or root mycorrhizal colonisation, and these correlations were never consistent between sorghum and leek. Thus, isotopic abundances in plant–AMF symbiosis were rather constrained by each AMF/plant interaction. Nevertheless, our data provide a valuable reference for future investigations of AMF communities and AM symbiosis in situ.     

Seasonal variations in natural 13C abundances in C3 and C4 plants collected in Thailand and the Philippines

Abstract

The natural ¹³C abundance (δ ¹³C) of plant leaves collected from fields in Thailand and the Philippines (Asian Monsoon tropics) was analyzed, and changes in the δ ¹³C values of C₃ and C₄ plants in wet and dry seasons were characterized. In Thailand, the δ ¹³C values of C₃ plants were ?29.2?±?1.04 (mean?±?standard deviation) ‰ in July and August (wet season) and ?28.6?±?1.05‰ in February and March (dry season): these values are not significantly different, whereas the values of C₄ plants were ?12.7?±?0.56‰ in the wet season and ?14.5?±?0.68‰ in the dry season (P?<?0.01, t-test). In the Philippines, where plants were collected only in October (late wet season), the δ ¹³C values of C₃ plants were ?29.5?±?1.28‰, whereas those of C₄ plants were ?12.6?±?1.11‰. These results suggest that under an Asian Monsoon climate, C₄ plants exhibit more negative δ ¹³C values in the dry season than in the wet season, whereas C₃ plants as a whole show no clear seasonal changes in δ ¹³C values.     

The initial rate of C substrate utilization and longer-term soil C storage

The initial reaction of microbial transformation and turnover of soil carbon inputs may influence the magnitude of longer-term net soil C storage. The objective of this study was to test the merit of the hypothesis that the more rapid substrates are initially utilized, the longer the residual products remain in the soil. We used simple model C compounds to determine their decomposition rates and persistence over time. Pure ¹⁴C compounds of glucose, acetate, arginine, oxalate, phenylalanine, and urea were incubated in soil for 125 days at 24°C. Total respired CO₂ and ¹⁴CO₂ was quantitatively measured every day for 15 days and residual soil ¹⁴C after 125 days. The percent ¹⁴C remaining in the soil after 125 days of incubation was positively and significantly correlated with the percent substrate utilized in the first day of incubation. The ¹⁴C in the microbial biomass ranged from 4–15% after 15 days and declined through day 125, contributing significantly to the ¹⁴C that evolved over the longer time period. Priming of ¹²C soil organic matter (SOM) was negative at day 3 but became positive, reaching a maximum on day 12; the total increase in soil C from added substrates was greater than the primed C. The primed C came from ¹²C SOM rather than the microbial biomass. This data supports the concept that the more rapidly a substrate is initially mineralized, the more persistent it will be in the soil over time.     

小麦温光型不育系C118S、C338S的选育

利用C49S-87为亲本材料,采用[(小黑麦×C49S-87)×绵农4号]×C49S-87、绵338×C49S-87杂交,通过分期播种试验,花粉镜检等手段,经过六年选育研究,转育出温光型不育系C118S、C338S。该不育系柱头外露率高、异交习性优于对照C49S-87。抗病性特别是抗条锈病有明显的改善,在永川自然条件下,表现为抗条锈病。在重庆地区正季播种不育可制种,晚播可繁殖不育系。     

Alkyl C and hydrophobicity in B and C horizons of an acid forest soil

Aliphatic C most probably derived from ester‐bound moieties was found to be present in sandy subsoil horizons. The hydrophobic nature of such compounds may increase their stabilization potential. Therefore, the aim of this study was to investigate the potential of aliphatic compounds in mineral soil horizons along a Dystric Cambisol profile under beech forest to increase hydrophobicity. The conceptual approach included the analyses of soil samples before and after solvent extraction and base hydrolysis for elemental and isotopic composition. Additionally, the advancing contact angle was measured to quantify hydrophobicity. Curie‐point pyrolysis GC/MS was carried out to characterize the nature of alkyl C present in subsoil samples. A close correlation between the ¹⁴C activity and the stable‐C‐isotope ratio (δ¹³C) indicates isotopic fractionation upon C stabilization in subsoils. Free lipids contributed less than 10% to the organic C found in subsoil horizons. Base hydrolysis revealed very high amounts of hydroxyalkanoic acids in the B horizons of the acid forest soil. Hydrophobicity of SOM was not found to be correlated to esterified‐ or free‐lipid content. The contact angle was in a similar range for all bulk soil horizons, suggesting greater hydrophobicity of organic matter in subsoil horizons considering their very low concentrations of organic C compared to the A horizon. The quantity and nature of pyrolysis products change with increasing depth in the soil profile. Aliphatic products cannot be detected in B and C horizons by Curie‐point pyrolysis GC/MS.     

长期施肥对土壤活性有机碳和碳库管理指数的影响

以长期定位试验为研究对象,分析了长期施肥对土壤不同层次活性碳的影响,并计算了各处理的碳库指数和碳库管理指数。结果表明,土壤活性碳均随土层的增加而减少0,～7.5 cm土层的活性有机碳平均比7.5～15 cm、15～30 cm、30～50 cm高出6.6%、23.4%、57.1%。施肥均可以提高土壤活性有机碳和碳库指数。单施有机肥培肥土壤和有机肥、化肥的配合施肥对提高土壤中活性有机碳的作用较单纯施用化肥更为显著。施肥除了绿+N、秸+N、农+绿+N处理的碳库管理指数降低外,其它各处理的碳库管理指数均有所增加。农业生产中必须重视有机肥的合理施用,使土壤碳库处于良性状态,最终才有可能维持土壤的可持续性利用。     

Root-derived carbon in soil respiration and microbial biomass determined by 14C and 13C

Two approaches to quantitatively estimating root-derived carbon in soil CO₂ efflux and in microbial biomass were compared under controlled conditions. In the ¹⁴C labelling approach, maize (Zea mays) was pulse labelled and the tracer was chased in plant and soil compartments. Root-derived carbon in CO₂ efflux and in microbial biomass was estimated based on a linear relationship between the plant shoots and the below-ground compartment. Since the maize plants were grown on C₃ soil, in a second approach the differences in ¹³C natural abundance between C₃ and C₄ plants were used to calculate root-derived carbon in the CO₂ efflux and in the microbial biomass. The root-derived carbon in the total CO₂ efflux was between 69% and 94% using the ¹⁴C labelling approach and between 86% and 94% in the natural ¹³C labelling approach. At a ¹³C fractionation measured to be 5.2‰ between soil organic matter (SOM) and CO₂, the root-derived contribution to CO₂ ranged from 70% to 88% and was much closer to the results of the ¹⁴C labelling approach. Root-derived contributions to the microbial biomass carbon ranged from 2% to 9% using ¹⁴C labelling and from 16% to 36% using natural ¹³C labelling. At a 3.2‰ ¹³C fractionation between SOM and microbial biomass, both labelling approaches yielded an equal contribution of root-derived C in the microbial biomass. Both approaches may therefore be used to partition CO₂ efflux and to quantify the C sources of microbial biomass. However, the assumed ¹³C fractionation strongly affects the contributions of individual C sources.     

Budgets for root-derived C and litter-derived C: comparison between conventional tillage and no tillage soils

Placement of plant residues in conventional tillage (CT) and no-tillage (NT) soils affects organic matter accumulation and the organization of the associated soil food webs. Root-derived C inputs can be considerable and may also influence soil organic matter dynamics and soil food web organization. In order to differentiate and quantify C contributions from either roots or litter in CT and NT soils, a ¹⁴C tracer method was used.To follow root-derived C, maize plants growing in the field were ¹⁴C pulse-labeled, while the plant litter in those plots remained unlabeled. The ¹⁴C was measured in NT and CT soils for the different C pools (shoots, roots, soil, soil respiration, microbial biomass). Litter-derived C was followed by applying ¹⁴C labeled maize litter to plots which had previously grown unlabeled maize plants. The ¹⁴C pools measured for the litter-derived CT and NT plots included organic matter, microbial biomass, soil respiration, and soil organic C.Of the applied label in the root-derived C plots, 35–55, 6–8, 3, 1.6, and 0.4–2.4% was recovered in the shoots, roots, soil, cumulative soil respiration, and microbial biomass, respectively. The ¹⁴C recovered in these pools did not differ between CT and NT treatments, supporting the hypothesis that the rhizosphere microbial biomass in NT and CT may be similar in utilization of root-derived C. Root exudates were estimated to be 8–13% of the applied label. In litter-derived C plots, the percentage of applied label recovered in the particulate organic matter (3.2–82%), microbial biomass (4–6%), or cumulative soil respiration (12.5–14.7%) was the same for CT and NT soils. But the percentage of ¹⁴C recovered in CT soil organic C (18–69%) was higher than that in NT (12–43%), suggesting that particulate organic matter (POM) leaching and decomposition occurred at a higher rate in CT than in NT. Results indicate faster turnover of litter-derived C in the CT plots.     

Reactivity of epicatechin in aqueous glycine and glucose maillard reaction models: quenching of C2, C3, and C4 sugar fragments

Mechanisms of how epicatechin alters the pathways of the Maillard reaction were investigated. Carbon-13 and nitrogen-15 labeling studies were utilized to define the reactivity of epicatechin with glucose, glycine, and/or reaction products in an aqueous model (pH 7, 125 degrees C for 30 min) via GC, GC/MS and HPLC/MS analysis. Quantification of the volatile reaction product isotopomers by GC/MS from a 1:1 labeled to unlabeled glucose (carbohydrate module labeling technique) plus glycine model system indicated the formation of 2,3-butanedione and acetol were primarily formed via intact C4 and C3 sugar fragments, whereas pyrazine, methylpyrazine, 2,5-dimethylpyrazine, 2,3,5-trimethylpyrazine, and cyclotene were primarily formed via intact C2/C2, C2/C3, C3/C3, C3/C3, and C3/C3 sugar fragment pairs, respectively. The formation of these seven compounds was also reported by GC analysis to be dramatically inhibited when epicatechin was added to the glucose/glycine model system (observed 9-113-fold reduction). HPLC/MS analysis of both the glucose-labeled and glycine-labeled model systems with and without epicatechin indicated that epicatechin reacted directly with C2, C3, and C4 sugar fragments, while epicatechin did not report any direct reactivity with glycine. In conclusion, the quenching of sugar fragmentation products via epicatechin was correlated with the observed inhibition on volatile compound formation when epicatechin was added to a glucose/glycine aqueous reaction model system.     

Near-infrared spectroscopy for determination of soil organic C,microbial biomass C and C and N fractions in a heterogeneous sample of German arable surface soils

Visual and near-infrared spectroscopy (vis-NIRS) is an established method to estimate soil properties. However, only limited information is available to estimate C and N fractions in a heterogeneous sample. The objectives of our study were to determine estimation accuracies of vis-NIRS using two software for soil organic carbon (SOC), total nitrogen (N_t), pH, texture, and C and N fractions (light (LF), mineral (MF), labile, intermediate and passive fractions) in a heterogeneous sample (consisting of 51 units with different mineralogy) and to compare these results with those obtained by mid-infrared spectroscopy (MIRS). Analyzing vis-NIRS spectra and the mentioned properties showed a possibility to distinguish between high and low values for SOC (residual prediction deviation (RPD) = 1.90) and N_t (RPD = 1.93). Sand and clay could be estimated, whereas pH and silt could not. No useful estimation was possible for N-LF, passive C, intermediate C or intermediate N. C-LF, C-MF and N-MF could be differentiated between high and low values, whereas for passive N the estimation was approximate quantitative. MIRS reached one or two times higher estimation categories than vis-NIRS for SOC, N_t, pH and texture, suggesting that MIRS has a higher potential to estimate soil properties in a heterogeneous sample.     

Mathematical description of the dynamics of 12C and 14C in the soil humus

Mathematical tools that can be applied to analyze the dynamics of ¹⁴C and ¹²C isotopes in soil humus are discussed. A system of two differential equations is suggested to describe the dynamics of the ¹²C and the ¹⁴C/¹²C ratio (normalized to the NBS standard) with due account for the initial conditions and for the ¹⁴C dynamics in the atmosphere (in particular, the bomb effect). Possible analytical solutions to these equations and algorithms that can be used in the case when these solutions are absent are discussed. Routine problems of the assessment of the dates of soil burying and the coefficient of the humus mineralization on the basis of the experimentally determined ¹⁴C/¹²C ratios in the soil humus are considered with the use of published data. In particular, the specific features of the assessment of the parameters of these equations in the case of the insufficiency of the experimental data are analyzed.     

施肥对黑土活性有机碳和碳库管理指数的影响

运用更有效的量化指标来了解和表征土壤碳库的变化是研究土壤碳库动态平衡的基础,也是评价土壤肥力和生态系统的可持续性的必要手段。我们采用土壤碳库管理指数,讨论了国家黑土肥力监测区内的不同施肥情况下土壤碳库的变化。结果显示:施肥与否、施肥种类和数量均对土壤活性有机碳和土壤碳库管理指数有非常显著的影响,施肥尤其高量有机肥与化肥(NPK)配施。更有助于土壤活性有机碳的增加,相应地也就提高了土壤碳库管理指数(CMPI),M2+CK、M4+CK、MO+NPK、M1+NPK、1.5M1+NPK、M2+NPK、M4+NPK各施肥处理对土壤活性有机碳提高的贡献率分别高达15.6％、24.8％、63.6％、135.1％、144.2％、185.9％和256.5％,对土壤碳库管理指数的提高系数达0.48、0.72、1.17、3.21、4.70、7.86和10.44。农业生产中必须切实地重视高量有机肥与化肥(NPK)的配施,以求保持土壤肥力,提高土壤质量,使土壤碳库处于良性状态,最终达到维持土壤的可持续利用之目的。     

C36多元醇改良C18多元醇包膜肥料缓控释性能

为探讨C36多元醇(简称C36)对C18多元醇(简称C18,对照)包膜材料缓控释性能的影响,该文以C18为对照膜材,将C36与C18分别按照质量比1:2、2:3、1:1共混聚合制备成3种新型包膜材料,在相同工艺下依次制造成包膜厚度均为2.5%(膜材占核芯肥料的百分比)的包膜尿素,分别记作PCU1、PCU2、PCU3、PCU4,其中PCU1为对照。通过坠落撞击前、后包膜尿素的释放率评价C36作为包膜材料的效果。研究结果表明:C36与C18共混聚合包膜尿素的外观及耐撞击性能明显优于对照处理的,表现为包膜层光泽明亮、膜质均匀、膜层韧性好,同期静水培养的颗粒表面积较对照的增加了23%。该文提出了肥效期保持率(tR)的概念,并用其评价包膜肥料耐撞击性能。共混聚合物C36:C18比例为2:3时,包膜尿素耐撞击性能最优,受撞击后肥效期保持率tR可高达88%左右,为对照处理的2.5倍以上。该处理包膜尿素的初期溶出率、微分溶出率最小,与对照PCU1处理的无明显差异。C36嵌入C18共混聚合生成的新包膜材料对包膜尿素肥效期的影响与两者的比例和包膜材料的用量有关,当C36:C18为2:3,包膜材料用量(2.5%)小于常规用量(3%~4%)时,虽然包膜尿素的肥效期没有延长,但是包膜完整、膜层均匀、韧性强,在包膜肥受到强烈撞击的情况下能够很好地保持原有肥料的肥效期,即能保持包膜肥料的缓控释性能。因此,在C18多元醇嵌入适量C36多元醇是改良C18多元醇包膜材料,增加其包膜肥肥效期保持率的有效方法。     

Organic C and N storage, and organic C fractions, in adjacent cultivated and forested soils of eastern Canada

As a major attribute of soil quality, organic matter is responsive to agricultural land use practices including tillage. A study was initiated in eastern Canada to characterize changes in the masses of organic C and total N, and organic matter fractions in forested and adjacent cultivated or forage sites. Generally, the cultivated and forage sites had denser soil profiles than the forest sites. Based on an equivalent soil mass, to accommodate differences in soil bulk density, the paired forest and cultivated sites showed that cultivation decreased the mass of organic C (35%) and total N (10%) in the soil profile of the Podzolic soils, but increased organic C (25%) and total N (37%) in the Brunisolic (Cambisol) and Gleysolic soils. For the Podzolic soils, use of forages increased soil stored organic C and N by 55% and 35%, respectively. Organic C fractions were mainly of significance in the A horizon. Soil microbial biomass C was greater in the forested, compared to the cultivated soil, but the proportion of soil organic C as microbial biomass C (1.3% to 1.6%) was similar. The proportion, however, was greater (2.1%) for the forage soil, compared to the corresponding cultivated (1.3%) soil, suggesting that organic C was continuing to increase under the former. The relatively large proportion (19%) of organic C found in the light fraction of forest soils in the A horizon was decreased (up to 70%) by cultivation. In contrast, the proportion of macro-organic C present in the soil sand fraction was not greatly influenced by cultivation. Overall, soils in eastern Canada have a relatively large potential to store organic matter. The study illustrates the importance of soil type and cultivation interactions for maintenance of soil organic matter storage, and the positive influence of forages in this regard in agroecosystems.     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.     

不同C N比堆肥碳素物质变化规律研究

针对堆肥化过程中常伴有少量CH4等温室气体排放造成环境污染等问题,采用密闭堆肥化装置,进行了不同碳氮比和通气条件下,堆肥过程中的气体释放规律、影响因子及其对堆肥理化性质的影响研究。结果表明,C/N=25、30堆肥处理的有机物降解率高于低碳氮比处理;全氮含量随着有机物的降解而浓缩,随着堆肥的进行而不断提高,到堆肥结束时,C/N=15、20、25和30处理的全氮含量分别为23.5、24、27.8 g.kg-1和28.4 g.kg-1;堆肥过程中,C/N=15、20、25和30的堆肥处理CH4累积排放为0.67、0.95、2.25g.kg-1和1.80 g.kg-1,损失比例占初始碳物质的0.39%、0.5%、1.24%和0.92%,并且CH4气体的排放主要集中在高温前期,高温期越长,排放的温室气体越多。高温期适当增大通气量,对于控制堆肥温度和减少温室气体生成有双重作用。     

造林恢复植被后土壤的化学计量学特征对其碳组分的影响

Afforestation is recognized as an important driving force for soil organic C(SOC) dynamics and soil element cycling.To evaluate the relationships between soil C:N:P stoichiometry and SOC fractions,soil C:N:P stoichiometry distributions at 0–200 cm soil depths were analyzed and the contents of SOC fractions were evaluated in 9 typical land-use systems on the Loess Plateau of China.The contents of light fraction organic C,particulate organic C(53,53–2 000,and2 000 μm),labile organic C,microbial biomass C,and dissolved organic C decreased with increasing soil depth and were higher in afforested soil than in slope cropland soil.Compared with the slope cropland,different vegetation types influenced soil C:N,C:P,and N:P ratios,especially when C:P and N:P ratios were significantly higher(P0.05).Moreover,SOC fractions at the 0–10 and 10–40 cm depths were particularly affected by soil C:P ratio,whereas those at the 40–100 and 100–200 cm soil depths were significantly affected(P0.05) by soil N:P ratio.These results indicate that changes in SOC fractions are largely driven by soil C:P and N:P ratios at different soil depths after afforestation.