Carbon flux from decomposing 13C-labeled transgenic and nontransgenic parental rice straw in paddy soil

Purpose

Genetic modification of Bt rice may affect straw decomposition and soil carbon pool under flood conditions. This study aims to assess the effects of cry gene transformation in rice on the residue decomposition and fate of C from residues under flooded conditions.

Materials and methods

A decomposition experiment was set up using ¹³C-enriched rice straws from transgenic and nontransgenic Bt rice to evaluate the soil C dynamics and CH₄ or CO₂ emission rates in the root and non-root zones. The concentrations and stable carbon isotope compositions of the soil organic carbon (SOC), dissolved organic carbon (DOC), microbial biomass carbon (MBC), CH₄, and CO₂ of the root and non-root zones were determined from 7 to 110 days after rice straw incorporation.

Results and discussion

Rice straw incorporation into soil significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates. The percentage of ¹³C-SOC remaining in the root zone was significantly lower than that in the non-root zone with rice straw decomposition. The DOC and MBC concentrations significantly increased in both the root and non-root zones between 0 and 80 days after rice straw incorporation. However, no significant differences were found after Bts (Bt rice straw added into soil) and Cks (nontransgenic Bt rice straw added into soil) incorporation in the root and non-root zones. This result may be attributed to the priming effects of sufficient oxygen and nutrients on straw degradation in the root zone.

Conclusions

Bt gene insertion did not affect the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates in both the root and non-root zones. However, rice straw incorporation and root exudation significantly increased the SOC, DOC, and MBC concentrations and the CH₄ and CO₂ emission rates.     

Using soil organic matter fractions as indicators of soil physical quality

The objective of this study was to evaluate the use of chemical and physical fractions of soil organic matter (SOM ), rather than SOM per se , as indicators of soil physical quality (SPQ ) based on their effect on aggregate stability (AS ). Chemically extracted humic and fulvic acids (HA and FA ) were used as chemical fractions, and heavy and light fractions (HF and LF ) obtained by density separation as physical fractions. The analyses were conducted on medium‐textured soils from tropical and temperate regions under cropland and pasture. Results show that soil organic carbon (SOC ), SOM fractions and AS appear to be affected by land use regardless of the origin of the soils. A general separation of structurally stable and unstable soils between samples of large and small SOC content, respectively, was observed. SOM fractions did not show a better relationship with AS than SOC per se . In both geographical regions, soils under cropland showed the smallest content of SOC , HA and carbon concentration in LF and HF , and the largest HF /LF ratio (proportion of the HF and LF in percent by mass of bulk soil). With significant associations between AS and SOC content (0.79**), FA /SOC (r  = −0.83*^*), HA /FA (r  = 0.58*^*), carbon concentration of LF (r  = 0.69*^*) and HF (r  = 0.70*^*) and HF /LF ratio (r  = 0.80*^*), cropland showed lowest AS . These associations indicate that SOM fractions provide information about differences in SOM quality in relation to AS and SPQ of soils from tropical and temperate regions under cropland and pasture.     

Climatic influences on active fractions of soil organic matter

Biologically active fractions of soil organic matter are important in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. We evaluated the quantitative relationships among potential C and net N mineralization, soil microbial biomass C (SMBC), and soil organic C (SOC) under four contrasting climatic conditions. Mean SOC values were 28±11 mg g⁻¹ (n=24) in a frigid–dry region (Alberta/British Columbia), 25±5 mg g⁻¹ (n=12) in a frigid–wet region (Maine), 11±4 mg g⁻¹ (n=117) in a thermic–dry region (Texas), and 12±5 mg g⁻¹ (n=131) in a thermic–wet region (Georgia). Higher mean annual temperature resulted in consistently greater basal soil respiration (1.7 vs 0.8 mg CO₂–C g⁻¹ SOC d⁻¹ in the thermic compared with the frigid regions, P<0.001), greater net N mineralization (2.8 vs 1.3 mg inorganic N g⁻¹ SOC 24 d⁻¹, P<0.001), and greater SMBC (53 vs 21 mg SMBC g⁻¹ SOC, P<0.001). Specific respiratory activity of SMBC was, however, consistently lower in the thermic than in the frigid regions (29 vs 34 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Higher mean annual precipitation resulted in consistently lower basal soil respiration (1.1 vs 1.3 mg CO₂–C g⁻¹ SOC d⁻¹ in the wet compared with the dry regions, P<0.01) and lower SMBC (31 vs 43 mg SMBC g⁻¹ SOC, P<0.001), but had inconsistent effects on net N mineralization that depended upon temperature regime. Specific respiratory activity of SMBC was consistently greater in the wet than the dry regions (≈33 vs 29 mg CO₂–C g⁻¹ SMBC d⁻¹, P<0.01). Although the thermic regions were not able to retain as high a level of SOC as the frigid regions, due likely to high annual decomposition rates, biologically active soil fractions were as high per mass of soil and even 2–3-times greater per unit of SOC in the thermic compared with the frigid regions. These results suggest that macroclimate has a large impact on the portion of soil organic matter that is potentially active, but a relatively small impact on the specific respiratory activity of SMBC.     

Decomposition temperature sensitivity of isolated soil organic matter fractions

The general consensus is that a warming climate will result in the acceleration of soil organic matter (SOM) decomposition, thus acting as a potential positive feedback mechanism. However, the debate over the relative temperature sensitivity of labile versus recalcitrant SOM has not been fully resolved. We isolated acid hydrolysis residues to represent a recalcitrant pool of SOM and particulate organic matter (POM) to represent a labile pool of SOM, and incubated each at different temperatures to determine temperature sensitivity of decomposition. Short-term incubations of POM generated results consistent with published experiments (i.e., greater proportion of C respired and lower Q₁₀ than whole soil), while incubations of acid hydrolysis residues did not. The contrasting results illustrate the difficulty in assessing temperature sensitivity of labile versus stable SOM decomposition, partly because of the inability to quantitatively isolate labile versus stable SOM pools and to be sufficiently certain that respiration responses to temperature are not masked by processes such as enhanced stabilization or microbial inhibition/adaptation. Further study on the temperature sensitivity of decomposition of isolated SOM fractions is necessary to better explain and predict temperature responses of bulk SOM decomposition.     

Evidence of peptides in low-molecular-weight fractions of soil organic matter

Summary Organic matter was extracted from three soils, a Berwick sandy loam, a Franklin loamy sand, and a Cumberland silty loam. The extracts were separated into high (>8000 daltons) and low-molecular-weight (<8000 daltons) fractions using gel filtration. Reverse-phase high performance liquid chromatography at 214 nm was used to separate the peptides into low-molecular-weight fractions. Peptide samples were collected with an integrated fraction collector and hydolyzed with an immobilized protease column reactor. High performance liquid chromatography with fluorescence detection was used to determine the amino-acid contents of the collected samples. The results indicated that peptide intermediates are present in soil size fractions. Greater quantities of several amino acids were released from the peptide hydrolyzates of the Berwick sandy loam and Franklin loamy sand, compared with the Cumberland silty loam, an uncultivated soil. These findings indicate that organic intermediates (e.g., peptides) are more prevalent in biologically active soils than in relatively inert soils.     

The quality of exogenous organic matter: short-term effects on soil physical properties and soil organic matter fractions

We examined the short-term effect of five organic amendments and compared them to plots fertilized with inorganic fertilizer and unfertilized plots on aggregate stability and hydraulic conductivity, and on the OC and ON distribution in physically separated SOM fractions. After less than 1 year, the addition of organic amendments significantly increased ( P  <   0.01) the aggregate stability and hydraulic conductivity. The stability index ranged between 0.97 and 1.76 and the hydraulic conductivity between 1.23 and 2.80 × 10⁻³ m/s for the plots receiving organic amendments, compared with 0.34–0.43, and 0.42–0.64 × 10⁻³ m/s, respectively, for the unamended plots. There were significant differences between the organic amendments (P <  0.01), although these results were not unequivocal for both soil physical parameters. The total OC and ON content were significantly increased ( P  <   0.05) by only two applications of organic fertilizers: between 1.10 and 1.51% OC for the amended plots versus 0.98–1.08% for the unamended and between 0.092 and 0.131% ON versus 0.092–0.098% respectively. The amount of OC and ON in the free particulate organic matter fraction was also significantly increased ( P  <   0.05), but there were no significant differences ( P  <   0.05) in the OC and ON content in the POM occluded in micro-aggregates and in the silt + clay-sized organic matter fraction. The results showed that even in less than 1 year pronounced effects on soil physical properties and on the distribution of OC and ON in the SOM fractions occurred.     

Land-use related organic matter dynamics in North Cameroon soils assessed by 13C analysis of soil organic matter fractions

Topsoil samples from cultivated and adjacent non‐cultivated fields on three major agricultural soils in North Cameroon were fractionated into particle‐size fractions that were analysed subsequently for their C and ¹³C contents. The aim was to obtain further insight into the dynamics of soil organic matter (SOM) in relation to land use in Cameroon. Since organic carbon contents of the fractions were often very small, samples and analyses were extensively replicated to obtain robust statistical estimates of observed differences. For each soil type, differences in δ¹³C values between fields could be related to changes in the input and decomposition of organic matter arising from soil type, land management and, for example, the nature and abundance of weeds. Turnover of organic matter appeared to be fastest in the sand fraction, which is in line with results from earlier studies. In the finer fractions, clear differences in reaction to changes in input and decomposition were observed, that seem to be linked to differences in clay mineralogy. The results illustrate that SOM in the various fractions is much less stable and more strongly affected by changes in land use than might be assumed on the basis of changes in total SOM contents alone. At the same time, they demonstrate the relevance of ¹³C isotope analyses of SOM for studies on the impact of land use on these savannah soils with little SOM that are highly susceptible to degradation.     

Management intensity affects density fractions of soil organic matter from grazed bahiagrass swards

Soil fertility and agricultural systems sustainability depend upon soil organic matter (SOM). The effects of pasture management intensity on SOM are not well understood. The objectives of this study were to determine the effect of management intensity of ‘Pensacola’ bahiagrass (Paspalum notatum Flügge) pastures on the light density fraction of SOM (LD-SOM), the fraction that responds most readily to changes in pasture management practices. Pastures were grazed from 2001-2004 at four management intensities, defined as the combination of stocking method, N fertilization, and stocking rate (SR). Treatments were continuously stocked (CS) Low (40 kg N ha⁻¹ yr⁻¹ and SR of 1.4 animal units ha⁻¹ (AU=500 kg live weight)); CS Moderate (120 kg N and SR of 2.8 AU); CS High (360 kg N and SR of 4.2 AU); and rotationally stocked with a 7-d grazing period and 21-d resting period (360 kg N and SR of 4.2 AU). Composite soil samples (0-8 cm) from each pasture were collected in 2004. Management intensity did not affect C and N concentration in the bulk soil, but it did impact C and N concentrations of size fractions of LD-SOM. In particles from 250 to 2000 μm, both C and N concentration were greater with increasing management intensity. In particles<53 μm, however, the lowest management intensity presented the greatest soil C and N concentrations. Increasing C and N in slow turn over SOM fractions with increased management intensity may result in greater C sequestration and potential soil fertility, but the increased likelihood of negative environmental impact and the questionable sustainability of high N fertilizer rates must also be considered.     

Composition of organic matter in particle-size fractions of an agricultural soil

The composition of organic matter was studied in clay (< 2 μm), fine silt (2-6.3 μm), medium silt (6.3-20 μm), coarse silt (20-63 μm) and sand (63-2000 μm) fractions of the Ap-horizon of a clay loam (Orthic Humic Gleysol) from Bainsville (Ottawa, Canada) by organic C and total N analyses and pyrolysis-field ionization mass spectrometry (Py-FIMS). The C and N contents were largest in fine silt and medium silt and smaller in coarse silt and sand. Differences in the contents of organic matter and absorbed water were significantly (r= 0.945***) reflected by the amounts of volatilized matter during Py-FIMS. The Py-FI therniograms and mass spectra showed clear differences in thermal stability and molecular composition of organic matter between the organo-mineral size-fractions. Abundances of carbohydrates, phenols and lignin monomers, alkylaromatics and N-containing compounds decreased, whereas abundances of lignin dimers and lipids increased with increasing equivalent diameters. An exception was the sand fraction which was dominated by the characteristic features of plant residues. The six compound classes, calculated using signals of biomarkers, accounted for 35% to 60% of the recorded total ion intensity. The thermal evolution of the selected compound classes, which are important constituents of soil organic matter (SOM), indicated the stability of humic and organo-mineral bonds in particle-size fractions, Moreover, the influence of mineral matrix on organic matter composition was shown by significant correlations between relative abundances of carbohydrates, N-containing compounds, lipids, lignin dimers, and proportions of phyllosilicates.     

东北黑土有机质组分与结构的研究进展

在全球气候变化背景下研究土壤有机质的转化过程对于评价陆地生态系统碳截获潜力具有重要意义,而土壤有机质的循环特征及其稳定性与土壤有机质的组成和结构密切相关.东北黑土区是我国重要的商品粮基地,近年来,黑土有机质含量呈显著下降趋势,造成黑土肥力和质量的严重退化.本文通过文献资料的整理,总结了不同农田管理措施下黑土土壤有机质的消长动态、组分变化以及结构特征的研究现状,并探讨了研究中存在的问题.开垦和耕作导致土壤有机质总量、活性组分以及腐殖物质含量的显著降低,而平衡施用化肥和有机肥是维持和提升土壤有机质数量和质量的有效途径,长期有机无机配施使土壤有机质结构趋于简单化,有利于土壤肥力的保持.黑土有机质组分化学结构变化的驱动机制是值得人们长期探索的问题.     

Calculating the annual input of organic matter to soil from measurements of total organic carbon and radiocarbon

Measurements of total organic C, δ¹³C and δ¹⁴C are given for topsoils taken from six experimental sites in southern England. At each site, some of the samples were collected before and some after the thermonuclear tests of the early 1960s, so that pre- and post-bomb samples could be compared for radiocarbon content. The current Rothamsted model for the turnover of organic C in soil gave an acceptable fit to the data from five of the sites, apart from one aberrant radiocarbon measurement. The annual input of C to the topsoil was calculated for the five sites from these fits; the values obtained were: 0.15 t C ha^?1 a^?l for a site on silty clay loam, kept bare by hand weeding since 1870; 0.2 for unmanured spring barley growing on a sandy loam; 2.95 for a fertilized all-arable rotation on a loamy sand; 1.9 for the same fertilized all-arable rotation on a silty loam and 2.5 from this rotation on a calcareous silty loam. The corresponding values for Net Primary Production at the five sites were 0.15, 0.76, 5.16, 5.71 and 5.46 t C ha^?1 a^?l. In fitting the model to the radiocarbon data it was necessary to postulate that all these sites contained substantial quantities of biologically-inert organic matter, ranging from 2.2 to 10.0 t C ha^?1.     

矿区复垦土壤碳组分对外源碳输入的响应特征

研究外源碳输入对复垦土壤有机碳及组分的影响,对于深入探究矿区复垦土壤有机碳提升及培肥管理具有重要意义。本文依托山西省襄垣县采煤沉陷复垦区的长期定位试验,研究了矿区复垦土壤碳组分对不同外源碳(生物炭、堆肥、沼渣、牛粪与秸秆)输入的响应特征。分别在2011年与2016年对矿区复垦土壤样品进行采集,测定0～20 cm土层土壤有机碳、易氧化有机碳、活性碳库Ⅰ、Ⅱ的含量。结果表明,生物炭处理的土壤有机碳增长率和年变化量较对照(CK,无外源碳添加)分别增加101.80%和0.56g·kg–1·a–1,且均显著高于其他有机物料处理;生物炭、牛粪处理土壤0～20 cm土层固碳量较CK分别提高100.52%和91.52%,二者间差异不显著,均显著高于其他有机物料处理,堆肥、沼渣和秸秆处理间对土壤固碳量的提升作用不显著。添加有机物料均能显著增加复垦土壤易氧化有机碳的增长率和年变化量,均表现为堆肥处理最高,较CK分别增加12.37%和0.16 g·kg–1·a–1。复垦土壤活性碳库Ⅰ、Ⅱ的增长率和年变化量均为牛粪处理显著高于其他有机物料处理。有机物料添加均能提高土壤稳定性有机碳含量,与CK相比,生物炭和牛粪处理的提高幅度最大,显著高于其他有机物料,而牛粪与生物碳之间差异不显著。生物炭处理碳库管理指数最高,分别较堆肥、沼渣、牛粪、秸秆处理提高36.30%、52.23%、41.50%、52.02%。施用生物炭、堆肥、沼渣、牛粪与秸秆都能显著提升复垦土壤各碳组分含量和碳库管理指数,施用生物炭的效果最优,因此施用生物炭可作为矿区复垦土壤有机碳提升的有效管理措施。     

Sources and composition of soil organic matter fractions between and within soil aggregates

It is generally accepted that particulate organic matter derives from plants. In contrast, the enriched labile fraction is thought by many to derive from microbes, especially fungi. However, no detailed chemical characterization of these fractions has been done. In this study, we wanted to assess the sources (plants or microbes; fungi or bacteria) and degree of microbial alteration of (i) three particulate organic matter fractions – namely the free light fraction (1.85 g cm^?3), the coarse (250–2000 μm) and the fine (53–250 μm) intra‐aggregate particulate organic matter fractions – and of (ii) three density fractions of fine‐silt associated carbon – namely < 2.0, 2.0–2.2 (i.e. enriched labile fraction) and > 2.2 g cm^?3– by analysing the amino sugars, by CuO oxidation analyses, and by ¹³C‐, ¹H‐ and ³¹P‐NMR analyses. Macroaggregates (250–2000 μm) were separated by wet‐sieving from a former grassland soil now under a no‐tillage arable regime. The three particulate organic matter fractions and the three density fractions were isolated from the macroaggregates by a combination of density flotation, sonication and sieving techniques. Proton NMR spectroscopy on alkaline extracts showed that the enriched labile fraction is not of microbial origin but is strongly degraded plant material that is enriched in aliphatic moieties partly bound to aromatics. In addition, the enriched labile fraction had a glucosamine content less than the whole soil, indicating that it is not enriched in carbon derived from fungi. Decreasing yields of phenolic CuO oxidation products and increasing side‐chain oxidation in the order coarse intra‐aggregate particulate organic matter < fine inter‐aggregate particulate organic matter < fine‐silt fractions indicate progressive alteration of lignin as particle size decreases. The light fraction was more decomposed than the coarse inter‐aggregate particulate organic matter, as indicated by (i) its larger ratio of acid‐to‐aldehyde of the vanillyl units released by CuO oxidation, (ii) the smaller contribution of H in carbohydrates to total extractable H as estimated by ¹H‐NMR spectroscopy, and (iii) a larger contribution of monoester P to total extractable P in the ³¹P‐NMR spectra. In conclusion, the four fractions are derived predominantly from plants, but microbial alteration increased as follows: coarse inter‐aggregate particulate organic matter < light fraction ≈ fine inter‐aggregate particulate organic matter < enriched labile fraction.     

Predicting soil N mineralization: Relevance of organic matter fractions and soil properties

Distinct extractable organic matter (EOM) fractions have been used to assess the capacity of soils to supply nitrogen (N). However, substantial uncertainty exists on their role in the N cycle and their functional dependency on soil properties. We therefore examined the variation in mineralizable N and its relationship with EOM fractions, soil physical and chemical properties across 98 agricultural soils with contrasting inherent properties and management histories. Mineralizable N was determined by aerobic incubation at 20 °C and optimum moisture content for 20 weeks. We used multivariate statistical modelling to account for multi-collinearity, an issue generally overlooked in studies evaluating the predictive value of EOM fractions. Mineralization of N was primarily related to the size of OM pools and fractions present; they explained 78% of the variation in mineralizable N whereas other soil variables could explain maximally 8%. Both total and extractable OM expressed the same soil characteristic from a mineralization perspective; they were positively related to mineralizable N and explained a similar percentage of the variation in mineralizable N. Inclusion of mineralizable N in fertilizer recommendation systems should be based on at least one OM variable. The most appropriate EOM fraction can only be identified when the underlying mechanisms are known; regression techniques are not suitable for this purpose. Combination of single EOM fractions is not likely to improve the prediction of mineralizable N due to high multi-collinearity. Inclusion of texture-related soil variables or variables reflecting soil organic matter quality may be neglected due to their limited power to improve the prediction of mineralizable N.     

Carbon and nitrogen in operationally defined soil organic matter pools

Humic substances [humic acid (HA), fulvic acid (FA), and insoluble humin], particulate organic matter (POM), and glomalin comprise the majority (ca 75%) of operationally defined extractable soil organic matter (SOM). The purpose of this work was to compare amounts of carbon (C) and nitrogen (N) in HA, FA, POM, and glomalin pools in six undisturbed soils. POM, glomalin, HA, and FA in POM, and glomalin, HA, and FA in POM-free soil were extracted in the following sequence: (1) POM fraction separation from the soil, (2) glomalin extraction from the POM fraction and POM-free soil, and (3) co-extraction of HA and FA from the POM fraction and POM-free soil. Only trace amounts of HA and FA were present in the POM fraction, while POM-associated glomalin (POM-glomalin) and POM alone contributed 2 and 12%, respectively, of the total C in the soil. Mean combined weights for chemically extracted pools from POM and from POM-free soil were 9.92 g glomalin, 1.12 g HA, and 0.88 g FA kg⁻¹ soil. Total protein and C, N, and H concentrations showed that glomalin and HA were, for the most part, separate pools, although protein was detected in HA extracts. Even though percentage carbon was higher in HA than in glomalin, glomalin was a larger (almost nine times) operationally defined pool of soil organic C. Glomalin was also the largest pool of soil N of all the pools isolated, but all pools combined only contained 31% of the total N in the soil.     

Temporal changes in distribution and composition of N from labeled fertilizer in soil organic matter fractions

Variations in the amount and composition of immobilized nitrogen (N) in major soil organic matter fractions were investigated in a 730-day soil incubation experiment using ¹⁵N-labeled urea and ¹⁵N nuclear magnetic resonance spectroscopy with the cross polarization/magic angle spinning (¹⁵N CPMAS NMR) method. After 730 days, 24.7% of the applied N was recovered from the soil as organic N. The urea-derived N recovered from humic acids and humin decreased from 11.2 and 33.8% of the applied amount after 14 days to 1.6 and 20.4% after 730 days, respectively. When these values were corrected for the microbial biomass (MB) N, they ranged from 9.0 to 1.2% and 28 to 18%, respectively. The proportion of urea-derived N recovered from fulvic acids was low, ranging between 0.4 and 5.8% (with MB N) or 5.6% (without MB N) of the applied amount, whereas that from water-soluble nonhumic substances (WS-NHS; NHS in the fulvic acid fraction) remained high, 28–33% of the applied amount after correction for the contribution of MB N up to day 365, and decreased to 0.9% thereafter. The ¹⁵N CPMAS NMR spectra of humic acids, fulvic acids, and humin showed the largest signal at −254 to −264 ppm, corresponding to peptide/amide N. The proportions of heterocyclic, peptide/amide, guanidine/aniline, and free amino N in the urea-derived humic acid N were 3–7, 83–90, 5–7, and 2–4%, respectively. More than 80% loss of the urea-derived humic acid N did not markedly alter their composition. No time-dependent variations were also observed for the proportions of respective N functional groups in humin N, which were 3–5, 71–78, 12–17, and 6–10% in the same order as above. These results suggest the greater importance of physical stability than structural variation for the initial accumulation of organic N in soil.     

Comparisons of soil organic matter and its fractions by pyrolysis mass-spectrometry

Pyrolysis mass-spectra from a sample of the A₁-horizon of a soil from southern Spain showed predominant peaks related to furan derivatives similar to those observed from complex polysaccharides in which not only hexoses but also pentoses and deoxyhexoses were constituent units. Smaller peaks, typical for protein materials and phenolic units, were also observed. On the other hand, typical peaks for the methoxyphenols of lignins were very small and indicated only limited amounts of undecomposed lignin residues in this soil sample. Peaks related to benzene or toluene were also very small.Humic acid samples from this soil showed much more prominent signals related to protein materials, benzene and phenolic derivatives and weaker polysaccharide-related signals than did the entire sample. Typical lignin related peaks were small or insignificant. Spectra from the grey or brown humidic acid fractions were much like those of the parent humic acid. Brown humic acid, however, showed stronger signals for nitrogen and sulphur compounds, indicating a higher content of protein-like materials in this fraction. Preparations of humic acid hydrolyzed by 6 N HCl showed in their pyrolysis products a marked increase in phenols and methoxyphenols.In its pyrogram, humin resembled humic acid, but signals for complex polysaccharides were more evident. Lignin-like materials seem not to be higher in this fraction. Hymatomelanic acid showed prominent signals related to polysaccharides and lignin. Pyrograms from the soil polysaccharides showed the characteristic pattern of a complex polysaccharide with the presence of fragments from polymers of amino acids or amino sugars. Fulvic acid spectra showed obvious dissimilarities to those from humic acid in that signals for protein, as well as those related to phenols, were low. Depending upon the isolation method, the fulvic acid preparations showed differing signals related to polysaccharide or phenolic materials.     

The distribution of the stable carbon isotope (13C/12C) in fractions of soil organic matter

The carbon-isotopic composition of fulvic and humic acid from the A horizons of eight soil types, developed under a wide variety of climatological conditions, was measured. The fulvic acid is always enriched in ¹³C as compared with the humic acid from the same soil by a rather constant factor of 0.9?. The fulvic acids are isotopically closer to the plant source of the organic matter and thus represent an intermediate stage in the formation of humic substances. Depth sections of peat soil showed that carbon isotopes can be used to evaluate the dynamic nature of the fulvic-acid fraction. With depth, a transfer of carbon groups from polysaccharides to fulvic acid is seen. Based on isotopic evidence it is shown that in addition to formation of β-humus, part of the fulvic acid is condensed with depth to a stable humic fraction — humin.     

Crop-weed competition changes the decomposition of soil organic matter fractions in the rhizosphere

ABSTRACT

Certain plant combinations can stimulate the mineralization of soil organic matter (SOM), thereby changing the storage of soil organic carbon and affecting the physical and chemical soil properties. The aim of this study was to evaluate whether competition between weeds and maize can stimulate SOM decomposition. Eight treatments were performed: monoculture of maize and weeds (Amaranthus viridis, Bidens pilosa, and Ipomoea grandifolia), maize in competition with weeds, and non-cultivated soil. During cultivation, the rhizosphere priming effect (RPE) on SOM was estimated. Additionally, at 60 days after planting, soil samples were collected to measure the C contents of particulate (POM) and mineral-associated (MAOM) organic matter. From the 43^rd day of cultivation onwards, the coexistence between maize and B. pilosa led to the highest RPE values, while maize vs. A. viridis showed negative RPE. Maize vs. A. viridis and maize vs. I. grandifolia caused increase in MAOM-C and decreases in POM-C. Ipomoea grandifolia monoculture and maize vs. B. pilosa led to the highest MAOM-C losses and reduced POM-C compared to the non-cultivated soils. Here it is demonstrated that competition between maize and B. pilosa increases SOM mineralization, while competition between maize and A. viridis or I. grandifolia retards this process.