首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The effects of atmospheric nitrogen (N) deposition on carbon (C) sequestration in terrestrial ecosystems are controversial. Therefore, it is important to evaluate accurately the effects of applied N levels and forms on the amount and stability of soil organic carbon (SOC) in terrestrial ecosystems. In this study, a multi‐form, small‐input N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station from 2007 to 2011. Three N fertilizers, NH4Cl, (NH4)2SO4 and KNO3, were applied at four rates: 0, 10, 20 and 40 kg N ha?1 year?1. One hundred and eight soil samples were collected at 10‐cm intervals to a depth of 30 cm in 2011. Contents and δ13C values of bulk SOC were measured, as well as three particle‐size fractions: macroparticulate organic C (MacroPOC, > 250 µm), microparticulate organic C (MicroPOC, 53–250 µm) and mineral‐associated organic C (MAOC, < 53 µm). The results show that 5 years of N addition changed SOC contents, δ13C values of the bulk soils and various particle‐size fractions in the surface 10‐cm layer, and that they were dependent on the amounts and forms of N application. Ammonium‐N addition had more significant effects on SOC content than nitrate‐N addition. For the entire soil profile, small additions of N increased SOC stock by 4.5% (0.43 kg C m?2), while medium and large inputs of N decreased SOC stock by 5.4% (0.52 kg C m?2) and 8.8% (0.85 kg C m?2), respectively. The critical load of N deposition appears to be about 20 kg N ha?1 year?1. The newly formed C in the small‐input N treatment remained mostly in the > 250 µm soil MacroPOC, and the C lost in the medium or large N treatments was from the > 53 µm POC fraction. Five years of ammonium‐N addition increased significantly the surface soil POC:MAOC ratio and increased the instability of soil organic matter (SOM). These results suggest that exogenous N input within the critical load level will benefit C sequestration in the alpine meadow soils on the Qinghai–Tibetan Plateau over the short term.  相似文献   

2.
Genetically modified plants and their residues may have direct effects on ecosystem processes. We aimed to determine the amount in soil of the insecticidal δ‐endotoxin, originally from the bacterium Bacillus thuringiensis, introduced into soil by root exudates and residues from genetically modified maize, to compare the short‐term rates of decay of Bt‐maize and non‐Bt‐maize, and to determine the rate at which the toxin in Bt‐maize leaves decomposes in soil. Intact soil, size fractions of soil, soluble fractions from soil and fractions of organic residues from a field where Bt‐maize had been cultivated for 4 years were analysed for the Btδ‐endotoxin. Traces of the δ‐endotoxin were detected in the whole (unfractionated) soil, the water‐soluble fractions, and some of the particle‐size fractions, but it was sufficiently concentrated only in the > 2000‐µm size fraction to be quantified. The δ‐endotoxin concentrations in this fraction ranged between 0.4 and 4.4 ng toxin g?1 fraction, which equated to 70, 6 and 50 mg toxin m?2 in the 0–15, 15–30 and 30–60 cm depths, respectively (or 126 mg toxin m?2 over the 0–60 cm depth) in the field in June (early summer). The > 2000‐µm size fraction was a mixture of light‐ and dark‐coloured organic material and mineral material comprising sand grains and stable aggregates. For samples collected early in the growing season, most of the detected δ‐endotoxin was present in the light‐coloured organic material, which was comprised of primarily live roots. However, recognizable maize residues, probably from previous years' crops, also contained δ‐endotoxin. In a laboratory incubation study, Bt‐ and non‐Bt‐maize residues were added to soil and incubated for 43 days. There was no detectable difference in the decomposition of plant material from the two lines of maize, as determined by CO2 production. The quantity of δ‐endotoxin in the decomposing plant material and soil mixtures declined rapidly with time during the incubation, with none being detectable after 14 days. The rapid disappearance of the δ‐endotoxin occurred at a rate similar to that of the water‐soluble components of the maize residues. The results suggested that much of the δ‐endotoxin in crop residues is highly labile and quickly decomposes in soil, but that a small fraction may be protected from decay in relatively recalcitrant residues.  相似文献   

3.
  相似文献   

4.
Intensive vegetable production in greenhouses has rapidly expanded in China since the 1990s and increased to 1.3 million ha of farmland by 2016, which is the highest in the world. We conducted an 11‐year greenhouse vegetable production experiment from 2002 to 2013 to observe soil organic carbon (SOC) dynamics under three management systems, i.e., conventional (CON), integrated (ING), and intensive organic (ORG) farming. Soil samples (0–20 and 20–40 cm depth) were collected in 2002 and 2013 and separated into four particle‐size fractions, i.e., coarse sand (> 250 µm), fine sand (250–53 µm), silt (53–2 µm), and clay (< 2 µm). The SOC contents and δ13C values of the whole soil and the four particle‐size fractions were analyzed. After 11 years of vegetable farming, ORG and ING significantly increased SOC stocks (0–20 cm) by 4008 ± 36.6 and 2880 ± 365 kg C ha?1 y?1, respectively, 8.1‐ and 5.8‐times that of CON (494 ± 42.6 kg C ha?1 y?1). The SOC stock increase in ORG at 20–40 cm depth was 245 ± 66.4 kg C ha?1 y?1, significantly higher than in ING (66 ± 13.4 kg C ha?1 y?1) and CON (109 ± 44.8 kg C ha?1 y?1). Analyses of 13C revealed a significant increase in newly produced SOC in both soil layers in ORG. However, the carbon conversion efficiency (CE: increased organic carbon in soil divided by organic carbon input) was lower in ORG (14.4%–21.7%) than in ING (18.2%–27.4%). Among the four particle‐sizes in the 0–20 cm layer, the silt fraction exhibited the largest proportion of increase in SOC content (57.8% and 55.4% of the SOC increase in ORG and ING, respectively). A similar trend was detected in the 20–40 cm soil layer. Over all, intensive organic (ORG) vegetable production increases soil organic carbon but with a lower carbon conversion efficiency than integrated (ING) management.  相似文献   

5.
Our goals in this study were to track the incorporation of plant residue into soil organic matter (SOM) and test the effectiveness of different fractionation methods to evaluate this transformation. We incubated soil amended with 13C‐labelled barley (Hordeum vulgare L.) residue and used three fractionation methods based on size (> 250, 53–250, 5–53 and < 5 µm) and density (< 1.7 g cm?3, i.e. light fraction (LF)) and determined its quantity and the rate of C loss or gain or both in these fractions as decomposition progressed. One method was based on size only, another involved density separation followed by size fractionation and a third separated organic matter fractions by size first and then by density. There were significant quantitative differences between the methods for the amount of residue in the fractions, but there was no effect of fractionation method on the rate of change in the residue that comprised the fractions. The density method did not appear to identify all of the most recently added (i.e. least decomposed) residue in the LF or that there was a redistribution of SOM among the fractions. The amount of residue C and the C:N ratio of the residue in the two smallest fractions increased early during the incubation (0–2 months), but subsequently decreased towards the end. The initially small C:N ratio in the clay fraction probably reflects the accumulation of microbial by‐products from the rapid decomposition of water‐soluble compounds. The subsequent increase and decrease in both residue C and C:N ratio reflects the balance of the accumulation of sorbed water‐soluble compounds and dense plant residue fragments and their mineralization over time. We conclude that clay is a sink for residue C (i.e. microbial metabolites) early during decomposition, and that there is a transfer among fractions and mineralization of residue C as decomposition proceeds. These findings indicate that the clay fraction contains a dynamic pool of C that can cycle within short time‐scales.  相似文献   

6.
The input to soils made by pollen and its subsequent mineralization has rarely been investigated from a soil microbiological point of view even though the small but significant quantities of C and N in pollen may make an important contribution to nutrient cycling. The relative resistance to decomposition of pollen exines (outer layers) has led to much of the focus of pollen in soil being on its preservation for archaeological and palaeo‐ecological purposes. We have examined aspects of the chemical composition and decomposition of pollen from birch (Betula alba) and maize (Zea mays) in soil. The relatively large N contents, small C‐to‐N ratios and large water‐soluble contents of pollen from both species indicated that they would be readily mineralized in soil. When added to soil and incubated at 16°C an amount of C equivalent to 22–26% of the added pollen C was lost as CO2 within 22 days, with the Z. mays pollen decomposing faster. For B. alba pollen, the water‐soluble fraction decomposed faster than the whole pollen and the insoluble fraction decomposed more slowly over 22 days. By contrast, there were no significant differences in the decomposition rates of the different fractions from Z. mays pollen. Solid‐state 13C nuclear magnetic resonance (NMR) revealed no gross chemical differences between the pollen of these two species, with strong resonances in the alkyl‐ and methyl‐C region (0–45 p.p.m.) indicative of aliphatic compounds, the O‐alkyl‐C (60–90 p.p.m.) and the acetal‐ and ketal‐C region (90–110 p.p.m.) indicative of polysaccharides, and the carbonyl‐C region indicative of peptides and carboxylic acids. In addition, both pollens gave a small but distinct resonance at 55 p.p.m. attributed to N‐alkyl‐C. The resonances attributed to polysaccharides were lost completely or substantially reduced after decomposition.  相似文献   

7.
This study uses diffuse reflectance infrared Fourier Transform (DRIFT) spectrometry and field emission scanning electron microscopy to investigate the vertical distribution of charcoal in a sandy soil from SE Australia. The soil was sampled to bedrock (120 cm) at varying depths and bulk samples were fractionated into three particle‐sizes: macro‐ (2000–200 µm), micro‐ (200–60 µm) and mineral‐associated organic matter (MAOM, < 60 µm). Charcoal was isolated from 0–30 and 30–60‐cm depths. Soil charcoal was detected by using a DRIFT band centred at 1590 cm?1 and scanning electron microscopy combined with energy dispersive spectroscopy. Charcoal content as a proportion of soil organic carbon (SOC) was estimated with linear regressions of cumulative DRIFT bands. At 0–30 cm, charcoal content as a portion of SOC did not differ significantly between particle‐size fractions, constituting 5–26% of SOC. At a depth of 30–60 cm, charcoal constituted 19–39% of SOC in the fractions. At 60–100 cm, charcoal was only detectable in the mid‐sized fraction, where it constituted about 17% of SOC. These results support our previous hypothesis of charcoal enrichment in the micro‐fraction inducing a greater SOC stability in this fraction as inferred from radiocarbon ages (Hobley et al., 2013). Our findings indicate that DRIFT spectra can be used to detect the presence and amount of charcoal in soil, which may prove to be a simple and low‐cost alternative to more laborious and costly detection methods.  相似文献   

8.
We investigated carbon (C) incorporation and sources of C in the surface CO2 flux at two sites in northern England on peaty (stagnohumic) gley soil, one afforested by Picea sitchensis, the other under continuous Molinia grassland cover. Radiocarbon (14C) derived from atmospheric nuclear weapons testing was used to trace the incorporation of C into the soil and sources of C in the soil CO2 flux from the soil surface and deeper layers. Larger values of 14CO2 in surface flux were found at the afforested site (109–110 per cent modern (pM) compared with 107–108 pM at the grassland site). Surface litter fractions (Oi horizon) from the afforested site showed larger 14C signatures than the equivalent fractions in the grassland (113–115 pM in the forest compared with 106–109 pM in the grassland). Fine root fractions (<2 mm, Oe horizon) had similar signatures at both sites (109 pM in the forest compared with 109–111 pM in the grassland). Humified fractions at 10‐cm depth (Oa horizon) showed smaller signatures (100–103 pM) in the forest than the equivalent fraction in the grassland soil (106–114 pM). According to a mixing model that takes into account pool size and 14C signature, the contributions to surface CO2 fluxes from slow turnover fractions that had resided in the soil for more than one year were greater at the forested site than the grassland site, but contributions from fast‐turnover C fixed within the year prior to study showed the opposite trend. The results, taken together with previous work indicating that both site preparation and clear‐felling lead to a net loss of C, indicate that long‐term fixation in deep soil organic fractions is limited on this soil type under plantation forest over 40–50‐year commercial rotations.  相似文献   

9.
Microbial biomass, respiratory activity, and in‐situ substrate decomposition were studied in soils from humid temperate forest ecosystems in SW Germany. The sites cover a wide range of abiotic soil and climatic properties. Microbial biomass and respiration were related to both soil dry mass in individual horizons and to the soil volume in the top 25 cm. Soil microbial properties covered the following ranges: soil microbial biomass: 20 µg C g–1–8.3 mg C g–1 and 14–249 g C m–2, respectively; microbial C–to–total organic C ratio: 0.1%–3.6%; soil respiration: 109–963 mg CO2‐C m–2 h–1; metabolic quotient (qCO2): 1.4–14.7 mg C (g Cmic)–1 h–1; daily in‐situ substrate decomposition rate: 0.17%–2.3%. The main abiotic properties affecting concentrations of microbial biomass differed between forest‐floor/organic horizons and mineral horizons. Whereas microbial biomass decreased with increasing soil moisture and altitude in the forest‐floor/organic horizons, it increased with increasing Ntot content and pH value in the mineral horizons. Quantities of microbial biomass in forest soils appear to be mainly controlled by the quality of the soil organic matter (SOM), i.e., by its C : N ratio, the quantity of Ntot, the soil pH, and also showed an optimum relationship with increasing soil moisture conditions. The ratio of Cmic to Corg was a good indicator of SOM quality. The quality of the SOM (C : N ratio) and soil pH appear to be crucial for the incorporation of C into microbial tissue. The data and functional relations between microbial and abiotic variables from this study provide the basis for a valuation scheme for the function of soils to serve as a habitat for microorganisms.  相似文献   

10.
The present review is focused on microbiological methods used in agricultural soils accustomed to human disturbance. Recent developments in soil biology are analyzed with the aim of highlighting gaps in knowledge, unsolved research questions, and controversial results. Activity rates (basal respiration, N mineralization) and biomass are used as overall indices for assessing microbial functions in soil and can be supplemented by biomass ratios (C : N, C : P, and C : S) and eco‐physiological ratios (soil organic C : microbial‐biomass C, qCO2, qNmin). The community structure can be characterized by functional groups of the soil microbial biomass such as fungi and bacteria, Gram‐negative and Gram‐positive bacteria, or by biotic diversity. Methodological aspects of soil microbial indices are assessed, such as sampling, pretreatment of samples, and conversion factors of data into biomass values. Microbial‐biomass C (µg (g soil)–1) can be estimated by multiplying total PLFA (nmol (g soil)–1) by the FPLFA‐factor of 5.8 and DNA (µg (g soil)–1) by the FDNA‐factor of 6.0. In addition, the turnover of the soil microbial biomass is appreciated as a key process for maintaining nutrient cycles in soil. Examples are briefly presented that show the direction of human impact on soil microorganisms by the methods evaluated. These examples are taken from research on organic farming, reduced tillage, de‐intensification of land‐use management, degradation of peatland, slurry application, salinization, heavy‐metal contamination, lignite deposition, pesticide application, antibiotics, TNT, and genetically modified plants.  相似文献   

11.
In Brazil, most Eucalyptus stands have been planted on Cerrado (shrubby savanna) or on Cerrado converted into pasture. Case studies are needed to assess the effect of such land use changes on soil fertility and C sequestration. In this study, the influence of Cerrado land development (pasture and Eucalyptus plantations) on soil organic carbon (SOC) and nitrogen (SON) stocks were quantified in southern Brazil. Two contrasted silvicultural practices were also compared: 60 years of short‐rotation silviculture (EUCSR) versus 60 years of continuous growth (EUCHF). C and N soil concentrations and bulk densities were measured and modelled for each vegetation type, and SOC and SON stocks were calculated down to a depth of 1 m by a continuous function. Changes in SOC and SON stocks mainly occurred in the forest floor (no litter in pasture and up to 0.87 kg C m?2 and 0.01 kg N m?2 in EUCSR) and upper soil horizons. C and N stocks and their confidence intervals were greatly influenced by the methodology used to compute these layers. C/N ratio and 13C analysis showed that down to a depth of 30 cm, the Cerrado organic matter was replaced by organic matter from newly introduced vegetation by as much as 75–100% for pasture and about 50% for EUCHF, poorer in N for Eucalyptus stands (C/N larger than 18 for Eucalyptus stands). Under pasture, 0–30 cm SON stocks (0.25 kg N m?2) were between 10 and 20% greater than those of the Cerrado (0.21 kg N m?2), partly due to soil compaction (limit bulk density at soil surface from 1.23 for the Cerrado to 1.34 for pasture). Land development on the Cerrado increased SOC stocks in the 0–30 cm layer by between 15 and 25% (from 2.99 (Cerrado) to 3.86 (EUCSR) kg C m?2). When including litter layers, total 0–30 cm carbon stocks increased by 35% for EUCHF (4.50 kg C m?2) and 53% for EUCSR (5.08 kg C m?2), compared with the Cerrado (3.28 kg C m?2), independently of soil compaction.  相似文献   

12.
Experiments were conducted between 2003 and 2008 to examine how N additions influence soil organic C (SOC) and its fractions in forests at different succession stages in the subtropical China. The succession stages included pine forest, pine and broadleaf mixed forest, and old‐growth monsoon evergreen broadleaf forest. Three levels of N (NH4NO3)‐addition treatments comprising control, low‐N (50 kg N ha–1 y–1), and medium‐N (100 kg N ha–1 y–1) were established. An additional treatment of high‐N (150 kg N ha–1 y–1) was established in the broadleaf mixed forest. Soil samples were obtained in July 2008 for analysis. Total organic C (TOC), particulate organic C (POC, > 53 μm), readily oxidizable organic C (ROC), nonreadily oxidizable organic C (NROC), microbial biomass C (MBC), and soil properties were analyzed. Nitrogen addition affected the TOC and its fractions significantly. Labile organic‐C fractions (POC and ROC) in the topsoil (0–10 cm) increased in all the three forests in response to the N‐addition treatments. NROC within the topsoil was higher in the medium‐N and high‐N treatments than in the controls. In the topsoil profiles of the broadleaf forest, N addition decreased MBC and increased TOC, while no significant effect on MBC and TOC occurred in the pine and mixed forests. Overall, elevated N deposition increased the availability of labile organic C (POC and ROC) and the accumulation of NROC within the topsoil irrespective of the forest succession stage, and might enhance the C‐storage capacity of the forest soils.  相似文献   

13.
Spatial location of carbon decomposition in the soil pore system   总被引：5，自引：0，他引：5  
We sought to examine the distribution of carbon (C) decomposition within the framework of the soil pore system. Soils were sampled from a transect having a natural gradient in pore‐size distribution. After the addition of labelled wheat straw (13C) the repacked soil columns were incubated (25°C) at soil water matric potentials of either ?75 kPa or ?5 kPa and for either 4 or 90 days. Pore‐size distribution was determined for each soil column after incubation and soils were then analysed for soluble C, label‐derived residual C, label‐derived and native biomass C, nematode abundance, and ergosterol concentration as an indicator of fungal biomass. Overall, the data suggested that pore‐size distribution and its interaction with soil water give rise to a highly stratified biogeography of organisms through the pore system. This results in different rates of decomposition in pores of different size. Added plant material seemed to decompose most rapidly in soils with a relatively large volume of pores with neck diameters c. 15–60 µm and most slowly in soils with large volumes of pores with neck diameters < 4 µm. Regression analysis suggested that at matric potentials of both ?75 kPa and ?5 kPa the fastest decomposition of organic substrate occurred close to the gas–water interface. This analysis also implied that slower rates of decomposition occur in the pore class 60–300 µm. Correlations between the mass of soil biota and the pore volume of each pore class point to the importance of fungi and possibly nematodes in the rapid decomposition of C in the pores c. 15–60 µm during the early stages of decomposition.  相似文献   

14.
As a source of organic matter, crop residues affect the behaviour of pesticides in agricultural soils. The fate of [U‐ring‐13C] and [U‐ring‐14C] atrazine (6‐chloro‐N‐ethyl‐N‐isopropyl‐1,3,5‐triazine‐2,4‐diamine) was investigated during laboratory incubation under controlled conditions in a loamy soil amended with wheat straw at two different states of decomposition: no preliminary decomposition or 6 months’ preliminary decomposition. After 3 months, non‐extractable, so‐called ‘bound’, 13C‐atrazine residues were recovered in three particle‐size fractions (> 200, 50–200 and < 50 μm), and investigated with solid‐state 13C‐NMR spectroscopy. Parallel incubations with [U‐ring‐14C] atrazine were carried out to quantify the bound residues as well as the extractable and mineralized fractions. The effect of straw residues on atrazine behaviour depended on whether they had been previously decomposed or not. When straw was decomposed for 6 months prior to incubation, atrazine mineralization was enhanced to 50% of the initial 14C in contrast to 15% of the initial 14C in soil alone and soil amended with fresh straw. In parallel, atrazine bound residues were formed in greater amount representing up to 20% of the initial 14C. CP/MAS 13C‐NMR on soil size fractions of soil–straw mixtures after incubation with 13C‐atrazine showed that bound residues contained mostly triazinic C, corresponding to atrazine or primary metabolites. Non‐humified organic materials recovered in size fractions > 200 and 50–200 μm contained significant amounts of bound residues, especially when straw was added to the soil. CP/MAS 13C‐NMR analysis of humic acids obtained from < 50‐μm fractions was difficult due to overlapping of the native carboxyl 13C signal with the 13C‐atrazine signal.  相似文献   

15.
Abstract

The pattern of carbon (C) storage in soils has implications for agriculture and the environment. Dynamics of organic C, in the 0‐ to 20‐cm soil depth along a toposequence in a peri urban site in Sierra Leone, West Africa, were studied. Organic C was determined by the dry‐combustion method on the following aggregate size fractions: whole soil (<2000 µm), 250–2000 µm, 53–250 µm, and <53 µm.

Mean organic C content of whole soil ranged from 4.8% on the backslope to 9.3% on the toeslope. Organic C content of aggregate size fractions increased with decreasing aggregate size. The amount of soil and organic C present in aggregate size fractions, at all positions on the toposequence, decreased with decreasing aggregate size. In general, convex upper slopes had lower contents and amounts of organic C compared to lower concave areas. This study provided benchmark levels and patterns against which changes resulting from imminent urbanization can be measured.  相似文献   

16.
The preservation of plant residues is important for sustainable arable cropping. Lignin is a marker for plant residues in soils. We have investigated influences of the length of cultivation on the dynamics of lignin. Composite samples were taken from the top 20 cm of soils that have been cropped for periods varying from 0 to 98 years in each of three different agro‐ecosystems in the Free State Province of South Africa. Lignin‐derived phenols were determined in the <2 µm (clay), 2–20 µm (silt), 20–250 µm (fine sand) and 250– 2000 µm (coarse sand) size separates. With increasing length of cultivation, the concentration of such phenols decreased to 36% of that in the grassland. The lignin contents as proportions of the total carbon did not change during cultivation, suggesting that there was no selective enrichment of lignin moieties as C was lost as a result of cultivation. The loss rate constants of lignin concentrations in particle‐size fractions increased in the order clay (0.17 year?1) ≤ silt (0.18 year?1) < fine sand (0.20 year?1) < coarse sand (0.22 year?1). Increasing ratios of phenolic acids to aldehydes in bulk soil, silt and fine sand fractions with increasing length of cultivation indicated that side chains were being oxidized. The ratios in the silt fraction, however, decreased after 10–20 years. We attribute this to a loss of lignin together with silt by wind erosion, resulting in a rejuvenation of lignin compounds in the remaining silt‐sized pools of C.  相似文献   

17.
The inability of physical and chemical techniques to separate soil organic matter into fractions that have distinct turnover rates has hampered our understanding of carbon (C) and nutrient dynamics in soil. A series of soil organic matter fractionation techniques (chemical and physical) were evaluated for their ability to distinguish a potentially labile C pool, that is ‘recent’ root and root‐derived soil C. ‘Recent’ root and root‐derived C was operationally defined as root and soil C labelled by 14CO2 pulse labelling of rye grass–clover pasture growing on undisturbed cores of soil. Most (50–94%) of total soil + root 14C activity was recovered in roots. Sequential extraction of the soil + roots with resin, 0.1 m NaOH and 1 m NaOH allocated ‘recent’ soil + root 14C to all fractions including the alkali‐insoluble residual fraction. Approximately 50% was measured in the alkali‐insoluble residue but specific activity was greater in the resin and 1 m NaOH fractions. Hot 0.5 m H2SO4 hydrolysed 80% of the 14C in the alkali‐insoluble residue of soil + roots but this diminished specific activity by recovering much non‐14C organic matter. Pre‐alkali extraction treatment with 30% H2O2 and post‐alkali treatment extractions with hot 1 m HNO3 removed organic matter with a large 14C specific activity from the alkali‐insoluble residue. Density separation failed to isolate a significant pool of ‘recent’ root‐derived 14C. The density separation of 14C‐labelled roots, and roots remixed with non‐radioactive soil, showed that the adhesion of soil particles to young 14C‐labelled roots was the likely cause of the greater proportion of 14C in the heavy fraction. Simple chemical or density fractionations of C appear unsuitable for characterizing ‘recent’ root‐derived C into fractions that can be designated labile C (short turnover time).  相似文献   

18.
Abstract

Two Ferralsols (350 and 600 g kg?1 clay) from the Brazilian Cerrado Region were evaluated for long‐term effects (5 and 8 years) of no tillage on carbon (C) stocks in particulate (>53 µm) and mineral‐associated (<53 µm) soil organic matter (SOM) fractions. Carbon stocks in particulate SOM increased under no tillage compared with conventional tillage, and the rate was higher in the clayey soil (0.62 Mg C ha?1 yr?1) than in the sandy clay loam soil (0.31 Mg C ha?1 yr?1). In contrast, the mineral‐associated SOM in the top soil layer (0–20 cm) was not affected by tillage system. Sequestration of atmospheric C in tropical no‐tillage soils seems to be due to accumulation of C in labile SOM fractions, with highest rates in clayey soils probably due to physical protection.  相似文献   

19.
Pesticides leaching through a soil profile will be exposed to changing environmental sorption and desorption conditions as different horizons with distinct physical and chemical properties are encountered. Soil cores were taken from a clay soil profile and samples taken from 0.0 to 0.3 m (surface), 1.0–1.3 m (mid) and 2.7–3.0 m (deep) and treated with the chloroacetanilide herbicide, acetochlor. Freundlich isotherms revealed that sorption and desorption behaviour varied with each depth sampled. As soil depth increased, the extent and strength of sorption decreased, indicating that the potential for leaching was increased in the subsoils compared with the surface soil. Hysteresis was evident at each of the three depths sampled, although no significant correlations between soil properties and the hysteresis coefficients were evident. Desorption studies using soil fractions with diameters of > 2000, 250–2000, 53–250, 20–53, 2–20, 0–2 and 0–1 µm separated from each of the three soil depths showed that differential desorption kinetics occurred and that the retention of acetochlor significantly correlated (R2 = 0.998) with organic matter content. A greater understanding of the influence of soil components on the overall sorption and desorption potential of surface and subsurface soils is required to allow accurate prediction of acetochlor retention in the soil. In addition, it is likely that the proportion of each size fraction in a soil horizon would influence acetochlor bioavailability and movement to groundwater.  相似文献   

20.
Geogenic CO2 emission on mofette sites may be a factor in soil formation. To demonstrate a CO2 effect, we studied soils (0–60 cm depth) along a transect across a mofette in the NW Czech Republic. We determined CO2 partial pressures (p(CO2)), and the contents in the soil of carbon (C), nitrogen (N), sulphur and dithionite‐ and oxalate‐extractable iron and manganese. X‐ray diffractometry (XRD) and Fourier‐transform infrared (FTIR) spectroscopy methods were applied to the soils' particle‐size fractions. The CO2 partial pressures varied considerably (0.001–1) along the transect and were positively correlated with both the Corg contents (5.5–432.9 g kg−1) and the C:N ratio (9.3–32.2), indicating a decreased turnover of organic parent material with increasing CO2. When the soil atmosphere was entirely composed of CO2, pedogenic Fe oxide contents were small (minimum 0.5 g dithionite‐extractable Fe kg−1) and poorly crystalline. XRD and FTIR spectroscopy revealed primary and secondary minerals such as quartz, feldspars, mica, illite, kaolinite and halloysite irrespective of CO2 contents. A pronounced effect of CO2 was found for soil organic matter (SOM), because the FTIR spectra did not reveal a normal accumulation of alkyl C and lipids of microbial origin in the clay fraction. This indicates that microbial synthesis and/or degradation of plant‐derived aliphatic species were reduced. We did not detect more organo–mineral associations, microbially formed polypeptides or pectin in clay fractions in comparison with the clay‐plus‐silt fractions at large p(CO2). This indicates relatively unaltered particulate OM in the clay fraction. At large p(CO2) values, the IR bands indicative of lignin became detectable and that of aryl ketones in lignin was positively correlated with p(CO2). Thus, we suggest that microbial formation of SOM and degradation of lignin is restricted under an increased CO2 atmosphere. We attribute less humification at increased CO2 in the soil atmosphere to a decrease in oxidative transformations and decreased microbial activity.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号