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1.
Four soils with 6, 12, 23 and 46% clay were fractionated according to particle size after incubation for 5–6 years with 14C labelled straw, hemicellulose or glucose: 6–23% of the 14C was still present and the amount increased with increasing content of fine particles. clay fractions contained 66–84% of the 14C and the silt fractions accounted for 4–19%. <2% was found in the sand fractions and 4–9% was water soluble. The distribution of the native C was: clay, 46–68%; silt, 20–31%; sand, 2–7%. The clay fractions had higher relative proportions of 14C than of native C, the reverse being true for the silt fractions. This distribution pattern was not directly related to soil clay content or to kind of organic amendment. The C enrichment factor of clay and silt fractions (per cent C in fraction/per cent C in whole soil) increased with decreasing fraction size for both native and 14C. However, clay enrichment factors were higher for 14C than for native C, whereas silt enrichment factors were lower. A soil (9% clay) that had been incubated in the field for 18 years with 14C labelled straw was also analysed. Labelled C content at sampling was 9% of the initial value. In contrast to the other soils the distribution of labelled and native C was similar in the clay and silt fractions, which contained 55% and 33% of the whole soil C, respectively. The results indicate that clay-bound organic matter may be important in mediumterm organic matter turnover, whereas silt-bound organic matter may participate in longer-term organic matter cycling.  相似文献   

2.
l3C–nuclear magnetic resonance (NMR) spectra taken using magic–angle spinning (MAS), cross polarization (CP) and with total suppression of side bands (TOSS) are reported for soils from two long–term field experiments. One set of soils was from the Broadbalk Experiment at Rothamsted, UK (monoculture of winter wheat since 1843) and the other was from the Lermarken site of the Askov Long–Term Experiment on Animal Manure and Mineral Fertilizers (arable rotation since 1894). At both sites soil samples were taken from three fertilizer treatments: nil, inorganic fertilizers, animal manure. Spectra were obtained from whole soil samples and from the size fractions clay (<2 μrn), silt (2–20 μm) and, in some cases, sand (20–2000 μm). Comparison of the total strengths of the 13C–NMR signal for each size separate in relation to its total organic C content shows that clay, particularly, contains large percentages of C not detected by NMR because of the large magnetic susceptibilities of the soil minerals. It is proposed that the observed signals come from the more labile pools of soil organic matter (SOM), on the presumption that these pools are less closely associated with soil minerals and iron oxides and are likely to be less protected from microbial or enzymic decomposition. For both Rothamsted and Askov, functional groups in the 45–110 ppm region (N– and O–alkyls) dominate in the spectra for whole soils, with aromatics (110–160 ppm) and alkyls (0–45 ppm) signals being the next prominent. In the Askov whole soil samples 13C–NMR revealed no differences between nil, inorganic fertilizer and animal manure treatments but in the Rothamsted whole soil there were some small differences. Clay and silt fractions from Askov contain more alkyls and less aromatics than those from Rothamsted. For both sites clay in enriched in alkyls and depleted in aromatics relative to silt. Clay from Askov, but not Rothamsted, contains more N–alkyls (45–65 ppm) and less acetals (90–110 ppm) than silt. O–alkyls (65–90 ppm) account for more than 20% of the total signal in clay and silt from both sites. Fertilization regimes have not significantly affected the chemical composition of SOM associated with clay– and silt–sized fractions in the soils at either site. We conclude that the chemical composition of SOM is determined primarily by the interaction between the organisms responsible for decomposition and the mineral soil matrix rather than the nature of substrate input.  相似文献   

3.
Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay. silt and sand-size fractions were held for 49 days at 20°C and the CO2 evolution measured on 14 dates by gas chromatography.Recovery of soil solids. C and N was 99, 98 and 93%, respectively. Most of the soil C and N was in the clay (<2μm). (loamy sand, 50% C and 56% N; sandy loam. 65% C and 68% N), the silt (2–20 μm) having smaller proportions (loamy sand, 41% C and 38% N; sandy loam. 29% C and 27% N). The sand fraction (20–6000 μm) accounted for 4–7% of the organic matter, and 1–2% of the C was water soluble. Straw incorporation generally increased the C and N content of whole soils and size fractions.The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils, the decomposability of the organic matter decreased in the order: sand > clay ⩾ whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small.Between 58 and 73% of the respiration was from clay, 21–25% from silt and 6–19% from the sand size fraction.  相似文献   

4.
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   

5.
Soil contains the major part of carbon in terrestrial ecosystems, but the response of this carbon to enriching the atmosphere in CO2 and to increased N deposition is not completely understood. We studied the effects of CO2 concentrations at 370 and 570 μmol CO2 mol?1 air and increased N deposition (7 against 0.7 g N m?2 year?1) on the dynamics of soil organic C in two types of forest soil in model ecosystems with spruce and beech established in large open‐top chambers containing an acidic loam and a calcareous sand. The added CO2 was depleted in 13C and thus the net input of new C into soil organic carbon and the mineralization of native C could be quantified. Soil type was the greatest determining factor in carbon dynamics. After 4 years, the net input of new C in the acidic loam (670 ± 30 g C m?2) exceeded that in the calcareous sand (340 ± 40 g C m?2) although the soil produced less biomass. The mineralization of native organic C accounted for 700 ± 90 g C m?2 in the acidic loam and for 2800 ± 170 g C m?2 in the calcareous sand. Unfavourable conditions for mineralization and a greater physico‐chemical protection of C by clay and oxides in the acidic loam are probably the main reasons for these differences. The organic C content of the acidic loam was 230 g C m?2 more under the large than under the small N treatment. As suggested by a negligible impact of N inputs on the fraction of new C in the acidic loam, this increase resulted mainly from a suppressed mineralization of native C. In the calcareous sand, N deposition did not influence C concentrations. The impacts of CO2 enrichment on C concentrations were small. In the uppermost 10 cm of the acidic loam, larger CO2 concentrations increased C contents by 50–170 g C m?2. Below 10 cm depth in the acidic loam and at all soil depths in the calcareous sand, CO2 concentrations had no significant impact on soil C concentrations. Up to 40% of the ‘new’ carbon of the acidic loam was found in the coarse sand fraction, which accounted for only 7% of the total soil volume. This suggests that a large part of the CO2‐derived ‘new’ C was incorporated into the labile and easily mineralizable pool in the soil.  相似文献   

6.
The relative contributions of sources of carbon in soils, such as throughfall, litter, roots, microbial decay products and stable organic fractions, to dissolved organic C are controversial. To identify the origin of dissolved organic C, we made use of a 4‐year experiment where spruce and beech, growing on an acidic loam and on a calcareous sand, were exposed to increased CO2 that was depleted in 13C. We traced the new C inputs from trees into dissolved organic C, into water‐extractable organic C, and into several particle‐size fractions. In addition, we incubated the labelled soils for 1 year and measured the production of dissolved organic C and CO2 from new and old soil C. In the soil solutions of the topsoil, the dissolved organic C contained only 5–10% new C from the trees. The δ13C values of dissolved organic C resembled those of C pools smaller than 50 µm, which strongly suggests that the major source of dissolved organic C was humified old C. Apparently, throughfall, fresh litter and roots made only minor contributions to dissolved organic C. Water‐extractable organic C contained significantly larger fractions of new C than did the natural dissolved organic C (25–30%). The δ13C values of the water‐extractable organic C were closely correlated with those of sand fractions, which consisted of little decomposed organic carbon. The different origin of dissolved and water‐extractable organic C was also reflected in a significantly larger molar UV absorptivity and a smaller natural 13C abundance of dissolved organic C. This implies that the sampling method strongly influences the characteristics and sources of dissolved organic C. Incubation of soils showed that new soil C was preferentially respired as CO2 and only a small fraction of new C was leached as dissolved organic C. Our results suggest that dissolved organic C is produced during incomplete decomposition of recalcitrant native C in the soils, whereas easily degradable new components are rapidly consumed by microbes and thus make only a minor contribution to the dissolved C fraction.  相似文献   

7.
14C-labelled cellulose and 15N-labelled (NH4)2SO4 were added to four soils with clay contents of 4, 11, 18 and 34%, respectively. Labelled cellulose was added to each soil in amounts corresponding to 1, 2 and 4 mg C g?1 soil, respectively, and labelled NH4+ at the rate of 1 mg N per 25 mg labelled C.After the first month of incubation at temperatures of 10, 20 and 30°C, respectively, from 38 to 65% of the labelled C added in cellulose had disappeared from the soils as CO2, and from 60 to nearly 100% of the labelled N added as NH4+ were incorporated into organic forms. The ratio of labelled C remaining in the soils to labelled N in organic forms was close to 25 after 10 days of incubation, decreasing to about 15 after 1 month and about 10 after 4 yr.The retention of total labelled C was largest in the soil with the highest content of clay where after 4 yr it was 25% of that added, compared to 12 in the soil with the lowest content of clay. The incorporation of labelled N in organic forms and its retention in these forms was not directly related to the content of clay in the soils, presumably because the two soils with the high content of clay had a relatively high content of available unlabelled soil-N which was used for synthesis of metabolic material.The proportionate retention of labelled C for a given soil was largely independent of the size of the amendments, whereas the proportionate amount of labelled N incorporated into organic forms increased in the clay-rich soils with increasing size of amendments. Presumably this is because the dilution with unlabelled soil-N was less with the large amendments.From 50 to 70% of the total labelled C remaining in the soils after the first month of incubation was acid hydrolyzable, as compared to 80–100% of the total remaining labelled organic N. This relationship held throughout the incubation and was independent of the size of the amendment and of the temperature of incubation.During the second, third and fourth year of incubation the half-life of labelled amino acid-N in the soils was longer than the half-life of labelled amino acid-C, presumably due to immobilization reactions. Some of the labelled organic N when mineralized was re-incorporated into organic compounds containing increasing proportions of native soil-C. whereas labelled C when mineralized as CO2 disappeared from the soils.In general, native C and native organic N were less acid hydrolyzable and were accounted for less in amino acid form than labelled C and N.The amount of labelled amino acid-C, formed during decomposition of the labelled cellulose, and retained in the soil, was proportional to the clay content. This amount was about three times as large in the soil with the highest content of clay as in the soil with the lowest content. This difference between the soils was established during the first 10 days of incubation when biological activity was most intense, and it held throughout the 4 yr of incubation; proportionally it was independent of the amount of cellulose added and the temperature.In contrast, the labelled amino acid-N content was not directly related to the amount of clay in the soil, presumably because more unlabelled soil-N was available for synthesis of metabolic material in the two clay-rich soils than in those soils with less clay. The wider ratio between labelled amino acid-C and labelled amino acid-N in the two clay-rich soils as compared with those obtained with the soils with less clay indicates this.The effect of clay in increasing the content of organic matter in soil is possibly caused by newly synthesized matter, extracellular metabolites, as well as cellular material, forming biostable complexes and aggregates with clay. The higher the concentration of clay the more readily the interactions take place. The presence of clay may also increase the efficiency of using substrate for synthesis.  相似文献   

8.
Solutions labelled with 15N were applied as (15NH4)2SO4 or K15NO3 to isolated microplots in the floor of mountain beech forest (Nothofagus solandri var. cliffortioides) and incubated for 135 days under field conditions of moisture and temperature. Solid state 15N CPMAS NMR spectra of the forest litter layer showed that more than 80% of the total signal intensity was attributable to the secondary amide-peptide peak. The degree of 15N enrichment or form of N did not alter the relative intensity of signals attributable to 15N in peptides, nucleic acids and aliphatic amine groups (amino sugars and free NH2 on amino acids). Combinations of 13C and 15N-NMR spectra, edited by a process that exploited differences in proton spin properties between distinct categories of organic matter, indicated incorporation of 15N in humified organic matter rather than partly degraded plant material. This application demonstrated that solid state 15N CPMAS NMR has potential for use in studies of N immobilization under field conditions and with materials containing little N and small 15N enrichment.  相似文献   

9.
Two field experiments in which straw has been removed or incorporated for 17 yr (loamy sand) and 10 yr (sandy clay loam) were sampled to examine the effect of straw on the C and N contents in whole soil samples, macro-aggregate fractions and primary particle-size separates. The particle size composition of the aggregate fractions was determined. Aggregates were isolated by dry sieving. Straw incorporation increased the number of 1–20 mm aggregates in the loamy sand but no effect was noted in the sandy clay loam. Straw had no effect on the particle size composition of the various aggregate fractions. After correction for loose sand that accumulated in the aggregate fractions during dry sieving, macro-aggregates appeared to be enriched in clay and silt compared with whole soil samples. Because of the possible detachment of sand particles from the exterior surface of aggregates during sieving operations, it was inferred that the particle size composition of macro-aggregates is similar to that of the bulk soil. The organic matter contents of the aggregate fractions were closely correlated with their clay + silt contents. Differences in the organic matter content of clay isolated from whole soil samples and aggregate fractions were generally small. This was also true for the silt-size separates. In both soils, straw incorporation increased the organic matter content of nearly all clay and silt separates; for silt this was generally twice that observed for clay. The amounts of soil C, derived from straw, left in the loamy sand and sandy clay loam at the time of sampling were 4.4 and 4.5 t ha?1, corresponding to 12 and 21% of the straw C added. The C/N ratios of the straw-derived soil organic matter were 11 and 12 for the loamy sand and sandy clay loam, respectively.  相似文献   

10.
The small organic matter content of mineral soils makes it difficult to obtain 13C and 15N nuclear magnetic resonance (NMR) spectra with acceptable signal-to-noise ratios. Subjecting such samples to hydrofluoric acid removes mineral matter and leads to a relative increase in organic material. The effect of treatment with 10% hydrofluoric acid on bulk chemical composition and resolution of solid-state 13C NMR spectra was investigated with six soils, some associated particle size fractions, plant litter and compost. The treatment enhanced the signal-to-noise ratio of the solid-state 13C NMR spectra. The improvement in spectrum quality was greatest in the clay fraction of soil contaminated with coal ash. The removal of paramagnetic compounds associated with the ash may be the main reason for the improvement. Based on total C, total N, C/N ratio and intensity distribution of the solid-state 13C NMR spectra, no changes in organic matter composition could be detected, except for a possible loss of carbohydrates. After treatment with HF, solid-state 15N NMR spectra of particle size fractions were obtained and indicated that the observable nitrogen is present mostly as peptides and free amino groups. Extraction with hydrofluoric acid is recommended as a routine treatment prior to solid-state 13C and 15N NMR on soil containing little C or N and soil samples containing paramagnetic compounds from natural or anthropogenic sources.  相似文献   

11.
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   

12.
The turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with 14C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of 14C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg 14C‐labelled glucose kg?1 and 200 mg NH4+‐N kg?1, and incubated aerobically. Production of 14CO2 and CO2, dynamics of NH4+, NO2– and NO3–, and microbial biomass 14C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because > 90 mg 14C kg?1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH4+ and NO3– in the NH4+‐amended soil could not be simulated unless an immobilization of NH4+ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH4+ was added to alkaline saline soils.  相似文献   

13.
The effect of barley plants on the rate of decomposition of soil organic matter over a 6-week period was studied using soil that had been previously labelled by incubation with 14C-labelled ryegrass for 1 year. The plants reduced the loss of 14CO2, from soil by 70 per cent over 42 days. About half of the reduction was accounted for by the uptake of labelled C by the plant roots, very little 14C label being associated with the shoot. Chemical fractionation of the root showed that the 14C was chemically incorporated into cell wall materials such as cellulose and holocellulose. The reduction in organic matter decomposition in the presence of plants has been explained by earlier workers in terms ofa reduction in microbial activity as a result of a soil moisture deficit caused by plant transpiration. This explanation does not account for all the reduction in decomposition noted in the present experiments. Control soil (without a plant, but amended with glucose or yeast extract to simulate the effect of root exudates) showed a small positive priming effect, the release of 14CO2, being increased. Thus the mechanism by which plants conserve organic matter is complex and cannot be explained merely by analogy to an increased level of nutrients available for microbial metabolism.  相似文献   

14.
Chemical fixation of NH3 to soil organic matter was studied in two Swedish soils with different contents of organic matter: a clay soil with 2.3% C and an organic soil with 36.6% C. 15N‐labelled urea was applied at different rates to both sterilized and non‐sterilized soils. After 10 days, the soils were extracted and washed with K2SO4 and determined for total N and atom% 15N excess. Urea N was recovered as non‐extractable N in sterilized soil corresponding to 9.7% of supplied l5N‐labelled urea in the organic soil and 2.2% in the clay soil. Since no biological immobilization is thought to occur in the sterile soil, this non‐extractable N is suggested to be chemically fixed to soil organic matter. Owing to urea hydrolysis in the clay soil, pH increased from 6.3 to 9.3 and in the organic soil from 5.7 to 6.9 and 8.8, respectively, at the low and high urea supply.  相似文献   

15.
The rates of decomposition of barley roots labelled with 14C were investigated in soil planted with maize or perennial ryegrass and in fallow controls. Evolution of 14CO2 was significantly less from the planted soils than from fallow controls. Roots of maize and ryegrass appeared to compete substantially with soil microbes for 14C-labelled materials. Simple competitive effects were, however, insufficient to explain all of the observed effects of root growth on soil organic matter decomposition. There was no indication that the detrimental effects of maize roots on aggregate stability could be associated with increased degradation of native soil organic materials; the broader significance of the results is also discussed.  相似文献   

16.
Soils from A horizons of Eutrochrepts under spruce forest (Sf), mixed deciduous forest (Df), permanent grassland (Gp), and arable rotation (Ar) were fractionated into clay- (<2 μm), silt-(2–20 μm) and sand- (20–2000 μm) sized separates. 13C NMR spectroscopy was used to compare SOM composition across size separates and between land-use regimes. CPMAS 13C NMR spectroscopy showed that the intensity of signals assigned to carbohydrates (representing most O-alkyl C) and lignin (phenolic and methoxyl C) declined with decreasing particle size. Concurrently, alkyl C and C-substitution of aromatic C increased in the order sand, silt, clay. The amount of alkyl C correlated well with microbial resynthesis of carbohydrates. Solution 13C NMR spectra suggested that humic acids (HA) extracted from the size separates were richer in carboxyl C and aromatic C than the bulk size separates. Also HA reflected increasing percentage of alkyl C with decreasing particle size. O-alkyl C were lower in silt HA than in clay HA whereas aromatic C tended to peak in silt HA. These results suggested that sand-sized separates were enriched in plant residues (primary resources) whereas clay-sized separates were dominated by products of microbial resynthesis (secondary resources). Silt was rich in selectively preserved and microbially transformed primary resources. 13C NMR spectroscopy showed only small differences in SOM composition between land-use regimes, except that silt and silt HA from Ar were richer in aromatic C than those from the other plots. But enrichment factors (E= content in fraction/content in whole soil) revealed differences in the distribution of C species across the size separates. Relatively high Earomatic (0.9) and Eo-alkyl (1.0) for sand from Gp indicated high amounts of plant residues, probably due to intense rhizodeposition and to occlusion of plant debris within aggregates. Low Earomatic (0.3) and Eo-alkyl (0.3) for sand from Ar suggested depletion of primary resources, which could be attributed to disintegration of soil aggregates upon cultivation. A pronounced enrichment of alkyl C in Ar clay-sized separates (Ealkyl= 3.1) suggested large amounts of microbial carbon. Microbial products attached to clay surfaces by a variety of physico-chemical bondings appeared more stable against mineralization induced by cultivation than plant residues sequestered in aggregates.  相似文献   

17.
The rate at which available nitrogen (N) is released from organic materials in soil is often measured by applying 15N and following its recovery by the growing crop. However, the turnover of labelled N in soil modifies the ratio of labelled to unlabelled available N and thereby affects the uptake of 15N by plants. The recovery of labelled N by maize was measured in a field experiment under three management systems, with one 15N‐labelled input in each: (1) conventional, with fertilizer side dressing, (2) low input, with vetch as a cover crop and fertilizer side dressing, and (3) organic, with vetch and composted manure. The NCSOIL model, which simulates C and N turnover in soil, was modified to include relevant processes related to the maize crop, and used to estimate the decomposition rate constant of vetch in the field by optimizing the simulated dynamics of labelled N uptake by maize against the measured results. A large input of C from mineralizable soil organic matter and root deposition was necessary to account for the recovery of fertilizer N by maize. Optimization of labelled N recovery in the low input system resulted in two optional rate constants for the decomposition of vetch: rapid decomposition (0.4 day?1) of a labile vetch pool (49% of total vetch N), or slow decomposition (0.008 day?1) of a single vetch pool. In the simulated organic system, where manure and vetch were incorporated at the same time, only a rapid decomposition of the labile component of vetch accounted well for the recovery of vetch N by maize. The prolonged recycling of N mineralized from the vetch, and its mixing with fertilizer side dressing in the low input system, reduced the recovery of vetch N even though it was mineralized rapidly. This demonstrates the difficulty in assessing the availability of N from organic materials.  相似文献   

18.
Investigating the biogeochemistry of plant material decomposition in soil has been restricted by difficulties extracting and identifying organic compounds. In this study the decomposition of 13C- and 15N-labelled Lolium perenne leaves mixed with mineral soil has been investigated over 224 days of incubation under laboratory conditions. Decomposition was followed using short-term rates of CO2 evolution, the amounts of 13C and 15N remaining were determined by mass spectrometry, and 13C and 15N solid-state nuclear magnetic resonance (NMR) spectroscopy was used to characterize chemically the plant material as it decomposed. After 224 days 48% of the added 13C had been lost with a rapid period of C02 evolution over the first 56 days. The fraction of cross-polarization magic angle spinning (CP MAS) 13C NMR spectra represented by O-alkyl-C signal probably in carbohydrates (chemical shift, 60–90 p.p.m.) declined from 60 to 20% of the spectrum (chemical shift, 0–200 p.p.m.) over 224 days. The rate of decline of the total 13C exceeded that of the 60–90 p.p.m. signal during the first 56 days and was similar thereafter. The fraction of the CP MAS 13C NMR spectra represented by the alkyl- and methyl-C (chemical shift, 10–45 p.p.m.) signal increased from 5 to 14% over the first 14 days and was 19% after 224 days. CP MAS 13C NMR of 13C- and 15N-L. perenne contained in 100-μm aperture mesh bags incubated in the soil for 56 days indicated that the remaining material was mainly carbohydrate but there was an increase in the alkyl- and methyl-C associated with the bag's contents. After 224 days incubation of the labelled 13C- and 15N-L. perenne mixed with the soil, 40% of the added N had been lost. Throughout the incubation there was only one signal centred around 100 p.p.m. detectable in the CP MAS 15N NMR spectra. This signal corresponded to amide 15N in peptides and may have been of plant or microbial origin or both. Although there had been substantial interaction between the added 15N and the soil microorganisms, the associated redistribution of 15N from plant to microbial tissues occurred within the amide region. The feasibility of following some of the component processes of plant material decomposition in soil using NMR has been demonstrated in this study and evidence that microbial synthesis contributes to the increase in alkyl- and methyl-C content of soil during decomposition has been represented.  相似文献   

19.
In semi‐arid northern Tanzania, the native woodland is being rapidly cleared and replaced by low input agriculture. This has resulted in pronounced environmental degradation, and in particular loss of phosphorus (P) from the soil. We have used sequential extraction and 31P‐NMR to investigate the effects of land use changes, i.e. native woodland, degraded woodland, cultivation for 3 and 15 years and homestead fields where manure was applied, on the amount and structural composition of P in this soil. Clearing and continuous cultivation reduced both organic and inorganic P in the soil. The difference in the amount of organic P from the bulk soil of the fields cultivated for 3 and 15 years was not statistically significant (P < 0.05), suggesting that most of the depletion in organic P occurred during the first 3 years of cultivation. By contrast, in the homesteads, there was much organic and inorganic P in the soil. The 31P‐NMR revealed that cultivation resulted in a 53% depletion of orthophosphate diester P, whereas only a 30% and 39% reduction of orthophosphate monoester P was found in the bulk soil after 3 and 15 years of cultivation, respectively. These results concur with the suggestion that diester P constitutes more easily mineralizable forms of organic P in soil than does monoester P. Our 31P‐NMR also showed that 70% of the inorganic orthophosphate P was depleted from the coarse and fine sand separates as a result of cultivation. The influence of clearing and subsequent cropping on the amount and forms of P was more pronounced in the coarse and fine sand than in the silt and clay, stressing the importance of particle size and chemical properties such as organic matter and oxides in the availability of P in this soil. Our results show that the current low input agricultural practice is not sustainable, and that practices must be developed to combat the ongoing degradation of the soil. A combined use of available organic materials such as animal manure with the judicious use of inorganic fertilizers can replenish the soil's fertility.  相似文献   

20.
Ryegrass uniformly labelled with I4C was incubated aerobically at 25°C for 62 days in two contrasting soils, a near-neutral (pH 6.8) palcudalf from England and a strongly acid (pH 3.6) haplorthox from Brazil. Decomposition of the labelled plant material was faster in the near-neutral soil throughout the whole of the incubation period. In neither soil did the addition of fresh plant material significantly accelerate the evolution of CO2 from organic matter already in the soil, i.e. there was no priming action. In the near-neutral soil there was a rapid build up of labelled microbial biomass in the first 6 days, followed by a much slower increase that continued throughout the whole incubation period. After 62 days 22.5% of the labelled C remaining in the near-neutral soil was in the biomass. The yield coefficient (the fraction of the incoming plant C converted to microbial C) of this stabilized or ‘resting’ biomass was 0.15. Much less labelled microbial biomass was formed in the acid soil than in the near-neutral soil. By the end of 62 days only 6.2% of the labelled carbon remaining in the acid soil was in the biomass. Biomass C measurements in strongly acid soils must however be treated with caution as the technique used has not yet been adequately validated for such soils.  相似文献   

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