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1.
L.H. Sorensen 《Soil biology & biochemistry》1981,13(4):313-321
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
2.
Four soils with a range of clay and silt contents were incubated for 5 a with 15N-labelled (NH4)SO4 and 14C-labelled hemicellulose and then fractionated according to particle size by ultrasonic dispersion and sedimentation. The distribution of labelled and native N between clay, silt and sand fractions was determined and elated to previous results on the C distributions. Between 29% and 48% of the added N was found in organic form. The 15N atom percentage excess decreased in the order: clay > whole soil > silt > sand. For both clay and silt, the enrichment factor for labelled and native N decreased with increasing fraction weight. Clay enrichment was higher for labelled than for native N, the converse being true for silt. The distribution of whole soil labelled organic N was: clay 77–91%, silt 4–11%, and sand <0.5%. Corresponding values for native N were 69–74%, 16–22%, and 1–2%, respectively. All soils had higher proportions of labelled than of native N in the clay, the converse was true for the silt. The C/N ratio of the native silt organic matter was higher and that of clay organic matter lower than whole soil C/N ratios. Differences between the C/N ratio distributions of native and labelled organic matter were small. The relative distribution of labelled N and C was very similar confirming that the turnover of C and N in soil organic matter is closely interrelated. 相似文献
3.
The turnover of N derived from rhizodeposition of faba bean (Vicia faba L.), pea (Pisum sativum L.) and white lupin (Lupinus albus L.) and the effects of the rhizodeposition on the subsequent C and N turnover of its crop residues were investigated in an incubation experiment (168 days, 15 °C). A sandy loam soil for the experiment was either stored at 6 °C or planted with the respective grain legume in pots. Legumes were in situ 15N stem labelled during growth and visible roots were removed at maturity. The remaining plant-derived N in soil was defined as N rhizodeposition. In the experiment the turnover of C and N was compared in soils with and without previous growth of three legumes and with and without incorporation of crop residues. After 168 days, 21% (lupin), 26% (faba bean) and 27% (pea) of rhizodeposition N was mineralised in the treatments without crop residues. A smaller amount of 15–17% was present as microbial biomass and between 30 and 55% of mineralised rhizodeposition N was present as microbial residue pool, which consists of microbial exoenzymes, mucous substances and dead microbial biomass. The effect of rhizodeposition on the C and N turnover of crop residues was inconsistent. Rhizodeposition increased the crop residue C mineralisation only in the lupin treatment; a similar pattern was found for microbial C, whereas the microbial N was increased by rhizodeposition in all treatments. The recovery of residual 15N in the microbial and mineral N pool was similar between the treatments containing only labelled crop residues and labelled crop residues + labelled rhizodeposits. This indicates a similar decomposability of both rhizodeposition N and crop residue N and may be attributable to an immobilisation of both N sources (rhizodeposits and crop residues) as microbial residues and a subsequent remineralisation mainly from this pool.Abbreviations C or Ndec
C or N decomposed from residues
- C or Nmic
microbial C or N
- C or Nmicres
microbial residue C or N
- C or Nmin
mineralised C or N
- C or Ninput
added C or N as crop residues and/or rhizodeposits
- dfr
derived from residues
- dfR
derived from rhizodeposition
- Ndfr
N derived from residues
- NdfR
N derived from rhizodeposition
- Nloss
losses of N derived from residues
- SOM
soil organic matter
- WHC
water holding capacity 相似文献
4.
An incubation experiment was conducted to determine the response of soil microbial biomass and activity to salinity when supplied with two different carbon forms. One nonsaline and three saline soils of similar texture (sandy clay loam) with electrical conductivities of the saturation extract (ECe) of 1, 11, 24 and 43 dS m?1 were used. Carbon was added at 2.5 and 5 g C kg?1 (2.5C, 5C) as glucose or cellulose; soluble N and P were added to achieve a C/N ratio of 20 and C/P ratio of 200. Soil microbial activity was assessed by measuring CO2 evolution continuously for 3 weeks; microbial biomass C and available N and P were determined on days 2, 7, 14 and 21. In all soils, cumulative respiration was higher with 5C than with 2.5C and higher with glucose than with cellulose. Cumulative respiration was highest in the nonsaline soil and decreased with increasing EC, whereas the decrease was gradual with glucose, there was a sharp drop in cumulative respiration with cellulose from the nonsaline soil to soil with EC11 with little further decrease at higher ECs. Microbial biomass C and available N and P concentrations were highest in the nonsaline soil but did not differ among the saline soils. Microbial biomass C was higher and available N was lower with 5C than with 2.5C. The C form affected the temporal changes of microbial biomass and available nutrients differentially. With glucose, microbial biomass was highest on day 2 and then decreased, whereas available N showed the opposite pattern, being lowest on day 2 and then increasing. With cellulose, microbial biomass C increased gradually over time, and available N decreased gradually. It is concluded that salinity reduced the ability of microbes to decompose cellulose more than that of glucose. 相似文献
5.
Turnover of 14C-labelled plant components and 15N-ammonium in soil The turnover of 14C-labelled glucose, cellulose, wheat straw, phenols or of lignin in soil was investigated in the presence of (15NH4)2SO4. The plant components were more or less rapidly degraded to 14CO2 and the amended nitrogen source became organically linked but was also remineralized to a variable extend. Also variable was the incorporation of the 14C or 15N into humified residues or microbial metabolites. During the turnover of carbohydrates and straw a rapid increase of 14C and 15N in amino-acids or unidentified components of soil hydrolysates occurred which was followed by a decrease. The turnover of phenols was mostly similar to that of carbohydrates but compared to their mineralization rates, a smaller incorporation occurred into the easily hydrolyzable soil fractions. Although lignin was considerably mineralized to CO2, the incorporation of the carbon remaining in soil into hydrolyzable components especially in amino-acids was, however, very small. A somewhat higher amount became incorporated into unidentified components of hydrolysates, but the bulk of the lignin carbon remained in the non-hydrolizable residue. 相似文献
6.
Estimation of the capacity of soils to supply N for crop growth requires estimates of the complex interactions among organic
and inorganic N components as a function of soil properties. Identification and measurement of active soil N forms could help
to quantify estimates of N supply to crops. Isotopic dilution during incubation of soils with added 15NH4
+ compounds could identify active N components. Dilution of 15N in KCl extracts of mineral and total N, non-exchangeable NH44
+, and N in K2SO4 extracts of fumigated and non-fumigated soil was measured during 7-week incubation. Samples from four soils varying in clay
content from 60 to 710 g kg–1 were used. A constant level of 15N enrichment within KCl and K2SO4 extracted components was found at the end of the incubation period. Total N, microbial biomass C and non-exchangeable NH4
+ contents of the soils were positively related to the clay contents. The mineralized N was positively related to the silt
plus clay contents. The active soil N (ASN) contained 28–36% mineral N, 29–44% microbial biomass N, 0.3–5% non-exchangeable
NH4
+ with approximately one third of the ASN unidentified. Assuming that absolute amounts of active N are related to N availability,
increasing clay content was related to increased N reserve for crop production but a slower turnover.
Received: 7 July 1998 相似文献
7.
The microbial population of a Brown Chernozemic soil was labelled in situ by adding 14C-glucose and 15NH415NO3 to the plow layer. The loss of 14C, nitrogen immobilization-mineralization reactions, bacterial numbers (plate count, direct count) and fungal hyphal lengths were determined periodically throughout the growing period in amended and unamended microplots and in the surrounding field soil. After 5 days, 90 per cent of the labelled N occurred in the organic form with little subsequent mineralization. Of the labelled C added, 63, 56 and 39 per cent, remained in the soil after 3, 14 and 104 days, respectively.The ratio of fungal C to bacterial C increased as soil moisture decreased. Viable (plate count) and total numbers of bacteria in samples from unamended plots and field soil were significantly correlated with each other and with soil moisture. Fungal hyphal lengths from amended soil were also significantly related to moisture but the rate of loss of 14C and mineralization of 15N were not. The synthesized microbial material (tissue and metabolites) exhibited a high degree of stability throughout the study. The half-life of labelled C remaining in the soil after 30 days was calculated to be 6 months compared to only 4 days for the added glucose C. The amount of energy used for maintenance by the soil population under field conditions was calculated from measurements of biomass C, respired labelled C and respired soil C. 相似文献
8.
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
9.
Two soils, one a sandy loam and the other of relatively high clay content, were incubated with [14C(U)]gtucose and [15N](NH4)2SO4 for 101 days, either under continuously moist conditions, or with intermittent drying of soils. Rates of evolution of 14CO2, decline in residual organic 14C, and net immobilization and mineralization of N and 15N in the sandy loam soil were more rapid than in the clay soil. First order decay rates for the decomposition of residual 14C, after 10 days, were consistently twice as fast in the sandy loam soil. By contrast, the efficiency with which glucose was utilized within the first few days, and the amounts of C, 14C, N and 15N present as soil biomass throughout the incubation, were greater in the clay soil than in the sandy loam. Biomass 14C as a percentage of residual organic 14C, was consistently 1.5 times greater in the clay soil. Compared with soils held continuously moist, soils which were intermittently dried and remoistened contained smaller amounts of isotope-labelled biomass C and N, but overall similar amounts of total residual organic 14C and 15N. Remoistening of dried soils caused a temporary (4 days) flush in C and N mineralization rates.A simulation model describes C and N behaviour in the two soils. Three features of the model are proposed to expain short-term differences between soils in the rates of C and N turnover, viz. the clay soil (a) has a greater capacity to preserve biomass C and N (b) holds a higher proportion of microbial decay products in the near vicinity of surviving cells, and, to a lesser extent, (c) utilizes glucose and metabolic products more efficiently for biosynthetic reactions. 相似文献
10.
《Soil biology & biochemistry》2001,33(4-5):583-591
Short-term effects of actively burrowing Octolasion lacteum (Örl.) (Lumbricidae) on the microbial C and N turnover in an arable soil with a high clay content were studied in a microcosm experiment throughout a 16 day incubation. Treatments with or without amendment of winter wheat straw were compared under conditions of a moistening period after summer drought. The use of 14C labeled straw allowed for analyzing the microbial use of different C components. Microbial biomass C, biomass N and ergosterol were only slightly affected by rewetting and not by O. lacteum in both cases. Increased values of soil microbial biomass were determined in the straw treatments even after 24 h of incubation. This extra biomass corresponded to the initial microbial colonization of the added straw. O. lacteum significantly increased CO2 production from soil organic matter and from the 14C-labeled straw. Higher release rates of 14C-CO2 were recorded shortly after insertion of earthworms. This effect remained until the end of the experiment. O. lacteum enhanced N mineralization. Earthworms significantly increased both mineral N content of soil and N leaching in the treatments without straw addition. Moreover, earthworms slightly reduced N immobilization in the treatments with straw addition. The immediate increase in microbial activity suggests that perturbation of soil is more important than substrate consumption for the effect of earthworms on C and N turnover in moistening periods after drought. 相似文献
11.
Influence of the rhizosphere on microbial biomass and recently formed organic matter 总被引：4，自引：0，他引：4
We have aimed to quantify the effect of roots on the size of the soil microbial biomass, and their influence on the turnover of soil organic matter and on the extent of the rhizosphere. We sampled a sandy clay loam topsoil from two subplots with different treatment histories. One had a normal arable fertilization record, the other had received only inorganic nitrogen fertilizer but no phosphorus and potassium for 30 years. Glucose labelled with 14C was added to both samples which were then incubated for 4 weeks before the soil was packed in cylinders and planted with ryegrass. In both soils, microbial biomass at the root surface doubled during the first 8 days. At day 15, the microbial biomass had further increased in the fertile soil, and the rhizosphere effect had extended 2.5 mm into the fertile soil, but to only 1 mm in the infertile soil. The microbial 14C increased threefold near the roots in the fertile soil as a result of assimilation of previously formed microbial residues, but in the infertile soil there was no increase. There was a close relation between the increase in microbial 14C and a decrease in 14C soluble in 2 m KCl, indicating that the microbial residues were more weakly adsorbed in the fertile soil. We conclude that the increased microbial population living near the root surfaces re‐assimilated part of the 14C‐labelled microbial residues in the fertile soil. In the infertile soil, microbial residues resisted decomposition because they were more strongly sorbed on to soil surfaces. 相似文献
12.
Summary An experiment was carried out to investigate how straw, cellulose and lignin affect the turnover and availability of inorganic labelled N in soil. The experiment comprised an incubation period in which the soil was incubated with 15NH4
15NO3 and organic materials followed by drying and by cropping the soil with Lolium perenne. The incubation period lasted 148 days during which soil samples were taken 36 and 148 days after the beginning of incubation. Addition of organic materials to the soil promoted the incorporation of inorganic N into organic matter and decreased apparent N denitrification losses during the first period of incubation (0–36 days after beginning of incubation). In this respect straw and cellulose were more effective than lignin. The organic materials also promoted the fixation of NH4
+ by clay minerals. In all treatments highest fixation of labelled NH4
+ by clay minerals was found at the end of the incubation period. During the cropping period high apparent denitrification losses were observed particularly in the straw and cellulose treatment. Hence the recovery of labelled N by Lolium was particularly low in these treatments while in the control treatment the 15N recovery was about twice as high. 相似文献
13.
Extractability of microbial N was estimated using in situ labelling of the microbial population with 15N. Four arable soils (one grey forest soil and three chernozems with different long-term fertilization) were amended with
(NH4)2SO4 (unlabelled or labelled with 15N) and d-glucose with a C : N ratio of 10 : 1 or 20 : 1 for the grey forest soil and 50 : 1 for the chernozems. d-glucose and labelled N with a C : N ratio of 20 : 1 did not cause microbial immobilization of unlabelled N. The use of substrates
with a C : N ratio of 50 : 1 led to a pronounced priming action on soil N and decreased the extractability of immobilized
15N. Values of the extractable biomass N fraction (k
EN
) assessed for the fumigation-extraction and rehydration procedures were similar and varied in inverse proportion to the C : N
ratio of the flush. The k
EN
factor was calculated using values of the C : N ratio in flushes and the fixed C : N ratio of structural cell components,
with the assumption that the C : N ratio of the extractable cytoplasmic cell fraction is variable. The ratio between the extractable
and non-extractable biomass N fraction (k
EC
) and the C : N ratio of non-extractable cell components were assessed as equation parameters optimized for the measured k
EN
and C : N ratio of flush data.
Received: 31 October 1997 相似文献
14.
Ingrid K Thomsen Per SchjønningJørgen E Olesen Bent T Christensen 《Soil biology & biochemistry》2003,35(6):765-774
The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg−1 were horizontially sliced and 15N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO2-C determined continuously for 177 d. Inorganic and microbial biomass N and 15N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO3-N present in soil before faeces was applied, net nitrification became negative indicating that NO3-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and 15N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces 15N was recovered in the microbial biomass in the amended soils. CO2-C evolution increased with clay content in amended and unamended soils. CO2-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content. 相似文献
15.
M. Pansu L. Sarmiento K. Metselaar D. Hervé & P. Bottner 《European Journal of Soil Science》2007,58(3):775-785
The mechanisms linking soil respiration to climate and soil physical properties are important for modelling transformation and sequestration of C and N in the soil. We investigated them by incubating 14C and 15N labelled straw in soils of the dry puna (Bolivian altiplano, semi‐arid shrubland at 3789 m above sea level) and the humid paramo (Venezuelan tropical alpine vegetation at 3400 m). These two ecosystems of the high Andes are comparable in terms of altitude, mean temperature and land use, but are very different regarding organic matter content, rainfall patterns and soil physical properties. Total 14C and 15N, microbial‐biomass 14C and 15N, soil moisture and meteorological data were recorded over 2 years. Daily soil moisture was predicted from a water balance model. The data from the paramo site were used to calibrate MOMOS‐6, a model of organic matter decomposition based on microbial activity and requiring only kinetic constant parameters to describe: (i) inputs to microbial biomass from plant debris and microbial metabolites, and (ii) losses from the biomass by mortality and respiration (respiration coefficient and microbial metabolic quotient qCO2). The simulated qCO2–14C agrees well with qCO2–14C and qCO2 measured at the calibration site and with published data. To apply MOMOS‐6 to the puna site, only the respiration coefficient of the biomass was re‐estimated. The dynamics of 14C and 15N were very different in the two systems. In the puna, the transformation processes stop during the long dry periods, though total annual mineralization is greater than in the paramo. The change in the value of the respiration coefficient enables us to predict that the amount of C and N sequestered in the stable humus is greater in the paramo than in the puna. The data in this paper can be used to estimate values of the respiration coefficient so that MOMOS‐6 can be applied to other systems. 相似文献
16.
14C-labelled glucose and 15N-labelled KNO3 were added to soil and the microbial biomass during 42 days' incubation was estimated using the chloroform fumigation-incubation method (CFIM). By day 1, most of the glucose (1577 μgCg?1 soil) was metabolized and 110 μg NO?3-Ng?1 soil were immobilized. In situ values for the proportions of biomass C (kC) and biomass N (kN) mineralized during the 10 days after CHCl3 fumigation were determined on the basis that the immobilized labelled C and N remaining in the soil at this time were present as living microbial cells and their associated metabolites. The tracer data indicated that biomass C could be calculated by applying a kc value of 0.41 to the CO2-C evolved from the fumigated sample without subtraction of an unfumigated “control”. Biomass N was estimated from the net NH4?-N accumulation during the fumigation-incubation. The problem of reimmobilization of NH+4-N where organisms of wide C:N ratio occur was overcome by adjusting the value of kN according to the ratio of CO2-C evolved: net NH4+-N accumulated during the fumigation-incubation (CF:NF).A CF:NF ratio of 6:1 resulted in a kN of 0.30 whereas a ratio of 13:1 indicated a kN of 0.20. 相似文献
17.
Wheat plants were grown on two soils of different texture, a sandy soil and a silty clay loam, in an atmosphere containing 14CO2. The 14C and total C content of the shoots, roots, soil rhizosphere CO2 and soil microbial biomass were measured 21, 28, 35 and 42 days after germination. There was a pronounced effect of soil texture on the turnover of root-derived C through the microbial biomass. Turnover was relatively fast and at a constant rate in the sandy soil but slowed down in the clay soil, following an initial high assimilation of root products into the microbial biomass.Four percent of the total fixed 14C was retained in the clay loam after 6 weeks compared with a corresponding value of 1.2% for the sandy soil. The proportion of fixed 14C recovered as rhizosphere CO2 at each of the sampling times was relatively constant for the sandy soil (ca 19%) but decreased from 17% at day 28 to 11% at day 42 in the clay soil. The proportion of total fixed 14C in the soil biomass as measured by a fumigation technique increased to a maximum value of 20% after 6 weeks in the sandy soil but decreased in the clay soil from 86% at day 21 to 26% after 42 days plant growth. 相似文献
18.
An arable soil with organic matter formed from C3-vegetation was amended initially with maize cellulose (C4-cellulose) and sugarcane sucrose (C4-sucrose) in a 67-day laboratory incubation experiment with microcosms at 25 °C. The amount and isotopic composition (13C/12C) of soil organic C, CO2 evolved, microbial biomass C, and microbial residue C were determined to prove whether the formation of microbial residues depends on the quality of the added C source adjusted with NH4NO3 to the same C/N ratio of 15. In a subsequent step, C3-cellulose (3 mg C g−1 soil) was added without N to soil to determine whether the microbial residues formed initially from C4-substrate are preferentially decomposed to maintain the N-demand of the soil microbial community. At the end of the experiment, 23% of the two C4-substrates added was left in the soil, while 3% and 4% of the added C4-cellulose and C4-sucrose, respectively, were found in the microbial biomass. The addition of the two C4-substrates caused a significant 100% increase in C3-derived CO2 evolution during the 5-33 day incubation period. The addition of C3-cellulose caused a significant 50% increase in C4-derived CO2 evolution during the 38-67 day incubation period. The decrease in microbial biomass C4-C accounted for roughly 60% of this increase. Cellulose addition promoted microorganisms strongly able to recycle N immediately from their own tissue by “cryptic growth” instead of incorporating NO3− from the soil solution. The differences in quality of the microbial residues produced by C4-cellulose and C4-sucrose decomposing microorganisms are also reflected by the difference in the rates of CO2 evolution, but not in the rates of net N mineralization. 相似文献
19.
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
20.
D. W. HOPKINS J. A. CHUDEK E. A. WEBSTER D. BARRACLOUGH 《European Journal of Soil Science》1997,48(4):623-631
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