ORIGINS AND STABILITY OF SOIL POLYSACCHARIDE

Recent work on the origin of soil polysaccharide and its biological stability in soil is reviewed. It is concluded that much of the constituent hexose and deoxyhexose sugars are of microbial origin, whereas the pentose sugars are derived from plant residues. The stability of soil polysaccharide in its native state is not related to its chemical composition but to its unavailability. This is caused by inaccessibility within undecomposed biological residues and to insolubility resulting from adsorption on clay, the formation of metal complexes, or tanning by soil humic substances. Complexing by metals and tanning may also inhibit enzymic hydrolysis.     

THE ORIGIN OF SOIL POLYSACCHARIDE: TRANSFORMATION OF SUGARS DURING THE DECOMPOSITION IN SOIL OF PLANT MATERIAL LABELLED WITH 14C

Incubation of soil with ¹⁴C-rye straw for 448 days resulted in the evolution of about 50 per cent of the carbon of the substrate as CO₂ The two main sugars of the straw, glucose and xylose, were degraded to approximately the same extent (70 per cent). The same results were obtained whether the soil was derived from granitic or basic igneous parent material. There was very little transformation of the substrate to galactose, mannose, arabinose, rhamnose, or fucose, and a much slower rate of degradation than with soil incubated with ¹⁴C-glucose over a similar period. Hydrolysis of the soil samples by a preliminary treatment with 5 N H₂SO₄, before treatment with 24 N H₂SO₄, followed by heating with N H₂SO₄ did not release significantly greater amounts of sugar than treatment with 24 N H₂SO₄ and N H₂SO₄ alone. Separate analysis of the hydrolysates showed that 90 per cent of each of galactose, mannose, arabinose, xylose, rhamnose, or fucose had been extracted by 5 N H₂SO₄, but only 50 per cent of the glucose. Fractionation of the straw-soil mixture after 224 days incubation showed that the specific activity of the glucose was higher in the humin fraction than in the fulvic acid, as would be expected if the remaining ¹⁴C were still in the form of unchanged plant material. This evidence that plant polysaccharide persists in soil could explain the presence of much of the xylose in the soil organic matter.     

THE ORIGIN OF THE PENTOSE FRACTION OF SOIL POLYSACCHARIDE

Incubation of soil with monosaccharide for 224 days resulted in the evolution of about 80 per cent of the substrate carbon as CO₂ and the transformation of 3 per cent to soil sugars whether the substrate was ¹⁴C-glucose or xylose and whether the soil was pH 7.4 or pH 5.0. There was no detectable change in the total amounts of individual sugars in the soil during incubation. ¹⁴C-glucose and xylose gave the same distribution of radioactivity among the soil sugars : hexoses and 6-deoxy-hexoses were initially well labelled, with glucose having twice the specific activity of the other sugars. As the incubation progressed some activity appeared in the pentoses (the activity in xylose became very low within the first 14 days of the ¹⁴C-xylose incubation) and that in the hexoses slowly declined, with glucose no longer predominant. Nevertheless after 448 days the hexoses were still 3–4 times more radioactive than the pentoses. The activity in rhamnose did not decline with time so that eventually it became the most strongly labelled sugar. Incubation of soil with glucose and ¹⁴C-acetate showed very little transformation of the acetate to sugars indicating that glucose is not metabolized to C₂ compounds before it is transformed to other sugars. Ammo-acids in soil incubated for 7 days with ¹⁴C-glucose had much lower levels of radioactivity than hexoses or 6-deoxy-hexoses. It is concluded that if soil pentose originates by microbial synthesis it must accumulate slowly by a long process of selective decomposition of a mixture of polysaccharides.     

STRUCTURAL STUDIES ON SOIL POLYSACCHARIDE

The polysaccharide extracted by alkali from a Countesswells series soil has been fully methylated and the hydrolysis products identified by GC-MS. The parent neutral sugars are galactose, glucose, mannose, arabinose, xylose, fucose and rhamnose and these constitute about 40 per cent of the polysaccharide. The analysis shows that hexose components are predominantly present in 1 → 3 and 1 → 4 linkages and pentose sugar in 1 → 4 linkages. About 20 per cent of the residues were in branching positions. From the number of non-reducing terminal groups present the average molecular weight of the methylated material has been calculated to be about 1460 compared with a value of 2700 obtained by vapour pressure osmometry. This contrasts with much higher values reported for unmethylated soil polysaccharides. The mixture of derivatives obtained supports the concept that soil polysaccharide originates in both plants and microorganisms.     

潮土中有机物质的分解与腐殖质积累

本文采用＾１４Ｃ标记示踪法研究我国北方潮土中有机物质的分解速率及其影响因素。潮土中有机物质的分解,除受气候条件影响外,还受土壤ＣａＣＯ３含量,水分及盐碱含量等因素的影响,并讨论了潮土有机质积累及提高潮土有机质含量的途径。     

STUDIES ON THE DECOMPOSITION OF PLANT MATERIAL IN SOIL

Soil samples taken during an experiment on the decomposition of ¹⁴C-labelled ryegrass in soil under field conditions (see Part I) were air-dried, irradiated, exposed to CHCl₃ or CH₃Br vapours, oven-dried or autoclaved. After these treatments the soils were inoculated, incubated, and the output of CO₂ measured. All these methods of partially (or, in some cases, completely) sterilizing soil rendered a small heavily labelled fraction of the soil organic matter decomposable. This fraction is postulated to be the soil biomass. Treatments involving heat or irradiation rendered small additional amounts of the soil organic matter decomposable (by processes other than the killing of organisms). Incubating unsterilized soil with partially sterilized soil did not decrease evolution of CO₂. This suggests that partial sterilization does not increase mineralization by destroying toxic substances that inhibit microbial growth, or by disturbing a host: predator balance in the unsterilized soil. The longer the labelled ryegrass was allowed to decompose in the field, the less labelled-CO₂ was evolved after partial sterilization. In contrast, the same amount of unlabelled-CO₂ was evolved from a soil that had been incubated 1 or 4 years with ryegrass. The labelled part of the biomass is considered to be largely zymogenic (with a half life of approximately 1.5 years), the unlabelled part largely autochthonous, remaining almost constant over the 3-year period. It is suggested that the size of the soil biomass can be roughly estimated from the size of the flush of CO₂ after CHCl₃ vapour treatment. Calculated on this basis, 2.3–3.5 Per cent the unlabelled-C in these soils (i.e. the C present in the soil before the labelled ryegrass was added) was in the biomass. Of the original ryegrass C added, 10–12 per cent was in the biomass after 1 year, decreasing to 4 per cent after 4 years.     

STUDIES ON THE DECOMPOSITION OF PLANT MATERIAL IN SOIL

The organic matter in soils containing decomposing ¹⁴C-labelled ryegrass was fractionated chemically. Earlier work on these soils had shown that they contained a small fraction, heavily labelled relative to the rest of the soil organic matter, that was mineralized when the partially sterilized soils were incubated. Reagents effective in extracting heavily labelled-C included cold o.in HC1, boiling saturated CaSO₄ solution, and o.in Ba(OH)₂, but neither these nor any other reagent tested could extract material as heavily labelled as that mineralized when partially sterilized soil was incubated. Reagents that extract heavily labelled-C are poor extractants for humified material and are not strongly hydrolytic: the more vigorous the hydrolysis the smaller the proportion of labelled-C in the hydrolysate. The amounts of labelled-C dissolved by Ba(OH)₂ from soils sampled after different periods in the field were directly proportional to the amounts of labelled-C mineralized by those soils when partially sterilized (by exposure to CHC1₃ vapour), inoculated and incubated. Balance sheets are presented for the distribution of labelled and unlabelled-C in fractions separated by hydrolysis with 6N HC1, by NaOH extraction, by neutral pyrophosphate extraction, and by oxidation with H₂O₂. The fraction remaining after hydrolysis with 6N HC1 was the most lightly labelled and had the widest C/N ratio. The percentage of labelled-C in the material dissolved by alkali or by pyrophosphate was little more than in the material not dissolved, despite the presence in the soil of fractions differing at least twenty-fold in intensity of labelling.     

USE OF A CHEMOSTAT TO STUDY DECOMPOSITION OF WHEAT STRAW IN SOIL SLURRIES

The breakdown of wheat straw in slurries of soil was studied under controlled physical conditions. In the absence of oxygen there was a reduction in the redox potential and pH of the slurries and there were concomitant increases in the concentrations in solution of total carbon compounds, acetic acid, iron and manganese. The temperature coefficients (Q₁₀) for these reactions and processes were between 2.5 and 2.9 but they did not take place in the presence of oxygen nor when nitrate was added. When no straw was mixed with the soil, similar changes took place but were much less pronounced and no acetic acid was formed.     

两种森林凋落物分解及其土壤效应的研究

本文对田林老山杉木林和常绿落叶阔叶混交林凋落物的分解状况、微生物数量及凋落物分解的土壤效应进行了初步研究。结果表明:经287d 杉木林和常绿落叶阔叶混交林凋落物的失重率地表样分别为23.8%和24.9%,埋置样分别为35.8%和37.2%;C:N 缩小地表样分别为41.0和32.4,埋置样分别为22.4和20.0。凋落物腐解过程中微生物数量明显上升,但冬季显著下降。凋落物腐解刺激相应土层土壤微生物增长,有机质含量和腐殖质 C,N 含量亦有提高。     

草木栖、麦秸和泥炭在黑土中腐解特点及对土壤肥力的影响

有机物料田间腐解试验表明,各地均以草木栖分解最快,麦秸次之,泥炭最慢;同一有机物料的分解速率,北部克山均比南部哈尔滨慢。腐殖化系数,各地均以泥炭最大,麦秸次之,草木栖最小;同一有机物料的腐殖化系数,北部克山均比南部哈尔滨大。三种有机物料处理黑土:1.提高了腐殖质的数量和品质,增加了养分贮量和保肥能力,泥炭处理优于其他处理。2.明显地提高了胶体复合有机炭和追加复合度,草木栖处理最好,麦秸次之,再次是泥炭。3.改善了胶体腐殖质结合形态,松结态显著增加,土壤腐殖质更加活化,复合胶体的特性也均得到了改善。     

覆膜对有机物料的腐解及土壤有机质特性的影响

砂滤管长期培养试验结果表明,有机物料和农肥施入土壤后,腐解残留率(Yt)与有机物料施入后的时间(t)遵循Yt=Y^λ.t_oe方程式,式中Yo为有机物料缓分解成分的碳理占加入总碳量的百分数,λ为缓解成分的分解速率。覆膜使Yt和Yo值明显降低。田间试验结果表明,覆膜降低了有机质的活性,使PQ值(胡敏酸占可提取腐殖酸的百分数)和A2/A3比值(胡敏酸在波长200nm和300nm处吸光度之比)增高,胡敏酸对光吸收增强。     

STUDIES ON THE DECOMPOSITION OF PLANT MATERIAL IN SOIL. IV. THE EFFECT OF RATE OF ADDITION

Different amounts of ryegrass roots and tops, both uniformly labelled with ^{1 4} C, were mixed with soil and allowed to decompose for 155 days under controlled conditions in the laboratory at 25°C. Initially the roots decomposed more slowly than the tops but by 155 days this difference had disappeared. About a third of the added plant C remained in the soil at the end of 155 days, about as much as when the same plant materials were incubated in the same soils for 6 months in the field. To a first approximation, the amount of labelled CO₂–C evolved was directly proportional to the amount of labelled plant C added. This held throughout the incubations. However, a slightly smaller percentage of the added plant C was evolved with small additions than with large, although this effect was on the limits of detection. Slightly more labelled plant C was retained in a soil rich in organic matter (2.43% C) than in an otherwise similar soil with less organic matter (0.97% C).     

利用δ13C方法研究添加玉米秸秆下红壤总有机碳和重组有机碳的分解速率

土壤有机碳分解速率是研究土壤碳动态变化的基础。本文利用13C自然丰度方法和同位素质谱分析技术,通过室内培育实验研究了红壤总有机碳和重组有机碳的分解速率,培养时间为180 d,培养温度为30℃。结果表明:在5%和10%秸秆用量下,红壤总有机碳的分解速率常数为8.2×10-4d-1~22.0×10-4d-1,而重组有机碳的分解速率常数为4.0×10-4d-1~15.6×10-4d-1。施用玉米秸秆明显地促进了红壤原有的总有机碳和重组有机碳的分解,施用量越多,原有机碳分解的越快,表明土壤中原有机碳的分解速率与进入到土壤中的新鲜有机碳量有关。     

石灰处理条件下土壤动物群落结构及其分解作用的响应

土壤动物在分解植物残体、改变土壤理化性质、促进土壤物质循环和转化过程中起着重要作用,同时土壤环境因子也影响着土壤动物的生存与活动。近年来国内外学者已从生理学、生态学、分子生物学等方面[1,2],研究了土壤动物与环境的密切关系,并采用试验模拟的方法,研究不同处理水平土壤动物群落的动态变化[3,4]。土壤pH是土壤动物群落的重要影响因子,对土壤动物的影响因环境条件的特异性而结果不尽一致[5,6],一般而言,酸碱度适宜的土壤环境土壤动物相对较丰富[7,8]。大兴安岭北部地区,由于气温低,降水少,物质分解缓慢,凋落物累积较多,构成火灾和虫灾的隐患。本研究试图通过人工处理方法,改变土壤微酸性环境,进而提     

秸秆碳的田间原位分解和微生物量碳的周转特征

应用14 C示踪技术研究了杂交狼尾草秸秆在稻麦轮作田中为期 1年的原位分解。结果表明 :秸秆用量对其分解率影响甚微 ,1年后秸秆C分解了 72 %左右 ,分解速率常数为 2 7× 1 0 - 3d- 1,但秸秆用量的多少与土壤原有碳的分解和土壤有机碳平衡密切相关。黄棕壤原有C年分解率为 5 4 5 %～ 6 0 7% ,分解速率常数在 1 0 4× 1 0 - 4～ 1 1 8× 1 0 - 4d- 1之间。随秸秆用量增加 ,黄棕壤原有C分解率和分解量均增加 ,土壤有机碳的亏缺减少。微生物量14 C占加入秸秆14 C的 3 79%～ 1 0 63% ,占土壤残留14 C的 1 2 2 7%～1 7 4 3% ,其大小变化及减少程度均较微生物量12 C显著。微生物量12 C约为微生物量14 C的 0 74～ 3 85倍 ,说明大多数情况下 ,土壤原有C仍是土壤微生物活动所需能量和养分的主要来源。微生物量14 C的周转率在 1 1 0～ 1 1 8a- 1之间 ,微生物量12 C的周转率在 0 97～ 1 0 6a- 1之间。增加秸秆用量可加快土壤微生物量C的周转速度 ,反过来微生物量C周转速度的加快又加速了秸秆C和土壤原有C的分解。土壤原有C和秸秆C的分解进程与微生物量12 C和微生物量14 C的动态变化趋势一致 ,说明有机碳分解的快慢是土壤微生物活动强弱的外在表现。     

FORMATION OF MICROBIAL BIOMASS DURING THE DECOMPOSITION OF 14C LABELLED RYEGRASS IN SOIL

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.     

用δ13C方法研究玉米秸秆分解期间土壤有机质数量动态变化

通过室内培养实验 ,应用δ1 3C方法研究了玉米秸秆分解期间 ,土壤中胡敏酸 (HA)和富里酸 (FA)数量的动态变化。结果表明 :培养期间 ,新加入的玉米秸秆以及原土有机C都减少 ,但后者分解速度较慢。培养初期 ,FA的形成速度大于HA ;随培养时间延长 ,FA转化为HA或相互转化。原土有机质中 ,HA、FA也发生了相互转化 ,但与新形成的HA、FA相比转化速度较慢。用δ1 3 C方法研究短期培养 (几个月～几年 )条件下新加入有机质在土壤中的分解动力学是可行的。     

旱地和水田有机碳分解速率的探讨与质疑

＾１４Ｃ标记的羊粪,杂交狼尾草及其根部分施于淋溶土中,在３０℃培养３６５天,土壤保持旱地和淹水两种状态,进行有机碳在土壤中分解的对比试验,另一组试验采用＾１４Ｃ标记的水稻秸和玉米秸和玉米分别施于老土和变性土中,在上述相同的水分条件下３０℃培养１１２天进行对比。所有加入的不同有机物料。不论是在何种土壤,其在淹水土壤中的有机碳分２解速率均快于旱地土壤,而前者的残留率却明显低于后者。淹水处理各种不同分解     

A VISUAL RECORD OF THE DECOMPOSITION OF 14C-LABELLED FRAGMENTS OF GRASSES AND RYE ADDED TO SOIL

The decomposition of grasses and rye labelled with ¹⁴C was studied using ground material and also fragments cut from intact leaves or roots either placed on the soil surface or buried in the soil incubated under various conditions. Autoradiography was used to observe the changes in the decaying plant tissue with a minimum of disturbance. Autoradiograms prepared before incubation and subsequently at intervals reveal an over-all fall in density of the images, a complete disappearance of ¹⁴C in small discrete sites, as well as a dispersion of ¹⁴C over distances of several cm from the plant residues. A photoelectric technique was devised by which changes in density could be expressed quantitatively. The log density of autoradiographic images of pellets of ground grasses shows a predominantly, though not completely, linear regression on time of incubation. The method has shown that the process of decomposition is very slow, that in the first stages of decomposition ryegrass decays faster than cocksfoot, and that ground material tends to behave in a different manner to fragments cut from intact tissues. Changes in the area of autoradiographic images with time of incubation could be used as an additional but less sensitive measure of the rate of decomposition. The participation of micro-organisms (especially fungi) in the breakdown of the plant tissues has been demonstrated by the presence of labelled organisms in the vicinity of plant residues. Labelled fungi are more numerous in the first 3 months of incubation, during which a marked fall in image density of the plant residues occurs. A further decrease in image density is frequently associated with the appearance of fungal resting structures with a greater concentration of ¹⁴C than the surrounding plant fragments. Because of their longevity these structures contribute to the fixing of ¹⁴C in a different fraction of the biomass. Faecal pellets of soil mesofauna also concentrate ¹⁴C and resist decomposition for very long periods of time.