Influence of drying and ageing on the stabilization of earthworm (Lumbricidae) casts

The influence of drying and ageing on the stabilization of casts produced by the endoge′ic earthworm, Aporrectodea rosea, from a soil, which was hard-setting and low in organic matter, were investigated in the laboratory. Casts and uningested soil were aged-most for up to 32 days, dried for up to 21 days, or subjected to different wetting and drying cycles over 30 days. The dispersion index of aged-moist casts decreased from 0.40 to 0.25 over 32 days, while dispersion index of dried casts decreased from 0.40 to 0.01 over 21 days. The dispersion index of air-dried casts was not significantly increased by five cycles of wetting and drying. The dispersion index of dried casts was not significantly less than that of dried soil. In soils wetter than a matric potential of approximately –35kPa, stabilization of casts was probably due to a combination of cohesion of soil particles, age-hardening and growth of microorganisms. However, in soils drier than –35kPa, cementation was probably the major mechanism of stabilization. The addition of wheat straw to the soil prior to ingestion by earthworms increased dispersion from aged-moist casts, but did not influence dispersion from dried casts. The addition of wheat straw decreased the number of air-dried casts which slaked severely. The concentration of soluble carbohydrate decreased with dispersion index as casts and uningested soil were each dried. This suggested that soluble carbohydrate may have been denatured with or without being bonded to soil particles during drying. Received: 7 May 1996     

Destabilization of soil during the production of earthworm (Lumbricidae) and artificial casts

The mechanisms by which soil is destabilized in the digestive tract of endogéic earthworms were investigated with artificial casts, which were moulded with a syringe from slurries of a silty loam with or without gypsum and organic matter treatments, and compared to casts produced by Aporrectodea rosea (Lumbricidae). Both types of casts generally had the same levels of mechanical dispersion, observations of slaking, and particle size distribution when the casts were fresh, aged-moist for 30 days, or air-dried. Fresh casts were significantly more dispersive than the uningested soil despite the addition of gypsum or organic matter to the soil. However, the dispersion from aged-moist or air-dried casts was not substantially greater than that of uningested soil. Air-drying was more effective than moist-ageing in increasing the stability of casts and uningested soil. The concentration of soluble carbohydrate was greater in artificial casts produced from soil treated with sheep dung or xanthan gum, and in earthworm casts produced from soil treated with xanthan, than in the uningested soil of the same treatments. An increase in the concentration of soluble carbohydrate was related to an increase in dispersion. An attempt was made to simulate the addition of mucus to soil in the digestive tract of earthworms, by the addition of sucrose or xanthan gum to the slurry during the production of artificial casts. The addition of xanthan, but not sucrose, to the slurry increased mechanical dispersion relative to that of the uningested soil in the fresh treatment. Although the production of artificial casts destabilized soil to the same degree as earthworm casts, the artificial casts did not simulate all chemical, biochemical, and microbiological aspects of digestion. Received: 24 November 1995     

干土效应对土壤生物组成及矿化与硝化作用的影响

将经过风干、过筛后的2种旱地红壤加水培养,并和新鲜土培养条件相比较,研究干土效应对土壤生物组成及矿化与硝化作用的影响.试验共4个处理(1)农田旱地风干土加水培养(RU);(2)农田旱地新鲜土培养(FU);(3)苗圃旱地风干土加水培养(RN);(4)苗圃旱地新鲜土培养(FN).结果表明红壤风干土加水预培养5 d后,细菌、放线菌、真菌数量比新鲜土显著增加(p＜0.01),细菌数量增加最为明显,农田旱地和苗圃旱地风干土处理分别是新鲜土的6.26倍和6.84倍,红壤风干土加水培养处理的微生物量碳、氮也随之增加.培养28 d后土壤中微生物数量趋于稳定,与预培养5 d时的数量相当或稍有下降,但风干后加水培养处理的微生物数量仍保持大于新鲜土的趋势(农田旱地的放线菌除外),微生物量碳、氮也存在同样的趋势.风干土加水培养后微生物数量的迅速增加,使得氮素矿化速度加快,由此导致NH+4-N量显著增加(p＜0.01),培养28 d后,NH+4-N量较预培养5 d时有所增加,且明显高于新鲜土培养处理;NO-3-N含量也增加,但新鲜土处理显著高于风干土处理.土壤风干处理对土壤自由生活线虫的影响比较大,农田旱地和苗圃旱地风干土加水培养28 d后,其自由生活线虫数量仅为新鲜土的16.0%和30.1%,显示风干土加水培养难以恢复土壤微型动物的数量.28 d的矿化和硝化培养试验结果显示,风干土加水培养处理的净矿化量和矿化率均高于新鲜土处理,苗圃旱地风干土处理的增量达到了显著水平(p＜0.05),但是硝化作用却刚好相反,农田旱地和苗圃旱地的净硝化量及硝化率均是新鲜土处理显著高于风干土处理(p＜0.05),其原因是对硝化作用起重要作用的硝化菌(氨氧化细菌和亚硝酸氧化细菌)数量在经历了风干过程后很难恢复到新鲜土水平.     

Effect of sample pretreatment on aggregate stability measured by wet sieving or turbidimetry on soils of different cropping history

The effects of cropping history (pasture or arable) and sample pretreatment (field-moist, air-dried or air-dried and then tension or vacuum rewetted) on aggregate stability as measured by wet sieving or turbidimetry were compared. When field-moist samples were used there was a tendency for aggregate stability, as measured by wet sieving, to decline with increasing time under arable cropping (i.e. decreasing soil organic matter content). Air-drying samples caused a pronounced decline in stability of soils from under arable management and as a consequence there was a marked decline in stability with increasing time under arable. Use of tension or vacuum rewetted samples resulted in high values of stability which were unaffected by cropping history. For turbidimetry, there was a marked decline in measured stability with increasing time under arable cropping when field-moist samples were used. Air-drying caused an increase in measured stability that was relatively greater for the less stable samples. In comparison with air-dried samples, tension and vacuum rewetting caused a decrease in stability values for relatively unstable soils. It is suggested that, upon air-drying (and contraction of aggregates), additional intermolecular associations were formed between soil constituents thus conferring greater stability on aggregates. This resulted in reduced dispersion (and the release of particles <0.04 mm in diameter) from the surfaces of aggregates and slaked aggregate fragments following rapid rewetting. As a consequence stability as measured by turbidimetry was increased by drying. For aggregates from a predominantly arable history, this stabilization was not great enough to prevent slaking occurring following rapid rewetting, with the formation of a large proportion of stabilized fragments <0.5 mm in diameter. The stability of these aggregates as measured by wet sieving was therefore decreased by drying.     

Effects of biomass and size of Millsonia anomala (Oligochaeta: Acanthodrilidae) on particle aggregation in a tropical soil in the presence of Panicum maximum (Gramineae)

Summary The present study assessed the effect of the tropical geophagous earthworm, Millsonia anomala, on the aggregate-size distribution of a sieved (2 mm), tropical ferruginous soil in the presence and absence of the perennial tropical grass Panicum maximum. The effect of two size groupings and graded biomass densities of M. anomala on soil aggregation was analysed in time and with depth in the containers within which the plants were grown. In the absence of earthworms, aggregation was rapid although limited (13.5% of soil as aggregates >2.0 mm), and probably arose from a combination of microbial activity and physical processes (interparticle bonding due to clays and other colloids). The roots of the test species contributed little to aggregation. In contrast, the effect of earthworm activity on soil aggregation was rapid and important. The effects of both biomass and, to a lesser extent, size were significant. After 79 days, aggregation reached a maximum with 3 g per container of small earthworms (ca. 59% of soil as aggregates >2.0 mm) and a minimum with 1 g per container of large earthworms (ca. 35% of aggregates >2.0 mm). Aggregation was considered to occur through three different mechanisms: (1) A rapid aggregation due to the interactions of colloids; (2) an intermediate aggregation due to a combination of unquantified processes related to earthworm activity (mucus secretion, development of fungus hyphae); (3) egestion of soil as earthworm casts which are stable macroaggregates.     

Fungal abundance and diversity in earthworm casts and in uningested soil

Earthworm casts and adjacent uningested soil from 30 different locations were compared to determine the abundance and diversity of fungal species. The casts contained larger fungal populations (g^-1 dry soil weight) and numbers of fungal species than the soil. Variations in these parameters between casts and soil were statistically significant (P=0.05). Fungal populations and the number of fungal species in casts and soil also varied significantly (P=0.05) between samples from different locations. A total of 27 fungal species were recorded from the casts and soil. Indices of dominance (0.084 casts; 0.14 soil) and general diversity (2.53 casts; 2.02 soil) demonstrated that the casts displayed more diverse fungal flora than the soil. The diversity of fungal species increased in earthworm casts after passing through the earthworm gut.     

干旱区盐碱地滴灌土壤基质势对土壤盐分分布的影

该文通过田间试验研究覆膜滴灌条件下土壤基质势对土壤盐分分布的影响.试验设3个水平的土壤基质势处理:-5 kPa(S1),-15 kPa(S2)和-25 kPa(S3),每个处理重复3次,按随机区组布置.试验结果表明:垄作覆膜滴灌条件下各盐分离子的迁移和分布特性不同.Na< 和Cl-.易被灌溉水淋洗,主要分布在湿润体的边缘,在滴头附近含量最低,远离滴头含量逐渐增大,土壤基质势越高,淋洗效果越明显,离子含量和土壤总盐量呈线性相关.Mg2 、Ca2 、HCO3含量受土壤基质势影响较小,在土壤剖面分布相对均匀,离了含量和土壤总盐量之间没有明显相关关系.盐渍化土壤中总盐分的淋洗主要受到作物播种时第一次大水量灌溉和苗期阶段较高土壤基质势控制的灌溉影响,作物生育中后期不同土壤基质势处理对土壤总盐分的淋洗影响较小.该研究结果对盐碱地水盐调控和灌溉制度的制定具有理论意义和应用价值.     

Mechanisms of stabilization of earthworm casts and artificial casts

Summary Fresh casts were collected from the earthworm species Aporrectodea caliginosa, and artificial casts were also made. The casts were subjected to ageing, drying-rewetting, and sterilization by hexanol vapour. Clay dispersion was determined, as a measure of the lack of stability of the casts. Two soils were used, the topsoil of a recently reclaimed polder soil in the Netherlands and the topsoil from a South Australian duplex soil. For both soils the fresh worm casts had higher dispersible clay than the artificial casts. During ageing, both types of casts became more stable. There are strong indications that this was mainly due to changes on the surface of the casts. Fungi developed on the surface of 6-day-old worm casts made of Australian soil. This gave a higher stability to the casts compared to artificial casts of the same age without fungal growth. With both types of casts, hexanol inhibited fungal growth on the surface of the casts, reducing the stabilizing effect of ageing. The fungus did not develop on Dutch soil casts until after 42 days, and the development of a higher stability with age was also less marked than in the Australian soil. When the casts were subjected to a drying and rewetting cycle before analysis, they became much more stable than the casts that were analyzed wet. The drying-rewetting cycle removed most of the differences between the worm casts and the artificial casts, and also removed any effects of ageing.Dedicated to the late Prof. Dr. W. Kühnelt     

Assessing the effects of air-drying and rewetting pre-treatment on soil microbial biomass,basal respiration,metabolic quotient and soluble carbon under Mediterranean conditions

Soil biochemical properties are useful indicators of soil quality as they are very sensitive to disturbance. Sample storage or pre-treatments could affect the results in these assays, which are normally determined on fresh samples, kept cold or frozen. The objectives of this study were to (i) evaluate the effect of air-drying or incubation of rewetted air-dried soil samples on microbial biomass carbon (MBC), basal soil respiration (BSR), qCO₂ and water soluble carbon (WSC), in soils from different locations, with different degradation status and sampling seasons, and (ii) assess if air-drying or incubation of rewetted air-dried soil samples is an accurate sample storage and pre-treatment procedure for these soil properties in soil quality evaluations under semiarid Mediterranean conditions. Our results showed that air-drying does not have the same effects on MBC, BSR, qCO₂ and WSC depending on the geographical situation and sampling date. It seems that the warmest and driest place and season show less variation when using air-dried soil samples, with values representative of those obtained under field-moist conditions. Short incubations (4, 8 and 12 days at 23 °C) provoked a general decrease in all properties, probably due to labile organic compounds depletion. Hence, air-dried soils can be used as part of soil quality analysis to estimate these biochemical properties in summer time in the semiarid region of South-East Spain, because they have not suffered severe affections. Moreover, MBC could also be determined using air-dried soil in the driest zones during all year. In contrast, estimations with incubated soil samples are not, in any case, representative of field-moist soil values.     

A comparison of aggregate stability and biological activity in earthworm casts and uningested soil as affected by amendment with wheat or lucerne straw

The mechanisms responsible for stabilization of earthworm casts were investigated in a laboratory study. Earthworms (Aporrectodea caliginosa) were fed soil or soil amended with either ground wheat straw or lucerne hay. Cast material and uningested soil material were incubated for 56 days, and changes in aggregate stability (measured by wet sieving), soil biological activity and macronutrient availability were measured periodically. In general, aggregate stability and microbial biomass C tended to increase during the incubation for both cast and soil material, whereas hot-water extractable carbohydrate content declined. For amended treatments, basal respiration rate and microbial metabolic quotient were large at the first sampling (7 days) but declined rapidly thereafter. There was a transitory increase in extractable P in fresh casts compared with uningested material, a sustained increase in mineral N concentrations but no change in exchangeable K content. For unamended treatments, the casts were less stable than soil material, but this difference diminished during incubation. Drying aggregates before analysing them did not reverse this trend. The casts contained more microbial biomass C than soil material did, but the basal respiratory rate, respiratory quotient and hot-water extractable carbohydrate content were less. By contrast, for wheat- and lucerne-amended treatments casts were more stable than soil material, and the microbial biomass was less at all sampling times. For wheat treatments, respiratory quotient and hot-water extractable carbohydrate content were larger for cast than soil material, but the opposite was the case for lucerne treatments. We attribute the stability of casts in amended treatments to the intimate mixing of part-decomposed organic fragments with comminuted soil particles, binding by microbial mucilage associated with the organic fragments and linking and binding by fungal hyphae.     

滴灌调控土壤水分对马铃薯生长的影响

研究了滴灌灌溉频率和土壤水势对马铃薯生长和水分利用效率的影响。研究结果表明，滴灌灌溉频率和土壤水势对土壤水分的分布有很大影响，灌水频率越低，灌水前的表层土壤干燥的范围越大，灌水后的土壤湿润范围越大；控制滴头下面20 cm处土壤水势明显影响到50 cm深度以上的土壤水势，20 cm深度处土壤水势越高，50 cm深度范围内的平均土壤水势越高；土壤表面土壤水势越低，以滴头为中心形成的干燥范围越大。当土壤基质势低于-45 kPa时，马铃薯的块茎膨大率会迅速下降，总产量、商品薯产量和水分利用效率高低顺序为：-25 kPa>-35 kPa>-15 kPa>-45 kPa>-55 kPa。不同灌溉频率下马铃薯的总产量、商品薯产量和水分利用效率的高低顺序为：1天1次>2天1次>3天1次>4天1次>6天1次>8天1次。就华北地区而言，采用滴灌对马铃薯进行灌溉，土壤基质势以-25 kPa左右为好，灌水频率以每天1次最优。     

A porous-matrix sensor to measure the matric potential of soil water in the field

The matric potential of soil water is probably the most useful assessment of soil water status. However, the water‐filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to ?85 kPa. In this paper, we report the development of a porous‐matrix sensor to measure matric potential in the approximate range ?50 to ?300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water‐filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than ?80 kPa. The porous‐matrix sensors appeared to give reliable readings of matric potential in soil down to ?300 kPa and also responded appropriately to repeated wetting and drying. This porous‐matrix sensor has considerable potential to help understand plant responses to drying soil.     

风干土保存时间和湿土培育时间对黄淮海平原潮土酶活性的影响

以经历18年不同施肥管理的土壤为研究对象,阐明它们经过4个不同时间保存或处理后的土壤脲酶、转化酶、脱氢酶、及FDA酶活性的动态变化。处理包括:风干保存30天或鲜土状态、风干保存210天、风干土湿润至田间持水量(25℃)条件下分别培育15天和51天;同时评估这些酶活性的变化程度与土壤本身有机碳含量之间的关系。结果表明,风干土保存时间和风干土湿润后短期培育均对脲酶活性影响很小,但风干土湿润培养51天后其活性则显著降低;随风干土保存时间延长,转化酶活性显著降低;与鲜土相比,风干土湿润培养15天后,脱氢酶活性显著提高,但继续湿润培养至51天后,其活性又降至与鲜土相当,因此风干土湿润培育一定时间后测定的脱氢酶活性可用来代表其田间自然湿度时的状态;FDA酶活性的变异程度最大,与其从鲜土状态至风干状态的活性急剧下降有关。土壤本身有机碳含量与脲酶和脱氢酶的活性变化程度成显著负相关关系,说明土壤有机碳含量是决定它们随环境条件改变而变化的主要因素之一。另外,土壤NH4+-N、NO3--N和可溶性有机碳含量对上述4种处理的响应程度也存在差异。其中风干状态土壤经湿润培育处理后,NH4+-N含量呈先降后升趋势,正好与脲酶活性变化趋势相反;而NO3--N含量整体上呈上升趋势,可溶性有机碳含量则正好相反。     

Burrowing ability of the earthworm Aporrectodea longa limited by soil compaction and water potential

Summary Adult earthworms (Aporrectodea longa) were maintained for 199 days in soil columns (h=30 cm; ø=10 cm) where the water potential ranged from -7 to-65 kPa and compaction from 50 to 350 kPa. The weight of casts on the soil surface was measured at the end of two periods of activity (spring and autumn). Cast production increased with bulk density, but the activity of earthworms was limited both by the mechanical strength of the soil and by decreasing water potential. The results obtained in the laboratory conditions of this study were consistent with field observations on casting and burrowing activities. The effects of water potential and soil compaction on these activities were estimated.     

Rainfall impact effects on ageing casts of a tropical anecic earthworm

We investigated the erodibility of surface casts produced by an anecic earthworm of the Colombian savannahs by means of indoor rain simulations. The kinetic energy applied to samples, 21.62 J minute⁻¹ m⁻², was estimated to be equivalent to 41% of the energy of the more intense period of a local storm. The erodibility of casts was assessed at different stages of their ageing along with the effects of repeated wetting‐drying cycles. Bare soil cores and soil cores of the same size with a cast on their surface (soil + cast) were used as controls. Saturated hydraulic conductivity (HC) was measured to test whether casts enhance soil water permeability. Fresh, almost liquid, individual casts were completely dispersed by a 2‐hour rainfall simulation at a dispersion rate of 0.9 ± 0.5% of sample minute⁻¹. After 5 hours drying at 32°C and 79% relative humidity, casts were as moist as fresh casts (non‐significant Mann–Whitney U‐test) but had a solid appearance and were no longer dispersed by raindrop impact. Under simulated rainfall, dry casts were very slowly fragmented into large aggregates (> 5 mm). The HC of casts was increased by repeated cycles of wetting and drying but unaffected by length of air‐drying and natural ageing, and averaged 7 (3, n = 6), 20 (3, n = 7) and 32 (1, n = 45) cm hour⁻¹ for bare soil, soil + cast and all casts, respectively (standard error, sample size). Except for bare soil, these values were greater than the greatest rainfall intensities recorded locally. The consequences for soil erosion, nutrient losses and water infiltration are discussed.     

Soil ciliates (Protozoa: Ciliophora) from evergreen rain forests of Australia, South America and Costa Rica: diversity and description of new species

This paper provides, for the first time, comprehensive data on alpha diversity of soil ciliates from evergreen tropical and temperate rain forests. Thirty-three samples were collected in Australia, Tasmania, Amazonia and Costa Rica and analysed with the non-flooded Petri dish method, which reactivates the ciliates’ resting cysts from air-dried samples. The 175 taxa found contained 34 new species, 4 of which are described in this paper, viz. Platyophrya paoletti n. sp., Lamtostyla abdita n. sp., L. granulifera n. sp., and Apoamphisiella tihanyiensis (Gellért and Tamás 1958) n. gen., n. comb. Although this is a considerable number, it is much lower than one would expect. The data would be even more perplexing if the four rich samples (up to 90 species/sample) from the Manaus floodplain were excluded. We then would be confronted with about 90 taxa in 29 samples, of which 13 contained fewer than ten species. A hypothesis is put forward that the non-flooded Petri dish method is inappropriate for studying soil ciliate diversity in evergreen rain forests because most species have a reduced capacity to produce dry-resistant (protective) resting cysts due to the permanent wetness of their habitats. This view is supported by a comparative analysis of a fresh (containing 40 species) and air-dried/rewetted (2 species only) sample from a cloud rain forest near Merida (Venezuela), and the observation that the capacity of soil ciliates to produce resting cysts often dramatically decreases after prolonged laboratory cultivation in liquid media. Direct microscopy of fresh samples seems to be an appropriate alternative because specimens can be easily collected due to their considerable abundance (≥1000 individuals/g wet mass of litter). Received: 8 April 1997     

Assessing air-drying and rewetting pre-treatment effect on some soil enzyme activities under Mediterranean conditions

Soil enzyme activities are useful indicators of soil quality as they are very sensitive to disturbance. Sample storage or pre-treatments could affect the results in these assays, which are normally determined in fresh samples, kept cold or frozen. The objectives of this study were to (i) evaluate the effect of air-drying or air-drying and rewetting on β-glucosidase, acid phosphatase and urease activities in soils from different locations, degradation status and sampling seasons, and (ii) assess if air-drying or air-drying and rewetting is an accurate sample storage and pre-treatment procedure for enzyme activities in soil quality evaluations under semiarid Mediterranean conditions. Our results showed that urease, phosphatase and β-glucosidase activities were hardly affected by air-drying of degraded and non-degraded soils from the two locations studied in all seasons. Short incubations (4, 8 and 12 d at 23 °C) of rewetted air-dried soil at 55% of water-holding capacity showed different patterns depending on the enzyme studied. Urease and β-glucosidase activities were relatively stable during incubation, with several significant (P<0.05) shifts up and down in some soils and samplings. However, acid phosphatase showed an increase in activity with incubation, of between 5% and 50% relative to air-dried samples. These increases followed no pattern and were unrelated to soil characteristics or sampling date. Hence, urease, phosphatase and β-glucosidase activities determined in air-dried soil samples seem to be representative of those obtained under field-moist conditions. In contrast, short incubations of rewetted soil samples can produce fluctuations in these enzyme activities, mainly of acid phosphatase, and estimations in these conditions are not so representative of field-moist soil values.     

Effect of Hormogaster elisae (Oligochaeta; Hormogastridae) on the stability of soil aggregates

The production and stability of soil aggregates produced by laboratory cultures of the endogeic earthworm Hormogaster elisae was studied using three different techniques: the determination of the soil mean weight diameter (MWD), the aggregate tensile strength, and by the Blanchart method, which involves three different tests. The MWD index of soils was higher in microcosms containing H. elisae. Tensile strength was significantly higher in earthworm casts than in naturally formed aggregates. The Blanchart method also showed aggregates produced by earthworms to be more stable. The results of all three methods concur in that aggregates produced by H. elisae are larger and more stable than those produced in control microcosms without earthworms.     

The effects of soil structure on protozoa in a clay-loam soil

Physical disruption of a clay-loam soil by slaking, grinding and compaction was used to determine the specific effects of soil structure on soil protozoa. Individual air-dry aggregates (1–2 cm diameter) were wetted slowly, or had their structure disrupted by slaking or grinding. They were then moistened with nutrient solution and incubated, at a matric potential of ?10 kPa. The nutrient solution had to supply at least 400 μg C g^?1 dry soil before protozoan populations increased measurably. Numbers of ciliate protozoa were enhanced by grinding, regardless of the amount of nutrient added. The moisture content and, therefore, the final nutrient concentration of the disrupted aggregates, changed markedly and could account for the observed increase in protozoan biomass from slowly wet to slaked to ground aggregates. There were no differences in protozoan biomass when the applied nutrient concentration was adjusted so that all treatments contained the same amount of nutrient. Soil cores were prepared from sieved (2–4 mm diameter), ground and ground/compacted soil. Thin sections revealed that the pore space accessible to protozoa decreased in these three treatments from 24% to 13% to 9%, respectively. Protozoan biomass was unaffected by grinding but showed a greater than 30-fold decrease following compaction that could not be accounted for solely by the reduced pore space. Grinding and compacting could have favoured anaerobic conditions in the core which would have reduced protozoan activity. Soil structure had no measurable direct effect on protozoan populations, but it had a much greater indirect effect through its influence on moisture content and aeration status.     

Effects of mixing and various temperature regimes on the respiration of fresh and air-dried coniferous raw humus materials

O₂ uptake of fresh and air-dried L. F and H layer materials from a coniferous-forest floor was studied in the laboratory under regimes of progressively-increasing temperature and also at constant temperature. Experiments were carried out on separate materials from each layer and on mixtures of these materials. A marked apparent “priming” of microbial activity was found when previously air-dried materials were mixed and then rewetted. Some evidence of “priming” was also found when fresh L. F and H layer materials which had not been air dried were mixed. There were large differences between the O₂ uptake patterns of the fresh and air-dried materials when incubated in the same manner. Microbial activity in fresh materials was found to be stimulated when the same number of accumulated heat units were provided under a regime of increasing temperatures as compared to a regime of constant temperature. Results of the respiration studies are discussed in the light of possible differences in species diversity of the soil microbial population resulting from treatment of these organic horizon materials.