Population and biomass of microorganisms in soils of pyrogenic succession in the northern taiga pine forests

In the organic horizons of the Al-Fe-humus podzols under the old pine forests of the northern taiga, the biomass of all the groups of microorganisms, the length of the fungal and actinomycete mycelium, the number of fungal spores, and the bacterial population were maximal (13 mg/g) irrespectively of the stage of pyrogenic succession. The share of fungi (mainly, of basidiomycetes) exceeded 90%. In the mineral root-inhabited soil horizons, the biomass of microorganisms was not greater than 1.0 mg/g. The soil under the lichen pine forest had the smallest biomass of microorganisms as compared to the soil under the pine forests that were not exposed to fire for a long time. At all the stages of the pyrogenic succession, the most favorable conditions for the functioning of microorganisms were in the root-inhabited horizons of the soils in near-stem sites due to the accumulation of nutrients there. In the soils of these zones, the basidiomycete biomass was greater than that in the soils of the gaps. In the mineral soil horizons, buckleless micromycetes demonstrated the same trend. No distinct parcella differences, with respect to the soil nutrient regime, were found only for the prokaryotes. The fungi in the Al-Fe-humus podzols may be used as indicators for the pyrogenic succession stages of forest ecosystems. At the early stages, micromycetes without buckles prevailed, and, in the course of succession, the share of basidiomycetes clearly increased. The density and structure of mycorrhiza were tightly related to the nutrient regime of the soils. The increase in the concentration of available biogenic elements in the root-inhabited soil horizons did not cause the necessity of developing complex mycorrhiza forms.     

Downed logs improve soil properties in old-growth temperate forests of northern Iran

Dead trees, particularly downed logs, play an important role in the dynamics of forest ecosystem. Contribution of decaying wood to C and nutrient pools of forest soils depends on the tree species and degree of wood decay. However, the extent to which the downed logs affect the soil properties of temperate forests has rarely been evaluated. In this study, a mixed beech forest was selected in Liresar region of Mazandaran Province, northern Iran, to investigate if and how the presence of downed logs affected soil quality and function by comparing soils underneath degraded logs and nearby soils of the two dominant tree species(beech and hornbeam). We then explored how these effects occurred as downed logs decomposed by comparing the woods of both tree species at four degrees of decomposition. Degree of decay of downed logs was classified into four classes(DC1–DC4). Eight dead trees of each tree species were selected at the center of each sample plot. Three composite soil samples underneath each decaying log and 100 cm away from a decaying log were collected at two soil depths(0–15 and 15–30 cm) to analyze soil main physicochemical properties and microbial activity. The results revealed that downed logs affected soil physical(5% wetter than control soils), chemical(2% lower pH, 100% increase in organic C and total N in the case of hornbeam, and 2% increase in P), and biological characteristics(soil microbial respiration enhanced by 10%, and microbial biomass C 620 and 351.5 mg kg~(-1) and microbial biomass N 66.47 and 32.18 mg kg~(-1), respectively, in the cases of beech and hornbeam), thus resulting in significantly different soil microsites from those without downed logs. Presence of downed logs increased soil microbial activity and soil fertility as wood decayed. Thus, the presence of downed logs is an important factor influencing forest soils and should be taken into consideration in forest management practices.     

Silicon concentrations in soil and bark in Irish Sitka spruce forests

The relationship between plant‐available silicon (Si) soil concentrations and bark Si concentrations in coniferous species is poorly understood. The objectives of this research were to generate baseline data on Si concentrations in soils and bark of Sitka spruce (Picea sitchensis) seedlings in Ireland and to understand better the relationship between soil and bark Si concentrations. Seedlings were harvested from eight plantation forestry sites and two tree nurseries, and Si concentrations in the bark tissue as well as plant‐available Si concentrations in soils (CaCl₂ extractant) were measured. Bark Si concentrations varied significantly between sites and were lowest [mean 790 (± 242 SD) mg kg^?1 dry plant tissue] on acidic, organic rich peat soils, while the highest Si concentrations occurred in seedlings [mean 3688 (± 633 SD) mg kg^?1 dry plant tissue] grown on soils with low C concentration and higher pH values (≈ 4.5 to 5.5 in H₂O). Plant‐available Si soil concentrations were not related to soil C concentrations. There was a negative (but statistically not significant) relationship between plant–soil concentrations and soil pH. A significant negative relationship was observed between plant‐available soil Si concentrations and bark Si concentrations, which may be related to the presence of soil from mixed soil horizons forming the mounds that seedlings were planted on. Uptake and sequestration of Si by seedlings may have been related to the rate of growth of the seedlings, as bark Si concentrations were highest on sites that were expected to have greater seedling growth rates. The negative relationship between bark and plant‐available Si soil concentrations suggest that uptake of Si by Sitka spruce is rejective at higher concentrations.     

Activity and biomass of soil microorganisms at different depths

We measured microbial biomass C and soil organic C in soils from one grassland and two arable sites at depths of between 0 and 90 cm. The microbial biomass C content decreased from a maximum of 1147 (0–10 cm layer) to 24 g g^-1 soil (70–90 cm layer) at the grassland site, from 178 (acidic site) and 264 g g^-1 soil (neutral site) at 10–20 cm to values of between 13 and 12 g g^-1 soil (70–90 cm layer) at the two arable sites. No significant depth gradient was observed within the plough layer (0–30 cm depth) for biomass C and soil organic C contents. In general, the microbial biomass C to soil organic C ratio decreased with depth from a maximum of between 1.4 and 2.6% to a minimum of between 0.5 and 0.7% at 70–90 cm in the three soils. Over a 24-week incubation period at 25°C, we examined the survival of microbial biomass in our three soils at depths of between 0 and 90 cm without external substrate. At the end of the incubation experiment, the contents of microbial biomass C at 0–30 cm were significantly lower than the initial values. At depths of between 30 and 90 cm, the microbial biomass C content showed no significant decline in any of the four soils and remained constant up to the end of the experiment. On average, 5.8% of soil organic C was mineralized at 0–30 cm in the three soils and 4.8% at 30–90 cm. Generally, the metabolic quotient qCO₂ values increased with depth and were especially large at 70–90 cm in depth.     

Contribution of fine roots to ecosystem biomass and net primary production in black spruce, aspen, and jack pine forests in Saskatchewan

Fine root (<2 mm) processes contribute to and exhibit control over a large pool of labile carbon (C) in boreal forest ecosystems because of the high proportion of C allocated to fine root net primary production (NPP), and the rapid decomposition of fine roots relative to aboveground counterparts. The objective of this study was to determine the contribution of fine roots to ecosystem biomass and NPP in a mature black spruce (Picea mariana Mill.) (OBS), aspen (Populus tremuloides Michx.) (OA), and jack pine (Pinus banksiana Lamb.) (OJP) stand, and an 11-year-old harvested jack pine (HJP) stand in Saskatchewan. Estimates of fine root biomass and NPP were obtained from nine minirhizotron (MR) tubes at each of the four Boreal Ecosystem Research and Monitoring Sites (BERMS). Fine root data were collected once a month for May–September in 2003 and 2004. Additional C biomass and NPP data for various components of the forest stands were obtained from Gower et al. (1997) and Howard et al. (2004). Annual fine root biomass averaged 3.10 ± 0.89, 1.71 ± 0.49, 1.62 ± 0.32, and 2.96 ± 0.67 Mg C ha⁻¹ (means ± S.D.) at OBS, OA, OJP, and HJP, respectively, comprising between 1 and 6% of total stand biomass. Annual fine root NPP averaged 2.66 ± 0.97, 2.03 ± 0.43, 1.44 ± 0.43, and 2.16 ± 0.81 Mg C ha⁻¹ year⁻¹ (means ± S.D.) at OBS, OA, OJP, and HJP, respectively, constituting between 41 and 71% of total stand NPP. Results of this study indicate that fine roots produce a large amount of C in boreal forests. It is speculated that fine root NPP may control a large amount of labile C-cycling in boreal forests and that fine root responses to environmental and anthropogenic stress may be an early indicator of impaired ecosystem functioning.     

Productivity increase in Northern Austria Norway spruce forests due to changes in nitrogen cycling and climate

We investigated the long‐term temporal trend in growth rate, soil chemical status, and nutrient content of needles of two Norway spruce (Picea abies) stands in the Bohemian Massif, Austria. The aim was to quantify changes in the site productivity over the last four decades as a consequence of the enriching effect of N deposition, rising CO₂ levels in the atmosphere, and changes in forest‐management practices. We used the data records of control plots from forest‐amelioration experiments that have been monitored for more than four decades. Both stands showed increased growth rates and a large deviation from the growth pattern of earlier applicable yield tables. The nutrient levels in the foliage remained unchanged and neither suggested luxury consumption nor nutrient imbalances. Results from soil chemistry analysis were inconclusive in respect of changes in soil conditions: an enrichment of the mineral soil with N and a decrease in the C : N ratio. Changes were confined to the uppermost part of the soil profile. Our data support the hypothesis that the sites are in a steady process of aggradation and that site productivity is rising.     

Nutrient limitations to soil microbial biomass and activity in loblolly pine forests

We performed an assay of nutrient limitations to soil microbial biomass in forest floor material and intact cores of mineral soil collected from three North Carolina loblolly pine (Pinus taeda) forests. We added solutions containing C, N or P alone and in all possible combinations, and we measured the effects of these treatments on microbial biomass and on microbial respiration, which served as a proxy for microbial activity, during a 7-day laboratory incubation at 22 °C. The C solution used was intended to simulate the initial products of fine root decay. Additions of C dramatically increased respiration in both mineral soil and forest floor material, and C addition increased microbial biomass C in the mineral soil. Additions of N increased respiration in forest floor material and increased microbial biomass N in the mineral soil. Addition of P caused a small increase in forest floor respiration, but had no effect on microbial biomass.     

Effects of silver nanoparticles on soil microorganisms and maize biomass are linked in the rhizosphere

Silver nanoparticles hold great promise as effective anti-microbial compounds in a myriad of applications but may also pose a threat to non-target bacteria and fungi in the environment. Because microorganisms are involved in extensive interactions with many other organisms, these partner species are also prone to indirect negative effects from silver nanoparticles.Here, we focus on the effects of nanosilver exposure in the rhizosphere. Specifically, we evaluate the effect of 100 mg kg⁻¹ silver nanoparticles on maize plants, as well as on the bacteria and fungi in the plant's rhizosphere and the surrounding bulk soil. Maize biomass measurements, microbial community fingerprints, an indicator of microbial enzymatic activity, and carbon use diversity profiles are used. Hereby, it is shown that 100 mg kg⁻¹ silver nanoparticles in soil increases maize biomass, and that this effect coincides with significant alterations of the bacterial communities in the rhizosphere. The bacterial community in nanosilver exposed rhizosphere shows less enzymatic activity and significantly altered carbon use and community composition profiles. Fungal communities are less affected by silver nanoparticles, as their composition is only slightly modified by nanosilver exposure. In addition, the microbial changes noted in the rhizosphere were significantly different from those noted in the bulk soil, indicated by different nanosilver-induced alterations of carbon use and community composition profiles in bulk and rhizosphere soil.Overall, microorganisms in the rhizosphere seem to play an important role when evaluating the fate and effects of silver nanoparticle exposure in soil, and not only is the nanosilver response different for bacteria and fungi, but also for bulk and rhizosphere soil. Consequently, assessment of microbial populations should be considered an essential parameter when investigating the impacts of nanoparticle exposure.     

Environmental effects on soil NO concentrations and root N uptake in beech and spruce forests

This study aimed to investigate the shifts in net nitrogen (N) uptake and N compounds of fine roots over the vegetation period (i.e., spring, summer, autumn) and correlate this with NO concentration in the soil. Soil NO concentration was measured using gas lysimeters for collection and a chemiluminescence analyzer for quantification. Net N uptake by the roots was determined using the ¹⁵N enrichment technique. N pools were quantified using spectrophotometric techniques. Soil NO concentrations at beech and spruce forest sites were highest in spring (June), and lowest in winter (December). Total N of the roots was similar during the seasons and between the two years under study despite considerable variation of different N compounds. Net N uptake generally increased with higher N supply. Correlation analysis revealed a positive relationship between soil NO concentration and net N uptake only for spruce trees. This relationship seemed to be modulated by environmental factors and tree species.     

Population dynamics of soil microorganisms associated with fungicide-dusted caladium seedpieces

The effects of three fungicidal dusts (captan ; a mixture of benomyl, thiram and streptomycin sulfate; and a mixture of chloroneb, thiram and streptomycin sulfate) on the short-term populations dynamics of fungi and bacteria associated with caladium seedpieces planted in raw muck soil were investigated. Both germinating seedpieces and seedpieces which were decomposing after the removal of eyes were used. To determine microbial populations, dilutions of a comminuted suspension of a seed-piece and surrounding soil were plated on selective media at 0,2,4, 8 and 12 weeks after seedpieces were planted. Addition of seedpieces to soil resulted in increases in populations in the following approximate chronological order: fluorescent Pseudomonas spp, Pythium spp, total bacteria, Fusarium spp and other genera of fungi. Most organisms assayed reached higher populations on decomposing than on germinating seedpieces. The effects of the fungicidal dusts on populations of microorganisms associated with seedpieces were not related to changes in populations that followed incorporation of the dusts into soil without seedpieces. The fungicidal dusts reduced total fungal population increases on germinating, but not on decomposing, seedpieces. Bacterial population increases were similar for all treatments of germinating seedpieces, but were prolonged on decomposing, fungicide-dusted seedpieces, as compared to controls.     

Microbial respiration and biomass (substrate-induced respiration) in soils of old-growth and regenerating forests on northern Vancouver Island,British Columbia

In studying the basal respiration, microbial biomass (substrate-induced respiration, SIR), and metabolic quotient (qCO₂) in western red cedar (Thuja plicata Donn ex D. Don)-western hemlock [(Tsuga heterophylla Raf.) Sarg.] ecosystems (old-growth forests, 3- and 10-year-old plantations) on northern Vancouver Island, British Columbia, Canada, we predicted that (1) soil basal respiration would be reduced by harvesting and burning, reflecting the reduction in microbial biomass and activities; (2) the microbial biomass would be reduced by harvesting and slash-burning, due to the excessive heat of the burning or due to reduced substrate availability; (3) microbial biomass in the plantations would tend to recover to the preharvesting levels with growth of the trees and increased substrate availability; and (4) microbial biomass measured by the SIR method would compare well with that measured by the fumigation-extraction (FE) method. Decaying litter layer (F), woody F (Fw) and humus layer (H) materials were sampled four times in the summer of 1992. The results obtained supported the four predictions. Microbial biomass was reduced in the harvested and slash-burned plots. Both SIR and FE methods provided equally good estimates of microbial biomass in the samples [SIR microbial C (mg g^-1)=0.227+0.458 FE microbial C (mg g^-1), r=0.63, P=0.0001] and proved suitable for microbial biomass measurements in this strongly acidic soil. Basal respiration was significantly greater in the old-growth forests than in the young plantations (P<0.05) in both F and H layers, but not in the Fw layer. For the 3- and 10-year-old plantations, there was no difference in basal respiration in F, Fw, and H layers. Basal respiration was related to changes in air temperature, precipitation, and the soil moisture contant at the time of sampling. The qCO₂ values were higher in the old-growth stands than in the plantations. Clear-cutting followed by prescribed burning did not increase soil microbial respiration, but CO₂ released from slash-burning and that contributed from other sources may be of concern to increasing atmospheric CO₂ concentrations.     

Influence of straw size on activity and biomass of soil microorganisms during decomposition

This study relates to the pattern of activity and biomass of soil microorganisms due to varying residue particle sizes during incubation. Wheat straw (8 t ha^–1) of different sizes (powdered, 0.9 cm, 1.8 cm, 2.9 cm and 4.4 cm) was incubated for 90 days at 50% water holding capacity in a loamy sand soil of Typic Camborthid. Dehydrogenase activity, an indicator of the total microbial activity, and microbial biomass were influenced by straw sizes during incubation. The peak dehydrogenase activity was recorded 21 days after incorporation of residue and it was highest in the powdered straw and decreased with increase in the straw length. The maximum biomass C build up was observed between 15 (< 1 cm) and 45 (> 1 cm) days after incorporation. The C:N ratio in the soil after 90 days of residue incorporation varied, with increase in straw size, between 12.1:1 and 20.8:1. The results reveal that for faster decomposition the length of the wheat straw should not exceed 1 cm.     

Plant community composition affects the biomass, activity and diversity of microorganisms in limestone grassland soil

The diversity and functional type of plants can affect the microbial biomass in the soil, its respiratory activity and the diversity of its bacterial population. We have studied these effects in microcosms of reconstituted limestone grassland containing (i) a 12‐species mixture of graminoids and forbs, (ii) a monoculture of the sedge Carex flacca, (iii) a monoculture of the grass Festuca ovina, and (iv) similar soil without plants. Microbial biomass was significantly greater in soil under monocultures of F. ovina than in the other microcosms. Basal respiration was largest in the F. ovina and mixed‐species treatments where values were more than double those in the C. flacca and bare soil microcosms. The basal respiration was strongly linearly related to plant productivity (r = 0.89). Analysis of the active bacterial population by denaturing gradient gel electrophoresis of 16S rRNA revealed its diversity to be significantly greater in the C. flacca and bare soil treatments than in the F. ovina or mixed‐species microcosms. This suggests that the functional type of plants has a strong influence on the composition of the bacterial community. We hypothesize that the discriminating functional attribute leading to a reduction of bacterial diversity in these microcosms was the presence in the F. ovina and mixed‐plant communities of an active arbuscular–mycorrhizal mycelium that is absent from bare soil and monocultures of C. flacca.     

Plant canopy effects on litter accumulation and soil microbial biomass in two temperate forests

The objective of this study was to determine whether differences in canopy structure and litter composition affect soil characteristics and microbial activity in oak versus mixed fir-beech stands. Mean litter biomass was greater in mixed fir-beech stands (51.9t ha⁻¹) compared to oak stands (15.7t ha⁻¹). Canopy leaf area was also significantly larger in mixed stands (1.96m² m⁻²) than in oak stands (1.73m² m⁻²). Soil organic carbon (C _org) and moisture were greater in mixed fir-beech stands, probably as a result of increased cover. Soil microbial biomass carbon (C _mic), nitrogen (N _mic), and total soil nitrogen (N _tot) increased slightly in the mixed stand, although this difference was not significant. Overall, mixed stands showed a higher mean C _org/N _tot ratio (22.73) compared to oak stands (16.39), indicating relatively low rate of carbon mineralization. In addition, the percentage of organic C present as C _mic in the surface soil decreased from 3.17% in the oak stand to 2.26% in the mixed stand, suggesting that fir-beech litter may be less suitable as a microbial substrate than oak litter.     

Earthworms increase soil microbial biomass carrying capacity and nitrogen retention in northern hardwood forests

Earthworms have been shown to produce contrasting effects on soil carbon (C) and nitrogen (N) pools and dynamics. We measured soil C and N pools and processes and traced the flow of ¹³C and ¹⁵N from sugar maple (Acer saccharum Marsh.) litter into soil microbial biomass and respirable C and mineralizable and inorganic N pools in mature northern hardwood forest plots with variable earthworm communities. Previous studies have shown that plots dominated by either Lumbricus rubellus or Lumbricus terrestris have markedly lower total soil C than uncolonized plots. Here we show that total soil N pools in earthworm colonized plots were reduced much less than C, but significantly so in plots dominated by contain L. rubellus. Pools of microbial biomass C and N were higher in earthworm-colonized (especially those dominated by L. rubellus) plots and more ¹³C and ¹⁵N were recovered in microbial biomass and less was recovered in mineralizable and inorganic N pools in these plots. These plots also had lower rates of potential net N mineralization and nitrification than uncolonized reference plots. These results suggest that earthworm stimulation of microbial biomass and activity underlie depletion of soil C and retention and maintenance of soil N pools, at least in northern hardwood forests. Earthworms increase the carrying capacity of soil for microbial biomass and facilitate the flow of N from litter into stable soil organic matter. However, declines in soil C and C:N ratio may increase the potential for hydrologic and gaseous losses in earthworm-colonized sites under changing environmental conditions.     

The response of soil microbial biomass and activity of a Norway spruce forest to liming and drought

Long-term effects of liming and short-term effects of an experimentally induced drought on microbial biomass and activity were investigated in samples from the O-layer (Of/Oh) and uppermost mineral soil (0—10 cm) in a spruce forest near Schluchsee (Black Forest, South-West Germany). Seven years after lime application a marked increase of pH values was restricted to the O-layer. The contents of C and N in the O-layer of the limed plot appeared to be lower, whereas in the A-horizon from the limed plot the contents of C and N appeared to be higher than on the control. However, these differences were statistically not significant due to a distinct spatial variability of topsoil conditions. On the limed plots C_mic, N_mic, and P_mic in the O-layer were lower in comparison to the control whereas differences in the A-horizon were negligible. In both sampling depths of the limed plot protease activity was higher while N-mineralization was lower. The other microbial activities studied (basal respiration, catalase activity) followed no consistent pattern after liming. Drought and drought in combination with liming, respectively, had no clear effects on microbial biomass and activity. Only in the A-horizon of the control, there is some evidence for drought stress for microorganisms. The high variability of results from the drought experiment (roof installation) is likely due to the marked spatial variability of top soil properties as well as imperfect and uneven achievement of experimental drought. Nevertheless, our study indicates that long-term effects of liming on microorganisms highly depend on site conditions. Thus, liming operations which currently affect vast areas of forest land should be accompanied by monitoring of soil organisms and their activities to reduce the possibility of a loss in functional diversity of soil organisms.     

Impact of faunal complexity on microbial biomass and N turnover in field mesocosms from a spruce forest soil

In a field study using soil mesocosms in an acid spruce forest soil we investigated the effects of mesofauna and macrofauna on microbial biomass, dissolved organic matter, and N cycling. Intact soil monoliths were taken from the ground, defaunated by deep-freezing, and wrapped in nets of various mesh-sizes to control re-immigration of different faunal size-classes. The monoliths were then replanted in the field. Three treatments of mesocosms were prepared: (1) with only microbiota, (2) microbiota and mesofauna, and (3) microbiota, mesofauna, and macrofauna (= complex fauna). After 8 months of exposure the mesocosms and the unmanipulated control plots (treatment 4) were destructively sampled. We estimated microbial biomass by substrate-induced respiration and the chloroform fumigation-extraction method. N cycling was measured by monitoring microbial N mineralization, the NH _inf4 ^sup+ content, and selected amino acids and the activities of protease, urease, and deaminase. The results from the L/F layer showed that the pool of the microbial biomass was not changed by the activity of the mesofauna. However, the mesofauna and macrofauna together enhanced SIR. An increase in microbial N mineralization was only observed in treatment 3 (microbiota + complex fauna). Protease activity and NH _inf4 ^sup+ content increased in treatments 2 (microbiota + mesofauna) and 3 (microbiota + complex fauna). The complex fauna induced a soil pH increase in treatment 3 as opposed to treatment 1 and the control. This increase was presumably due to excretory NH _inf4 ^sup+ . Principal component analysis revealed that the complex fauna in treatment 3 caused a significantly higher N turnover per unit of microbial biomass.     

The influence of lead on the respiration and biomass of microorganisms in gray forest soil in a long-term field experiment

The effects of lead oxide and lead nitrate on soil microorganisms were studied in a field experiment. As soon as the soil was treated with lead, a response of the microbial community was expressed in the higher rate of the basal respiration irrespectively of the dose and form of the lead compounds applied. At the same time, the microbial biomass decreased in the variants contaminated with 100–1000 mg Pb/kg. The long-lasting influence of lead lowered the basal respiration and the microbial biomass in the variant with the application of 400–1000 mg Pb/kg in the form of nitrate and 1000 mg Pb/kg as oxide. The experiments proved that the content of mobile lead in the soil, irrespectively of the form and solubility of the lead compounds, controls the functional relationship between the effect of the lead and the microbiological indices of the soil. The suppression of the soil microbial activity by more than 25% took place when the mobile lead content exceeded 170 mg Pb/kg.     

Analysis of the soil organic matter stability in spruce forests of Krkonose in Czechia on the basis of the ROMUL mathematical model

Computational experiments with the ROMUL mathematical model were performed for studying the dynamics of soil organic matter (SOM) in spruce forests of northeastern Czechia that were disturbed because of the atmospheric sulfur deposition in the second half of the 20th century. The effect of the soil acidification on the decomposition dynamics of the forest die-back in the model is of importance. Conditions of the forest productivity were found under which the SOM pool could be preserved. It was shown that, later on, the content of the litter will decrease because of the forest degradation, and the succession changes due to the effect of the contamination will affect the type of vegetation, as well as the type of soil organic matter. The total SOM content will decrease in this case. However, the maintenance of the grass productivity can slow down this process. It was noted that the quantitative prediction of the SOM dynamics requires measurements of the productivity parameters of the forest as a whole and the living ground cover, including the content of root litter, and the hydrothermal regime of the soil determining the transformation of the litter and humus.     

Cultivation and the soil biomass

The fumigation-respirometric determination of soil biomass was modified to assess the effects of cultivation on the biomass in the upper 5 cm of intact soil samples. We found the method suitable for comparative studies, but not for providing an absolute measure. The soil biomass increased during the growth of a wheat crop and then decreased to an approximately constant amount. The biomass was significantly greater where the soil had been direct-drilled than where it had been ploughed, probably because plant roots were more abundant after direct drilling. The size of the oil biomass in relation to substrate input is discussed.