Relationships of soil respiration to microbial biomass, substrate availability and clay content

The roles of microbial biomass (MBC) and substrate supply as well as their interaction with clay content in determining soil respiration rate were studied using a range of soils with contrasting properties. Total organic C (TOC), water-soluble organic carbon, 0.5 M K₂SO₄-extractable organic C and 33.3 mM KMnO₄-oxidisable organic carbon were determined as C availability indices. For air-dried soils, these indices showed close relationship with flush of CO₂ production following rewetting of the soils. In comparison, MBC determined with the chloroform fumigation-extraction technique had relatively weaker correlation with soil respiration rate. After 7 d pre-incubation, soil respiration was still closely correlated with the C availability indices in the pre-incubated soils, but poorly correlated with MBC determined with three different techniques—chloroform fumigation extraction, substrate-induced respiration, and chloroform fumigation-incubation methods. Results of multiple regression analyses, together with the above observations, suggested that soil respiration under favourable temperature and moisture conditions was principally determined by substrate supply rather than by the pool size of MBC. The specific respiratory activity of microorganisms (CO₂-C/MBC) following rewetting of air-dried soils or after 7 d pre-incubation was positively correlated with substrate availability, but negatively correlated with microbial pool size. Clay content had no significant effect on CO₂ production rate, relative C mineralization rate (CO₂-C/TOC) and specific respiratory activity of MBC during the first week incubation of rewetted dry soils. However, significant protective effect of clay on C mineralization was shown for the pre-incubated soils. These results suggested that the protective effect of clay on soil organic matter decomposition became significant as the substrate supply and microbial demand approached to an equilibrium state. Thereafter, soil respiration would be dependent on the replenishment of the labile substrate from the bulk organic C pool.     

Seasonal variations of soil microbial biomass and activity in warm- and cool-season turfgrass systems

Plant growth can be an important factor regulating seasonal variations of soil microbial biomass and activity. We investigated soil microbial biomass, microbial respiration, net N mineralization, and soil enzyme activity in turfgrass systems of three cool-season species (tall fescue, Festuca arundinacea Schreb., Kentucky bluegrass, Poa pratensis L., and creeping bentgrass, Agrostis palustris L.) and three warm-season species (centipedegrass, Eremochloa ophiuroides (Munro.) Hack, zoysiagrass, Zoysia japonica Steud, and bermudagrass, Cynodon dactylon (L.) Pers.). Microbial biomass and respiration were higher in warm- than the cool-season turfgrass systems, but net N mineralization was generally lower in warm-season turfgrass systems. Soil microbial biomass C and N varied seasonally, being lower in September and higher in May and December, independent of turfgrass physiological types. Seasonal variations in microbial respiration, net N mineralization, and cellulase activity were also similar between warm- and cool-season turfgrass systems. The lower microbial biomass and activity in September were associated with lower soil available N, possibly caused by turfgrass competition for this resource. Microbial biomass and activity (i.e., microbial respiration and net N mineralization determined in a laboratory incubation experiment) increased in soil samples collected during late fall and winter when turfgrasses grew slowly and their competition for soil N was weak. These results suggest that N availability rather than climate is the primary determinant of seasonal dynamics of soil microbial biomass and activity in turfgrass systems, located in the humid and warm region.     

Interactions between microclimatic variables and the soil microbial biomass

Summary Soil moisture, temperature, microbial substrate-induced respiration and basal respiration were monitored in two plots in an agricultural field from April 30 to September 25, 1987, and in a further two plots from May 26 to August 27, 1988. An attempt to relate biological variables to microclimatic variables was made through the use of correlation analysis. The microbial substrate-induced and basal respiration were both strongly positively correlated with the soil moisture content, and to a lesser extent positively related to soil temperature, especially when partial correlation was used to control for variation in soil moisture. Short-term changes in substrate-induced and basal respiration were correlated with changes in soil moisture but were largely independent of soil temperature. The ratio of basal to substrate-induced respiration (indicating the respiration: biomass ratio and therefore ecosystem stability or persistence) was negatively associated with the soil moisture content and in some instances with soil temperature when partial correlation analysis (correcting for soil moisture variation) was used. This suggests that the climatic conditions which contributed to the lowest ecosystem stability were low temperature, low moisture conditions.     

Short-term effects of tillage systems on active soil microbial biomass

 Conservation tillage, and especially no-tillage, induce changes in the distribution of organic pools in the soil profile. In long-term field experiments, marked stratification of the total soil microbial biomass and its activity have been observed as consequence of the application of no-tillage to previously tilled soils. Our objective was to study the evolution of the total and active soil microbial biomass and mineralized C in vitro during the first crop after the introduction of no-tillage to an agricultural soil. The experiment was performed on a Typic Hapludoll from the Argentinean Pampa. Remaining plant residues, total and active microbial biomass and mineralized C were determined at 0–5 cm and 5–15 cm depths, at three sampling times: wheat tilling, silking and maturity. The introduction of no-tillage produced an accumulation of plant residues in the soil surface layer (0–5 cm), showing stratification with depth at all sampling dates. Active microbial biomass and C mineralization were higher under no-tillage than under conventional tillage in the top 5 cm of the profile. The total soil microbial biomass did not differ between treatments. The active soil biomass was highly and positive correlated with plant residues (r ²=0.617;P<0.01) and with mineralized C (r ²=0.732;P<0.01). Consequently, the active microbial biomass and mineralized C reflected immediately the changes in residue management, whereas the total microbial biomass seemed not to be an early indicator of the introduction of a new form of soil management in our experiment. Received: 23 February 1999     

Influence of polychlorinated biphenyls on microbial biomass and its activity in grassland soil

Microbial biomass content, soil respiration and biomass specific respiration rate were measured in two parts of an area polluted by a municipal waste incinerator [polychlorinated biphenyls (PCBs) from combustion processes]. The soils in the studied parts differed significantly only in their levels of PCBs. The concentration of PCBs found in a control plot (4.4 ng g^-1 soil) can be regarded as a background value while the polluted plot contained an increased amount of PCBs (14.0 ng g^-1 soil). A significantly lower microbial biomass (decreased by 23%, based on the chloroform-fumigation extraction technique) and a lower specific respiration rate (decreased by 14%) were observed in the polluted plot in comparison with the control plot at the end of experimental period (1992–1994). Furthermore, a lower ability of microorganisms in the polluted plot to convert available C_org into new biomass was found in laboratory incubations with glucose-amended samples.     

Ratios of microbial biomass estimates to evaluate microbial physiology in soil

Soil microbial biomass data derived from fumigation–extraction (FE), substrate-induced respiration (SIR) and ATP estimations differed significantly and were significantly correlated, which agrees to previous studies. In a second step, the SIR/FE, ATP/FE and SIR/ATP ratios were calculated to evaluate the glucose-responsive and active component of the microbial (active and resting) biomass and the glucose-responsive component of the active microbiota. Soils were sampled along gradients within and between associated ecosystems in Northern Germany, Denmark and along a gradient of heavy metal pollution in Finland. The ratios indicated that the active portion and glucose-responsive component decreased with proceeding litter decomposition, higher degree of sustainable land management practices and higher degree of heavy metal contamination. This work was presented at the workshop ‘Non-molecular manipulation of soil microbial communities’ at the University of Udine, Udine, Italy, 17–20 October 2004; convened by P.C. Brookes and M. De Nobili and supported by European Science Foundation.     

The effect of tropical forest conversion on soil microbial biomass

We investigated the effects of converting forest to savanna and plough land on the microbial biomass in tropical soils of India. Conversion of the forest led to a significant reduction in soil organic C (40–46%), total N (47–53%), and microbial biomass C (52–58%) in the savanna and the plough land. Among forest, savanna, and plough land, basal soil respiration was maximum in the forest, but the microbial metabolic quotient (qCO₂ was estimated to be at a minimum in the forest and at a maximum in the plough land.     

Effect of temperature on soil microbial biomass and its metabolic quotient in situ under different tillage systems

Variations in the microbial biomass and the in situ metabolic quotient (qCO₂) due to climatic conditions were determined in a typical soil from the Argentine Rolling Pampa. Microbial C was evaluated by fumigation-incubation and qCO₂ was calculated using soil respiration in the field. An inverse relationship between microbial C and soil temperature was fitted to a model (r ²=0.90, P=0.01). No significant association with the soil water content was detected because the soil was generally near field capacity and thus water availability did not limited microbial growth and activity. Values of qCO₂ increased (r ²=0.89, P=0.01) as the result of metabolic activatìon, likely induced by a higher maintenance energy requirement at high temperatures. The highest values of qCO₂ were obtained when microbial C was the lowest, which was attributed to self consumption of microbial C in the presence of high temperatures. Consequently, microbial C was generally higher (P=0.05) in winter than in summer. Therefore, when microbial C is used as an index of soil biological activity, the influence of temperature should be taken into account.     

The influence of tillage on the proportion of organic carbon and nitrogen in the microbial biomass of medium-textured soils in a humid climate

Summary Microbial biomass C and N respond rapidly to changes in tillage and soil management. The ratio of biomass C to total organic C and the ratio of mineral N flush to total N were determined in the surface layer (0–5 cm) of low-clay (8–10%), fine sandy loam, Podzolic soils subjected to a range of reduced tillage (direct drilling, chisel ploughing, shallow tillage) experiments of 3–5 years' duration. Organic matter dynamics in the tillage experiments were compared to long-term conditions in several grassland sites established on the same soil type for 10–40 years. Microbial biomass C levels in the grassland soils, reduced tillage, and mouldboard ploughing treatments were 561, 250, and 155 g g^-1 soil, respectively. In all the systems, microbial biomass C was related to organic C (r=0.86), while the mineral N flush was related to total N (r=0.84). The average proportion of organic C in the biomass of the reduced tillage soils (1.2) was higher than in the ploughed soils (0.8) but similar to that in the grassland soils (1.3). Reduced tillage increased the average ratio of mineral N flush to total soil N to 1.9, compared to 1.3 in the ploughed soils. The same ratio was 1.8 in the grassland soils. Regression analysis of microbial biomass C and percent organic C in the microbial biomass showed a steeper slope for the tillage soils than the grassland sites, indicating that reduced tillage increased the microbial biomass level per unit soil organic C. The proportion of organic matter in the microbial biomass suggests a shift in organic matter equilibrium in the reduced tillage soils towards a rapid, tillage-induced, accumulation of organic matter in the surface layer.     

一种测定淹水土壤中微生物生物量碳的方法:液氯熏蒸浸提—水浴法

针对氯仿熏蒸浸提法测定淹水土壤微生物生物量碳时存在的问题,进行了测定方法的研究,建立了适合淹水土壤微生物生物量碳测定的新方法：液氯熏蒸浸提-水浴法,即在淹水土壤加入一定量的液态氯仿后,直接置于常压下熏蒸,然后用0．5μmolL^-1K2SO4溶液浸提,随后将浸提液放置于100℃水浴中以排除其中的残余氯仿,最后采用TOC分析仪测定浸提液中的有机碳含量。本方法既符合氯仿熏蒸法的原理,在操作上又简便可行,是一种测定淹水土壤微生物生物量碳的理想方法。     

Examination of microbial biomass in beech forest moder profiles

Microbial biomass C, ATP, and substrate-induced respiration were measured in the organic layers and the mineral A horizon of three beech forest soils with moder humus differing in Ca and Mg supply. Analyses of variance showed that horizon-specific differences explained most of the variance in the three microbial parameters. All three were significantly interrelated, with Spearman rank correlation coefficients of between 0.86 and 0.93. However, differences in the decline of these parameters with depth led to horizon-specific differences in their ratios. Thus, the ratios were not markedly interrelated. The mean ATP: microbial C ratio was 5.2 mol ATP g^-1 C in the L 2 layer, 19.5 in the F layers, and 9.6 in the H and A horizons. The ratio of substrate-induced respiration to microbial C varied between 39.3 and 82.2 O₂h^-1 g^-1 C in the F1 layers and between 5.3 and 32.1 l in the other layers. It is concluded that the use of different parameters can help to analyze both horizonand site-specific differences in microbial performance.     

Organic matter, microbial biomass and enzyme activity of soils under different crop rotations in the tropics

Soil organic matter level, soil microbial biomass C, ninhydrin-N, C mineralization, and dehydrogenase and alkaline phosphatase activity were studied in soils under different crop rotations for 6 years. Inclusion of a green manure crop of Sesbania aculeata in the rotation improved soil organic matter status and led to an increase in soil microbial biomass, soil enzyme activity and soil respiratory activity. Microbial biomass C increased from 192 mg kg^–1 soil in a pearl millet-wheat-fallow rotation to 256 mg kg^–1 soil in a pearl millet-wheat-green manure rotation. Inclusion of an oilseed crop such as sunflower or mustard led to a decrease in soil microbial biomass, C mineralization and soil enzyme activity. There was a good correlation between microbial biomass C, ninhydrin-N and dehydrogenase activity. The alkaline phosphatase activity of the soil under different crop rotations was little affected. The results indicate the green manuring improved the organic matter status of the soil and soil microbial activity vital for the nutrient turnover and long-term productivity of the soil. Received: 7 January 1996     

Prolonged simulated acid rain treatment in the subarctic: Effect on the soil respiration rate and microbial biomass

Humus chemistry and respiration rate, ATP, ergosterol, and muramic acid concentration as measures of chemical properties, microbial activity, biomass, and indicators of fungal and bacterial biomass were studied in a long-term acid rain experiment in the far north of Finnish Lapland. The treatments used in this study were dry control, irrigated control (spring water, pH 6), and two levels of simulated acid rain (pH 4 and pH 3). Originally (1985–1988), simulated acid rain was prepared by adding both H₂SO₄ and HNO₃ (1.9:1 by weight). In 1989 the treatments were modified as follows. In subarea 1 the treatments continued unchanged (H₂SO₄+HNO₃ in rain to pH 4 and pH 3), but in subarea 2 only H₂SO₄ was applied. The plots were sampled in 1992. The acid application affected humus chemistry by lowering the pH, cation exchange capacity, and base saturation (due to a decrease in Ca and Mg) in the treatment with H₂SO₄+HNO₃ to pH 4 (total proton load over 8 years 2.92 kmol ha^-1), whereas the microbial variables were not affected at this proton load, and only the respiration rate decreased by 20% in the strongest simulated acid rain treatment (total proton load 14.9 kmol ha^-1). The different ratios of H₂SO₄+HNO₃ in subareas 1 and 2 did not affect the results.     

Long-term tillage and rotation effects on soil microbial biomass,carbon and nitrogen

Summary Three mollisols, typical of the Palouse winter wheat region of eastern Washington and northern Idaho, were analyzed for microbial biomass, total C and total N after 10 years of combined tillage and rotation treatments. Treatments included till, no-till and three different cereal-legume rotations. All crop phases in each rotation were sampled in the same year. Microbial biomass was monitored from April to October, using a respiratory-response method. Microbial biomass, total C and total N were highest under no-till surface soils (0–5 cm), with minimal differences for tillage or depth below 5 cm. Microbial biomass differences among rotations were not large, owing to the relative homogeneity of the treatments. A rotation with two legume crops had the highest total C and N. Microbial biomass was significantly higher in no-till surface soils where the current crop had been preceded by a high-residue crop. The opposite was true for the tilled plots. There was little change in microbial biomass over the seasons until October, when fresh crop residues and rains had a strong stimulatory effect. The seasonal pattern of biomass in no-till surface soils reflected the dry summer/winter rainfall climate of the region. The results of this study show that numerous factors affect soil microbial biomass and that cropping history and seasonal changes must be taken into account when microbial biomass data are compared.Scientific paper no. 7634     

Native carbon mineralization of an acid organic soil after use of the chloroform-fumigation method to estimate microbial biomass

Summary Two experiments were carried out on a dysmoder humus sampled from a depth of 2–5 cm from a mixed hardwood forest. In acid soil, the chloroform fumigation-incubation method failed to estimate the microbial biomass, not because bacterial growth was inhibited after fumigation but because a labile C source was taken up which differed from the killed biomass C.     

Influence of soil properties on microbial populations, activity and biomass in humid subtropical mountainous ecosystems of India

 Microbial populations, biomass, soil respiration and enzyme activities were determined in slightly acid organic soils of major mountainous humid subtropical terrestrial ecosystems, along a soil fertility gradient, in order to evaluate the influence of soil properties on microbial populations, activity and biomass and to understand the dynamics of the microbial biomass in degraded ecosystems and mature forest. Although the population of fungi was highest in the undisturbed forest (Sacred Grove), soil respiration was lowest in the 7-year-old regrowth and in natural grassland (approximately 373 μg g^–1 h^–1). Dehydrogenase and urease activities were high in "jhum" fallow, and among the forest stands they were highest in the 7-year-old regrowth. Microbial biomass C (MBC) depended mainly on the organic C status of the soil. The MBC values were generally higher in mature forest than in natural grassland, 1-year-old jhum fallow and the 4-year-old alder plantation. The MBC values obtained by the chloroform-fumigation-incubation technique (330–1656 μg g^–1) did not vary significantly from those obtained by the chloroform-fumigation-extraction technique (408–1684 μg g^–1), however, the values correlated positively (P<0.001). The enzyme activities, soil respiration, bacterial and fungal populations and microbial biomass was greatly influenced by several soil properties, particularly the levels of nutrients. The soil nutrient status, microbial populations, soil respiration and dehydrogenase activity were greater in Sacred Grove, while urease activity was greater in grassland. Received: 14 October 1998     

Natural abundance radiocarbon in soil microbial biomass: Results from a glacial foreland

We present a method for determining the natural abundance radiocarbon (¹⁴C) content of soil microbial biomass (SMB) based on existing fumigation-extraction procedures. We applied the technique to soils from the foreland of the Ödenwinkelkees glacier in the Austrian Alps, which has a well-characterised chronosequence of soils at different stages of development. Across the chronosequence, SMB contained post-bomb levels of ¹⁴C, suggesting it was substantially composed of carbon that had been fixed since the 1960s. Comparison of our results with previous findings from the same site showed that at most stages in the sequence the SMB had a similar ¹⁴C content to the bulk soil organic matter (SOM). However, soil respired CO₂ was ¹⁴C-depleted relative to SMB, indicating that at least a component of the microbial community was mineralising some older carbon. In the most recently exposed soils, SMB was ¹⁴C-enriched compared to both soil respiration and SOM, suggesting that a small component of the microbial biomass that utilises older carbon contributes disproportionately more to the CO₂ efflux. Although other interpretations are possible, this explanation is consistent with the notion that early on in the succession a large proportion of the microbial biomass is dormant.     

Biochemical analysis of soil organic matter and microbial biomass composition—a pilot study

Biochemical composition of both intracellular (biomass) and extracellular soil organic matter was determined after extraction with 0.5 M K₂SO₄. Extractable carbon, hexoses, pentoses, total reducing sugars, ninhydrin-reactive nitrogen (NRN), proteins and DNA content were colorimetrically determined. The objective of the pilot study was to examine the information potential included in newly measured biochemical characteristics, their environmental variance and the relationships with main soil properties. Correlation analysis and PCA showed independence between biochemical parameters and physico-chemical properties of the soil. Thus, the parameters characterising biochemical composition of the soil biomass and extracellular matter seem to bring new information about the soils beyond the physico-chemical parameters. They also seem to reveal a more detailed view on microbial biomass or extracellular organic matter pool than C_bio or C_ext alone, respectively. The variance, which occurred in biochemical characteristics, also displayed a high discrimination potential between the defined soil categories. Three types of indices were newly proposed: index I (“substrate quantity index”)—the biomass-specific amount of the extracellular organic compounds, index II (“immobilisation ratio”)—the portion of the organic compound immobilised in microbial biomass, and index III (“substrate quality index”)—the extracellular organic compound content related to extracellular organic carbon. The indices displayed a higher potential than both soil biotic and abiotic parameters to discriminate soil characters and soil types.     

The significance of microbial biomass sulphur in soil

The soil microbial biomass S fraction of total organic S in soil is considered to be relatively labile and the most active S pool for S turnover in soil. Its significance has been demonstrated in studies of S deficiency in agronomic situations and in those of S pollution from high atmospheric inputs. The utility of the CHCl₃ fumigation-extraction technique for the measurement of microbial S has been proved for a range of soils and conditions. The various methodologies currently available are discussed, including the need for determination of the conversion (K _s) factor. Microbial S values, summarized from the available literature, ranged from 3 to 300 g S g^-1 dry weight soil. They were generally greater in grassland than in arable systems, though the greatest values were obtained in the few examples from forest and peatland soil systems. Microbial S values showed direct relationships with both microbial C and with total soil organic S. Again, there were significant differences between arable and grassland systems. The effect of factors such as organic and inorganic inputs as well as soil physical conditions on microbial S are described. Microbial S turnover rates were estimated from seasonal, ³⁵S-labelling and modelling studies. These rates varied between an approximately annual turnover rate in undisturbed soils up to 80 year^-1 following the addition of readily available substrates. Prospective future research areas are also outlined.     

Evaluating soil microbial biomass carbon as an indicator of long-term environmental change

The aim is to assess whether soil microbial biomass carbon (biomass C) could be used as an indicator of environmental change in natural and semi-natural ecosystems. Biomass C was measured by fumigation-extraction in soils from two sites at Rothamsted. One was a plot from the Broadbalk Wheat Experiment, given inorganic fertiliser and chalk, which has been in continuous cultivation for more than 150 yr. The other was a similar sized area, from Geescroft Wilderness, which has been left to revert to woodland since 1885, after being an arable field. Other soil properties (pH, soil organic C and exchangeable cations) were also measured to compare with biomass C. The coefficients of variation (cvs) of the properties measured were calculated for comparison, little difference was found between the cvs for biomass C from each site: cv=26% for Broadbalk and 23% for Geescroft. The cvs for the other, chemical properties, were mostly <10% for Broadbalk and generally >25% for Geescroft, as expected, given the different cultivation histories. Statistical analysis of the variation in biomass C concentration revealed that such measurements would not be valid indicators of environmental change, without processing impossibly large numbers of samples. To decrease the least significant percentage change to less than 5% after three samplings, 320 samples would have to be taken each time. This would be also be true of the other chemical properties in Geescroft Wilderness, where the measured background variation would mask any subtle environmental change. This indicates that, for some properties at least, statistically significant changes will only be detected in the longer term with regular sampling, e.g. 30-40 yr.