Microbial response to the addition of glucose in low-fertility soils

Addition of soluble organic substrates to soil has been shown to either increase or restrict the rate of microbial CO₂–C evolution. This has been attributed to a priming effect resulting from accelerated or decreased turnover of the soil organic matter including the soil microflora. We investigated microbial responses to small glucose-C additions (10–50 μg C g¹ soil) in arable soils either amended or not with cellulose. An immediate CO₂–C release between 0 and 69 h (equivalent to 59% of glucose-C applied) was measured. However, only half of the CO₂–C respired could be attributed to the utilisation of glucose-C substrate, based on the percentage of ¹⁴C–CO₂ evolved after the addition of a ¹⁴C-labelled glucose tracer. Thus, although no evidence of an immediate release of ‘extra’ C above the rate applied as glucose-C was observed, the pattern of decomposition for ¹⁴C-glucose suggested utilisation of an alternate C source. Based on this, a positive priming effect (1.5 to 4.3 times the amount CO₂–C evolved that was attributed to glucose-C decomposition) was observed for at least 170 h in non-cellulose-amended soil and 612 h in cellulose-amended soil. Two further phases of microbial activity in cellulose-amended soils were attributed to either activation of different microbial populations or end-product inhibition of cellulase activity after glucose addition. During these subsequent phases, a negative priming effect of between 0.1 and 1.5 times was observed. Findings indicate that the response of the microbial community to small additions of soluble organic C substrate is not consistent and support the premise that microbial response varies in a yet to be predicted manner between soil type and ecosystems. We hypothesise that this is due to differences in the microbial community structure activated by the addition of organic C and the timing of soluble organic substrate addition with respect to the current dissolved organic C status of the soil.     

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 proportional mineralisation of microbial biomass and organic matter caused by air-drying and rewetting of a grassland soil

During the first few days after rewetting of an air-dried soil (AD-RW), microbial activity increases compared to that in the original moist soil, causing increased mineralisation (a flush) of soil organic carbon (C) and other nutrients. The AD-RW flush is believed to be derived from the enhanced mineralisation of both non-biomass soil organic matter (due to its physical release and enhanced availability) and microbial biomass killed during drying and rewetting. Our aim was to determine the effects of AD-RW on the mineralisation of soil organic matter and microbial biomass during and after repeated AD-RW cycles and to quantify their proportions in the CO₂-C flushes that resulted. To do this, a UK grassland soil was amended with ¹⁴C-labelled glucose to label the biomass and then given five AD-RW cycles, each followed by 7 d incubation at 25 °C and 50% water holding capacity. Each AD-RW cycle increased the amount of CO₂-C evolved (varying from 83 to 240 μg g⁻¹ soil), compared to the control with, overall, less CO₂-C being evolved as the number of AD-RW cycles increased. In the first cycle, the amount of biomass C decreased by 44% and microbial ATP by 70% while concentrations of extractable C nearly doubled. However, all rapidly recovered and within 1.3 d after rewetting, biomass C was 87% and ATP was 78% of the initial concentrations measured prior to air-drying. Similarly, by 2 d, extractable organic C had decreased to a similar concentration to the original. After the five AD-RW cycles, the amounts of total and ¹⁴C-labelled biomass C remaining in the soil accounted for 60 and 40% of those in the similarly incubated control soil, respectively. Soil biomass ATP concentrations following the first AD-RW cycle remained remarkably constant (ranging from about 10 to 14 μmol ATP g⁻¹ biomass C) and very similar to the concentration in the fresh soil prior to air-drying. We developed a simple mathematical procedure to estimate the proportion of CO₂-C derived from biomass C and non-biomass C during AD-RW. From it, we estimate that, over the five AD-RW cycles, about 60% of the CO₂-C evolved came from mineralisation of non-biomass organic C and the remainder from the biomass C itself.     

Soil-released carbon dioxide from microbial biomass carbon in the cultivated soils of karst areas of southwest China

 Soil microbial biomass and the emission of CO₂ from the soil surface were measured in yellow soils (Ultisols) of the karst areas of southwest China. The soils are relatively weathered, leached and impoverished, and have a low input of plant residues. The measurements were made for a 1-year period and show a reciprocal relationship between microbial biomass and surface CO₂ efflux. The highest (42.6±2.8 mg CO₂-C m^–2 h^–1) and lowest (15.6±0.6 mg CO₂-C m^–2 h^–1) CO₂ effluxes are found in the summer and winter, respectively. The cumulative CO₂ efflux is 0.24 kg CO₂-C m^–2 year^–1. There is also a marked seasonal variation in the amount of soil microbial biomass carbon, but with the highest (644±71 μg C g^–1 soil) and lowest (270±24 μg C g^–1 soil) values occurring in the winter and summer, respectively. The cumulative loss of soil microbial biomass carbon in the top 10 cm of the soil was 608 μg C g^–1 year^–1 soil over 17 sampling times. The mean residence time of microbial biomass is estimated at 105 days, suggesting that the carbon in soil microbial biomass may act as a source of the CO₂ released from soils. Received: 13 July 1999     

Microbial biomass and activity indices after organic substrate addition to a selenium-contaminated soil

High concentrations of Se in soil might have negative effects on microorganisms. For this reason, the effect of organic substrate addition (glucose + maize straw) on Se volatilisation in relation to changes in microbial biomass and activity indices was investigated using an artificially Se-contaminated soil. Microbial biomass N was reduced on average by more than 50% after substrate addition, but adenylate energy charge (AEC) and metabolic quotient qCO₂ were both increased. The Se content decreased by nearly 30% only with the addition of the organic substrate at 25°C. No significant Se loss occurred without substrate at 25°C or with substrate at 5°C. In the two treatments with substrate addition, the substrate-derived CO₂ evolution was about 30% lower with Se addition than without. In contrast, Se had no effect on any of the other soil microbial indices analysed, i.e. microbial biomass C, microbial biomass N, adenosine triphosphate (ATP), AEC, ATP-to-microbial biomass C, and qCO₂.     

Microbial biomass and C mineralization in agricultural soils as affected by atrazine addition

This study examines the effects of atrazine on both microbial biomass C and C mineralization dynamics in two contrasting agricultural soils (organic C, texture, and atrazine application history) located at Galicia (NW Spain). Atrazine was added to soils, a Humic Cambisol (H) and a Gleyic Cambisol (G), at a recommended agronomic dose and C mineralization (CO₂ evolved), and microbial biomass measurements were made in non-treated and atrazine-treated samples at different time intervals during a 12-week aerobic incubation. The cumulative curves of CO₂–C evolved over time fit the simple first-order kinetic model [Ct = Co (1 − e ^−kt )], whose kinetic parameters were quantified. Differences in these parameters were observed between the two soils studied; the G soil, with a higher content in organic matter and microbial biomass C and lower atrazine application history, exhibited higher values of the total C mineralization and the potentially mineralizable labile C pool than those for the H soil. The addition of atrazine modified the kinetic parameters and increased notably the C mineralized; by the end of the incubation the cumulative CO₂–C values were 33–41% higher than those in the corresponding non-added soils. In contrast, a variable effect or even no effect was observed on the soil microbial biomass following atrazine addition. The data clearly showed that atrazine application at normal agricultural rates may have important implications in the C cycling of these two contrasting acid soils.     

Shifts in bacterial community structure associated with inputs of low molecular weight carbon compounds to soil

Low molecular weight carbon (C) substrates are major drivers of bacterial activity and diversity in the soil environment. However, it is not well understood how specific low molecular weight C compounds, which are frequently found in root exudates and litter leachates, influence bacterial community structure or if there are specific groups of soil bacteria that preferentially respond to these C inputs. To address these knowledge gaps, we added three simple C substrates representative of common root exudate compounds (glucose, glycine, and citric acid) to microcosms containing three distinct soils from a grassland, hardwood forest, and coniferous forest. CO₂ production was assessed over a 24 h incubation period and, at the end of the incubation, DNA was extracted from the samples for assessment of bacterial community structure via bar-coded pyrosequencing of the 16S rRNA gene. All three C substrates significantly increased CO₂ production in all soils; however, there was no relationship between the magnitude of the increase in CO₂ production and the shift in bacterial community composition. All three substrates had significant effects on overall community structure with the changes primarily driven by relative increases in β-Proteobacteria, γ-Proteobacteria, and Actinobacteria. Citric acid additions had a particularly strong influence on bacterial communities, producing a 2-5-fold increase in the relative abundance of the β-Proteobacteria subphylum. These results suggest that although community-level responses to substrate additions vary depending on the substrate and soil in question, there are specific bacterial taxa that preferentially respond to the substrate additions across soil types.     

Capacity of fatty acid profiles and substrate utilization patterns to describe differences in soil microbial communities associated with increased salinity or alkalinity at three locations in South Australia

 Fatty acid methyl ester (FAME) profiles, together with Biolog substrate utilization patterns, were used in conjunction with measurements of other soil chemical and microbiological properties to describe differences in soil microbial communities induced by increased salinity and alkalinity in grass/legume pastures at three sites in SE South Australia. Total ester-linked FAMEs (EL-FAMEs) and phospholipid-linked FAMEs (PL-FAMEs), were also compared for their ability to detect differences between the soil microbial communities. The level of salinity and alkalinity in affected areas of the pastures showed seasonal variation, being greater in summer than in winter. At the time of sampling for the chemical and microbiological measurements (winter) only the affected soil at site 1 was significantly saline. The affected soils at all three sites had lower organic C and total N concentrations than the corresponding non-affected soils. At site 1 microbial biomass, CO₂-C respiration and the rate of cellulose decomposition was also lower in the affected soil compared to the non-affected soil. Biomarker fatty acids present in both the EL- and PL-FAME profiles indicated a lower ratio of fungal to bacterial fatty acids in the saline affected soil at site 1. Analysis of Biolog substrate utilization patterns indicated that the bacterial community in the affected soil at site 1 utilized fewer carbon substrates and had lower functional diversity than the corresponding community in the non-affected soil. In contrast, increased alkalinity, of major importance at sites 2 and 3, had no effect on microbial biomass, the rate of cellulose decomposition or functional diversity but was associated with significant differences in the relative amounts of several fatty acids in the PL-FAME profiles indicative of a shift towards a bacterial dominated community. Despite differences in the number and relative amounts of fatty acids detected, principal component analysis of the EL- and PL-FAME profiles were equally capable of separating the affected and non-affected soils at all three sites. Redundancy analysis of the FAME data showed that organic C, microbial biomass, electrical conductivity and bicarbonate-extractable P were significantly correlated with variation in the EL-FAME profiles, whereas pH, electrical conductivity, NH₄-N, CO₂-C respiration and the microbial quotient were significantly correlated with variation in the PL-FAME profiles. Redundancy analysis of the Biolog data indicated that cation exchange capacity and bicarbonate-extractable K were significantly correlated with the variation in Biolog substrate utilization patterns. Received: 8 March 2000     

Salinity and sodicity effects on respiration and microbial biomass of soil

An understanding of the effects of salinity and sodicity on soil carbon (C) stocks and fluxes is critical in environmental management, as the areal extents of salinity and sodicity are predicted to increase. The effects of salinity and sodicity on the soil microbial biomass (SMB) and soil respiration were assessed over 12weeks under controlled conditions by subjecting disturbed soil samples from a vegetated soil profile to leaching with one of six salt solutions; a combination of low-salinity (0.5dSm⁻¹), mid-salinity (10dSm⁻¹), or high-salinity (30dSm⁻¹), with either low-sodicity (sodium adsorption ratio, SAR, 1), or high-sodicity (SAR 30) to give six treatments: control (low-salinity low-sodicity); low-salinity high-sodicity; mid-salinity low-sodicity; mid-salinity high-sodicity; high-salinity low-sodicity; and high-salinity high-sodicity. Soil respiration rate was highest (56–80mg CO₂-C kg⁻¹ soil) in the low-salinity treatments and lowest (1–5mg CO₂-C kg⁻¹ soil) in the mid-salinity treatments, while the SMB was highest in the high-salinity treatments (459–565mg kg⁻¹ soil) and lowest in the low-salinity treatments (158–172mg kg⁻¹ soil). This was attributed to increased substrate availability with high salt concentrations through either increased dispersion of soil aggregates or dissolution or hydrolysis of soil organic matter, which may offset some of the stresses placed on the microbial population from high salt concentrations. The apparent disparity in trends in respiration and the SMB may be due to an induced shift in the microbial population, from one dominated by more active microorganisms to one dominated by less active microorganisms.     

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.     

Microbial immobilisation of C rhizodeposits in rhizosphere and root-free soil under continuous C labelling of oats

A greenhouse experiment was conducted by growing oats (Avenasativa L.) in a continuously ¹³CO₂ labeled atmosphere. The allocation of ¹³C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO₂ emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of ¹³C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO₂ emissions at the earing and ripeness stages were revealed. These ¹³C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ¹³C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of ¹³C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of ¹³C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO₂ emission from soil varied from 61 to 92% of total CO₂ evolved, including 4-23% attributed to rhizomicrobial respiration. While 81-91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9-19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural ¹³C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.     

Experimental warming effects on the microbial community of a temperate mountain forest soil

Soil microbial communities mediate the decomposition of soil organic matter (SOM). The amount of carbon (C) that is respired leaves the soil as CO₂ (soil respiration) and causes one of the greatest fluxes in the global carbon cycle. How soil microbial communities will respond to global warming, however, is not well understood. To elucidate the effect of warming on the microbial community we analyzed soil from the soil warming experiment Achenkirch, Austria. Soil of a mature spruce forest was warmed by 4 °C during snow-free seasons since 2004. Repeated soil sampling from control and warmed plots took place from 2008 until 2010. We monitored microbial biomass C and nitrogen (N). Microbial community composition was assessed by phospholipid fatty acid analysis (PLFA) and by quantitative real time polymerase chain reaction (qPCR) of ribosomal RNA genes. Microbial metabolic activity was estimated by soil respiration to biomass ratios and RNA to DNA ratios. Soil warming did not affect microbial biomass, nor did warming affect the abundances of most microbial groups. Warming significantly enhanced microbial metabolic activity in terms of soil respiration per amount of microbial biomass C. Microbial stress biomarkers were elevated in warmed plots. In summary, the 4 °C increase in soil temperature during the snow-free season had no influence on microbial community composition and biomass but strongly increased microbial metabolic activity and hence reduced carbon use efficiency.     

Root activity and carbon metabolism in soils

Summary Two different soils were amended with ¹⁴C-labelled plant material and incubated under controlled laboratory conditions for 2 years. Half the samples were cropped with wheat (Triticum aestivum) 10 times in succession. At flowering, the wheat was harvested and the old roots removed from the soil, so that the soil was continuously occupied by predominantly active root systems. The remaining samples were maintained without plants under the same conditions. During the initial stages of high microbial activity, due to decomposition of the labile compounds, the size of the total microbial biomass was comparable for both treatments, and the metabolic quotient (qCO₂-C = mg CO₂-C·mg^–1 Biomass C·h^–1) was increased by the plants. During the subsequent low-activity decomposition stages, after the labile compounds had been progressively mineralized, the biomass was multiplied by a factor of 2–4 in the presence of plants compared to the bare soils. Nevertheless, qCO₂-C tended to reach similar low values with both treatments. The ¹⁴C-labelled biomass was reduced by the presence of roots and qCO₂-¹⁴C was increased. The significance of these results obtained from a model experiment is discussed in terms of (1) the variation in the substrate originating from the roots and controlled by the plant physiology, (2) nutrient availability for plants and microorganisms, (3) soil biotic capacities and (4) increased microbial turnover rates induced by the roots.     

Decomposition of pea and maize straw in Pakistani soils along a gradient in salinity

Maize straw and pea straw were added to five Pakistani soils from a gradient in salinity to test the following hypotheses: Increasing salinity at high pH decreases proportionally (1) the decomposition of added straw and (2) the resulting net increase in microbial biomass. In the non-amended control soils, salinity had depressive effects on microbial biomass C, biomass N, but not on biomass P and ergosterol. The ratios microbial biomass C-to-N and biomass C-to-P decreased consistently with increasing salinity. In contrast, the ergosterol-to-microbial biomass C ratio was constant in the four soils at pH>8.9, but nearly doubled in the most saline, but least alkaline, soil (pH 8.2). The addition of the maize and pea straw always increased the contents of microbial biomass C, biomass N, biomass P and ergosterol, but without clear effects of salinity. Highest mean contents of microbial biomass C and biomass N were measured at day 0, immediately after the straw was added. Straw amendments increased the CO₂ evolution rates of all five soils without any effect of salinity. The same was true for total C and total N in the two fractions of particulate organic matter (POM) 63–400 μm and >400 μm. Lowest percentage of straw-derived CO₂-C and highest recoveries of POM-C and POM-N were observed in the maize straw treatment and the reverse in the pea straw treatment. Yield coefficients were calculated for maize and pea straw based on the assumption that the balance gap between CO₂ and the amount of POM can be fully assigned to microbial products.     

Effects of Cd,Zn, or both on soil microbial biomass and activity in a clay loam soil

We investigated Cd, Zn, and Cd + Zn toxicity to soil microbial biomass and activity, and indigenous Rhizobium leguminosarum biovar trifolii, in two near neutral pH clay loam soils, under long-term arable and grassland management, in a 6-month laboratory incubation, with a view to determining the causative metal. Both soils were amended with Cd- or Zn-enriched sewage sludge, to produce soils with total Cd concentrations at four times (12 mg Cd g⁻¹ soil), and total Zn concentrations (300 mg Zn kg⁻¹ soil) at the EU upper permitted limit. The additive effects of Cd plus Zn at these soil concentrations were also investigated. There were no significant differences in microbial biomass C (B _C), biomass ninhydrin N (B _N), ATP, or microbial respiration between the different treatments. Microbial metabolic quotient (defined as qCO₂ = units of CO₂–C evolved unit⁻¹ biomass C unit⁻¹ time) also did not differ significantly between treatments. However, the microbial maintenance energy (in this study defined as qCO₂-to-μ ratio value, where μ is the growth rate) indicated that more energy was required for microbial synthesis in metal-rich sludge-treated soils (especially Zn) than in control sludge-treated soils. Indigenous R. leguminosarum bv. trifolii numbers were not significantly different between untreated and sludge-treated grassland soils after 24 weeks regardless of metal or metal concentrations. However, rhizobial numbers in the arable soils treated with metal-contaminated sludges decreased significantly (P < 0.05) compared to the untreated control and uncontaminated sludge-treated soils after 24 weeks. The order of decreasing toxicity to rhizobia in the arable soils was Zn > Cd > Cd + Zn.     

Microwave irradiation of soil for routine measurement of microbial biomass carbon

 Microwave irradiation was evaluated as a non-toxic alternate to chloroform fumigation for routine measurement of soil microbial biomass C. Microwave energy was applied to moist soil to disrupt microbial cells. The flush of C released was then measured after extraction or incubation. Microwave irradiation at 800 J g^–1 soil was optimal because this level resulted in an almost instantaneous rise in soil temperature (≥80  °C), an abrupt reduction in microbial activity, maximal release of biomass C, and minimal solubilization of humic substances. Both incubation-CO₂ titration and extraction-colorimetry methods were used on separate 20-g subsamples to compare the labile C in the microwave-treated and untreated soil samples. The incubation-titration method was also used to measure C in chloroform-fumigated soil samples. Averaged across soils, the chloroform fumigation yielded 123.3±5.1 mg CO₂-C kg^–1. Microwave irradiation yielded 93.6±3.9 mg CO₂-C kg^–1 soil determined by incubation and 52.4±2.4 mg C kg^–1 soil determined by extraction, accounting for 76% and 42% of the net flush of C measured by the chloroform fumigation. Microwave-stimulated net flushes of C were correlated closely (r ²=0.974 for incubation or 0.908 for extraction) with microbial biomass C measured by the chloroform fumigation. Little correlation was found with the total soil organic C (r ²=0.241 for incubation or for 0.166 extraction). Mean efficiency factors for incubation (K _MI) or extraction (K _ME) were used to calculate microbial biomass C from net flushes of C between microwaved and unmicrowaved soils. Values of K _MI and K _ME were not affected by soil pH, bulk density or clay contents. Extraction of microwaved soil by 0.5M K₂SO₄ proved to be a simple, fast, precise, reliable, and safe method to measure soil microbial biomass C. Received: 12 September 1997     

Influence of soil phosphorus status and nitrogen addition on carbon mineralization from 14C-labelled glucose in pasture soils

 This study examines the effect of soil P status and N addition on the decomposition of ¹⁴C-labelled glucose to assess the consequences of reduced fertilizer inputs on the functioning of pastoral systems. A contrast in soil P fertility was obtained by selecting two hill pasture soils with different fertilizer history. At the two selected sites, representing low (LF) and high (HF) fertility status, total P concentrations were 640 and 820 mg kg^–1 and annual pasture production was 4,868 and 14,120 kg DM ha^–1 respectively. Soils were amended with ¹⁴C-labelled glucose (2,076 mg C kg^–1 soil), with and without the addition of N (207 mg kg^–1 soil), and incubated for 168 days. During incubation, the amounts of ¹⁴CO₂ respired, microbial biomass C and ¹⁴C, microbial biomass P, extractable inorganic P (P_i) and net N mineralization were determined periodically. Carbon turnover was greatly influenced by nutrient P availability. The amount of glucose-derived ¹⁴CO₂ production was high (72%) in the HF and low (67%) in the LF soil, as were microbial biomass C and P concentrations. The ¹⁴C that remained in the microbial biomass at the end of the 6-month incubation was higher in the LF soil (15%) than in the HF soil (11%). Fluctuations in P_i in the LF soil during incubation were small compared with those in HF soil, suggesting that P was cycling through microbial biomass. The concentrations of P_i were significantly greater in the HF samples throughout the incubation than in the LF samples. Net N mineralization and nitrification rates were also low in the LF soils, indicating a slow turnover of microorganisms under limited nutrient supply. Addition of N had little effect on biomass ¹⁴C and glucose utilization. This suggests that, at limiting P fertility, C turnover is retarded because microbial biomass becomes less efficient in the utilization of substrates. Received: 18 October 1999     

Microbial use of maize cellulose and sugarcane sucrose monitored by changes in the C/C ratio

An arable soil with organic matter formed from C₃-vegetation was amended initially with maize cellulose (C₄-cellulose) and sugarcane sucrose (C₄-sucrose) in a 67-day laboratory incubation experiment with microcosms at 25 °C. The amount and isotopic composition (¹³C/¹²C) of soil organic C, CO₂ evolved, microbial biomass C, and microbial residue C were determined to prove whether the formation of microbial residues depends on the quality of the added C source adjusted with NH₄NO₃ to the same C/N ratio of 15. In a subsequent step, C₃-cellulose (3 mg C g⁻¹ soil) was added without N to soil to determine whether the microbial residues formed initially from C₄-substrate are preferentially decomposed to maintain the N-demand of the soil microbial community. At the end of the experiment, 23% of the two C₄-substrates added was left in the soil, while 3% and 4% of the added C₄-cellulose and C₄-sucrose, respectively, were found in the microbial biomass. The addition of the two C₄-substrates caused a significant 100% increase in C₃-derived CO₂ evolution during the 5-33 day incubation period. The addition of C₃-cellulose caused a significant 50% increase in C₄-derived CO₂ evolution during the 38-67 day incubation period. The decrease in microbial biomass C₄-C accounted for roughly 60% of this increase. Cellulose addition promoted microorganisms strongly able to recycle N immediately from their own tissue by “cryptic growth” instead of incorporating NO₃⁻ from the soil solution. The differences in quality of the microbial residues produced by C₄-cellulose and C₄-sucrose decomposing microorganisms are also reflected by the difference in the rates of CO₂ evolution, but not in the rates of net N mineralization.     

Relative importance of substrate type and previous soil management in synthesis of microbial biomass and substrate mineralization

Our aim was to determine whether the soil microbial biomass, which has developed naturally over many years in a given ecosystem, is specially adapted to metabolize the plant‐derived substrate C of the ecosystem within which it developed or whether the nature of recently added substrate is the more important factor. To examine this, soils from three sites in close proximity (woodland, grassland and arable from the Broadbalk Experiment at Rothamsted Research, Harpenden, UK) were each amended with air‐dried wheat straw (Triticum aestivum), ryegrass leaves (Lolium perenne) or woodland leaf litter (mainly Quercus robur and Fagus sylvatica) in a fully replicated 3 × 3 factorial laboratory experiment. The initial mineralization rates (evolved CO₂‐C) were determined during the first 6.5 hours and again, together with the amount of microbial biomass synthesized (microbial biomass C), at 7, 14, 21, 30 and 49 days of incubation. The hourly rate of CO₂‐C production during the first 6.5 hours was slowest following leaf litter addition, while the added grass gave the fastest rates of CO₂‐C evolution both within and between soils. Ryegrass addition to the arable soil led to approximately four times more CO₂‐C being evolved than when it was added to the woodland soil, at an overall rate in the arable soils of 41 μg C g^?1 soil hour^?1. In each soil, the net amounts of CO₂‐C produced were in the order grass > straw > leaf litter. In each case, the amount produced by the added leaf litter was significantly less (P < 0.05) than either the added grass or straw. Overall, the trend was for much slower rates of mineralization of all substrates in the woodland soil than in either the arable or grassland soils. During 49 days of incubation in the woodland and grassland soils, the net total amounts of CO₂‐C evolved differed significantly (P < 0.01), with grass > straw > leaf litter, respectively. In the arable soil, the amounts of CO₂‐C evolved from added grass and straw were significantly larger (P < 0.01) than from the leaf litter treatment. Our findings indicated that the amounts of CO₂‐C evolved were not related to soil management or to the size of the original biomass but to the substrate type. The amount of biomass C synthesized was also in the order grass > straw > leaf litter, at all stages of incubation in the woodland and grassland soil. In the arable soil, the same effect was observed up to 14 days, and for the rest of the incubation the biomass C synthesized was in the order grass > straw > leaf litter. Up to three times more biomass C was synthesized from the added grass than from the other substrates in all soils throughout the incubation. The maximum biomass synthesis efficiency was obtained with grass (7% of added C). Overall, the woodland soil was most efficient at synthesizing biomass C and the arable soil the least. We conclude that substrate type was the overriding factor that determined the amount of new soil microbial biomass synthesized. Mineralization of substrate C by soil microorganisms was also influenced mainly by substrate type and less by soil management or size of original biomass.     

The spatial distribution of soil microbial biomass in a permanent row crop

The effects of soil texture (silt loam or sandy loam) and cultivation practice (green manure) on the size and spatial distribution of the microbial biomass and its metabolic quotient were investigated in soils planted with a permanent row crop of hops (Humulus lupulus). The soil both between and in the plant rows was sampled at three different depths (0–10, 10–20, and 20–30 cm). The silt loam had a higher overall microbial biomass C concentration (260 g g^-1) than the sandy loam (185 g g^-1), whereas the sandy loam had a higher (3.1 g CO₂-C mg^-1 microbial Ch^-1) metabolic quotient than the silt loam (2.6 g CO₂-C mg^-1 microbial C h^-1), on average over depth (0–30 cm) and over all treatments. There was a sharp decrease in the microbial biomass with increasing depth for all plots. However, this was more pronounced in the silt loam than in the sandy loam. There was no distinct influence of sampling depth on the metabolic quotient. The microbial biomass was considerably higher in the rows than between the rows, especially in the silt loam plots. There was no significant difference between plots without green manure and plots with green manure for either the microbial biomass or the metabolic quotient.