Fungal Substrate-induced Respiration Research Articles

Dynamics, Structure, and Functional Activity of Microbial Biomass in Soils of Restoring Felled Areas in Fir Forests of the Yenisei Ridge

The dynamics, structure, and functional activity of microbial biomass have been studied in soils of felling areas in southern taiga fir forests (the Yenisei Ridge, Krasnoyarsk krai), which are at different stages of natural regenerative successions. The top organomineral horizons of soddy-podzolic soil (Albic Glossic Retisol (Loamic, Cutanic, Ochric)) of the after-felling regeneration succession (with the stage of secondary birch forests) is characterized by an increase in the content of carbon of microbial biomass (Cmic) and in the intensity of basal respiration from new felling to the stage of dense young forest. In the soil of the dense young-wood stage, the total reserves of Cmic (170 g C/m3) and microbial CO2 production (528 mg СО2–С/m3 per hour) are the highest and exceed the control by 80–85%. In the humus-accumulative soil horizon of the studied succession series, there is a tendency for a decrease in the portion of fungal substrate-induced respiration and in the fungi to bacteria ratio in comparison with the control. In the middle-aged deciduous forests with dark-coniferous undergrowth, all the considered parameters of microbial complexes decrease but remain higher than the control. A trend for a decrease in Cmic and basal respiration is revealed in the profile of soddy-podzolic soil of the regeneration series without species change over an eight-year period. At the stage of fir dense young forest, all ecological-functional parameters of the microbial complex become closer to the control small-grass–true-moss fir forest.

Effects of tillage and winter cover cropping on microbial substrate-induced respiration and soil aggregation in two Japanese fields

We hypothesized that cover cropping could increase soil microbial activities under various tillage systems and that increased microbial activities would improve soil properties. Soil sampling was conducted at two fields in Japan in 2009. At the Ibaraki field (Andosol, clay loam), three tillage practices (no-tillage, plowing to 30 cm, and rotary tillage to 15 cm) and three types of winter cover cropping [bare fallow as control, hairy vetch (Vicia villosa Roth), and rye (Secale cereale L.)] were conducted from 2003 to 2009. At the Hokkaido field (Fluvisol, light clay), two tillage practices (autumn tillage and rotary tillage with a rotary tiller to a depth of 15 cm once in autumn and twice in a year, respectively), and four types of winter cover cropping (bare fallow, hairy vetch, bristle oat (Avena strigosa L.), and a mixture of hairy vetch and bristle oat) were conducted from 2006 to 2009. Soil microbial activities and the fungal-to-bacterial activity ratio (F/B ratio) were estimated by the substrate-induced respiration (SIR) method with the use of selective antibiotics. At the Ibaraki field, rye cover cropping showed higher microbial SIR than bare fallow at depths of 0–30 cm and rotary tillage maintained higher microbial SIR than no-tillage or plowing at depths of 7.5–15 cm. There was no meaningful interaction effect between cover cropping and tillage on microbial SIR. At the Hokkaido field, cover cropping and tillage had only limited effects on microbial SIR. High F/B ratios (indicating fungal dominance) were recorded with the use of cover crops in both fields. Fungal SIR, estimated from the microbial SIR and F/B ratio, was closely related to the content of total soil organic carbon (SOC) and the mean weight diameter (MWD) of water-stable aggregates. Based on SOC, fungal SIR was significantly higher under rye cover cropping. The relationship between fungal SIR and MWD was affected by tillage. We conclude that rye cover cropping and rotary tillage were very effective in increasing fungal SIR, SOC, and MWD in the Ibaraki soil. Field practices that enhance fungal activities might be effective in improving certain types of arable soil.

Dynamics of soil biota at different depths under two contrasting tillage practices

One aim of conservation tillage is to preserve soil biological properties. This study was conducted to examine the effects of two contrasting tillage treatments on soil biota at different depths. We investigated the population dynamics and vertical distributions of microbes and several soil faunal groups for 2 years in field Andosols in northeastern Japan. The experimental plots were under no tillage (NT) or conventional tillage (CT, rotary tilled to 20 cm) management. In the 0–10-cm soil layer, bacterial and fungal substrate-induced respiration (SIR) and the population density of enchytraeids were higher under NT than under CT, but the population densities of protozoa, mites, and collembolans did not differ significantly. In contrast, at 10–20 cm, both SIR values were higher under CT, where larger populations of mites and collembolans were recorded. At both depths, nematodes were more abundant under CT. Thus, the effects of tillage on these soil organisms differed according to soil depth, and negative impacts of tillage were smaller in the deeper layer. Larger amounts of earthworm casts at the soil surface in NT plots showed a greater biomass of earthworms than in CT. To evaluate the activities of soil biota, we buried litterbags with three different mesh sizes at the two depths and examined the rate of decomposition. The daily decay constant of litter in the surface soil layer (1.5–8.5 cm) was greater under NT. We suppose that the activities of soil biota in this layer were stimulated under NT, and that especially microbes and enchytraeids, which were abundant at 0–10 cm, contributed greatly to the decomposition.

Abundance and activity of soil organisms in fields of maize grown with a white clover living mulch

The effects of a white clover (Trifolium repens) living mulch on the soil detritus food-web during maize (Zea mays) culture were studied on Andosols for 2 years. In the first year, the use of white clover (W) and Italian ryegrass (I; Lolium multiflorum) was compared against the control treatment in the absence of fertilization. In the second year, combinations of white clover and inorganic fertilizers were studied. Bacterial and fungal substrate-induced respiration and the population densities of protozoa (flagellates, naked amoebae, ciliates), nematodes, and microarthropods (mites, collembolans) increased with the use of white clover living mulch. In the first year, nematode and microarthropod populations increased throughout the growing season in the living mulch plots. In the second year, the communities of soil organisms under the white clover living mulch had reached a more mature successional stage and were characterized by increased populations of organisms in the fungal pathway of organic matter decomposition and by a high population density of higher trophic groups (e.g., mesostigmatid mites and ciliates). The higher litter decomposition rate in the living mulch plots suggested that the function of the detritus food-web was enhanced under the living mulch system. On the other hand, inorganic fertilization only increased populations of nematodes and mesostigmatid mites in mid-summer, possibly owing to better maize growth, and had no effect on other soil organisms.

Potential Physiological Activities of Fungi and Bacteria in Relation to Plant Litter Decomposition along a Gap Size Gradient in a Natural Subtropical Forest

The dynamics of fungal and bacterial potential physiological activities during leaf, branch, and bark litter decomposition along a gap size gradient in a subtropical forest was determined using substrate-induced respiration (SIR) with antibiotics selective for fungi and bacteria, respectively. A gap size gradient (1) was under closed canopy; (2) had small gaps with a diameter (≤5m); (3) had small to intermediate gaps (5–15 m diameter); (4) had intermediate to large gaps (15–30 m diameter); and (5) had large gaps (≥30 m diameter). Litter decomposition was studied using a litter bag technique. Fungi had higher SIR than bacteria for each type of litter in any size class of gaps. Gaps 1, 2, and 3 had higher fungal and bacterial SIRs than gaps 4 and 5. Moreover, decomposing leaf litter exhibited higher fungal and bacterial SIRs than branch, and branch higher than bark. Simple correlation analysis indicated that fungal SIR was a reliable index of decomposition rates. Fungal SIR was significantly and positively correlated with soil moisture, whereas bacterial SIR was not significantly correlated with soil moisture. The relationships among microclimatic factors, fungal and bacterial physiological activities, and rates of plant litter decomposition suggest that, in subtropical ecosystems, fungal community activities were strongly and directly regulated by the environmental heterogeneity within gaps, and an important regulator of rates of plant litter decomposition rates.

Estimating crop residue decomposition coefficients using substrate-induced respiration

Modeling of crop residue decomposition for nutrient cycling and effectiveness of residues to control soil erosion requires information on crop-specific decomposition coefficients ( k). Respiration of decomposing residues reflects the activity of the microbial community and should give an indication of the residue decomposition rate. A method for estimating k using substrate-induced respiration (SIR) of plant residues was evaluated. Basal respiration, total SIR, fungal SIR and bacterial SIR were measured for five crop residues monthly for 1 y. In general, total SIR and basal respiration declined for the more decomposable residues, but were somewhat constant for the more resistant residues. Mass loss was used to determine k for a single exponential decay function. Prediction of k from SIR using an equation proposed by Neely et al. (1991) ( Soil Biology & Biochemistry 23, 947–954) was unsatisfactory for the five crops. A new equation ( k = − 6.07 × 10 −4 + 6.23 × 10 −6 × SIR) was determined using the data of Neely et al. (1991) and data from the current study. Prediction of k using the 60-day SIR measurement was significantly improved with the new equation. Predicting k from SIR could greatly reduce the labor and time involved in evaluating decomposition differences between residues and locations.

Measuring bacterial and fungal substrate-induced respiration in dry soils

The substrate-induced respiration inhibition (SIRIN) method of Anderson and Domsch for partitioning bacterial and fungal contributions to soil respiration was modified for application to dry soils. This new method also provided a comparative basis when measuring SIRIN in soils of different moisture contents. Soil was incubated under optimum moisture conditions (55% water-filled pore space) to maximize microbial activity and to ensure homogeneous incorporation of substrate and inhibitors into soil. Soil samples were packed to a uniform bulk density prior to measurement of CO 2 evolution by gas chromatography. Glucose (3 mg g −1) was added together with streptomycin (0.5 or 1.0 mg g −1) and/or cycloheximide (15 mg g −1) for selective respiratory inhibition. The procedure included conditioning for 16 h at 4°C, followed by 1.5-h equilibration and 2-h incubation. The method yielded consistent and reproducible CO 2 respiration measurements for soils from a semi-arid region having gravimetric moisture contents ranging between 7.5 and 23.2%. Method sensitivity was not sufficient to detect variations in the fungal-to-bacterial ratio due to management practice for the soil under study. Measured fungal-to-bacterial ratios of 29:1 and 15:1, for conventionally and no-till managed soil, were not significantly different at a probability level of 5%.

Relationships between fungal and bacterial substrate-induced respiration, biomass and plant residue decomposition

Residues of six plant species were incubated in the field and analyzed for decomposition rates, fungal, bacterial and total substrate-induced respiration (SIR), total fungal and bacterial biomass and changes in residue composition during 161 days. Plant residues included crimson clover ( Trifolium incarnatum L.), hairy vetch ( Vicia villosa Roth), crabgrass [ Digitaria sanguinalis, (L.) Scop.], winter rye ( Secale cereale L.), grain sorghum ( Sorghum bicolor L. Moench) and chestnut oak ( Quercus prinus L.) leaves. Plant residues were incubated in litterbags placed on the soil surface in no-tillage ( T. incarnatum, V. villosa, S. bicolor, S. cereale), old-field ( D. sanguinalis) or hardwood forest ( Q. prinus) plots at the Horseshoe Bend Experimental Area in Athens, Ga and collected periodically for analyses. Decomposition rate constants ( k) were greatest for V. villosa followed by T. incarnatum, D. sanguinalis, S. cereale, S. bicolor and Q. prinus. Net N loss generally followed the pattern of dry matter loss. Net N gain was observed after 100 days of decay for those residues with high initial C:N ratios. Initial N concentration was exponentially related with the annual decay rale constant ( r 2 = 0.93) for all species; however, on an individual species basis, lignin content was best correlated to dry matter weight loss. Total SIR was greatest on T. incarnatum and V. villosa followed by D. sanguinalis, S. bicolor, S. cereale and Q. prinus. Across all sample dates, residue carbon-to-nitrogen ratio was the best predictor of total SIR. Measurements of potential fungal and bacterial activity by SIR as well as biomass-C estimates by direct counts indicated that fungi were the dominant decomposers of these surface residues. For most residues, lignin content through time exerted the greatest influence on fungal SIR and fungal biomass-C ( r = −0.56 to −0.93). Total SIR, fungal SIR and total fungal biomass tended to decrease through time as residues decomposed. Total SIR, on any given sample date, was significantly correlated with residue dry weight remaining and annual decay rate constants were exponentially related to overall mean values of total SIR for all residues excluding S. cereale ( r 2 = 0.99). Residue SIR rates as a measure of the potentially active microbial biomass reflected the resource qualities of the plant residues investigated here and were positively correlated to their decomposition rates.