Effects of root and litter exclusion on soil CO2 efflux and microbial biomass in wet tropical forests

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass at a soil depth of 25 cm in a secondary forest (dominated by Tabebuia heterophylla) and a pine (Pinus caribaea) plantation in the Luquillo Experimental Forest in Puerto Rico. The experimental plots were initially established in 1990, when root, forest floor mass and new litterfall were excluded for 7 y since then. Soil respiration was significantly reduced in the litter and root exclusion plots in both the secondary forest and the pine plantation compared with the control. Root exclusion had a greater effect on soil CO₂ efflux than the litter exclusion in the plantation, whereas a reversed pattern was observed in the secondary forest. The reduction of microbial biomass in the root exclusion plot was greater in the secondary forest (59%) than in the plantation (31%), while there was no difference of the reduction in the litter exclusion plots between these forests. Our results suggest that above-ground input and roots (root litter and exudates) differentially affect soil CO₂ efflux under different vegetation types.     

Species richness of ectomycorrhizal hyphal necromass increases soil CO2 efflux under laboratory conditions

The ectomycorrhizal mycelium is a large component of boreal and temperate forest soil microbial biomass and the resulting necromass is likely to be an important source of nutrients for saprotrophic microorganisms. Here we test the effects of species richness of ectomycorrhizal mycelial biomass on short-term CO₂ efflux by amending forest soil with necromass from 8 fungal species added separately and in mixtures of 2, 4 and 8 species. All additions of necromass rapidly increased soil CO₂ efflux compared to unamended controls but CO₂ efflux increased significantly with species richness. Efflux of CO₂ did not correlate with the carbon (C) or nitrogen (N) contents or the C:N ratio of the added necromass. The study demonstrates that species diversity of dead ectomycorrhizal fungal hyphae can have important consequences for soil CO₂ efflux, and suggests decomposition of hyphae is regulated by specific constituents of the nutrient pools in the necromass rather than the total quantities added.     

Long-term effects of seasonal and diurnal temperature fluctuations on carbon dioxide efflux from a forest soil

Temperature fluctuations are a fundamental entity of the soil environment in the temperate zone and show fast (diurnal) and slow (seasonal) dynamics. Responses of soil respiration to temperature fluctuations were investigated in a root-free soil of a mid-European beech-oak forest. First, in laboratory we analysed the efflux of CO₂ from soil microcosms exposed to seasonal (±5 °C of the annual mean) and diurnal fluctuations (±5 °C of the seasonal levels) in a two-factorial design. Second, in field microcosms we investigated effects of smoothing diurnal temperature fluctuations in soil (simulating a possible global trend) on CO₂ efflux. Third, the natural temperature regime was simulated in laboratory microcosms and their CO₂ efflux was compared to the one in the field. The experiments lasted for 1 year to differentiate seasonal and annual responses.Dynamics of CO₂ efflux, microbial basal respiration, biomass and qO₂ varied with seasonal temperature regime. However, in the laboratory the annual cumulative CO₂-C production did not differ between treatments and varied between 10.9% and 11.7% of the total microcosm C, disregarding seasonal and/or diurnal fluctuations. The similarity of cumulative C production suggests that the availability of microbially mobilisable carbon pools rather than the temperature regime limited soil respiration. Diurnal fluctuations generally did not affect CO₂ efflux and microbial activity, though winter Q₁₀ values were increased in their absence. Simulation of the natural temperature regime in the laboratory resulted in CO₂ efflux similar to field microcosms. In the field, rates of CO₂ efflux and microbial activity, seasonal and annual cumulative CO₂-C production were significantly higher at smoothed than at natural temperature conditions (annually 13.1% and 11.0% of total C was respired, respectively). Facing global climate changes the mechanisms regulating responses of soil respiration to temperature fluctuations need further investigation.     

Nitrogen fertilization increases rhizodeposit incorporation into microbial biomass and reduces soil organic matter losses

Agricultural soils receive large amounts of anthropogenic nitrogen (N), which directly and indirectly affect soil organic matter (SOM) stocks and CO₂ fluxes. However, our current understanding of mechanisms on how N fertilization affects SOM pools of various ages and turnover remains poor. The δ¹³C values of SOM after wheat (C₃)-maize (C₄) vegetation change were used to calculate the contribution of C₄-derived rhizodeposited C (rhizo-C) and C₃-derived SOM pools, i.e., rhizo-C and SOM. Soil (Ap from Haplic Luvisol) sampled from maize rhizosphere was incubated over 56 days with increasing N fertilization (four levels up to 300 kg N ha^?1), and CO₂ efflux and its δ¹³C were measured. Nitrogen fertilization decreased CO₂ efflux by 27–42% as compared to unfertilized soil. This CO₂ decrease was mainly caused by the retardation of SOM (C₃) mineralization. Microbial availability of rhizo-C (released by maize roots within 4 weeks) was about 10 times higher than that of SOM (older than 4 weeks). Microbial biomass and dissolved organic C remained at the same level with increasing N. However, N fertilization increased the relative contribution of rhizo-C to microbial biomass by two to five times and to CO₂ for about two times. This increased contribution of rhizo-C reflects strongly accelerated microbial biomass turnover by N addition. The decomposition rate of rhizo-C was 3.7 times faster than that of SOM, and it increased additionally by 6.5 times under 300 kg N ha^?1 N fertilization. This is the first report estimating the turnover and incorporation of very recent rhizo-C (4 weeks old) into soil C pools and shows that the turnover of rhizo-C was much faster than that of SOM. We conclude that the contribution of rhizo-C to CO₂ and to microbial biomass is highly dependent on N fertilization. Despite acceleration of rhizo-C turnover, the increased N fertilization facilitates C sequestration by decreasing SOM decomposition.     

Microbial biomass and C and N transformations in forest floors under European beech, sessile oak, Norway spruce and Douglas-fir at four temperate forest sites

The purpose of this research was to compare soil chemistry, microbially mediated carbon (C) and nitrogen (N) transformations and microbial biomass in forest floors under European beech (Fagus sylvatica L.), sessile oak (Quercus petraea (Mattuschka) Lieblein), Norway spruce (Picea abies (L.) Karst) and Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) at four study sites. We measured soil chemical characteristics, net N mineralization, potential and relative nitrification, basal respiration, microbial and metabolic quotient and microbial biomass C and N under monoculture stands at all sites (one mixed stand). Tree species affected soil chemistry, microbial activities and biomass, but these effects varied between sites. Our results indicated that the effect of tree species on net N mineralization was likely to be mediated through their effect on soil microbial biomass, reflecting their influence on organic matter content and carbon availability. Differences in potential nitrification and relative nitrification might be related to the presence of ground vegetation through its influence on soil NH₄ and labile C availability. Our findings highlight the need to study the effects of tree species on microbial activities at several sites to elucidate complex N cycle interactions between tree species, ground vegetation, soil characteristics and microbial processes.     

Timing patterns of nitrogen application alter plant production and CO2 efflux in an alpine meadow on the Tibetan Plateau,China

Nitrogen (N) availability is an important factor that determines ecosystem productivity and respiration, especially in N-limited alpine ecosystems. However, the magnitude of this response depends on the timing and amounts of N input. Moreover, we have only a limited understanding of the potential effects of the timing of N fertilization on ecosystem carbon (C) and N processes, and activities of the soil microbes. A nitrogen fertilization experiment was conducted in an alpine meadow on the Tibetan Plateau to determine how plant productivity and ecosystem respiration (RE) respond to the timing and amount of N application. In this study, half of the N was added either in the early spring (ES), before the growing season, or in the late fall (LF), after the growing season. All treatments received the other half of the N in mid-July. Three N levels (10, 20, 40 kg N hm⁻² yr⁻¹) were used for each of two N treatments, with no N addition used as a control. Plant aboveground biomass, ecosystem respiration (RE) and soil respiration (RS) were measured for the 2011 and 2012 growing seasons. The LF treatment enhanced ecosystem CO₂ efflux compared with the ES treatment at high N addition levels, resulting from an increase of soil dissolved organic C (DOC) and soil microbial activity. The ES treatment resulted in increased plant aboveground biomass when compared with LF during both growing seasons, although this increase accounted for little variation in ecosystem and soil respiration. Overall, the ES treatment is likely to increase the ecosystem C pool, while the LF treatment could accelerate ecosystem C cycling, especially for the high N treatment. Our results suggest that supplying N during the early stage of the growing season benefits both forage production and soil C sequestration in this alpine ecosystem.     

Soil carbon dioxide flux, carbon sequestration and crop productivity in a tropical dryland agroecosystem: Influence of organic inputs of varying resource quality

In view of the significance of agricultural soils in affecting global C balance, the impact of manipulation of the quality of exogenous inputs on soil CO₂–C flux was studied in rice–barley annual rotation tropical dryland agroecosystem. Chemical fertilizer, Sesbania shoot (high quality resources), wheat straw (low quality resource) and Sesbania + wheat straw (high + low quality), all carrying equivalent recommended dose of N, were added to soil. A distinct seasonal variation in CO₂–C flux was recorded in all treatments, flux being higher during rice period, and much reduced during barley and summer fallow periods. During rice period the mean CO₂–C flux was greater in wheat straw (161% increase over control) and Sesbania + wheat straw (+129%) treatments; however, during barley and summer fallow periods differences among treatments were small. CO₂–C flux was more influenced by seasonal variations in water-filled pore space compared to soil temperature. In contrast, the role of microbial biomass and live crop roots in regulating soil CO₂–C flux was highly limited. Wheat straw input showed smaller microbial biomass with a tendency of rapid turnover rate resulting in highest cumulative CO₂–C flux. The Sesbania input exhibited larger microbial biomass with slower turnover rate, leading to lower cumulative CO₂–C flux. Addition of Sesbania to wheat straw showed higher cumulative CO₂–C flux yet supported highest microbial biomass with lowest turnover rate indicating stabilization of microbial biomass. Although single application of wheat straw or Sesbania showed comparable net change in soil C (18% and 15% relative to control, respectively) and crop productivity (32% and 38%), yet they differed significantly in soil C balance (374 and −3 g C m⁻² y⁻¹ respectively), a response influenced by the recalcitrant and labile nature of the inputs. Combining the two inputs resulted in significant increment in net change in soil C (33% over control) and crop yield (49%) in addition to high C balance (152 g C m⁻² y⁻¹). It is suggested that appropriate mixing of high and low quality inputs may contribute to improved crop productivity and soil fertility in terms of soil C sequestration.     

Factors controlling soil CO2 effluxes and the effects of rewetting on effluxes in adjacent deciduous, coniferous, and mixed forests in Korea

To better understand the factors that control forest soil CO₂ efflux and the effects of rewetting on efflux, we measured soil CO₂ efflux in adjacent deciduous, coniferous, and mixed forests in the central part of the Korean Peninsula over the course of one year. We also conducted laboratory rewetting experiments with soil collected from the three sites using three different incubation temperatures (4 °C, 10 °C, and 20 °C). Soil moisture (SM), soil organic matter (SOM), and total root mass values of the three sites were significantly different from one another; however, soil temperature (ST), observed soil CO₂ efflux and sensitivity of soil CO₂ efflux to ST (i.e., Q₁₀ = 3.7 ± 0.1) were not significantly different among the three sites. Soil temperature was a dominant control factor regulating soil CO₂ efflux during most of the year. We infer that soil CO₂ efflux was not significantly different among the sites due to similar ST and Q₁₀. Though a significant increase in soil CO₂ efflux following rewetting of dry soil was observed both in the field observations (60-170%) and laboratory incubation experiments (100-1000%), both the increased rates of soil CO₂ efflux and the magnitude of change in SM were not significantly different among the sites. The increased rates of soil CO₂ efflux following rewetting depended on the initial SM before rewetting. During drying phase after rewetting, a significant correlation between SM and soil CO₂ efflux was found, but the effect of ST on increased soil CO₂ efflux was not clear. Cumulative peak soil CO₂ efflux (11.3 ± 0.7 g CO₂ m⁻²) following rewetting in the field was not significantly different among the sites. Those evidences indicate that the observed similar rewetting effects on soil CO₂ efflux can be explained by the similar magnitude of change in SM after rewetting at the sites. We conclude that regardless of vegetation type, soil CO₂ efflux and the effect of rewetting on soil CO₂ efflux do not differ among the sites, and ST is a primary control factor for soil CO₂ efflux while SM modulates the effect of rewetting on soil CO₂ efflux. Further studies are needed to quantify and incorporate relationship of initial dryness of the soil and the frequency of the dry-wet cycle on soil CO₂ efflux into models describing carbon (C) processes in forested ecosystems.     

Changes in soil microbial biomass carbon and enzyme activities under elevated CO2 affect fine root decomposition processes in a Mongolian oak ecosystem

The relationships between soil microbial properties and fine root decomposition processes under elevated CO₂ are poorly understood. To address this question, we determined soil microbial biomass carbon (SMB-C) and nitrogen (SMB-N), enzymes related to soil carbon (C) and nitrogen (N) cycling, the abundance of cultivable N-fixing bacteria and cellulolytic fungi, fine root organic matter, lignin and holocellulose decomposition, and N mineralization from 2006 to 2007 in a Mongolian oak (Quercus mongolica Fischer ex Ledebour) ecosystem in northeastern China. The experiment consisted of three treatments: elevated CO₂ chambers, ambient CO₂ chambers, and chamberless plots. Fine roots had significantly greater organic matter decomposition rates under elevated CO₂. This corresponded with significantly greater SMB-C. Changes in the activities of protease and phenol oxidase under elevated CO₂ could not explain the changes in fine root N release and lignin decomposition rates, respectively, while holocellulose decomposition rate had the same response to experimental treatments as did cellulase activity. Changes in cultivable N-fixing bacterial and cellulolytic fungal abundances in response to experimental treatments were identical to those of N mineralization and lignin decomposition rates, respectively, suggesting that the two indices were closely related to fine root N mineralization and lignin decomposition. Our results showed that the increased fine root organic matter, lignin and holocellulose decomposition, and N mineralization rates under elevated CO₂ could be explained by shifts in SMB-C and the abundance of cellulolytic fungi and N-fixing bacteria. Enzyme activities are not reliable for the assessment of fine root decomposition and more attention should be given to the measurement of specific bacterial and fungal communities.     

Effects of tree harvesting, forest floor removal, and compaction on soil microbial biomass, microbial respiration, and N availability in a boreal aspen forest in British Columbia

The effects of timber harvesting and the resultant soil disturbances (compaction and forest floor removal) on relative soil water content, microbial biomass C and N contents (C_mic and N_mic), microbial biomass C:N ratio (C_mic-to-N_mic), microbial respiration, metabolic quotient (qCO₂), and available N content in the forest floor and the uppermost mineral soil (0-3 cm) were assessed in a long-term soil productivity (LTSP) site and adjacent mature forest stands in northeastern British Columbia (Canada). A combination of principal component analysis and redundancy analysis was used to test the effects of stem-only harvest, whole tree harvest plus forest floor removal, and soil compaction on the studied variables. Those properties in the forest floor were not affected by timber harvesting or soil compaction. In the mineral soil, compaction increased soil total C and N contents, relative water content, and N_mic by 45%, 40%, 34% and 72%, respectively, and decreased C_mic-to-N_mic ratio by 29%. However, these parameters were not affected by stem only harvesting or whole tree harvesting plus forest floor removal, contrasting the reduction of white spruce and aspen growth following forest floor removal and soil compaction reported in an earlier study. Those results suggest that at the study site the short-term effects of timber harvesting, forest floor removal, and soil compaction are rather complex and that microbial populations might not be affected by the perturbations in the same way as trees, at least not in the short term.     

Rhizosphere priming effect: Its functional relationships with microbial turnover, evapotranspiration, and C–N budgets

Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural ¹³C tracer method to separately measure SOM-derived CO₂ from root-derived CO₂, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to mechanistically link the rhizosphere priming effect to soil microbial turnover and evapotranspiration. Results indicated that the magnitude of the rhizosphere priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control, and was largely influenced by plant species and phenology. Balancing the extra soil C loss from the strong rhizosphere priming effect in the planted treatments with C inputs from rhizodeposits and root biomass, the whole plant–soil system remained with a net carbon gain at the end of the experiment. The increased soil microbial biomass turnover rate and the enhanced evapotranspiration rate in the planted treatments had clear positive relationships with the level of the rhizosphere priming effect. The rhizosphere enhancement of soil carbon mineralization in the planted treatments did not result in a proportional increase in net N mineralization, suggesting a possible de-coupling of C cycling with N cycling in the rhizosphere.     

Carbon stocks, soil respiration and microbial biomass in fire-prone tropical grassland, woodland and forest ecosystems

A thorough understanding of the role of microbes in C cycling in relation to fire is important for estimation of C emissions and for development of guidelines for sustainable management of dry ecosystems. We investigated the seasonal changes and spatial distribution of soil total, dissolved organic C (DOC) and microbial biomass C during 18 months, quantified the soil CO₂ emission in the beginning of the rainy season, and related these variables to the fire frequency in important dry vegetation types grassland, woodland and dry forest in Ethiopia. The soil C isotope ratios (δ¹³C) reflected the 15-fold decrease in the grass biomass along the vegetation gradient and the 12-fold increase in woody biomass in the opposite direction. Changes in δ¹³C down the soil profiles also suggested that in two of the grass-dominated sites woody plants were more frequent in the past. The soil C stock ranged from being 2.5 (dry forest) to 48 times (grassland) higher than the C stock in the aboveground plant biomass. The influence of fire in frequently burnt wooded grassland was evident as an unchanged or increasing total C content down the soil profile. DOC and microbial biomass measured with the fumigation-extraction method (C_mic) reflected the vertical distribution of soil organic matter (SOM). However, although SOM was stable throughout the year, seasonal fluctuations in C_mic and substrate-induced respiration (SIR) were large. In woodland and woodland-wooded grassland C_mic and SIR increased in the dry season, and gradually decreased during the following rainy season, confirming previous suggestions that microbes may play an important role in nutrient retention in the dry season. However, in dry forest and two wooded grasslands C_mic and SIR was stable throughout the rainy season, or even increased in this period, which could lead to enhanced competition with plants for nutrients. Both the range and the seasonal changes in soil microbial biomass C in dry tropical ecosystems may be wider than previously assumed. Neither SIR nor C_mic were good predictors of in situ soil respiration. The soil respiration was relatively high in infrequently burnt forest and woodland, while frequently burnt grasslands had lower rates, presumably because most C is released through dry season burning and not through decomposition in fire-prone systems. Shifts in the relative importance of the two pathways for C release from organic matter may have strong implications for C and nutrient cycling in seasonally dry tropical ecosystems.     

Carbon mineralization is promoted by phosphorus and reduced by nitrogen addition in the organic horizon of northern hardwood forests

Limitations to the respiratory activity of heterotrophic soil microorganisms exert important controls of CO₂ efflux from soils. In the northeastern US, ecosystem nutrient status varies across the landscape and changes with forest succession following disturbance, likely impacting soil microbial processes regulating the transformation and emission of carbon (C). We tested whether nitrogen (N) or phosphorus (P) limit the mineralization of soil organic C (SOC) or that of added C sources in the Oe horizon of successional and mature northern hardwood forests in three locations in central New Hampshire, USA. Added N reduced mineralization of C from SOC and from added leaf litter and cellulose. Added P did not affect mineralization from SOC; however, it did enhance mineralization of litter- and cellulose- C in organic horizons from all forest locations. Added N increased microbial biomass N and K₂SO₄-extractable DON pools, but added P had no effect. Microbial biomass C increased with litter addition but did not respond to either nutrient. The direction of responses to added nutrients was consistent among sites and between forest ages. We conclude that in these organic horizons limitation by N promotes mineralization of C from SOC, whereas limitation by P constrains mineralization of C from new organic inputs. We also suggest that N suppresses respiration in these organic horizons either by relieving the N limitation of microbial biomass synthesis, or by slowing turnover of C through the microbial pool; concurrent measures of microbial growth and turnover are needed to resolve this question.     

Turnover of soil organic matter and of microbial biomass under C3-C4 vegetation change: Consideration of C fractionation and preferential substrate utilization

Two processes contribute to changes of the δ¹³C signature in soil pools: ¹³C fractionation per se and preferential microbial utilization of various substrates with different δ¹³C signature. These two processes were disentangled by simultaneously tracking δ¹³C in three pools - soil organic matter (SOM), microbial biomass, dissolved organic carbon (DOC) - and in CO₂ efflux during incubation of 1) soil after C₃-C₄ vegetation change, and 2) the reference C₃ soil.The study was done on the Ap horizon of a loamy Gleyic Cambisol developed under C₃ vegetation. Miscanthus giganteus - a perennial C₄ plant - was grown for 12 years, and the δ¹³C signature was used to distinguish between ‘old’ SOM (>12 years) and ‘recent’ Miscanthus-derived C (<12 years). The differences in δ¹³C signature of the three C pools and of CO₂ in the reference C₃ soil were less than 1‰, and only δ¹³C of microbial biomass was significantly different compared to other pools. Nontheless, the neglecting of isotopic fractionation can cause up to 10% of errors in calculations. In contrast to the reference soil, the δ¹³C of all pools in the soil after C₃-C₄ vegetation change was significantly different. Old C contributed only 20% to the microbial biomass but 60% to CO₂. This indicates that most of the old C was decomposed by microorganisms catabolically, without being utilized for growth. Based on δ¹³C changes in DOC, CO₂ and microbial biomass during 54 days of incubation in Miscanthus and reference soils, we concluded that the main process contributing to changes of the δ¹³C signature in soil pools was preferential utilization of recent versus old C (causing an up to 9.1‰ shift in δ¹³C values) and not ¹³C fractionation per se.Based on the δ¹³C changes in SOM, we showed that the estimated turnover time of old SOM increased by two years per year in 9 years after the vegetation change. The relative increase in the turnover rate of recent microbial C was 3 times faster than that of old C indicating preferential utilization of available recent C versus the old C.Combining long-term field observations with soil incubation reveals that the turnover time of C in microbial biomass was 200 times faster than in total SOM. Our study clearly showed that estimating the residence time of easily degradable microbial compounds and biomarkers should be done at time scales reflecting microbial turnover times (days) and not those of bulk SOM turnover (years and decades). This is necessary because the absence of C reutilization is a prerequisite for correct estimation of SOM turnover. We conclude that comparing the δ¹³C signature of linked pools helps calculate the relative turnover of old and recent pools.     

Long-term fertilization and residue return affect soil stoichiometry characteristics and labile soil organic matter fractions

Ecological stoichiometry provides the possibility for linking microbial dynamics with soil carbon (C), nitrogen (N), and phosphorus (P) metabolisms in response to agricultural nutrient management. To determine the roles of fertilization and residue return with respect to ecological stoichiometry, we collected soil samples from a 30-year field experiment on residue return (maize straw) at rates of 0, 2.5, and 5.0 Mg ha^-1 in combination with 8 fertilization treatments:no fertilizer (F0), N fertilizer, P fertilizer, potassium (K) fertilizer, N and P (NP) fertilizers, N and K (NK) fertilizers, P and K (PK) fertilizers, and N, P, and K (NPK) fertilizers. We measured soil organic C (SOC), total N and P, microbial biomass C, N, and P, water-soluble organic C and N, KMnO₄-oxidizable C (KMnO₄-C), and carbon management index (CMI). Compared with the control (F0 treatment without residue return), fertilization and residue return significantly increased the KMnO₄-C content and CMI. Furthermore, compared with the control, residue return significantly increased the SOC content. Moreover, the NPK treatment with residue return at 5.0 Mg ha^-1 significantly enhanced the C:N, C:P, and N:P ratios in the soil, whereas it significantly decreased the C:N and C:P ratios in soil microbial biomass. Therefore, NPK fertilizer application combined with residue return at 5.0 Mg ha^-1 could enhance the SOC content through the stoichiometric plasticity of microorganisms. Residue return and fertilization increased the soil C pools by directly modifying the microbial stoichiometry of the biomass that was C limited.     

Effects of ammonium and nitrate additions on carbon mineralization in wetland soils

Wetlands have been recognized as a soil carbon (C) sink due to low decomposition. As decomposition is largely controlled by the availability of soil nitrogen (N), an elevated anthropogenic N input could influence the C balance in wetlands. However, the effects of the form of N on decomposition are poorly understood. Here, a 54-day laboratory incubation experiment was conducted, with a diel cycle (day: 22 °C for 13 h; night: 17 °C for 11 h) in order to determine how the dominant N form influences the mineralization of soil C in two adjacent wetland soils, with distinct physicochemical characteristics. Three combinations of N compounds were added at three different rates (0, 30, 60 kg N ha⁻¹ yr⁻¹): Ammonium dominant (NH₄Cl + NH₄NO₃); nitrate dominant (NH₄NO₃ + NaNO₃); and ammonium nitrate treatments (NH₄NO₃). In the acidic soil, the CO₂ efflux was reduced with N additions, especially with NH₄NO₃ treatment. In addition, decreases in the microbial enzyme activities (β-glucosidase, N-acetyl-glucosaminidase, phosphatase, and phenol oxidase) and soil pH were observed with NH₄NO₃ and -dominant treatment. Under alkaline conditions, marginal changes in response to N additions were observed in the soil CO₂ efflux, extractable DOC, simple substrate utilization, enzyme activities and pH. A regression analysis revealed that the changes in pH and enzyme activities after fertilization significantly influenced the soil CO₂ efflux. Our findings suggest that the form of N additions could influence the rate of C cycling in wetland soils via biological (enzyme activities) and chemical (pH) changes.     

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.     

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.     

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.     

Responses of microbial biomass and respiration of soil to topography,burning, and nitrogen fertilization in a temperate steppe

Temporal dynamics of microbial biomass and respiration of soil and their responses to topography, burning, N fertilization, and their interactions were determined in a temperate steppe in northern China. Soil microbial indices showed strong temporal variability over the growing season. Soil microbial biomass C (MBC) and N (MBN) were 14.8 and 11.5% greater in the lower than upper slope, respectively. However, the percentage of organic C present as MBC and the percentage of total N present as MBN were 16.9 and 26.2% higher in the upper than lower slope, respectively. Neither microbial respiration (MR) nor metabolic quotient (qCO₂) was affected by topography. Both MBC and MBN were increased by burning, on average, by 29.8 and 14.2% over the growing season, and MR and qCO₂ tended to reduce depending on the sampling date, especially in August. Burning stimulated the percentage of organic C present as MBC and the percentage of total N present as MBN in the upper slope, but did not change these two parameters in the lower slope. No effects of N fertilization on soil microbial indices were observed in the first growing season after the treatment. Further research is needed to study the long-term relationships between changes in soil microbial diversity and activity and plant community in response to burning and N fertilization.