Changing sources of soil respiration with time since fire in a boreal forest

Radiocarbon signatures (Δ¹⁴C) of carbon dioxide (CO₂) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ¹⁴C of soil respiration and soil CO₂ in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ¹⁴C with Δ¹⁴C of CO₂ evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ¹⁴C of respired CO₂ indicated marked variation in respiration sources in space and time. The ¹⁴C signature of respired CO₂ respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ¹⁴C greater (averaging ～120‰) than autotrophic respiration. The Δ¹⁴C of autotrophically respired CO₂ in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004). CO₂ respired by black spruce roots in stands >40 years old had Δ¹⁴C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants. Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ～50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO₂ had Δ¹⁴C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ¹⁴C of soil respiration in younger successional stands dropped below those of the atmospheric CO₂.     

Mechanisms influencing changes in lake area in Alaskan boreal forest

During the past ～50 years, the number and area of lakes have declined in several regions in boreal forests. However, there has been substantial finer‐scale heterogeneity; some lakes decreased in area, some showed no trend, and others increased. The objective of this study was to identify the primary mechanisms underlying heterogeneous trends in closed‐basin lake area. Eight lake characteristics (δ¹⁸O, electrical conductivity, surface : volume index, bank slope, floating mat width, peat depth, thaw depth at shoreline, and thaw depth at the forest boundary) were compared for 15 lake pairs in Alaskan boreal forest where one lake had decreased in area since ～1950, and the other had not. Mean differences in characteristics between paired lakes were used to identify the most likely of nine mechanistic scenarios that combined three potential mechanisms for decreasing lake area (talik drainage, surface water evaporation, and terrestrialization) with three potential mechanisms for nondecreasing lake area (subpermafrost groundwater recharge through an open talik, stable permafrost, and thermokarst). A priori expectations of the direction of mean differences between decreasing and nondecreasing paired lakes were generated for each scenario. Decreasing lakes had significantly greater electrical conductivity, greater surface : volume indices, shallower bank slopes, wider floating mats, greater peat depths, and shallower thaw depths at the forest boundary. These results indicated that the most likely scenario was terrestrialization as the mechanism for lake area reduction combined with thermokarst as the mechanism for nondecreasing lake area. Terrestrialization and thermokarst may have been enhanced by recent warming which has both accelerated permafrost thawing and lengthened the growing season, thereby increasing plant growth, floating mat encroachment, transpiration rates, and the accumulation of organic matter in lake basins. The transition to peatlands associated with terrestrialization may provide a transient increase in carbon storage enhancing the role of northern ecosystems as major stores of global carbon.     

Tree growth and soil acidification in response to 30 years of experimental nitrogen loading on boreal forest

Relations among nitrogen load, soil acidification and forest growth have been evaluated based on short‐term (<15 years) experiments, or on surveys across gradients of N deposition that may also include variations in edaphic conditions and other pollutants, which confound the interpretation of effects of N per se. We report effects on trees and soils in a uniquely long‐term (30 years) experiment with annual N loading on an un‐polluted boreal forest. Ammonium nitrate was added to replicated (N=3) 0.09 ha plots at two doses, N1 and N2, 34 and 68 kg N ha^?1 yr^?1, respectively. A third treatment, N3, 108 kg N ha^?1 yr^?1, was terminated after 20 years, allowing assessment of recovery during 10 years. Tree growth initially responded positively to all N treatments, but the longer term response was highly rate dependent with no gain in N3, a gain of 50 m³ ha^?1 stemwood in N2 and a gain of 100 m³ ha^?1 stemwood in excess of the control (N0) in N1. High N treatments caused losses of up to 70% of exchangeable base cations (Ca²⁺, Mg²⁺, K⁺) in the mineral soil, along with decreases in pH and increases in exchangeable Al³⁺. In contrast, the organic mor‐layer (forest floor) in the N‐treated plots had similar amounts per hectare of exchangeable base cations as in the N0 treatment. Magnesium was even higher in the mor of N‐treated plots, providing evidence of up‐lift by the trees from the mineral soil. Tree growth did not correlate with the soil Ca/Al ratio (a suggested predictor of effects of soil acidity on tree growth). A boron deficiency occurred on N‐treated plots, but was corrected at an early stage. Extractable NH₄⁺ and NO₃^?were high in mor and mineral soils of on‐going N treatments, while NH₄+ was elevated in the mor only in N3 plots. Ten years after termination of N addition in the N3 treatment, the pH had increased significantly in the mineral soil; there were also tendencies of higher soil base status and concentrations of base cations in the foliage. Our data suggest the recovery of soil chemical properties, notably pH, may be quicker after removal of the N‐load than predicted. Our long‐term experiment demonstrated the fundamental importance of the rate of N application relative to the total amount of N applied, in particular with regard to tree growth and C sequestration. Hence, experiments adding high doses of N over short periods do not mimic the long‐term effects of N deposition at lower rates.     

Partitioning sources of soil respiration in boreal black spruce forest using radiocarbon

Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ¹⁴C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently‐fixed C that fuels plant or microbial metabolism has Δ¹⁴C values close to that of current atmospheric CO₂, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ¹⁴C of C respired by recently excised black spruce roots averaged 14‰ greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ¹⁴C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60‰ higher than the contemporary atmosphere Δ¹⁴CO₂, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ¹⁴C end members with measurements of the Δ¹⁴C of total soil respiration, we calculated that 47–63% of soil CO₂ emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ¹⁴C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration.     

Termination of belowground C allocation by trees alters soil fungal and bacterial communities in a boreal forest

The introduction of photosynthates through plant roots is a major source of carbon (C) for soil microbial biota and shapes the composition of fungal and bacterial communities in the rhizosphere. Although the importance of this process, especially to ectomycorrhizal fungi, has been known for some time, the extent to which plant belowground C allocation controls the composition of the wider soil community is not understood. A tree-girdling experiment enabled studies of the relationship between plant C allocation and microbial community composition. Girdling involves cutting the phloem of trees to prevent photosynthates from entering the soil. Four years after girdling, fungal and bacterial communities were characterized using DNA-based profiles and cloning and sequencing. Data showed that girdling significantly altered fungal and bacterial communities compared with the control. The ratio of ectomycorrhizal to saprobic fungal sequences significantly decreased in girdled treatments, and this decline was found to correlate with the fungal phospholipid fatty acid biomarker 18:2ω6,9. Bacterial communities also varied in the abundance of the two dominant phyla Acidobacteria and Alphaproteobacteria . Concomitant changes in fungal and bacterial communities suggest linkages between these two groups and point toward plant belowground C allocation as a key determinant of microbial community composition.     

Winter soil frost conditions in boreal forests control growing season soil CO2 concentration and its atmospheric exchange

The impact of changes in winter soil frost regime on soil CO₂ concentration and its atmospheric exchange in a boreal Norway spruce forest was investigated using a field‐scale soil frost manipulation experiment. The experiment comprised three treatments: deep soil frost, shallow soil frost and control plots (n= 3). Winter soil temperatures and soil frost distribution were significantly altered by the different treatments. The average soil CO₂ concentrations during the growing season were significantly lower in plots with deep soil frost than in plots with shallow soil frost. The average CO₂ soil–atmosphere exchange rate exhibited the same pattern, and differences in soil respiration rates among the treatments were statistically significant. Both the variation in soil CO₂ concentration and the CO₂ soil–atmosphere exchange rate could statistically be explained by the differences in the maximum soil frost depth during the previous winter. A response model for growing season soil respiration rates suggests that every 1 cm change in winter soil frost depth will change the emission rates by ca. 0.01 g CO₂ m^?2 day^?1, corresponding to 0.2–0.5% of the estimated net ecosystem productivity (NEP). This suggests that the soil frost regime has a significant influence on the C balance of the system, because interannual variations in soil frost up to 60 cm have been recorded at the site. We conclude that winter climate conditions can be important in controlling C balances in northern terrestrial ecosystems, and also that indirect effects of the winter season must be taken into account, because these can affect the prevailing conditions during the growing season.     

Effects of post-fire conditions on soil respiration in boreal forests with special reference to Northeast China forests

Forest fires frequently occur in boreal forests,and their effects on forest ecosystems are often significant in terms of carbon flux related to climate changes.Soil respiration is the second largest carbon flux in boreal forests and the change in soil respiration is not negligible.Environmental factors controlling the soil respiration,for example,soil temperature,are altered by such fires.The abnormal increase in soil temperature has an important negative effect on soil microbes by reducing their activities or even by killing them directly with strong heat.On the other hand,although vegetation is directly disturbed by fires,the indirect changes in soil respiration are followed by changes in root activities and soil microbes.However,there is very limited information on soil respiration in the forests of Northeast China.This review,by combining what is known about fire influence on soil respiration in boreal forests from previous studies of post-fire effects on soil conditions,soil microbes,and forest regeneration,presents possible scenarios of the impact of anticipated post-fire changes in forest soil respiration in Northeast China.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.     

Effects of post-fire conditions on soil respiration in boreal forests with special reference to Northeast China forests

Forest fires frequently occur in boreal forests, and their effects on forest ecosystems are often significant in terms of carbon flux related to climate changes. Soil respiration is the second largest carbon flux in boreal forests and the change in soil respiration is not negligible. Environmental factors controlling the soil respiration, for example, soil temperature, are altered by such fires. The abnormal increase in soil temperature has an important negative effect on soil microbes by reducing their activities or even by killing them directly with strong heat. On the other hand, although vegetation is directly disturbed by fires, the indirect changes in soil respiration are followed by changes in root activities and soil microbes. However, there is very limited information on soil respiration in the forests of Northeast China. This review, by combining what is known about fire influence on soil respiration in boreal forests from previous studies of post-fire effects on soil conditions, soil microbes, and forest regeneration, presents possible scenarios of the impact of anticipated post-fire changes in forest soil respiration in Northeast China.     

Mineral nitrogen and microbial dynamics in the forest floor of clearcut or partially harvested successional boreal forest stands

The effects of clearcut and partial harvesting of early-seral trembling aspen plots were compared to conventional clearcut harvesting in mid-seral mixedwood and late-seral conifer plots. Twice a year, for three consecutive years, we assessed mineral N and microbial dynamics in the forest floor of these plots to test three hypotheses related to the higher litter quality of aspen leaves and to the sustained inputs of available C on partially harvested plots: (1) the post-clearcutting mineral N flush and the net [(NO₃^–): (NO₃^– + NH₄⁺)] production ratio (RNI) are higher in aspen plots than in black spruce plots, with intermediate values occurring in mixedwood plots; (2) net N mineralization rates in aspen plots are higher in spring than in autumn; and (3) compared to clearcutting, partial harvesting reduces potential ammonification and nitrification rates. Initial NH₄⁺ and NO₃^– concentrations respectively ranged between 1.7–4.4 and 0.2–1.5 g N kg^–1 N_total, net ammonification and nitrification rates (30 d incubations) respectively ranged between 5.3–17.8 and 0.1–27.6 g N kg^–1 N_total, basal respiration ranged between 20.9–38.9 mg CO₂-C kg^–1 h^–1, and microbial biomass ranged between 6.1–8.7 g C_mic kg^–1. Although clearcutting increased NO₃^– concentrations in aspen plots, the balance of our results did not support our first hypothesis, because NH₄⁺ concentrations increased in conifer plots only, potential ammonification was unaffected by clearcutting, potential nitrification increased in mixedwood plots only, and RNI increased in all plots. In each seral stage, basal respiration, microbial biomass, and metabolic quotient either increased or were unaffected by clearcutting, suggesting that increases in RNI after disturbance were not related to lower microbial immobilisation of NO₃^– due to lower available C. Forest floors in mid-seral mixedwood plots exhibited a distinct combination of mineral N and microbial properties, suggesting that the functional richness of the forest is enhanced not only by the number of species, but also by the diversity of assemblages that are present. Results supported our second hypothesis and showed, furthermore, that net N mineralization in conifer stands is greater in autumn than in spring. Partial harvesting in aspen stands resulted in lower potential mineralization of N and lower RNI, compared to clearcutting. Further lysimetry studies are needed to confirm whether partial harvesting mitigates NO₃^– leaching following disturbance.     

Mycorrhizal fungi have a potential role in soil carbon storage under elevated CO2 and nitrogen deposition

In this review, we discuss the potential for mycorrhizal fungi to act as a source or sink for carbon (C) under elevated CO₂ and nitrogen deposition. Mycorrhizal tissue has been estimated to comprise a significant fraction of soil organic matter and below-ground biomass in a range of systems. The current body of literature indicates that in many systems exposed to elevated CO₂, mycorrhizal fungi might sequester increased amounts of C in living, dead and residual hyphal biomass in the soil. Through this process, the fungi might serve as a negative feedback on the rise in atmospheric CO₂ levels caused by fossil fuel burning and deforestation. By contrast, a few preliminary studies suggest that N deposition might increase turnover rates of fungal tissue and negate CO₂ effects on hyphal biomass. If these latter responses are consistent among ecosystems, C storage in hyphae might decline in habitats surrounding agricultural and urban areas. When N additions occur without CO₂ enrichment, effects on mycorrhizal growth are inconsistent. We note that analyses of hyphal decomposition under elevated CO₂ and N additions are extremely sparse but are critical in our understanding of the impact of global change on the cycling of mycorrhizal C. Finally, shifts in the community composition of arbuscular and ectomycorrhizal fungi with increasing CO₂ or N availability are frequently documented. Since mycorrhizal groups vary in growth rate and tissue quality, these changes in species assemblages could produce unforeseeable impacts on the productivity, survivorship, or decomposition of mycorrhizal biomass.     

淹水培养条件下土壤微生物生物量碳、氮和可溶性有机碳、氦的动态

以洞庭湖区2个典型水稻土（红黄泥和紫潮泥）为对象,研究了25℃、淹水培养条件下稻草-硫铵配施和单施硫铵处理土壤微生物生物量碳、氮（SMBC、SMBN）和可溶性有机碳、氦（SDOC、SDON）的动态变化．结果表明,SMBC、SMBN和SDOC、SDON在培养前期达到峰值,之后降低,并趋于稳定．添加底物后,2种土壤不同处理土壤微生物生物量碳与有机碳（SMBC／TC）和土壤微生物生物量氮与全氮（SMBN／TN）的平均值都在2％-3％之间变化;可溶性碳与全碳（SDOC／TC）的平均值为1％左右,可溶性氮与全氮（SDON／TN）平均值为5％-6％．2种土壤中SMBC峰值单施硫铵处理最大,但与稻草-硫铵配施处理差异均不显著;SMBN、SDOC和SDON峰值稻草-硫铵配施最大．稻草．硫铵配施与单施硫铵处理中,低肥力红黄泥的SMBN、SDOC和SDON峰值差异显著;而高肥力紫潮泥SMBN和SDOC峰值差异不显著．前7d,SMBC／SMBN〈10;14d后,同一时刻单施硫铵处理SMBC／SMBN〉稻草．硫铵配施．不同处理的SDOC!SDON3d时最大．28d时最小．     

Seasonal changes in the contribution of root respiration to total soil respiration in a cool-temperate deciduous forest

A trenching method was used to determine the contribution of root respiration to soil respiration. Soil respiration rates in a trenched plot (R _trench) and in a control plot (R _control) were measured from May 2000 to September 2001 by using an open-flow gas exchange system with an infrared gas analyser. The decomposition rate of dead roots (R _D) was estimated by using a root-bag method to correct the soil respiration measured from the trenched plots for the additional decaying root biomass. The soil respiration rates in the control plot increased from May (240–320 mg CO₂ m^–2 h^–1) to August (840–1150 mg CO₂ m^–2 h^–1) and then decreased during autumn (200–650 mg CO₂ m^–2 h^–1). The soil respiration rates in the trenched plot showed a similar pattern of seasonal change, but the rates were lower than in the control plot except during the 2 months following the trenching. Root respiration rate (R _r) and heterotrophic respiration rate (R _h) were estimated from R _control, R _trench, and R _D. We estimated that the contribution of R _r to total soil respiration in the growing season ranged from 27 to 71%. There was a significant relationship between R _h and soil temperature, whereas R _r had no significant correlation with soil temperature. The results suggest that the factors controlling the seasonal change of respiration differ between the two components of soil respiration, R _r and R _h.     

油松-辽东栎混交林地表凋落物与氮添加对土壤微生物生物量碳、氮及其活性的影响

2010年9月-2011年10月,在山西省灵空山油松和辽东栎混交林样地采取随机区组设计,研究了地表凋落物和氮添加处理对土壤微生物生物量碳、氮和微生物活性的影响.凋落物处理包括:剔除凋落物(N)、叶凋落物加倍(L)、枝果凋落物加倍(B)和混合凋落物加倍(LB);氮添加量分别为0(N0)、5 g· m-2·a-1(N1)和10 g·m-2·a-1(N2).结果表明:剔除地表凋落物且无氮添加时,油松和辽东栎混交林地的土壤有机碳(SOC)含量显著降低,其他试验处理间对SOC的影响无显著差异.土壤微生物生物量碳(MBC)、氮(MBN)及其活性(MR)的变化范围依次为:262.42 ～ 873.16 mg·kg-1、73.55 ～ 173.85 mg·kg-1和2.38～3.68mg·kg-1·d-1.MBC、MBN和MR两两间呈极显著正相关.氮添加对MBC、MBN和MR均无显著影响;凋落物处理对MR影响显著,表现为混合凋落物加倍处理的MR最高,叶凋落物加倍处理次之,剔除凋落物处理最低,而对MBC和MBN无显著影响.凋落物和氮添加处理在整个试验过程中未表现出交互作用.短期的氮添加处理和森林地表凋落物变化对土壤微生物过程的影响有限.     

Effects of forest fertilization on nitrogen leaching and soil microbial properties in the Northern Calcareous Alps of Austria

Several boreal and alpine forests are depleted in nutrients due to acidification. Fertilization may be a remedy, but rapidly-soluble salts (N, P, K, Mg) may pose nitrate problems for the groundwater or decrease microbial activity.With the aim to investigate potential nitrogen leaching after fertilization we set up an experiment employing intact soil cores (11 cm diameter, 20–40 cm long) from a mixed forest and a Picea abies stand (soil type Rendsina) in the Northern Calcareous Alps of Austria. The cores were fertilized with a commercial NPK fertilizer or a methylene-urea-apatite-biotite (MuAB) fertilizer at a rate corresponding to 300 kg N ha^-1 and incubated for 28 weeks together with unfertilized controls. Both soil water (retrieved 5 cm below the soil surface) and leachate were analyzed for nitrate and ammonium in regular intervals. After the incubation, soil microbial biomass and basal repiration were determined and a nitrogen mineralization assay was performed.For the control, in the soil water and leachate maximum NH₄ ⁺ and NO₃ ^- concentrations of 5 and 11 mg N L^-1, respectively, were found. Compared to that, MuAB fertilizer resulted in a slow increase of NH₄ ⁺ and NO₃ ^- in the soil water (up to 11 and 35 mg N L^-1 respectively) and in the leachate (4 mg NH₄ ⁺-N L^-1 and 44 mg NO₃ ^--N L^-1). Highest nitrogen loads were found for the fast release NPK fertilizer, with NH₄ ⁺ and NO₃ ^- concentrations up to 170 and 270 mg N L^-1, respectively, in the soil water. NH₄ ⁺-N levels in the leachate remained below 5, while NO₃-N levels were up to 190 mg L^-1. Fast- release NPK caused a significant decrease of microbial biomass and basal respiration. These parameters were not affected by MuAB fertilizer.The results suggest that the MuAB fertilizer may be an ecologically appropriate alternative to fast-release mineral fertilizers for improving forest soils.     

Carbon and nitrogen cycling in North American boreal forests

A model of boreal forest dynamics was adapted to examine the factors controlling carbon and nitrogen cycling in the boreal forests of interior Alaska. Empirical relationships were used to simulate decomposition and nitrogen availability as a function of either substrate quality, the soil thermal regime, or their interactive effects. Test comparisons included black spruce forests growing on permafrost soils and black spruce, birch, and white spruce forests growing on permafrost-free soils. For each forest, simulated above-ground tree biomass, basal area, density, litterfall, moss biomass, and forest floor mass, turnover, thickness, and nitrogen concentration were compared to observed data. No one decay equation simulated forests entirely consistent with observed data, but over the range of upland forest types in interior Alaska, the equation that combined the effects of litter quality and the soil thermal regime simulated forests that were most consistent with observed data. For black spruce growing on permafrost soils, long-term simulated forest dynamics in the absence of fire resulted in unproductive forests with a thick forest floor and low nitrogen mineralization. Fires were an important means to interrupt this sequence and to restart forest succession.     

A comparative account of the microbiological characteristics of soils under natural forest,grassland and cropfield from Eastern India

Microbiological and physico-chemical characteristics of tropical forest, grassland and cropfield soils from India were investigated. The study revealed that the conversion of natural forest led to a reduction of soil organic C (26–36%), total N (26–35%), total P (33–44%), microfungal biomass (44–66%) and total microbial biomass C, N and P (25–60%) over a period of 30–50 years. Comparative analysis of microbial activity in terms of basal soil respiration revealed maximum activity in the forest and minimum in the cropfield soil. Analysis of microbial metabolic respiratory activity (qCO₂) indicated relatively greater respiratory loss of CO₂-C per unit microbial biomass in cropfield and grassland than in forest soil. Considering the importance of the microbial component in soil, we conclude that the conversion of the tropical forest to different land uses leads to the loss of biological stability of the soil.     

Impact of fire on active layer and permafrost microbial communities and metagenomes in an upland Alaskan boreal forest

Permafrost soils are large reservoirs of potentially labile carbon (C). Understanding the dynamics of C release from these soils requires us to account for the impact of wildfires, which are increasing in frequency as the climate changes. Boreal wildfires contribute to global emission of greenhouse gases (GHG—CO₂, CH₄ and N₂O) and indirectly result in the thawing of near-surface permafrost. In this study, we aimed to define the impact of fire on soil microbial communities and metabolic potential for GHG fluxes in samples collected up to 1 m depth from an upland black spruce forest near Nome Creek, Alaska. We measured geochemistry, GHG fluxes, potential soil enzyme activities and microbial community structure via 16SrRNA gene and metagenome sequencing. We found that soil moisture, C content and the potential for respiration were reduced by fire, as were microbial community diversity and metabolic potential. There were shifts in dominance of several microbial community members, including a higher abundance of candidate phylum AD3 after fire. The metagenome data showed that fire had a pervasive impact on genes involved in carbohydrate metabolism, methanogenesis and the nitrogen cycle. Although fire resulted in an immediate release of CO₂ from surface soils, our results suggest that the potential for emission of GHG was ultimately reduced at all soil depths over the longer term. Because of the size of the permafrost C reservoir, these results are crucial for understanding whether fire produces a positive or negative feedback loop contributing to the global C cycle.