The influence of acid irrigation and liming on the soil microbial biomass in a Norway spruce (Picea abies [L.] K.) stand

Over a period of three years (1990–1992) microbial biomass-C (C_mic), CO₂ evolution, the C_mic:C_org ratio and the metabolic quotient for CO₂ (qCO₂) were determined in a Norway spruce stand (Höglwald) with experimentally acid-irrigated and limed plots since 1984. A clear relationship between soil pH and the level of microbial biomass-(C_mic) was noted, C_mic increasing with increasing soil pH in O_h or A_h horizons. More microbial biomass-C per unit C_{org} (C_mic:C_org ratio) was detected in limed plots with elevated pH of O_h or A_h horizons as compared to unlimed plots with almost 3 times more C_mic per unit C_org in the limed O_h horizon. Differences here are significant at least at the p=0.05 level. The positive effects of liming (higher pH) on the C_mic:C_org ratio was more pronounced in the upper horizon (O_h)). The total CO₂ evolution rate of unlimed plots was only half of that noted for limed plots which corresponded to the low microbial biomass levels of unlimed plots. The specific respiratory activity, qCO₂, was similar and not significantly different between the unlimed control plot and the limed plot.Acid irrigation of plots with already low pH did not significantly affect the level of microbial biomass, the C_mic:C_org ratio or qCO₂. An elevated qCO₂ could be seen, however, for the limed + acid irrigated plot. The biomass seemed extremely stressed, showing with 3.8 g CO₂-C mg_-1 C_mic h_-1 (O_h) the highest qCO₂ value of all treatments. This was interpreted as a reflection of the continuous adaptation processes to the H⁺ ions by the microflora. The negative effect of acid irrigation of limed plots was also manifested in a decreased C_mic:C_org ratio.     

Interactions of tropospheric CO2 and O3 enrichments and moisture variations on microbial biomass and respiration in soil

Soil microbial biomass C (C_mic) is a sensitive indicator of trends in organic matter dynamics in terrestrial ecosystems. This study was conducted to determine the effects of tropospheric CO₂ or O₃ enrichments and moisture variations on total soil organic C (C_org), mineralizable C fraction (C_Min), C_mic, maintenance respiratory (qCO₂) or C_mic death (qD) quotients, and their relationship with basal respiration (BR) rates and field respiration (FR) fluxes in wheat‐soybean agroecosystems. Wheat (Triticum aestivum L.) and soybean (Glycine max. L. Merr) plants were grown to maturity in 3‐m dia open‐top field chambers and exposed to charcoal‐filtered (CF) air at 350 μL CO₂ L^?1; CF air + 150 μL CO₂ L^?1; nonfiltered (NF) air + 35 nL O₃ L^?1; and NF air + 35 nL O₃ L^?1 + 150 μL CO₂ L^?1 at optimum (? 0.05 MPa) and restricted soil moisture (? 1.0 ± 0.05 MPa) regimes. The + 150 μL CO₂ L^?1 additions were 18 h d^?1 and the + 35 nL O₃ L^?1 treatments were 7 h d^?1 from April until late October. While C_org did not vary consistently, C_Min, C_mic and C_mic fractions increased in soils under tropospheric CO₂ enrichment (500 μL CO₂ L^?1) and decreased under high O₃ exposures (55 ± 6 nL O₃ L^?1 for wheat; 60 ± 5 nL O₃ L^?1 for soybean) compared to the CF treatments (25 ± 5 nL O₃ L^?1). The qCO₂ or qD quotients of C_mic were also significantly decreased in soils under high CO₂ but increased under high O₃ exposures compared to the CF control. The BR rates did not vary consistently but they were higher in well‐watered soils. The FR fluxes were lower under high O₃ exposures compared to soils under the CF control. An increase in C_mic or C_mic fractions and decrease in qCO₂ or qD observed under high CO₂ treatment suggest that these soils were acting as C sinks whereas, reductions in C_mic or C_mic fractions and increase in qCO₂ or qD in soils under elevated tropospheric O₃ exposures suggest the soils were serving as a source of CO₂.     

Land cover change effects on soil chemical and biological properties after planting Mongolian pine (Pinus sylvestris var. mongolica) in sandy lands in Keerqin, northeastern China

We compared soil moisture content, pH, total organic carbon (C _org), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH₄ ⁺–N, NO₃ ^?–N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (C _mic), soil metabolic quotient (qCO₂), soil microbial quotient (C _mic/C _org) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)–grass savanna in southeastern Keerqin, in northeast China. Soil C _org, TN and TP concentrations (0–10, 10–20, 20–40 and 40–60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH₄ ⁺–N and NO₃ ^?–N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0–10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on C _mic and qCO₂ of surface soil were not significant; but C _mic/C _org were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type?×?sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.     

Phytoremediation affects microbial development on a limestone quarry

Phytoremediation was used to regenerate a limestone quarry area. Plant growth mixed medium added over the quarry surface, consisting of a mixture of pyrolusite byproducts, natural soil, sand, and rice husk. Three different plant species: pine, cypress, and broom were planted at 9 randomized plots in order to assess the effects of vegetation on the microbial development, which was measured for the following 3 years. Substrate samples were analyzed for organic carbon content (C_org), microbial biomass (C_mic), basal CO₂ respiration activity (BR), alkaline phosphatase (ALP), and acid phosphatase activities at each plant specie and year. Furthermore, the ratio C_mic/C_org, the metabolic quotient (qCO₂), and the C mineralization quotient (qM) were determined. The highest survival rates occurred for broom (93.52%), followed by cypress and pine (82.41%) at the final year, while the content of C_mic, BR, and ALP was increased significantly under plants (pine, cypress, and broom) compared with control. C_mic content and BR was plant dependent. Cypress sites had the highest values of C_mic (214.9 μgCg^?1) and BR (112.8 μgCO₂-Cg^?1d^?1) at the 3^rd year. The plant root environment clearly enhances and regulates the microbial community, in correspondence to the species used. Below ground enhanced activity could fulfill the scope of phytoremediation strategies.     

Microbial diversity of topographical gradient profiles in Fushan forest soils of Taiwan

To evaluate the microbial diversity of Fushan forest soils, the variation of soil properties, microbial populations, and soil DNA with soil depth in three sites of different altitude were analyzed. Microbial population, moisture content, total organic carbon (C_org), and total nitrogen (N_tot) decreased with increasing soil depth. The valley site had the lowest microbial populations among the three tested sites due to the low organic matter content. Bacterial population was the highest among the microbial populations. The ratios of cellulolytic microbes to the total bacteria in organic layers were high, implying their roles in the carbon cycle. The microbial biomass carbon (C_mic) and nitrogen (N_mic) contents ranged from 130.5 to 564.1 μg g⁻¹ and from 16.7 to 95.4 μg g⁻¹, respectively. The valley had the lowest C_mic and N_mic. The organic layer had the highest C_mic and N_mic and decreased with soil depth. Analysis using denaturing gradient gel electrophoresis (DGGE) of polymerase chain reaction (PCR) amplicons of 16S rDNA showed that the bacterial diversity of the three sites were very similar to each other in the major bands, and the variation was in the minor bands. However, the patterns in PCR-DGGE profile through gradient horizons were different, indicating the prevalence of specific microbes at different horizons. These results suggest that the microbial diversity in the deeper horizons is not simply the diluted analogs of the surface soils and that some microbes dominate only in the deeper horizons. Topography influenced the quantity and diversity of microbial populations.     

Variables of Microbial Response in Natural Soil Aggregates for Soil Characterization in Different Fluvial Land Shapes

The main objective of this study was to determine changes in microbial response in natural soil aggregates for soil characterization in different fluvial land shapes. This study was carried out in fluvial lands formed on accumulated sediment depositions carried by K?z?l?rmak River. The majority soils of the study area were classified as Typic Ustifluvent and Typic Haplustept in Soil Taxonomy. It was found that macroaggregates (especially >6300 μm and 2000–4750 μm diameters) of all soil samples were higher than microaggregate of soils. In addition, it was determined that the C_org content varies between 0.41–0.91% in soil samples. C_mic content was also found higher level in aggregates involved <250 and 250–425 μm diameters as compared to other aggregate size classes. Moreover, we detected that C_org:C_mic ratio was much higher in macroaggregates than in microaggregate fractions. BR levels were also greater in macroaggregates of >6,300, 4,750–6,300 and 2,000–4,750 μm than in the other macroaggregates sizes and microaggregates. Consequently, macroaggregates have relatively more C_org level than the C_org level in microaggregates, even if the absolute values of C_mic were the lower. This study thus evidenced contrasting microbial habitats and their response in different soil aggregate size formed in various developed soils.     

The variation of soil microbial respiration with depth in relation to soil carbon composition

The vertical variation in soil microbial respiratory activity and its relationship to organic carbon pools is critical for modeling soil C stock and predicting impacts of climate change, but is not well understood. Mineral soil samples, taken from four Scottish soils at different depths (0–8, 8–16, 16–24, 24–32 cm), were analyzed and incubated in the laboratory under constant temperature and environmental conditions. The vegetation type/plant species showed significant effects on the absolute concentration of C components and microbial activity, but the relative distribution of C and respiration rate with soil depth are similar across sites. Soil C pools and microbial respiratory activity declined rapidly with soil depth, with about 30% of total organic carbon (TOC) and dissolved organic carbon (DOC), and about half microbial carbon (C_mic) and respired CO₂ observed in the top 8 cm. The ratio of CO₂:TOC generally decreased with soil depth, but CO₂:DOC was significantly higher in the top 8 cm of soil than in the subsoil (8–32 cm). No general pattern between qCO₂ (CO₂:C_mic) and soil depth was found. The vertical distributions of soil C pools and microbial respiratory activity were best fitted with a single exponential equation. Compared with TOC and DOC, C_mic appears to be an adequate predictor for the variation in microbial respiration rate with soil depth, with 95% of variation in normalized respiration rate accounted for by a linear relationship.     

Soil Amendment with <Emphasis Type="Italic">Pseudomonas fluorescens</Emphasis> CHA0: Lasting Effects on Soil Biological Properties in Soils Low in Microbial Biomass and Activity

Pseudomonas fluorescens strains are used in agriculture as plant growth-promoting rhizobacteria (PGPR). Nontarget effects of released organisms should be analyzed prior to their large-scale use, and methods should be available to sensitively detect possible changes in the environments the organism is released to. According to ecological theory, microbial communities with a greater diversity should be less susceptible to disturbance by invading organisms. Based on this principle, we laid out a pot experiment with field-derived soils different in their microbial biomass and activity due to long-term management on similar parent geological material (loess). We investigated the survival of P. fluorescens CHA0 that carried a resistance toward rifampicine and the duration of potential changes of the soil microflora caused by the inoculation with the bacterium at the sowing date of spring wheat. Soil microbial biomass (C _mic, N _mic) basal soil respiration (BR), qCO₂, dehydrogenase activity (DHA), bacterial plate counts, mycorrhiza root colonization, and community level substrate utilization were analyzed after 18 and 60 days. At the initial stage, soils were clearly different with respect to most of the parameters measured, and a time-dependent effect between the first and the second set point were attributable to wheat growth and the influence of roots. The effect of the inoculum was small and merely transient, though significant long-term changes were found in soils with a relatively low level of microbial biomass. Community level substrate utilization as an indicator of changes in microbial community structure was mainly changed by the growth of wheat, while other experimental factors were negligible. The sensitivity of the applied methods to distinguish the experimental soils was in decreasing order N _mic, DHA, C _mic, and qCO₂. Besides the selective enumeration of P. fluorescens CHA0 rif⁺, which was only found in amended soils, methods to distinguish the inoculum effect were DHA, C _mic, and the ratio of C _mic to N _mic. The sampling time was most sensitively indicated by N _mic, DHA, C _mic, and qCO₂. Our data support the hypothesis—based on ecosystem theory—that a rich microflora is buffering changes due to invading species. In other words, a soil-derived bacterium was more effective in a relatively poor soil than in soils that are rich in microorganisms.     

Relationship between soil organic carbon and microbial biomass on chronosequences of reclamation sites

The interrelationship between soil microorganisms and soil organic carbon was studied on an agricultural and on a forest chronosequence of open-pit mine reclamation soils. Thirty years after reclamation, soil carbon levels of 0.8% on the agricultural sites and 1.7% on the forest sites (A-horizon) were reached. Microbial biomass rose very fast to levels characteristic of undisturbed soils. Microbial carbon (C_mier) was 57 mg·100 g^–1 soil after 15 years on the agricultural sites and 43 mg·100 g^–1 on the forest sites. The contribution of C_mier to the total organic carbon (C_org) decreased with time, more rapidly on the forest sites than on the agricultural ones. From the C_mierr/C_org ratio it became evident that both chronosequences had not yet reached a steady state within the 50 years of reclamation. A significant decrease of the metabolic quotient qCO₂ (microbial respiration per unit biomass) with time was observed on the agricultural sites but not on the forest sites. The C_mier/C_org ratio proved to be a reliable soil microbial parameter for describing changes in man-made ecosystems. For evaluating reclamation efforts, the C_mier/C_org ratio can be considered superior to its single components (C_mier or C_org) and to other parameters.     

Fungal to bacterial biomass ratio in the forests soil profile

The carbon content in microbial biomass (C_mic-MB) was determined in various horizons of the soil profile (sod-podzo, gray, podzol, and rzhavozem) of various forests (oak, spruce archangel, spruce moss, aspen, spruce broadleaf) in the southern taiga of European Russia (Moscow and Kaluga regions) by the substrate-induced respiration (SIR) and direct microscopy (DM) methods. The fungi-to-bacteria ratio was measured by the selective inhibition technique and DM. A quantitative differentiation of the fungal mycelium was suggested. The C_mic-DM / C_mic-SIR in various horizons of the soil profile was about 98%. The fungal contribution to MB was 52–74% and 92–99% according to the SIR and DM methods, respectively. The microbial parameters were associated with the CO₂ and N₂O production by the soils. The contradictory data about the fungal portion in the MB of soils of various ecosystems were discussed.     

Vegetation and climate controls on potential CO2, DOC and DON production in northern latitude soils

Climatic change may influence decomposition dynamics in arctic and boreal ecosystems, affecting both atmospheric CO₂ levels, and the flux of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) to aquatic systems. In this study, we investigated landscape‐scale controls on potential production of these compounds using a one‐year laboratory incubation at two temperatures (10° and 30 °C). We measured the release of CO₂, DOC and DON from tundra soils collected from a variety of vegetation types and climatic regimes: tussock tundra at four sites along a latitudinal gradient from the interior to the north slope of Alaska, and soils from additional vegetation types at two of those sites (upland spruce at Fairbanks, and wet sedge and shrub tundra at Toolik Lake in northern Alaska). Vegetation type strongly influenced carbon fluxes. The highest CO₂ and DOC release at the high incubation temperature occurred in the soils of shrub tundra communities. Tussock tundra soils exhibited the next highest DOC fluxes followed by spruce and wet sedge tundra soils, respectively. Of the fluxes, CO₂ showed the greatest sensitivity to incubation temperatures and vegetation type, followed by DOC. DON fluxes were less variable. Total CO₂ and total DOC release were positively correlated, with DOC fluxes approximately 10% of total CO₂ fluxes. The ratio of CO₂ production to DOC release varied significantly across vegetation types with Tussock soils producing an average of four times as much CO₂ per unit DOC released compared to Spruce soils from the Fairbanks site. Sites in this study released 80–370 mg CO₂‐C g soil C^?1 and 5–46 mg DOC g soil C^?1 at high temperatures. The magnitude of these fluxes indicates that arctic carbon pools contain a large proportion of labile carbon that could be easily decomposed given optimal conditions. The size of this labile pool ranged between 9 and 41% of soil carbon on a g soil C basis, with most variation related to vegetation type rather than climate.     

Increases in soil organic carbon sequestration can reduce the global warming potential of long‐term liming to permanent grassland

The application of calcium‐ and magnesium‐rich materials to soil, known as liming, has long been a foundation of many agro‐ecosystems worldwide because of its role in counteracting soil acidity. Although liming contributes to increased rates of respiration from soil thereby potentially reducing soils ability to act as a CO₂ sink, the long‐term effects of liming on soil organic carbon (C_org) sequestration are largely unknown. Here, using data spanning 129 years of the Park Grass Experiment at Rothamsted (UK), we show net C_org sequestration measured in the 0–23 cm layer at different time intervals since 1876 was 2–20 times greater in limed than in unlimed soils. The main cause of this large C_org accrual was greater biological activity in limed soils, which despite increasing soil respiration rates, led to plant C inputs being processed and incorporated into resistant soil organo‐mineral pools. Limed organo‐mineral soils showed: (1) greater C_org content for similar plant productivity levels (i.e. hay yields); (2) higher ¹⁴C incorporation after 1950s atomic bomb testing and (3) lower C : N ratios than unlimed organo‐mineral soils, which also indicate higher microbial processing of plant C. Our results show that greater C_org sequestration in limed soils strongly reduced the global warming potential of long‐term liming to permanent grassland suggesting the net contribution of agricultural liming to global warming could be lower than previously estimated. Our study demonstrates that liming might prove to be an effective mitigation strategy, especially because liming applications can be associated with a reduced use of nitrogen fertilizer which is a key cause for increased greenhouse gas emissions from agro‐ecosystems.     

帽儿山地区不同土地利用方式下土壤-微生物-矿化碳氮化学计量特征

土地利用方式的变化导致土壤碳氮含量及其化学计量关系的变化,然而土壤微生物化学计量及其驱动的碳氮矿化过程如何响应这种变化仍不明确。以帽儿山地区天然落叶阔叶林、人工红松林、草地和农田4种不同土地利用类型为对象,测定其土壤有机碳(C_(soil))、全氮(N_(soil))、微生物生物量碳和氮(C_(mic)和N_(mic))、土壤碳和氮矿化速率(C_(min)和N_(min)),旨在比较不同土地利用方式对土壤、微生物碳氮化学计量特征及矿化速率的影响,探索土壤-微生物-矿化之间碳氮化学计量特征的相关性,揭示微生物对土壤碳氮化学计量变化的响应和调控机制。结果显示:C_(soil)、N_(soil)、C_(mic)、N_(mic)和C_(min)均呈现天然落叶阔叶林人工红松林草地农田,而天然落叶阔叶林和草地的N_(min)显著高于人工红松林和农田。土地利用方式显著影响土壤和微生物碳氮比(C∶N_(soil)和C∶N_(mic)),均呈现农田最高。不同土地利用方式的数据综合分析发现:碳氮矿化速率比与C∶N_(mic)呈负相关,而和微生物与土壤碳氮化学计量不平衡性(C∶N_(imb))显著正相关。单位微生物生物量的碳矿化速率(qCO_2)随着C∶N_(mic)的增加而降低,而单位微生物生物量的氮矿化速率(qAN)随着C∶N_(mic)的增加而增加。C∶N_(imb)与qCO_2正相关,与qAN负相关。以上结果表明,微生物会通过改变自身碳氮化学计量、调整碳氮之间相对矿化速率,以适应土地利用变化导致的土壤碳氮及其化学计量的变异性,以满足自身生长和代谢的碳氮需求平衡。     

Differences of Soil Microbial Biomass and Nitrogen Transformation under Two Forest Types in Central Germany

In order to observe the tree species effect on soil N status, soil microbial biomass C and N (C_mic, N_mic), potential N mineralization and potential nitrification (under laboratory incubation conditions, 22 °C) in different subhorizons (LOf₁, Of₂, Oh and mineral soil at 0–10 cm depth) were determined at three forest sites in central Germany. At each site, two contrasting stands (Beech, Norway spruce or Scots pine) were selected, where the initial soil conditions were similar. Three sampling dates that represented different stages of tree growth were selected: growing season - August, dormant season - November, after budbreak – April. In organic layers, C_mic-to-total C (C_t) ratios under beech and under conifer were 0.72–4.74% and 0.34–2.11%, respectively. N_mic-to-total N (N_t) ratios were 2.47–11.61% and 0.71–5.77%, respectively. Both concentrations of C_mic and N_mic were significantly affected by the stand type and sampling time. Potential N mineralization rates, ranging from 3.7 to 19.7 mg N kg⁻¹ d⁻¹, showed no clear pattern in relation to stand type. However, potential nitrification rates were mostly significantly higher under beech than under contrasting conifer. In mineral soils, concentrations of C_mic and N_mic showed a clear temporal pattern in the order: August>November>April. The average N_mic and N_mic-to-N_t were higher in soils from beech than conifer, while C_mic and C_mic-to-C_t ratios were similar between the two forest types. In organic layers, the highest values of C_mic-to-N_mic ratio and C_mic were found in November samples, especially under beech. By contrast, in mineral soils the highest value of C_mic-to-N_mic ratios were found in April samples, and at that time the C_mic concentrations were the lowest, especially under conifer. These results revealed the differences in microbial growth form and survival strategy associated with different tree species and soil layers.     

Seagrass losses since mid‐20th century fuelled CO2 emissions from soil carbon stocks

Seagrass meadows store globally significant organic carbon (C_org) stocks which, if disturbed, can lead to CO₂ emissions, contributing to climate change. Eutrophication and thermal stress continue to be a major cause of seagrass decline worldwide, but the associated CO₂ emissions remain poorly understood. This study presents comprehensive estimates of seagrass soil C_org erosion following eutrophication‐driven seagrass loss in Cockburn Sound (23 km² between 1960s and 1990s) and identifies the main drivers. We estimate that shallow seagrass meadows (<5 m depth) had significantly higher C_org stocks in 50 cm thick soils (4.5 ± 0.7 kg C_org/m²) than previously vegetated counterparts (0.5 ± 0.1 kg C_org/m²). In deeper areas (>5 m), however, soil C_org stocks in seagrass and bare but previously vegetated areas were not significantly different (2.6 ± 0.3 and 3.0 ± 0.6 kg C_org/m², respectively). The soil C_org sequestration capacity prevailed in shallow and deep vegetated areas (55 ± 11 and 21 ± 7 g C_org m^?2 year^?1, respectively), but was lost in bare areas. We identified that seagrass canopy loss alone does not necessarily drive changes in soil C_org but, when combined with high hydrodynamic energy, significant erosion occurred. Our estimates point at ~0.20 m/s as the critical shear velocity threshold causing soil C_org erosion. We estimate, from field studies and satellite imagery, that soil C_org erosion (within the top 50 cm) following seagrass loss likely resulted in cumulative emissions of 0.06–0.14 Tg CO_2‐eq over the last 40 years in Cockburn Sound. We estimated that indirect impacts (i.e. eutrophication, thermal stress and light stress) causing the loss of ~161,150 ha of seagrasses in Australia, likely resulted in the release of 11–21 Tg CO_2‐eq since the 1950s, increasing cumulative CO₂ emissions from land‐use change in Australia by 1.1%–2.3% per annum. The patterns described serve as a baseline to estimate potential CO₂ emissions following disturbance of seagrass meadows.     

Pedogenesis, permafrost, and soil moisture as controlling factors for soil nitrogen and carbon contents across the Tibetan Plateau

We investigated the main parameters [e.g. mean annual air temperature , mean annual soil temperature, mean annual precipitation, soil moisture (SM), soil chemistry, and physics] influencing soil organic carbon (C_org), soil total nitrogen (N_t) as well as plant available nitrogen (N_min) at 47 sites along a 1200 km transect across the high‐altitude and low‐latitude permafrost region of the central‐eastern Tibetan Plateau. This large‐scale survey allows testing the hypothesis that beside commonly used ecological variables, diversity of pedogenesis is another major component for assessing carbon (C) and nitrogen (N) cycling. The aim of the presented research was to evaluate consequences of permafrost degradation for C and N stocks and hence nutrient supply for plants, as the transect covers all types of permafrost including heavily degraded areas and regions without permafrost. Our results show that SM is the dominant parameter explaining 64% of C_org and 60% of N variation. The extent of the effect of SM is determined by permafrost, current aeolian sedimentation occurring mostly on degraded sites, and pedogenesis. Thus, the explanatory power for C and N concentrations is significantly improved by adding CaCO₃ content (P=0.012 for C_org; P=0.006 for N_t) and soil texture (P=0.077 for C_org; P=0.015 for N_t) to the model. For soil temperature, no correlations were detected indicating that in high‐altitude grassland ecosystems influenced by permafrost, SM overrides soil temperature as the main driving parameter at landscape scale. It was concluded from the current study that degradation of permafrost and corresponding changes in soil hydrology combined with a shift from mature stages of pedogenesis to initial stages, have severe impact on soil C and plant available N. This may alter biodiversity patterns as well as the development and functioning of the ecosystems on the Tibetan Plateau.     

Impact of Atmospheric Nitrogen Deposition on Carbon Dynamics in Two Scots Pine Forest Soils of Northern Germany

The impact of atmospheric N deposition on the dynamics of various carbon fractions was investigated in two Scots pine forest soils (cambisol, podzol) of Northern Germany in microcosm experiments. Total organic carbon (TOC), CO₂ emission, microbial carbon (C_mic) as well as organic hot- and coldwater extractable carbon fractions (C_hwe, C_cwe) were analyzed before, during, and after soil incubation in microcosms, run in three treatments: 0, +45, and +90 kg N ha⁻¹a⁻¹. On both sites, the N treatment showed no response to total organic carbon (TOC) contents in most of the investigated soil layers. Microbial carbon (C_mic) was significantly increased in the organic layer of both soil types by the N application. Subsequent to the N application, the CO₂ emission increased in all mineral soil layers of the cambisol but remained almost unaffected in the podzol. After the N application, a remarkable increase of hotwater extractable C (C_hwe) was detected for the organic layer of the cambisol but not for the podzol, whereas coldwater extractable C (C_cwe) concentrations decreased at both sites. The N application did not have a significant impact on the leachate concentrations of total organic carbon (TOC), dissolved organic carbon (DOC), and particulate organic carbon (POC) in the podzol, whereas the concentrations of these C fractions were decreased in the organic layer and the 35–70~cm mineral soil layer of the cambisol. The N treatment changed the contents of most of the investigated C fractions in both soil types and resulted in a considerable C~mobilization. But the processes of the C~mobilization between the cambisol and the podzol were completely different. According to the presented data, the cambisol obtaining moderate atmospheric N loads is much more sensitive to additional N inputs than the podzol that already received high amounts of atmospheric N.     

Microbial biomass and growth kinetics of microorganisms in chernozem soils under different land use modes

The carbon content of microbial biomass and the kinetic characteristics of microbial respiration response to substrate addition have been estimated for chernozem soils under different land use: arable lands used for 10, 46, and 76 years, mowed meadow, natural forest, and forest shelter belt. Microbial biomass and the content of microbial carbon in humus (C_mic /C_org) decreased in the following order: soils under forest cenoses—mowed meadow—10-year arable land—46- and 75-year arable land. The amount of microbial carbon in the long-plowed horizon was 40% of its content in the upper horizon of natural forest. Arable soils were characterized by a lower metabolic diversity of microbial community and by the highest portion of microorganisms able to grow directly on glucose introduced into soil. The effects of different scenarios of carbon sequestration in soil on the amounts and activity of microbial biomass are discussed.     

A pan‐Arctic synthesis of CH4 and CO2 production from anoxic soil incubations

Permafrost thaw can alter the soil environment through changes in soil moisture, frequently resulting in soil saturation, a shift to anaerobic decomposition, and changes in the plant community. These changes, along with thawing of previously frozen organic material, can alter the form and magnitude of greenhouse gas production from permafrost ecosystems. We synthesized existing methane (CH₄) and carbon dioxide (CO₂) production measurements from anaerobic incubations of boreal and tundra soils from the geographic permafrost region to evaluate large‐scale controls of anaerobic CO₂ and CH₄ production and compare the relative importance of landscape‐level factors (e.g., vegetation type and landscape position), soil properties (e.g., pH, depth, and soil type), and soil environmental conditions (e.g., temperature and relative water table position). We found fivefold higher maximum CH₄ production per gram soil carbon from organic soils than mineral soils. Maximum CH₄ production from soils in the active layer (ground that thaws and refreezes annually) was nearly four times that of permafrost per gram soil carbon, and CH₄ production per gram soil carbon was two times greater from sites without permafrost than sites with permafrost. Maximum CH₄ and median anaerobic CO₂ production decreased with depth, while CO₂:CH₄ production increased with depth. Maximum CH₄ production was highest in soils with herbaceous vegetation and soils that were either consistently or periodically inundated. This synthesis identifies the need to consider biome, landscape position, and vascular/moss vegetation types when modeling CH₄ production in permafrost ecosystems and suggests the need for longer‐term anaerobic incubations to fully capture CH₄ dynamics. Our results demonstrate that as climate warms in arctic and boreal regions, rates of anaerobic CO₂ and CH₄ production will increase, not only as a result of increased temperature, but also from shifts in vegetation and increased ground saturation that will accompany permafrost thaw.     

Soil-surface carbon dioxide efflux and microbial biomass in relation to tree density 13 years after a stand replacing fire in a lodgepole pine ecosystem

The effects of fire on soil‐surface carbon dioxide (CO₂) efflux, F_S, and microbial biomass carbon, C_mic, were studied in a wildland setting by examining 13‐year‐old postfire stands of lodgepole pine differing in tree density (< 500 to > 500 000 trees ha^?1) in Yellowstone National Park (YNP). In addition, young stands were compared to mature lodgepole pine stands (～110‐year‐old) in order to estimate ecosystem recovery 13 years after a stand replacing fire. Growing season F_S increased with tree density in young stands (1.0 µmol CO₂ m^?2 s^?1 in low‐density stands, 1.8 µmol CO₂ m^?2 s^?1 in moderate‐density stands and 2.1 µmol CO₂ m^?2 s^?1 in high‐density stands) and with stand age (2.7 µmol CO₂ m^?2 s^?1 in mature stands). Microbial biomass carbon in young stands did not differ with tree density and ranged from 0.2 to 0.5 mg C g^?1 dry soil over the growing season; C_mic was significantly greater in mature stands (0.5–0.8 mg C g^?1 dry soil). Soil‐surface CO₂ efflux in young stands was correlated with biotic variables (above‐ground, below‐ground and microbial biomass), but not with abiotic variables (litter and mineral soil C and N content, bulk density and soil texture). Microbial biomass carbon was correlated with below‐ground plant biomass and not with soil carbon and nitrogen, indicating that plant activity controls not only root respiration, but C_mic pools and overall F_S rates as well. These findings support recent studies that have demonstrated the prevailing importance of plants in controlling rates of F_S and suggest that decomposition of older, recalcitrant soil C pools in this ecosystem is relatively unimportant 13 years after a stand replacing fire. Our results also indicate that realistic predictions and modeling of terrestrial C cycling must account for the variability in tree density and stand age that exists across the landscape as a result of natural disturbances.