Long-term microbial control of nutrient availability and plant biomass in a subarctic-alpine heath after addition of carbon, fertilizer and fungicide

A long-term field experiment lasting more than a decade was conducted on a subarctic fellfield to investigate effects of changes in nutrient availability on soil microbial C, N and P, soil nutrients, vascular plant biomass and plant-microbial interactions. Additions of NPK fertilizer, labile C (sugar) and fungicide (benomyl) were done in a fully factorial design, replicated in six blocks. The treatments were run for ten years and soil and vegetation samples were collected four years after initiating the experiment, and again after an additional 12 years, to evaluate the long-term effects. Labile C addition resulted in increased microbial biomass and nutrient immobilization after four years, and a long-term decrease in vascular plant biomass, thus suggesting the microorganisms to strongly control soil nutrient availability in periods of high microbial biomass. Fertilization increased the inorganic and total soil nutrient pools of N and P and the fine root biomass, but not the total aboveground vascular plant biomass. The vascular plant biomass increased due to benomyl addition thus indicating the plants to be strongly affected by the microbial community. Overall, the effects of benomyl resulted in more lasting changes in the soil compared to labile C and fertilizer addition. In relation to environmental changes, the indicated strong microbial control of the available nutrients in the fellfield ecosystem might limit ecosystem changes due to increased soil nutrient availability as otherwise expected in arctic soils.     

Importance of rhizodeposition in the coupling of plant and microbial productivity

Plant roots influence the biological, chemical and physical properties of rhizosphere soil. These effects are a consequence of their growth, their activity and the exudation of organic compounds from them. In natural ecosystems, the linkages between inputs of carbon from plants and microbial activity driven by these inputs are central to our understanding of nutrient cycling in soil and the productivity of these systems. This coupling of plant and microbial productivity is also of increasing importance in agriculture, where the shift towards low‐input systems increases the dependence of plant production on nutrient cycling, as opposed to fertilizers. This review considers the processes by which plants can influence the cycling of nutrients in soil, and in particular the importance of organic inputs from roots in driving microbially mediated transformations of N. This coupling of plant inputs to the functioning of the microbial community is beneficial for acquisition of N by plants, particularly in low‐input systems. This occurs through stimulation of microbes that produce exoenzymes that degrade organic matter, and by promoting cycling of N immobilized in the microbial biomass via predation by protozoa. Also, plants increase the cycling of N by changes in exudation in response to nitrogen supply around roots, and in response to browsing by herbivores. Plants can release compounds in exudates that directly affect the expression of genes in microbes, and this may be an important way of controlling their function to the benefit of the plant.     

Snow removal alters soil microbial biomass and enzyme activity in a Tibetan alpine forest

Projected future decreases in snow cover associated with global warming in alpine ecosystems could affect soil biochemical cycling. To address the objectives how an altered snow removal could affect soil microbial biomass and enzyme activity related to soil carbon and nitrogen cycling and pools, plastic film coverage and returning of melt snow water were applied to simulate the absence of snow cover in a Tibetan alpine forest of western China. Soil temperature and moisture, nutrient availability, microbial biomass and enzyme activity were measured at different periods (before snow cover, early snow cover, deep snow cover, snow cover melting and early growing season) over the entire 2009/2010 winter. Snow removal increased the daily variation of soil temperature, frequency of freeze–thaw cycle, soil frost depth, and advanced the dates of soil freezing and melting, and the peak release of inorganic N. Snow removal significantly decreased soil gravimetric water, ammonium and inorganic N, and activity of soil invertase and urease, but increased soil nitrate, dissolve organic C (DOC) and N (DON), and soil microbial biomass C (MBC) and N (MBN). Our results suggest that a decreased snow cover associated with global warming may advance the timing of soil freezing and thawing as well as the peak of releases of nutrients, leading to an enhanced nutrient leaching before plant become active. These results demonstrate that an absence of snow cover under global warming scenarios will alter soil microbial activities and hence element biogeochemical cycling in alpine forest ecosystems.     

农田潮土养分耦合循环的微生物学机理研究进展

科学施肥是实现农业增产和土壤健康的重要手段。深入认知土壤微生物驱动的养分元素循环过程对于评价和指导科学施肥具有极为重要的意义。近年来不同施肥策略对土壤微生物影响的认知虽取得长足进步,但多限于单一养分元素的影响。土壤养分元素循环过程及与碳循环过程具有不可分割性,互作和协同也是土壤微生物的重要特性之一。因此,揭示土壤微生物介导的养分元素的耦合会更有助于我们全面了解农田土壤养分循环过程和指导科学施肥。笔者研究团队多年来以中国科学院封丘农业生态试验站养分平衡长期定位试验为平台,利用土壤微生物学、分子生态学、微量热和同位素测定等技术,系统研究了不同施肥策略下潮土微生物介导的碳、氮、磷元素循环及耦合过程。本文综述相关研究成果。对于缺磷潮土,磷肥施用能够提高微生物生物量碳氮、转化酶活性、脲酶活性、呼吸强度及微生物代谢活性等,使得微生物能够保障土壤养分转化与作物养分吸收,增加潮土综合碳氮汇效应,“高效低排”地为生态系统服务;与之相反,缺施磷肥条件下,即使长期施用其它养分如氮肥,潮土微生物代谢效率仍然低下,且代谢过程会损失更多碳氮素,不利于潮土碳氮累积,致使土壤质量无法提高,还加剧了生态环境的污染。在阐明了微生物对潮土养分的响应与反馈后,还进一步揭示了潮土碳磷耦合的微生物学机制,提出了“缺磷耗碳,增碳活磷”的理论框架。长期缺磷使得外源碳经由微生物转化进入潮土有机碳库的比例减少,却增加了外源碳的净矿化量,使得更多外源碳以CO₂形式排放到大气,即缺磷潮土不利于外源碳向潮土有机碳的转化。但是,外源碳的添加能够长效刺激缺磷潮土中微生物的增殖,特别是解磷微生物。该过程可以使得土壤化学固持的磷素转移到微生物体内,增加了潮土中潜在有效磷含量,后续可以提供给植物利用,即外源碳的添加能够通过解磷微生物活化土壤中不可利用态磷素。最后,对今后农田土壤养分耦合循环的微生物学研究方向和内容进行了展望。这些结果会加深对科学施肥重要性的认知,有助于指导调控土壤微生物更好地服务农田生态系统。     

Litter, warming and plants affect respiration and allocation of soil microbial and plant C, N and P in arctic mesocosms

In a mesocosm experiment, we studied decomposition rates as CO₂ efflux and changes in plant mass, nutrient accumulation and soil pools of nitrogen (N) and phosphorus (P), in soils from a sub-arctic heath. The soil was incubated at 10 °C and 12 °C, with or without leaf litter and with or without plants present. The purpose of the experiment was to analyse decomposition and nutrient transformations under simulated, realistic conditions in a future warmer Arctic.Both temperature enhancement and litter addition increased respiration rates. Temperature enhancement and surprisingly also litter addition decreased microbial biomass carbon (C) content, resulting in a pronounced increase of specific respiration. Microbial P content increased progressively with temperature enhancement and litter addition, concomitant with increasing P mineralisation, whereas microbial N increased only in the litter treatment, at the same time as net N mineralisation decreased. In contrast, microbial biomass N decreased as temperature increased, resulting in a high mobilisation of inorganic N.Plant responses were closely coupled to the balance of microbial mineralisation and immobilisation. Plant growth and N accumulation was low after litter addition because of high N immobilisation in microbes and low net mineralisation, resulting in plant N limitation. Growth increased in the temperature-enhanced treatments, but was eventually limited by low supply of P, reflected in a low plant P concentration and high N-to-P ratio. Hence, the different microbial responses caused plant N limitation after litter addition and P limitation after temperature enhancement. Although microbial processes determined the main responses in plants, the plants themselves influenced nutrient turnover. With plants present, P mobilisation to the plant plus soil inorganic pools increased significantly, and N mobilisation non-significantly, when litter was added. This was presumably due to increased mineralisation in the rhizosphere, or because the nutrients in addition to being immobilised by microbes also could be absorbed by plants. This suggests that the common method of measuring nutrient mineralisation in soils incubated without plants may underestimate the rates of nutrient mobilisation, which probably contributes to a commonly observed discrepancy of measured lower rates of net nutrient mineralisation than uptake rates in arctic soils.     

Exotic annuals reduce soil heterogeneity in coastal sage scrub soil chemical and biological characteristics

Exotic plant invasions alter ecosystem structure and function above- and below-ground through plant–soil feedbacks. The resistance of ecosystems to invasion can be measured by the degree of change in microbial communities and soil chemical pools and fluxes, whereas their resilience can be measured by the ability to recover following restoration. Coastal sage scrub (CSS) is one of the most highly invaded ecosystems in the US but the response of CSS soils to exotic plant invasion is little known. We examined resistance and resilience of CSS soil chemical and biological characteristics following invasion of exotic annual grasses and forbs and restoration of the native plant community. We hypothesized that invasion of exotic plant species would change biological and chemical characteristics of CSS soils by altering soil nutrient inputs. Additionally, we expected that if exotic plants were controlled and native plants were restored, native soil characteristics would recover. We sampled two locations with invaded, restored and native CSS for plant community composition, soil chemistry and microbial communities, and phospholipid fatty acid (PLFA) profiles. Communities invaded by exotic annuals were resistant to some measured parameters but not others. Extractable nitrogen pools decreased, nitrogen cycling rates increased, and microbial biomass and fungal:bacterial ratios were altered in invaded soils, and these effects were mediated by the phenological stage of the dominant plant species. The largest impact of invasion on soils was an overall reduction of spatial heterogeneity in soil nutrients, nutrient cycling and microbial communities. Restored plots tended to recover in most biotic and chemical parameters including increased resource heterogeneity compared to invaded plots, suggesting that CSS soils are resilient but not resistant to invasion.     

Tree girdling provides insight on the role of labile carbon in nitrogen partitioning between soil microorganisms and adult European beech

Nitrogen (N) cycling in terrestrial ecosystems is complex since it involves the closely interwoven processes of both N uptake by plants and microbial turnover of a variety of N metabolites. Major interactions between plants and microorganisms involve competition for the same N species, provision of plant nutrients by microorganisms and labile carbon (C) supply to microorganisms by plants via root exudation. Despite these close links between microbial N metabolism and plant N uptake, only a few studies have tried to overcome isolated views of plant N acquisition or microbial N fluxes. In this study we studied competitive patterns of N fluxes in a mountainous beech forest ecosystem between both plants and microorganisms by reducing rhizodeposition by tree girdling. Besides labile C and N pools in soil, we investigated total microbial biomass in soil, microbial N turnover (N mineralization, nitrification, denitrification, microbial immobilization) as well as microbial community structure using denitrifiers and mycorrhizal fungi as model organisms for important functional groups. Furthermore, plant uptake of organic and inorganic N and N metabolite profiles in roots were determined.Surprisingly plants preferred organic N over inorganic N and nitrate (NO₃⁻) over ammonium (NH₄⁺) in all treatments. Microbial N turnover and microbial biomass were in general negatively correlated to plant N acquisition and plant N pools, thus indicating strong competition for N between plants and free living microorganisms. The abundance of the dominant mycorrhizal fungi Cenococcum geophilum was negatively correlated to total soil microbial biomass but positively correlated to glutamine uptake by beech and amino acid concentration in fine roots indicating a significant role of this mycorrhizal fungus in the acquisition of organic N by beech. Tree girdling in general resulted in a decrease of dissolved organic carbon and total microbial biomass in soil while the abundance of C. geophilum remained unaffected, and N uptake by plants was increased. Overall, the girdling-induced decline of rhizodeposition altered the competitive balance of N partitioning in favour of beech and its most abundant mycorrhizal symbiont and at the expense of heterotrophic N turnover by free living microorganisms in soil. Similar to tree girdling, drought periods followed by intensive drying/rewetting events seemed to have favoured N acquisition by plants at the expense of free living microorganisms.     

Ecosystem partitioning of N-glycine after long-term climate and nutrient manipulations, plant clipping and addition of labile carbon in a subarctic heath tundra

Low temperatures and high soil moisture restrict cycling of organic matter in arctic soils, but also substrate quality, i.e. labile carbon (C) availability, exerts control on microbial activity. Plant exudation of labile C may facilitate microbial growth and enhance microbial immobilization of nitrogen (N). Here, we studied ¹⁵N label incorporation into microbes, plants and soil N pools after both long-term (12 years) climate manipulation and nutrient addition, plant clipping and a pulse-addition of labile C to the soil, in order to gain information on interactions among soil N and C pools, microorganisms and plants. There were few effects of long-term warming and fertilization on soil and plant pools. However, fertilization increased soil and plant N pools and increased pool dilution of the added ¹⁵N label. In all treatments, microbes immobilized a major part of the added ¹⁵N shortly after label addition. However, plants exerted control on the soil inorganic N concentrations and recovery of total dissolved ¹⁵N (TD¹⁵N), and likewise the microbes reduced these soil pools, but only when fed with labile C. Soil microbes in clipped plots were primarily C limited, and the findings of reduced N availability, both in the presence of plants and with the combined treatment of plant clipping and addition of sugar, suggest that the plant control of soil N pools was not solely due to plant uptake of soil N, but also partially caused by plants feeding labile C to the soil microbes, which enhanced their immobilization power. Hence, the cycling of N in subarctic heath tundra is strongly influenced by alternating release and immobilization by microorganisms, which on the other hand seems to be less affected by long-term warming than by addition or removal of sources of labile C.     

Simulated grazer effects on microbial respiration in a subarctic meadow: Implications for nutrient competition between plants and soil microorganisms

Plant-microbial competition for nutrients is considered to be a strong mechanism affecting nutrient distribution in subarctic ecosystems, but the role of grazers on the distribution of nutrients between the plants and soil microorganisms remain poorly understood. We designed a factorial fertilization and clipping experiment to study the potential competition between plants and soil microorganisms for soil nitrogen in an ecosystem under grazing. We assumed that clipping reduces plant photosynthetic capacity and C flux to the soil, which ultimately results in lower microbial substrate availability and reduced potential for N immobilization. In concurrence with microbial substrate availability, increased nutrient availability through fertilization was expected to enhance microbial N in the unclipped but not in the clipped treatment.Clipping significantly reduced microbial respiration, suggesting that grazing reduces the labile C available for soil microbes in the system. Clipping had no effect on microbial C and N and the amount of NH₄-N captured in ion exchange resin bags, which was used as an index of net N mineralization. Microbial potential for N immobilization thus seemed insensitive to grazer-mediated changes in microbial availability of labile substrates. Fertilization had no effects or interactions with clipping on microbial C and N. By contrast, we found a close negative correlation between the plant root biomass and microbial N, indicating that plants had a negative impact on the microbial nutrient acquisition. The subarctic grassland vegetation seemed superior to the soil microorganisms in the competition for nutrients even when the plants were subjected to artificial grazing. We suggest that nutrient competition by higher plants constrained the microbial N immobilization in the system, which could explain why the reduction in microbial C availability by clipping had little effects on microbial N acquisition. In this subarctic system, grazing has significant influences on soil C cycling, but due to plant predominance in the competition for nutrients, does not affect N allocation between the plants and the soil microorganisms.     

Dynamics of soil microbial biomass C, N and P in disturbed and undisturbed stands of a tropical wet-evergreen forest

To understand the spatial and temporal dynamics of soil microbial biomass and its role in soil organic matter and nutrient flux in disturbed tropical wet-evergreen forests, we determined soil microbial biomass C, N and P at two soil depths (0–15 and 15–30 cm), along a disturbance gradient in Arunachal Pradesh, northeastern India. Disturbance resulted in considerable increase in air temperature and light intensity in the forest and decline in the soil nutrients concentration, which affected the growth of microbial populations and soil microbial biomass. There were significant correlations between bacterial and fungal populations and microbial biomass C, N and P. Soil microbial population was higher in the undisturbed (UD) forest stand than the disturbed forest stands during post-monsoon and less during rainy season due to heavy rainfall. Greater demand for nutrients by plants during rainy season limited the availability of nutrients to soil microbes and therefore, low microbial biomass C, N and P. Microbial biomass was negatively correlated with soil temperature and pH in all the forest stands. However, there were significant positive relationships among microbial biomass C, N and P. Percentage contribution of microbial C to soil organic C was higher in UD forest, whereas percentage contribution of microbial biomass N and P to total N and total P was higher in the moderately disturbed site than in the highly disturbed (HD) site. These results reveal that the nutrient retention by soil microbial biomass was greater in the selective logged stand and would help in the regeneration of the forest upon protection. On the other hand, the cultivated site (HD) that had the lowest labile fractions of soil organic matter may recover at a slower phase. Further, minimum and maximum microbial biomass C, N and P during rainy and winter seasons suggest the synchronization between nutrient demand for plant growth and nutrient retention in microbial biomass that would help in ecosystem recovery following disturbance.     

Soil microbial and nutrient dynamics in a wet Arctic sedge meadow in late winter and early spring

Microbial activity is known to continue during the winter months in cold alpine and Arctic soils often resulting in high microbial biomass. Complex soil nutrient dynamics characterize the transition when soil temperatures approach and exceed 0 °C in spring. At the time of this transition in alphine soils microbial biomass declines dramatically together with soil pools of available nutrients. This pattern of change characterizes alpine soils at the winter-spring transition but whether a similar pattern occurs in Arctic soils, which are colder, is unclear. In this study amounts of microbial biomass and the availability of carbon (C), nitrogen (N) and phosphorus (P) for microbial and plant growth in wet peaty soils of an Arctic sedge meadow have been determined across the winter-spring boundary. The objective was to determine the likely causes of the decline in microbial biomass in relation to temperature change and nutrient availability. The pattern of soil temperature at depths of 5-15 cm can be divided into three phases: below −10 °C in late winter, from −7 to 0 °C for 7 weeks during a period of freeze-thaw cycles and above 0 °C in early spring. Peak microbial biomass and nutrient availability occurred early in the freeze-thaw phase. Subsequently, a steady decrease in inorganic N occurred, so that when soil temperatures rose above 0 °C, pools of inorganic nutrients in soils were very low. In contrast, amounts of microbial C and soluble organic C and N remained high until the end of the period of freeze-thaw cycles, when a sudden collapse occurred in soluble organic C and N and in phosphatase activity, followed by a crash in microbial biomass just prior to soil temperatures rising consistently above 0 °C. Following this, there was no large pulse of available nutrients, implying that competition for nutrients from roots results in the collapse of the microbial pool.     

Temporal variability in plant and soil nitrogen pools in a high-Arctic ecosystem

This study determined temporal variability in N pools, both aboveground and belowground, across two contrasting plant communities in high-Arctic Spitsbergen, Svalbard (78°N). We measured N pools in plant material, soil microbial biomass and soil organic matter in moist (Alopecurus borealis dominated) and dry (Dryas octopetala dominated) meadow communities at four times during the growing season. We found that plant, microbial and dissolved inorganic and organic N pools were subject to significant, but surprisingly low, temporal variation that was controlled primarily by changes in temperature and moisture availability over the short growing season. This temporal variability is much less than that experienced in other seasonally cold ecosystems such as alpine tundra where strong seasonal partitioning of N occurs between plant and soil microbial pools. While only a small proportion of the total ecosystem N, the microbial biomass represented the single largest of the dynamic N pools in both moist and dry meadow communities (3.4% and 4.6% of the total ecosystem N pool, respectively). This points to the importance of soil microbial community dynamics for N cycling in high-Arctic ecosystems. Microbial N was strongly and positively related to soil temperature in the dry meadow, but this relationship did not hold true in the wet meadow where other factors such as wetter soil conditions might constrain biological activity. Vascular live belowground plant parts represented the single largest plant N pool in both dry and moist meadow, constituting an average of 1.6% of the total N pool in both systems; this value did not vary across the growing season or between plant communities. Overall, our data illustrate a surprisingly low growing season variability in labile N pools in high-Arctic ecosystems, which we propose is controlled primarily by temperature and moisture.     

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.     

The effect of warming on the vulnerability of subducted organic carbon in arctic soils

Arctic permafrost soils contain large stocks of organic carbon (OC). Extensive cryogenic processes in these soils cause subduction of a significant part of OC-rich topsoil down into mineral soil through the process of cryoturbation. Currently, one-fourth of total permafrost OC is stored in subducted organic horizons. Predicted climate change is believed to reduce the amount of OC in permafrost soils as rising temperatures will increase decomposition of OC by soil microorganisms. To estimate the sensitivity of OC decomposition to soil temperature and oxygen levels we performed a 4-month incubation experiment in which we manipulated temperature (4–20 °C) and oxygen level of topsoil organic, subducted organic and mineral soil horizons. Carbon loss (C_LOSS) was monitored and its potential biotic and abiotic drivers, including concentrations of available nutrients, microbial activity, biomass and stoichiometry, and extracellular oxidative and hydrolytic enzyme pools, were measured. We found that independently of the incubation temperature, C_LOSS from subducted organic and mineral soil horizons was one to two orders of magnitude lower than in the organic topsoil horizon, both under aerobic and anaerobic conditions. This corresponds to the microbial biomass being lower by one to two orders of magnitude. We argue that enzymatic degradation of autochthonous subducted OC does not provide sufficient amounts of carbon and nutrients to sustain greater microbial biomass. The resident microbial biomass relies on allochthonous fluxes of nutrients, enzymes and carbon from the OC-rich topsoil. This results in a “negative priming effect”, which protects autochthonous subducted OC from decomposition at present. The vulnerability of subducted organic carbon in cryoturbated arctic soils under future climate conditions will largely depend on the amount of allochthonous carbon and nutrient fluxes from the topsoil.     

Seasonal variations and effects of nutrient applications on N and P and microbial biomass under two temperate heathland plants

Eutrofication is a threat against nutrient-poor habitats as increased amounts of nutrients in ecosystems may cause changes in the vegetation. Nitrogen (N) deposition leads to conversion of Calluna heathlands into graminoid dominated heath, but low availability of P may hinder or slow down this process.In this study the soil properties under two dominant heathland plants, the dwarf shrub Calluna vulgaris and the grass Deschampsia flexuosa, were investigated, with focus on nutrient content in the organic top soil and soil microbes during the main growing season and effects of nutrient amendments. The concentration of inorganic and dissolved organic N was significantly higher under D. flexuosa than C. vulgaris all though there were the same amounts of total N in the soil below the two species. N and P amendment enhanced available N and P in the soil, but added nutrients had little direct effects on microbes. The microbial biomass on the other hand was positively related to soil water content in fertilized plots indicating that this was due to an indirect effect of enhanced nutrient availability. Microbial N and P pools were respectively 1000 and 100 times higher than the pool of inorganic N and P, and microbes therefore may play an important role in regulating plant nutrient supply. Judged from responses of inorganic and microbial N and P concentrations to added N and P, N seemed to limit C. vulgaris and soil microbes below while P seemed to limit D. flexuosa and soil microbes below this species. There were lower rates of net nitrification, net ammonification and DOC and DON production rates during winter in the soil under C. vulgaris than below D. flexuosa, although all these rates were equal under the two species on an annual basis. This indicates that these microbial processes were taking place during winter but were affected by exudates from C. vulgaris.     

Effects of litter addition and warming on soil carbon, nutrient pools and microbial communities in a subarctic heath ecosystem

Climatic warming leads to the expansion of deciduous shrubs and trees in the Arctic. This leads to higher leaf litter inputs, which together with warming may alter the rate of carbon and nutrient cycling in the arctic ecosystems. We assessed effects of factorial warming and additional litter on the soil ecosystem of a subarctic heath in a 7-year-long field experiment. Fine root biomass, dissolved organic carbon (DOC) and total C concentration increased in response to warming, which probably was a result of the increased vegetation cover. Litter addition increased the concentration of inorganic P in the uppermost 5 cm soil, while decreasing the pool of total P per unit area of the organic profile and having no significant effects on N concentrations or pools. Microbial biomass C and N were unaffected by the treatments, while the microbial biomass P increased significantly with litter addition. Soil ergosterol concentration was also slightly increased by the added litter in the uppermost soil, although not statistically significantly. According to a principal component analysis of the phospholipid fatty acid profiles, litter addition differed from the other treatments by increasing the relative proportion of biomarkers for Gram-positive bacteria. The combined warming plus litter addition treatment decreased the soil water content in the uppermost 5 cm soil, which was a likely reason for many interactions between the effects of warming and litter addition. The soil organic matter quality of the combined treatment was also clearly different from the control based on a near-infrared reflectance (NIR) spectroscopic analysis, implying that the treatment altered the composition of soil organic matter. However, it appears that the biological processes and the microbial community composition responded more to the soil and litter moisture conditions than to the change in the quality of the organic matter.     

Above- and belowground carbon inputs affect seasonal variations of soil microbial biomass in a subtropical monsoon forest of southwest China

Soil microbial activity drives carbon and nutrient cycling in terrestrial ecosystems. Soil microbial biomass is commonly limited by environmental factors and soil carbon availability. We employed plant litter removal, root trenching and stem-girdling treatments to examine the effects of environmental factors, above- and belowground carbon inputs on soil microbial C in a subtropical monsoon forest in southwest China. During the experimental period from July 2006 through April 2007, 2 years after initiation of the treatments, microbial biomass C in the humus layer did not vary with seasonal changes in soil temperature or water content. Mineral soil microbial C decreased throughout the experimental period and varied with soil temperature and water content. Litter removal reduced mineral soil microbial C by 19.0% in the ungirdled plots, but only 4.0% in girdled plots. Root trenching, stem girdling and their interactions influenced microbial C in humus layer. Neither root trenching nor girdling significantly influenced mineral soil microbial C. Mineral soil microbial C correlated with following-month plant litterfall in control plots, but these correlations were not observed in root-trenching plots or girdling plots. Our results suggest that belowground carbon retranslocated from shoots and present in soil organic matter, rather than aboveground fresh plant litter inputs, determines seasonal fluctuation of mineral soil microbial biomass.     

Mineralization and carbon turnover in subarctic heath soil as affected by warming and additional litter

Arctic soil carbon (C) stocks are threatened by the rapidly advancing global warming. In addition to temperature, increasing amounts of leaf litter fall following from the expansion of deciduous shrubs and trees in northern ecosystems may alter biogeochemical cycling of C and nutrients. Our aim was to assess how factorial warming and litter addition in a long-term field experiment on a subarctic heath affect resource limitation of soil microbial communities (measured by thymidine and leucine incorporation techniques), net growing-season mineralization of nitrogen (N) and phosphorus (P), and carbon turnover (measured as changes in the pools during a growing-season-long field incubation of soil cores in situ). The mainly N limited bacterial communities had shifted slightly towards limitation by C and P in response to seven growing seasons of warming. This and the significantly increased bacterial growth rate under warming may partly explain the observed higher C loss from the warmed soil. This is furthermore consistent with the less dramatic increase in the contents of dissolved organic carbon (DOC) and dissolved organic N (DON) in the warmed soil than in the soil from ambient temperature during the field incubation. The added litter did not affect the carbon content, but it was a source of nutrients to the soil, and it also tended to increase bacterial growth rate and net mineralization of P. The inorganic N pool decreased during the field incubation of soil cores, especially in the separate warming and litter addition treatments, while gross mineralized N was immobilized in the biomass of microbes and plants transplanted into the incubates soil cores, but without any significant effect of the treatments. The effects of warming plus litter addition on bacterial growth rates and of warming on C and N transformations during field incubation suggest that microbial activity is an important control on the carbon balance of arctic soils under climate change.     

Short and long-term effects of elevated CO2 on Lolium perenne rhizodeposition and its consequences on soil organic matter turnover and plant N yield

It is still unclear whether elevated CO₂ increases plant root exudation and consequently affects the soil microbial biomass. The effects of elevated CO₂ on the fate of the C and nitrogen (N) contained in old soil organic matter pools is also unclear. In this study the short and long-term effects of elevated CO₂ on C and N pools and fluxes were assessed by growing isolated plants of ryegrass (Lolium perenne) in glasshouses at elevated and ambient atmospheric CO₂ and using soil from the New Zealand FACE site that had >4 years exposure to CO₂ enrichment. Using ¹⁴CO₂ pulse labelling, the effects of elevated CO₂ on C allocation within the plant-soil system were studied. Under elevated CO₂ more root derived C was found in the soil and in the microbial biomass 48 h after labelling. The increased availability of substrate significantly stimulated soil microbial growth and acted as priming effect, enhancing native soil organic matter decomposition regardless of the mineral N supply. Despite indications of faster N cycling in soil under elevated CO₂, N availability to plants stayed unchanged. Soil previously exposed to elevated CO₂ exhibited a higher N cycling rate but again there was no effect on plant N uptake. With respect to the difficulties of extrapolating glasshouse experiment results to the field, we concluded that the accumulation of coarse organic matter observed in the field under elevated CO₂ was probably not created by an imbalance between C and N but was likely to be due to more complex phenomena involving soil mesofauna and/or other nutrients limitations.     

Interactions between plants, litter and microbes in cycling of nitrogen and phosphorus in the arctic

Estimated nutrient mineralization in northern nutrient-poor ecosystems, measured as differences in soil inorganic nutrients before and after a period of soil incubation in the absence of plants and litter, usually shows a discrepancy of much lower rates than plant nutrient uptake rates. In plots that had been pre-treated by 12 year of warming and fertilizer addition, we incubated soils together with litter and plants added and examined whether the absence of plants and litter in ‘traditional’ incubations could explain the discrepancy. The pre-treatment had no effect on nitrogen (N) mineralization but increased phosphorus (P) mineralization, while litter addition decreased N and increased P mineralization but without any effect on plant and microbial N and P sequestration. Incubations of soils with plants increased N mobilization to the soil inorganic plus plant pools several-fold as compared to the net mineralization in soils without plants. Hence, the presence of plants stimulated mobilization of the growth-limiting N. The growth-sufficient P was not affected by the presence of plants, however. Furthermore, increased plant and microbial N uptake correlated positively, which speaks against competition for plant available N from soil microbes in N-constrained ecosystems, at least during the time-span of 10 weeks the experiment lasted, and instead suggests facilitation.