Responses of Plant Community Composition and Biomass Production to Warming and Nitrogen Deposition in a Temperate Meadow Ecosystem

Climate change has profound influences on plant community composition and ecosystem functions. However, its effects on plant community composition and biomass production are not well understood. A four-year field experiment was conducted to examine the effects of warming, nitrogen (N) addition, and their interactions on plant community composition and biomass production in a temperate meadow ecosystem in northeast China. Experimental warming had no significant effect on plant species richness, evenness, and diversity, while N addition highly reduced the species richness and diversity. Warming tended to reduce the importance value of graminoid species but increased the value of forbs, while N addition had the opposite effect. Warming tended to increase the belowground biomass, but had an opposite tendency to decrease the aboveground biomass. The influences of warming on aboveground production were dependent upon precipitation. Experimental warming had little effect on aboveground biomass in the years with higher precipitation, but significantly suppressed aboveground biomass in dry years. Our results suggest that warming had indirect effects on plant production via its effect on the water availability. Nitrogen addition significantly increased above- and below-ground production, suggesting that N is one of the most important limiting factors determining plant productivity in the studied meadow steppe. Significant interactive effects of warming plus N addition on belowground biomass were also detected. Our observations revealed that environmental changes (warming and N deposition) play significant roles in regulating plant community composition and biomass production in temperate meadow steppe ecosystem in northeast China.     

Earthworm and belowground competition effects on plant productivity in a plant diversity gradient

Diversity is one major factor driving plant productivity in temperate grasslands. Although decomposers like earthworms are known to affect plant productivity, interacting effects of plant diversity and earthworms on plant productivity have been neglected in field studies. We investigated in the field the effects of earthworms on plant productivity, their interaction with plant species and functional group richness, and their effects on belowground plant competition. In the framework of the Jena Experiment we determined plant community productivity (in 2004 and 2007) and performance of two phytometer plant species [Centaurea jacea (herb) and Lolium perenne (grass); in 2007 and 2008] in a plant species (from one to 16) and functional group richness gradient (from one to four). We sampled earthworm subplots and subplots with decreased earthworm density and reduced aboveground competition of phytometer plants by removing the shoot biomass of the resident plant community. Earthworms increased total plant community productivity (+11%), legume shoot biomass (+35%) and shoot biomass of the phytometer C. jacea (+21%). Further, phytometer performance decreased, i.e. belowground competition increased, with increasing plant species and functional group richness. Although single plant functional groups benefited from higher earthworm numbers, the effects did not vary with plant species and functional group richness. The present study indicates that earthworms indeed affect the productivity of semi-natural grasslands irrespective of the diversity of the plant community. Belowground competition increased with increasing plant species diversity. However, belowground competition was modified by earthworms as reflected by increased productivity of the phytometer C. jacea. Moreover, particularly legumes benefited from earthworm presence. Considering also previous studies, we suggest that earthworms and legumes form a loose mutualistic relationship affecting essential ecosystem functions in temperate grasslands, in particular decomposition and plant productivity. Further, earthworms likely alter competitive interactions among plants and the structure of plant communities by beneficially affecting certain plant functional groups.     

Long‐term study of root biomass in a biodiversity experiment reveals shifts in diversity effects over time

Biodiversity experiments generally report a positive effect of plant biodiversity on aboveground biomass (overyielding), which typically increases with time. Various studies also found overyielding for belowground plant biomass, but this has never been measured over time. Also, potential underlying mechanisms have remained unclear. Differentiation in rooting patterns among plant species and plant functional groups has been proposed as a main driver of the observed biodiversity effect on belowground biomass, leading to more efficient belowground resource use with increasing diversity, but so far there is little evidence to support this. We analyzed standing root biomass and its distribution over the soil profile, along a 1–16 species richness gradient over eight years in the Jena Experiment in Germany, and compared belowground to aboveground overyielding. In our long‐term dataset, total root biomass increased with increasing species richness but this effect was only apparent after four years. The increasingly positive relationship between species richness and root biomass, explaining 12% of overall variation and up to 28% in the last year of our study, was mainly due to decreasing root biomass at low diversity over time. Functional group composition strongly affected total standing root biomass, explaining 44% of variation, with grasses and legumes having strong overall positive and negative effects, respectively. Functional group richness or interactions between functional group presences did not strongly contribute to overyielding. We found no support for the hypothesis that vertical root differentiation increases with species richness, with functional group richness or composition. Other explanations, such as stronger negative plant–soil feedbacks in low‐diverse plant communities on standing root biomass and vertical distribution should be considered.     

Responses of phenology and biomass production of boreal fens to climate warming under different water‐table level regimes

Climate change affects peatlands directly through increased air temperatures and indirectly through changes in water‐table level (WL). The interactions of these two still remain poorly known. We determined experimentally the separate and interactive effects of temperature and WL regime on factors of relevance for the inputs to the carbon cycle: plant community composition, phenology, biomass production, and shoot:root allocation in two wet boreal sedge‐dominated fens, “southern” at 62°N and “northern” at 68°Ν. Warming (1.5°C higher average daily air temperature) was induced with open‐top chambers and WL drawdown (WLD; 3–7 cm on average) by shallow ditches. Total biomass production varied from 250 to 520 g/m², with belowground production comprising 25%–63%. Warming was associated with minor effects on phenology and negligible effects on community composition, biomass production, and allocation. WLD clearly affected the contribution of different plant functional types (PFTs) in the community and the biomass they produced: shrubs benefited while forbs and mosses suffered. These responses did not depend on the warming treatment. Following WLD, aboveground biomass production decreased mainly due to reduced growth of mosses in the southern fen. Aboveground vascular plant biomass production remained unchanged but the contribution of different PFTs changed. The observed changes were also reflected in plant phenology, with different PFTs showing different responses. Belowground production increased following WLD in the northern fen only, but an increase in the contributions of shrubs and forbs was observed in both sites, while sedge contribution decreased. Moderate warming alone seems not able to drive significant changes in plant productivity or community composition in these wet ecosystems. However, if warming is accompanied by even modest WL drawdown, changes should be expected in the relative contribution of PFTs, which could lead to profound changes in the function of fens. Consequently, hydrological scenarios are of utmost importance when estimating their future function.     

Effects of Warming and Nitrogen Addition on Plant Photosynthate Partitioning in an Alpine Meadow on the Tibetan Plateau

Quantifying plant carbon (C) allocation among different pools is critical for understanding and predicting how C turnover responds to global climate change in terrestrial ecosystems. A field experiment with increasing warming and nitrogen (N) was established to investigate interactive effects on plant C allocation in alpine meadows. Open-top chambers (OTCs) were used to simulate warming. In OTCs, daytime air and soil temperature at 5 cm depth increased by 2.0 and 1.6 °C, respectively, compared with ambient conditions, but soil moisture at 5 cm depth decreased by 4.95% (v/v) from 2012 to 2014. Warming reduced aboveground biomass by 38, 36, and 43% in 2012, 2013, and 2014, respectively, and increased belowground biomass by 64% and 29% in 2013 and 2014, respectively, and the root-to-shoot ratio was significantly increased. Specifically, warming increased the proportion of plant roots in the deep layers (10–20 cm). Both N addition and its combination with warming substantially enhanced belowground biomass. Pulse-labeling experiments for ¹³C revealed that warming reduced the translocation of assimilated C to shoots by 8.8% (38.7% in warming, and 47.5% in the control [CK]), and increased the allocation to root by 12.2% (55.5% in warming, and 43.3% in CK) after 28 days labeling. However, N addition increased the proportion of assimilated C allocated to shoots by 6.5% (54.0% in N addition, and 47.5% in CK), whereas warming combined with N addition reduced this proportion by 10.9%. A decline in soil water content in the surface layer may be the main cause of plants allocating more newly fixed photosynthate to roots. Therefore, plants promoted root growth to draw water from deeper soil layers (10–20 cm). We concluded that climate warming will change the allocation patterns of plant photosynthates by affecting soil water availability, whereas N addition will increase plant photosynthates aboveground in alpine meadows and thus will significantly affect C turnover under future climate change scenarios.

     

Density may alter diversity–productivity relationships in experimental plant communities

Contemporary biodiversity experiments, in which plant species richness is manipulated and aboveground productivity of the system measured, generally demonstrate that lowering plant species richness reduces productivity. However, we propose that community density may in part compensate for this reduction of productivity at low diversity. We conducted a factorial experiment in which plant functional group richness was held constant at three, while plant species richness increased from three to six to 12 species and community density from 440 to 1050 to 2525 seedlings m⁻². Response variables included density, evenness and above- and belowground biomass at harvest. The density gradient converged slightly during the course of the experiment due to about 10% mortality at the highest sowing density. Evenness measured in terms of aboveground biomass at harvest significantly declined with density, but the effect was weak. Overall, aboveground, belowground and total biomass increased significantly with species richness and community density. However, a significant interaction between species richness and community density occurred for both total and aboveground biomass, indicating that the diversity–productivity relationship was flatter at higher than at lower density. Thus, high species richness enabled low-density communities to reach productivity levels otherwise seen only at high density. The relative contributions of the three functional groups C₃, C₄ and nitrogen-fixers to aboveground biomass were less influenced by community density at high than at low species richness. We interpret the interaction effects between community density and species richness on community biomass by expanding findings about constant yield and size variation from monocultures to plant mixtures.     

The variable effects of soil nitrogen availability and insect herbivory on aboveground and belowground plant biomass in an old-field ecosystem

Nutrient availability and herbivory can regulate primary production in ecosystems, but little is known about how, or whether, they may interact with one another. Here, we investigate how nitrogen availability and insect herbivory interact to alter aboveground and belowground plant community biomass in an old-field ecosystem. In 2004, we established 36 experimental plots in which we manipulated soil nitrogen (N) availability and insect abundance in a completely randomized plot design. In 2009, after 6 years of treatments, we measured aboveground biomass and assessed root production at peak growth. Overall, we found a significant effect of reduced soil N availability on aboveground biomass and belowground plant biomass production. Specifically, responses of aboveground and belowground community biomass to nutrients were driven by reductions in soil N, but not additions, indicating that soil N may not be limiting primary production in this ecosystem. Insects reduced the aboveground biomass of subdominant plant species and decreased coarse root production. We found no statistical interactions between N availability and insect herbivory for any response variable. Overall, the results of 6 years of nutrient manipulations and insect removals suggest strong bottom-up influences on total plant community productivity but more subtle effects of insect herbivores on aspects of aboveground and belowground production.     

Diversity and abundance of macro‐invertebrates on abandoned cattle kraals in a semi‐arid savanna

Abandoned cattle (Bos taurus) kraals are sources of habitat heterogeneity in dystrophic semi‐arid African savannas with a strong positive effect on soil nutrients and plant productivity. However, little is known regarding how macro‐invertebrate assemblages vary between abandoned kraals and the surrounding savanna matrix. We tested whether herbaceous biomass and basal and aerial covers and soil nutrients have an effect on aboveground and belowground macro‐invertebrate assemblages. Twelve abandoned kraals were contrasted with their paired control plots for soil characteristics, herbaceous productivity, and macro‐invertebrate assemblages in Save Valley Conservancy, Zimbabwe. Abandoned kraals had significantly higher concentrations of soil nitrogen (N), phosphorus (P), potassium (K), and calcium (Ca) as well as herbaceous biomass and basal and aerial covers than control plots. Both aboveground and belowground macro‐invertebrate species richness were higher on abandoned kraals. However, only belowground macro‐invertebrate diversity (Shannon H′ and Hill number 1) was significantly higher on abandoned kraals. Soil nutrients and herbaceous productivity had positive and significant correlations with the dominant taxa (Coleoptera, Hymenoptera, Hemiptera, Isoptera, and Myriapoda) on abandoned kraals. These results add to the growing body of evidence that abandoned kraals exert significant effects on savanna spatial heterogeneity years later, with implications on ecosystem processes and functioning.     

A review of belowground biomass allocation and its response to global climatic change in grassland ecosystems北大核心CSCD

Biomass allocations between aboveground and belowground organs provide pivotal information for connecting aboveground productivity and belowground carbon sequestration. As accurate measurement of belowground biomass is essential for determining the biomass allocation, we first reviewed the methods in quantifying belowground biomass and their merits. We then presented the major advances on plant biomass allocations between aboveground and belowground organs, as well as the potential drivers such as precipitation, warming, atmospheric CO2 concentration, and nitrogen deposition. We finally provided a list of challenges in studying belowground biomass allocation for the future. This review has important implications for studies on carbon cycling in grassland ecosystems under the changing climate.     

Functional dominance rather than taxonomic diversity and functional diversity mainly affects community aboveground biomass in the Inner Mongolia grassland

The relationship between biodiversity and productivity has been a hot topic in ecology. However, the relative importance of taxonomic diversity and functional characteristics (including functional dominance and functional diversity) in maintaining community productivity and the underlying mechanisms (including selection and complementarity effects) of the relationship between diversity and community productivity have been widely controversial. In this study, 194 sites were surveyed in five grassland types along a precipitation gradient in the Inner Mongolia grassland of China. The relationships between taxonomic diversity (species richness and the Shannon–Weaver index), functional dominance (the community‐weighted mean of four plant traits), functional diversity (Rao's quadratic entropy), and community aboveground biomass were analyzed. The results showed that (1) taxonomic diversity, functional dominance, functional diversity, and community aboveground biomass all increased from low to high precipitation grassland types; (2) there were significant positive linear relationships between taxonomic diversity, functional dominance, functional diversity, and community aboveground biomass; (3) the effect of functional characteristics on community aboveground biomass is greater than that of taxonomic diversity; and (4) community aboveground biomass depends on the community‐weighted mean plant height, which explained 57.1% of the variation in the community aboveground biomass. Our results suggested that functional dominance rather than taxonomic diversity and functional diversity mainly determines community productivity and that the selection effect plays a dominant role in maintaining the relationship between biodiversity and community productivity in the Inner Mongolia grassland.     

Responses of High Arctic wet sedge tundra to climate warming since 1980

The global climate is changing rapidly and Arctic regions are showing responses to recent warming. Responses of tundra ecosystems to climate change have been examined primarily through short‐term experimental manipulations, with few studies of long‐term ambient change. We investigated changes in above‐ and belowground biomass of wet sedge tundra to the warming climate of the Canadian High Arctic over the past 25 years. Aboveground standing crop was harvested from five sedge meadow sites and belowground biomass was sampled from one of the sites in the early 1980s and in 2005 using the same methods. Aboveground biomass was on average 158% greater in 2005 than in the early 1980s. The belowground biomass was also much greater in 2005: root biomass increased by 67% and rhizome biomass by 139% since the early 1980s. Dominant species from each functional group (graminoids, shrubs and forbs) showed significant increases in aboveground biomass. Responsive species included the dominant sedge species Carex aquatilis stans, C. membranacea, and Eriophorum angustifolium, as well as the dwarf shrub Salix arctica and the forb Polygonum viviparum. However, diversity measures were not different between the sample years. The greater biomass correlated strongly with increased annual and summer temperatures over the same time period, and was significantly greater than the annual variation in biomass measured in 1980–1983. Increased decomposition and mineralization rates, stimulated by warmer soils, were likely a major cause of the elevated productivity, as no differences in the mass of litter were found between sample periods. Our results are corroborated by published short‐term experimental studies, conducted in other wet sedge tundra communities which link warming and fertilization with elevated decomposition, mineralization and tundra productivity. We believe that this is the first study to show responses in High Arctic wet sedge tundra to recent climate change.     

A review of belowground biomass allocation and its response to global climatic change in grassland ecosystems

Biomass allocations between aboveground and belowground organs provide pivotal information for connecting aboveground productivity and belowground carbon sequestration. As accurate measurement of belowground biomass is essential for determining the biomass allocation, we first reviewed the methods in quantifying belowground biomass and their merits. We then presented the major advances on plant biomass allocations between aboveground and belowground organs, as well as the potential drivers such as precipitation, warming, atmospheric CO₂ concentration, and nitrogen deposition. We finally provided a list of challenges in studying belowground biomass allocation for the future. This review has important implications for studies on carbon cycling in grassland ecosystems under the changing climate.     

Linking soil food web structure to above‐ and belowground ecosystem processes: a meta‐analysis

Soil fauna can be an important regulator of community parameters and ecosystem processes, but there have been few quantitative syntheses of the role of soil fauna in terrestrial soil communities and ecosystems. Here, we conducted a meta‐analysis to investigate the impacts of invertebrate soil micro‐ and mesofauna (grazers and predators) on plant productivity and microbial biomass. Overall our results indicate that an increase in the biomass of soil fauna increased aboveground plant productivity across ecosystems by 35% and decreased microbial biomass by 8%. In addition, we found no evidence for trophic cascades in terrestrial soil food webs, but the bacterivorous component of soil fauna influenced plant productivity and microbial biomass more than did the fungivorous component. Furthermore, changes in the biomass of soil fauna differentially affected plant productivity among plant functional groups: a higher biomass of soil fauna increased aboveground productivity by 70% in coniferous systems. However, in ecosystems dominated by legumes, a functional group with lower inorganic nitrogen requirements, there was no response of aboveground productivity to increases in the biomass of soil fauna. In sum, the results of this meta‐analysis indicate that soil fauna help to regulate ecosystem production, especially in nutrient‐limited ecosystems.     

Nitrogen availability increases in a tundra ecosystem during five years of experimental permafrost thaw

Perennially frozen soil in high latitude ecosystems (permafrost) currently stores 1330–1580 Pg of carbon (C). As these ecosystems warm, the thaw and decomposition of permafrost is expected to release large amounts of C to the atmosphere. Fortunately, losses from the permafrost C pool will be partially offset by increased plant productivity. The degree to which plants are able to sequester C, however, will be determined by changing nitrogen (N) availability in these thawing soil profiles. N availability currently limits plant productivity in tundra ecosystems but plant access to N is expected improve as decomposition increases in speed and extends to deeper soil horizons. To evaluate the relationship between permafrost thaw and N availability, we monitored N cycling during 5 years of experimentally induced permafrost thaw at the Carbon in Permafrost Experimental Heating Research (CiPEHR) project. Inorganic N availability increased significantly in response to deeper thaw and greater soil moisture induced by Soil warming. This treatment also prompted a 23% increase in aboveground biomass and a 49% increase in foliar N pools. The sedge Eriophorum vaginatum responded most strongly to warming: this species explained 91% of the change in aboveground biomass during the 5 year period. Air warming had little impact when applied alone, but when applied in combination with Soil warming, growing season soil inorganic N availability was significantly reduced. These results demonstrate that there is a strong positive relationship between the depth of permafrost thaw and N availability in tundra ecosystems but that this relationship can be diminished by interactions between increased thaw, warmer air temperatures, and higher levels of soil moisture. Within 5 years of permafrost thaw, plants actively incorporate newly available N into biomass but C storage in live vascular plant biomass is unlikely to be greater than losses from deep soil C pools.     

Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau

Grazing is an important modulator of both plant productivity and biodiversity in grassland community, yet how to determine a suitable grazing intensity in alpine grassland is still controversy. Here, we explore the effects of different grazing intensities on plant biomass and species composition, both at community level and functional group level, and examines the productivity–species richness relationship under four grazing patterns: no grazing (CK), light grazing (LG), moderate grazing, (MG) and heavy grazing (HG), attempt to determine a suitable grazing intensity in alpine grassland. The results were as follows. The total aboveground biomass (AGB) reduced with increasing grazing intensity, and the response of plant functional groups was different. AGB of both sedges and legumes increased from MG to HG, while the AGB of forbs reduced sharply and the grass AGB remained steady. There was a significant positive relationship between productivity and species richness both at community level and functional group level. In contrast, the belowground biomass (BGB) showed a unimodal relationship from CK to HG, peaking in MG (8,297.72 ± 621.29 g/m²). Interestingly, the grassland community tends to allocate more root biomass to the upper soil layer under increasing grazing intensities. Our results suggesting that moderate levels of disturbance may be the optimal grassland management strategy for alpine meadow in terms of root production.     

Comparative impacts of aboveground and belowground enemies on an invasive thistle

Most research examining how herbivores and pathogens affect performance of invasive plants focuses on aboveground interactions. Although important, the role of belowground communities remains poorly understood, and the relative impact of aboveground and belowground interactions is still debated. As well, most studies of belowground interactions have been carried out in controlled environments, so little is known about the role of these interactions under natural conditions or how these relationships may change across a plant's range. Using the invasive plant Cirsium arvense, we performed a reciprocal transplant experiment to test the relative impacts of above‐ and belowground interactions at three sites across a 509‐km latitudinal gradient in its invaded range in Ontario, Canada. At each site, C. arvense seedlings were protected with above‐ and/or belowground exclosures in a factorial design. Plant performance (biomass, height, stem thickness, number of leaves, length of longest leaf, maximum rhizome length) was greatest when both above‐ and belowground exclosures were applied and lowest when no exclosures were applied. When only one type of exclosure was applied, biomass generally improved more with belowground exclosures than with aboveground exclosures. Despite site‐to‐site differences in foliar damage, root damage, and mesofaunal populations, belowground interactions generally had a greater negative impact on performance than aboveground herbivory alone. These results stress the importance of including both aboveground enemy interactions and plant–soil interactions in studies of plant community dynamics and invader performance.     

Soil amendment interacts with invasive grass and drought to uniquely influence aboveground versus belowground biomass in aridland restoration

Water‐holding soil amendments such as super‐absorbent polymer (SAP) may improve native species establishment in restoration but may also interact with precipitation or invasive species such as Bromus tectorum L. (cheatgrass or downy brome) to influence revegetation outcomes. We implemented an experiment at two sites in Colorado, U.S.A., in which we investigated the interactions of drought (66% reduction of ambient rainfall), B. tectorum seed addition (BRTE, 465 seeds/m²), and SAP soil amendment (25 g/m²) on initial plant establishment and 3‐year aboveground and belowground biomass and allocation. At one site, SAP resulted in higher native seeded species establishment but only with ambient precipitation. However, by the third year, we detected no SAP effects on native seeded species biomass. Treatments interacted to influence aboveground and belowground biomass and allocation differently. At one site, a SAP × precipitation interaction resulted in lower belowground biomass in plots with SAP and drought (61.7 ± 7.3 g/m²) than plots with drought alone (91.6 ± 18.1 g/m²). At the other site, a SAP × BRTE interaction resulted in higher belowground biomass in plots with SAP and BRTE (56.6 ± 11.2 g/m²) than BRTE alone (35.0 ± 3.7 g/m²). These patterns were not reflected in aboveground biomass. SAP should be used with caution in aridland restoration because initial positive effects may not translate to long‐term benefits, SAP may uniquely influence aboveground versus belowground biomass, and SAP can interact with environmental variables to impact developing plant communities in positive and negative ways.     

Plant diversity and identity effects on predatory nematodes and their prey

There is considerable evidence that both plant diversity and plant identity can influence the level of predation and predator abundance aboveground. However, how the level of predation in the soil and the abundance of predatory soil fauna are related to plant diversity and identity remains largely unknown. In a biodiversity field experiment, we examined the effects of plant diversity and identity on the infectivity of entomopathogenic nematodes (EPNs, Heterorhabditis and Steinernema spp.), which prey on soil arthropods, and abundance of carnivorous non‐EPNs, which are predators of other nematode groups. To obtain a comprehensive view of the potential prey/food availability, we also quantified the abundance of soil insects and nonpredatory nematodes and the root biomass in the experimental plots. We used structural equation modeling (SEM) to investigate possible pathways by which plant diversity and identity may affect EPN infectivity and the abundance of carnivorous non‐EPNs. Heterorhabditis spp. infectivity and the abundance of carnivorous non‐EPNs were not directly related to plant diversity or the proportion of legumes, grasses and forbs in the plant community. However, Steinernema spp. infectivity was higher in monocultures of Festuca rubra and Trifolium pratense than in monocultures of the other six plant species. SEM revealed that legumes positively affected Steinernema infectivity, whereas plant diversity indirectly affected the infectivity of Heterorhabditis EPNs via effects on the abundance of soil insects. The abundance of prey (soil insects and root‐feeding, bacterivorous, and fungivorous nematodes) increased with higher plant diversity. The abundance of prey nematodes was also positively affected by legumes. These plant community effects could not be explained by changes in root biomass. Our results show that plant diversity and identity effects on belowground biota (particularly soil nematode community) can differ between organisms that belong to the same feeding guild and that generalizations about plant diversity effects on soil organisms should be made with great caution.     

Experimentally increased nutrient availability at the permafrost thaw front selectively enhances biomass production of deep‐rooting subarctic peatland species

Climate warming increases nitrogen (N) mineralization in superficial soil layers (the dominant rooting zone) of subarctic peatlands. Thawing and subsequent mineralization of permafrost increases plant‐available N around the thaw‐front. Because plant production in these peatlands is N‐limited, such changes may substantially affect net primary production and species composition. We aimed to identify the potential impact of increased N‐availability due to permafrost thawing on subarctic peatland plant production and species performance, relative to the impact of increased N‐availability in superficial organic layers. Therefore, we investigated whether plant roots are present at the thaw‐front (45 cm depth) and whether N‐uptake (¹⁵N‐tracer) at the thaw‐front occurs during maximum thaw‐depth, coinciding with the end of the growing season. Moreover, we performed a unique 3‐year belowground fertilization experiment with fully factorial combinations of deep‐ (thaw‐front) and shallow‐fertilization (10 cm depth) and controls. We found that certain species are present with roots at the thaw‐front (Rubus chamaemorus) and have the capacity (R. chamaemorus, Eriophorum vaginatum) for N‐uptake from the thaw‐front between autumn and spring when aboveground tissue is largely senescent. In response to 3‐year shallow‐belowground fertilization (S) both shallow‐ (Empetrum hermaphroditum) and deep‐rooting species increased aboveground biomass and N‐content, but only deep‐rooting species responded positively to enhanced nutrient supply at the thaw‐front (D). Moreover, the effects of shallow‐fertilization and thaw‐front fertilization on aboveground biomass production of the deep‐rooting species were similar in magnitude (S: 71%; D: 111% increase compared to control) and additive (S + D: 181% increase). Our results show that plant‐available N released from thawing permafrost can form a thus far overlooked additional N‐source for deep‐rooting subarctic plant species and increase their biomass production beyond the already established impact of warming‐driven enhanced shallow N‐mineralization. This may result in shifts in plant community composition and may partially counteract the increased carbon losses from thawing permafrost.     

Effects of Soil Nutrient Heterogeneity and Earthworms on Aboveground Biomass of Experimental Plant Communities

Soil nutrients are commonly heterogeneously distributed and earthworms are one of the most common soil organisms. While effects of both soil nutrient heterogeneity and earthworms have been well studied, their interactive effect on plant community productivity has rarely been tested. In a greenhouse experiment, we constructed experimental plant communities by sowing seed mixtures of four grasses, two legumes and two forbs in either a heterogeneous soil consisting of low and high nutrient soil patches or a homogeneous soil where the low and high nutrient soil patches were evenly mixed. The earthworm Eisenia fetida was either added to these soils or not. Aboveground biomass of the whole communities, grasses and legumes did not differ between the homogeneous and heterogeneous soils or between the soils with and without earthworms. However, soil nutrient heterogeneity reduced aboveground biomass of forbs, and such an effect did not interact with earthworms. In response to soil heterogeneity and earthworms, biomass ratio of the three functional groups showed similar patterns as that of their biomass. At the patch level, aboveground biomass of the whole community, grasses and legumes were greater in the high than in the low nutrient soil patches within the heterogeneous soil. A similar pattern was found for the forbs, but this was only true in the absence of earthworms. Our results suggest that soil nutrient heterogeneity and earthworms may not influence aboveground biomass of plant communities, despite the fact that they may modify the growth of certain plant functional groups within the community.