β-diversity in temperate grasslands is driven by stronger environmental filtering of plant species with large genomes

Elucidating mechanisms underlying community assembly and biodiversity patterns is central to ecology and evolution. Genome size (GS) has long been hypothesized to potentially affect species' capacity to tolerate environmental stress and might therefore help drive community assembly. However, its role in driving β-diversity (i.e., spatial variability in species composition) remains unclear. We measured GS for 161 plant species and community composition across 52 sites spanning a 3200-km transect in the temperate grasslands of China. By correlating the turnover of species composition with environmental dissimilarity, we found that resource filtering (i.e., environmental dissimilarity that includes precipitation, and soil nitrogen and phosphorus concentrations) affected β-diversity patterns of large-GS species more than small-GS species. By contrast, geographical distance explained more variation of β-diversity for small-GS than for large-GS species. In a 10-year experiment manipulating levels of water, nitrogen, and phosphorus, adding resources increased plant biomass in species with large GS, suggesting that large-GS species are more sensitive to the changes in resource availability. These findings highlight the role of GS in driving community assembly and predicting species responses to global change

Plant microbiomes : do different preservation approaches and primer sets alter our capacity to assess microbial diversity and community composition?

The microbial communities associated with plants (the plant microbiome) play critical roles in regulating plant health and productivity. Because of this, in recent years, there have been significant increase in studies targeting the plant microbiome. Amplicon sequencing is widely used to investigate the plant microbiome and to develop sustainable microbial agricultural tools. However, performing large microbiome surveys at the regional and global scales pose several logistic challenges. One of these challenges is related with the preservation of plant materials for sequencing aiming to maintain the integrity of the original diversity and community composition of the plant microbiome. Another significant challenge involves the existence of multiple primer sets used in amplicon sequencing that, especially for bacterial communities, hampers the comparability of datasets across studies. Here, we aimed to examine the effect of different preservation approaches (snap freezing, fresh and kept on ice, and air drying) on the bacterial and fungal diversity and community composition on plant leaves, stems and roots from seven plant species from contrasting functional groups (e.g. C3, C4, N-Fixers, etc.). Another major challenge comes when comparing plant to soil microbiomes, as different primers sets are often used for plant vs. soil microbiomes. Thus, we also investigated if widely used 16S rRNA primer set (779F/1193R) for plant microbiome studies provides comparable data to those often used for soil microbiomes (341F/805R) using 86 soil samples. We found that the community composition and diversity of bacteria or fungi were robust to contrasting preservation methods. The primer sets often used for plants provided similar results to those often used for soil studies suggesting that simultaneous studies on plant and soil microbiomes are possible. Our findings provide novel evidence that preservation approaches do not significantly impact plant microbiome data interpretation and primer differences do not impact the treatment effect, which has significant implication for future large-scale and global surveys of plant microbiomes

Myristate and the ecology of AM fungi : significance, opportunities, applications and challenges

A recent study by Sugiura and coworkers reported the nonsymbiotic growth and spore production of an arbuscular mycorrhizal (AM) fungus, Rhizophagus irregularis, when the fungus received an external supply of certain fatty acids, myristates (C:14). This discovery follows the insight that AM fungi receive fatty acids from their hosts when in symbiosis. If this result holds up and can be repeated under nonsterile conditions and with a broader range of fungi, it has numerous consequences for our understanding of AM fungal ecology, from the level of the fungus, at the plant community level, and to functional consequences in ecosystems. In addition, myristate may open up several avenues from a more applied perspective, including improved fungal culture and supplementation of AM fungi or inoculum in the field. We here map these potential opportunities, and additionally offer thoughts on potential risks of this potentially new technology. Lastly, we discuss the specific research challenges that need to be overcome to come to an understanding of the potential role of myristate in AM ecology

Testing nitrogen and water co-limitation of primary productivity in a temperate steppe

Background and aims: Primary productivity in the temperate steppe is assumed to be co-limited by nitrogen (N) and water availability, but empirical evidence is scarce. We examined the N and water limitation status of primary productivity from the species scale to community scale under the framework of resource co-limitation. Methods: We compared the responses of aboveground net primary productivity (ANPP) at different ecological levels to factorial N and water addition in two years in a temperate steppe of northern China. Results: Water addition significantly enhanced total ANPP by 46%, with stronger effects in the dry year. Total ANPP was sub-additively co-limited by N and water availability, being more sensitive to water addition than to N addition in the dry year and equally sensitive to both resources in the year with normal precipitation. The responses of total ANPP to resource additions were largely driven by the changes of grasses rather than the forbs. Species level ANPP showed conservative responses to resource additions. Conclusions: Our results highlight the hierarchical patterns of limitation status in primary productivity at different biological organization levels in this temperate steppe. The sub-additive limitation by N and water in this ecosystem deserves more attention in modelling the dynamics of ecosystem carbon cycle under global change scenarios

Carbon isotope fractionation including photosynthetic and post-photosynthetic processes in C3 plants : low [CO2] matters

Carbon isotope ratios of plants are highly informative for the reconstruction of ancient environments and for the interpretation of plant physiological processes to climate, but their responses to changing atmospheric CO2 concentration are currently debated. Moreover, plants in the geological past have experienced long-term low CO2 concentration (LC). However, the effects of LC on the plant C isotope ratios are still elusive. To investigate effects of atmospheric CO2 concentration ([CO2]) and drought on isotope ratios of plant metabolites we grew winter wheat (Triticum aestivum) in climate-controlled chambers under different [CO2] covering glacial, pre-industrial, and present concentrations (170, 280, and 400 ppm) and water regimes (well-watered and drought). First, we quantified total C isotope discrimination between plant and atmosphere (Δ) using 13C on-line measurements of plant gas exchange and 13C values of plant metabolites, i.e., cellulose, n-alkane, and phospholipid fatty acids (PLFA). We found that LC yielded a higher Δ regardless of water regime, i.e. more 13C-depleted values were found under LC; the effect was stronger for n-C29 alkane (1.5‰/100 ppm) and C16:0 PLFA (1.1‰/100 ppm) than that for cellulose (0.6‰/100 ppm). We then calculated post-photosynthetic C isotope shift (ɛ) between specific metabolites and plant bulk isotope values. δ13Cn -C29 alkane and δ13CPLFA were 8.3‰ and 7.3‰ lighter than the δ13Cbulk under 400 ppm; these depletions became higher (9.8‰ and 8.2‰ lighter than the δ13Cbulk for n-C29 alkane and PLFA, respectively) under 170 ppm. In contrast, δ13Ccellulose was 1.2‰ heavier than the δ13Cbulk under 400 ppm while this enrichment became higher (1.6‰) under 170 ppm. Changes in atmospheric [CO2] affected C fractionation not only via photosynthetic but also post-photosynthetic processes and thus must be taken into account when interpreting C isotopes for paleoclimate reconstruction and future global C cycle prediction

Plant carbon limitation does not reduce nitrogen transfer from arbuscular mycorrhizal fungi to Plantago lanceolata

Aims: The stress-gradient-hypothesis predicts that interactions among organisms shift from competition to facilitation as environmental stress increases. Whether the strength of mutualism will increase among symbiotically associated organisms when partners are forced into resource limitation remains unknown. Plants exchange photosynthetic carbohydrates (plant C) for nutrients in mycorrhizal symbiosis but how this exchange varies with plant C limitation is not fully understood. Methods: We investigated the influence of plant C availability and of arbuscular mycorrhizal fungi (AMF) on plant nitrogen (N) uptake and resource allocation using 13C and 15N labeling. We grew Plantago lanceolata with and without AMF Rhizophagus irregularis under ambient (400 pm, AC) and low (100 ppm, LC) atmospheric [CO2] and physically restricted plant root but not mycorrhizal access to soil N. Results: We found that plants grown under LC used AMF to obtain the same amount of N as those grown under AC, but the amount of newly fixed C correlated with the acquisition of N only under LC. The LC plants allocated more of their C to aboveground tissues. Conclusions: Overall our results suggest a more beneficial role of symbiosis under C limitation. The tight reciprocal control on N transfer and C allocation under C limited conditions supports the stress-gradient hypothesis of mutualistic symbiotic functioning

The fate of fertilizer nitrogen in a high nitrate accumulated agricultural soil

Well-acclimatized nitrifiers in high-nitrate agricultural soils can quickly nitrify NH4 + into NO3 - subject to leaching and denitrifying loss. A 120-day incubation experiment was conducted using a greenhouse soil to explore the fates of applied fertilizer N entering into seven soil N pools and to examine if green manure (as ryegrass) co-application can increase immobilization of the applied N into relatively stable N pools and thereby reduce NO3 - accumulation and loss. We found that 87-92% of the applied 15N-labelled NH4 + was rapidly recovered as NO3 - since day 3 and only 2-4% as microbial biomass and soil organic matter (SOM), while ryegrass co-application significantly decreased its recovery as NO3 - but enhanced its recovery as SOM (17%) at the end of incubation. The trade-off relationship between 15N recoveries in microbial biomass and SOM indicated that ryegrass co-application stabilized newly immobilized N via initial microbial uptake and later breakdown. Nevertheless, ryegrass application didn't decrease soil total NO3 - accumulation due to its own decay. Our results suggest that green manure co-application can increase immobilization of applied N into stable organic N via microbial turnover, but the quantity and quality of green manure should be well considered to reduce N release from itself

Fungal genome size and composition reflect ecological strategies along soil fertility gradients

Genomic traits reflect the evolutionary processes that have led to ecological variation among extant organisms, including variation in how they acquire and use resources. Soil fungi have diverse nutritional strategies and exhibit extensive variation in fitness along resource gradients. We tested for trade-offs in genomic traits with mycelial nutritional traits and hypothesize that such trade-offs differ among fungal guilds as they reflect contrasting resource exploitation and habitat preferences. We found species with large genomes exhibited nutrient-poor mycelium and low GC content. These patterns were observed across fungal guilds but with varying explanatory power. We then matched trait data to fungal species observed in 463 Australian grassland, woodland and forest soil samples. Fungi with large genomes and lower GC content dominated in nutrient-poor soils, associated with shifts in guild composition and with species turnover within guilds. These findings highlight fundamental mechanisms that underpin successful ecological strategies for soil fungi

[In Press] Short-term drought is a stronger driver of plant morphology and nutritional composition than warming in two common pasture species

Under warmer and drier future conditions, global livestock and dairy production are threatened by impacts on the productivity and nutritional quality of pastures. However, morphological and nutritional adjustments within plants in response to warming and drought vary among species and less is known how these relate to production and forage quality. To investigate this, we grew two common pasture species, tall fescue (Festuca arundinacea: grass) and lucerne (Medicago sativa: legume), in a climate-controlled facility, under different temperatures (ambient and elevated) and watering regimes (well-watered and droughted). We found that drought had a strong negative impact on biomass production, morphology and nutritional quality while warming only significantly affected both species when response metrics were considered in concert, although to a lesser degree than the drought. Furthermore, interactions between warming and drought were only seen for lucerne, with a reduction in biomass and an increase in dead material and dry matter. In tall fescue, drought had bigger impacts on nutritional composition than morphological traits, while in lucerne, drought affected all morphological traits and most of the nutritional parameters. These findings suggest that in future climate scenarios, drought may be a stronger driver of changes in the morphology and nutritional composition of pasture grasses and legumes, compared to modest levels of warming

5300-year-old soil carbon is less primed than young soil organic matter

Soils harbor more than three times as much carbon (C) as the atmosphere, a large fraction of which (stable organic matter) serves as the most important global C reservoir due to its long residence time. Litter and root inputs bring fresh organic matter (FOM) into the soil and accelerate the turnover of stable C pools, and this phenomenon is termed the “priming effect” (PE). Compared with knowledge about labile soil C pools, very little is known about the vulnerability of stable C to priming. Using two soils that substantially differed in age (500 and 5300 years before present) and in the degree of chemical recalcitrance and physical protection of soil organic matter (SOM), we showed that leaf litter amendment primed 264% more organic C from the young SOM than from the old soil with very stable C. Hierarchical partitioning analysis confirmed that SOM stability, reflected mainly by available C and aggregate protection of SOM, is the most important predictor of leaf litter-induced PE. The addition of complex FOM (i.e., leaf litter) caused a higher bacterial oligotroph/copiotroph (K-/r-strategists) ratio, leading to a PE that was 583% and 126% greater than when simple FOM (i.e., glucose) was added to the young and old soils, respectively. This implies that the PE intensity depends on the chemical similarity between the primer (here FOM) and SOM. Nitrogen (N) mining existed when N and simple FOM were added (i.e., Glucose+N), and N addition raised the leaf litter-induced PE in the old soil that had low N availability, which was well explained by the microbial stoichiometry. In conclusion, the PE induced by FOM inputs strongly decreases with increasing SOM stability. However, the contribution of stable SOM to CO2 efflux cannot be disregarded due to its huge pool size