Empirical and Earth system model estimates of boreal nitrogen fixation often differ: A pathway toward reconciliation

Limited dependence on soil nitrogen fixation as subtropical forests develop

Soil nitrogen (N) fixation, driven by microbial reactions, is critical to support the entrance of nitrogen in nutrient poor and pioneer ecosystems. However, how and why N fixation and soil diazotrophs evolve as forests develop remain poorly understood. Here, we used a 60-year forest rewilding chronosequence and found that soil N fixation activity gradually decreased with increasing forest age, experiencing dramatic drops of 64.8% in intermediate stages and 93.0% in the oldest forests. Further analyses revealed loses in diazotrophic diversity and a significant reduction in the abundance of important diazotrophs (e.g., Desulfovibrio and Pseudomonas) as forest develops. This reduction in N fixation, and associated shifts in soil microbes, was driven by acidification and increases in N content during forest succession. Our results provide new insights on the life history of one of the most important groups of soil organisms in terrestrial ecosystems, with consequences for understanding the buildup of nutrients as forest soil develops.

Warming influences carbon and nitrogen assimilation between a widespread Ericaceous shrub and root-associated fungi

High-latitude ecosystems are warming faster than other biomes and are often dominated by a ground layer of Ericaceous shrubs, which can respond positively to warming. The carbon-for-nitrogen (C-for-N) exchange between Ericaceous shrubs and root-associated fungi may underlie shrub responses to warming, but has been understudied. In a glasshouse setting, we examined the effects of warming on the C-for-N exchange between the Ericaceous shrub Empetrum nigrum ssp. hermaphroditum and its root-associated fungi. We applied different 13 C and 15 N isotope labels, including a simple organic N form (glycine) and a complex organic N form (moss litter) and quantified their assimilation into soil, plant biomass, and root fungal biomass pools. We found that warming lowered the amount of 13 C partitioned to root-associated fungi per unit of glycine 15 N assimilated by E. nigrum, but only in the short term. By contrast, warming increased the amount of 13 C partitioned to root-associated fungi per unit of moss 15 N assimilated by E. nigrum. Our study suggests that climate warming affects the short-term exchange of C and N between a widespread Ericaceous shrub and root-associated fungi. Furthermore, while most isotope tracing studies use labile N sources, we demonstrate that a ubiquitous recalcitrant N source may produce contrasting results.

The trait co-variation regulates the response of bryophytes to nitrogen deposition: A meta-analysis

The nitrogen deposition has the potential to alter the trait composition of plant communities by affecting the fitness and physiological adaptation of species, consequently exerting an influence on ecosystem processes. Despite the importance of bryophytes in nutrient and carbon dynamics across different ecosystems, there is a lack of research examining the relationship between nitrogen deposition and the co-variation of bryophyte traits. To address this gap, a meta-analysis was conducted using data from 27 independent studies to investigate potential associations between trait co-variation of bryophytes and nitrogen deposition. The results revealed that interspecific variability regulates the influence of nitrogen deposition on bryophytes by affecting trait co-variation. Multiple correspondence analysis identified six combinations of closely related traits. For example, species with unbranched main stems frequently exhibit robust leaf midribs, leading to leaf wrinkling and leaf clasping around the stem as a response to water loss. Some weft or mat species tend to obtain resources (nitrogen) through their scale hairs on the main stem. Some species with narrow leaves require leaf teeth to maintain a normal leaf shape. The subgroup analyses indicated that certain traits, including unbranched main stem, changes in leaf morphology, robust leaf midrib, main stem without scale hairs, narrow leaf, leaf margin with teeth, undeveloped apophysis, and erect capsule minimize interaction with pollutants and represent a resource strategy. Conversely, functional traits representing a resource acquisition strategy, such as branched main stem, no changes in leaf morphology, short and weak leaf midrib, main stem with scale hairs, broad leaf, leaf margin without teeth, developed apophysis, and non-erect capsule increase pollutant exposure. Overall, our results suggest that anthropogenic global change may significantly impact bryophytes due to changes in their individual physiology and colony ecological indicators caused by increased nitrogen deposition.

Agricultural drift depositional simulation of a copper-based fungicide and its effects on non-target terrestrial and freshwater compartments

Our research investigated the potential impacts of the fungicide Bordeaux mixture drift processes on off-target species representing terrestrial vegetation and fluvial-lacustrine zooplankton. The simulation of drift events was carried out by a predictive scaling analysis of the quantities potentially exported to a predetermined area adjacent to an agricultural field. The theoretical rate of deposition on a terrestrial species, the lichen Pseudevernia furfuracea, was calculated following high (4 kg ha^-1) and low (2 kg ha^-1) rate treatments using anti-drift nozzles and non-anti-drift nozzles. The experimental set up consisted in 40 boxes holding lichen thalli, all stored in a climatic chamber for 40 days. Spraying of the fungicide was alternated with rainfall simulations to reproduce scenarios related to agricultural practices. Following a single simulation, anti-drift nozzles resulted in a higher overall load deposited per unit of lichen surface area compared to non-anti-drift nozzles, although both loads significantly differed from controls. However, only anti-drift nozzles, associated with the high rate, caused a remarkable impairment of several ecophysiological parameters, differing (p < 0.05) from controls. Rainfalls promoted activation of lichen metabolism, mitigating the cell damage, but exported only 2.5% of the copper deposited on the thalli surfaces. Nevertheless, the exposure of Daphnia magna neonates to leachates showed significant outcomes for the two rates. After only 24 h, leachates resulting from the high application rate led to widespread mortality, which appeared to be extremely relevant after 48 h, whereas the lower rate induced much lower toxicity for both exposure times.

Effects of Multiple Global Change Factors on Symbiotic and Asymbiotic N2 Fixation: Results Based on a Pot Experiment

Biological N2 fixation, a major pathway for new nitrogen (N) input to terrestrial ecosystems, largely determines the dynamics of ecosystem structure and functions under global change. Nevertheless, the responses of N2 fixation to multiple global change factors remain poorly understood. Here, saplings of two N2-fixing plant species, Alnus cremastogyne and Cajanus cajan, were grown at rural and urban sites, respectively, with the latter representing an environment with changes in multiple factors occurring simultaneously. Symbiotic N2 fixation per unit of nodule was significantly higher at the urban site than the rural site for A. cremastogyne, but the rates were comparable between the two sites for C. cajan. The nodule investments were significantly lower at the urban site relative to the rural site for both species. Symbiotic N2 fixation per plant increased by 31.2 times for A. cremastogyne, while that decreased by 88.2% for C. cajan at the urban site compared to the rural site. Asymbiotic N2 fixation rate in soil decreased by 46.2% at the urban site relative to the rural site. The decrease in symbiotic N2 fixation per plant for C. cajan and asymbiotic N2 fixation in soil was probably attributed to higher N deposition under the urban conditions, while the increase in symbiotic N2 fixation per plant for A. cremastogyne was probably related to the higher levels of temperature, atmospheric CO2, and phosphorus deposition at the urban site. The responses of N2 fixation to multiple global change factors and the underlying mechanisms may be divergent either between symbiotic and asymbiotic forms or among N2-fixing plant species. While causative evidence is urgently needed, we argue that these differences should be considered in Earth system models to improve the prediction of N2 fixation under global change.

Habitat-adapted microbial communities mediate Sphagnum peatmoss resilience to warming

Sphagnum peatmosses are fundamental members of peatland ecosystems, where they contribute to the uptake and long-term storage of atmospheric carbon. Warming threatens Sphagnum mosses and is known to alter the composition of their associated microbiome. Here, we use a microbiome transfer approach to test if microbiome thermal origin influences host plant thermotolerance. We leveraged an experimental whole-ecosystem warming study to collect field-grown Sphagnum, mechanically separate the associated microbiome and then transfer onto germ-free laboratory Sphagnum for temperature experiments. Host and microbiome dynamics were assessed with growth analysis, Chla fluorescence imaging, metagenomics, metatranscriptomics and 16S rDNA profiling. Microbiomes originating from warming field conditions imparted enhanced thermotolerance and growth recovery at elevated temperatures. Metagenome and metatranscriptome analyses revealed that warming altered microbial community structure in a manner that induced the plant heat shock response, especially the HSP70 family and jasmonic acid production. The heat shock response was induced even without warming treatment in the laboratory, suggesting that the warm-microbiome isolated from the field provided the host plant with thermal preconditioning. Our results demonstrate that microbes, which respond rapidly to temperature alterations, can play key roles in host plant growth response to rapidly changing environments.