Global changes in climate, atmospheric composition, and pollutants are altering ecosystems and the goods and services they provide. Among approaches for predicting ecosystem responses, long-term ...observations and manipulative experiments can be powerful approaches for resolving single-factor and interactive effects of global changes on key metrics such as net primary production (NPP). Here we combine both approaches, developing multidimensional response surfaces for NPP based on the longest-running, best-replicated, most-multifactor global-change experiment at the ecosystem scale—a 17-y study of California grassland exposed to full-factorial warming, added precipitation, elevated CO₂, and nitrogen deposition. Single-factor and interactive effects were not time-dependent, enabling us to analyze each year as a separate realization of the experiment and extract NPP as a continuous function of global-change factors. We found a ridge-shaped response surface in which NPP is humped (unimodal) in response to temperature and precipitation when CO₂ and nitrogen are ambient, with peak NPP rising under elevated CO₂ or nitrogen but also shifting to lower temperatures. Our results suggest that future climate change will push this ecosystem away from conditions that maximize NPP, but with large year-to-year variability.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Course assessments reflect authentic scientific communication * The final paper should follow the format of an influential journal in the given field, and students should receive multiple iterations ...of feedback from peers and instructors. * Students should present their findings in a conference-like presentation format at the end of the course. 6.\n Despite the simple techniques used, students were able to generate and test a variety of hypotheses on ecological interactions (see video at http://www.bio-link.org/home/summer-fellows-forum-2011/mimulus). ...the course resulted in a source of novel data and hypotheses that are being used by the instructor's research lab to both guide and answer research questions 15,16.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Global climate models predict that the frequency and intensity of precipitation events will increase in many regions across the world. However, the biosphere‐climate feedback to elevated ...precipitation (eP) remains elusive. Here, we report a study on one of the longest field experiments assessing the effects of eP, alone or in combination with other climate change drivers such as elevated CO2 (eCO2), warming and nitrogen deposition. Soil total carbon (C) decreased after a decade of eP treatment, while plant root production decreased after 2 years. To explain this asynchrony, we found that the relative abundances of fungal genes associated with chitin and protein degradation increased and were positively correlated with bacteriophage genes, suggesting a potential viral shunt in C degradation. In addition, eP increased the relative abundances of microbial stress tolerance genes, which are essential for coping with environmental stressors. Microbial responses to eP were phylogenetically conserved. The effects of eP on soil total C, root production, and microbes were interactively affected by eCO2. Collectively, we demonstrate that long‐term eP induces soil C loss, owing to changes in microbial community composition, functional traits, root production, and soil moisture. Our study unveils an important, previously unknown biosphere‐climate feedback in Mediterranean‐type water‐limited ecosystems, namely how eP induces soil C loss via microbe‐plant–soil interplay.
Eco‐responses to long‐term elevated precipitation (eP), combined with other climate change factors, have not been well understood. This work, conducted in one of the longest eP experiments, unveils eco‐responses in a timescale of 14 years. Soil total carbon decreased after 9 years of eP treatment, while plant root production decreased after only 2 years since the experiment began. Changes in microbial taxonomic composition and functional traits of resource acquisition, viral shunt, and stress response contributed to explain this asynchrony. Our study unveils an important biosphere‐climate feedback via microbe‐plant–soil interplay in the Mediterranean grassland ecosystem, which has been overlooked so far.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Soil microbial communities regulate and respond to key biogeochemical cycles and influence plant community patterns. However, microbial communities also respond to disturbance events, motivating an ...assessment of the relative roles of decadal multi‐factor global change, disturbance and plant community structure on microbial community responses.
We used high‐throughput amplicon sequencing to characterize the diversity and composition of bacterial and fungal communities in bulk soil (0–7 cm) collected in 2014 from the Jasper Ridge Global Change Experiment, a full‐factorial field experiment in which ambient and elevated levels of nitrogen deposition (+7 g N m−2 yr−1 calcium nitrate), CO2 concentration (+275 ppm), temperature (+1–2°C) and precipitation (+50% volume with +3 weeks duration) were applied to a California annual grassland from 1998 to 2014. We used linear mixed‐effects modelling to test for the effects of global change on microbial diversity (observed richness, Shannon index). We also used generalized dissimilarity modelling (GDM) to study controls on compositional dissimilarity in fungal and bacterial communities.
Bacterial community composition was best explained by exposure to fires in 2003 and 2011, whereas fungal community composition was best explained by plant community composition. The richness of fungi increased under elevated nitrogen deposition; bacterial diversity metrics decreased under warmer temperatures. Interactions between global change factors were statistically insignificant or weak.
Synthesis. Our results indicate that even on decadal time‐scales, the effects of fire history and plant community composition on bacterial and fungal community composition, respectively, outweigh the effects of multi‐factor global change. Furthermore, global change factors have mostly additive effects on microbial diversity patterns. Our results show that highly variable mediators such as fire history and plant community composition limit the generalizability of soil microbial responses to long‐term global change.
Even on decadal timescales, the effects of fire history and plant community composition on bacterial and fungal community composition, respectively, outweigh the effects of multi‐factor global change. Global change factors have mostly additive effects on microbial diversity patterns. Our results show that highly variable mediators such as fire history and plant community composition limit the generalizability of soil microbial responses to long‐term global change.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Global change drivers (GCDs) are expected to alter community structure and consequently, the services that ecosystems provide. Yet, few experimental investigations have examined effects of GCDs on ...plant community structure across multiple ecosystem types, and those that do exist present conflicting patterns. In an unprecedented global synthesis of over 100 experiments that manipulated factors linked to GCDs, we show that herbaceous plant community responses depend on experimental manipulation length and number of factors manipulated. We found that plant communities are fairly resistant to experimentally manipulated GCDs in the short term (10 y). In contrast, long-term (<10 y) experiments show increasing community divergence of treatments from control conditions. Surprisingly, these community responses occurred with similar frequency across the GCD types manipulated in our database. However, community responses were more common when 3 or more GCDs were simultaneously manipulated, suggesting the emergence of additive or synergistic effects of multiple drivers, particularly over long time periods. In half of the cases, GCD manipulations caused a difference in community composition without a corresponding species richness difference, indicating that species reordering or replacement is an important mechanism of community responses to GCDs and should be given greater consideration when examining consequences of GCDs for the biodiversity–ecosystem function relationship. Human activities are currently driving unparalleled global changes worldwide. Our analyses provide the most comprehensive evidence to date that these human activities may have widespread impacts on plant community composition globally, which will increase in frequency over time and be greater in areas where communities face multiple GCDs simultaneously.
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Shifting plant phenology (i.e., timing of flowering and other developmental events) in recent decades establishes that species and ecosystems are already responding to global environmental change. ...Earlier flowering and an extended period of active plant growth across much of the northern hemisphere have been interpreted as responses to warming. However, several kinds of environmental change have the potential to influence the phenology of flowering and primary production. Here, we report shifts in phenology of flowering and canopy greenness (Normalized Difference Vegetation Index) in response to four experimentally simulated global changes: warming, elevated CO₂, nitrogen (N) deposition, and increased precipitation. Consistent with previous observations, warming accelerated both flowering and greening of the canopy, but phenological responses to the other global change treatments were diverse. Elevated CO₂ and N addition delayed flowering in grasses, but slightly accelerated flowering in forbs. The opposing responses of these two important functional groups decreased their phenological complementarity and potentially increased competition for limiting soil resources. At the ecosystem level, timing of canopy greenness mirrored the flowering phenology of the grasses, which dominate primary production in this system. Elevated CO₂ delayed greening, whereas N addition dampened the acceleration of greening caused by warming. Increased precipitation had no consistent impacts on phenology. This diversity of phenological changes, between plant functional groups and in response to multiple environmental changes, helps explain the diversity in large-scale observations and indicates that changing temperature is only one of several factors reshaping the seasonality of ecosystem processes.
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Fire is a crucial event regulating the structure and functioning of many ecosystems. Yet few studies have focused on how fire affects taxonomic and functional diversities of soil microbial ...communities, along with changes in plant communities and soil carbon (C) and nitrogen (N) dynamics. Here, we analyze these effects in a grassland ecosystem 9 months after an experimental fire at the Jasper Ridge Global Change Experiment site in California, USA. Fire altered soil microbial communities considerably, with community assembly process analysis showing that environmental selection pressure was higher in burned sites. However, a small subset of highly connected taxa was able to withstand the disturbance. In addition, fire decreased the relative abundances of most functional genes associated with C degradation and N cycling, implicating a slowdown of microbial processes linked to soil C and N dynamics. In contrast, fire stimulated above‐ and belowground plant growth, likely enhancing plant–microbe competition for soil inorganic N, which was reduced by a factor of about 2. To synthesize those findings, we performed structural equation modeling, which showed that plants but not microbial communities were responsible for significantly higher soil respiration rates in burned sites. Together, our results demonstrate that fire ‘reboots’ the grassland ecosystem by differentially regulating plant and soil microbial communities, leading to significant changes in soil C and N dynamics.
Fire significantly increased environmental selection pressure on soil microbial community, where a small subset of highly connected taxa was able to withstand the disturbance. Fire decreased the relative abundances of most functional genes associated with C degradation and N cycling, but stimulated above‐ and belowground plant growth, likely enhancing plant–microbe competition for soil inorganic N. Plants but not microbial communities were responsible for significantly higher soil respiration rates in burned sites.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The continuously increasing concentration of atmospheric CO2 has considerably altered ecosystem functioning. However, few studies have examined the long-term (i.e. over a decade) effect of elevated ...CO2 on soil microbial communities. Using 16S rRNA gene amplicons and a GeoChip microarray, we investigated soil microbial communities from a Californian annual grassland after 14 years of experimentally elevated CO2 (275 ppm higher than ambient). Both taxonomic and functional gene compositions of the soil microbial community were modified by elevated CO2. There was decrease in relative abundance for taxa with higher ribosomal RNA operon (rrn) copy number under elevated CO2, which is a functional trait that responds positively to resource availability in culture. In contrast, taxa with lower rrn copy number were increased by elevated CO2. As a consequence, the abundance-weighted average rrn copy number of significantly changed OTUs declined from 2.27 at ambient CO2 to 2.01 at elevated CO2. The nitrogen (N) fixation gene nifH and the ammonium-oxidizing gene amoA significantly decreased under elevated CO2 by 12.6% and 6.1%, respectively. Concomitantly, nitrifying enzyme activity decreased by 48.3% under elevated CO2, albeit this change was not significant. There was also a substantial but insignificant decrease in available soil N, with both nitrate (NO3−) (−27.4%) and ammonium (NH4+) (−15.4%) declining. Further, a large number of microbial genes related to carbon (C) degradation were also affected by elevated CO2, whereas those related to C fixation remained largely unchanged. The overall changes in microbial communities and soil N pools induced by long-term elevated CO2 suggest constrained microbial N decomposition, thereby slowing the potential maximum growth rate of the microbial community.
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•Effects of 14 years of experimentally elevated CO2 on soil microbes in a semi-arid grassland were examined.•The abundance-weighted average rrn copy number of significantly changed OTUs declined by elevated CO2.•The nitrogen fixation gene nifH and the ammonium-oxidizing gene amoA significantly decreased by elevated CO2.•Elevated CO2 constrained microbial N decomposition, thereby slowing potential maximum growth rate of microbial community.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
In this century, increasing concentrations of carbon dioxide (CO2) and other greenhouse gases in the Earth's atmosphere are expected to cause warmer surface temperatures and changes in precipitation ...patterns. At the same time, reactive nitrogen is entering natural systems at unprecedented rates. These global environmental changes have consequences for the functioning of natural ecosystems, and responses of these systems may feed back to affect climate and atmospheric composition. Here, we report plant growth responses of an ecosystem exposed to factorial combinations of four expected global environmental changes. We exposed California grassland to elevated CO2, temperature, precipitation, and nitrogen deposition for five years. Root and shoot production did not respond to elevated CO2 or modest warming. Supplemental precipitation led to increases in shoot production and offsetting decreases in root production. Supplemental nitrate deposition increased total production by an average of 26%, primarily by stimulating shoot growth. Interactions among the main treatments were rare. Together, these results suggest that production in this grassland will respond minimally to changes in CO2 and winter precipitation, and to small amounts of warming. Increased nitrate deposition would have stronger effects on the grassland. Aside from this nitrate response, expectations that a changing atmosphere and climate would promote carbon storage by increasing plant growth appear unlikely to be realized in this system.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Background
Anthropogenic activities have increased the inputs of atmospheric reactive nitrogen (N) into terrestrial ecosystems, affecting soil carbon stability and microbial communities. ...Previous studies have primarily examined the effects of nitrogen deposition on microbial taxonomy, enzymatic activities, and functional processes. Here, we examined various functional traits of soil microbial communities and how these traits are interrelated in a Mediterranean-type grassland administrated with 14 years of 7 g m
−2
year
−1
of N amendment, based on estimated atmospheric N deposition in areas within California, USA, by the end of the twenty-first century.
Results
Soil microbial communities were significantly altered by N deposition. Consistent with higher aboveground plant biomass and litter, fast-growing bacteria, assessed by abundance-weighted average rRNA operon copy number, were favored in N deposited soils. The relative abundances of genes associated with labile carbon (C) degradation (e.g.,
amyA
and
cda
) were also increased. In contrast, the relative abundances of functional genes associated with the degradation of more recalcitrant C (e.g.,
mannanase
and
chitinase
) were either unchanged or decreased. Compared with the ambient control, N deposition significantly reduced network complexity, such as average degree and connectedness. The network for N deposited samples contained only genes associated with C degradation, suggesting that C degradation genes became more intensely connected under N deposition.
Conclusions
We propose a conceptual model to summarize the mechanisms of how changes in above- and belowground ecosystems by long-term N deposition collectively lead to more soil C accumulation.
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