Lilia Serrano Grijalva

The size, frequency, and timing of precipitation events are predicted to become more variable worldwide. Despite these predictions, the importance of changes in precipitation in driving multiple above- and belowground ecosystem attributes simultaneously remains largely underexplored. Here, we carried out 3 yr of rainfall manipulations at the DRI-Grass facility, located in a mesic grassland in eastern Australia. Treatments were implemented through automated water reapplication and included +50% and −50% amount, reduced frequency of events, and an extreme summer drought. We evaluated the spatiotemporal responses of multiple ecosystem attributes including microbial biomass, community composition and activity, soil nutrient content and availability, and plant nutritional status to altered rainfall regimes. We found that changing precipitation patterns resulted in multiple direct and indirect changes in microbial communities and soil and plant nutrient content. Main results included greater availability of soil macronutrients and reduced availability of micronutrients under drought, and taxon-specific changes in the composition of soil microbial communities in response to altered rainfall. Moreover, using structural equation modeling, we showed that, in summer 2015, plant macronutrient contents, a widely used ecological indicator of pasture quality, were simultaneously explained by greater soil nutrient availability and the structure of soil microbial communities, and significantly reduced by lower rainfall. Plant micronutrients were also reduced by lower rainfall and explained by changes in microbial attributes. Despite treatment effects on many of the soil, microbial, and plant variables analyzed across the 3 yr of study, many of these ecosystem attributes varied greatly across sampling events. This resulted in many significant interactions between the rainfall treatments and experimental duration, suggesting complex system-level responses to changing rainfall in our grassland, and a high natural buffering capacity of the ecosystem to varying rainfall conditions. Some interactions manifested as changes in the coefficient of variation of ecosystem attributes, particularly in response to changes in the timing of precipitation events and the extreme summer drought. Finally, we posit that a detailed understanding of plant–soil–microbial interactions, and the role of climate in modifying these linkages, will be key for adapting the sustainability of grasslands to a future that will be shaped by climate change.

Experiments employing free-air CO2 enrichment (FACE) facilities have indicated that elevated atmospheric carbon dioxide (eCO2) stimulates growth in diverse terrestrial ecosystems. Studies of the effects of eCO2 on wetland plants have indicated a similar response, but these studies were mostly performed in growth chambers. We conducted a 2-year FACE experiment [CO2 ≈ 582 µmol mol−1] in a marsh in Spain to test whether the common reed (Phragmites australis) responds to carbon enrichment, as previously reported in other macrophytes. More specifically, we tested the effect of eCO2 on P. australis growth, photosynthesis, transpiration, and biomass, its effect on modifying plant and soil ratios of carbon, nitrogen, and phosphorus, and whether the strong environmental variability of this wetland modulates these responses. Our findings show that effects of eCO2 in this wetland environment are more complex than previously believed, probably due to hydrological effects. The effects of eCO2 on reed plants were cumulative and manifested at the end of the growing season as increased 38–44% instantaneous transpiration efficiency (ratio of net photosynthesis to transpiration), which was dependent on plant age. However, this increase did not result in a significant increase in biomass, because of excessive root exudation of carbon. These observations contrast with previous observations of wetland plants to increased atmospheric CO2 in growth chambers and shed new light on the role of wetland plants as a carbon sink in the face of global climate change. The combined effects of water stress, eCO2, and soil carbon processes must be considered when assessing the function of wetlands as a carbon sink under global change

Biocrusts are key drivers of the structure and functioning of drylands and are very sensitive to disturbance, including atmospheric nitrogen (N) deposition. We studied the impacts of simulated N deposition on biocrust community composition and soil photosynthetic and photoprotective pigment content after five years of N application in a European semiarid Mediterranean shrubland. The experiment consisted in six experimental blocks with four plots, each receiving 0, 10, 20, or 50 kg NH4NO3-N ha−1 year−1 + 6–7 kg N ha−1 year−1 background. After 5 years of N application, total lichen cover decreased up to 50% compared to control conditions and these changes were only clearly evident when evaluated from a temporal perspective (i.e. as the percentage of change from the first survey in 2008 to the last survey in 2012). In contrast, moss cover did not change in response to N, suggesting that biocrust community alterations operate via species- and functional group-specific effects. Interestingly, between-year variations in biocrust cover tracked variations in autumnal precipitation, showing that these communities are more dynamic than previously thought. Biocrust species alterations in response to N were, however, often secondary when compared to the role of ecologically relevant drivers such as soil pH and shrub cover, which greatly determined the composition and inter-annual dynamics of the biocrust community. Similarly, cyanobacterial abundance and soil pigment concentration were greatly determined by biotic and abiotic interactions, soil pH for pigments, and organic matter content and shrub cover for cyanobacteria. Biocrusts, and particularly the lichen component, are highly sensitive to N deposition and their responses to pollutant N can be best understood when evaluated from a temporal and multivariate perspective, including impacts mediated by interactions with biotic and abiotic drivers.

The Free Air CO2 Enrichment (FACE) system has proved suitable for exposing plants to elevated [CO2] with minimal disturbance of their natural environment. Here we describe a FACE facility in a floodplain wetland in detail and, additionally, its performance after the first year of operation (2012). The FACE system consisted of six 3-m diameter emission rings in which Phragmitesaustralis was grown. The target [CO2] was 550 μmol mol−1 and fertilization was carried out continuously. Daily temporal [CO2] performance was adequate with 61 and 83 % of air samples at the ring’s centre having a [CO2] within 10 and 20 % of the target, respectively, with values closest to their target during summer months and daytime. Spatial [CO2] distribution showed no significant gradients across the ring. Increased wind speed improved the system’s spatial performance, as [CO2] was within ±10 % of the target in the whole ring. Across the entire fertilization season, CO2 requirements for maintaining a mean [CO2] of 582 μmol mol−1 in wetland plots averaged 17.4 kg CO2 ring−1 day−1. Our requirements (2.5 kg CO2 m−2 day−1) were very low compared to other FACE systems, demonstrating its high potential to study the effects of elevated CO2 in wetlands at low cost.

Although numerous studies have reported the negative effects of shrimp aquaculture on water quality, little is known about the ecological effects of these practices in coastal lagoons and near-shore marine habitats. The impact of shrimp-farm effluents on the food webs of an impacted subtropical coastal lagoon in the Gulf of California was evaluated through measurements of isotopic (δ13C, δ15N) signatures in sediments, plants and animals, and compared with the results of a near-pristine reference site. Degradation was manifested in a strong reduction on fish diversity at the perturbed site. δ13C signatures provided ambiguous evidence of degradation while δ15N was a better descriptor of shrimp-farm effluent impact on coastal lagoon food webs. The site receiving nutrient-rich discharges showed significant enrichment of δ15N (≈ 5‰) in sediments, macroalgae, benthic algae, filterfeeders and omnivorous feeders, resulting in qualitative differences in foodweb structure between both lagoons. The food web in the perturbed site was sustained by sediment detritus and dominated by opportunistic species. The lowest influence on δ15N signatures by aquaculture discharges recorded in the upper trophic levels could be explained by the shift in the composition of biotic communities, and associated feeding strategies. While alterations in resource availability do not affect directly food chain length, trophic linkages between food web compartments can be reduced as a result of shrimp farm impacts. Our study demonstrates that nutrient-enriched discharges from shrimp-farm aquaculture generate changes in the availability of food sources, which reduce biodiversity and alter structural and functional food web characteristics.