Effects of long-term nighttime warming on extractable soil element composition in a Mediterranean shrubland.

Understanding the soil biogeochemical responses to increasing global warming in the near future is essential for improving our capacity to mitigate the impacts of climate change on highly vulnerable Mediterranean ecosystems. Previous studies have primarily focused on the effects of warming on various biogeochemical processes. However, there is limited knowledge about how the changes in water availability associated to high temperatures can alter the bioavailability and dynamics of soil elements, thereby impacting ecosystem productivity, species composition, and pollution through soil biogeochemical and hydrological processes. In this study, we investigated the effects of long-term nighttime warming on the extractable concentrations of organic carbon (EOC), total nitrogen (ETN), total phosphorus (ETP), and 17 mineral elements (arsenic (As), calcium (Ca), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), potassium (K), magnesium (Mg), manganese (Mn), molybdenum (Mo), nickel (Ni), lead (Pb), sulfur (S), strontium (Sr), vanadium (V), and zinc (Zn)) through environmental experiments in a semi-arid Mediterranean shrubland. We explored the potential biotic and abiotic mechanisms underlying the seasonal and long-term changes in extractable-mobilizable elemental composition and concentrations. Our findings revealed that prolonged warming led to higher mean annual soil temperature (with an average increase of 0.67 °C from 1999 to 2014), accumulation of soil organic matter (EOC) and extractable concentrations of soil elements (particularly increased ETP and extractable Ca, Mg, Cu, Sr, Mn, and As). These changes were attributed to uniformly higher activities of extracellular soil enzymes and/or lower plant photosynthetic and nutrient uptake capacity linked to more water deficit under warmer conditions. Seasonality unevenly altered element extractable concentrations, with soil microclimate (temperature and water content) and biological (soil microbial and plant) activity being the main drivers of this variability, thus influencing soil element composition. These results suggest significant fluctuations in the extractable concentrations of specific mineral elements in these soils, implying potential future variations in soil element composition as well as the loss of total element concentrations/contents in semi-arid Mediterranean ecosystems due to increasing warming. Therefore, these findings enhance our ability to predict ecosystem management strategies and mitigate the observed negative impacts on plant-soil systems and water quality in the context of climate change.

Effects of seasonal and decadal warming on soil enzymatic activity in a P-deficient Mediterranean shrubland.

Soil enzymes are central in the response of terrestrial ecosystems to climate change, and their study can be crucial for the models' implementation. We investigated for 1 year the effects of warming and seasonality on the potential activities of five soil extracellular enzymes and their relationships with soil moisture, phosphorus (P) concentration, and other soil parameters in a P-limited Mediterranean semiarid shrubland. The site was continuously subjected to warming since 1999, and we compared data from this study to analogous data from 2004. Warming uniformly increased all enzymes activities, but only when a sufficient amount of soil water was available. Seasonality unevenly altered enzyme activities, thus affecting enzymatic stoichiometry. P deficiency affected enzymatic stoichiometry, favoring the activities of the phosphatases. The effect of warming was stronger in 2014 than 2004, excluding the hypothesis of acclimation of rhizospheric responses to higher temperatures and suggesting that further increases in extracellular enzymatic activities are to be expected if sufficient water is available. Climatic warming will likely generally stimulate soil enzymatic activities and accelerate nutrient mineralization and similar ecological processes such as the production and degradation of biomass and changes in community composition, but which will be limited by water availability, especially in Mediterranean soils in summer. Winters in such ecosystems will benefit from a general increase in activity and production, but biological activity could even decrease in summer, potentially leading to a negative overall balance of nutrient mineralization. This study suggests that a general increase in activity due to warming could lead to faster mineralization of soil organic matter and water consumption in colder climates, until one of these factors in turn becomes limiting. Such trade-offs between water and temperature in relation with enzyme activity should be considered in biogeochemical models.

Increased aridity drives post-fire recovery of Mediterranean forests towards open shrublands.

Recent observations suggest that repeated fires could drive Mediterranean forests to shrublands, hosting flammable vegetation that regrows quickly after fire. This feedback supposedly favours shrubland persistence and may be strengthened in the future by predicted increased aridity. An assessment was made of how fires and aridity in combination modulated the dynamics of Mediterranean ecosystems and whether the feedback could be strong enough to maintain shrubland as an alternative stable state to forest. A model was developed for vegetation dynamics, including stochastic fires and different plant fire-responses. Parameters were calibrated using observational data from a period up to 100 yr ago, from 77 sites with and without fires in Southeast Spain and Southern France. The forest state was resilient to the separate impact of fires and increased aridity. However, water stress could convert forests into open shrublands by hampering post-fire recovery, with a possible tipping point at intermediate aridity. Projected increases in aridity may reduce the resilience of Mediterranean forests against fires and drive post-fire ecosystem dynamics toward open shrubland. The main effect of increased aridity is the limitation of post-fire recovery. Including plant fire-responses is thus fundamental when modelling the fate of Mediterranean-type vegetation under climate-change scenarios.

Volatiles and Tannins in Pistacia lentiscus and Their Role in Browsing Behavior of Goats (Capra hircus).

Goat herding is an important tool in the ecologically sound management of Mediterranean shrublands and woodlands, although effective levels of woody biomass removal by the goats is neither guaranteed nor easy to predict. Preliminary observations indicated that one reason for this may be poor understanding of plant-herbivore interactions that operate intraspecifically at the local spatial scale. We asked, whether goats show intraspecific preferences among neighboring plants when foraging a small local population of Pistacia lentiscus, a dominant tall shrub. First, we characterized and quantified the profile of stored and emitted volatile organic compounds (VOCs) and the PEG-binding capacity of tannins (a proxy for protein binding capacity) in the foliage of P. lentiscus shrubs, sampled within an area of 0.9 ha. We then tested goat preference between pairs of these shrubs that differed in chemical composition. Almost all sampled P. lentiscus shrubs were allocated to one of two distinct VOC chemotypes: one dominated by germacrene D and limonene (designated chemotype L) and the other by germacrene D and α-pinene (chemotype P). In contrast, continuous moderate variability was found in the binding capacity of tannins in the foliage. Goats showed preference for shrubs of chemotype L over those of chemotype P, and their preference was negatively correlated with the binding capacity of tannins. Possible influences of VOCs on goat preference that may explain the observed patterns are discussed in the light of possible context-dependent interpretation of plant VOC signals by large mammalian herbivores.

Estimating belowground plant abundance with DNA metabarcoding.

Most work on plant community ecology has been performed above ground, neglecting the processes that occur in the soil. DNA metabarcoding, in which multiple species are computationally identified in bulk samples, can help to overcome the logistical limitations involved in sampling plant communities belowground. However, a major limitation of this methodology is the quantification of species' abundances based on the percentage of sequences assigned to each taxon. Using root tissues of five dominant species in a semi-arid Mediterranean shrubland (Bupleurum fruticescens, Helianthemum cinereum, Linum suffruticosum, Stipa pennata and Thymus vulgaris), we built pairwise mixtures of relative abundance (20%, 50% and 80% biomass), and implemented two methods (linear model fits and correction indices) to improve estimates of root biomass. We validated both methods with multispecies mixtures that simulate field-collected samples. For all species, we found a positive and highly significant relationship between the percentage of sequences and biomass in the mixtures (R2  = .44-.66), but the equations for each species (slope and intercept) differed among them, and two species were consistently over- and under-estimated. The correction indices greatly improved the estimates of biomass percentage for all five species in the multispecies mixtures, and reduced the overall error from 17% to 6%. Our results show that, through the use of post-sequencing quantification methods on mock communities, DNA metabarcoding can be effectively used to determine not only species' presence but also their relative abundance in field samples of root mixtures. Importantly, knowledge of these aspects will allow us to study key, yet poorly understood, belowground processes.

Postfire nitrogen balance of Mediterranean shrublands: Direct combustion losses versus gaseous and leaching losses from the postfire soil mineral nitrogen flush.

Fire is a major factor controlling global carbon (C) and nitrogen (N) cycling. While direct C and N losses caused by combustion have been comparably well established, important knowledge gaps remain on postfire N losses. Here, we quantified both direct C and N combustion losses as well as postfire gaseous losses (N2 O, NO and N2 ) and N leaching after a high-intensity experimental fire in an old shrubland in central Spain. Combustion losses of C and N were 9.4 Mg C/ha and 129 kg N/ha, respectively, representing 66% and 58% of initial aboveground vegetation and litter stocks. Moreover, fire strongly increased soil mineral N concentrations by several magnitudes to a maximum of 44 kg N/ha 2 months after the fire, with N largely originating from dead soil microbes. Postfire soil emissions increased from 5.4 to 10.1 kg N ha-1  year-1 for N2 , from 1.1 to 1.9 kg N ha-1  year-1 for NO and from 0.05 to 0.2 kg N ha-1  year-1 for N2 O. Maximal leaching losses occurred 2 months after peak soil mineral N concentrations, but remained with 0.1 kg N ha-1  year-1 of minor importance for the postfire N mass balance. 15 N stable isotope labelling revealed that 33% of the mineral N produced by fire was incorporated in stable soil N pools, while the remainder was lost. Overall, our work reveals significant postfire N losses dominated by emissions of N2 that need to be considered when assessing fire effects on ecosystem N cycling and mass balance. We propose indirect N gas emissions factors for the first postfire year, equalling to 7.7% (N2 -N), 2.7% (NO-N) and 5.0% (N2 O-N) of the direct fire combustion losses of the respective N gas species.