Background In the Mediterranean climate, plants have evolved under conditions of low soil-water and nutrient availabilities and have acquired a series of adaptive traits that, in turn exert strong ...feedback on soil fertility, structure, and protection. As a result, plant-soil systems constitute complex interactive webs where these adaptive traits allow plants to maximize the use of scarce resources. Scope It is necessary to review the current bibliography to highlight the most know characteristic mechanisms underlying Mediterranean plant-soil feed-backs and identify the processes that merit further research in order to reach an understanding of the plant-soil feedbacks and its capacity to cope with future global change scenarios. In this review, we characterize the functional and structural plant-soil relationships and feedbacks in Mediterranean regions. We thereafter discuss the effects of global change drivers on these complex interactions between plants and soil. Conclusions The large plant diversity that characterizes Mediterranean ecosystems is associated to the success of coexisting species in avoiding competition for soil resources by differential exploitation in space (soil layers) and time (year and daily). Among plant and soil traits, high foliar nutrient re-translocation and large contents of recalcitrant compounds reduce nutrient cycling. Meanwhile increased allocation of resources to roots and soil enzymes help to protect against soil erosion and to improve soil fertility and capacity to retain water. The long-term evolutionary adaptation to drought of Mediterranean plants allows them to cope with moderate increases of drought without significant losses of production and survival in some species. However, other species have proved to be more sensitive decreasing their growth and increasing their mortality under moderate rising of drought. All these increases contribute to species composition shifts. Moreover, in more xeric sites, the desertification resulting from synergic interactions among some related process such as drought increases, torrential rainfall increases and human driven disturbances is an increasing concern. A research priority now is to discern the effects of long-term increases in atmospheric CO₂ concentrations, warming, and drought on soil fertility and water availability and on the structure of soil communities (e.g., shifts from bacteria to fungi) and on patching vegetation and root-water uplift (from soil to plant and from soil deep layers to soil superficial layers) roles in desertification.
This study examined the literature in ISI Web of Science to identify the effects that the main drivers of global change have on the nutrient concentrations and C:N:P stoichiometry of organisms and ...ecosystems, and examined their relationship to changes in ecosystem structure and function. We have conducted a meta-analysis by comparing C:N:P ratios of plants and soils subjected to elevated CO2 with those subjected to ambient CO2. A second meta-analysis compared the C:N:P ratios of plants and soils that received supplemental N to simulate N deposition and those that did not receive supplemental N. On average, an experimental increase in atmospheric CO2 increased the foliar C:N ratios of C3 grasses, forbs, and woody plants by 22%, but the foliar ratios of C4 grasses were unaffected. This trend may be enhanced in semi-arid areas by the increase in droughts that have been projected for the coming decades which can increase leaf C:N ratios. The available studies show an average 38% increase in foliar C:P ratios in C3 plants in response to elevated atmospheric CO2, but no significant effects were observed in C4 grasses. Furthermore, studies that examine the effects of elevated atmospheric CO2 on N:P ratio (on a mass basis) are warranted since its response remains elusive. N deposition increases the N:P ratio in the plants of terrestrial and freshwater ecosystems, and decreases plants and organic soil C:N ratio (25% on average for C3 plants), reducing soil and water N2 fixation capacity and ecosystem species diversity. In contrast, in croplands subjected to intense fertilization, mostly, animal slurries, a reduction in soil N:P ratio can occur because of the greater solubility and loss of N. In the open ocean, there are experimental observations showing an ongoing increase in P-limited areas in response to several of the factors that promote global change, including the increase in atmospheric CO2 which increases the demand for P, the warming effect that leads to an increase in water column stratification, and increases in the N:P ratio of atmospheric inputs. Depending on the type of plant and the climate where it grows, warming can increase, reduce, or have no effect on foliar C:N ratios. The results suggest that warming and drought can increase C:N and C:P ratios in warm-dry and temperate-dry terrestrial ecosystems, especially, when high temperatures and drought coincide. Advances in this topic are a challenge because changes in stoichiometric ratios can favour different types of species and change ecosystem composition and structure.
Soil phosphatase levels strongly control the biotic pathways of phosphorus (P), an essential element for life, which is often limiting in terrestrial ecosystems. We investigated the influence of ...climatic and soil traits on phosphatase activity in terrestrial systems using metadata analysis from published studies. This is the first analysis of global measurements of phosphatase in natural soils. Our results suggest that organic P (P
), rather than available P, is the most important P fraction in predicting phosphatase activity. Structural equation modeling using soil total nitrogen (TN), mean annual precipitation, mean annual temperature, thermal amplitude and total soil carbon as most available predictor variables explained up to 50% of the spatial variance in phosphatase activity. In this analysis, P
could not be tested and among the rest of available variables, TN was the most important factor explaining the observed spatial gradients in phosphatase activity. On the other hand, phosphatase activity was also found to be associated with climatic conditions and soil type across different biomes worldwide. The close association among different predictors like P
, TN and precipitation suggest that P recycling is driven by a broad scale pattern of ecosystem productivity capacity.
Longer and more severe drought periods are expected in the near future for Mediterranean ecosystems. Soil enzymes play an essential role in the nutrient mineralization and their activity is an ...exceptional sensor in predicting the capacity of nutrient supply to plants. We conducted an experiment of water availability manipulation in evergreen oak mountain stands with the aim to study the effects of enhanced drought on the activity of five soil enzymes. The drought treatment consisted of runoff exclusion by a ditch along the entire top edge of the upper part of treatment plots and partial rain exclusion by suspending PVC strips and funnels. The reduction of 10% of soil moisture produced by runoff exclusion decreased urease activity by 10–67%, protease activity by 15–66% and β-glucosidase activity by 10–80%, depending on annual period and soil depth. The reduction of 21% of soil moisture produced by runoff and rainfall exclusion together reduced urease activity by 42–60%, protease activity by 35–45%, β-glucosidase activity by 35–83% and acid phosphatase activity by 31–40%. No significant effects were observed on alkaline phosphatase activity. The activities of the enzymes involved in the nitrogen cycle, protease and urease, were the most affected by drought. In all cases, the activities of these enzymes strongly decreased with soil depth and they were greater in spring than in autumn. These results show the link between drought and a slower nutrient turn-over, which decreases the nutrient supply to plants.
We studied the impacts of anthropogenic changes in land use on the stoichiometric imbalance of soil carbon (C), nitrogen (N), phosphorus (P) and potassium (K) in Phragmites australis wetlands in the ...Minjiang River estuary. We compared five areas with different land uses: P. australis wetland (control), grassland, a mudskipper breeding flat, pond aquaculture and rice cropland. Human activity has affected the elemental and stoichiometric compositions of soils through changes in land use. In general, soil C and N concentrations were lower and total soil K concentrations were higher at the sites under human land uses relative to the control site, and total soil P concentrations were generally not significantly different. The close relationship between total soil C and N concentrations in all cases, including fertilization with N, suggested that N was the most limiting nutrient in these wetlands. Lower soil N concentrations and similar soil P concentrations and higher soil K concentrations under human land-use activities suggest that human activity has increased the role of N limitation in these wetlands. Only grassland use increases soil N contents (only in the 0–10cm of soil). Despite N fertilization, lower soil N concentrations were also observed in the rice cropland, indicating the difficulty of avoiding N limitation in these wetlands. The observed lower soil N:P ratio, together with higher soil P and K availabilities in rice croplands, is consistent with the tendency of human activity to change the competitive relationships of plants, in this case favoring species adapted to high rates of growth (low N:P ratio) and/or favoring plants with high demands for P and K. Both, soil C storage and respiration were higher in grasslands, likely due to the introduction of grasses, which led to a high density of plants, increased grazing activity and soil compaction. Soil C storage and respiration were lower under human land uses, except in the rice cropland, with respect to natural wetland. Using overall data, soil C storage and respiration were correlated, indicating that soil respiration was correlated with plant productivity. In this wetland area the impacts of different human land-uses on soil stoichiometry and C-cycle can be very different depending on the activity. Further regeneration of natural communities can be determined by the previous type of land-use.
•Human activity increases total soil K and available-P concentrations.•Despite low soil N:P ratio, human activities decrease even more the soil N:P ratios.•These decreases of soil N:P are linked with the presence of high growth rate species.•Human activities decrease soil C and N concentrations affecting C storing capacity.•These soil stoichiometry changes can influence the regeneration of natural vegetation.
In a Mediterranean shrubland, we investigated the effects of the projected warming and drought on soil urease, protease and β-glucosidase activities and the relation of the possible changes in the ...activities of these enzymes with the observed changes in soil moisture, soil pH and in C and N stocks in soils, leaves and leaf litter during 1 year (April 2004–May 2005). This investigation was conducted in a long-term experiment of warming and drought manipulation that began in 1999 and is lasting until now. Warming increased soil urease activity by 10% in the study period, mainly by increasing soil urease activity 30% in winter and 10% in spring, and increased β-glucosidase activity 38% in spring. Soil urease and β-glucosidase activities were positively correlated with soil temperatures in winter and negatively in summer. Warming increased soil enzyme activities in winter when soil moisture was highest and in spring coinciding with the greatest biological activity. Warming decreased NH
4
+ soil concentration in the spring of 2004 (by 30%) and 2005 (by 72%), in consonance with the increase in N uptake by plants. Warming decreased N concentration in
Globularia alypum leaf litter, increasing C/N leaf ratio by 30% showing an increase in N mobilization and contributing to a greater total N accumulation in plants. However, the greater NO
3
− availability in soil observed under warming, probably by an increase in nitrification, may lead to a net N loss by leaching under the torrential rainfalls typical of the Mediterranean climate regions. Drought reduced soil protease activity (9%) in the study period, mainly by decreasing it in spring by 13–21%, but did not affect N soil contents because N turn-over reduction was counterbalanced by a decrease in N leaf concentrations. Soil protease activity was positively correlated with soil water content showing a strong dependence of this enzyme on soil water content. Drought did not affect β-glucosidase activity but tended to increase C contents in soils, which together with the increase in C/N in leaves indicate a reduction of C turn-over and a trend to increase C stocks in soil at long term. The effects of warming and drought on soil enzyme activities were due to a direct effect on soil temperature and soil water content, respectively, and not to changes on soil organic matter quantity and nutritional quality.
► We analyzed leaf concentrations and stoichiometry in 3587 forest plots of Catalonia. ► Nutrient leaf C:nutrients ratios decreased with increasing MAPs and decreasing MATs. ► Water availability, ...which drives productivity, was related with low leaf N:P ratio. ► Forest type explained part of element concentrations and stoichiometry variability. ► Leaf element concentrations and stoichiometry are also related to human driven pollution.
Although some studies have observed significant correlations between latitude and climate gradients and tree leaf nutrient concentration and stoichiometry, others have not. This study examined the nutrient concentrations of tree leaves in 3530 plots of the Catalonian Forest Inventory. Catalonia is a Mediterranean region located in NE Iberian Peninsula. It has a long land-use history and includes the large industrial-urban area of Barcelona but still contains a large forest area (42%). In the forests of Catalonia, leaf nutrient concentration increased and leaf C:nutrient ratios decreased from south to north, which paralleled the increase in MAP (mean annual precipitation) and the decrease in MAT (mean annual temperature), which was expected in a Mediterranean climate where the availability of water is the most limiting factor for plant nutrient uptake. In addition, the availability of water, which influences productivity, was associated with low leaf N:P content ratios, which is consistent with the Growth Rate Hypothesis. At a regional scale, the results support the Soil-Age Hypothesis because the youngest soils in the Pyrenees had the lowest leaf N:P ratios. Furthermore, the type of forest (evergreen, deciduous, or coniferous) explained some of the variation in leaf nutrient concentrations and stoichiometry. Nutrient concentrations were highest in deciduous trees and lowest in coniferous trees. Leaf nutrient concentrations and stoichiometry were mainly correlated with climate, but other factors such as the chemical properties of soil and rock, phylogenetics, and different ecological histories and anthropogenic factors such as pollution, had an effect.
We investigated the effect of soil microclimate on the structure and functioning of soil microbial communities in a Mediterranean Holm-oak forest subjected to 10 years of partial rain exclusion ...manipulations, simulating average drought conditions expected in Mediterranean areas for the following decades. We applied a high throughput DNA pyrosequencing technique coupled to parallel measurements of microbial respiration (RH) and temperature sensitivity of microbial respiration (Q10). Some consistent changes in the structure of bacterial communities suggest a slow process of community shifts parallel to the trend towards oligotrophy in response to long-term droughts. However, the structure of bacterial communities was mainly determined by short-term environmental fluctuations associated with sampling date (winter, spring and summer) rather than long-term (10 years) shifts in baseline precipitation. Moreover, long-term drought did not exert any chronic effect on the functioning of soil microbial communities (RH and Q10), emphasizing the functional stability of these communities to this long-term but mild shifts in water availability. We hypothesize that the particular conditions of the Mediterranean climate with strong seasonal shifts in both temperature and soil water availability but also characterized by very extreme environmental conditions during summer, was acting as a strong force in community assembling, selecting phenotypes adapted to the semiarid conditions characterizing Mediterranean ecosystems. Relations of climate with the phylogenetic structure and overall diversity of the communities as well as the distribution of the individual responses of different lineages (genera) to climate confirmed our hypotheses, evidencing communities dominated by thermotolerant and drought-tolerant phenotypes.
•Seasonality was the stronger force driving microbial community assembling, diversity and functioning.•Changes in bacterial community composition associated with long-term drought-induced oligotrophy.•Bacterial communities show strong functional stability to long-term drought.•Mediterranean bacterial communities acclimated to severe droughts.
Aim: Mediterranean terrestrial ecosystems serve as reference laboratories for the investigation of global change because of their transitional climate, the high spatiotemporal variability of their ...environmental conditions, a rich and unique biodiversity and a wide range of socio-economic conditions. As scientific development and environmental pressures increase, it is increasingly necessary to evaluate recent progress and to challenge research priorities in the face of global change. Location: Mediterranean terrestrial ecosystems. Methods: This article revisits the research priorities proposed in a 1998 assessment. Results: A new set of research priorities is proposed: (1) to establish the role of the landscape mosaic on fire-spread; (2) to further research the combined effect of different drivers on pest expansion; (3) to address the interaction between drivers of global change and recent forest management practices; (4) to obtain more realistic information on the impacts of global change and ecosystem services; (5) to assess forest mortality events associated with climatic extremes; (6) to focus global change research on identifying and managing vulnerable areas; (7) to use the functional traits concept to study resilience after disturbance; (8) to study the relationship between genotypic and phenotypic diversity as a source of forest resilience; (9) to understand the balance between storage and water resources; (10) to analyse the interplay between landscape-scale processes and biodiversity conservation; (11) to refine models by including interactions between drivers and socio-economic contexts; (12) to understand forest-atmosphere feedbacks; (13) to represent key mechanisms linking plant hydraulics with landscape hydrology. Main conclusions: (1) The interactive nature of different global change drivers remains poorly understood. (2) There is a critical need for the rapid development of regional-and global-scale models that are more tightly connected with largescale experiments, data networks and management practice. (3) More attention should be directed to drought-related forest decline and the current relevance of historical land use.
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