Manuel Delgado-Baquerizo

Plant species richness and ecosystem multifunctionality in global drylands

Global Ecosystem Analysis The relationship between species richness and the functional properties of their ecosystems has often been studied at small scales in experimental plots. Maestre et al. (p. 214; see the Perspective by Midgley) performed field measurements at 224 dryland sites from six continents and assessed 14 ecosystem functions related to carbon, nitrogen, and phosphorus cycling. Positive relationships were observed between perennial plant species richness and ecosystem functionality. The relative importance of biodiversity was found to be as large as, or larger than, many key abiotic variables. Thus, preservation of plant biodiversity is important to buffer negative effects of climate change and desertification in drylands, which collectively cover 41% of Earths land surface and support over 38% of the human population. Plant species richness is positively related to ecosystem multifunctionality in drylands at a global scale. Experiments suggest that biodiversity enhances the ability of ecosystems to maintain multiple functions, such as carbon storage, productivity, and the buildup of nutrient pools (multifunctionality). However, the relationship between biodiversity and multifunctionality has never been assessed globally in natural ecosystems. We report here on a global empirical study relating plant species richness and abiotic factors to multifunctionality in drylands, which collectively cover 41% of Earth’s land surface and support over 38% of the human population. Multifunctionality was positively and significantly related to species richness. The best-fitting models accounted for over 55% of the variation in multifunctionality and always included species richness as a predictor variable. Our results suggest that the preservation of plant biodiversity is crucial to buffer negative effects of climate change and desertification in drylands.

Changes in biocrust cover drive carbon cycle responses to climate change in drylands.

Dryland ecosystems account for ca. 27% of global soil organic carbon (C) reserves, yet it is largely unknown how climate change will impact C cycling and storage in these areas. In drylands, soil C concentrates at the surface, making it particularly sensitive to the activity of organisms inhabiting the soil uppermost levels, such as communities dominated by lichens, mosses, bacteria and fungi (biocrusts). We conducted a full factorial warming and rainfall exclusion experiment at two semiarid sites in Spain to show how an average increase of air temperature of 2-3 °C promoted a drastic reduction in biocrust cover (ca. 44% in 4 years). Warming significantly increased soil CO2 efflux, and reduced soil net CO2 uptake, in biocrust-dominated microsites. Losses of biocrust cover with warming through time were paralleled by increases in recalcitrant C sources, such as aromatic compounds, and in the abundance of fungi relative to bacteria. The dramatic reduction in biocrust cover with warming will lessen the capacity of drylands to sequester atmospheric CO2 . This decrease may act synergistically with other warming-induced effects, such as the increase in soil CO2 efflux and the changes in microbial communities to alter C cycling in drylands, and to reduce soil C stocks in the mid to long term.

Biological soil crusts modulate nitrogen availability in semi-arid ecosystems: insights from a Mediterranean grassland

Biological soil crusts (BSCs) greatly influence the N cycle of semi-arid ecosystems, as some organisms forming them are able to fix atmospheric N. However, BSCs are not always taken into account when studying biotic controls on N cycling and transformations. Our main objective was to understand how BSCs modulate the availability of N in a semi-arid Mediterranean ecosystem dominated by the tussock grass Stipa tenacissima. We selected the six most frequent soil cover types in the study area: S. tenacissima tussocks (ST), Retama sphaerocarpa shrubs (RS), and open areas with very low (BS), low (LC) medium (MC) and high (HC) cover of well developed and lichen-dominated BSCs. The temporal dynamics of available N dynamics followed changes in soil moisture. Available NH4+-N did not differ between microsites, while available NO3--N was substantially higher in the RS than in any other microsite. No significant differences in the amount of available NO3--N were found between ST and BS microsites, but these microsites had more NO3--N than those dominated by BSCs (LC, MC and HC). Our results suggest that BSCs may be inhibiting nitrification, and highlight the importance of this biotic community as a modulator of the availability of N in semi-arid ecosystems.

Side‐effects of plant domestication: ecosystem impacts of changes in litter quality

Domestication took plants from natural environments to agro-ecosystems, where resources are generally plentiful and plant life is better buffered against environmental risks such as drought or pathogens. We hypothesized that predictions derived from the comparison of low vs high resource ecosystems (faster-growing plants promoting faster nutrient cycling in the latter) extrapolate to the process of domestication. We conducted the first comprehensive assessment of the consequences of domestication on litter quality and key biogeochemical processes by comparing 24 domesticated crops against their closest wild ancestors. Twelve litter chemistry traits, litter decomposability and indicators of soil carbon (C) and nitrogen (N) cycling were assessed in each domesticated vs wild ancestor pair. These assessments were done in microbial-poor and microbial-rich soils to exemplify intensively and extensively managed agricultural soils, respectively. Plant domestication has increased litter quality, encouraging litter decomposability (36% and 44% increase in the microbial-rich and microbial-poor soils, respectively), higher soil NO3 - availability and lower soil C : N ratios. These effects held true for the majority of the crops surveyed and for soils with different microbial communities. Our results support ecological theory predictions derived from the comparison of low- and high-resource ecosystems, suggesting a parallelism between ecosystem-level impacts of natural and artificial selection.

Biological soil crusts increase the resistance of soil nitrogen dynamics to changes in temperatures in a semi-arid ecosystem

AimsBiological soil crusts (BSCs), composed of mosses, lichens, liverworts and cyanobacteria, are a key component of arid and semi-arid ecosystems worldwide, and play key roles modulating several aspects of the nitrogen (N) cycle, such as N fixation and mineralization. While the performance of its constituent organisms largely depends on moisture and rainfall conditions, the influence of these environmental factors on N transformations under BSC soils has not been evaluated before.MethodsThe study was done using soils collected from areas devoid of vascular plants with and without lichen-dominated BSCs from a semi-arid Stipa tenacissima grassland. Soil samples were incubated under different temperature (T) and soil water content (SWC) conditions, and changes in microbial biomass-N, dissolved organic nitrogen (DON), amino acids, ammonium, nitrate and both inorganic N were monitored. To evaluate how BSCs modulate the resistance of the soil to changes in T and SWC, we estimated the Orwin and Wardle Resistance index.ResultsThe different variables studied were more affected by changes in T than by variations in SWC at both BSC-dominated and bare ground soils. However, under BSCs, a change in the dominance of N processes from a net nitrification to a net ammonification was observed at the highest SWC, regardless of T.ConclusionsOur results suggest that the N cycle is more resistant to changes in T in BSC-dominated than in bare ground areas. They also indicate that BSCs could play a key role in minimizing the likely impacts of climate change on the dynamics of N in semi-arid environments, given the prevalence and cover of these organisms worldwide.

Biocrusts control the nitrogen dynamics and microbial functional diversity of semi-arid soils in response to nutrient additions

AimsHuman activities are causing imbalances in the nutrient cycles in natural ecosystems. However, we have limited knowledge of how these changes will affect the soil microbial functional diversity and the nitrogen (N) cycle in drylands, the biggest biome on Earth. Communities dominated by lichens, mosses and cyanobacteria (biocrusts) influence multiple processes from the N cycle such as N fixation and mineralization rates. We evaluated how biocrusts modulate the effects of different N, carbon (C) and phosphorus (P) additions on theN availability, the dominance of different available N forms and the microbial functional diversity in dryland soils.MethodsSoil samples from bare ground (BG) and biocrust-dominated areas were gathered from the center of Spain and incubated during seven or 21xa0days under different combinations of N, C and P additions (N, C, P, Nu2009+u2009C, Nu2009+u2009P, Pu2009+u2009C, and Cu2009+u2009Nu2009+u2009P).ResultsThe relative dominance of dissolved organic N (DON) and the microbial functional diversity were higher in biocrust than in BG microsites when C or P were added. Changes in the C to N ratio, more than N availability, seem to modulate N transformation processes in the soils studied. In general, biocrusts increased the resilience to N impacts (N, Cu2009+u2009N, Nu2009+u2009P, Cu2009+u2009Nu2009+u2009P) of the total available N, ammonium, nitrate and DON when C was present.ConclusionsOur results suggest that biocrusts may buffer the effects of changes in nutrient ratios on microbial functional diversity and DON dominance in dryland soils. Thus, these organisms may have an important role in increasing the resilience of the N cycle to imbalances in C, N and P derived from human activities.

Aridity Modulates N Availability in Arid and Semiarid Mediterranean Grasslands

While much is known about the factors that control each component of the terrestrial nitrogen (N) cycle, it is less clear how these factors affect total N availability, the sum of organic and inorganic forms potentially available to microorganisms and plants. This is particularly true for N-poor ecosystems such as drylands, which are highly sensitive to climate change and desertification processes that can lead to the loss of soil nutrients such as N. We evaluated how different climatic, abiotic, plant and nutrient related factors correlate with N availability in semiarid Stipa tenacissima grasslands along a broad aridity gradient from Spain to Tunisia. Aridity had the strongest relationship with N availability, suggesting the importance of abiotic controls on the N cycle in drylands. Aridity appeared to modulate the effects of pH, plant cover and organic C (OC) on N availability. Our results suggest that N transformation rates, which are largely driven by variations in soil moisture, are not the direct drivers of N availability in the studied grasslands. Rather, the strong relationship between aridity and N availability could be driven by indirect effects that operate over long time scales (decades to millennia), including both biotic (e.g. plant cover) and abiotic (e.g. soil OC and pH). If these factors are in fact more important than short-term effects of precipitation on N transformation rates, then we might expect to observe a lagged decrease in N availability in response to increasing aridity. Nevertheless, our results suggest that the increase in aridity predicted with ongoing climate change will reduce N availability in the Mediterranean basin, impacting plant nutrient uptake and net primary production in semiarid grasslands throughout this region.

Vascular plants mediate the effects of aridity and soil properties on ammonia-oxidizing bacteria and archaea.

An integrated perspective of the most important factors driving the abundance of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in natural ecosystems is lacking, especially in drylands. We evaluated how different climatic, abiotic, and nutrient-related factors determine AOA and AOB abundance in bare and vegetated microsites from grasslands throughout the Mediterranean Basin. We found a strong negative relationship between the abundance of AOA genes and soil fertility (availability of C, N, and P). Aridity and other abiotic factors (pH, sand content, and electrical conductivity) were more important than soil fertility in modulating the AOA/AOB ratio. AOB were more abundant under vegetated microsites, while AOA, highly resistant to stressful conditions, were more abundant in bare ground areas. These results suggest that AOA may carry out nitrification in less fertile microsites, while AOB predominate under more fertile conditions. Our results indicate that the influence of aridity and pH on the relative dominance of AOA and AOB genes is ultimately determined by local-scale environmental changes promoted by perennial vegetation. Thus, in spatially heterogeneous ecosystems such as drylands, there is a mutual exclusion and niche division between these microorganisms, suggesting that they may be functionally complementary.

On the Importance of Shrub Encroachment by Sprouters, Climate, Species Richness and Anthropic Factors for Ecosystem Multifunctionality in Semi-arid Mediterranean Ecosystems

One of the most important changes taking place in drylands worldwide is the increase of the cover and dominance of shrubs in areas formerly devoid of them (shrub encroachment). A large body of research has evaluated the causes and consequences of shrub encroachment for both ecosystem structure and functioning. However, there are virtually no studies evaluating how shrub encroachment affects the ability of ecosystems to maintain multiple functions and services simultaneously (multifunctionality). We aimed to do so by gathering data from ten ecosystem functions linked to the maintenance of primary production and nutrient cycling and storage (organic C, activity of β-glucosidase, pentoses, hexoses, total N, total available N, amino acids, proteins, available inorganic P, and phosphatase activity), and summarizing them in a multifunctionality index (M). We assessed how climate, species richness, anthropic factors (distance to the nearest town, sandy and asphalted road, and human population in the nearest town at several historical periods) and encroachment by sprouting shrubs impacted both the functions in isolation and M along a regional (ca. 350xa0km) gradient in Mediterranean grasslands and shrublands dominated by a non-sprouting shrub. Values of M were higher in those grasslands and shrublands containing sprouting shrubs (43 and 62%, respectively). A similar response was found when analyzing the different functions in isolation, as encroachment by sprouting shrubs increased functions by 2–80% compared to unencroached areas. Encroachment was the main driver of changes in M along the regional gradient evaluated, followed by anthropic factors and species richness. Climate had little effects on M in comparison to the other factors studied. Similar responses were observed when evaluating the functions in isolation. Overall, our results showed that M was higher at sites with higher sprouting shrub cover, longer distance to roads and higher perennial plant species richness. Our study is the first documenting that ecosystem multifunctionality in shrublands is enhanced by encroaching shrubs differing in size and leaf attributes. Our findings reinforce the idea that encroachment effects on ecosystem functioning cannot be generalized, and that are largely dependent on the traits of the encroaching shrub relative to those of the species being replaced.

Rhizosphere-driven increase in nitrogen and phosphorus availability under elevated atmospheric CO2 in a mature Eucalyptus woodland

Background and aimsRhizosphere processes are integral to carbon sequestration by terrestrial ecosystems in response to rising concentrations of atmospheric CO2. Yet, the nature and magnitude of rhizosphere responses to elevated CO2, particularly in nutrient and water-limited forest ecosystems, remain poorly understood.MethodsWe investigated rhizosphere responses (enzyme activities and nutrient availability) to atmospheric CO2 enrichment (ambient +150xa0μmol CO2 mol−1) in a phosphorus-limited mature eucalypt woodland in south-eastern Australia (the EucFACE experiment).ResultsFollowing 17xa0months of treatment, the activity of rhizosphere soil exoenzymes related to starch and cellulose degradation decreased between 0 and 10xa0cm and increased from 10 to 30xa0cm depth under elevated CO2. This response was concurrent with increases in nitrogen and phosphorus availability and smaller C:P nutrient ratios in rhizosphere soil under elevated CO2.ConclusionsThis nutrient-poor eucalypt woodland exhibited rhizosphere responses to atmospheric CO2 enrichment that increased nutrient availability in rhizosphere soil and suggest accelerated rates of soil organic matter decomposition, both of which may, in turn, promote plant growth under elevated CO2 concentrations.