Eli Zaady

Decoupling of soil nutrient cycles as a function of aridity in global drylands

The biogeochemical cycles of carbon (C), nitrogen (N) and phosphorus (P) are interlinked by primary production, respiration and decomposition in terrestrial ecosystems. It has been suggested that the C, N and P cycles could become uncoupled under rapid climate change because of the different degrees of control exerted on the supply of these elements by biological and geochemical processes. Climatic controls on biogeochemical cycles are particularly relevant in arid, semi-arid and dry sub-humid ecosystems (drylands) because their biological activity is mainly driven by water availability. The increase in aridity predicted for the twenty-first century in many drylands worldwide may therefore threaten the balance between these cycles, differentially affecting the availability of essential nutrients. Here we evaluate how aridity affects the balance between C, N and P in soils collected from 224 dryland sites from all continents except Antarctica. We find a negative effect of aridity on the concentration of soil organic C and total N, but a positive effect on the concentration of inorganic P. Aridity is negatively related to plant cover, which may favour the dominance of physical processes such as rock weathering, a major source of P to ecosystems, over biological processes that provide more C and N, such as litter decomposition. Our findings suggest that any predicted increase in aridity with climate change will probably reduce the concentrations of N and C in global drylands, but increase that of P. These changes would uncouple the C, N and P cycles in drylands and could negatively affect the provision of key services provided by these ecosystems.

Infiltration, penetration resistance and microphytic crust composition in contrasted microsites within a Mediterranean semi-arid steppe

In semi-arid areas with sparse vegetation cover, runoff generated in the open areas is crucial for the maintenance of vegetated patches. Microphytic crusts play a major role in this redistribution of water, thus influencing ecosystem functioning and dynamics. We investigated the effects of alpha grass (Stipa tenacissima L.) on the composition of the microphytic crusts, surface soil compaction, and infiltration in a semi-arid steppe of SE Spain. The microphytic crust composition differed between the upslope of S. tenacissima tussocks (tussock microsites) and the inter-tussock areas with sparse vascular plant cover (open microsites), with more moss cover in the tussock microsite, and more cyanobacteria and lichens in the open microsite. The surface soil compaction was higher in the open microsite. Variables related with infiltration showed a clear microsite effect, with higher infiltration rate and less time required by first drop to percolate in the tussock microsite. Partial correlation analysis showed a significant negative relationship between the cyanobacteria cover and the infiltration rate, and both the cyanobacteria cover and the percentage of bare soil showed a significant positive relationship with the time required for first drop to percolate. Our results reinforce the idea that open microsites act as sources of water for S. tenacissima tussocks. This study helps to understand the interactions between microphytic crusts and vascular plants in semi-arid environments.

Litter as a regulator of N and C dynamics in macrophytic patches in Negev desert soils

Abstract In desert ecosystems, nutrient cycling activity is concentrated in brief periods of intense biological activity following wetting events. Release and uptake of N from litter and microbial biomass may be important regulators of N availability to plants and N loss to denitrification and NH 3 volatilization. Litter and microbial biomass dynamics may also be important to the maintenance of shrub-dominated patches of high fertility in desert ecosystems. We have measured soil C and N cycling processes (respiration, NH 4 + and NO 3 − dynamics, denitrification and microbial biomass C and N dynamics) in rewetted Negev desert soil treated with different size classes of desert plant litter in 30-day laboratory incubations. The results suggest that litter plays a strong role in conserving N following wetting events in Negev soils. Amounts of soil NH 4 + and NO 3 − and microbial biomass N were reduced in litter-amended treatments, suggesting that significant quantities of N were sequestered in litter, especially the largest size classes of litter. Denitrification was a significant sink for N, and was stimulated by the presence of litter, but was less important than immobilization of N in litter. Immobilization and release of N by litter may be especially important in the N cycle in desert ecosystems, moderating seasonal patterns of N availability and regulating patch interactions that facilitate the development of “islands of fertility” in these ecosystems.

The effect of microphytes on the spectral reflectance of vegetation in semiarid regions

The normalized difference vegetation index (NDVI), which is derived from satellite sensor images, is widely used as a measure of vegetation and ecosystem dynamics, change in land use, desertification, and climatic change processes on a regional or global scale. Surprisingly, in semiarid regions, relatively high values of NDVI were measured in landscapes where little, if any, photosynthetic activity of higher plants exists. We tested the hypothesis that the high NDVI values may be caused by the photosynthetic activity of microphytes (lower plants), consisting of mosses, lichens, algae, and cyanobacteria, which cover most of the rock and soil surfaces in semiarid regions. We found that the spectral reflectance curves of lower plants can be similar to those of the higher ones and their derived NDVI values can be as high as 0.30 units. We conclude that, in semiarid environments, the reflectance of lower plant communities may lead to misinterpretation of the vegetation dynamics and overestimation of ecosystem productivity.

Assessment of the spatial distribution of soil microbial communities in patchy arid and semi‐arid landscapes of the Negev Desert using combined PLFA and DGGE analyses

Arid and semi-arid ecosystems are often characterized by vegetation patchiness and variable availability of resources. Phospholipid fatty acid (PLFA) and 16S rRNA gene fragment analyses were used to compare the bulk soil microbial community structure at patchy arid and semi-arid landscapes. Multivariate analyses of the PLFA data and the 16S rRNA gene fragments were in agreement with each other, suggesting that the differences between bulk soil microbial communities were primarily related to shrub vs intershrub patches, irrespective of climatic or site differences. This suggests that the mere presence of a living shrub is the dominant driving factor for the differential adaptation of the microbial communities. Lipid markers suggested as indicators of Gram-positive bacteria were higher in soils under the shrub canopies, while markers suggested as indicators of cyanobacteria and anaerobic bacteria were elevated in the intershrub soils. Secondary differences between soil microbial communities were associated with intershrub characteristics and to a lesser extent with the shrub species. This study provides an insight into the multifaceted nature of the factors that shape the microbial community structure in patchy desert landscapes. It further suggests that these drivers not only act in concert but also in a way that is dependent on the aridity level.

Temporal dynamics of soil and vegetation spectral responses in a semi-arid environment

This paper discusses several difficulties encountered in detecting and monitoring temporal changes in vegetation using multispectral imagery from airborne or spaceborne sensors. These difficulties are due to (1) temporal change in the vegetation state; (2) temporal change in the soil/rock signature; and (3) difficulty in discriminating vegetation from soil or rock background. The seasonal dynamics of soil and vegetation was investigated over two years on permanent sample plots in a natural fenced-off area in the semi-arid region (200 mm annual average rainfall) of the Northern Negev, Israel. Results show that temporal analysis of natural vegetation in semi-arid regions should take into account three ground features--perennials, annuals, and biological soil crusts; all having phenological cycles with the same basic elements--oscillation from null (or low) to full photosynthetic status. However, these cycles occur in successive periods throughout the year. The phenological cycle of perennial plants is related to the adaptation of desert plants to scarcity of water. Annuals are green only for a relatively short period during the wet season and turn into dry organic matter during the summer. The microphytic communities (lower plants) of the biological soil crusts are rapidly affected by moisture and turn green immediately after the first rain, in a timescale of minutes. In arid environments, where the higher plants are sparse, this type of plant has considerable importance in the overall production of the greenness signal. However, crust-covered areas are visually similar to bare soil throughout the dry period. This paper concludes that a priori knowledge of the phenological changes in desert plants (lower and higher) is valuable in the interpretation of remote sensing data of arid environments. It is shown that rainfall amount and regime are the keys for understanding the dynamic processes of the different ground features. Through polynomial fitting, simple functions describing the annual variations in the NDVI of the different cover types have been formulated and validated; showing the feasibility and viability of modelling the processes. Although fluctuations in the rainfall regime between years poses a problem to designing a unique model, it is believed that such a problem can be overcome with long-term observations.

Size asymmetry of resource competition and the structure of plant communities

Summary nPlant communities show two general responses to gradients of soil resources: a decrease in species richness at high levels of resource availability and an associated shift in species composition from small and slow-growing species to large and fast-growing species. Models attempting to explain these responses have usually focused on a single pattern and provided contradicting predictions concerning the underlying mechanisms. nWe use an extension of Tilmans resource competition model to investigate the hypothesis that both patterns may originate from the size-asymmetric nature of light exploitation by competing plants. The only mechanism producing changes in species richness and species composition in our model is mortality due to competition. nUnder the framework of the model, asymmetric light exploitation is a necessary and sufficient condition to obtain the empirically observed responses of species richness and species composition to soil resource gradients. This theoretical result is robust to relaxing the simplifying assumptions of the model. nOur model shows that the traits enhancing competitive superiority depend on the mode of resource exploitation: under symmetric exploitation, competitive superiority is achieved by tolerance of low resource levels, while under asymmetric exploitation, it is achieved by the ability to grow fast and attain a large size. This result indicates that a long-standing debate concerning the traits that enhance competitive superiority in plant communities (the ‘Grime–Tilman debate’) can be reduced into a single parameter of our model – the degree of asymmetry in resource competition. nThe model also explains the observed shift from below-ground to above-ground competition with increasing productivity, the associated increase in the asymmetry of competitive interactions and the increasing likelihood of competitive exclusion under high levels of productivity. None of these patterns could be obtained under symmetric competition in our model. nSynthesis. The ability of the model to explain a wide range of observed patterns and the robustness of these predictions to its simplifying assumptions suggest that the size asymmetry of competition for light is a fundamental factor in determining the structure and diversity of plant communities.

Spatial and temporal diversity and abundance of ammonia oxidizers in semi-arid and arid soils: indications for a differential seasonal effect on archaeal and bacterial ammonia oxidizers

Besides water, nitrogen is the limiting factor for biomass production in arid ecosystems. Global climatic changes are exacerbating aridity levels, and the response of nitrogen-transforming microorganisms to these changes is not clear yet. Using semi-arid and arid ecosystems as surrogates for conditions of increased aridity, we investigated the activity, abundance, and diversity of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in arid and semi-arid soils. Ammonia oxidation potentials were higher during the winter in both sites than in the summer, and higher nitrate concentrations were measured in the arid soil than in the semi-arid soil. Denaturing gradient gel electrophoresis (DGGE) patterns of AOB 16S rRNA gene fragments were similar for the arid and semi-arid soils with no seasonal variations. In contrast, the DGGE patterns of the AOA amoA gene fragments differed between the sites and a soil transfer experiment suggested that these differences are possibly associated with soil type. AOB numbers were higher during the winter than in the summer, while AOA numbers were higher during the summer. The results indicate the resistance of AOB and AOA community structure to arid conditions, albeit with seasonal variations in their abundance. Together, the results suggest the resilience of nitrification activity to increased aridity level.

Release and consumption of nitrogen by snail feces in Negev Desert soils

Snail grazing and feces production have been shown to be major components of the nitrogen (N) budget of Negev Desert ecosystems. However, the movement of N from feces into soil N cycling processes has not been studied. In this study, we measured immediate N release from different types of snail feces following wetting of dry desert soils, and characterized potential net N mineralization and nitrification and soil respiration over a 12-day incubation under laboratory conditions. The dynamics of morganic N exhibited two distinct phases during the 12-day incubation: (1) immediate release of inorganic N following wetting of the soil and (2) decline of inorganic N from day 1 today 12 of the incubation. The immediate pulse of N release from this one wetting event (6–25 mg N m-2) was larger than annual atmospheric inputs of N to Negev Desert ecosystems (<2 mg N m-2); however, from 50 to 80% of the N released upon wetting was consumed by the end of the incubation. There were differences in inorganic N release and respiration from feces from different kinds of snails, and from feces from the same species of snail fed different plants. The results suggest that while snail feces contribute significant amounts of plant available N to Negev ecosystems, plants must compete with other “sinks” for this N.

Soil functions and ecosystem services in conventional, conservation, and integrated agricultural systems. A review

Soil tillage, crop residue management, nutrient management, and pest management are among the core farming practices. Each of these practices impacts a range of soil functions and ecosystem services, including water availability for crops, weed control, insect and pathogen control, soil quality and functioning, soil erosion control, soil organic carbon pool, environmental pollution control, greenhouse gas refuse, and crop yield productivity. In this study, we reviewed relevant bibliography and then developed a simple conceptual model, in which these soil functions and ecosystem services were scored and compared between conventional, conservation, and integrated agricultural systems. Using this conceptual model revealed that the overall agro-environmental score, excluding crop yield productivity, is largest for conservation systems (71.9xa0%), intermediate for integrated systems (68.8xa0%), and the smallest for conventional systems (52.1xa0%). At the same time, the crop yield productivity score is largest for integrated systems (83.3xa0%), intermediate for conventional systems (66.7xa0%), and the smallest for conservation systems (58.3xa0%). This study shows the potential of moderate-intensity and integrated farming systems in carrying on global food security while adequately sustaining environmental quality and ecosystem services.