Competition-induced downregulation of symbiotic nitrogen fixation.

Controlled experiments at the level of individual plants show that legume species use different strategies for the regulation of symbiotic dinitrogen fixation in response to nitrogen availability. These strategies were suggested to improve legume fitness in the context of the plant community, although rarely studied at this level. We evaluated how nitrogen availability and conspecific vs heterospecific interactions influenced the strategy of regulation of nitrogen fixation. We grew two species of herbaceous legumes representing two different strategies of regulation without interaction, under treatments of deficient and sufficient nitrogen availability, with conspecific or heterospecific interaction. We found that Hymenocarpus circinnatus maintained a facultative strategy of downregulating nitrogen fixation when nitrogen was available under both con- and heterospecific interactions, as was also found for this species when grown alone. Vicia palaestina also downregulated nitrogen fixation under both con- and heterospecific interactions but did not regulate fixation when grown alone. Our results showed that under nitrogen limitation, interaction with a neighboring plant reduced fitness, reflecting a competitive effect. Our findings suggest that when interacting with other plants, downregulation of nitrogen fixation is more likely, therefore reducing the energetic cost of fixation, and improving plant performance in competitive ecological communities, especially when nitrogen is available.

Comparative study of bacterial community dynamics in different soils following application of the herbicide atrazine.

Microbial communities in cultivated soils control the fate of pollutants associated with agricultural practice. The present study was designed to explore the response of bacterial communities to the application of the widely-used herbicide atrazine in three different crop fields that differ significantly in their physicochemical structure and nutritional content: the nutrient-rich (with relatively high carbon and nitrogen content) Newe Yaar (NY) and Ha-Ogen (HO) soils and the nutrient-poor, sandy Sde-Eliyahu (SE) soil. The 16 S rRNA gene amplicon sequencing revealed the nutrient poor HO soil differs in its response to atrazine in comparison to the two nutrient-rich soils both in the shortest persistence of atrazine and its effect on community structure and composition. Potential reported bacterial degraders of atrazine such as Pseudomonas, Clostridium and Bacillus were more abundant in contaminated sandy/poor soils (HO) whereas bacteria known for nitrogen cycling such as Azospirillum, Sinorhizobium, Nitrospira and Azohydromonas were significantly more abundant in the nutrient rich contaminated SE soils. No significant increase of potential indigenous degrader Arthrobacter was detected in SE and NY soils whereas a significant increase was recorded with HO soils. An overall shift in bacterial community composition following atrazine application was observed only in the nutrient poor soil. Understanding atrazine persistence and microbiome response to its application of in dependence with soil types serve the design of precision application strategies.

The nitrogen economic spectrum of legume stoichiometry and fixation strategy.

Leaf nitrogen concentration often is higher in leguminous plants, which associate with dinitrogen-fixing bacteria, compared with nonlegume plants. However, the range of nitrogen concentrations in legumes is wide, likely related to the range of nitrogen fixation strategies. We evaluated how carbon and nitrogen allocation to roots, stems and leaves is influenced by the type of strategy of nitrogen fixation regulation. We grew herbaceous annual legumes (Medicago truncatula, Hymenocarpos circinnatus and Vicia palaestina) under two nitrogen availability treatments (none/sufficient), with and without bacterial inoculation. We found facultative downregulation of the rate of nitrogen fixation when nitrogen was available in H. circinnatus, and an obligate similar fixation rate in both nitrogen treatments in M. truncatula and V. palaestina. Uninoculated plants invested more biomass in roots and contained lower nitrogen concentrations. However, nitrogen concentration in the entire plant and in the leaves was lower and more plastic in the species with a facultative fixation strategy, whereas species with an obligate fixation strategy also maintained high nitrogen concentrations. Our results suggest a suite of functional traits associated with the strategies of allocation and symbiotic nitrogen fixation. This suite of traits probably shapes successional and functional niches of different leguminous species in specious plant communities.

Symbiotic dinitrogen fixation is seasonal and strongly regulated in water-limited environments.

Plants, especially perennials, growing in drylands and seasonally dry ecosystems are uniquely adapted to dry conditions. Legume shrubs and trees, capable of symbiotic dinitrogen (N2 ) fixation, often dominate in drylands. However, the strategies that allow symbiotic fixation in these ecosystems, and their influence on the nitrogen cycle, are largely unresolved. We evaluated the climatic, biogeochemical and ontogenetic factors influencing nitrogen fixation in an abundant Mediterranean legume shrub, Calicotome villosa. We measured nodulation, fixation rate, nitrogen allocation and soil biogeochemistry in three field sites over a full year. A controlled experiment evaluated differences in plant regulation of fixation as a function of soil nutrient availability and seedling and adult developmental stages. We found a strong seasonal pattern, shifting between high fixation rates during the rainy season at flowering and seed-set times to almost none in the rainless season. Under controlled conditions, plants downregulated fixation in response to soil nitrogen availability, but this response was stronger in seedlings than in adult shrubs. Finally, we did not find elevated soil nitrogen under N2 -fixing shrubs. We conclude that seasonal nitrogen fixation, regulation of fixation, and nitrogen conservation are key adaptations influencing the dominance of dryland legumes in the community, with broader consequences on the ecosystem nitrogen cycle.

A new functional ecological model reveals the nature of early plant management in southwest Asia.

The protracted domestication model posits that wild cereals in southwest Asia were cultivated over millennia before the appearance of domesticated cereals in the archaeological record. These 'pre-domestication cultivation' activities are widely understood as entailing annual cycles of soil tillage and sowing and are expected to select for domestic traits such as non-shattering ears. However, the reconstruction of these practices is mostly based on indirect evidence and speculation, raising the question of whether pre-domestication cultivation created arable environments that would select for domestic traits. We developed a novel functional ecological model that distinguishes arable fields from wild cereal habitats in the Levant using plant functional traits related to mechanical soil disturbance. Our results show that exploitation practices at key pre-domestication cultivation sites maintained soil disturbance conditions similar to untilled wild cereal habitats. This implies that pre-domestication cultivation did not create arable environments through regular tillage but entailed low-input exploitation practices oriented on the ecological strategies of the competitive large-seeded grasses themselves.