Can we understand and predict the regulation of biological N2 fixation in grassland ecosystems?

Abstract

We discuss results from controlled environment studies including mesocosms, grazing experiments and long term field experiments which show how biological N 2 fixation in legume based systems is tightly coupled to the N demand at scales ranging from the individual plant to the grassland ecosystem. We further test the consequences of this hypothesis of a feedback regulation of biological N 2 fixation by N demand with a mechanistic model linking plant community dynamics and ecosystem functioning. Results confirm the heuristic power of this hypothesis which accounts for a number of observations concerning changes in the relative abundance and N 2 fixation rate of legumes in managed grasslands. Then we show how nitrogen and carbon fluxes are affected by plant-plant (e.g. competition and facilitation), plant-soil and plant-herbivore interactions and by climate and management changes.

Asparagine: an amide of particular distinction in the regulation of symbiotic nitrogen fixation of legumes

“…apoplastic transport is only possible between infected cells (Abd-Alla et al, 2000). Recently, detailed investigation dealing with simulation mechanistic models strongly supports the correlation between the plant N-demand and N 2 ase activity (Soussana & Tallec, 2010). Several supporting lines of evidence for the N-demand concept were revealed which were basically established to address the question of how symbiotic N 2 fixation is regulated.…”

“…Indeed, as the carbohydrate status will determine the growth rate of the plant, and hence its N-requirements from the nodules, it seems reasonable to hypothesize that overall control of nodule activity is going to operate through the N-status of the host plant (Schulze, 2003;Fischinger et al, 2006;Sulieman & Schulze, 2010b). Now it is becoming widely accepted that N 2 fixation rate is regulated by the N-demand concept (Schubert, 2007;Soussana & Tallec, 2010). This concept is defined as the balance between the actual N-amount in plant tissues and N-requirement with the later being described as the product between optimal N-content and growth rate (Touraine, 2004).…”

“…N 2 or NO 3 , it was revealed that plant N supplementation is finely tuned to reach critical N concentrations of the shoot. The later is being defined as the minimum plant N concentration allowing maximum growth rate (Soussana & Tallec, 2010). The physiological findings suggest that (i) N 2 ase activity is more sensitive to the surrounding environment and the manipulation of the N-status in the shoot resulted in pronounced effects on the N 2 fixation rate (Schulze, 2003;Fischinger et al, 2006), (ii) combined-N application showed a negative correlation with N 2 fixation (Sulieman & Schulze, 2010a), (iii) external phloem feeding of various AAs resulted in remarkable changes in N 2 ase activity (Sulieman & Schulze, 2010b;Sulieman et al, 2010;Sulieman, 2011), (iv) application of nitrate to one side of the split root system led to a significant reduction in the N 2 fixation of the untreated and physically separated side (EA Omer & S Sulieman, unpublished data), (v) a rapid translocation of shoot applied 15 N to the nodules (Sulieman et al, 2010) with strong tendency for increasing nodule radiolabelled proportion after partial darkening of the leaves (Fischinger et al, 2006), and (vi) remarkable increment of phloem AAs when growing plants were exposed to certain diverse treatments (NH 4 NO 3 and P-deficiency) (Sulieman et al, 2010).…”

The Rengen Grassland Experiment: relationship between soil and biomass chemical properties, amount of elements applied, and their uptake

“…A negative nutrient balance is in agreement with the results of repeated soil analyses indicating a continuous decrease in plant available P concentration in these treatments (Hejcman et al 2010a). The negative values for the N balance in the A and B treatments must be taken with caution because additional N was delivered by atmospheric deposition (at least 10 kg N ha −1 annually, Solga et al 2006) and by N 2 fixation by legumes (Soussana and Tallec 2010). In the A and B treatments, the cover of legumes was more than 20% (Hejcman et al 2007b) with an estimated annual N fixation of more than 30 kg ha −1 .…”

Nitrogen-Fixing Plant-Microbe Symbioses

“…In the presence of abundant nitrogen, legumes tend to exclude nitrogen fi xing microorganisms because biological nitrogen fi xation is bio-energetically costly (Houlton et al 2008 ). In fact nitrogen fi xation depends on a feedback control mechanism between legume demand and nitrogen availability in a particular ecosystem (Soussana and Tallec 2010 ). Moreover, rhizobial infection process can be hampered by a number of factors, like the absence of appropriate rhizobia or too few rhizobia in the soil, the presence of competing rhizobia, a late response of the plant due to a lack of nutrients, or through altered soil metabolism, for example nitrate reduction in soil high in available nitrogen.…”