Nitrogen fixation

About: Nitrogen fixation is a research topic. Over the lifetime, 7940 publications have been published within this topic receiving 232921 citations. The topic is also known as: GO:0009399.

Light regulates tropical symbiotic nitrogen fixation more strongly than soil nitrogen

Abstract: Nitrogen limits primary production in almost every biome on Earth1,2. Symbiotic nitrogen fixation, conducted by certain angiosperms and their endosymbiotic bacteria, is the largest potential natural source of new nitrogen into the biosphere3, influencing global primary production, carbon sequestration and element cycling. Because symbiotic nitrogen fixation represents an alternative to soil nitrogen uptake, much of the work on symbiotic nitrogen fixation regulation has focused on soil nitrogen availability4-8. However, because symbiotic nitrogen fixation is an energetically expensive process9, light availability to the plant may also regulate symbiotic nitrogen fixation rates10,11. Despite the importance of symbiotic nitrogen fixation to biosphere functioning, the environmental factors that most strongly regulate this process remain unresolved. Here we show that light regulates symbiotic nitrogen fixation more strongly than does soil nitrogen and that light mediates the response of symbiotic nitrogen fixation to soil nitrogen availability. In a shadehouse experiment, low light levels (comparable with forest understories) completely shut down symbiotic nitrogen fixation, whereas soil nitrogen levels that far exceeded plant demand did not fully downregulate symbiotic nitrogen fixation at high light. For in situ forest seedlings, light was a notable predictor of symbiotic nitrogen fixation activity, but soil-extractable nitrogen was not. Light as a primary regulator of symbiotic nitrogen fixation is a departure from decades of focus on soil nitrogen availability. This shift in our understanding of symbiotic nitrogen fixation regulation can resolve a long-standing biogeochemical paradox12, and it will improve our ability to predict how symbiotic nitrogen fixation will fuel the global forest carbon sink and respond to human alteration of the global nitrogen cycle.

Arbuscular mycorrhizal fungi colonize nonfixing root nodules of several legume species.

Abstract: Many legumes form tripartite symbiotic associations with rhizobia and arbuscular mycorrhizal fungi (AMF). Rhizobia are located in root nodules and provide the plant with fixed atmospheric nitrogen, while AMF colonize plant roots and deliver several essential nutrients to the plant. Recent studies showed that AMF are also associated with root nodules. This might point to interactions between AMF and rhizobia inside root nodules. Here, we test whether AMF colonize root nodules in various plant-AMF combinations. We also test whether nodules that are colonized by AMF fix nitrogen. Using microscopy, we observed that AMF colonized the root nodules of three different legume species. The AMF colonization of the nodules ranged from 5% to 74% and depended on plant species, AMF identity and nutrient availability. However, AMF-colonized nodules were not active, that is, they did not fix nitrogen. The results suggest that AMF colonize old senescent nodules after nitrogen fixation has stopped, although it is also possible that AMF colonization of nodules inhibits nitrogen fixation.

Nitrogen Fixation and the Nitrogen Balance in Peatland Ecosystems

Abstract: A survey of heterotrophic nitrogen fixation was carried out using the acetylene reduction assay on peat from mires in Canada, England, Scotland, Ireland, Norway, Germany, Italy, and Malaysia. A detailed study of the relationship between nitrogen fixation and peatland ecology was carried out on the German sites. Nitrogenase activity was detected in all 16 fen sites examined, with the highest rates of activity being >100 nmol C2H4°mL peat—1°d—1 . Lower rates were found in both extreme rich fen and poor fen sites, with the lower rates found in bogs. A multiple regression analysis indicated pH and K to be positively correlated with nitrogenase activity whilst the correlation with Ca was negative. These results suggest that the systems as a whole may be in balance with regard to nitrogen. The rate of nitrogenase in peat from lower latitudes is greater than in peat from higher latitudes when temperatures are allowed for. The concentration of soluble nitrogen was higher, and the rate of nitrogenase activity lowe...

Characterization of diazotrophic bacteria associated with maize: effect of plant genotype, ontogeny and nitrogen-supply

Abstract: The population size of diazotrophic bacteria naturally associated with the maize rhizosphere can be affected by biotic and environmental factors. In this work we have evaluated the effect of two genotypes of maize, with and without nitrogen fertilization, on the population dynamics and distribution of diazotrophic bacteria associated with maize plants over different plant ontogenic stages. The study was carried out in a field experiment with and without nitrogen fertilization, using two maize cultivars (Santa Helena 8447 and Santa Rosa 3063) previously selected from 32 maize cultivars for the lowest and highest response to nitrogen fertilization, respectively. Microbiological and molecular approaches were used to characterize the diazotrophic bacterial population structure. Bacterial population was assessed by the most probable number using semi-solid N-free media, and by DNA isolation from soil and plant tissue followed by amplification of nifH gene fragments using nested PCR, and by RFLP analysis using the restriction endonucleases TaqI and HaeIII. The dynamics of the diazotrophic bacterial population were affected by the ontogenic stage of the maize plants, but not by the cultivar type. Roots were the preferred site of colonization, independent of cultivar type and growth stage. During the first stage of maize growth, addition of nitrogen fertilizer negatively affected the diazotrophic bacterial population.

Below-ground nitrogen cycling in relation to net canopy production in mangrove forests of southern Thailand

Abstract: Rates of accumulation, transformation and availability of sediment nitrogen in four mangrove forests of different age and type in southern Thailand were examined in relation to forest net canopy production. Net ammonification (range: 0.3–2.3 mmol N m–2 day–1), nitrification (range: 0–0.7 mmol N m–2 day–1) and nitrogen fixation (range: 0–0.6 mmol N m–2 day–1) in surface sediments equated to <10% of canopy nitrogen demand (range: 7.5–32 mmol N m–2 day–1). By mass balance, we estimated that most of the nitrogen required for tree growth must be derived from root-associated nitrogen fixation and/or mineralisation processes occurring possibly to the maximum depth of live root penetration (75–100 cm). Denitrification, nitrification, rainfall and tidal exchange were comparatively small components of sediment nitrogen flow. Denitrification (range: 0–3.8 mmol N m–2 day–1) removed 3–6% of total nitrogen input at three Rhizophora forests, but removed 23% of total nitrogen input in a high-intertidal Ceriops forest. Nitrogen burial ranged from 4% to 12% of total nitrogen input, with the greatest burial rates in two forests receiving the least tidal inundation. Inputs of nitrogen to the forests were rapid (range: 11–37 mmol N m–2 day–1), likely originating from upstream sources such as agricultural and industrial lands, sewage and shrimp ponds. Our results indicate that ~70% to 90% of the nitrogen supplied to the forest floor is shunted via the ammonium pool to trees to sustain the rapid rates of net canopy production measured in these forests. Differences in plant–sediment nitrogen relations between the forests appeared to be a function of the interaction between intertidal position and stand age.