Georg Carlsson

Nitrogen fixation in perennial forage legumes in the field

Nitrogen acquisition is one of the most important factors for plant production, and N contribution from biological N2 fixation can reduce the need for industrial N fertilizers. Perennial forages are widespread in temperate and boreal areas, where much of the agriculture is based on livestock production. Due to the symbiosis with N2-fixing rhizobia, perennial forage legumes have great potential to increase sustainability in such grassland farming systems. The present work is a summary of a large number of studies investigating N2 fixation in three perennial forage legumes primarily relating to ungrazed northern temperate/boreal areas. Reported rates of N2 fixation in above-ground plant tissues were in the range of up to 373 kg N ha−1 year−1 in red clover (Trifolium pratense L.), 545 kg N ha−1 year−1 in white clover (T. repens L.) and 350 kg N ha−1 year−1 in alfalfa (Medicago sativa L.). When grown in mixtures with grasses, these species took a large fraction of their nitrogen from N2 fixation (average around 80%), regardless of management, dry matter yield and location. There was a large variation in N2 fixation data and part of this variation was ascribed to differences in plant production between years. Studies with experiments at more than one site showed that also geographic location was an important source of variation. On the other hand, when all data were plotted against latitude, there was no simple correlation. Climatic conditions seem therefore to give as high N2 fixation per ha and year in northern areas (around 60°N) as in areas with a milder climate (around 40°N). Analyzing whole plants or just above-ground plant parts influenced the estimate of N2 fixation, and most reported values were underestimated since roots were not included. Despite large differences in environmental conditions, such as N fertilization and geographic location, N2 fixation (Nfix; kg N per ha and year) was significantly (P<0.001) correlated to legume dry matter yield (DM; kg per ha and year). Very rough, but nevertheless valuable estimations of Nfix in legume/grass mixtures (roots not considered) are given by Nfix = 0.026ċDM + 7 for T. pratense, Nfix = 0.031ċDM + 24 for T. repens, and Nfix = 0.021ċDM + 17 for M. sativa.

Discrimination against 15N in three N2-fixing Trifolium species as influenced by Rhizobium strain and plant age

Abstract When estimating N2 fixation with the 15N natural abundance method, the 15N abundance in plants grown with N2 in air as only N source (B), must be known in order to account for discrimination against 15N during N2 fixation. Trifolium hybridum L., T. pratense L. and T. repens L. were grown without N in the nutrient solution in a greenhouse. The aim was to establish B values in these species in symbioses with Scandinavian Rhizobium genotypes, and to test the hypothesis that B is related to N2 fixation efficiency and plant age. Choice of Rhizobium strain significantly influenced B in shoots of T. hybridum and T. repens, and N content in all Trifolium species. B in shoots was partly correlated to amounts of N2 fixed, while plant age only had a marginal effect. Soil suspension inoculum or single strain inocula gave very similar B in shoots of T. pratense.

Does nitrogen transfer between plants confound 15N-based quantifications of N2 fixation?

Background and aimsTransfer of fixed N from legumes to non-legume reference plants may alter the 15N signature of the reference plant as compared to the soil N available to the legume. This study investigates how N transfer influences the result of 15N-based N2 fixation measurements.MethodsWe labelled either legumes or non-legumes with 15N and performed detailed analyses of 15N enrichment in mixed plant communities in the field. The results were used in a conceptual model comparing how different N transfer scenarios influenced the 15N signatures of legumes and reference plants, and how the resulting N2 fixation estimate was influenced by using reference plants in pure stand or in mixture with the legume.ResultsBased on isotopic signatures, N transfer was detected in all directions: from legume to legume, from legume to non-legume, from non-legume to legume, from non-legume to non-legume. In the scenario of multidirectional N transfer, N2 fixation was overestimated by using a reference plant in pure stand.ConclusionsFixed N transferred to neighbouring reference plants modifies the 15N signature of the soil N available both to the reference plant and the N2-fixing legume. This provides strong support for using reference plants growing in mixture with the legumes for reliable quantifications of N2 fixation.

Large genotypic variation but small variation in N2 fixation among rhizobia nodulating red clover in soils of northern Scandinavia.

Aims:  To analyse the symbiotic variations within indigenous populations of rhizobia nodulating red clover (Trifolium pratense L.) in soils of northern Norway and Sweden at different times of the growing season.

Perennial species mixtures for multifunctional production of biomass on marginal land

Multifunctional agriculture provides noncommodity functions and services along with food, feed and bioenergy feedstocks, for example by preserving or promoting biodiversity, improving soil fertility, mitigating climate change and environmental degradation, and contributing to the socio‐economic viability of rural areas. Producing biomass for bioenergy from low‐input perennial species mixtures on marginal land has the potential to support biodiversity and soil carbon sequestration in synergy with greenhouse gas mitigation. We compared biomass production in species‐rich mixtures of perennial grasses, legumes and forbs with pure‐stand grasses and relatively species‐poor mixtures under different nitrogen fertilization regimes. Field experiments were performed on different types of marginal land, that is agricultural field margins and land with poor soil fertility, at four sites in southernmost and western Sweden. Biomass production was measured for three years in perennial grasses grown as pure stands, in legume‐grass mixtures, and legume‐grass‐forb mixtures across a species richness gradient. In unfertilized species‐rich mixtures, average biomass yields per experimental site and year were in the range from 3 to 9 metric ton DM ha−1 yr−1. While the most productive pure‐stand grasses fertilized with 60–120 kg N ha−1 yr−1 often produced higher biomass yields than unfertilized mixtures, these differences were generally smaller than the variations between years and sites. Calculations of climate impact using the harvested biomass for conversion to biogas as vehicle fuel showed that the average greenhouse gas emissions per energy unit were about 50% lower in unfertilized systems than in treatments fertilized with 100–120 kg N ha−1 yr−1. Our findings thereby show that unfertilized species‐rich perennial plant mixtures on marginal land provide resource‐efficient biomass production and contribute to the mitigation of climate change. Perennial species mixtures managed with low inputs thus promote synergies between productivity and biodiversity in the perspective of climate‐smart and multifunctional biomass production.

Physiological and Molecular Aspects of Tolerance to Environmental Constraints in Grain and Forage Legumes

Despite the agronomical and environmental advantages of the cultivation of legumes, their production is limited by various environmental constraints such as water or nutrient limitation, frost or heat stress and soil salinity, which may be the result of pedoclimatic conditions, intensive use of agricultural lands, decline in soil fertility and environmental degradation. The development of more sustainable agroecosystems that are resilient to environmental constraints will therefore require better understanding of the key mechanisms underlying plant tolerance to abiotic constraints. This review provides highlights of legume tolerance to abiotic constraints with a focus on soil nutrient deficiencies, drought, and salinity. More specifically, recent advances in the physiological and molecular levels of the adaptation of grain and forage legumes to abiotic constraints are discussed. Such adaptation involves complex multigene controlled-traits which also involve multiple sub-traits that are likely regulated under the control of a number of candidate genes. This multi-genetic control of tolerance traits might also be multifunctional, with extended action in response to a number of abiotic constraints. Thus, concrete efforts are required to breed for multifunctional candidate genes in order to boost plant stability under various abiotic constraints.

Discrimination against 15N among recombinant inbred lines of Phaseolus vulgaris L. contrasting in phosphorus use efficiency for nitrogen fixation

Although isotopic discrimination processes during nitrogen (N) transformations influence the outcome of (15)N based quantification of N2 fixation in legumes, little attention has been given to the effects of genotypic variability and environmental constraints such as phosphorus (P) deficiency, on discrimination against (15)N during N2 fixation. In this study, six Phaseolus vulgaris recombinant inbred lines (RILs), i.e. RILs 115, 104, 34 (P deficiency tolerant) and 147, 83, 70 (P deficiency sensitive), were inoculated with Rhizobium tropici CIAT899, and hydroaeroponically grown with P-sufficient (250 μmol P plant(-1) week(-1)) versus P-deficient (75 μmol P plant(-1) week(-1)) supply. Two harvests were done at 15 (before nodule functioning) and 42 (flowering stage) days after transplanting. Nodulation, plant biomass, P and N contents, and the ratios of (15)N over total N content ((15)N/Nt) for shoots, roots and nodules were determined. The results showed lower (15)N/Nt in shoots than in roots, both being much lower than in nodules. P deficiency caused a larger decrease in (15)N/Nt in shoots (-0.18%) than in nodules (-0.11%) for all of the genotypes, and the decrease in shoots was greatest for RILs 34 (-0.33%) and 104 (-0.25%). Nodule (15)N/Nt was significantly related to both the quantity of N2 fixed (R(2)=0.96***) and the P content of nodules (R(2)=0.66*). We conclude that the discrimination against (15)N in the legume N2-fixing symbiosis of common bean with R. tropici CIAT899 is affected by P nutrition and plant genotype, and that the (15)N/Nt in nodules may be used to screen for genotypic variation in P use efficiency for N2 fixation.

Legume Performance and Nitrogen Acquisition Strategies in a Tree-Based Agroecosystem

Legume N2 fixation is a critical plant-mediated nutrient pathway highly dependent on site and management conditions, yet little is known of annual crop N2 fixation associated with established trees in tree-based intercropping systems. Our objective was to investigate the performance and N acquisition strategies of soybean in a tree-based intercropping system with four tree species (genera Juglans, Fraxinus, Acer, and Populus) at two distances from the tree row (1- and 4-m tree-crop interface) in comparison to monocropped soybean. Soil nitrification rates were inversely related to soybean nodule production at the 1-m tree-crop interface (r = −0.64; p = 0.0260). However, regardless of inhibited nodule production, soybeans at the 1-m tree-crop interface were fixing N at high rates as compared to monocropped soybeans, possibly stimulated by belowground interspecies competition; nitrogen derived from atmosphere (Ndfa) in soybeans was highest when intercropped with spruce (75% Ndfa) and poplar (65% Ndfa) at the 1- and 4-m tree-crop interface, respectively. We show a shift in crop N acquisition strategies, as modified by tree species and distance. We suggest that an often overlooked nutrient pathway, nitrate deposition via tree canopy wash, may be an unaccounted abiotic factor influencing nodule formation of associated legume crops.

Phosphorous deficiency decreases nitrogenase activity but increases proton efflux in N2-fixing Medicago truncatula

Effects of Sinorhizobium strain and P nutrition on N(2)-dependent growth, nitrogenase activity and proton efflux by nodulated roots were investigated in the model legume Medicago truncatula cultivar Jemalong grown in hydroaeroponic culture in symbioses with Sinorhizobium meliloti strains 102F51 and 2011. Sinorhizobium strain had strong effects on nitrogenase activity and N(2)-dependent growth, with S. meliloti 102F51 being the more efficient strain. Apparent and total nitrogenase activities, measured by hydrogen evolution in air and argon, respectively, were drastically reduced in plants supplied with 5 μmol P plant(-1) week(-1) as compared with 15 μmol P plant(-1) week(-1). There was a net proton efflux as soon as 2 weeks after inoculation and, in contrast to the effect of P nutrition on nitrogenase activity, P deficiency increased total and specific proton effluxes, irrespective of Sinorhizobium strain.

How to Quantify Biological Nitrogen Fixation in Forage Legumes in the Field

Reliable measurements of the input of fixed-N from forage legumes are essential in estimating the need for N fertilization in grasslands. Such measurements rely on methods that can be applied in the field for either the entire growing season or parts of it. High precision and reliability should always be aimed at when measuring N2 fixation. However, depending on the aims and availability of resources (time and equipment), methods with varying precision can be chosen. Methods with high precision are needed if the aim is to reveal factors that affect the N2-fixation rate, whereas N balances at the field or farm level may be based on N2 fixation estimates obtained with lower precision. Of the methods listed in Table 1, we only recommend N natural abundance (NA) and N isotope dilution (ID) for measurements with high precision. In addition, we propose that simple formulas based on legume biomass can be used for rapid estimates with lower precision. Nitrogen difference is not a reliable method for forage legumes because forage grasses usually are more efficient than legumes in taking up soil N (Table 1; Carlsson and Huss-Danell, 2003 and references therein).