No abstract available

[en] An attempt to correlate the positive nitrogen balance in rice-soil system with the ¹⁵N dilution in rice plants was made to see if isotope dilution can be used to assess the contribution of nitrogen fixation to the nitrogen nutrition of rice. ¹⁵N ammonium sulfate and sucrose were added to the moist soil in pots to label biomass nitrogen fraction. The rice-soil system with higher nitrogen gain had lower ¹⁵N content in the rice plants. When the surface of pots was covered with black cloths to suppress photodependent N₂ fixation, no significant nitrogen gain was observed. Significant gain was found in the rice-flooded soil system exposed to light, and the ¹⁵N content of plants decreased in allowing the photodependent N₂ fixation by blue-green algae symbiosis. The contribution of plant nitrogen derived from photodependent N₂ fixation was estimated to be 20-30 % of the positive nitrogen gain in the system by the ¹⁵N dilution technique using the rice-covered soil as reference system. (Mori, K.)

[en] Symbiotic N_2 fixation is of primordial significance in sustainable agro-forestry management as it allows reducing the use of mineral N in the production of mixed stands and by protecting the soils from degradation. Thereby, on a 2-year basis, N_2 fixation was evaluated in four oak woodlands under Mediterranean conditions using a split-plot design and three replicates. "1"5N technique was used for determination of N_2 fixation rate. Variations in environmental conditions (temperature, rainfall, radiation) by the cork tree canopy as well as the age of stands and pasture management can cause great differences in vegetation growth, legume N_2 fixation, and soil rhizobial abundance. In the present study, non-legumes dominated the swards, in particular beneath the tree canopy, and legumes represented only 42% of total herbage. A 2-fold biomass reduction was observed in the oldest sown pasture in relation to the medium-age sward (6 t DW ha"−"1 yr"−"1). Overall, competition of pasture growth for light was negligible, but soil rhizobial abundance and symbiotic N_2 fixation capacity were highly favored by this environmental factor in the spring and outside the influence of tree canopy. Nitrogen derived from the atmosphere was moderate to high (54–72%) in unsown and sown swards. Inputs of fixed N2 increased from winter to spring due to more favorable climatic conditions (temperature and light intensity) for both rhizobia and vegetation growths. Assuming a constant fixation rate at each seasonal period, N_2 fixation capacity increased from about 0.10 kg N ha"−"1 per day in the autumn–winter period to 0.15 kg N ha"−"1 per day in spring. Belowground plant material contributed to 11% of accumulated N in pasture legumes and was not affected by canopy. Size of soil fixing bacteria contributed little to explain pasture legumes N. - Highlights: • Legumes fixation in oak woodlands was quantified in terms of biomass and N_2 fixed. • Cork oak canopy did not influence pasture biomass, but decreased the plant N_2 fixed. • The native rhizobial population size declined beneath the tree canopy. • Symbiotic fixation rate was moderate to high, lowering with pasture age. • Total N_2 fixed by pasture legumes was weakly explained by soil rhizobial abundance

No abstract available

[en] Isotopic and non-isotopic methods were used to quantify below ground nitrogen (BGN) for two winter legumes, fababean (Vicia faba) and chickpea (Cicer arietinum), under glasshouse and field conditions. In the glasshouse study, estimates of BGN for fababean and chickpea, respectively, were 13 and 10% of total plant N (physical recovery), 11 and 52% (soil ¹⁵N dilution), 30 and 52% (mass N balance), 39 and 53% (¹⁵N-shoot labelling), 37 and 42% (adjusted 15N shoot labelling), and 33 and 43 % (15N balance). In the field experiment, values were 25 and 77% (15N-shoot labelling), 24 and 68% (adjusted ¹⁵N shoot labelling) and 29 and 60% (15N balance). When averaged across all estimates (other than physical recovery), BGN of glasshouse-grown plants represented 31% of total plant N for fababean and 48% for chickpea. By comparison, the mean values for BGN as percent of total plant N in the field study using the two methods considered likely to give the most reliable results (adjusted ¹⁵N shoot labelling and ¹⁵N balance) were 27% for fababean and 64% for chickpea. (author)

[en] Isotopic and non-isotopic methods were used to quantify below ground nitrogen (BGN) for two winter legumes, fababean (Vicia faba) and chickpea (Cicer arietinum), under glasshouse and field conditions. In the glasshouse study, estimates of BGN for fababean and chickpea, respectively, were 13 and 10% of total plant N (physical recovery), 11 and 52% (soil ¹⁵N dilution), 30 and 52% (mass N balance), 39 and 53% (¹⁵N-shoot labelling), 37 and 42% (adjusted ¹⁵N shoot labelling), and 33 and 43 % (¹⁵N balance). In the field experiment, values were 25 and 77% (¹⁵N-shoot labelling), 24 and 68% (adjusted ¹⁵N shoot labelling) and 29 and 60% (¹⁵N balance). When averaged across all estimates (other than physical recovery), BGN of glasshouse-grown plants represented 31% of total plant N for fababean and 48% for chickpea. By comparison, the mean values for BGN as percent of total plant N in the field study using the two methods considered likely to give the most reliable results (adjusted ¹⁵N shoot labelling and ¹⁵N balance) were 27% for fababean and 64% for chickpea. (author)

[en] To assess the inhibitory effect of nitrate on the contribution of symbiotic N fixation to total plant N, cultivars of different nodulation capacity were monitored in a growth chamber study. Plants inoculated with Bradyrhizobium sp. (Arachis) strain NC 70.1 were grown in a nutrient solution containing 0, 2.5, 5 or 10 mM NO₃ enriched with 2.5 atom % ¹⁵N. Plant harvests at 30 and 60 DAP provided tissue for measurement of growth, total N, NO₃ and ¹⁵N partitioning. Nitrogenase activity was estimated via C₂H₂ reduction. Data indicates that plant growth was associated to NO₃ concentration. Average nodule weight and N plant^-1 decreased in excess of 2.5mM NO₃. Specific nitrogenase activity diminished markedly with application of NO₃ with a decline from 40.2 to 25.0 μmoles C₂H₂ g hr^-1 at 0 and 2.5mM NO₃, respectively. Nitrate and fixed N assimilation patterns will be elucidated by ¹⁵N analysis

[en] Mineralization of organic-N to soil samples of an Oxisol as ¹⁵N-labeled bean straw, with and without N from fertilizer (urea) was studied, as well as the effect of expanded vermiculite in the production and absorption of the mineralized-N by a grass. The experiment was conducted in plastic pots. The fertilizer urea (46,64%N) utilized was labelled (5,2% of ¹⁵N) atoms). All experimental pots received 150 ppm of P and K as simple superphosphate (18% P₂O₅) and 26% CaO) and potassium sulphate (60% K₂O), respectively. The grass was planted by putting 8 small pieces by pot. The aerial part was harvested at 30 days intervals. Grass production was a function of the N available and bean straw behaved as an important N source for the plants; at 30 days (first sampling) the production N extraction and efficiency of utilization of the organic N were at their maximum, decreasing (p=0,01) at each following harvest; after the first sampling the mineralization rate of organic N was very low, decreasing significantly the grass production; N fertilizer favoured significantly the mineralization and the efficiency of utilization of the organic-N applied; vermiculite did not affect either production or the N extraction by the grass; in the soil mineral-N, after the culture, the percentage of N from labelled sources was two times that of the total-N and lower than in the plant in the final harvest. (Author)

[en] In samples from the plough horizon of five soils taken after cereal harvest, denitrification was measured as volatilization of N₂ and N₂O from ¹⁵N nitrate in the absence of O₂. Nitrate contents lower than 50 ppm N (related to soil dry matter) had only a small effect on denitrification velocity in four of the five soils. In a clay soil dependence on nitrate concentration corresponded to a first-order reaction. Available C was no limiting factor. Even at zero temperatures remarkable N amounts (on average 0.2 ppm N per day) were still denitrified. The addition of daily turnover rates in relation to soil temperatures prevailing from December to March revealed potential turnovers in the 0-to-30-cm layer of the soils to average 28 ± 5 ppm N. (author)

[en] In mature fruit trees, internal recycling is an important source of N for the growth of new wood, leaves and fruits. Using ¹⁵N-depleted fertilizer, i.e. ¹⁴N-enriched, N-uptake efficiency and the magnitude of internal N cycling were studied in mature walnut trees. Two kg of ¹⁴N-labelled ammonium sulfate N were applied per tree, and compartmentation of N was followed over a period of 6 years by analyzing catkins, pistillate flowers, leaves and fruits each year for total N content and isotopic composition. Subsequently, two of the six labelled trees were excavated and analyzed for labelled-N content. The data indicate that mature walnut uses most of the N accumulated from soil and fertilizer for storage purposes, to be remobilized for new growth within 2 years, and about half of the total-N pool in a mature tree is present as non-structural compounds, available for recycling. (author)