Comparative effects of organic and inorganic nitrogen sources applied to a flooded soil on rice yield and availability of N

A pot experiment was conducted to study the effect of organic and inorganic nitrogen (N) sources on the yield and N uptake of rice from applied and native soil-N. The residual effect of these N sources on a succeeding wheat crop was also studied. Organic N was applied in the form of ¹⁵N-labelled Sesbania aculeata L., a legume, and inorganic N in the form of ¹⁵N-labelled ammonium sulphate. The two sources were applied to the soil separately or together at the time of transplanting rice. Recovery of N by rice from both the applied sources was quite low but both sources caused significant increases in biomass and N yield of rice. Maximum increase was recorded in soil treated with organic N. The residual value of the two materials as source of N for wheat was not significant; the wheat took up only a small fraction of the N initially applied. Loss of N occurred from both applied N sources, the losses being more from inorganic N. Both applied N sources caused a substantial increase in the availability of soil-N to rice and wheat; most of this increase was due to organic N and was attributed to the so-called ‘priming’ effect or ANI (added nitrogen interaction) of the applied material.     

Nitrogen fertilization effects on dinitrogen fixation as influenced by legume species and proportion in legume-grass mixtures in Uruguay

Grasses grown in mixture with nodulated legumes often are N-limited, but N fertilization may result in reductions of N₂ fixation and legume stands. We studied N-fertilizer effects on N₂ fixation for three binary legume-grass mixtures in Uruguay. Replicated swards of white clover (Trifolium repens L.) (WC), red clover (Trifolium pratense L.) (RC), or birdsfoot trefoil (Lotus corniculatus L.) (BT), each in combination with tall fescue (Festuca arundinacea Schreb) (TF) at two legume proportions were sown in 1983 (Exp. 1) and 1984 (Exp. 2). In the fall of 1984, N treatments at 100 kg ha⁻¹ and controls were randomly assigned to subplots in Exp. 1 (established swards) and in Exp. 2 (at seeding). The soil for both experiments was a fine, montmorillonitic, mesic, Typic Argiudolls. Herbage fixed-N was estimated by ¹⁵N isotope-dilution with pure stands of TF as reference. In both experiments, N fertilization reduced the proportion of legume N derived from air (% Ndfa) and increased herbage yield only during the first 18 to 20 weeks after application. Fertilizer-N reduced annual fixed-N yield from 178 to 148 kg ha⁻¹ in Exp. 1 and from 65 to 29 kg ha⁻¹ in Exp. 2 Fixed-N yield for BT was markedly reduced by N in both experiments (33 to 53%), whereas for the clovers reduction was lesser in Exp. 1 (9 to 13%) than in Exp. 2 (46 to 64%). Negative effects of N on % Ndfa were more evident for the high legume proportion. We conclude that fertilization with 100 kg N ha⁻¹ reduced % Ndfa only for the immediate 18 to 20 weeks after application. Fertilizer-induced reduction of fixed-N yields lasted longer because of a more prolonged depression of legume proportion, especially for BT and for newly seeded swards. Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012). Journal Paper no. J.-13327 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, U.S.A. (Project 2281). Supported in part by the Facultad de Agronomía, Montevideo, Uruguay; and the International Atomic Energy Agency, Vienna, Austria (Project URU/5/012).     

Growth and reproduction of Arabidopsis thaliana in relation to storage of starch and nitrate in the wild-type and in starch-deficient and nitrate-uptake-deficient mutants

We have investigated the interactions between resource assimilation and storage in rosette leaves, and their impact on the growth and reproduction of the annual species Arabidopsis thaliana. The resource balance was experimentally perturbed by changing (i) the external nutrition, by varying the nitrogen supply; (ii) the assimilation and reallocation of resources from rosette leaves to reproductive organs, by cutting or covering rosette leaves at the time of early flower bud formation, and (iii) the internal carbon and nitrogen balance of the plants, by using isogenic mutants either lacking starch formation (PGM mutant) or with reduced nitrate uptake (NU mutant). When plants were grown on high nitrogen, they had higher concentrations of carbohydrates and nitrate in their leaves during the rosette phase than during flowering. However, these storage pools did not significantly contribute to the bulk flow of resources to seeds. The pool size of stored resources in rosette leaves at the onset of seed filling was very low compared to the total amount of carbon and nitrogen needed for seed formation. Instead, the rosette leaves had an important function in the continued assimilation of resources during seed ripening, as shown by the low seed yield of plants whose leaves were covered or cut off. When a key resource became limiting, such as nitrogen in the NU mutants and in plants grown on a low nitrogen supply, stored resources in the rosette leaves (e.g. nitrogen) were remobilized, and made a larger contribution to seed biomass. A change in nutrition resulted in a complete reversal of the plant response: plants shifted from high to low nutrition exhibited a seed yield similar to that of plants grown continuously on a low nitrogen supply, and vice versa. This demonstrates that resource assimilation during the reproductive phase determines seed production. The PGM mutant had a reduced growth rate and a smaller biomass during the rosette phase as a result of changes in respiration caused by a high turnover of soluble sugars ( Caspar et al. 1986 ; W. Schulze et al. 1991 ). During flowering, however, the vegetative growth rate in the PGM mutant increased, and exceeded that of the wild-type. By the end of the flowering stage, the biomass of the PGM mutant did not differ from that of the wild-type. However, in contrast to the wild-type, the PGM mutant maintained a high vegetative growth rate during seed formation, but had a low rate of seed production. These differences in allocation in the PGM mutant result in a significantly lower seed yield in the starchless mutants. This indicates that starch formation is not only an important factor during growth in the rosette phase, but is also important for whole plant allocation during seed formation. The NU mutant resembled the wild-type grown on a low nitrogen supply, except that it unexpectedly showed symptoms of carbohydrate shortage as well as nitrogen deficiency. In all genotypes and treatments, there was a striking correlation between the concentrations of nitrate and organic nitrogen and shoot growth on the one hand, and sucrose concentration and root growth on the other. In addition, nitrate reductase activity (NRA) was correlated with the total carbohydrate concentration: low carbohydrate levels in starchless mutants led to low NRA even at high nitrate supply. Thus the concentrations of stored carbohydrates and nitrate are directly or indirectly involved in regulating allocation.     

Identification and inter-relationship analysis of Bradyrhizobium japonicum strains by restriction fragment length polymorphism (RFLP) and random amplified polymorphic DNA (RAPD)

Genomic DNA of 13 Bradyrhizobium japonicum strains was prepared and analysed by restriction fragment length polymorphism (RFLP) with nif and nod probes, and by random amplified polymorphic DNA (RAPD) with 11 primers of arbitrary nucleotide sequence. Polymorphism was observed in both analyses. The RFLP and RAPD banding patterns of different strains were used to calculate genetic divergence and to construct phylogenetic trees, allowing studies on the relationships between the strains. RFLP with nif and nod probes permitted the separation of the strains into two divergent groups, whereas RAPD separated them into four main groups. RAPD allowed closely related strains to be distinguished.     

Oxidative stress and aging in Caenorhabditis elegans

Much attention has been focused on the hypothesis that oxidative damage plays in cellular and organismal aging. A mev-1 (kn1) mutant of Caenorhabditis elegans, isolated on the basis of its methyl viologen (paraquat) hypersensitivity, is also hypersensitive to elevated oxygen levels. Unlike the wild type, its life span decreases dramatically as oxygen concentrations are increased from 1% to 60%. Strains, which bear this mutation, accumulate fluorescent materials and protein carbonyl groups, markers of aging, at faster rates than the wild type. We have cloned mev-1 gene by transformation rescue and found that it is, in fact, the previously sequenced gene (cyt-1) that encodes succinate dehydrogenase cytochrome b. A missense mutation abolishes complex II activity in the mitochondrial membrane but not succinate dehydrogenase enzyme activity per se. These data suggest that CYT-1 directly participates in electron transport from FADH₂ to coenzyme Q. Moreover, mutational inactivation of this process renders animals susceptible to oxidative stress and, as a result, leads to premature aging.     

On the causes of trends in the seasonal amplitude of atmospheric CO2

No consensus has yet been reached on the major factors driving the observed increase in the seasonal amplitude of atmospheric CO₂ in the northern latitudes. In this study, we used atmospheric CO₂ records from 26 northern hemisphere stations with a temporal coverage longer than 15 years, and an atmospheric transport model prescribed with net biome productivity (NBP) from an ensemble of nine terrestrial ecosystem models, to attribute change in the seasonal amplitude of atmospheric CO₂. We found significant (p < .05) increases in seasonal peak‐to‐trough CO₂ amplitude (AMP_P‐T) at nine stations, and in trough‐to‐peak amplitude (AMP_T‐P) at eight stations over the last three decades. Most of the stations that recorded increasing amplitudes are in Arctic and boreal regions (>50°N), consistent with previous observations that the amplitude increased faster at Barrow (Arctic) than at Mauna Loa (subtropics). The multi‐model ensemble mean (MMEM) shows that the response of ecosystem carbon cycling to rising CO₂ concentration (eCO₂) and climate change are dominant drivers of the increase in AMP_P‐T and AMP_T‐P in the high latitudes. At the Barrow station, the observed increase of AMP_P‐T and AMP_T‐P over the last 33 years is explained by eCO₂ (39% and 42%) almost equally than by climate change (32% and 35%). The increased carbon losses during the months with a net carbon release in response to eCO₂ are associated with higher ecosystem respiration due to the increase in carbon storage caused by eCO₂ during carbon uptake period. Air‐sea CO₂ fluxes (10% for AMP_P‐T and 11% for AMP_T‐P) and the impacts of land‐use change (marginally significant 3% for AMP_P‐T and 4% for AMP_T‐P) also contributed to the CO₂ measured at Barrow, highlighting the role of these factors in regulating seasonal changes in the global carbon cycle.     

Responses to ocean acidification and diurnal temperature variation in a commercially farmed seaweed,Pyropia haitanensis (Rhodophyta)

To investigate carbon and nitrogen metabolism in Pyropia haitanensis in response to the combined conditions of ocean acidification and diurnal temperature variation, maricultured thalli were tested in acidified culture under different temperature treatments. The results showed a combined effect of ocean acidification and diurnal temperature difference on the C and N metabolism and growth of P. haitanensis. In acidifed culture, algal growth, maximum photosynthetic rate, nitrate reductase (NR) activity, amino acid (AA) content and AA score (AAS) were more significantly enhanced in seaweed under diurnal temperature variation than in seaweed at constant temperature. In acidified seawater, soluble carbohydrates in P. haitanensis increased due to greater dissolved inorganic carbon (DIC), whereas soluble proteins decreased. Under the diurnal temperature treatment, higher temperature during the light period enhanced accumulation of algal photosynthates, whereas lower temperature in the dark period reduced energy consumption, resulting in enhanced algal growth, AA content and AAS. We concluded that suitable diurnal temperature difference would be conducive to C fixation and N assimilation under ocean acidification. However, excessively high temperatures would depress algal photosynthesis and increase energy consumption, thereby exerting a negative effect on algal growth.     

Effect of temperature on growth and nitrogen mineralization of fungi and fungal-feeding nematodes

Dish and column microcosms containing alfalfa-sand medium were established to determine effect of temperature on growth and N mineralization ability of two fungi (Rhizoctonia solaniand Botrytis cinerea) and two nematodes (Aphelenchus avenaeand Aphelenchoides composticola). The microcosms were incubated at 15, 20, 25 and 29 °C for 21 days. In the dish microcosms, hyphal growth rates of both fungal species increased with temperature in the range of 15–25 °C. Above that temperature range, the growth rate of R. solani remained almost constant while that of B. cinereadecrease considerably. The population growth rate of A. avenae increased with temperature between 15 and 29 °C on colonies of R. solani and B. cinerea in dish microcosms. The growth rate of A. composticola also increased in the range of 15–25 °C but decreased greatly beyond that temperature range independent of the fungal species as food source. Inorganic N (NH₄ ⁺ + NO₃ ^–) was collected from each column microcosm by leaching every 3 days. In the columns containing R. solani, there was a significant effect of temperature on the amount of N detected in the fungus+A avenaeor A. composticolabut not in the fungus alone columns. The total amount of N was greatest at 29 °C for A. avenaeand at 20 °C for A. composticola columns, concurrent with the population growth rates of the nematodes. In the columns containing B. cinerea, the effect of temperature on the amount of inorganic N was not significant in either the fungus alone or fungus+nematode columns, although the population growth rates of the both nematode species were highest at 20 °C. For B. cinerea, the N amount across temperatures was the same or larger for the fungus alone as for the fungus+nematode columns. In general, the contribution of fungal-feeding nematodes to N mineralization was small in any combinations of fungus and nematode species at any temperature. Similarity in C/N ratio of the fungal and nematode biomass, organic substrate C/N ratios too low for measurable increase in net mineralization by the nematodes and small reproduction of the nematodes in the column microcosms were probable contributory factors.     

Superoxide dismutase, catalase and nitrogenase activities of symbiotic Frankia (Alnus incana) in response to different oxygen tensions

Presence and activity of the enzymes superoxide dismutase (SOD) and catalase were studied in Frankia in symbiosis with Alnus incana (L.) Moench. Analysis on native PAGE gels indicated that symbiotic Frankia contained an FeSOD and catalase. The activity of the enzymes was in the same range as reported for cultured Frankia . Attempts to characterize SOD by western blots with antisera from Escherichia coli and Azotobacter vinelandii did not give clear-cut results with the antibodies used. Alnus incana plants were grown with the root system in 5, 10, 21 or 40% O₂ for up to 6 days. Nitrogenase activity, measured as ARA (acetylene reducing activity) dropped within 3 h when roots were exposed to low or high oxygen. At 40% O₂ ARA was almost completely lost while at 5 and 10% O₂ ARA decreased to 69 and 74% of the inital value, respectively, Nitrogenase activity recovered at ail oxygen tensions. Recovery rates resembled the continuous increase in ARA in plants continuosly kept at 21% O₂, and suggests that new vesicles with envelopes of appropriate thickness were formed. The ARA measurements confirm results from an earlier study where nitrogenase activity was measured as H₂ evolution. There was a tendency for increased SOD and catalase activities in Frankia from root systems exposed to 40% O₂ for 24 h but not earlier or later than this. When data from all experimental times were pooled. SOD activity increased significantly with increased oxygen tension whereas catalase activity decreased. Although ARA per plant varied with oxygen tension, there was no statistically significant correlation between ARA and SOD or between ARA and catalase. It seems that being linked to nitrogenase activity is only one role of SOD and catalase in this symbiotic Frankia .