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Notes: Summary A dry season field experiment conducted for two consecutive years highlighted problems of achieving increased populations of N2-fixing blue-green algae (BGA) in wetland rice fields. Inoculation of non-indigenous BGA strains, either dried or as fresh viable inocula even at high levels of application, was unsuccessful. A limiting effect of grazing invertebrate populations on BGA establishment was evident, but other factors were involved. Reducing grazer pressure did not permit establishment of inoculated BGA; interspecific competition and environmental factors may explain the inoculation failure. Grazer regulation permitted the establishment of a fast-growing indigenous N2-fixing Anabaena and the doubling of N2-fixing activity over a control. Neither inoculation nor grazer control affected grain yields significantly.

Notes: Summary The dynamics of five inoculated strains of heterocystous blue-green algae (BGA) and indigenous algae were studied for 1 month in 1-m2 microplots of five soils previously air-dried or oven-dried. The same soils were then dried and resubmerged for another 2 months to study the effect of controlling algal grazers with neem (Azardirachta indica) seeds on the revival and dynamics of indigenous and inoculated algae. During the month following inoculation, inoculated BGA multiplied to some extend in all soils but never dominated the total algal flora. They rarely dominated the indigeneous heterocystous BGA, and did so only when the growth of N2-fixing BGA was poor or after the decline of blooms of indigenous strains. Once the soils were dried, two of the five inoculated strains did not reappear. During the 1st month following rewetting, the remaining inoculated strains again exhibited poor growth; however, after 2 months of submergence, inoculated Aulosira fertilissima developed an agronomically significant bloom in neem-treated plots of two soils. Correlations between acetylene-reducing activity and heterocystous BGA populations indicated a major contribution by indigenous BGA and a minor contribution by inoculated BGA to the N2-fixing activity of the soils during the first experiment and the 1st month of the second experiment. The establishment of inoculated BGA exhibited clear differences among strains but was less affected by the nature of the soil and heat treatment. Neem application might have had a delayed positive effect on the late establishment of inoculated A. fertilissima and favored BGA growth and N2 fixation by the total algal population.

Notes: Summary Laboratory cultures, soil cultures, and natural samples of N2-fixing blue-green algae (BGA) from rice fields were analyzed for dry matter, ash, N, C, P, and a few other constituents. Results show a very large variability of the composition. Dry matter contents ranged from 0.28% to 13.6% (average 3.3%). Ash contents ranged from 15.6% to 71.3%. Nitrogen contents ranged from 1.9% to 11.8% on an ash-free basis (average 6%). Carbon content was less variable, ranging from 37% to 72% and averaging 43.7%. A decrease in N and pigment contents, and an increase in reducing sugars, was observed in aging laboratory cultures. Large differences in composition were observed between field samples and material grown in artificial medium. Soil-grown BGA and field samples were characterized by very high ash contents, N contents lower than those in laboratory cultures, and P deficiency. Extrapolation from (1) average dry matter, ash, and N contents and (2) records of BGA biomass in rice fields indicates that an algal bloom has a potentiality of about 15–25 kg N per hectare and that a BGA biomass of agronomic significance is visible to the naked eye.

Notes: Summary Algal populations were quantified (as colony-forming units [CFU] per square centimetre) in 102 samples of rice soils from the Philippines, India, Malaysia and Portugal, and in 22 samples of soil-based inocula from four countries. Heterocystous blue-green algae (BGA) were present in all samples. Nostoc was the dominant genus in most samples, followed by Anabaena and Calothrix. In soils, heterocystous BGA occurred at densities ranging from 1.0 × 102 to 8.0 × 106 CFU/cm2 (median 6.4 × 104) and comprised, on average, 9% of the total CFU of algae. Their abundance was positively correlated with the pH and the available P content of the soils. In soil-based inocula, heterocystous BGA occurred at densities ranging from 4.6 × 104 to 2.8 × 107 CFU/g dw (dry weight), comprising only a moderate fraction (average 13%) of the total algae. In most soils, the density of indigenous N2-fixing BGA was usually higher than that attained by applying recommended rates of soil-based inoculum. Whereas research on the practical utilization of BGA has been mostly directed towards inoculation with foreign strains, our results suggest that attention should also be given to agricultural practices that enhance the growth of indigenous strains already adapted to local environmental conditions.

Notes: Summary The effect of different methods of nitrogen fertilizer application on the algal flora and biological nitrogen fixation (Acetylene-reducing activity) in a wetland rice soil was studied in pot and field experiments. Broadcast application of urea inhibited nitrogen fixation and favored the growth of green algae. In contrast, deep placement of urea supergranules (1–2 g urea granules) did not suppress the growth of N2-fixing blue-green algae and permitted acetylene-reducing activity on the soil surface to continue virtually uninhibited.

Notes: Abstract This paper 1) reviews improvements and new approaches in methodologies for estimating biological N2 fixation (BNF) in wetland soils, 2) summarizes earlier quantitative estimates and recent data, and 3) discusses the contribution of BNF to N balance in wetland-rice culture. Measuring acetylene reducing activity (ARA) is still the most popular method for assessing BNF in rice fields. Recent studies confirm that ARA measurements present a number of problems that may render quantitative extrapolations questionable. On the other hand, few comparative measures show ARA's potential as a quantitative estimate. Methods for measuring photodependent and associative ARA in field studies have been standardized, and major progress has been made in sampling procedures. Standardized ARA measurements have shown significant differences in associative N2 fixation among rice varieties. The 15N dilution method is suitable for measuring the percentage of N derived from the atmosphere (% Ndfa) in legumes and rice. In particular, the 15N dilution technique, using available soil N as control, appears to be a promising method for screening rice varieties for ability to utilize biologically fixed N. Attempts to adapt the 15N dilution method to aquatic N2 fixers (Azolla and blue-green algae [BGA]) encountered difficulties due to the rapid change in 15N enrichment of the water. Differences in natural 15N abundance have been used to show differences among plant organs and species or varieties in rice and Azolla, and to estimate Ndfa by Azolla, but the method appears to be semi-quantitative. Recent pot experiments using stabilized 15N-labelled soil or balances in pots covered with black cloth indicate a contribution of 10–30 kg N ha-1 crop-1 by heterotrophic BNF in flooded planted soil with no or little N fertilizer used. Associative BNF extrapolated from ARA and 15N incorporation range from 1 to 7 kg N ha-1 crop-1. Straw application increases heterotrophic and photodependent BNF. Pot experiments show N gains of 2–4 mg N g-1 straw added at 10 tons ha-1. N2 fixation by BGA has been almost exclusively estimated by ARA and biomass measurements. Estimates by ARA range from a few to 80 kg N ha-1 crop-1 (average 27 kg). Recent extensive measurements show extrapolated values of about 20 kg N ha-1 crop-1 in no-N plots, 8 kg in plots with broadcast urea, and 12 kg in plots with deep-placed urea. Most information on N2 fixed by Azolla and legume green manure comes from N accumulation measurements and determination of % Ndfa. Recent trials in an international network show standing crops of Azolla averaging 30–40 kg N ha-1 and the accumulation of 50–90 kg N ha-1 for two crops of Azolla grown before and after transplanting rice. Estimates of % Ndfa in Azolla by 15N dilution and delta 15N methods range from 51 to 99%. Assuming 50–80% Ndfa in legume green manures, one crop can provide 50–100 kg N ha-1 in 50 days. Few balance studies in microplots or pots report extrapolated N gains of 150–250 kg N ha-1 crop-1. N balances in long-term fertility experiments range from 19 to 98 kg N ha-1 crop-1 (average 50 kg N) in fields with no N fertilizer applied. The problems encountered with ARA and 15N methods have revived interest in N balance studies in pots. Balances are usually highest in flooded planted pots exposed to light and receiving no N fertilizer; extrapolated values range from 16 to 70 kg N ha-1 crop-1 (average 38 kg N). A compilation of balance experiments with rice soil shows an average balance of about 30 kg N ha-1 crop-1 in soils where no inorganic fertilizer N was applied. Biological N2 fixation by individual systems can be estimated more or less accurately, but total BNF in a rice field has not yet been estimated by measuring simultaneously the activities of the various components in situ. As a result, it is not clear if the activities of the different N2-fixing systems are independent or related. A method to estimate in situ the contribution of N2 fixed to rice nutrition is still not available. Dynamics of BNF during the crop cycle is known for indigenous agents but the pattern of fixed N availability to rice is known only for a few green manure crops.

Notes: Abstract The effects of the algicides terbutryn and copper sulfate on the potential for reducing the gaseous loss of NH3 from urea applied to rice were examined in experiments with 2 methods of N fertilizer management, 2 or 3 N rates, and 3 algicide treatments. The experiments were conducted during the 1986 dry and wet seasons in an experimental field at Pila, Laguna, Philippines. Copper sulfate had little effect as an algicide at the rate used, but terbutryn immediately reduced algal growth. The populations of species resistant to terbutryn probably increased, but terbutryn had no long-term effect on the total number of colony-forming units of algae. There was some evidence that terbutryn reduced photodependent N2 fixation as estimated by acetylene reduction assay. Terbutryn, when applied with urea 10 days after transplanting, reduced the maximum floodwater pH by 0.9 units or more for 7 d in the DS and by about 0.5 units for 8 d in the WS. Terbutryn increased the ammoniacal-N (AN) concentration in the floodwater 100% or more in the DS and 60% in the WS. The combined effect of terbutryn on the floodwater pH and AN concentration was reduced photodependent NH3 partial pressure (ρNH3), about 25% in the DS and 38% in the WS.

Notes: Abstract The review summarizes the current status of the utilization of N2-fixing organisms as biofertilizer in rice cultivation. Heterotrophic bacteria, free-living cyanobacteria,Azolla, and legume green manures are considered with regard to their potential for increasing yield, their current use and the prospects for their use with regard to the identified limiting factors. Biological N2 fixation has been the most effective system for sustaining production in low-input traditional rice cultivation. On the other hand, the utilisation of N2-fixing organisms in intensified rice production encounters serious limitations. The utilization of free-living bacteria and cyanobacteria is refrained by their modest potential and the non establishment of inoculated strains.Azolla and legumes used as green manures have a high potential as N source, but their utilization is severely limited by socio-economic factors.