Soil Chemical Analysis

Soil microorganisms as controllers of atmospheric trace gases (H2, CO, CH4, OCS, N2O, and NO).

Recent molecular studies on endophytic bacterial diversity have revealed a large richness of species. Endophytes promote plant growth and yield, suppress pathogens, may help to remove contaminants, solubilize phosphate, or contribute assimilable nitrogen to plants. Some endophytes are seed-borne, but others have mechanisms to colonize the plants that are being studied. Bacterial mutants unable to produce secreted proteins are impaired in the colonization process. Plant genes expressed in the presence of endophytes provide clues as to the effects of endophytes in plants. Molecular analysis showed that plant defense responses limit bacterial populations inside plants. Some human pathogens, such as Salmonella spp., have been found as endophytes, and these bacteria are not removed by disinfection procedures that eliminate superficially occurring bacteria. Delivery of endo-phytes to the environment or agricultural fields should be carefully evaluated to avoid introducing pathogens.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-19-0827

Bacterial Endophytes and Their Interactions with Hosts

Plants exposed to environmental stress factors, such as drought, chilling, high light intensity, heat, and nutrient limitations, suffer from oxidative damage catalyzed by reactive oxygen species (ROS), e.g., superoxide radical (O2 -), hydrogen peroxide (H2O2) and hydroxyl radical (OH ). Reactive O2 species are known to be primarily responsible for impairment of cellular function and growth depression under stress conditions. In plants, ROS are predominantly produced during the photosynthetic electron transport and activation of membrane-bound NAD(P)H oxidases. Increasing evidence suggests that improvement of potassium (K)-nutritional status of plants can greatly lower the ROS production by reducing activity of NAD(P)H oxidases and maintaining photosynthetic electron transport. Potassium deficiency causes severe reduction in photosynthetic CO2 fixation and impairment in partitioning and utilization of photosynthates. Such disturbances result in excess of photosynthetically produced electrons and thus stimulation of ROS production by intensified transfer of electrons to O2. Recently, it was shown that there is an impressive increase in capacity of bean root cells to oxidize NADPH when exposed to K deficiency. An increase in NADPH oxidation was up to 8-fold higher in plants with low K supply than in K-sufficient plants. Accordingly, K deficiency also caused an increase in NADPH-dependent O2 - generation in root cells. The results indicate that increases in ROS production during both photosynthetic electron transport and NADPH-oxidizing enzyme reactions may be involved in membrane damage and chlorophyll degradation in K-deficient plants. In good agreement with this suggestion, increases in severity of K deficiency were associated with enhanced activity of enzymes involved in detoxification of H2O2 (ascorbate peroxidase) and utilization of H2O2 in oxidative processes (guaiacol peroxidase). Moreover, K-deficient plants are highly light-sensitive and very rapidly become chlorotic and necrotic when exposed to high light intensity. In view of the fact that ROS production by photosynthetic electron transport and NADPH oxidases is especially high when plants are exposed to environmental stress conditions, it seems reasonable to suggest that the improvement of K-nutritional status of plants might be of great importance for the survival of crop plants under environmental stress conditions, such as drought, chilling, and high light intensity. Several examples are presented here emphasizing the roles of K in alleviating adverse effects of different abiotic stress factors on crop production.

The role of potassium in alleviating detrimental effects of abiotic stresses in plants

In the absence of human activities, biotic fixation is the primary source of reactive N, providing about 90–130 Tg N yr−1 (Tg = 1012 g) on the continents. Human activities have resulted in the fixation of an additional ≈140 Tg N yr−1 by energy production (≈20 Tg N yr−1 ), fertilizer production (≈80 Tg N yr−1), and cultivation of crops (e.g., legumes, rice) (≈40 Tg N yr−1 ). We can only account for part of this anthropogenic N. N2O is accumulating in the atmosphere at a rate of 3 Tg N yr−1. Coastal oceans receive another 41 Tg N yr−1 via rivers, much of which is buried or denitrified. Open oceans receive 18 Tg N yr−1 by atmospheric deposition, which is incorporated into oceanic N pools (e.g., NO3−, N2). The remaining 80 Tg N yr−1 are either retained on continents in groundwater, soils, or vegetation or denitrified to N2. Field studies and calculations indicate that uncertainties about the size of each sink can account for the remaining anthropogenic N. Thus although anthropogenic N is clearly accumulating on continents, we do not know rates of individual processes. We predict the anthropogenic N-fixation rate will increase by about 60% by the year 2020, primarily due to increased fertilizer use and fossil-fuel combustion. About two-thirds of the increase will occur in Asia, which by 2020 will account for over half of the global anthropogenic N fixation.

Nitrogen fixation: Anthropogenic enhancement‐environmental response

Acid- and gas-producing nitrogen-fixing bacteria associated with rice roots and leaf sheaths were isolated. These isolates along with reference enterobacteria strains were characterized biochemical...

Isolation and identification of nitrogen fixing Enterobacter cloacae and Klebsiella planticola associated with rice plants

To explain the mechanism of iron toxicity, greenhouse and growth chamber (14CO2 atmosphere) experiments were carried out. In pot experiments (with a typical iron-toxic soil and a fertile clay) we studied the effect of N, P, K and Ca+Mg fertilization (alone or in combination) on dehydrogenase activity, Fe++ formation, and the populations of iron-reducing bacteria in the rhizosphere of rice IR22 and IR42. Fe uptake by the plants was measured at regular intervals. Dehydrogenase activity, the number of N2-fixing iron-reducing bacteria, and the formation and uptake of Fe++ decreased with increased supply of K, Ca, and Mg. This effect was clearer with IR22 (susceptible to iron toxicity) than with IR42 (releatively tolerant). Increased exudation and Fe uptake by IR36 at low nutrient and high Fe supply were recorded in a growth chamber experiment. Nutritional conditions, exudation rate (a measure of metabolic root leakage), the iron-reducing activity of the rhizosphere, and Fe++ uptake by wetland rice appear to be clearly related. Iron toxicity is considered a physiological disorder caused by multiple nutritional soil stress rather than by a low pH and high Fe supply per se.

Effect of fertilization on exudation, dehydrogenase activity, iron-reducing populations and Fe++ formation in the rhizosphere of rice (Oryza sativa L.) in relation to iron toxicity

This paper summarizes recent achievements in exploiting new biological nitrogen fixation (BNF) systems in rice fields, improving their management, and integrating them into rice farming systems. The inoculation of cyanobacteria has been long recommended, but its effect is erratic and unpredictable. Azolla has a long history of use as a green manure, but a number of biological constraints limited its use in tropical Asia. To overcome these constraints, the Azolla-Anabaena system as well as the growing methods were improved. Hybrids between A. microphylla and A. filiculoides (male) produced higher annual biomass than either parent. When Anabaena from high temperature-tolerant A. microphylla was transferred to Anabaena-free A. filiculoides, A. filiculoides became tolerant of high temperature. Azolla can have multiple purposes in addition to being a N source. An integrated Azolla-fish-rice system developed in Fujian, China, could increase farmers' income, reduce expenses, and increase ecological stability. A study using Azolla labeled with 15N showed the reduction of N losses by fish uptake of N. The Azolla mat could also reduce losses of urea N by lowering floodwater-pH and storing a part of applied N in Azolla. Agronomically useful aquatic legumes have been explored within Sesbania and Aeschynomene. S. rostrata can accumulate more than 100kg N ha-1 in 45 d. Its N2 fixation by stem nodules is more tolerant of mineral N than that by root nodules, but the flowering of S. rostrata is sensitive to photoperiod. Aquatic legumes can be used in rainfed rice fields as N scavengers and N2 fixers. The general principle of integrated uses of BNF in rice-farming systems is shown.

Improving nitrogen-fixing systems and integrating them into sustainable rice farming

Azolla spp. and Sesbania spp. can be used as green manure crops for wetland rice. A long-term experiment was started in 1985 to determine the effects of organic and urea fertilizers on wetland rice yields and soil fertility. Results of 10 rice croppings are reported. Azolla sp. was grown for 1 month and then incorporated before transplanting the rice and 3–4 weeks after transplanting the rice. Sesbania rostrata was grown for 7–9 weeks and incorporated only before transplanting the rice. Sesbania sp. grew more poorly before dry season rice than before wet season rice. Aeschynomene afraspera, which was used in one dry season rice trial, produced a larger biomass than the Sesbania sp. The quantity of N produced by the Azolla sp. ranged from 70 to 110 kg N ha-1. The Sesbania sp. produced 55–90 kg N ha-1 in 46–62 days. Rice grain yield increases in response to the green manure were 1.8–3.9 t ha-1, similar to or higher than that obtained in response to the application of 60 kg N ha-1 as urea. Grain production per unit weight of absorbed N was lower in the green manure treatments than in the urea treatment. Without N fertilizer, N uptake by rice decreased as the number of rice crops increased. For similar N recoveries, Sesbania sp. required a lower N concentration than the Azolla sp. did. Continuous application of the green manure increased the organic N content in soil on a dry weight basis, but not on a area basis, because the application of green manure decreased soil bulk density. Residual effects in the grain yield and N uptake of rice after nine rice crops were found with a continuous application of green manure but not urea.

Green manure production of Azolla microphylla and Sesbania rostrata and their long-term effects on rice yields and soil fertility.

A field experiment in concrete-based plots was conducted to estimate the contribution of N derived from air (Ndfa) or biological N2 fixation in Sesbania rostrata and S. cannabina (syn. S. aculeata), using various references, by the 15N dilution method. The two Sesbania species as N2-fixing reference plants and four aquatic weed species as non-N2-fixing references were grown for 65 days after sowing in two consecutive crops, in the dry and the wet seasons, under flooded conditions. Soil previously labeled with 15N at 0.26 atom % 15N excess in mineralizable N was further labeled by ammonium sulfate with 3 and 6 atom % 15N excess. The results showed that 15N enrichment of soil NH
 4
 +
 -N dropped exponentially in the first crop to half the original level in 50 days while in the second crop, it declined gradually to half the level in 130 days. The decline in 15N enrichment, in both N2-fixing and non-fixing species, was also steeper in the first crop than in the second crop. Variations in 15N enrichment among non-fixing species were smaller in the second crop. The ratio of the uptake of soil N to that of fertilizer N in N2-fixing and non-fixing species was estimated by the technique of varying the 15N level. In the second crop, this ratio in non-fixing species was higher than that in N2-fixing species. Comparable estimates of % Ndfa were obtained by using 15N enrichment of various non-fixing species. There was also good agreement between the estimates obtained by using 15N enrichment of non-fixing species and those by using soil NH
 4
 +
 -N, particularly in the second crop. By 25 days after sowing, the first crop of both Sesbania spp. had obtained 50% of total N from the atmosphere and the second crop had obtained 75%. The contribution from air increased with the age of the plant and ranged from 70% to 95% in 45–55 days. S. rostrata fixed substantially higher amounts of N2 due to its higher biomass production compared with S. cannabina. Mathematical considerations in applying the 15N dilution method are discussed with reference to these results.

I. Watanabe

Papers

Isolation and identification of nitrogen fixing Enterobacter cloacae and Klebsiella planticola associated with rice plants

Effect of fertilization on exudation, dehydrogenase activity, iron-reducing populations and Fe++ formation in the rhizosphere of rice (Oryza sativa L.) in relation to iron toxicity

Improving nitrogen-fixing systems and integrating them into sustainable rice farming

Green manure production of Azolla microphylla and Sesbania rostrata and their long-term effects on rice yields and soil fertility.

Estimating N2 fixation by Sesbania rostrata and S. cannabina (syn. S. aculeata) in lowland rice soil by the 15N dilution method