Showing papers in "Plant and Soil in 1977"

Measurement of amount of nitrogen fixed by a legume crop

Abstract: The amount of nitrogen fixed by a legume crop can be calculated from the relationship % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaqGbb% GaaeyBaiaab+gacaqG1bGaaeOBaiaabshacaqGGaGaae4BaiaabAga% caqGGaGaaeOzaiaabMgacaqG4bGaaeyzaiaabsgacaqGGaGaaeOtai% aabccacaqG9aaabaGaaeypaiaabccadaqadaqaaiaaigdacqGHsisl% daWcaaqaaiGacggacaGG0bGaai4Baiaac2gacaGGGaGaaiyjaiaacc% cacaGGobGaaiylaiaacgdacaGG1aGaaiiiaiaacwgacaGG4bGaai4y% aiaacwgacaGGZbGaai4CaiaacccacaGGPbGaaiOBaiaacccacaGGSb% GaaiyzaiaacEgacaGG1bGaaiyBaiaacwgacaGGGaGaai4yaiaackha% caGGVbGaaiiCaaqaaiGacggacaGG0bGaai4Baiaac2gacaGGGaGaai% yjaiaacccacaGGobGaaiylaiaacgdacaGG1aGaaiiiaiaacwgacaGG% 4bGaai4yaiaacwgacaGGZbGaai4CaiaacccacaGGPbGaaiOBaiaacc% cacaqGYbGaaeyzaiaabAgacaqGLbGaaeOCaiaabwgacaqGUbGaae4y% aiaabwgacaGGGaGaai4yaiaackhacaGGVbGaaiiCaaaaaiaawIcaca% GLPaaacaqI4bGaaKiiaiaabshacaqGVbGaaeiDaiaabggacaqGSbGa% aeiiaiaab6eacaqGGaGaaeyAaiaab6gacaqGGaGaaeiBaiaabwgaca% qGNbGaaeyDaiaab2gacaqGLbGaaeiiaiaabogacaqGYbGaae4Baiaa% bchaaaaa!9A78!\[\begin{gathered} {\text{Amount of fixed N = }} \hfill \\ {\text{ = }}\left( {1 - \frac{{\operatorname{atom} \% N - 15 excess in legume crop}}{{\operatorname{atom} \% N - 15 excess in {\text{reference}} crop}}} \right)\user1{x }{\text{total N in legume crop}} \hfill \\ \end{gathered} \]when a suitable reference crop is chosen. The implications and interpretation of this method of measurement are described.

A simple ion-exchange resin procedure for extracting plant-available elements from soil

Abstract: A simplified procedure for extracting ions from soil with ion-exchange resins is reported. The resin, placed in nylon-netting bags, were immersed in soil suspensions and shaken. Compared to the conventional procedure, where the resin beads are freely suspended in the soil-water mixture, this bag-procedure allowed quicker separation of resin from the soil suspensions. Furthermore, the severe soil grinding as done in the conventional procedure was eliminated. Phosphate was extracted from 4 Danish soils using both procedures. The bag-procedure resulted in slightly higher values than did the conventional procedure. re]19760408

The effect of fallow and continuous cultivation on the chemical and physical properties of an alfisol in western Nigeria

Abstract: Soil properties under continuous cropping were compared with those under planted fallows and natural bush regrowth for three years after forest clearing. The cropping treatments consisted of continuous maize with and without stover returned as surface mulch, continuous soybean, and maize and cassava intercropped. The fallow treatments included pigeon pea, leucaena, Guinea grass and natural bush regrowth. In the continuous soybean and unmulched maize plots, soil organic matter and pH declined rapidly; whereas the mulched maize plots maintained a soil organic matter level comparable to the fallow treatments. To maintain soil organic matter in the surface soil at a level comparable to soil under secondary forest, two to three applications of a total amount of 16 MT/ha/annum of dry plant materials (maize stover or grass) are required when the material is applied as surface mulch. In the cropped plots, favorable physical characteristics in the surface soil were also maintained when sufficient plant residue was returned; whereas the deterioration of subsoil structure of the forest soil occurred in all cropping treatments. Guinea grass fallow has a distinct advantage in recycling mineral nutrients and maintaining soil physical properties and organic matter. It is suggested that soils may be planted with a combination of Guinea grass and pigeon pea fallow for one or two years after three or four years of arable cultivation.

Effect of aluminum on growth and mineral elements of al-tolerant and Al-intolerant corn

Abstract: Growth and P, K, Ca, Mg, Mn, Zn, Fe, and Cu concentrations and contents were determined in Al-tolerant and Al-intolerant corn (Zea mays L.) inbreds when grown at various levels of Al. B57 was more tolerant to Al than was Oh40B. Relatively low Al levels (up to 5 mg/l) enhanced B57 growth but inhibited Oh40B growth. With few exceptions, Oh40B root and leaf concentrations of the elements decreased with added Al. The decreases in element concentrations were not as large for B57 as they were for Oh40B. The Mg concentrations and contents decreased more than the other elements in all inbreds with added Al. Root Mg decreased more than leaf Mg. Total uptake of some elements were higher at low Al than with no Al. Inasmuch as Mg has a pronounced effect on root growth, low Mg may be an important response in plants sensitive to Al.

Microbial abundance in the rhizosphere: A computer model

Abstract: A mathematical model is described which can predict the abundance of microorganisms in the rhizosphere (as μg microbial dry weight/cm3 soil) in relation to distance from the root surface and time since the root started exuding substrate. The growth rate of the microorganisms at each point in the soil is assumed to be controlled by the concentration of soluble organic substrate. The concentration of substrate changes due to (1) its production by the root and diffusion through the soil, (2) its production in the soil by breakdown of insoluble organic matter, and (3) its use by the microorganisms. Values for all of the required input parameters have been obtained from the literature.

Growth of a sitka spruce plantation: Spatial distribution and seasonal fluctuations of lengths, weights and carbohydrate concentrations of fine roots

Abstract: As part of an investigation into the primary production of a forest the activity of fine roots was estimated by taking weekly soil cores from 24 May to 27 September in the 11 year of growth of a plantation of Picea sitchensis. Distinct maxima were found in 1) starch and soluble carbohydrate concentration in the root, mid-June, 2) root weight/soil volume, early July, and 3) root length/soil volume, late July with a second maximum in early September. However, root concentrations in the soil were the same at the end as at the start of the period and it is suggested that the fine root system of the forest had reached a dynamic equilibrium.

Copper and cobalt uptake by Haumaniastrum species

Abstract: Herbarium specimens of the genus Haumaniastrum (mainly from Shaba Province Zaire) were analysed for copper and cobalt.H. homblei showed elevated copper levels and indicated its tolerance to mineralized ground.H. robertii (previously-known as a “copper flower”) showed the expected high copper concentrations, but contained abnormally high concentrations of cobalt (av. 4304 μg/g dry weight) far exceeding those of copper. This species has the highest cobalt content of any phanerogam and its distribution may be controlled by cobalt rather than by copper. re]19761220

Nitrogen accumulation in kaolin mining wastes in cornwall I. Natural communities

Abstract: Plant succession was investigated on sand waste heaps produced by kaolin mining in central Cornwall. It was found that relatively even aged, monospecific stands of vegetation were frequently present. The principal colonists were woody leguminous plants which, in some situations, were superceded by a massive growth of rhododendrons (Rhododendron ponticum) or native woodland species. Where legumes were absent, the waste was slowly colonised by Calluna vulgaris and other heathland species.

Evaluation of the size of a plant's root system using its electrical capacitance

Abstract: This paper extends an earlier one5. The relationship between electrical capacitance of a plant root system (C) and size (weight, volume, surface) of the root system (x) can be expressed by equation of the line $$C = a + bx$$ ifa equals size of parasitic capacitance of measurement, i.e. capacitance of soil, wires,etc. andb equals regression coefficient influenced by type of soil plant species, measuring frequency and voltage,etc.

Soybean growth on fly ashamended strip mine spoils

Abstract: The use of fly ash as an amendment for strip mine soils has been studied under field conditions. Spoils ranging in pH from 4.0–6.0 were tested. The addition of fly ash in all cases was effective as an acid soil neutralizer and substantially enhanced the growth and development of all experimental plants. The parameters used in growth analyses were plant height, dry weight, root/shoot ratios, nodulation, pod production, and nitrogen fixing capacity for legumes. re]19761004

Studies on mineral ion absorption by plants

Abstract: The effect of Al and P on the growth of lucerne (Medicago sativa) was studied in nutrient solutions in which aluminium phosphate did not precipitate. Al and P retained in the free space of the roots was washed out with 0.1N HCl/O4 at 5°C. The inhibitory effect of Al on growth was much less at pH 5 than at pH 4.5, although 3 to 4 times as much Al was found in the roots and shoots of the pH 5 plants. It is suggested that the low toxicity of high contents of Al was due to a portion of the uptake at pH 5 being in the form of polymeric aluminophosphate complexes of low net charge density. The optimum pH for the formation and polymerization of such complexes is around 5, and their composition depends on the P/Al mole ratio of the initial solutions. Washing32P-labelled roots in unlabelled P solutions containing Ca showed that 12–43 per cent more of the total label diffused out of the Al-treated roots at pH 5 than from control roots. This was consistent with estimates by solution analysis of 16–36 per cent (depending on the P/Al mole ratio) of the P present in the original uptake solutions being complexed with Al.

Comparative lime requirements of tropical and temperate legumes

Abstract: In a field trial using a continuous function design, calcium carbonate was applied to a nitrogen deficient Hawaiian Oxisol at rates which increased along the plant row from 0 to 22 tons/ha, producing a gradient in soil pH from 4.7 to 7.1. Inoculated legumes representing 18 species were grown.

Nitrification inhibitors I. Studies with karanjin, A furanolflavonoid from karanja (Pongamia glabra) seeds

Abstract: Karanjin, the major crystalline principle of karanja (Pongamia glabra Vent.) seeds, is shown to be a potent nitrification inhibitor. It compares well with N-serve in pot-culture experiments with rice crop. Structure activity relationship studies with chemically altered molecules from karanjin show that its furan ring is essential for showing the inhibitory effect.

Subsoil gravel horizon and maize root growth

Abstract: A series of factoral experiments involving the effects of gravel concentration in the subsoil horizon, the effect of inter-gravel bulk density, the depth of the surface soil above the gravel horizon and the interaction between soil moisture regime and gravel concentration, on growth and development of maize roots were conducted in the greenhouse. Increasing sub soil gravel concentration decreased the total porosity and the available waterholding capacity. Though the root growth of maize seedlings was adversely affected by gravel concentration above 20 per cent, lower concentration of gravels had a beneficial effect on roots. The root development and the leaf concentration of N, P and K improved with increasing depth of surface soil above the gravel horizon. The symptoms of mechanical impedance, such as thickening of root tips and profuse branching behind the tip were commonly observed on the root axis penetrating the gravelly horizon. re]19750916

Nitrogen accumulation in kaolin mining wastes in cornwall

Abstract: Nitrogen accumulation was studied in mica and sand mining wastes of Cornwall after twelve forage legume varieties were established with the use of lime and fertilizers containing phosphorus and potassium. In the finetextured mica waste legume productivity and nitrogen accumulation were similar to those for upland pastures in Cornwall; but legume growth was limited by summer drought on the coarse-textured sand waste. Native perennial legumes well adapted to the British climate were the most productive and showed the highest potential for nitrogen fixation. More than 500 kg N/ha accumulated during the 2-year period whenTrifolium pratense andLotus corniculatus were established on mica waste, and more than 250 kg N/ha was accumulated by these legumes on sand waste.Trifolium pratense andLotus corniculatus were persistent on both the mica and sand waste, andMedicago lupulina showed an unusually high tolerance for competition from invading and sown grass on the sand waste.Trifolium repens andT. hybridum are recommended for waste sites where grazing is a part of management. Nitrogen accumulation on mica waste was consistent with N fixation rates expected from climatic conditions. Nitrogen fixation by free-living bacteria is limited by the low organic matter content of the wastes. The nitrogen fixation potential by legumes on sand wastes has been underestimated because leaching losses were not adequately evaluated. Since nitrogen accumulation rates by earlyTrifolium pratense andT. repens were 70 percent higher than the maximum rate estimated for natural legumesi.e. (Ulex europaeus) on sand waste, the use of forage legumes should reduce reclamation time considerably. re]19760512

Iron-nutritional aspects of the ionic balance of plants

Abstract: The effect of iron on the ionic balance of several plant species and cultivars was studied. Those plants which normally excrete relatively low amounts of hydroxyl ions respond to iron stress by lowering the pH of the nutrient medium and decreasing anion uptake. These plants may be considered as being Fe efficient. Plants which normally excrete relatively high amounts of hydroxyl ions and which continue to increase the pH of a nutrient medium when under iron stress may be considered as Fe inefficient. Iron deficiency tends to increase carboxylate accumulation and to decrease anion uptake. When cation uptake is depressed by iron deficiency this is mainly a non specific depression of potassium uptake, on the other hand when iron stress stimulates cation uptake this is mainly due to a specific stimulation of the divalent cations Ca and Mg. Additional key words: Iron, efficiency of uptake, ionic balance, hydroxyl and hydrogen ion excretion.

Ammonia volatilization from a flooded tropical soil

Abstract: Ammonia volatilization, which follows upon the application of nitrogenous fertilizers to a flooded tropical soil, was directly measured in the greenhouse and in the field. Most of the ammonia volatilization losses occurred during the first 9 days after nitrogen application. Ammonia volatilization increased markedly with increases in soil pH. Nitrogen losses from ammonium sulfate applied to soils whose pH values were below 7.5 were very small. The losses from urea were much greater than those from ammonium sulfate. Mixing the fertilizer materials with the puddled soil reduced the losses. Ammonia losses from flooded soil were larger than from dry soil, and drying of a flooded soil reduced the duration and magnitude of ammonia volatilization. It is suggested that only a small amount of nitrogen is being lost through ammonia volatilization from many lowland rice soils. re]19750820

Influence of lime on nitrogen fixation by tropical and temperate legumes

Abstract: A nitrogen deficient Oxisol which had been fertilized with P, K, Zn and Mo received CaCO3 at rates which increased continuously from zero to 22 tons/ha. Liming produced a range of pH in the saturation paste from 4.7 to 7.1; a range of calcium in the saturation extract from 0.3 to 3 meq/l; and a significant decline in available manganese. Responses of 9 tropical and 7 temperate legumes were compared across the pH variable.

Effects of acetic, propionic, and butyric acids on rice seedling growth and nutrition

Abstract: Soils which are flooded for lowland rice culture shift from aerobic to anaerobic organic matter transformations. Anaerobic carbon transformations, involving chiefly rice crop residues, are characterized by the formation of various organic acids. These may accumulate after prolonged incubations in amounts sufficient to be toxic to developing rice seedlings. In these experiments the effects of acetic, propionic, and butyric acids were studied at 1, 5, and 10 mN on the growth and nutrition of 14 day old (Oryza sativa L.) cultivar ‘Earlirose’ rice seedlings. Nutrient solutions were used in the experiments with pH controlled at 6.5 in one experiment and in another the acid concentrations were allowed to attain equilibrium pH with the nutrient solution (1 mN=4.6, 5 mN=3.9, and 10 mN=3.8).

Application of Beaufils' diagnostic indices to maize data published in the literature irrespective of age and conditions

Abstract: Recently Beaufils published a monograph entitled Diagnosis and Recommendation Integrated System (DRIS) in which he presented complete details of his experimental approach and calibration technique for making fertilizer and other practice recommendations for crops. The relevant literature on maize published by various workers throughout the world is studied in an attempt to show the general validity of this particular system. The data of various workers have been recalculated in terms of DRIS indices from which it can be shown that these indices introduce a distinct refinement in foliar diagnosis. This refinement consists in being able to diagnose intensities of plant requirements in terms of yield responses for any particular nutrient at any stage of plant growth. In all cases DRIS indices have been superior to those criteria used by the various authors in diagnosing nutrient requirements and uptake in maize. re]19750922

Ecological aspects of nitrogen fixation by purshia tridentata

Abstract: This study examines several aspects of nitrogen fixation by Purshia tridentata (Pursh) D.C., a rosaceous shrub widespread in the Central Oregon pumice region, especially as an understory species in Pinus ponderosa and Pinus contorta forests. Acetylene reduction was used to assay nodule activity in both field and greenhouse plants. The maximum rates were observed at 20°C, although summer soil temperatures were frequently around 15°C, at which a much lower rate was observed. Acetylene reduction by excised nodules was linear for 5 h and then slowly declined, finally ceasing after 19 h. Nodule activity declined in water stressed plants, essentially ceasing in plants with xylem pressure potentials below −25 bars. Field studies at five sites revealed that nodule activity began in mid-May or early June when soil temperature at 20 cm increased to above 10°C. Activity began later and remained lower until July 20 in plants located under Pinus contorta, probably because of the cooler temperatures at this site. Nodule activity at all sites was maximum in June and July. In late July, nodule activity declined sharply, corresponding with moisture stress readings in the −25 bar range. Acetylene reduction rates declined sharply during the night; this decline was even more severe late in the season. Only 46 per cent of Purshia plants were nodulated. Several possible explanations for this low nodulation are discussed, but the primary reasons appear to be low soil temperature and unfavorable moisture conditions. Previous speculations that Purshia may contribute significant amounts of nitrogen to the ecosystems in which it occurs are disputed using estimates based on seasonal acetylene reduction rates and a determination of nodule biomass at one site. The estimated nitrogen accretion rate was only 0.057 kg N/ha.yr. re]19760310

Mineral nutrition and reduction processes in the rhizosphere of rice

Abstract: The influence of nitrogen, phosphorus, and potassium on the reduction processes in the rhizosphere of rice grown in solution culture and of rice under lowland conditions was studied. In solution culture the redox potential in the complete nutrient solution was highest, indicating that fully nourished roots have the highest oxidizing power. When the supply of only one element was interrupted, the lack of potassium in the nutrient solution caused the greatest decline in redox potential. Redox potential was further decreased when, besides nitrogen, either phosphorus or potassium was discontinued. Simultaneous deficiencies of nitrogen and potassium lowered redox potential even more severely than did deficiency of all three elements. A long-term nitrogen fertilizer trial under lowland conditions, however, revealed that an abundant supply of nitrogen can decrease redox potential. Redox potential was higher in the soil near plants than in the soil away from plants. In solution culture, at low Eh levels, the increase in iron reducing power of the solution was correlated with the decrease in redox potential. The total number of bacteria and iron reducing bacteria increased almost parallel to the decrease in redox potential and increase in iron reducing power. These relationships show that the nutritional status of the rice plant essentially influences bacterial activity and, thus, oxidation-reduction conditions around the roots. Since sufficient potassium nutrition seems important in maintaining the oxidising power of rice roots, root growth in a potassium deficient soil with K application was compared with root growth without K application. Without potassium the fine lateral roots far from the root base showed black coloration due to ferrous sulfide, indicating a loss of oxidising power. With increasing potassium supply, this phenomenon disappeared and the iron content of the rice plants decreased. re]19751208

The effect of vesicular-arbuscular mycorrhizal associations on growth of cereals: III. Effects on barley growth

Abstract: The growth of barley seedlings which were already mycorrhizal when planted in the field in soil deficient in phosphorus was much better than that of the non-mycorrhizal plants. Mycorrhizal plants removed more phosphorus from the soil and had greater dry matter. Yield was increased four fold by the fungus, but weight of individual grain was not affected. Differences between mycorrhizal and non-mycorrhizal barley plants were almost eliminated by the application of phosphate fertilizer indicating that the fungus does not enhance barley growth in soils containing enough available phosphate. It seems that the mycorrhizal effect is primarily to improve the supply of phosphate. Endogone spore number, mycorrhizal development, root infection and increased growth showed positive correletion with each other. The extent of root infection was greatest in mycorrhizal plants in soil not supplemented with phosphate and it decreased in inoculated plants in the plot supplemented with super phosphate. Number of tillers and ears per plant was more than double in mycorrhizal plants as compared to non-mycorrhizal plants. re]19751006

Regrowth of western wheatgrass utilizing 14C-labeled assimilates stored in belowground parts

Abstract: Results of this study showed that carbohydrates stored in the roots of western wheatgrass are utilized for regrowth following clipping of the aboveground foliage. Shoots remained dependent on carbohydrates stored in roots until sufficient photosynthetic leaf surface was developed to supply carbon to the shoots. During early phenophases, the partitioning of carbohydrates between shoots and roots was identical, indicating equal metabolic demands for carbon from both the shoot and root systems. Subsequent fluctuations in root and shoot carbohydrates may be caused by selected pressure imposed on either the root or shoot systems by physiological changes in these organs.

Effect of clay, organic matter and CaCO3 content on zinc adsorption by soils

Abstract: Zinc adsorption was studied in suspensions of six soils of different physicochemical characteristics in dilute ZnSO4 solutions. At low concentrations, Zn2+ adsorption was described by the Langmuir adsorption equation. The calculated Langmuir adsorption maxima were related positively to clay and carbonate content and negatively with organic matter content of soils. Multiple regression analysis revealed that zinc adsorption maxima can be predicted with good precision from information in soil survey reports.

Changes in n and p availability and p fractions in iwo soil from nigeria under intensive cultivation

Abstract: Two N-P factorial experiments, one on a site previously in bush fallow and the other previously in arable crops, were used to follow changes in the P status of Iwo soils under continuous cultivation. Marked yield response of maize to N was found at both sites, with the response being relatively small in the early crop at the fallowed site. Yield response to P was found only at the cropped site and then only if N was applied. About 25% of the organic P in the surface soil was mineralized during the two cropping periods of the rainy season which resulted in release of about 3 times as much P as was taken up by the maize. Most of the mineralized P as well as any added P was converted to Fe and Al-P. Apparently the rate of organic P mineralization in this soil is rapid enough for a few years following clearing to maintain adequate P activity for plant growth whereas N deficiency shows up very early.

Micronutrient availability to cereals from calcareous soils

Abstract: The mechanism of higher grain production of wheat (Triticum aestivum L.) by four tillage methods was explored on a sandy calcareous soil of Sahl-Al Jafara, Libya, during the year 1976–77. Tillage methods increased grain yield and dry matter weight at the boot stage in the order of no-tillage

A comparative study of metal levels in plants from two contrasting lead mine sites

Abstract: An investigation into the levels of metals in plants growing on metalliferous soils was carried out. The exchangeable metal concentration of soils from Tyndrum and Trelogan was found to be extremely variable from sample to sample and the influence of soil pH on the exchangeable lead and zinc concentration is discussed. Large differences in heavy metal levels were found between species and may be indicative of different mechanisms of tolerance to lead and zinc excess. Large differences in calcium levels between plant tissues were also found. These may result from the use of calcium in ameliorating lead and zinc toxicity in some species.

Variations among woody plants in stomatal conductance and photosynthesis during and after drought

Abstract: Acer saccharum, Fraxinus americana, Juglans nigra, Acer rubrum, Cornus amomum, and Ulmus americana seedlings were subjected to a soil drying cycle and then rewatered. At frequent intervals during the drying cycle and following rewatering, determinations were made of equilibrium photosynthesis rates, leaf conductances and leaf water potentials. As the drying cycle progressed, leaf water potentials decreased, stomata closed, and rates of transpiration and photosynthesis were reduced. Stomata of the two Acer species initially were more sensitive to water stress than were those of the other species. At low leaf water potentials, stomata of Juglans and Cornus were more open than those of the other species. Photosynthesis of Acer saccharum, Fraxinus and Juglans was significantly reduced by plant water stress, while photosynthetic water use efficiency of Cornus and Juglans was most unfavourable. Photosynthesis/leaf conductance ratios in water stressed leaves were higher in Fraxinus than in the other species. Immediately after rewatering, only limited stomatal opening occurred in Acer saccharum and Cornus with recovery of stomatal opening most protracted in Fraxinus and Ulmus. There was extended reduction of photosynthesis of all species as a result of the soil drying treatment. This effect was most significant in Acer saccharum and Juglans. Survival of plants on moist and dry sites is discussed in relation to stomatal control of transpiration and metabolic responses to water stress.

Influence of some environmental factors on nitrogen fixation in the rhizosphere of rice

Abstract: Nitrogen fixers make up a large percentage of the total microflora in the rhizosphere of lowland rice. There are more aerobic nitrogen fixers than there are anaerobic ones.