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CHAPTER 2. HOW MUCH NITROGEN DO LEGUMES FIX?

CHAPTER 2. HOW MUCH NITROGEN DO LEGUMES FIX?

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16



THOMAS A. LARUE AND THOMAS G. PATTERSON



part to the ability of legumes, in symbiosis with rhizobia, to obtain nitrogen from

the air.

But how much nitrogen is obtained? The direction of research and the future

management of legumes in agriculture will require accurate knowledge of the

amounts fixed by crops in the field. If fixation is less than we now think, then we

will not achieve the expected returns of N. If fixation is as high or higher than

some reports say, we must search out the reason why some legume crops can

deplete soil N.

Recent work in plant physiology indicates that symbiotic fixation is not ‘‘free

fertilizer”; the plant must provide energy in the form of photosynthate. Whether

a legume crop “pays” for fixation with decreased yield is not yet determined.

Legumes require more phosphate fertilizer than cereals, and many are more

demanding of water. The cost of inoculant will not be negligible to cash-poor

farmers in developing countries. Does the N fixed compensate for these inputs?

Ultimately, plant breeders and agronomists will require accurate estimates of the

amount of fixation, and of its cost, to determine whether increasing fixation is

economically justified.

B. THEENERGYCOSTOF SYMBIOTIC

FIXATION

VERSUS

NITRATEUTILIZATION



Despite apparent differences, there is a fundamental similarity between symbiotic nitrogen fixation and the industrial production of nitrogen fertilizer.

Energy is required for both methods, for thermodynamics requires this of all

possible methods of fixation. Nitrogenase requires energy in the form of ATP

and electrons. In addition, there are energy costs associated with nodule formation and maintenance, hydrogen loss, and incorporation and transport of newly

fixed N. There must also be an energy cost for using soil nitrate, but comparisons

of the two have been difficult to examine experimentally.

Silsbury (1977, 1979) estimated the respiratory burden of subterranean clover

grown under artificial light with nitrate or NZ.He calculated the growth coefficients (the fraction of net C02uptake in light associated with the synthesis of new

dry matter) and found them constant over a 50-day period. For nodulated plants

they were significantly higher (0.189) than for plants using nitrate (0.137).

Nodulated plants used 810 mg C02 for the synthesis of 1 g dry weight, while

nonnodulated plants used only 510 mg C02.

Mahon (1977, 1978) calculated the energy cost of fixation after measuring root

respiration and estimating N fixation by acetylene reduction. He compared the

respiration of plants grown on N, with similar plants treated a few days previously with ammonium nitrate. It was assumed that the respiration due to



HOW MUCH NITROGEN DO LEGUMES FIX?



17



growth and maintenance was the same in both populations, and that the decreased root respiration in the treated plants was associated with decreased nitrogenase activity. Mahon obtained a value of about 6.7 g C/g N fixed with

soybeans, cowpeas, Phaseolus, and peas.

Ryle and his co-workers (1979) compared growth, photosynthesis, and shoot

and root respiration of soybean, clover, and cowpea grown on N2 or nitrate under

bright light. The plants provided with nitrate grew larger. Gross photosynthesis

did nor differ in the two populations, nor did shoot respiration. The fixing plants

had respiration rates about twice those of nitrate-grown plants. Expressed as a

percentage of the gross photosynthesis, the root respiration of fixing and nonfixing plants was 22 versus 11 for soybean, 27 versus 14 for cowpea, and 34 versus

21 for clover; that is, for three species, plants fixing their nitrogen respire

11-13% more of their photosynthate.

The relationship between root respiration and N fixed varied during plant

growth. In all species it was highest (- 15 g C/g N) in young plants. Presumably,

energy was used in forming nodules. In soybean and cowpea it dropped to a

minimum of 3-5 g C/g N just before the nodules senesced during pod fill. The

average cost was 6.3 g C/g N-a figure very close to Mahon’s.

It is remarkable that the same respiratory cost, approximately 6.5 g C/g N, was

determined by two investigators using five legumes. The close similarity

amongst species suggests that it may not be easy to find significant differences in

efficiency within a single species.

There is good evidence that photosynthate supply to nodules is a major limitation to symbiotic fixation (Hardy and Havelka, 1975). The carbon cost of 6.5 g

C/g N estimated by Ryle et al. and Mahon suggests that fixation of 1 kg NH,

would cost 15-20 kg dry weight. In evaluating some of the extraordinarily high

claims for hundreds of kilograms of N per hectare from symbiotic fixation, one

should question whether the crop is capable of fixing the required carbon and

translocating it to the root.

Would a nodulated legume crop ever yield less than one obtaining all its N

from soil? The common observation on soils very low in available N is that

effective nodulation increases yield compared to uninoculated controls. In soils

of very high fertility, the soil N suppresses nodulation and fixation, and yields

are generally equal. The aforementioned experiments, however, indicate that on

soils of intermediate fertility, the energy cost of nodule formation and fixation

might lower the yield of the crop obtaining some of its N via the symbiosis,

compared to an uninoculated control.

Such a result was observed in a Brazilian soil in which soil N was not limiting

to growth. With Phaseolus cultivars there was a positive correlation of nodule

weight with plant N content but a negative correlation with grain production

(Pessanha et al., 1972). This is what is expected if fixation requires more



18



THOMAS A . LARUE AND THOMAS G. PATTERSON



photosynthate than nitrate utilization. We did not find similar results elsewhere in

the literature. It may be that the difference in energy cost between nitrate and N2

usage is not so great in the field as in lab experiments.



C. PUBLISHED

ESTIMATES

OF NITROGENFIXATION

BY

LEGUME

CROPS



In reviews and texts there are many published tabulations of the amount of

fixation by legume crops. Most are derived from a very few sources. Two

favorites are publications by Erdman (1949) and Lyon and Bizzell(l933, 1934).

The first was an extension pamphlet promoting inoculation, and containing data

that were not substantiated by methodology. Erdman stated that his estimates had

been calculated in most cases from controlled pot experiments which when

magnified to an acre basis gave results higher than the actual figures. Unfortunately others citing his figures did not include his caveat. The study of Lyon and

Bizzell, as we will see, did not approximate the field crop condition and probably

overestimated fixation.

Bums and Hardy (1975) averaged a great many published estimates to arrive at

an average figure of 140 kg N fixed per year per hectare of arable land under

legumes. Shortly thereafter, that figure was considered by a group of scientists

attending a conference on nitrogen-fixing microbes. They concluded (Burris,

1978) that a realistic figure would be half the Bums-Hardy calculation. This

reassessment, however, was apparently based as much on intuition as on new

data.

In the literature much of the work reported on estimating fixation cannot be

extrapolated to field conditions. A very common procedure, especially when

isotopic N is used, is to grow legumes in small pots, often in the greenhouse. The

substrate is so unlike the soil and the conditions of plant growth so different from

the field that the results cannot be used to calculate fixation by crops. Many

experimenters, including those using field plots, give results only as milligrams

N per plant or percentage N of the yield or percentage plant N derived from

fixation. In the absence of information about yield per area or planting density, it

is impossible to calculate the fixed N per hectare.

The published results of fixation as a percentage of plant N may, however, serve

in estimating a realistic figure for fixation in farm crops. It is a common observation that yields in experimental or demonstration plots are much higher than the

average crop yields in the area. It is likely true that symbiotic fixation is also less

on the farm. Nodule formation and function are depressed by a variety of environmental factors-water stress, flooding, herbicides, improper fertilizer

placement, etc. It is unlikely that the percentage N from fixation will be higher

on farms than on well-tended research plots. Therefore the percentage N fixed in



HOW MUCH NITROGEN DO LEGUMES FIX?



19



test plots might be used in calculating a realistic upper limit on fixation in

agriculture.



II. METHODS OF ESTIMATING FIXATION BY CROPS

A. NITROGEN

ACCUMULATION



The standard procedure for nitrogen analysis is the Kjeldahl determination

(Bremner, 1965). Its major advantages are simplicity and low expense.

The simplest estimate of N fixation is by total N accumulation of the crop.

This is based on the intuitive assumption that the crop derives all its N via

symbiotic fixation. We can find no published evidence that this is ever the case

under field conditions. The many estimates based on total plant N certainly

overestimate fixation.

Growth on low-fertility soils or on soils artificially impoverished in available

N is no guarantee that all N is obtained by fixation. Kohl et al. (1980) decreased

available N in a soil by the addition of 34 tonnesha of corn cobs. The nodulated

soybean neverthelessobtained 47% of its plant N from this soil, and a nomodulated

isoline yielded 2106 kg/ha of grain. These results demonstrate the ability of a

legume to scavenge soil N.

A closer approximation to N fixed may be achieved by analyzing changes in

soil N as well as that removed in the crop. The frequently cited study by Lyon

and Bizzel (1933, 1934) was such an approach. These workers placed a mixture

of 60% silty clay loam and 40% sand in outdoor frames of unstated dimensions.

Various crops were alternated or grown together for 10 years and the N in the

crops was assayed at each harvest. The N in the top 28 cm of substrate was

analyzed at the beginning and end of the trial, and the “apparent fixation of

nitrogen” was calculated on an annual basis.

There was an accumulation of soil N under clovers, alfalfa, and vetch, and a

decrease with soybeans, peas, beans, barley, rye, and oats. The clovers had an

apparent fixation averaging 166-200 kg N/ha. Alfalfa accumulated 296-330 kg

N/ha. Soybeans, peas, and beans averaged 125, 57, and 70 kg N h a annually,

respectively. Nonlegumes accreted 2 1-37 kg Nha.

This study is considered a classic for documenting the advantage to topsoil N

of proper forage-cereal rotations. However, its limitations are that the change in

soil N has based on only two estimates 10 years apart and that only the top layer

was analyzed. Legume roots may extend downwards 3 m (Weaver, 1926). The

unaccountably high “apparent fixation” by nonlegumes suggests that there was

appreciably mobilization of nutrients from below the sampled zone.



20



THOMAS A . LARUE AND THOMAS G . PATTERSON



Experiments on legumes growing on artificially impoverished soils are not

uncommon in the literature. The substrate used by Lyon and Bizzell was an

artificial one and for that reason the data they obtained should not be extrapolated

to estimate fixation by farm crops.

Long-Term Nitrogen Balance Studies Using Lysimeters



A lysimeter is an enclosed soil system in which the addition and removal of

nutrients, water, and plant material can be controlled and measured. Although

many designs have been described (Kohnke et al., 1940), the basic construction

consists of a tank or box placed in the ground and filled with soil. Leachate from

the soil in the tank is collected from the bottom of the lysimeter. Lysimeters have

been used in long-term studies on nitrogen balance in crops under different

management systems (Chapman et al., 1949). In addition, lysimeters can be

used to compare and calibrate different techniques for estimating N fixation

under controlled conditions (Williams et al., 1977).

Several disadvantages are inherent in lysimeter studies. They are expensive to

install because they require the excavation of large volumes of soil and must be

constructed of materials resistant to corrosion. Due to their expense, the size and

number of lysimeters, and thus the number of treatments and replications, is

limited.

In some lysimeter studies, long-term nitrogen balances have shown unaccounted losses of nitrogen (Collison et al., 1933). This is attributed to volatilization of N from the lysimeters (Chapman et al., 1949; Patwary and Raikavich,

1979), although this assumption has been challenged (Craswell and Martin,

1975a,b).

B. DIFFERENCE

METHODS



An adjusted measure of fixation by the nitrogen accumulation technique is

obtained when the contribution of soil N to the total N of legumes is estimated.

This correction for the contribution of soil N is obtained by growing a nonfixing

plant in comparison with the N-fixing legume. Total N content of the nonfixing

crop (derived solely from soil N) is subtracted from the total N content of the

N-fixing legume. The difference between the values is assumed to be the quantity

of N derived by N fixation. This procedure is often referred to as the “difference” method (Williams et al., 1977). Three versions of the difference

method are commonly used.

1 . Comparison of a Legume with a Nonlegume

Soil N contribution to a fixing legume is estimated by growing a nonlegume

concurrently with the legume. Annual small grains such as wheat and oats have



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