This paper reviews a growing body of literature on the use of variations in the natural abundance of 15N to estimate the fractional contribution of N2-fixation to N2-fixing systems. This method is based on the small difference in 15NN abundance which frequently occurs between N derived from N2-fixation and N derived from other sources. The requirement of the method is that this difference be significant. Whether this requirement is met is site specific and must be empirically established at each site of interest. Advantages and disadvantages of this method are compared with those of more conventional methods. Sources of error, including heterogeneity of 15NN abundance of non-atmospheric N sources are considered. Tests of the method, under both greenhouse and field conditions, are described. Estimates based on this method compare favourably with other methods for field evaluation of N2-fixation, provided that the site and the sampling strategy are appropriate for application of the method. Applications of the method in several ecosystems are described.

Foliar samples were obtained from symbiotic nitrogen-fixers and control plants (non-fixers) along elevational and primary successional gradients in volcanic sites in Hawai'i. Most control plants had negative δN values (range-10.1 to +0.7‰), while most nitrogen-fixers were near 0‰. Foliar N in the native tree Metrosideros polymorpha did not vary with elevation (from sea level to tree-line), but it did increase substantially towards 0‰ on older soils. The soil in an 197-yr-old site had a δN value of approximately-2‰, while in a ∼67000-yr-old site it was +3.6‰. We suggest that inputs of N-depleted nitrogen from precipitation coupled with very low nitrogen outputs cause the strongly negative δN values in non-nitrogen-fixing plants on early successional sites.

The N abundance of tissues of five Prosopis specimens at our primary study site (a Prosopis woodland at Harper's Well in the Sonoran desert of Southern California) was determined over two growing seasons 1980 and 1981. TheN abundance of soil and of tissues of presumed non-N-fixing (control) plants was also measured. Prosopis tissues were significantly lower in N than either soil N or corresponding tissues of presumed non-N-fixing plants which derive their N entirely from soil. Soil N was also significantly higher in N than atmospheric N. We conclude that it is feasible to use variations in the natural abundance of N as an index of N-fixation in this kind of ecosystem, and that N-fixation is of considerable importance to Prosopis growing at this site.We also determined the N abundance of leaf tissue of presumed N-fixing and control plants growing at the same site at six additional sites (five in the Sonoran desert of southern California and one in Baja California, Mexico near the town of Catavina). Four of these additional sites were dominated by Prosopis and two were mixed communities. There were statistically significant differences between the N abundances of the pooled legume population and control plants at all sites, although not every legume specimen exhibited this difference. FromN abundance data we estimated the fractional contribution of biologically fixed N to the N economy of desert legumes. We concluded that N-fixation is very important to Prosopis at six of seven sites in the Sonoran Desert. At the site where Prosopis did not appear to be fixing N, N-fixation was important only for legumes of the sub-family Papilionoideae, Lupinus, Dalea, Astragalus and Lotus.

N2-fixing root nodules of soybean (Glycine max L. Merr.) convert atmospheric N2 to ammonia(um) in an energy-intensive enzymatic reaction. These nodules synthesize large quantities of purines because nitrogen fixed by bacteria contained within this tissue is transferred to the shoots in the form of ureides, which are degradation products of purines. In animal systems, it has been proposed that proline biosynthesis by pyrroline-5-carboxylate reductase (P5CR) is used to generate the NADP+ required for the synthesis of the purine precursor ribose 5-phosphate. We have examined the levels, properties, and location of P5CR and proline dehydrogenase (ProDH) in soybean nodules. Nodule P5CR was found in the plant cytosol. Its activity was substantially higher than that reported for other animal and plant tissues and is 4-fold higher than in pea (Pisum sativum) nodules (which export amides).The Km for NADPH was lower by a factor of 25 than the Km for NADH, while the Vm. with NADPH was one-third of that with NADH. P5CR activity was diminished by NADP+ but not by proline. These characteristics are consistent with a role for P5CR in supporting nodule purine biosynthesis rather than in producing proline for incorporation into protein. ProDH activity was divided between the bacteroids and plant cytosol, but <2% was in the mitochondria-rich fractions. The specific activity of ProDH in soybean nodule bacteroids was comparable to that in rat liver mitochondria. In addition, we propose that some of the proline synthesized in the plant cytosol by P5CR is catabolized within the bacteroids by ProDH and that this represents a novel mechanism for transferring energy from the plant to its endosymbiont. N2 fixation in legumes is a symbiotic process in which bacteria of the genus Bradyrhizobium or Rhizobium infect root cells and form specialized organs (nodules) within which N2 is reduced to NH'. Fixation of N2 is an energyintensive process, requiring a total of 25-30 ATP per N atom fixed. Of this total, 12-14 ATP per N are required within the bacteroid to reduce N2. As much as 10-30% of the total photosynthetic capacity of the plant is used to support this process (1). The energy-yielding metabolite(s) supplied by the host to the bacteroid, the symbiotic form of the bacterium, is not known. However, one attractive suggestion is that bacteroids import and oxidize a nitrogenous compound, such as glutamate (2). The nitrogen fixed in the bacteroid is exported as ammonia(um) to the infected host cell, where it is packaged for export to the rest of the plant. Legumes of temperate origin (e.g., peas) export nitrogen as amides (principally asparagine), whereas legumes of tropical origin (e.g., soybeans) export the ureides, allantoin, and allantoic acid. Ureide biogenesis proceeds by way of synthesis of purine ribonucleotides (3). This pathway is the same as that found in microorganisms, fungi, and animals. The purines so formed are then oxidatively degraded to ureides (3). The estimated peak rate of de novo purine biosynthesis necessary t...

A small minority of farmers in the Midwest produces crops on a commercial scale without using modern fertilizers and pesticides. On the basis of a 5-year study, it appears that these farmers have more in common with the majority of farmers in the region than with certain stereotypes of organic farmers. Their farming practices (other than chemical use), the size and labor requirements of their farms, and the production and profitability they achieve differ from those of conventional farmers by considerably less than might be expected on the basis of the fundamental importance of chemicals in modern agricultural production. Compared to conventional methods, organic methods consume less fossil energy and cause less soil erosion, but have mixed effects on soil nutrient status and grain protein content.

Measurements of nitrate concentration and relative enrichment in nitrogen-15 were made on samples of the surface waters of a typical Illinois corn belt watershed and the effluent of the subterranean tiles that drain the cropped land in the region. From these measurements, we estimate that at the time of peak nitrate concentration in the spring of 1970 a minimum of 55 to 60 percent of the nitrogen found as nitrate in the surface waters of this watershed originated from fertilizer nitrogen

One hundred thirty‐nine soil samples from 20 states were analyzed for 15N abundance. Soil characteristics and environmental conditions at the sampling sites varied widely. The total N of surface soil samples had a mean δ15N value (per mill 15N excess) slightly but significantly higher than the mean value for soils collected from deeper layers, although the relationship between δ15N and soil depth was not consistent from location to location. Differences among mean δ15N values for the total N of soil samples collected from cropland, pasture, uncultivated land (with native herbs), and forest were not striking. There was no systematic effect of variation in the rate of application of N fertilizer on the δ15N of the total N of soils. The mean δ15N value of surface soils, with respect to atmospheric N, was +9.22 and the standard deviation was 2.10 δ15N units. The range within which 90% of the samples fell was +5.1 to +12.3 δ15N units. Less than half of the variation among the soils in δ15N of the total N could be accounted for by differences in environmental variables or soil characteristics.

Isotopic fractionation associated with N2 fixation and NO3-uptake by plants are relevant to the accuracy of estimates of N2 fixation based on differences in the natural abundance of "N between N2 fixing and nonfixing plants. The isotope effect on N2 fixation by soybeans (Glyine max ILI Merrill, variety Harosoy) and red clover (Trifolium pratense ILI) was determined from the difference in "N abundance between atmospheric N2 and the total N of plants grown hydroponically with N-free nutrient solution. In soybeans the isotope effect was found to be +0.98 ± 0.18%o (f? = 0.99902). In clover the isotope effect was +1.88 ± 0.14%o (( = 0.99812). The magnitude of these inverse isotope effects is small. However, they would lead to an underestimation of the amount of N2 fixed, since the N of atmospheric origin which finally appears in the plant is made richer in "5N by the inverse isotope effects than is atmospheric N2, and, to that degree, is attributed to soil-derived N in the calculation.Isotopic fractionation associated with NO3-uptake by plants does not have a critical effect on estimates of N2 fixation which are based on natural abundance of "5N since the l"N abundance of soil-derived N in plants is measured directly (Le. after the N has undergone fractionation). Nevertheless, such fractionation is of some interest from the point of view of deciding upon the most appropriate sampling time. The isotope effect on NO3-uptake by a nonnodulating isoline of soybeans (variety Harosoy), marigold (Tagetes erecta ILl) and ryegrass (Lolium perenne ILI) was estimated from the difference between the "N abundance of the total N of plants grown hydroponically and that of NO3-supplied in the medium. It was found to be about -5%o (8 = 1.005).A number of investiators have reported a small, but measureable, difference in the 5N abundance of symbiotic N2-fixing and nonfLxing plants growing at the same site (1,2,6,7,12,15,18). The difference is always in the same direction, with the N2-fixing plant being less abundant in "N than nonfixing plants. This difference has led to the suggestion (6) that the "N abundance of plants may be used as an index of the contribution of atmospheric N2 to the N economy of plants or plant communities.Such an index may be useful in estimating the contribution of symbiotic N2 fixation integrated over time despite the fact that the acetylene reduction assay (8) is considerably more sensitive in detecting the presence of symbiotic N2-fixing activity. Estimating the contribution of symbiotically fixed N2 over the lifetime of the plant or within an ecosystem poses very different problems from estimating the instantaneous rate of nitrogenase activity from the rate of acetylene reduction. Among the problems in using the 'The work reported in this paper was supported by Grant DEB 77-01869 from the National Science Foundation. 51 acetylene assay for the former purpose are (a) conversion of acetylene reduced to N2 reduced (4); (b) large environmental, diurnal and seasonal variations (8); (c) the determination...