Estimates of N2-fixation from variation in the natural abundance of 15N in Sonoran desert ecosystems

Abstract

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.

Tansley Review No. 9515N natural abundance in soil‐plant systems

“…Deiwiche et al (1979), Virginia & Delwiche (1982), Shearer et al (1983), Hansen & Pate(1987), Yoneyama e? a/.…”

Imprint of denitrifying bacteria on the global terrestrial biosphere

“…Here, we use the isotope composition of N ( 15 N/ 14 N) to constrain the transfer of this nutrient from the land to the water and atmosphere. We report that the integrated 15 N/ 14 N of the natural terrestrial biosphere is elevated with respect to that of atmospheric N inputs. This cannot be explained by preferential loss of 14 N to waterways; rather, it reflects a history of low 15 N/ 14 N gaseous N emissions to the atmosphere owing to denitrifying bacteria in the soil.…”

“…We report that the integrated 15 N/ 14 N of the natural terrestrial biosphere is elevated with respect to that of atmospheric N inputs. This cannot be explained by preferential loss of 14 N to waterways; rather, it reflects a history of low 15 N/ 14 N gaseous N emissions to the atmosphere owing to denitrifying bacteria in the soil. Parameterizing a simple model with global N isotope data, we estimate that soil denitrification (including N2) accounts for Ϸ1/3 of the total N lost from the unmanaged terrestrial biosphere.…”

“…We show in Fig. 1B that the ␦ 15 N of bulk nitrate deposition observed for various latitudes (Table S4), altitudes and biomes (from deserts to tropical rainforests; n ϭ 187) approach a median of Ϫ1.3‰, with most values clustered between Ϫ3 and 1. Although ammonium and dissolved organic N compounds can also contribute deposition, their 15 N/ 14 N ratios either overlap with or are somewhat 15 Ndepleted with respect to that of nitrate in bulk precipitation (7,12,13). We therefore infer that there must be a pathway of N loss that preferentially removes 14 N from the land, acting to elevate the terrestrial biosphere's 15 N/ 14 N over the long term.…”

“…NЈs isotopes, 14 N and 15 N, are stable but display a high degree of variation in biogenic materials owing to isotope fractionation, particularly kinetic fractionation, whereby organisms' enzymes more rapidly transfer the light isotope from substrates to products. 15 N is rare (Ϸ0.37%), and small deviations in its concentration relative to 14 N have been widely used in the examination of N cycling on land and in the sea (1). In the case of the N isotope composition of the terrestrial biosphere, we consider a steady-state approximation to examine the partitioning of global N losses: Fig.…”