Ancient CO2 levels favor nitrogen fixing plants over a broader range of soil N compared to present

Haoran Chen,John Markham

doi:10.1038/s41598-021-82701-7

Abstract

Small inreases in CO2 stimulate nitrogen fixation and plant growth. Increasing soil N can inhibit nitrogen fixation. However, no studies to date have tested how nitrogen fixing plants perform under ancient CO2 levels (100 MYA), when nitrogen fixing plants evolved, with different levels of N additions. The aim of this study was to assess if ancient CO2, compared to present, favors nitrogen fixers over a range of soil nitrogen concentrations. Nitrogen fixers (Alnus incana ssp. rugosa, Alnus viridis ssp. crispa, and Alnus rubra) and their close non-nitrogen fixing relatives (Betula pumila, Betula papyrifera, Betula glandulosa) were grown at ancient (1600 ppm) or present (400 ppm) CO2 over a range of soil N levels, equivalent to 0, 10, 50, and 200 kg N ha−1 year−1. The growth of non-N fixing plants increased more than N fixing plants in response to the increasing N levels. When grown at an ancient CO2 level, the N level at which non-nitrogen fixing plant biomass exceeded nitrogen fixing plant biomass was twice as high (61 kg N ha−1 year−1) as the N level when plants were grown at the ambient CO2 level. Specific nodule activity was also reduced with an increasing level of soil N. Our results show there was a greater advantage in being a nitrogen fixer under ancient levels of CO2 compared with the present CO2 level.

Highlights

Small inreases in CO2 stimulate nitrogen fixation and plant growth
The effect of nitrogen on plant mass varied with CO2 level (P = 0.006, F = 8.03 for the nitrogen by CO2 level interaction in the least squares model), and differed between the nitrogen fixing and non-nitrogen fixing plants (P < 0.0001, F = 28.22 for the plant group by nitrogen level interaction), but the model showed no effect of species within plant groups (P = 0.20, F = 1.55, Supplementary Figure S1)
At 400 ppm CO2, plant biomass of the nitrogen fixing and non-nitrogen fixing plants was equal at a soil N deposition rate of 27 kg N ha−1 year−1, while at 1600 ppm C O2, the N level at which plant biomass was equal was at 61 kg N ha−1 year−1, more than double the N when plants were grown at 400 ppm C O2

Summary

Introduction

Small inreases in CO2 stimulate nitrogen fixation and plant growth. Increasing soil N can inhibit nitrogen fixation. The aim of this study was to assess if ancient CO2, compared to present, favors nitrogen fixers over a range of soil nitrogen concentrations. Crispa, and Alnus rubra) and their close non-nitrogen fixing relatives (Betula pumila, Betula papyrifera, Betula glandulosa) were grown at ancient (1600 ppm) or present (400 ppm) CO2 over a range of soil N levels, equivalent to 0, 10, 50, and 200 kg N ha−1 year−1. One reason for the present lack of success of nitrogen fixing plants may relate to the high energy cost of nitrogen fixation, especially at higher latitudes[9]. The loss of the nitrogen fixing trait with decreasing CO2 levels over geological time may relate to the energetic cost of fixation, with decreased C O2 limiting energy capture, and to plants ability to balance carbon and nitrogen acquisition. There is no study comparing nitrogen fixing plant growth at ambient and ancient C O2 levels

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