Effects of Multiple Global Change Factors on Symbiotic and Asymbiotic N2 Fixation: Results Based on a Pot Experiment

Abstract

Biological N2 fixation, a major pathway for new nitrogen (N) input to terrestrial ecosystems, largely determines the dynamics of ecosystem structure and functions under global change. Nevertheless, the responses of N2 fixation to multiple global change factors remain poorly understood. Here, saplings of two N2-fixing plant species, Alnus cremastogyne and Cajanus cajan, were grown at rural and urban sites, respectively, with the latter representing an environment with changes in multiple factors occurring simultaneously. Symbiotic N2 fixation per unit of nodule was significantly higher at the urban site than the rural site for A. cremastogyne, but the rates were comparable between the two sites for C. cajan. The nodule investments were significantly lower at the urban site relative to the rural site for both species. Symbiotic N2 fixation per plant increased by 31.2 times for A. cremastogyne, while that decreased by 88.2% for C. cajan at the urban site compared to the rural site. Asymbiotic N2 fixation rate in soil decreased by 46.2% at the urban site relative to the rural site. The decrease in symbiotic N2 fixation per plant for C. cajan and asymbiotic N2 fixation in soil was probably attributed to higher N deposition under the urban conditions, while the increase in symbiotic N2 fixation per plant for A. cremastogyne was probably related to the higher levels of temperature, atmospheric CO2, and phosphorus deposition at the urban site. The responses of N2 fixation to multiple global change factors and the underlying mechanisms may be divergent either between symbiotic and asymbiotic forms or among N2-fixing plant species. While causative evidence is urgently needed, we argue that these differences should be considered in Earth system models to improve the prediction of N2 fixation under global change.