Nitrogen addition alters photosynthetic carbon fixation, allocation of photoassimilates, and carbon partitioning of Leymus chinensis in a temperate grassland of Inner Mongolia

Abstract

Atmospheric nitrogen (N) deposition is sharply increasing, and this may have substantial impact on carbon (C) fixation and allocation of plants. Knowledge of the effect of N addition on C fixation and allocation patterns of assimilated C is therefore critical to understand the effects on C cycles in terrestrial ecosystems. We conducted a field experiment to examine the effects of N addition (0 to 25 g N m−2 yr−1) on photosynthetic C fixation and C allocation by Leymus chinensis using 13CO2 pulse-labeling and measurements of the percentage of assimilation allocated to nonstructural carbohydrates (NSCs), secondary metabolites (SMs), and growth in a temperate semi-arid grassland. Moderate N addition (5 and 10 g N m−2 yr−1) significantly increased the δ13C value and 13C fixation of plant leaves by increasing Pn and biomass of L. chinensis. However, high N addition (25 g N m−2 yr−1) did not result in a further increase in Pn and total biomass, suggesting excess N inputs was harmful to photosynthetic C fixation in plants. Moderate N addition (10 g N m−2 yr−1) also significantly increased the root/shoot ratio and the proportion of assimilated 13C allocated to roots, but decreased the proportion of leaves and stems allocated, indicating more C was allocated to roots with N addition. We further found that moderate N addition increased the overall NSC and SM concentrations, but C allocation to growth decreased with increasing N, possibly indicating the existence of a trade-off between the C allocation to defense and the allocation to storage and growth. Our findings demonstrated that N addition will alter photosynthetic C fixation and the allocation patterns of photoassimilates, and thus will significantly affect the C cycle and C balance of terrestrial ecosystems under predicted future global changes.