Divergence of above- and belowground C and N pool within predominant plant species along two precipitation gradients in North China

Abstract

The coupling of carbon cycle and nitrogen cycle drives the food web structure and biogeochemistry of an ecosystem. However, across precipitation gradients, there may be a shift in C pool and N pool from above- to belowground because of shifting plant stoichiometry and allocation. Based on previous evidence, biomass allocation to roots should increase with aridity, while leaf [N] should increase. If their effect sizes are equal, they should cancel each other out, and the above- and belowground proportions of the N would remain constant. Here, we present the first study to explicitly compare above- and belowground pool sizes of N and C within predominant plant species along precipitation gradients. Biomass and nutrient concentrations of leaves, stems and roots of three predominant species were measured along two major precipitation gradients in Inner Mongolia, China. Along the two gradients, the effect sizes of the biomass shifts were remarkably consistent among three predominant species. However, the size of the shift in aboveground [N] was not, leading to a species-specific pattern in above- and belowground pool size. In two species (Stipa grandis and Artemisia ordosica) the effect sizes of biomass allocation and [N] were equal and the proportion of N of above- and belowground did not change with aridity, but in S. bungeana the increase in leaf [N] with aridity was much weaker than the biomass shift, leading to a decrease in the proportion of N aboveground at dry sites. We have found examples of consistent N pool sizes above- and belowground and a shift to a greater proportion of belowground N in drier sites depending on the species. We suggest that precipitation gradients do potentially decouple the C and N pool, but the exact nature of the decoupling depends on the dominant species' capacity for intraspecific variation.