Arid community responses to nitrogen and carbon addition depend on dominant species traits and are decoupled between above- and below-ground biomass

Abstract

Questions: Arid communities are strongly limited by soil resources including water and nitrogen (N). Plants compete for N with other plants and microorganisms, which are also limited by carbon (C). We propose that above- and below-ground plant responses to soil resources are modulated by community structure (species relative abundances, “mass ratio hypothesis”) and species traits (relative growth rates — RGRs). We evaluated the single and combined effects of soil N and C addition on the above- and below-ground biomass accumulation of perennial grass patches in an arid community, and the mechanisms involved in their responses. Location: Patagonian steppe, Argentina. Methods: We added N (2 g N m−2; NH4NO3) and C (330 g C m−2; sucrose) to 1-m2 field plots in a factorial design. After two years, we harvested above-ground (n = 5 plots) and below-ground biomass (n = 10 soil cores) and sorted it by species. We measured potential soil respiration as a proxy of microbial activity. Results: Total above-ground biomass increased by 55% as a result of N and decreased by 45% as a result of C addition, in relation to controls. C addition reduced total below-ground biomass by 42%. The above-ground differences were associated with changes in the biomass of dominant species according to their RGRs. Poa ligularis (dominant, high RGR) increased by 92% as a result of N addition while Pappostipa speciosa (dominant, low RGR) decreased by 55% as a result of C addition. Intermediate and subordinate grasses did not modify their biomass, independently of their RGR. Potential soil respiration was three times higher in plots with C addition than in control plots. Conclusions: Community biomass was explained by a combination of mass ratio hypothesis and specific RGR, as dominant grasses controlled above-ground community responses to N (high-RGR species) and C addition (low-RGR species). Our findings highlight the independence between the above- and below-ground processes and the importance of considering community equitability and species characteristics to predict plant community responses to changes in soil resources.