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

Prepared by Sofía Campana, María Fernanda Reyes and Martín R. Aguiar

Graphical abstract of the paper. At the top-left, there is also a picture of the Patagonian steppe (Chubut province, Argentina), where we conducted the experiment to test the individual and combined effects of soil C and N addition on the above- and below-ground biomass accumulation of the perennial grass community. From the original paper.

The scarcity of soil resources plays a vital role in shaping plant community dynamics, particularly in arid ecosystems. These communities are mostly composed of species with a high lignin/N ratio, which controls the balance between labile carbon (C) and nitrogen (N) availability and the competition among plants and between plants and soil microorganisms. We postulate that dominance and relative growth rate (RGR) control the response of above- and below-ground plant biomass. Firstly, the “mass ratio hypothesis”, which suggests that the most abundant species, dominants, have a significant impact on the overall community response of biomass accumulation, while subordinate species play a minor role in this process. Secondly, plant trait specific relative growth rate (RGR), which measures the increase in plant biomass within a specific time interval, controls the rate of nutrient absorption. In our study in the Patagonian steppe, we assessed the individual and combined effects of soil C and N addition on the above- and below-ground biomass accumulation of perennial grass patches. Furthermore, we aimed to elucidate the underlying mechanisms driving these responses in natural conditions.

Surprisingly, we observed different responses to soil C and N addition. Labile C addition reduced above- and below-ground plant community biomass by more than 40% in both portions, in relation to control plots. Conversely, N addition increased by 55% total above-ground biomass. These changes in the perennial grass community resulted from complex responses among different species. Only dominant plant species responded to C and N addition, aligning with the mass ratio hypothesis. Even more, the specific RGRs of these dominant species played a crucial role in determining the above- and below-ground biomass they produced. For instance, a dominant species with a high RGR (Poa ligularis) showed a 92% increase due to N addition, while a dominant species with lower RGR (Pappostipa speciosa) experienced a 55% reduction in response to C addition. Intermediate and subordinate grasses did not change their biomass, regardless of their RGR values. Notably, potential soil respiration, a proxy for soil microorganism activity, was three times higher in plots with C addition compared to control plots, suggesting that microbial N immobilization could account for the reduction in total above- and below-ground biomass of the perennial grass community in response to labile C availability.

One of the experimental plots (1 m2) in the Patagonian steppe. The picture shows the limits of the plot (white twine), the high proportion of bare soil, and the presence of different perennial grass species. Photo credit: Sofía Campana

One strength of our study was that we considered not only individual plant traits but also community attributes to accurately predict plant community dynamics in response to changes in soil resource availability for microorganisms (C) or for microorganisms and plants (N). Furthermore, the observed increase in above-ground biomass of the dominant species with a high RGR in response to N fertilization has direct implications for management practices. This species is a crucial forage resource for domestic grazing in these arid rangelands. However, overgrazing drives down its abundance, which, in turn, may potentially diminish the grass community’s response to changes in N availability.

This is a plain language summary for the paper of Campana et al. published in the Journal of Vegetation Science (