Interacting effects of grazing and nutrient addition on temporal stability of grassland productivity

CAMPANA, MS; TOGNETTI, PM; YAHDJIAN, ML

Congreso; 105th ESA Annual Meeting. Virtual.; 2020

Background/Question/Methods: Human activities are increasing the density of domestic grazers and global nutrient loads, modifying the main determinants of vegetation community dynamics. Changes in grazing intensity and plant nutrient availability may interact to control plant productivity. Additionally, the response of plant productivity to grazing and nutrient availability interaction it is expected to change over the years and to affect forage temporal (inter-annual) stability, which is particularly important in grasslands devoted to livestock production. However, the specific mechanisms that modulate the combined effects of grazing and fertilization on plant productivity and its temporal stability are still poorly understood. Our objective was to study the interaction between livestock grazing and nutrient addition on grassland plant productivity and temporal stability, and to evaluate the mechanisms involved in their response. We carried out a factorial experiment of grazing (cattle exclusion) and fertilization (10 g.m-2.year-1 of nitrogen, phosphorus, and potassium + micronutrients; n = 6 blocks) during 5 consecutive years in a natural grassland devoted to livestock production (Flooding Pampa, Argentina), following the Nutrient Network experimental protocol (https://nutnet.org/). Results/Conclusions: We found that different mechanisms modulate plant productivity and stability responses to the combined effect of grazing and fertilization. Grazing reduced live aboveground plant biomass by 54%, and when combined with fertilization this reduction was 70%. Nutrient addition under grazing increased plant productivity (ANPP) by 103%, plant tissue nutrients, and promoted legume abundance. These changes in turn produced a ten-fold increase in plant consumption by cattle. By contrast, fertilization did not produce any significant effect on plant biomass or ANPP inside the exclosures, probably because plant growth was light limited. Grazing increased ANPP temporal stability by 188%, which was related to higher plant species diversity. Finally, the abundance of Schedonorus arundinaceae (tall fescue, a perennial exotic species), which increased an order of magnitude by grazing exclusion and 41% by fertilization, were related to lower ANPP stability. Fertilization is a recommended strategy to improve forage production. However, our results suggest that this practice could have negative consequences on long-term grassland sustainability, as fertilization intensified plant consumption by cattle and promoted the abundance of an exotic grass that reduced stability of plant productivity. Therefore, our work represents a fundamental step toward the development of effective conservation and management strategies in grasslands.