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 increasing grazing and nutrient availability is expected to change over time and to affect forage temporal (inter-annual) stability, which is particularly important in the sustainability of grasslands devoted to livestock production. So far, the combined effects of grazing and fertilization on plant productivity and its temporal stability are still poorly understood.
Our objective was to evaluate the mechanisms involved in the interaction between livestock grazing and nutrient addition on grassland plant productivity and its temporal stability. We carried out a factorial experiment of 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
Grazing reduced aboveground plant biomass by 54%, and when combined with fertilization, this reduction reached 70%. However, nutrient addition in the grazed grassland increased aboveground net primary productivity (ANPP) by 103% and promoted legume abundance and plant nutrient concentration. These changes in turn produced a ten-fold increase in plant consumption by cattle, which explained the reduction in the aboveground biomass. By contrast, fertilization did not produce a significant effect on plant biomass or ANPP inside exclosures. Grazing increased ANPP temporal stability by 188%, which was explained by higher plant species diversity and lower dominance of tall fescue (Schedonorus arundinaceae), a perennial exotic species that dominated inside exclosures and increased by 41% with fertilization.
Here, we demonstrated that the interaction of grazing and fertilization modified ANPP temporal stability through changes in plant diversity and the abundance of an exotic grass species. Fertilization improved forage production, but our results suggest that this practice could have negative consequences on the sustainability of livestock production because it decreases ANPP temporal stability. Therefore, our study represents a fundamental step toward the development of effective conservation and management strategies in temperate grasslands.