Plant quality and primary productivity modulate plant biomass responses to the joint effects of grazing and fertilization in a mesic grassland

Prepared by Sofía Campana & Laura Yahdjian

(a) Experimental exclosures used in the study site (Flooding Pampa, Argentina) to evaluate the effect of domestic grazing. Plant biomass accumulates inside the exclosure (left) in comparison with the grazed grassland (right). (b) Mobile cage used to estimate cattle consumption and aboveground net primary productivity in the grazed grassland. Photo credit: Sofía Campana.

Grassland ecosystems are used for livestock production worldwide, being one of the human activities responsible for land-use changes. Grasslands are also exposed to the global alteration of nutrient budgets. In theory, the increments of limiting nutrients increase aboveground plant biomass. We challenged this idea for modern grasslands used for livestock production, where grazing may counterbalance the response of plant production to nutrients with uncertain results for grassland standing crop.

To explain plant biomass responses to increased nutrient loads in grazed grasslands, we established an experiment of fertilization and cattle exclosures in the Flooding Pampas, a vast 90,000 km2 region in the Buenos Aires province, east-central Argentina, which supports national livestock production. In the field, we studied the rate of plant production, plant nutritional quality, and herbivores consumption for four years.

Temperate grassland of the Flooding Pampas devoted to livestock production (Buenos Aires province, Argentina). Photo credit: Sofía Campana.

As predicted, grazing significantly reduced standing crop, but when combined with fertilization, this reduction was even greater. Fertilization promoted aboveground net primary productivity as expected, but this effect was only evident in the presence of cows that increased ground-level light in comparison with the exclosures. Moreover, fertilization also increased leaf N and P concentration, a measure of plant nutritional quality for herbivore consumption. Grazers selected and consumed the extra biomass of higher nutritional quality produced in the fertilized plots, which led to a higher herbivore load in these nutrient-rich patches. Thus, grazers neutralized the plant production induced by fertilization, which explained the greater reduction in the standing crop of fertilized grazing plots compared with those unfertilized. In synthesis, the observed changes in aboveground plant biomass across experimental plots were mainly accounted for by differences in plant quality and rates of aboveground plant biomass production.

These findings illustrate how global change drivers may synergistically interact to produce novel outcomes from the controls of grassland standing crop. Contrary to theory, fertilization of natural grasslands devoted to livestock production led to decreases in standing crop, challenging the sustainability of temperate grasslands.

This is a plain language summary for the paper of Campana and Yahdjian published in Applied Vegetation Science (