Grazing exclusion reduces below-ground biomass of temperate subhumid grasslands of South America: A meta-analysis and a database

Abstract

There are several plant below-ground biomass databases for grasslands, but the contribution from the southern hemisphere is minimal. Particularly, no study has compiled the published values of plant below-ground biomass for the temperate subhumid grasslands of South America. In these grasslands, called Río de la Plata grasslands, grazing is the most frequent disturbance. We set two objectives: (1) to compile and synthesize data of four variables: below-ground biomass, fraction of total biomass allocated below-ground, below-ground net primary productivity and below-ground biomass turnover rate; (2) to estimate the effect of grazing exclusion on below-ground biomass by means of a meta-analysis. There are 15 studies published on six out of eight regional sub-divisions of the Río de la Plata grasslands. The most evaluated variable was below-ground biomass, followed by below-ground net primary productivity, while the other two variables were scarcely registered. Below-ground biomass in the shallow portion of the soil (0–10 cm) was on average 877 g m−2. Below-ground net primary productivity resulted in an average of 245 g m−2 year−1 (estimated using the ingrowth core method) or 560 g m−2 year−1 (core method). The fraction of total biomass allocated below-ground was on average 0.75. Below-ground biomass turnover rate presented an average of 2.5 years. On average, the effect of grazing exclusion on below-ground biomass depended on exclusion time. When exclusion time was shorter than or equal to 3 years, grazing exclusion did not affect below-ground biomass. In contrast, when exclusion time was longer than 3 years, grazing exclusion reduced below-ground biomass by 62% (0–10 cm depth) and by 40% (total depth). Excluding South American grasslands from the global databases may introduce some biases in the observed patterns.