Laura Yahdjian, Pedro M. Tognetti, Enrique J. Chaneton
Land-use changes often involve the abandonment and natural re-vegetation of former agricultural fields, a process perceived as an opportunity for conservation of biodiversity and ecosystem services such as carbon sequestration. In theory, the speed of plant litter decay and nutrient cycling should decelerate as weedy plant species colonizing old fields are replaced by more competitive, longer-lived plant species. We challenged this idea for modern grasslands where invasion by fast-growing exotic grasses interferes with the recovery of native perennial species, creating ‘novel’ ecosystems with accelerated carbon and nutrient cycling rates.
A set of experimental plant communities was established from seed to mimic early, mid and late stages of the vegetation succession that occurs after field abandonment in the eastern Inland Pampas of Argentina. We had previously shown that such post-agricultural grasslands may end up being dominated by native or exotic grasses depending on site management. In the field, we studied the rate of decomposition and nitrogen recycling for senescent material (litter) produced by the dominant plant species in each community using the ‘litterbag’ technique, which measures mass loss through time (as determined by the amount of carbon consumed by soil microbes) for a known litter type and quantity.
As predicted, litter decay rates decreased, while nitrogen retention increased, from early through mid to late community stages dominated by broad-leaved weeds, annual grasses and native perennial grasses, respectively. However, litter decomposition and nitrogen cycling were faster in communities dominated by exotic perennial grasses than by their native perennial counterparts, but similar to those in annual grass communities. The observed changes in litter carbon and nitrogen cycling across experimental plots were mainly accounted for by differences in the species’ litter chemical quality for soil microbes. Thus, contrary to theory, a dominance shift from native to exotic perennial grasses prevented the deceleration of materials cycling expected with time following abandonment from agriculture. These findings illustrate how invasion by fast-growing, nutrient-rich exotic grasses may create novel grasslands that are less prone to tie-up carbon and nutrients in plant and soil compartments.
Image captions: Experimental plant communities set up to mimic stages of vegetation succession that occurs in old field grasslands (Inland Pampa, Argentina), and litterbags used in one of the decomposition experiments (upper right). Photo provided by authors.