Effects of monoculture-conditioned soils on common tallgrass prairie species productivity

Abstract

Within biodiversity–ecosystem function experiments, it is widely understood that yields of some species rapidly decline when planted in monoculture. This effect may arise due to decreased access to soil nutrients or an increase in detrimental soil pathogens within monoculture plantings. To determine whether or not soil conditioning affects tall grass prairie species biomass production, we conducted a field experiment to assess species growth in conspecifically and heterospecifically conditioned soils and a greenhouse experiment to elucidate how conspecific soil biota affected species growth. To test for species-specific soil effects, seedlings of the legume Astragulus canadensis, the cool-season grass Elymus canadensis, the forb Helianthus maximiliani and the warm-season grass Panicum virgatum were grown in field plots that had either been conspecifically or heterospecifically conditioned over 2 years. Plant growth was recorded over a single growing season, and soils were assessed for differences in their nematode (mesofauna) communities. Seedlings of these species were additionally grown over a 6-week period in conspecifically conditioned soil that was either untreated, heated to 60°C, sterilized (autoclaved at 120°C) or heated to 60°C and reinoculated with conspecific soil biota. The two heating treatments were used to compare growth responses between a low- and high-temperature soil treatment. The reinoculation treatment was used to assess the effect of soil biota in light of any nutrient changes that may have occurred with soil heating. Elymus canadensis, H. maximiliani and P. virgatum growth was improved in field plots conditioned by the legume A. canadensis compared with their growth in conspecifically conditioned (home) soils. Despite variation (grass versus nongrass) in their soil nematode communities, there was no evidence to suggest that these three species were inhibited by conspecific or functionally conspecific soil conditioning in the field. Astragulus canadensis was the only species whose growth was reduced in conspecifically conditioned field soil. In the greenhouse, E. canadensis growth increased in all of the heat-treated soils, likely a response to a fertilization effect associated with soil heating. Panicum virgatum growth also increased among the heated soils. However, its growth decreased in heated soils where conspecific soil mesofauna were reintroduced, indicating that this grass may be inhibited by soil mesofauna. Finally, A. canadensis growth decreased in soils treated to fully remove soil biota and was not affected by reintroduction of soil mesofauna, suggesting that this species negatively responds to soil changes that occur with extreme heating. At least for the suite of tallgrass prairie species evaluated within this experiment, it appears that changes in soil chemistry and generalist soil biota, as opposed to increasing species-specific soil pathogens, more strongly contribute to temporal disparities in their performance.