Primary production in herbaceous systems is influenced by soil nutrient availability. Generally primary production increases with nutrient additions across many herbaceous systems. However the strength of this response can vary greatly across systems and the mechanisms driving this response remain unclear. We examined the effects of multiple resource additions (nitrogen, phosphorous, and potassium) on primary production at three sites spanning the precipitation gradient of the North American Central Great Plains. These sites are located within shortgrass steppe (driest), mixed grass prairie, and tallgrass prairie (wettest) ecosystems. Our objectives were to: (1) identify potential interactive effects between nutrient and water availability on primary production and (2) determine the effects of nutrient additions on the dominant grass and forb species at each site. In this way we were able to identify species-specific responses to nutrient additions and determine whether these responses are driving the overall response in primary production to nutrient additions observed across the Great Plains.
Across all sites, we found that the effect of nutrient additions was highly dependent on water availability, with nutrient additions resulting in an increase in primary production only when water was not limiting. Primary production increased with N and P additions at the high precipitation site (tallgrass prairie), but not in the lower precipitation sites (mixed grass prairie and shortgrass steppe). However, primary production did increase with both N and P additions during an above-average precipitation year in shortgrass steppe. In tallgrass prairie, the increase in biomass with N and P additions appeared to be driven by an increase in biomass of a dominant forb (Aster ericoides) and a dominant grass (Schizachyrium scoparius). In mixed grass prairie, nutrient additions did not result in an increase in total productivity. However, we did observe a tradeoff in the responses of the two co-dominant grass species. S. scoparius, which has been shown to be a strong competitor for N, decreased in biomass with N additions, while Sporobolous heterolepis increased with N additions. In shortgrass steppe the increase in production with nutrient additions during the high-precipitation year was driven by an increase in biomass of the dominant grass Bouteloua gracilis. Thus, multiple resource additions appear to result in an increase in primary productivity only when water is not limiting. Furthermore, this differential response appears to be driven primarily by the responses of dominant plant species.