Professor Colorado State University Fort Collins, Colorado, United States
Abstract: Increased nitrogen fertilization and deposition has altered ecosystems worldwide since the rise of industrial agriculture and fossil fuel combustion. Concurrently, climate change threatens these systems with intensifying weather patterns, such as severe droughts and extreme rainstorms (deluges). Under the “Pulse-Press” framework, sustained resource alteration such as chronic nitrogen addition is considered a “press,” while “pulses” such as deluges are discrete in time. When combined with long-term resource change, such pulses have the potential to interact, causing an effect greater than the sum of their parts. Despite the importance of such perturbations, there is insufficient research to consistently predict their potential to interact, particularly with respect to the magnitude of the resource press. To address this knowledge gap, we ask: Does the magnitude of a nitrogen addition press determine ecosystem response to a deluge pulse in grassland communities? This research was conducted in two grasslands that represent opposite ends of a precipitation gradient in the Great Plains: a semi-arid shortgrass prairie in northeastern Colorado and a mesic tallgrass prairie in eastern Kansas. We expected that given that water is more limiting at the semi-arid shortgrass prairie site that interactions between increasing nitrogen press amount and a pulsed deluge event would be more profound than at the mesic tallgrass prairie site. To address our research questions and hypotheses, nitrogen was added at 8 levels, ranging from 0-30g m-2 yr-1 since 2014 and a deluge event (95th percentile) was applied in mid-to-late summer 2021 during the 8th year of fertilization. During the year of the deluge and the year after the event, we measured aboveground net primary productivity (ANPP) at both study sites.
The effect of the deluge treatment was modest; production increased at both sites but was significant only at the semi-arid shortgrass site (p < 0.001). We also observed a legacy of the deluge event, whereby that production was increased irrespective of nitrogen addition amount. However, there was no evidence of interaction between the nitrogen press and deluge pulse treatment at any time during this experiment (p > 0.5). We conclude that short-term deluges cause an increase in average production and may result in production legacies, irrespective of nitrogen addition amount. Though previous research has demonstrated interactions among press-pulse perturbation, nitrogen press and deluge pulse were merely additive in these Great Plains grasslands and these effects were surprisingly not influenced by the magnitude of the nitrogen press.
