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ARS Home » Midwest Area » St. Paul, Minnesota » Soil and Water Management Research » Research » Publications at this Location » Publication #228898
Title: Long-lasting effects on nitrogen cycling 12 years after treatments cease despite minimal long-term nitrogen retention

Author
item CLARK, CHRISTOPHER - ARIZONA STATE UNIVERSITY
item HOBBIE, SARAH - UNIVERSITY OF MINNESOTA
item Venterea, Rodney - Rod
item TILMAN, DAVID - UNIVERSITY OF MINNESOTA

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2008
Publication Date: 7/1/2009
Citation: Clark, C.M., Hobbie, S., Venterea, R.T., Tilman, D. 2009. Long-lasting effects on nitrogen cycling 12 years after treatments cease despite minimal long-term nitrogen retention. Global Change Biology. 15(7):1755-1766.

Interpretive Summary: The deposition of biologically active nitrogen (N) from the atmosphere is known to have important effects on ecosystem function and soil N cycling processes. However, few studies have examined the effects of multiple decades of N deposition, and even fewer have examined N-cycling patterns after inputs have ceased. This study reports the results of a long-term N deposition experiment conducted in a Minnesota grassland in plots that received N addition for 10 years and then none for 12 years, and for plots that received annual N treatment for 22 years. We found weak evidence for long-term N-retention in plots not receiving treatment; and, in plots that received N continuously, retention that was high after 12 years (50-100%) was low after 22 years (15%). In spite of this apparent recovery of N pools to control levels, net N mineralization rates remained elevated in plots that ceased receiving treatment 12 years prior, likely from N rich litter maintaining higher nutrient cycling rates. These results suggest (1) some systems do not retain much deposited N, with potentially large impacts on downstream habitats, (2) the previously reported high retention efficiencies for this and many other terrestrial ecosystems may be relatively short lived as N sinks become saturated over time, and (3) the effects of even small amounts retained N in N-limited environments may be particularly long lasting. In total, these findings highlight the importance of long-term studies in evaluating the impacts of chronic N deposition to ecosystems, and urge additional research examining dynamics following N cessation to evaluate the reversibility of these impacts. Results of this study will be useful to scientists and policy-makers involved in evaluating and developing regulations related to atmospheric emissions of N oxides and its impacts on downwind ecosystems.

Technical Abstract: Atmospheric deposition of biologically active nitrogen (N) has increased dramatically over the past 60 years, with far reaching impacts on the structure and function of many ecosystems. Much research has examined the initial impacts of N enrichment; however, few studies have been multi-decadal, and even fewer have examined N-cycling patterns after inputs cease. These limitations restrict our inference on the long-term impacts of N-enrichment, rendering the total size of ecosystem N-sinks as uncertain, and the longevity of N-induced impacts as largely unknown should inputs be reduced. Here we fill this gap by reporting the state of key N pools and fluxes in a Minnesota grassland for plots that received N addition for 10 years and then none for 12 years, and for plots that received annual N treatment for 22 years. We found weak evidence for long-term N-retention in plots not receiving treatment; and, in plots that received N continuously, retention that was high after 12 years (50-100%) was low after 22 years (15%). In spite of this apparent recovery of N pools to control levels, net N mineralization rates remained elevated in plots that ceased receiving treatment 12 years prior, likely from N rich litter maintaining higher nutrient cycling rates. These results suggest (1) some systems do not retain much deposited N, with potentially large impacts on downstream habitats, (2) the previously reported high retention efficiencies for this and many other terrestrial ecosystems may be relatively short lived as N sinks become saturated over time, and (3) the effects of even small amounts retained N in N-limited environments may be particularly long lasting. In total, these findings highlight the importance of long-term studies in evaluating the impacts of chronic N deposition to ecosystems, and urge additional research examining dynamics following N cessation to evaluate the reversibility of these impacts.