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Title: Impact of precipitation dynamics on net ecosystem exchange

item PARTON, WILLIAM - Colorado State University
item Morgan, Jack
item Smith, David
item Del Grosso, Stephen - Steve
item PRIHODKO, LARA - Colorado State University
item Lecain, Daniel
item KELLY, ROBIN - Colorado State University

Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2011
Publication Date: 2/18/2012
Citation: Parton, W., Morgan, J.A., Smith, D.P., Del Grosso, S.J., Prihodko, L., Lecain, D.R., Kelly, R. 2012. Impact of precipitation dynamics on net ecosystem exchange. Global Change Biology. 28:915-927.

Interpretive Summary: The continued net release of greenhouse gases to Earth’s atmosphere is largely acknowledged to be the major factor behind world-wide climate change. Research is underway to better understand both natural and anthropogenic causes behind such terrestrial releases of greenhouse in order to better understand and manage their emissions. This research conducted in the shortgrass prairie of eastern Colorado endeavors to understand how precipitation patterns affect the biological responses of plants and soil microorganism which are responsible for the land-atmosphere exchange of the greenhouse gas carbon dioxide (CO2). The results indicate that annual uptake of CO2 by the shortgrass prairie is largely due to CO2 assimilated in Spring, and that the amount of CO2 assimilated each year is dependent not only on the amount of rainfall received in Spring, but also the size of rainfall events. These findings suggest that managers who hope to increase C sequestration will want to ensure stocking rates promote abundant forage in early spring to optimize photosynthetic responses to rainfall at that time of year.

Technical Abstract: Net ecosystem carbon dioxide (CO2) exchange (NEE) was measured on shortgrass steppe (SGS) vegetation at the USDA Central Plains Experimental Range in northeastern Colorado from 2001-2003. Large year-to-year differences were observed in annual NEE, with > 95% of the net carbon uptake occurring during the April to June time period. Low precipitation during the 2002 April to June time period greatly reduced annual net carbon uptake. Large precipitation events (> 10.0 mm day-1) promoted carbon uptake, while small precipitation events (< 5.0 mm day-1) enhanced heterotrophic respiration and resulted in a net loss of carbon from the system. Large precipitation event enhanced carbon uptake was attributed to increased soil water content (SWC), which promotes plant photosynthesis. Live aboveground plant biomass and SWC were the major variables that controlled daytime net CO2 uptake (NEEd) and nighttime respiration losses (NEEn). NEEd and NEEn were negatively correlated to the live plant biomass. These results suggest that the major factor controlling growing season nighttime respiration flux is the amount of carbon fixed via photosynthesis during the day. Heterotrophic soil respiration is greatly enhanced for one to two days following rainfall events. A regression model was developed to simulate daytime and nighttime net carbon exchange as a function of soil water and temperature and live plant biomass (r2 = 0.76).