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Abstract

Interactions between the biosphere and atmosphere are an important part of the global carbon cycle, and quantifying the carbon dioxide exchanges between them is helpful in predicting the uptake of carbon dioxide from anthropogenic sources by the biosphere in the future. In the Midwestern United States, agricultural systems cover a large part of the landscape, so understanding their role in influencing the global carbon budget is crucial as anthropogenic sources of carbon dioxide grow larger. Carbon dioxide exchanges can be measured by eddy covariance at the ecosystem level (bottom-up approach) or regionally by inversion techniques (top-down approach). Here we describe a novel approach to estimate the exchange at an intermediate spatial scale using weather balloons. Two different techniques were used to collect data. In the first-generation method used from 2012 to 2013, a single balloon launch was conducted per launch date and the rate of uptake between the ascent and descent was compared. In the second-generation method used in the summer of 2014, two launches were conducted in one day and the rate of uptake between the two ascents was calculated. The carbon dioxide concentrations measured during the ascents were converted to a molar difference using the observed temperature and pressure of the atmosphere, and a flux was calculated by summing the molar differences and dividing by the time difference between flights. This value is the Net Ecosystem Exchange (NEE). The first-generation method found that the peak uptake by the biosphere occurred in mid-July. The second-generation method found that uptake was highest in mid-July as well, with uptake decreasing throughout August and September. Only four data points were collected using the second-generation methodology, so significance of this trend is limited. During peak growing season over the summer, uptake rates of -30 to -50 μmol m-2 s-1 were observed, while as fall approached this rate became positive. The methodology established here will be used to explore new hypotheses related to the NEE of crops.