Partitioning CO2 Fluxes in Transitional Bioenergy CROPS:EFFECT of Land Use Change

The demand for alternatives to petroleum is increasing the production of bioenergy. Undisturbed ecosystems in different part of the globe were converted to bioenergy cultivations. In this study we examined the effect of land conversion on C Pools and fluxes using the Eddy Covariance (EC) technique in seven sites in southwestern Michigan undergoing such conversions. Of the seven sites, four had been managed for the Conservation Reserve Program (CRP) during the last 20 years to maintain them as grasslands. The other three were cultivated in a corn/soybean rotation. The effects of land use change were studied during 2009 when six of the fields (three CRP and three crop fields) were converted to soybean cultivation, with the 7th site remained as a grassland reference. Daytime estimates of ecosystem respiration (Reco) were obtained from the night NEE-temperature relationship. An Arrhenius-type model was used to describe the temperature dependence of Reco. The Gross Primary Productivity (GPP) was then obtained by subtracting Reco from NEE. Soil CO2 fluxes (SRR) were measured in all sites with a portable EGM-4 infrared gas analyzer (PP-Systems, UK). SRR, soil temperature, and soil moisture were analyzed using a two-way ANOVA with repeated measures analyses on one factor. SRR was modeled using a nonlinear regression function to describe SRR as dependant on soil temperature and soil moisture, expressed as soil water content relative to the soil water content at field capacity (RSWC). Standard errors of nonlinear regression parameters were estimated by a bootstrapping algorithm. During winter the agricultural sites were essentially carbon (C) neutral while the grasslands were C sources, with average emissions of 15 g C m-2 month-1. The annual NEP at sites converted from CRP to soybeans had a net emission of 156 (± 25) - 128 (± 27) g C m-2 year-1. The sites previously cultivated as corn/soybean rotation was a net C uptake, with NEP ranging from -91 (± 26) to -57 (± 21) g C m-2 year-1. At one site characterized by a lower soil C content and lower biomass production, the NEP was 31 (± 10) g C m-2 year-1. Annual changes in SRR were significantly different between CRP and the agricultural sites, while there were no significant differences among the three sites of each group. The annual SRR ranged from 831 to 749 g C m-2 year-1 at the CRP sites and from 311 to 274 g C m-2 year-1 at the agricultural site. Cumulative SRR showed a strong correlation (r2 = 0.88; p = 0.001) with soil organic matter (SOC). Annual average GPP was 579 (± 47) g C m-2 year-1 at the CRP sites and 685 (± 31) g C m-2 year-1 at the agricultural sites. Overall, the conversion of CRP to soybean cultivation in preparation for biofuel crop production induced net C emissions in all sites investigated.