Land use versus climate change impact on regional carbon cycle and carbon isotope signatures

Agricultural crops with a C4 photosynthetic pathway rapidly expanded across North America as early as 800 A.D. Their distribution continues to expand globally as demands for food and biofuel production increase. These systems are highly productive, having a significant impact on carbon and water exchange between the land and atmosphere. Based on isotopic measurement, we can estimate the different photosynthetic pathways (C3 and C4) influence on the regional biosphere-atmosphere carbon exchange. However, it is hard to distinguish land use change (changes of C3 and C4 vegetation fractions) and climate variation's impact on the regional carbon cycle and carbon isotope signatures. Here, we use a land surface model CLM4 to investigate the relative impact of agricultural C4 vegetation on the regional CO2 budget and atmospheric isotopic forcing in the Upper Midwest, United States.

This study integrated CLM4 with a C4 plants discrimination model and a canopy kinetic fractionation model, to simulate regional carbon isotope exchange between land and atmosphere from 2007 to 2009. The model simulated discrimination was compared to the discrimination derived from EC-TDL measurement at two towers with homogeneous underlying surface and a Tall Tower (44°41'19” N, 93°4'22” W, 290m ASL). The land cover data is derived from Landsat 30m resolution data using a hybrid supervised/un-supervised land use classification scheme.

Based on this research we address four climatology questions:

1. What is the relative importance of C3 and C4 species to the regional CO2 budget?

2. How do these different photosynthetic pathways influence the regional biosphere atmosphere isotope discrimination?

3. To what extent do changes in C4 vegetation impact atmospheric isotopic forcing and the isotopic signature of the atmosphere?

4. To what extent do changes in Climate factors impact atmospheric isotopic forcing and the isotopic signature of the atmosphere?