<b>Hristov</b>, A. N., J. M. F. <b>Johnson</b>, C. W. Rice, M. E. Brown, R. T. Conant, S. J. Del Grosso, N. P. Gurwick, C. A. Rotz, U. M. Sainju, R. H. Skinner, T. O. West, B. R. K. Runkle, H. Janzen, S. C. Reed, N. Cavallaro, and G. Shrestha, 2018: Chapter 5: Agriculture. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 229-263, https://doi.org/10.7930/SOCCR2.2018.Ch5.
Between 1960 and 2000, global crop net primary production (NPP) more than doubled, and global cropland area in 2011 was estimated to be 1.3 billion ha (Wolf et al., 2015). Global crop NPP in 2011 was estimated at 5.25 Pg C, of which 2.05 Pg was harvested and respired offsite (Wolf et al., 2015). Global livestock feed intake was 2.42 Pg C, of which 52% was grazed and the rest was either harvested biomass or residue collected from croplands. Global human food intake was 0.57 Pg C in 2011 (Wolf et al., 2015). The global agricultural carbon budget indicates a general increase in NPP, harvested biomass, and movement of carbon among global regions. At the global scale, cereal crops declined and have been replaced primarily with corn, soybean, and oil crops. While total NPP and yield (i.e., biomass per area) have increased in nearly all global regions since 1960, the most pronounced increase has been in southern and eastern Asia where harvested biomass has tripled. Also, cropland NPP in the former Soviet Union significantly declined in 1991, with the level of production recovering around 2010 (Wolf et al., 2015).
Annual crop cultivation and crop burning often is considered carbon neutral (IPCC 2006; U.S. EPA 2018) because biomass is harvested and regrown annually. Although biomass itself is technically carbon neutral, this assumption does not necessarily account for changes in soil carbon that may be associated with production practices, which affect the carbon cycle and net emissions. The impact of non-CO2 emissions is accounted for in the other categories. The increased global uptake of carbon by croplands influences the annual oscillation of global atmospheric carbon (Gray et al., 2014), as more carbon is taken up and released annually than would occur without extensive global cropland production. The cycling of cropland biomass into soils and the cultivation of soils influence how much of the carbon in crop biomass is respired back to the atmosphere versus remaining in the soil, ultimately determining if a cropping system is a net source or sink.
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