## 4 Discussion
Carbon source areas are generally aligned with the drier western edge of the Great Plains with small grain cropping in the central and western Northwestern Glaciated Plains ecoregion (9.3.1), primarily dry grasslands in the Southwestern Tablelands (9.43) and grassland and shrubland in the High Plains (9.41) ecoregion. Extreme carbon source areas (\(<\)\(-\)300 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)) tended to be in warmer southern croplands of the Great Plains in the southeastern High Plains (9.4.1), southwestern Central Great Plains (9.4.2), and southern croplands of the Western Gulf Coastal Plain (9.51) ecoregions. These extreme flux regions are distant from the nearest crop non-wheat flux tower (Figure 1, Table S1) and may have moderate to high flux mapping uncertainties (Figure 8). Corn production dominates the Western Corn Belt Plains ecoregion (9.2.3) transitioning from moderate carbon sinks (200 to 100 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)) in northern and central parts of this ecoregion to moderate sources (\(-\)200 to \(-\)100 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)) in the southern and eastern part of this ecoregion where pasture land use becomes more common in erodible soils and landscapes. The productivity gradient analysis used regression intercept terms to quantify carbon sink advantages of grass over non-irrigated crop (Figure 12), but equilibrium NEP discussed here is from 2000 to 2008, not a long-term NEP. Given that the long-term spatial and temporal weather variations and weather extremes (very wet to strong drought years) are high in the Great Plains [72], 2000-2008 equilibrium NEP are expected to vary from the 30-year climate record (1981-2010 used in this study) and to future conditions. However, NEP productivity relationships with 30-year climate precipitation (Table 7) can help quantify expected ecosystem service benefits and consequences related to grass and non-irrigated land cover changes. Extending the carbon flux mapping period forward in time and adding more dryland crop flux tower datasets would further improve these estimates. Regional Great Plains NEP means and minimum to maximum years for each majority land cover (Figure S2) through the study period agreed well with published flux tower estimates with most crop types and grass estimates, with the possible exception of alfalfa (Figure 13). Generally, individual flux tower site and year variability in NEP tended to be greater than the inter-annual variability in regional mean NEP through regional averaging and generalized fitting of the NEP mapping models for crops and grass not capturing all of the occasional flux variations. Corn-dominated regional flux means (218-385 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)), which likely included some soybean years in the crop rotations, encompass the mean annual corn NEP from flux towers from [PERSON] et al. (2016) of 333 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\) (Figure 13). The range of corn flux tower annual NEP observations was 121-548 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\). Soybean-dominated areas had a long-term NEP of \(-\)56 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\) and ranged from \(-\)137 to \(-\)12 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\). Soybean flux tower estimates from [PERSON], et al. (2016) agreed closely with a mean NEP of \(-\)77 and a data range of \(-\)220-208 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\). Similarly, winter wheat mean NEP was consistent (27 and 13 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)) for regional and flux tower means, respectively. The range of annual flux estimates from regional yearly means and flux towers were comparable (\(-\)64-102 and \(-\)193-128 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\), respectively). Grassland flux tower mean NEP was greater than the regional inter-annual mean (189 versus 75 g C\(\cdot\)m\({}^{-2}\cdot\)year\({}^{-1}\)).