NPP Cropland: Gridded Estimates for the Central U.S.A., 1982-1996

Net primary productivity (NPP) of agricultural regions, where most of the land is sown with a few well-studied crops, was estimated from crop harvested yield, as recorded in national agricultural statistics. The magnitudes and inter-annual variations in NPP of croplands in the US Mid-West were estimated using crop area and yield data from the US National Agricultural Statistics Service (NASS). Total NPP, including estimates of both above and below-ground components, was calculated from harvested yield data by (1) conversion from reporting units of yield of the crop product, usually in volume, to mass; (2) conversion from fresh weight to dry weight; (3) estimation of above-ground yield using crop harvest indices, defined as the ratio of economic product (e.g. grain) dry weight to plant above-ground dry weight; and (4) estimation of below-ground yield as a function of above-ground biomass. This approach was applied to corn, soybean, sorghum, sunflower, oats, barley, wheat and hay in Illinois, Indiana, Iowa, Michigan, Minnesota, North Dakota, Ohio and Wisconsin for 1992, and in Iowa from 1982 to 1996. Many counties in these eight states had over 70 per cent coverage of these crops. In Iowa, corn and soybean accounted for over 50 per cent of the land area in most counties. County-level NPP in 1992 ranged from 4 Mg/ha/yr (400 g/m2/yr, dry biomass, or 200 gC/m2/yr in terms of carbon content) in North Dakota, Wisconsin and Minnesota, to over 17 Mg/ha/yr (1700 g/m2/yr, dry biomass, or 850 gC/m2/yr) in central Iowa, Illinois and Ohio. Areas of highest NPP were dominated by corn and soybean cultivation. NPP for counties in Iowa varied between years by a factor of 2, with the lowest NPP in 1983 which had an unusually wet Spring, in 1988 which was a drought year, and in 1993 which experienced floods. A sensitivity analysis, conducted by varying harvest index and root:shoot ratio by 10-50 per cent, indicated that the limit of accuracy of the method is about 1 Mg/ha/yr (100 g/m2/yr, dry biomass).