NET PRIMARY PRODUCTION OF U.S. MIDWEST CROPLANDS FROM AGRICULTURAL HARVEST YIELD DATA

We studied crop harvested yield, as recorded in national agricultural statistics, to estimate net primary production (NPP) in agricultural regions where most of the land area is sown with a few, well-studied crops. We estimated the magnitudes and interannual variations in NPP in croplands in the U.S. Midwest using crop area and yield data obtained from the U.S. National Agricultural Statistics Service (NASS). Total NPP, including estimates of the above- and belowground 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 mass to dry mass, (3) estimation of aboveground yield using crop harvest indices, defined as the ratio of economic product (e.g., grain) dry mass to plant aboveground dry mass, and (4) estimation of belowground yield as a function of aboveground biomass. This approach is applied to corn, soybean, sorghum, sunflower, oats, barley, wheat, and hay in Illinois, Indiana, I...