One hundred years of nutrient loading to Lake Michigan watersheds

We estimated nutrient loading to 25 watersheds of the Lake Michigan Basin from 1880 to 2002, documenting changes in anthropogenic inputs over time and the influence of changing land use and agricultural practices on spatial-temporal variation in anthropogenic N and P.  Based on an accounting of net anthropogenic N inputs (NANI) due to fertilizer, net trade of N in food and feed, crop fixation, and atmospheric deposition, NANI to the entire LMB increased nearly six-fold over the 20^th Century, from ~ 400 kg-N km^-2 yr^-1 in 1880 to a peak of ~ 2,400 kg-N km^-2 yr^-1 in 1987, and then declined to ~1,930 kg-N km^-2 yr^-1 in 2002.  Net anthropogenic P input (NAPI) estimated as the sum of P fertilizer and net import of P in food, feed and detergent, increased eight-fold from ~50 kg-P km^-2 yr^-1 in 1880 to 450 kg-P km^-2 yr^-1 in 1974, and then declined to 188 kg-P km^-2 yr^-1 in 2002, mainly due to a reduction in fertilizer use.  Prior to 1944 inputs were governed by a single source (e.g. crop N fixation for N), but input sources became more heterogeneous after the 1980s.  River nutrient loads show evidence of varying with watershed inputs.