A phosphorus budget for the Lake Mendota watershed

A phosphorus (P) budget was calculated for the agriculture-dominated Lake M endota watershed located in Dane and Columbia Counties, Wisconsin, USA. P i nputs included fertilizer for agricultural crops and lawns, dietary supplem ents for dairy cattle, and natural inputs such as dry and wet deposition. O utputs included agricultural crops, livestock and livestock products, and h ydrologic export to Lake Mendota. The total P input to the watershed (1,307 ,000 kg year(-1)) and total output (732,000 kg year(-1)) are large relative to the average of 34,000 kg P washing into the lake each year, indicating that the P flux that eutrophies Lake Mendota is a very minor component of t he total watershed P budget. Using the formula inputs - outputs = change in storage, we found that 575,000 kg P accumulated in the watershed in 1995. This estimate was corroborated by long-term soil P concentration data, whic h showed an average annual increase in soil P of over 450,000 kg year(-1). Future management programs designed to reduce P inputs to Lake Mendota will be compelled to cope with the large amount of P being stored in the waters hed.