Impact of Manure Management on Soil Test Phosphorus Dynamics in a Long-Term Corn-Alfalfa Rotation.

Multi-year repeated dairy manure application based on nitrogen (N) needs of corn (Zea mays L.) increases soil test phosphorus (STP) and risk of P runoff, accordingly. A P-removal based application of manure adds less P but could impact yield. A corn field trial was implemented in New York in 2001 and rotated into alfalfa (Medicago sativa L.) in 2006 to examine STP dynamics over a corn-alfalfa rotation (2001 to 2011). Treatments were annual spring applications of N-based versus P-removal based compost (74 and 46 Mg ha^-1, respectively), and liquid dairy manure (196 and 68 kL ha^-1, respectively), in addition to two control treatments (zero N and 112 kg ha^-1 sidedress N). No manure or compost was applied during alfalfa years. A change from N-based to P-based manure and compost management decreased corn DM yield by 13% and 7%, respectively. Alfalfa yields in N-based manure and both compost treatments were 21% higher than the average DM yield of P-based manure and controls reflecting benefits of manure and compost addition during the corn years for alfalfa that follows in the rotation. In the corn years, STP remained unchanged in both control treatments but increased 4- and 2-fold for N-based and P-based manure, and 6- and 4-fold for N-based and P-based compost, respectively, consistent with greater P balances possibly combined with differences in P fractions in each source. Over the 10-yr rotation, STP increased by 7.8 and 3.9 mg kg^-1 per 100 kg P balance in compost and manure amended plots, respectively. We conclude a shift from N- to P-based manure can maintain STP over the rotation but additional N should be applied to compensate for the yield loss. For higher application rates, a greater number of years in alfalfa than corn might be needed to prevent STP from building over the rotation.