Nutrient Concentrations in Soil Water As Influenced By Crop Rotation in Iowa.

Nutrient leaching differs between annual and perennial crops, but nutrient losses from rotations containing both annual and perennial crops are not well documented.  This study compared NO₃-N and total phosphorus (TP) concentrations in soil water under three crop rotations in Iowa, including two-year (corn-soybean; 2YR), three year (corn-soybean-small grain/red clover; 3YR), and four-year (corn-soybean-small grain/alfalfa-alfalfa;  4YR) systems.  Using spring soil-test results, nutrient applications comprised synthetic fertilizers to 2YR corn, and composted manure to supplement legume-residue sources for 3YR and 4YR.  Soil water was collected from suction samplers at 1.2 m depth during eight years (2004-2011).  The 4YR rotation, during the alfalfa and following corn years, showed smaller NO₃-N concentrations and less frequent detection of TP.  Mean concentrations of NO₃-N were 1.1 mg NO₃-N L^-1 under alfalfa and 6.5 mg NO₃-N L^-1 during the following corn crop, compared to average concentrations between 8.7 and 18.1 mg NO₃-N L^-1 among all other crop-years.  Seasonal NO₃-N dynamics were distinct among rotations due to differences in mineralization-immobilization of N from decomposing crop residues.  Total P results showed the importance of landscape hydrology, as the greatest total P concentrations occurred in one 2YR plot where preferential flow was evident.  Among the remaining plots, detection of total P (i.e., >0.02 mg L^-1) occurred less frequently (p<0.01) under the 4YR alfalfa and corn crops (0.12) than all crop-years (0.34).  Results suggest nutrient management may not always involve a tradeoff between N and P losses, and that crop rotations including alfalfa may reduce losses of both nutrients.