Nutrients in soil water under three rotational cropping systems, Iowa, USA

Subsurface nutrient losses differ between annual and perennial crops; however, nutrient losses from cropping systems that rotate annual and perennial crops are poorly documented. This study tracked NO3-N and P in soil water under three cropping systems suited for the U.S. Midwest, including two-year (corn–soybean; 2YS), three-year (corn–soybean–small grain/red clover; 3YS), and four-year (corn–soybean–small grain/alfalfa–alfalfa; 4YS) systems. Nutrient applications were based on soil-test results, and solely comprised inorganic fertilizers to 2YS corn, whereas, in the 3YS and 4YS systems, nutrients were provided by legume residues, and a managed balance of composted manure, and inorganic fertilizers. Soil water was collected from 2004 through 2011 using suction samplers. The 4YS system had smaller concentrations of NO3-N and a lower frequency of P detection (p < 0.05) during the established alfalfa and subsequent corn crop than other crop-years in the experiment. Mean concentrations of NO3-N were 1.1 mg NO3-N L−1 under alfalfa and 6.5 mg NO3-N L−1 under the following 4YS 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 among the three systems were explainable based on mineralization and – immobilization of N from decomposing crop residues. Large P concentrations (0.23–1.02 mg P L−1) occurred in one plot that occasionally ponded, indicating P transport to depth by preferential flow is a risk in some soils on this landscape. Among the remaining plots, the proportion of samples with detectable P concentrations (i.e., >0.02 mg L−1) was less (p < 0.05) under the 4YS system (0.26) than observed for the experiment overall (0.35), which resulted from infrequent detection (0.17) of P in soil water during the alfalfa and corn crops. Results provide evidence that rotational systems including alfalfa with annual crops and use of soil testing in nutrient management can reduce movement of N and P below the root zone.