Management and soil-quality effects on fertilizer-use efficiency and leaching

We tested the hypothesis that particulate organic matter (POM) and aggregate dry mean weight diameter (DMWD) are related to fertilizer-use efficiency (FUE) and leaching susceptibility. Soil cores (15 cm diam. by 50 cm depth) were collected from 12 farm fields representing three cropping systems: conventional (CT) and no-tillage (NT) management of corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotations, and CT applied to more diversified corn-soybean-based rotations (R-CT). Three of the four R-CT farms were organically managed. In a 95-d greenhouse trial, cores were seeded with corn, amended with ¹⁵N-labeled urea applied at four rates (0, 75, 150, 225 kg N ha^-1), and subjected to a stressful moisture regime. Aggregate DMWD, which serves as a rough index of pore-size distribution, was greater in NT cores. Although total leached N was similar in all cropping systems, increased macropore flow in NT cores led to greater leaching of fertilizer N and less leaching of soil-derived N, as well as greater moisture stress and decreased plant N uptake. The R-CT cores had more POM and organic C in the top 30 cm of soil and higher crop biomass and biomass-N content. However, FUE in R-CT cores was relatively low since FUE does not account for contributions of indigenous N. For the same reason, FUE remained relatively high in CT systems despite less labile organic matter. Both FUE and SOM conservation declined with increasing N application rates. Increasing labile sources of N, reflected in POM pools, through crop diversification can substitute for incremental increases in fertilizer N and improve long-term productivity on Illinois Mollisols.