Ridge Tillage Concentrates Potentially Mineralizable Soil Nitrogen, Facilitating Maize Nitrogen Uptake

Ridge tillage (RT) can promote increases in soil C and aggregation at greater rates than conventional tillage, but few studies have investigated how RT may affect soil N distributions across the row/inter-row space. Using a spatially intensive sampling design, we monitored soil potentially mineralizable N (PMN), inorganic N, and plant tissue N in a field study comparing RT and chisel plow (CP) systems. Experiments were fully replicated at two sites in Urbana, IL and Mason, MI during the 2012 growing season. At both sites, a strong interaction effect of tillage x row position was observed for PMN (Illinois, p = 0.005; Michigan, p = 0.02) with higher levels of PMN in the in-row (IR) position than off-row (OR) and between-row (BR) positions of RT treatments following re-ridging. Plant tissue analyses indicated a significant RT advantage at both sites (Illinois, p = 0.04; Michigan, p = 0.02), and a structural equation modeling (SEM) analysis indicated that PMN at the 0- to 5-cm depth in the IR position following re-ridging had a significant effect on inorganic N at the same position and, in turn, a strong influence on plant tissue N (comparative fit index = 0.86, standardized root mean square residual = 0.11, Akaike wt. = 1). Overall, our results suggest that RT can establish soil functional zones (SFZ) with distinct N profiles and that the relocation of PMN in-row may increase the spatial efficiency of N provisioning relative to conventional tillage.