RESUMEN
Enrichment of soluble P on the surface layer of long-term no-till (NT) soils, and consequent increase in dissolved P losses, is a concern for which occasional plowing has been suggested as a remedy. We measured the effect of such strategic tillage (ST) on surface and subsurface P losses from 0.5-ha field plots on clay soil for 4 years. Two NT plots had discharged threefold dissolved molybdate-reactive P (DRP) losses compared to annually plowed soil conventional tillage (CT). ST by plowing to 20-cm depth was applied on one of the NT plots, whereas the other remained under NT. ST done in July was sown with canola (Brassica napus ssp. oleifera) to establish plant cover before winter. Summed 4-year DRP loss from ST treatment was 60% lower compared to NT (0.78 vs. 1.96 kg ha-1), accompanied with 11% higher particulate P (PP) loss (4.39 vs. 3.97 kg ha-1). CT plots produced slightly lower DRP losses (0.53-0.76 kg ha-1) than ST, but higher PP losses (6.02-7.96 kg ha-1). Bioavailable P (BAP) losses from ST were lower than from the other treatments if >7% of PP turns bioavailable. After ST, soil P stratification first vanished, but started to develop again when NT was resumed. Occasional tillage of NT soils mitigates DRP losses over several years, and it was at the study site the preferred mitigation option in reducing BAP losses.