Strategic tillage of no-till decreased surface and subsurface losses of dissolved phosphorus

Enrichment of soluble P on the surface layer of long-term no-till (NT) soils, andconsequent increase in dissolved P losses, is a concern for which occasional plowinghas 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) lossescompared to annually plowed soil conventional tillage (CT). ST by plowing to 20-cmdepth was applied on one of the NT plots, whereas the other remained under NT. STdone in July was sown with canola (Brassica napus ssp. oleifera) to establish plantcover before winter. Summed 4-year DRP loss from ST treatment was 60% lowercompared to NT (0.78 vs. 1.96 kg ha−1), accompanied with 11% higher particulateP (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). BioavailableP (BAP) losses from ST were lower than from the other treatments if >7% of PP turnsbioavailable. After ST, soil P stratification first vanished, but started to develop againwhen NT was resumed. Occasional tillage of NT soils mitigates DRP losses overseveral years, and it was at the study site the preferred mitigation option in reducingBAP losses. Plain Language Summary No-till is a very effective erosion control option and therefore a widely recommendedwater protection measure. However, categorical application of no-till, at sites thatare not very erosion-prone, may be more detrimental than beneficial in eutrophica-tion point of view, because dissolved P losses from no-till tend to be higher thanin regular inversion plowing. This study showed that occasional plowing of no-tillhelps controlling dissolved P losses for several years, and it would be the preferredeutrophication control measure at the site of this study.