Reduced weeding shows potential to regulate nutrient leaching in a cabbage (Brassica oleracea, var. capitata) lysimeter trial

Agricultural land is the main contributor to nitrogen and phosphorus pollution of groundwater sources. Adopting agroecological management practices can support the transition towards sustainable farming systems. We investigated if reduced weeding could support nutrient retention in a nutrient demanding vegetable crop (Brassica oleracea, var. capitata), without causing unacceptable yield losses. We conducted a three-year study (2019–2021) with two cropping seasons (autumn and spring) using 1 m3 above-ground lysimeters, each containing four cabbage plants. Different vegetation covers, each with four replicates, were considered: 1) crop only 2) weeds only 3) crop + weeds from crop transplanting 4) crop + weeds after 20 days from crop transplanting 5) bare soil with fertilizer 6) bare soil without fertilizer. Each system received the recommended dose of mineral fertilizer for cabbage (130 kg ha−1 N, 80 kg ha−1 P, 150 kg ha−1 K), except for two bare soil lysimeters which were not fertilized. Water samples were taken throughout the growing season, in particular after each fertilization event, and analysed for nitrate (NO3-N), phosphorus (P) and potassium (K) concentrations. Cabbage yield in both weedy treatments was significantly lower compared to the weed-free plots only in spring 2020 and fall 2021. Weed cover contributed to explain NO3-N and K leaching, while P leaching was affected by crop cover. The results suggest that it is possible to reduce weed management intensity in cabbage while also obtaining some benefits concerning nutrient losses.