Effect of sewage sludge-derived amendments on the nutrient uptake by Chinese cabbage from Mediterranean soils

The utilization of sewage sludge (SS) as a fertilizer for agricultural land is increasingly seen by many as the best option for recycling. This study investigates the effects of SS-derived compost (SSC) or biochar (SSB) amendments on N, P, K, Ca, Mg, Na, Fe, Mn and Mo uptake by Chinese cabbage (Brassica rapa L. subsp. pekinensis (Lour.) Hanelt) from terra rossa and rendzina soils. A two-factorial experiment was performed under greenhouse conditions whereby elemental concentrations (substrate, leachates and plants), edible tissue uptake (ETU), concentration ratios (CR) and soil characteristics were determined. The pH (7.03 to 7.32) and P content (286 to 421 mg/kg d.w.) in SSC- and SSB-amended soils increased, while EC (450 to 253 mu S/cm) decreased. Phosphorous levels were higher in SSB-amended soils, although P leaching indicates a low environmental risk in both soils. In leaves, P (1.02 to 4.15 g/kg d.w.), K (12.8 to 21.8 g/kg d.w.), Mg (1.05 to 1.55 g/kg d.w.), Na (660 to 1350 mg/kg d.w.), and Mo (1.20 to 2.65 mg/kg d.w.) levels increased with amendment rate and were higher in SSC-amended soils. Nutrient bioavailability also increased with amendment rate, while cabbage biomass production increased proportionally to the dose. The P, K, and Mg levels were higher in leaves, while roots contained more Ca, Fe, and Mn. Despite amendment rates 10x the permitted dose of SS, they did not affect soil leachate, plant growth and biomass yield. Overall, adopting the cautionary principle is advised until the long-term effects are better understood.