The intensification of agriculture has been distorting the perception of fertility, with a great attention on its chemical component, reinforced by the use of chemical fertilizers as a major option to manage soil fertility. The consequent decline in soil organic matter content is a threat to the sustainability of intensive agricultural systems. Improving soil quality by increasing soil organic carbon (SOC) is seen as a potential way of improving crop yield and quality. In this paper, we studied the effects of two compost-based fertilization strategies on soil fertility and on yields of the following seven vegetable crops: eggplant (Solanum melongena L.), Endive (Cichorium endivia L.), Tomato (Lycopersicon lycopersicum (L.) Karsten Exfarw), Cauliflower (Brassica oleracea L), Onion (Allium cepa L. var. cepa), Muskmelon (Cucumis melo L), Fennel (Pheniculum vulgare L). The results of a seven-year experiment carried out under open field conditions in Southern Italy are presented. Four different fertilization treatments were applied: a) unfertilized control (CNT) b) supplying of mineral N, P2O5 and K2O by chemical fertilizers (CF); c) biowaste compost amendment at 30 Mg ha−1 year−1 as dry matter (d.m.) (≈ 8.4 Mg ha−1 organic C) in the first three years, and 15 Mg ha−1 year−1 as d.m. (≈ 4.2 Mg ha−1 organic C) from the fourth year onwards (Com); d) biowaste compost application of 15 Mg ha−1 year−1 as d.m. integrated with ½ of mineral N supplied in the mineral fertilization (ComN). We demonstrated that the loss of the productive ability in unfertilized control corresponded to SOC concentration ≤ 1%, the same occurred with CF fertilization where, however, the addition of chemical fertilizers concealed soil degradation. On the contrary, SOC concentration ranging between 1.2 and 1.4% in compost amended treatments granted vegetable crops yields always higher than or similar to mineral fertilization. Such SOC concentrations were far from the 2% threshold considered as the minimum SOC concentration of not degraded soil. SOC content increased 8.1 and 5.7 Mg ha−1 due to Com and ComN strategies. The supplying of 4-5 Mg C ha−1 year−1 as in ComN resulted in a positive efficiency (15%) in stabilizing C in soil compared to the supplying of higher doses as in Com. As concerns the risks of groundwater pollution due to N-NO3 releases in amended soils, we ascertained in three annual monitoring campaigns that releases measured in the 0-30 cm soil layer did not exceed values of 20-25 mg kg−1 considered as the treshold above which vegetable crop response to additional fertilization was unlikely.
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