Two soil amendments, biochar and zeolites, were evaluated in their potential for increasing crop productivity and agro-system sustainability. The effect of biochar and zeolites, in combination with four nitrogen (N) rates [0 (N0), 50 (N50), 100 (N100), and 200 (N200) kg ha −1 ], on crop yield, N use efficiency, and soil properties was evaluated in a cropping system of irrigated forage maize (Zea mays L.) grown in summer and oats (Avena sativa L.) grown in winter as a catch crop. Biochar increased soil organic carbon (C), pH, cation exchange capacity (CEC), and extractable phosphorus (P), but strongly reduced N recovery in the set of the four cropping cycles. In biochar-amended plots, N50 had a negative apparent N recovery (− 21%), indicating that less N was recovered by the plants than in the N0 treatment without biochar. Biochar reduced maize dry matter (DM) yield by 15.6% in comparison to the untreated control, indicating N immobilization by biochar at low N rates (N0 and N50). Zeolites did not influence crop productivity or soil properties, except for the increase in extractable K, probably the result of its initial K content. N application to maize significantly increased the productivity of both crops, including that of the non-fertilized oats. Under the conditions of this experiment, biochar and zeolites did not prove to be useful soil amendments to increase crop DM yield in the short-term. The use of biochar increased soil organic C, which associated to a high N rate can enable the dual objective of maintaining productivity and the sustainability of the agro-system. The results stressed also the important role of oats as a cover crop to reduce the risk of nitrate leaching and denitrification during winter.

The composting process of winery wastewater sludge with grape stalks enables these wastes to be recycled as soil organic amendments or pot substrate components. However, it is necessary to assess the potential of this compost as crop fertilizer. This assessment is also needed for the use of biochar as soil amendment because negative effects on crop growth and nutrient uptake may occur depending on application rates and biochar physical and chemical characteristics. With this aim, a randomized block design pot experiment with lettuce was set up with four treatments including: (i) 5% (w/w) of winery waste compost; (ii) 4% (w/w) biochar (commercial charcoal); (iii) 5% (w/w) winery waste compost with 4% (w/w) biochar; and (iv) a control treatment without soil amendments (T0). Lettuce shoot yield increased 13% with compost application but decreased 18% with biochar in comparison to T0 and 25% with compost and biochar compared to compost alone. Similarly, lettuce root weight increased with compost, but not with biochar. Lettuce shoot dry matter (DM), N and Ca contents clearly increased with compost application compared to lettuce grown in the treatment with biochar. The same happened for root Ca and Mg contents. The lowest P and K contents of lettuce shoots and roots were recorded in T0. However, lettuce root N content decreased with biochar compared to T0. Therefore, root and shoot growth and nutrient uptake decreased with soil application of 4% (w/w) of biochar whereas the winery waste compost showed positive effects as soil fertilizer.