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.

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The application of fertilizers as a topdressing in maize raises serious concerns because too much fertilizer is retained in the upper leaves, causing burning to the tissues. In this study, the use of a controlled-release and a stabilized fertilizer (with 3, 4-dimethylpyrazole phosphate) was compared with the application of a conventional fertilizer split into two equivalent applications in a forage maize-oats cropping system. In maize, 100 and 200 kg N ha À1 of different fertilizers were used in addition to an unfertilized control. The oat crop was not fertilized, since it served only as a winter catch crop. Maize dry matter (DM) yield increased significantly with N rate only in 2019, being the second growing season, with the control showing the lowest average value (7.1 t ha À1 ). The most fertilized treatments (200 kg N ha À1 ) gave the highest DM yields, ranging between 14.2 and 16.7 t ha À1 , but with no significant differences between them. Oats had a relevant role as a catch crop recovering residual N that could have potentially been lost from the soil. Stalk nitrate concentration proved to be very sensitive to N fertilization (varying from 150.4 to 1945.6 mg kg À1 in 2018 and 494.9 to 1574.9 mg kg À1 in 2019), showing great potential as a tool of N management. These three fertilization strategies seem to be valid options that farmers can consider, after incorporating technical-economic information related to equipment suitability and the price of fertilizers.