Green manure-15N absorbed by broccoli and zucchini in sequential cropping

•Green manure-N provide the cultivation of at least two vegetable crops.•Residual N is dependent on sunn hemp amount with greatest effects in larger doses.•N use by plants can contribute to scientific understanding in green manure processes.

Short-cycle crops use part of the N from green manure fertilization, while the remainder continues to be available for the subsequent ones. The aim of the present work was to evaluate the N use efficiency of sunn hemp, Crotalaria juncea L. (Fabaceae) in the sequential cropping of Brassica oleracea var. italica (Brassicaceae) followed by zucchini, Cucurbita pepo L. (Cucurbitaceae). Three doses of C. juncea green manure 3, 6, and 9 t ha−1 were evaluated, each one added with 12 t ha−1 of organic compost. The N use efficiency was based on the 15N isotope dilution technique. The total N accumulated in the shoots of broccoli and zucchini with the highest dose of C. juncea was 78.25 and 70.42 kg ha−1, respectively. The green manure-N recovery efficiency and quantity with the highest doses of C. juncea in broccoli were 74.39 and 47.42 kg ha−1 and in zucchini 58.82 and 31.95 kg ha−1, respectively. The green manure-N supplied 28.50% of N for broccoli and 16.22% for zucchini. The N residual effect from C. juncea green manure is enough to provide for the cultivation of at least two vegetable crops - broccoli followed by zucchini.