Aggregation index, carbon, nitrogen, and natural abundance of 13 C and 15 N in soil aggregates and bulk soil cultivated with onion under crop successions and rotations Lucas Dupont Giumbelli A , Arcângelo Loss A,F , Barbara Santos Ventura A , Elano dos Santos Junior A , Janaine Almeida A , Marisa de Cássia Piccolo B , Álvaro Luiz Mafra C , Claudinei Kurtz D , Gustavo Brunetto E , and Jucinei José Comin A A Universidade Federal de Santa Catarina, Centro de Ciências Agrárias, Itacorubi, Florianopolis, SC 88034000, Brazil. B Universidade de São Paulo, Centro de Energia Nuclear na Agricultura, Piracicaba, SP 13416970, Brazil. C Universidade do Estado de Santa Catarina, Centro de Ciências Agroveterinárias, Lages, SC 88520000, Brazil. D Empresa de Pesquisa Agropecuária e Extensão Rural de Santa Catarina, Ituporanga, SC 88400000, Brasil. E Universidade Federal de Santa Maria, Centro de Ciências Rurais, Camobi, Santa Maria, RS 97105900, Brasil. F Corresponding author. Email: arcangelo.loss@ufsc.br Abstract. Use of soil cover crops of different families in crop rotation or succession under no-tillage system (NTS) for onion production results in higher soil quality compared to land use systems with less plant diversity. The objective was to evaluate the effect of using different combinations of plant species from different botanical families in rotation and succession of soil cover crops in NTS for onion production on formation of macroaggregates, mesoaggregates, and microaggregates, and on total organic C (TOC) and N (TN) contents, including isotopic forms of C and N, in soil aggregates and bulk soil. The treatments (T) evaluated were maize/onion (NTS-T1); cover plants (winter)/onion (NTS- T2); maize/winter grasses/onion (NTS-T3); velvet bean/onion (NTS-T4); millet/cover plants (winter)/onion (NTS-T5); velvet bean/rye/onion (NTS-T6); maize/onion in conventional tillage system (CTS-T7); and intercrop cover plants (summer)/onion (NTS-T8). We evaluated macroaggregates (8.0–0.25 mm), microaggregates (<0.25 mm), and bulk soil (<2.0 mm) at depths of 0–5, 5–10, and 10–20 cm, in a nine-year field experiment. The greater plant diversity in T2–T6 and T8 resulted in higher geometric mean diameter (GMD) of aggregates compared to T1 and T7. The T8 was more efficient in increasing GMD in the 10–20 cm soil depth than the other treatments. The T1 was more efficient in improving the evaluated soil physical and chemical attributes than T7. The use of NTS with plants of the Poaceae and Fabaceae families in single or intercrop systems for onion production resulted in higher TOC and TN contents in the 0–5 and 5–10 cm soil depths compared to CTS. Isotope 15 N measurements showed that C and N were more protected in microaggregates in all evaluated treatments and depths compared to macroaggregates and bulk soil. Macroaggregates had more TOC and TN than microaggregates. Additional keywords: conventional tillage system, macroaggregates and microaggregates, natural abundance of 15 N, no-tillage system, onion production, soil cover plants. Received 26 November 2019, accepted 17 June 2020, published online 23 July 2020 Introduction Onion (Allium cepa) is a member of the Amaryllidaceae family and has wide use in human food (Souza and Lorenzi 2012). Onion is grown all over the world; China, India, and the USA are the main producing countries. Brazil is the ninth largest onion-producing country, with an estimated production of 1.72 million tonnes in 2017, which decreased to 1.66 million tonnes in 2018 (The Daily Records 2019; IBGE 2019). The state of Santa Catarina (SC) in Brazil has the largest national production, with ~630 000 tonnes, in an area of more than 20 000 ha in 2017 (SEAP 2017). Onion production in SC is predominantly in the Upper Itajai River Valley, especially in the municipality of Ituporanga, which has the largest national onion production in the country (Menezes Junior et al. 2013). The conventional tillage system (CTS) of the soil is traditionally used in SC for onion production, with intensive soil turning and use of highly soluble fertilisers and agrochemicals. Therefore, the soil physical, chemical, and biological attributes show intense degradation, making it necessary to seek alternatives for soil management, such as the no-tillage system (NTS) for onion production (Souza et al. 2013; Loss et al. 2015; Santos et al. 2017; Ferreira et al. 2018). One of the principles of NTS is protection of the soil surface and structure, thus avoiding the effects of surface Journal compilation Ó CSIRO 2020 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research, 2020, 58, 622–635 https://doi.org/10.1071/SR19346