Carbon and nitrogen contents and aggregation index of soil cultivated with onion for seven years using crop successions and rotations

Onion is usually cultivated using conventional tillage system (CTS), with excessive soil turning, leaving it with low or no vegetation cover. This favors erosive processes and impacts negatively diverse edaphic attributes. Adopting soil management systems with conservationist bases that use permanent soil coverage and crop rotation can maintain or improve these attributes (for example, soil aggregation and soil organic matter). The objective of this work was to evaluate the total organic carbon (TOC) and total nitrogen (TN) contents, aggregation index, and aggregate mass distribution of a Humic Cambisol cultivated with onions in succession or rotation with other species in no-tillage system (NTS) and CTS. The treatments 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 (CTS-T7); intercrops cover plants (summer)/onion (NTS-T8). Seven years after the implementation of the experiment, the weighted mean diameter (WMD) of the aggregates, distribution of macroaggregates, mesoaggregates, and microaggregates, and TOC, and TN contents of the soil (0-0.05, 0.05-0.10 and 0.10-0.20 m layers) were evaluated. Periodic soil turning (CTS) in the succession of maize, and onion (T7) reduces TOC and TN contents in the soil surface layer, compared to succession and rotation systems with onion crops in NTS. This negative effect on soil quality is connected to the reduction of aggregate stability, especially the decrease in the amount of macroaggregates. The use of grasses, especially winter grasses in rotation with maize (T3), preceding onion crops in NTS increases TOC content in the soil surface layer. Higher TN accumulation in the surface layer is found in areas with more soil cover plant species in rotation or succession with onion in NTS (T2, T3, T5, T6 and T8). The use of NTS for onion crops generates high soil aggregate stability, with predominance of macroaggregates, regardless of the crop succession or rotation system used. Treatments with no winter soil cover plants (T1, T4 and T7) reduce soil TOC contents and the mass of water-stable macroaggregates and increase the amount of microaggregates in the soil surface layer when compared to the other treatments.