Organic carbon and nitrogen contents and their fractions in soils with onion crops in different management systems

The use of plant species in rotation or succession of crops can increase C and N contents and their fractions in the soil. The objective of this study was to evaluate the effect of using soil cover crops in succession and rotation with onion crops in different soil management systems on the total organic carbon (TOC), total nitrogen (TN), and C and N fractions in soil aggregates, and bulk soil. The experiment was implemented in April 2007 with eight treatments: succession of onion and maize in a no-tillage system (NTS) (T1); rotation of soil cover crops (winter) and biennial onion in a NTS (T2); rotation of maize, winter grasses, and onion in a NTS (T3); succession of summer legume and annual onion in a NTS (T4); rotation of summer grass, winter grasses, and annual onion in a NTS (T5); succession of summer legume, winter grass, and annual onion in a NTS (T6); succession of maize and onion in a conventional tillage system (CTS) (T7); and succession of intercrops of soil cover crops (summer), and annual onion in a NTS (T8). Undisturbed soil samples were collected in the 0.0-5.0, 5.0-10.0, and 10.0-20.0 cm soil layers in July 2014, and their aggregate (8.0 to 2.0 mm) and bulk soil (<2 mm) fractions were separated to evaluate their TOC, TN, particulate organic carbon and particulate organic nitrogen (OCP and ONP respectively), and mineral-associated organic carbon and mineral-associated organic nitrogen (OCM and ONM respectively). Soil turning due to the CTS in T7 (0.0-5.0 cm) reduced TOC, OCP, OCM, TN, ONP, and ONM, in the soil aggregates and in the bulk soil, when compared with the NTS with the use of soil cover crops in succession or rotation with onion crops (T1-T6 and T8). T6 increased the TOC, TN, OCP, OCM, ONP, and ONM contents in the soil aggregates and bulk soil when compared with the successions with only grasses or only legumes. T1 increased the soil TOC and TN contents in aggregates compared with the same succession in CTS. T8 had higher OCP (0.0-20.0 cm) and ONP (5.0-10.0 cm) contents in aggregates than in the bulk soil. In general, aggregates had higher TOC and OCM contents, and bulk soil had higher TN, OCP, ONP and ONM contents. The main changes resulting from the management systems and soil cover crop combinations used were observed in the particulate fraction, especially in the soil aggregates.