Aggregation index, carbon, nitrogen, and natural abundance of 13C and 15N in soil aggregates and bulk soil cultivated with onion under crop successions and rotations

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 15N 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.

Combinations of Plant Species for Rotation With Onion Crops: Effects on the Light Fraction, Carbon, and Nitrogen Contents in Granulometric Fractions of the Soil Organic Matter

“…More than 80% of COT is composed of MOC; thus, the addition of plant materials for the maintenance and increase of COT contents increases the compartments of MOC in the soil (Bayer et al 2004;Conceiç ã o et al, 2013). The higher MOC contents in T4 and T5 corroborates the higher COT contents in these treatments (Giumbelli et al, 2020).…”

“…The higher MOC contents in aggregates in T2, T4, T5, and T6 (0-5 cm), T5 (5-10 cm), and T8 (10-20 cm) were due to the preservation of the soil structure in the NTS (Giumbelli et al, 2020). This favors the formation of stable aggregates, the release of exudates rich in polysaccharides and cementitious agents by the soil cover plants, and the permanence of plant residues that protect the aggregates and decrease the soil surface temperature, leading to the cycling of carbon.…”

“…The samples were then placed in plastic bags and sent to the Laboratory of Soil Management and Classification of the Federal University of Santa Catarina, where they were air-dried. Undisturbed samples were manually disaggregated following cracks or weak points, and sieved in 8.00 mm and 2.00 mm mesh sieves to obtain the soil aggregate indexes (Giumbelli et al, 2020). In general, each undisturbed sample weighed approximately 900 to 1,000 g. The aggregates with diameters of 8.00 mm > Ø ≥ 2.0 mm, which were used for the evaluation of chemical properties of soils, represented approximately 60% of the total weight of soils under NTS.…”

“…The aggregates of soils under CTS did not exceed 30% to 35% of the total soil weight. The amounts referring to aggregate stability and aggregation indexes are described in Giumbelli et al (2020). Disturbed samples were sieved in a 2.00 mm mesh sieve to obtain the air-dried fine earth (bulk soil).…”

“…The higher POC contents found in aggregates of the 10-20 cm layer in the treatment T7 was due to the incorporation of plant residues to the soil by plowing and harrowing, which also explains why T7 had the lowest POC contents in aggregates of the 0-5 cm soil layer. POC contents in T8 and T5 were similar for the layer 10-20 cm because of the intercrop of soil cover crops, which resulted in higher DMG and POC contents in macroaggregates (Giumbelli et al, 2020), creating an environment that provided higher protection for POC.…”