Abstract
The no-till system with complex cropping sequences may improve the structural quality and carbon (C) sequestration in soils of the tropics. Thus, the objective of this study was to evaluate the effects of cropping sequences after eight years under the no-till system on the physical properties and C sequestration in an Oxisol in the municipality of Jaboticabal, Sao Paulo, Brazil. A randomized split-block design with three replications was used. The treatments were combinations of three summer cropping sequences - corn/corn (Zea mays L.) (CC), soybean/soybean (Glycine max L. Merryll) (SS), and soybean-corn (SC); and seven winter crops - corn, sunflower (Helianthus annuus L.), oilseed radish (Raphanus sativus L.), pearl millet (Pennisetum americanum (L.) Leeke), pigeon pea (Cajanus cajan (L.) Millsp), grain sorghum (Sorghum bicolor (L.) Moench), and sunn hemp (Crotalaria juncea L.). Soil samples were taken at the 0-10 cm depth after eight years of experimentation. Soil under SC and CC had higher mean weight diameter (3.63 and 3.55 mm, respectively) and geometric mean diameter (3.55 and 2.92 mm) of the aggregates compared to soil under SS (3.18 and 2.46 mm). The CC resulted in the highest soil organic C content (17.07 g kg-1), soil C stock (15.70 Mg ha-1), and rate of C sequestration (0.70 Mg ha-1 yr-1) among the summer crops. Among the winter crops, soil under pigeon pea had the highest total porosity (0.50 m³ m-3), and that under sunn hemp had the highest water stable aggregates (93.74 %). In addition, sunn hemp did not differ from grain sorghum and contained the highest soil organic C content (16.82 g kg-1) and also had the highest rate of C sequestration (0.67 Mg ha-1 yr-1). The soil resistance to penetration was the lower limit of the least limiting water range, while the upper limit was air-filled porosity for soil bulk densities higher than 1.39 kg dm-3 for all cropping sequences. Within the SC sequence, soil under corn and pigeon pea increased least limiting water range by formation of biopores because soil resistance to penetration decreased with the increase in soil bulk density.
Highlights
Oxisols are highly weathered and have a strong microstructure due to the presence of Fe- and Al-oxides and hydroxides (Six et al, 2002)
A higher amount of above-ground biomass of the winter crops was measured in the soybean/soybean and soybean/corn sequences, as summer crops, when compared to those observed in the corn/corn sequence (Table 1)
In another experiment conducted at the same site in the 2007/2008 and 2008/2009 growing seasons, Marcelo et al (2012) observed that the biomass of cereals such as pearl millet and grain sorghum increased by 56 and 69 % when grown after soybean than after corn/corn in the summer
Summary
Oxisols are highly weathered and have a strong microstructure due to the presence of Fe- and Al-oxides and hydroxides (Six et al, 2002). A no-till system is widely recommended to minimize these adverse effects and improve soil quality. The no-till system improves structural properties and can enhance C sequestration in soils of tropical regions at a rate of 0.35 Mg ha-1 yr-1 C in the 0-0.20 m depth (Bayer et al, 2006), and 0.43 Mg ha-1 yr-1 C in the 0-0.10 m depth (Six et al, 2002), contributing to mitigation of global warming (Bayer et al, 2006). The intensity of no-till effects on soil quality depends on specific cropping sequences. Establishment of cropping sequences in regions of the Cerrado (tropical savanna) biome, especially of winter crops, is limited by a relatively dry season from April to September. Winter crops must have a deep and vigorous root system (Marcelo et al, 2009) and exhibit fast establishment to produce the maximum biomass by late autumn, after which the rains cease (Bayer et al, 2006)
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