Cultivating cotton in a direct planting system increases the carbon stock in the soil, increases the nitrogen content and also increases productivity compared to the conventional soil preparation system. This was demonstrated by a study carried out over nine years by scientists from Embrapa Algodão (PB) in the Brazilian Cerrado.
After almost a decade, the direct planting system increased the carbon content in the first five centimeters of soil depth by 55%, and the carbon stock in the layer up to 20 cm deep by 40%. “This rate of increase in soil carbon is almost five times greater than the global target proposed by the ‘4 per 1000’ initiative, presented during the 21st United Nations Conference on Climate Change (COP21). This shows the potential of cotton cultivation in a direct planting system to contribute to the commitments made by Brazil with several other countries in favor of reducing global warming”, assesses Embrapa researcher Alexandre Cunha de Barcellos Ferreira, who coordinated the study.
The conservation system of soil preparation also resulted in nitrogen accumulation and gains in cotton fiber productivity. The nitrogen content in the soil in the direct planting system, in the first five centimeters, was 50% higher than the levels found in other systems. The productivity recorded was around 10% higher and has the potential to increase by up to 30%.
The experiment compared the direct planting system and conventional soil preparation, with crop succession or rotation - cotton, soybeans, corn and brachiaria (Urochloa ruziziensis). The best results were obtained in the direct planting system of cotton in rotation with soybean intercropped with brachiaria, and with corn in intercropped with brachiaria.
According to Ferreira, the research results would be a stimulus to expand the adoption of the practice among cotton farmers, contributing to the mitigation of global warming. “In addition to increasing cotton productivity and accumulating more carbon in the soil, the direct planting system provides greater productive resilience and subsidies to maintain or expand world trade for a society that is increasingly demanding on sustainable production processes”, he argues.
The research was conducted under rainfed conditions in the experimental area of Goiás Foundation, municipality of Santa Helena de Goiás. In the last seven years of the study, after the consolidation of cultivation and soil management systems, total fiber productivity was 13.958 kilos per hectare in the direct planting system, while in monoculture with conventional tillage soil was 12.698 kilograms per hectare. “In the direct planting system there was an increase in fiber productivity of 1.260 kilos per hectare, which is practically equivalent to a new harvest”, observes Ferreira.
“The greater accumulation of carbon in the surface layer of the soil in the direct planting system is due to harvest residues and the abundant root system of grasses that form pastures. The cultivation of Urochloa ruziziensis in the second harvest, as a cover plant for the formation of straw and direct sowing, it protects the soil and improves its chemical, physical and biological characteristics”, says the Embrapa researcher.
In the plot with direct planting, the carbon content found in the most superficial soil layer, from zero to five centimeters, was around 55% higher than that found in plots with conventional soil preparation. In the deeper layers, up to 30 cm, there were no significant variations.
Ferreira reports that, in conventional soil preparation, organic matter from crop residues from each production system studied was incorporated into the soil, up to approximately 20 cm deep, through plowing and harrowing. “These processes favor the decomposition of organic matter, especially less stable organic compounds. As a result, conventional tillage treatments, regardless of the soy-cotton or soy-corn-cotton rotation, were not sufficient to increase the carbon content in the most superficial layer,” he explains.
In layers from 11 cm to 30 cm there was no difference between the plots due to the influence of the crop roots, which are found at that depth. However, in layers from 31 cm to 40 cm, the soil with no-tillage system had a higher carbon content compared to conventional tillage. “This is probably due to the high capacity for development of the roots of Urochloa ruziziensis to greater depths”, says the scientist.
The study also revealed that there was a loss of organic matter in plots with conventional soil preparation over the nine years. “We measured organic matter in the soil before the start of work, in 2005, and at the end, in 2014, and we found that it decreased by 7% in the layer from zero to 20 centimeters”, says Ferreira.
Over the nine years of research, 996 kilograms per hectare of mineral nitrogen were added to the cotton monoculture system, while in the direct planting system it was 649 kilograms per hectare. “Despite having received 347 kilograms per hectare less of mineral nitrogen, the maintenance of nitrogen in the soil under the direct planting system was greater, equivalent to 320 kilograms per hectare. This accumulation of nitrogen can be attributed not only to the lack of soil disturbance, but also to the potential for nutrient cycling by Urochloa ruziziensis”, observes the researcher, highlighting that the inclusion of soybeans in the rotation was inefficient in increasing the nitrogen content in the soil with conventional tillage.
“Production systems based on conventional soil preparation with plows and harrows, with or without rotation of cotton, soybeans and corn, did not positively influence the carbon balance and were not sufficient to improve carbon stocks. In the direct planting system, the stock increased by 20% after nine years”, concludes Ferreira.
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