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Decomposition dynamics altered by straw removal management in the sugarcane-expansion regions in Brazil

Varanda, Letícia L., Cherubin, Maurício R., Cerri, Carlos E. P.
Soil research 2019 v.57 no.1 pp. 41-52
biofuels, carbon, carbon nitrogen ratio, cellulose, climate, decision making, field experimentation, harvesting, hemicellulose, industry, models, nitrogen, raw materials, scanning electron microscopy, soil, soil quality, straw, sugarcane, Brazil
Brazilian bioenergy production based on sugarcane is globally known as a sustainable energy matrix. In order to supply the growing demand for biofuels, the sugarcane area expanded by 46% in the last decade in Brazil. In addition, the industry’s interest in using sugarcane straw as raw material for bioenergy production has recently increased. Thus, understanding the straw decomposition dynamics in these new frontiers is imperative to support decision-making that will define a sustainable straw removal rate. A field experiment was conducted in three sites under a climate and soil gradient (from central to south) within the sugarcane-expansion region (i.e. Goiás, Mato Grosso do Sul and Paraná states) to evaluate the straw decomposition. The amounts of 3, 6 and 12 Mg ha-1 of straw (dry mass, DM) were left on the soil surface after harvesting, representing the straw removal rates of about 75, 50 and 0% respectively. We quantified DM loss, carbon (C) and nitrogen (N) contents and biochemical composition of the straw throughout a crop cycle (300 days). In addition, visual changes in the decomposing straw were evaluated through scanning electron microscopy. An exponential model was efficiently fitted to data of DM loss, describing the pattern of straw decomposition over time. The DM, C losses and cellulose and hemicellulose contents of straw changed over time among sites, reflecting the local edaphoclimatic conditions favourable for decomposition. The C:N ratio decreased from 106:1 to 65:1 for the lowest straw amount and 41:1 for the highest straw amount, and were correlated with the lowest (65%) and highest (76%) DM loss respectively. After the crop cycle, sugarcane straw changed visually, possible due to loss of biochemical compounds. In conclusion, lower straw removal induced an increase in the straw decomposition rate (higher DM loss over time), which could boost positive agronomic impacts such as increased C stock and improved soil quality.