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Sugarcane (Saccharum officinarum L.) straw is a promising feedstock for bioelectricity and cellulosic ethanol in Brazil, but some straw is also needed to sustain nutrient cycling and crop yields. A literature review and two field studies were conducted to quantify above-ground macronutrient accumulation by sugarcane. Five straw removal scenarios using different amounts of biomass and nutrient composition data of top (green) and bottom (dry) leaves were constructed to estimate fertilizer requirements associated with straw removal in central southern Brazil. Calculated straw removal for five scenarios (S1 to S5) was 3.5, 7, 3, 6, and 12 Mg ha−1, respectively. By removing 12 Mg ha−1 of straw (S5), the potential N, P, K, Ca, Mg, and S removal was 69, 7, 92, 45, 16, and 14 kg ha−1, respectively. It represents a nutrient (NPK) replacement cost of US$ 90.00 ha−1 or US$ 7.60 per Mg of removed straw. The scenario analysis shows that less-impacting strategies are those that maintain top leaves in the field and harvesting only bottom leaves (S1 and S2), whereas total removal (S5) could double NPK-fertilizer consumption in sugarcane fields by 2050. Our findings provide stakeholder guidelines for improved straw management, establish an economic value for accumulated plant nutrients, and promote both socio-economic and environmental benefits of more sustainable bioenergy production in Brazil. [ABSTRACT FROM AUTHOR]

Global promotion of bioenergy for mitigating climate changes has arisen the interest of the Brazilian sugarcane industry to use crop residue (straw) as an important source of biomass for bioelectricity and cellulosic ethanol production. However, the sugarcane straw influences several soil properties, supporting soil quality and crop yields. Thus, defining an optimal removal rate would keep the benefits of the sugarcane straw in soil, and also maximize the bioenergy production. Shifts on soil bacterial structure have been used as a sensitive indicator of land management and could help to prescribe an optimal removal rate. We conducted a field study at two sites in São Paulo state to investigate how rates of sugarcane straw removal are associated with soil bacterial community changes over 1 year. Four sugarcane straw removal rates were evaluated: no removal (~ 14 Mg ha−1 of dry mass left) and 50% (~ 7.0 Mg ha−1), 75% (~ 3.5 Mg ha−1), and 100% of straw removal. The soil bacterial community structure was evaluated by the terminal restriction fragment length polymorphism (T-RFLP). Our results indicated that soil bacteria communities change over time, regardless of site conditions, and their changes are more strongly associated with changes on straw composition. A similar straw decomposition dynamics was observed under moderate (50%) and no removal treatments. Moderate straw removal induced the lowest modification of the bacterial niche occupancy and highest microbial interaction when compared with the no removal. Therefore, the identification of changes in soil bacterial structure community is useful to provide guidance for sugarcane straw removal. [ABSTRACT FROM AUTHOR]

Growing global demand for bioenergies has raised the interest in using sugarcane crop residues (straw) as feedstock to produce cellulosic ethanol and/or bioelectricity in Brazil. However, indiscriminate straw removal may deplete soil quality, particularly induced by its negative effects on soil microbial communities. A 2-year field study was conducted in soils classified as Oxisol (site 1) and Ultisol (site 2) in southeastern Brazil. The experiment started after plant cane cycle, establishing three straw removal rates (treatments), as follows: total removal, moderated removal, and no removal of sugarcane straw from the field. The amount of straw left on the soil surface and removed in the treatments varied according to the straw yield of each year and location. Soil samples were collected at 0–5- and 5–10-cm layers. The total abundance of bacteria, archaea, and fungi were analyzed by real-time quantitative polymerase chain reaction (qPCR). Bulk density, pH, base saturation, cation exchange capacity, phosphorus (P), and total organic C and soil microbial biomass C (MBC) were also measured. The soil chemical results indicate that the Oxisol presents higher inherent soil fertility compared with the Ultisol. In the first year, the straw had a different significant effect for bacterial and archaeal abundance in both soils and for fungi in the Oxisol site. In the second year, the numbers of gene copies of bacteria, archaea, and fungi in the 0–5-cm layer were higher under moderated and no removal when compared with those under total removal, regardless of site. The MBC decreased significantly under total removal of straw at the Ultisol site. Moderate straw removal has no significant effect on the abundance of soil microbes, but total straw removal may induce sharp reductions (from 23 to 54%) on microbial abundance. Soil microbes are a sensitive indicator to assess soil changes induced by crop residue management and should be taken into account for defining sugarcane straw management towards a sustainable bioenergy production. [ABSTRACT FROM AUTHOR]

Burning sugarcane (Saccharum officinarum) prior to manual harvest has been gradually replaced by a mechanized, green harvesting system that returns large amounts of crop residue (straw) to the soil surface in Brazil. A portion of the residue is required to protect soil against erosion, but some may be available as a feedstock for bioelectricity and cellulosic ethanol. Since excessive straw removal will degrade soil resources, it is imperative to determine the amount of straw needed for soil protection and to incorporate that information into decision-support tools so that straw management is sustainable. Our objective was to determine the amount of sugarcane straw needed to completely cover the soil surface. Photographic image analysis was used to quantify surface soil cover under 0 to 21 Mg ha−1 of straw. Photos of 1-m2 areas where various quantities of straw were homogeneously distributed were taken at a resolution of 8 megapixels (3264 × 2448 pixels) and then processed with ImageJ 1.52a software to obtain soil cover fraction. Straw layer thickness was also measured to the nearest millimeter using a ruler mounted in a rectangular acrylic box (0.5 m2, 0.15 m height) into which the equivalent of 1 to 21 Mg ha−1 of straw was placed. Soil cover increased exponentially, reaching more than 90% with 3 Mg ha−1 and full coverage with approximately 7 Mg ha−1. Straw layer thickness increased linearly by 0.5 cm per Mg of straw left on the soil surface. Our findings will lead to more sustainable straw management practices that balance soil conservation and provision of feedstock for bioenergy production. [ABSTRACT FROM AUTHOR]

In Brazilian sugarcane (Saccharum spp.) production systems, the practice of moving harvesting residue from row to inter‐row positions (i.e., raking) has increased in response to producer concerns over the potential negative effects of sugarcane straw on crop establishment and stalk yield. Despite increasing adoption among sugarcane farmers, the impacts of straw raking practices on plant growth and yield remain unclear. A 2‐yr experiment that included both dry and wet seasons was conducted at two sites in southeastern Brazil to evaluate straw management strategy effects on plant tillering, phytomass accumulation, plant nutritional status, and stalk yield. The experiments were established at the Bom Retiro mill and the Univalem mill. Experimental treatments included raking straw to inter‐rows (raked), total straw removal (bare soil), and no straw removal (straw cover). Raked and bare soil treatments improved plant tillering but did not influence final plant population. Straw management had a slight effect on phytomass accumulation. Reduction of phytomass yield was observed from the first to the second ratoon during both seasons at both sites. At Bom Retiro, phytomass yield decreased 37% for stands established during the dry season and 19% for stands established during the wet season. At Univalem, phytomass yield decreased 20% for stands established during the dry season and 30% for stands established during the wet season. Retaining straw in the field (regardless of treatment) increased leaf tissue P content but not stalk yield. Raking straw from row to inter‐row positions at these locations in southeastern Brazil had no benefit on sugarcane yield but may result in soil compaction and higher production costs over time. [ABSTRACT FROM AUTHOR]

• The Soil Management Assessment Framework (SMAF) detected soil health changes induced by straw removal in sugarcane areas. • Sandy and clayey soils were functioning at 41–56% and 67–86% of their full potential, respectively. • Intensive straw removal rates induced soil health degradation. • Healthier soils were associated with higher sugarcane stalk yields. • Soil health assessments are crucial to design sustainable straw management in Brazil. Sugarcane straw removal is a promising strategy to increase bioenergy production in Brazil. However, if straw removal is not properly managed, it may lead to soil health degradation, thus threatening sustainability of the entire bioenergy production system. In this context, a comprehensive assessment was conducted in 12 field experiments located in central-southern Brazil, the world's largest sugarcane-producing region, to investigate the impacts of straw removal scenarios on soil health, using the Soil Management Assessment Framework (SMAF). Two groups of experiments were arranged in this study, consisting of three (no–NR; moderate–MR; and total removal–TR) and four straw removal treatments (NR; low–LR; high–HR; and TR). Soil samples were collected (0–10 and 10−20 cm) to evaluate a selected set of soil health indicators (i.e., pH, available phosphorus, exchangeable potassium, bulk density and soil organic carbon). Indicators were individually scored and then integrated into an overall Soil Health Index (SHI) and its chemical, physical, and biological components. Datasets were analyzed by site, experiment groups and soil texture (clayey and sandy soils) and the relationships among straw removal, soil health, and sugarcane yield were also investigated. Our findings showed that soil health responses to straw removal were properly detected by SMAF scoring curves. Based on SHI, sandy and clayey soils were functioning at 41–56% and 67–86% of their full potential, respectively. Soil health degradation was mostly induced by TR and HR, while LR and MR resulted in minimal detrimental effects on a short-term basis compared with NR. However, sandy soils were more prone to soil health degradation, indicating that straw removal should be avoided on those soils. For clayey soils, decline in soil health was driven by physical degradation. Healthier soils were associated with higher sugarcane stalk yields, and physical degradation was the main driver for crop yield losses. Conclusions drawn from this study suggest that comprehensive soil health assessments by integrating chemical, physical and biological indicators are fundamental to design more sustainable straw management and bioenergy production systems in Brazil. [ABSTRACT FROM AUTHOR]