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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]

Green harvest sugarcane management has increased soil organic C and N stocks over time. However, emerging sugarcane straw removal to meet increasing bioenergy demands has raised concerns about soil C and N depletions. Thus, we conducted a field study in southeast Brazil over nearly three years (1100 days) for assessing soil C and N responses to increasing sugarcane straw removal rates. In order to detect the C input as a function of the different amounts of straw over three years, a field simulation was performed, where the original soil layer (0-0.30 m) was replaced by another from an adjacent area with low total C and δ13C. The treatments tested were as follows: (i) 0 Mg ha−1 year−1 (i.e., 100% removal), (ii) 3.5 Mg ha−1 year−1 (i.e., 75% removal), (iii) 7.0 Mg ha−1 year−1 (i.e., 50% removal), (iv) 14.0 Mg ha−1 year−1 (i.e., no removal), and (v) 21.0 Mg ha−1 year−1 (i.e., no removal + extra 50% of the straw left on the field). The results showed that sugarcane straw removal affected the soil C and total N pools. In the first 45 days of straw decomposition, a small but important straw-derived C portion enters into the soil as dissolved organic carbon (DOC). The lower the straw removal rate, the higher was straw-derived DOC content found into the soil, down to 0.50 m depth. After 3 years of management, keeping sugarcane straw on soil surface significantly increased C and N stocks within surface soil layer (0-0.025 m). Our findings suggest that under no straw removal management (i.e., 14 Mg ha−1), approximately 364 kg ha−1 of C and 23 kg ha−1 of N are annually stored into this low-C soil. The contribution of the straw-derived C (C-C4) to the total soil C increases over time, which accounted for about 60% under no straw removal rate. The greatest contribution of the C storage preferentially occurs into the fraction of organic matter (< 0.53 μm) associated with soil clay minerals. We concluded that indiscriminate sugarcane straw removal to produce cellulosic ethanol or bioelectricity depletes soil C stocks and reduces N cycling in sugarcane fields, impairing environmental gains associated with bioenergy production. Therefore, this information, linked with other agronomic and environmental issues, should be taken into account towards a more sustainable straw removal management for bioenergy production in Brazil. [ABSTRACT FROM AUTHOR]

Ethanol derived from sugarcane (Saccharum officinarum) could replace a substantial amount of fossil fuel in Brazil, provided the greenhouse gas (GHG) budget is balanced. Green harvest systems have significantly increased the potential amount of sugarcane residue (straw) available for harvest and bioenergy production, but several questions about how those practices will affect field GHG fluxes. In this sense, we quantified GHG emissions associated with four straw harvest rates (12 [no removal]; 6 [moderate removal]; 3 [high removal] and 0 Mg ha−1 [total removal] of dry matter of straw left on the soil), with and without 100 L ha−1 vinasse + N-fertilizer [80 kg ha−1 (NH 4) 2 SO 4 ]. GHG fluxes were measured for 60 days using static chambers. Our results showed that total straw removal reduced CO 2 and N 2 O emissions by 15% and 25%, respectively. After vinasse and fertilizer additions, CO 2 emissions were 2.5-fold higher, and N 2 O were 5-fold higher, regardless of straw rates. In additional, synergic effect of vinasse + fertilizer and straw removal decreased 60% CH 4 uptakes. Direct N2O emission factor was estimated at 0.32%. Net GHG emissions induced by vinasse + fertilizer applied in the sugarcane plantation were 220 g m−2 CO 2 e (or 60 g m−2 C-equivalent), disregarding straw harvest rate. Therefore, we conclude that maintaining at least 6 Mg ha-1 of sugarcane straw in the field will result in a 'win-win situation' by mitigating GHG emissions, sustaining soil, and still saving part of the raw-material for bioenergy production. • Total removal reduced emissions associated with CO2 by 15% and N2O, by 15%. • Vinasse + fertilizer increase CO2 emissions in 2.5 times and N2O, 5 times, regardless of straw harvest rates. • Direct N2O emission factor was estimated at 0.32%. • Net GHG emissions induced by vinasse + N-fertilizer were 220 g m-2 CO2e disregarding straw harvest rate. • Straw harvest rate did not alter the GHG emission. [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]

The sugarcane (Saccharum spp. L.) mechanical harvesting system leaves a large amount of straw mulch on the soil surface. The straw mulch may affect soil conditions, root regrowth, and sugarcane yield. Thus, this study assessed the response of sugarcane root system growth and stalk yield to different rates of straw removal. An experiment was conducted in a Rhodic Kandiudox with sand clay loam texture to test the impact of four rates of straw removal: no removal (18.9 Mg ha−1 of dry mass); moderate removal (8.7 Mg ha−1); high removal (4.2 Mg ha−1) and total removal on sugarcane root system and stalk yield. Higher concentrations of roots (60%) were found in the first 40 cm of soil. Moderate straw removal resulted in higher root mass (3.6 Mg ha−1) and stalk production (23 Mg ha−1 of dry mass). However, no straw removal reduced root mass by <40% (2099 kg ha−1) and reduced stalk yield by >20% (105 Mg ha−1). Through regression analysis, it was estimated that retaining between 8.5 and 13 Mg ha−1 of straw resulted in the highest root mass and stalk yield. Managing straw removal to retain a moderate amount enables producers to sustain suitable soil conditions for sugarcane root growth and stalk production while providing straw for industrial use. [ABSTRACT FROM AUTHOR]

• Soil compaction parameters were examined at around field capacity. • Operations for straw removal intensify soil compaction in sugarcane fields. • Baling operation may transmit stress greater than 400 kPa near the soil surface. • Initial soil bulk density govern the magnitude of impact by straw removal traffic. Straw removal requires a complex set of mechanized operations, including machines for raking, baling, and transporting the straw, which may intensify soil physical degradation in sugarcane fields. A field experiment was conducted to estimate the stress transmitted caused by a set of machines used for sugarcane straw removal and to quantify the impact of these stresses on soil physical indicators. The treatments consisted of reproducing the field traffic of individual and sequential operations used for straw removal, including raking; raking + baling; and raking + baling + transporting operations. Soil bulk density (BD), soil penetration resistance (SPR), macroporosity (MaP) and microporosity (MiP) at the depths of 2.5, 5, 15, and 30 cm were measured before and after the field operations. Contact area of the tires and stress transmitted into the soil induced by the straw removal operations were simulated by SoilFlex model and then used to assess the magnitude of the changes in soil physical indicators. Results showed that baling operation transmitted the highest stress (∼400 kPa), followed by straw raking and transporting, whose transmitted stress level exceed 300 kPa near the soil surface. The sum of sequential operations (i.e., raking + bale + transporting) showed the highest negative impact on soil physical quality. The magnitude of the changes in soil physical indicators occurred according to initial conditions of BD and transmitted stress. Our results show that operations for straw removal intensify soil compaction, which can be detrimental to sustain soil health and plant growth. Therefore, we advocate that traffic control strategies should be adopted during the straw removal operations to improve soil quality towards greater sustainability of sugarcane production in Brazil. [ABSTRACT FROM AUTHOR]