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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]

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]

Exploiting native soil phosphorus (P) and the large reservoirs of residual P accumulated over decades of cultivation, namely "legacy P", has great potential to overcome the high demand of P fertilisers in Brazilian cropping systems. Long-term field experiments have shown that a large proportion (> 70%) of the surplus P added via fertilisers remains in the soil, mainly in forms not readily available to crops. An important issue is if the amount of legacy P mobilized from soil is sufficient for the crop nutritional demand and over how long this stored soil P can be effectively 'mined' by crops in a profitable way. Here we mapped the spatial–temporal distribution of legacy P over the past 50 years, and discussed possible agricultural practices that could increase soil legacy P usage by plants in Brazil. Mineral fertiliser and manure applications have resulted in ~ 33.4 Tg of legacy P accumulated in the agricultural soils from 1967 to 2016, with a current annual surplus rate of 1.6 Tg. Following this same rate, soil legacy P may reach up to 106.5 Tg by 2050. Agricultural management practices to enhance soil legacy P usage by crops includes increasing soil pH by liming, crop rotation, double-cropping, inter-season cover crops, no-tillage system and use of modern fertilisers, in addition to more efficient crop varieties and inoculation with P solubilising microorganisms. The adoption of these practices could increase the use efficiency of P, substantially reducing the new input of fertilisers and thus save up to 31.8 Tg of P fertiliser use (US$ 20.8 billion) in the coming decades. Therefore, exploring soil legacy P is imperative to reduce the demand for mineral fertilisers while promoting long-term P sustainability in Brazil. [ABSTRACT FROM AUTHOR]

The original version of this article was published with a wrong heading by mistake. The correct heading is Original Paper. [ABSTRACT FROM AUTHOR]