Sugarcane Straw Removal Research Articles

Carbon savings from sugarcane straw-derived bioenergy: Insights from a life cycle perspective including soil carbon changes

Sugarcane straw removal for bioenergy production will increase substantially in the next years, but this may deplete soil organic carbon (SOC) and exacerbate greenhouse gas (GHG) emissions. These aspects are not consistently approached in bioenergy life cycle assessment (LCA). Using SOC modeling and LCA approach, this study addressed the life cycle GHG balance from sugarcane agroindustry in different scenarios of straw removal, considering the potential SOC changes associated with straw management in sugarcane-cultivated soils in Brazil. Long-term simulations showed SOC losses of up to −0.5 Mg ha−1 yr−1 upon complete straw removal, whereas the moderate removal had little effects on SOC and the maintenance of all straw in the field increased SOC accumulation by up to 0.4 Mg ha−1 yr−1. Our analysis suggests that accounting for SOC changes in LCA calculations could lower the net GHG benefits of straw-derived bioenergy, whose emissions intensity varied according to soil type. Overall, SOC depletion induced by complete straw removal increased the life cycle GHG emissions of straw-derived bioenergy by 26 % (3.9 g CO2eq MJ−1) compared to a scenario without taking SOC changes into account. Straw removal for cellulosic ethanol could be effective for mitigating GHG emissions relative to gasoline, but it was not advantageous for bioelectricity generation depending on the energy sources that are displaced. Therefore, straw-induced change of SOC stocks is a critical factor to model life cycle GHG emissions of straw-derived bioenergy.

Sugarcane Straw Removal Enhances Short-Term N Source and N Source Efficiencies Under Sandy Soil Condition

Sugarcane Straw Removal Enhances Short-Term N Source and N Source Efficiencies Under Sandy Soil Condition

Quantity, quality and physical protection of soil carbon associated with sugarcane straw removal in southern Brazil

Sugarcane straw is identified as a potential energy feedstock to increase bioenergy production, but advances are required in understanding the straw removal impacts on soil organic carbon (SOC) stability and storage. The main objective of this study was to evaluate the quantity, quality and physical protection of SOC under areas with different straw removal rates over a six-year period. A field experiment was conducted in randomized blocks with four replications, including the following straw removal treatments: total removal (TR), high removal (HR), low removal (LR), and no removal (NR), corresponding to the quantities of 0, 5, 10 and 15 Mg ha−1 of dry straw maintained on soil surface, respectively. Effects of straw removal on SOC stocks and its temporal dynamics, as well as the rates of carbon (C) incorporated into the soil were evaluated to a 0.3-m depth. The effects on particulate organic matter (POM) and mineral-associated organic matter (MAOM) fractions were evaluated through physical fractionation of SOM. Finally, it was evaluated the distribution of aggregates, the C content of aggregates and the C preservation capacity (CPC) in fractions of aggregates. Our findings revealed that TR and HR decreased SOC stocks at 0.0–0.1 m and 0.0–0.3 m depths. The data indicated that about 24% of the C added via straw were retained in soil, and NR showed a SOC accumulation rate of 1.42 Mg ha-¹ year-¹ relative to baseline. Low straw removal increased POM (46.2%) and MAOM (12.4%) in surface layer as compared to TR. The straw maintenance in the field increased the proportion of soil macroaggregates, resulting in higher amounts of C preserved in this fraction. Our findings suggest that maintaining 10 Mg ha-¹ of straw was enough to favor physical protection and sustain storage of SOC over time. Therefore, the surplus of straw could be removed to produce bioenergy, although this management may hinder soil C sequestration and its benefits for sugarcane production and climate change mitigation.

Linking soil physical quality to shoot and root biomass production in scenarios of sugarcane straw removal

Understanding how sugarcane straw removal affects soil physical attributes and root development over successive cultivation years is critical for sustainable straw-based bioenergy production. The aim of this study was to evaluate the effects of straw removal on soil physical quality and its relationship with sugarcane root development and crop yield in soils with contrasting textures. Three straw removal rates corresponding to total removal (TR), moderate removal (MR), and no removal (NR) were arranged in a randomized block design with four replicates. Soil was sampled at depths of 0–0.10, 0.10–0.20, and 0.20–0.40 m to determine soil bulk density, macro- and microporosity, soil penetration resistance, and carbon stock. Annual assessments of sugarcane root and stalk dry biomass were conducted. Overall, excessive straw removal rates were detrimental to soil porosity, C stock, and increased bulk density in the surface layer over successive years compared to moderate and no removal rates. Higher soil penetration resistance levels were observed under total removal, reaching values of 2.6 MPa in the 0–0.10 m layer in both soils. Average reductions in root biomass of 23–33% were verified at moderate and total removal rates in both soils, which affected sugarcane stalk yield, with maximum losses of 24% over crop cycles in the clayey soil. In the sandy loam soil, increases in soil C were correlated with reduced soil penetration resistance, which positively influenced root biomass production. Soil compaction over the years was particularly detrimental in the clayey soil, where the biomass production performance of sugarcane roots and stalk yield decreased in response to increased soil bulk density. Our results suggest that reductions in root biomass may occur in response to straw removal, as well as the loss of soil physical quality induced by soil compaction, with negative effects on C accumulation and soil resistance to root penetration.

Changes in soil temperature and moisture due to sugarcane straw removal in central-southern Brazil

Crop residues left in the field cover and protect the soil surface, and regulate key processes and functions, such as gas and water exchanges. However, the Brazilian sugarcane (Saccharum officinarum L.) sector has begun to use straw as feedstock to produce bioenergy. We conducted a field study to evaluate the effects of sugarcane straw removal in soil temperature and moisture changes at three sites (with different soil textures: Site 1 - clayey Oxisol, Site 2 - medium texture Oxisol, and Site 3 - sandy Ultisol) in the state of São Paulo, Brazil. The experimental design was randomized blocks with four rates of straw removal: i) no removal (NR); ii) moderate removal (MR); iii) substantial removal (SR), and iv) total removal (TR). Soil temperature was measured by sensors in the 0- to 5- and 5- to 10-cm soil layers. Undisturbed soil samples were collected from the 0- to 20- and 20- to 40-cm layers to determine soil moisture. Intensive straw removal (HR and TR) increased the soil temperature between 2 and 3 °C and the thermal amplitude between 5 and 9 °C in the 0- to 5-cm layer, compared to MR and NR. Soil moisture decreased between 0.03 and 0.07 g g–1 in the 0- to 20-cm layer with intensive straw removal. The sandy soil was more susceptible to straw removal. Therefore, straw maintenance on the soil surface plays an essential role in temperature regulation and preservation of higher soil moisture, especially in regions with severe water deficits and long periods of water stress.

Machinery traffic in sugarcane straw removal operation: Stress transmitted and soil compaction

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.

Multilocation changes in soil carbon stocks from sugarcane straw removal for bioenergy production in Brazil

AbstractSugarcane (Saccharum spp.) straw has been identified as a valuable asset for bioenergy production, but it is also a primary carbon input for sustaining soil resources. Therefore, excessive removal of this crop residue can potentially decrease soil organic carbon (SOC) and have negative environmental consequences. Most crop residue management studies focus on site‐specific responses which are important, but do not provide comprehensive assessments across the multiple edaphoclimatic conditions needed by decision makers to guide sustainable sugarcane straw management. Temporal modifications in SOC stocks induced by straw removal were quantified in 10 field experiments located in south‐central of Brazil. Each field experiment was arranged in a randomized block design with four replications, including four straw removal rates: total (TR), high (HR), low (LR) and no removal (NR). Soil sampling was performed to a 30 cm depth at the trial establishment (baseline) and after 2 and 4 consecutive years of straw removal. The data indicate that about 19% and 25% of the C added via straw were retained in areas of sandy and clay soils, respectively, and show that excessive removal rates depleted SOC stocks across the evaluated sites. Furthermore, regardless of removal rate, maintenance of straw on sandy soils was not sufficient to sustain SOC stocks, which were depleted by 2.3, 1.9, 1.5, and 1.4 Mg ha−1 year−1 for the TR, HR, LR, and NR. Our measured data provide scientific information to support policy and management decisions for straw‐derived bioenergy in Brazil. They also suggest straw removal from clay soils will be less deleterious to SOC stocks than from sandy soils, where removal should be avoided.

Soil health response to sugarcane straw removal 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.

Sugarcane straw preservation results in limited immobilization and improves crop N-fertilizer recovery

Current knowledge indicates that sugarcane straw is a short-term sink of fertilizer nitrogen (N) due to N immobilization during the decomposition process. This study aimed to determine whether sugarcane straw removal from the field benefits the N-fertilizer recovery by plant (NRP), and if legume cultivation will further improve NRP. Half of a field was planted with sunn hemp (Crotalaria spectabilis) and half was left fallow. In the second ratoon, treatments with 0, 50, and 100% of sugarcane straw maintenance levels were installed, receiving 120 kg ha−1 N over straw. A microplot of ammonium nitrate labeled with 1.99% atoms 15N was included. NRP was quantified at 92, 164, and 264 days after fertilization (DAF). At harvest (264 DAF), soil samples and straw (<4 mm and >4 mm fractions) were collected to determine N-fertilizer recovery in the soil (NRS) and in the straw (NRSt). Maintaining 50 and 100% of straw improved NRP over the sugarcane cycle. NRP was not improved by crop rotation cultivation and NRS averaged 50% in the entire soil profile. NRSt at harvest averaged 1–3% in the 50% and 100% straw levels. Contrary to the current view, N-fertilizer immobilization in the straw was negligible and keeping straw could improve NRP. The findings indicate that increasing N rates in green cane trash blanket system to compensate for the N-fertilizer immobilization is not necessary, and that straw removal for bioenergy production will change the short-term demand for N-fertilizer.

Long term sugarcane straw removal affects soil phosphorus dynamics

Brazil is the world's largest sugarcane producer. This crop leaves huge amounts of straw behind which have to be managed with innovative approaches. In this study, a field experiment was carried out over six successive years of sugarcane harvests to evaluate the effect of straw maintenance on the field on changes in soil phosphorus (P) fractions. Four straw maintenance rates were tested: (i) maintaining 15 Mg ha−1 (SM15); (ii) maintaining 10 Mg ha−1 (SM10); (iii) maintaining 5 Mg ha−1 (SM5), and (iv) maintaining 0 Mg ha−1 (SM0). Our results showed that low maintenance of straw (SM5) resulted in the highest amount of P extracted by anion exchange resin and inorganic P extracted by 0.5 mol L−1 NaHCO3 in 0−5 and 5−10 cm soil depths. While total removal of residues resulted in soil P deficiency after six crop cycles, keeping straw on the soil surface at the rate of 5 Mg ha−1 supports plant growth without the need for P fertilizer application. We detected inorganic P extracted by 0.1 mol L−1 NaOH as the sink of P derived from straw since it was the only P fraction which increased with increasing straw on the soil surface. Non-labile P fractions were not influenced by straw maintenance rates in all depths. This is a promising result since it shows that P derived from straw does not accumulate in non-labile P fractions which are not plant available. Inorganic P was not influenced by straw removal rates and total P was only changed in 0−5 cm soil layer, while organic P was affected in all depths. We observed that organic P is available to sugarcane plants under SM0, SM5, and SM10, while under SM15, organic P does not play a role in sugarcane nutrition.

Trade-Offs between Sugarcane Straw Removal and Soil Organic Matter in Brazil

Environmental benefits from bioenergy production derived from sugarcane crop residues (straw) can be lost by soil organic matter depletion resulting from excessive straw removal rates from fields. Soil organic carbon stock is the core for sustaining soil health, supporting nutrient cycling, and sequestering carbon dioxide. To find out that how much sugarcane straw can be removed from the field to produce bioenergy without changes in soil C concentrations, we investigated effects of straw removal rates (total, moderate, and no removal of sugarcane straw) on soil carbon and nitrogen fractions in an Oxisol and an Ultisol in southeastern Brazil for two years. Soil C and N fractions were affected by increased rates of straw removal at the second year. In the Oxisol, total straw removal decreased labile and microbial-C by ~30% and soil C stock by 20% compared to no straw removal. No removal decreased microbial-N and total N stock by ~15% and ~20%, respectively. In the Ultisol, no straw removal resulted in increases in C stock by &gt;10% and labile and microbial-C by ~20% related to total straw removal. Total straw removal showed more microbial-N (~10%) and total-N stock (~25%) compared to no straw removal. The moderate straw removal intensity (i.e., 8 to 10 Mg ha−1 of straw) may control the straw-C release to soil by straw decomposition. This study suggests that excessive straw removal rates should be avoided, preventing SOM depletion and consequently, soil health degradation. Moderate straw removal seems to be a promising strategy, but long-term soil C monitoring is fundamental to design more sustainable straw management and bioenergy production systems.

Soil dissolved organic carbon responses to sugarcane straw removal

AbstractGlobal demand for bioenergy increases interest in biomass‐derived fuels, as ethanol from sugarcane straw. However, straw is the main carbon source to soil and its removal reduces C input, affecting active fractions (dissolved organic carbon, DOC) and C storage. To quantify the effects of straw removal on DOC and C stocks, we built lysimeter system using soil (Rhodic Kandiudox) from sugarcane field. We evaluated four soil depths (1, 20, 50 and 100 cm) and four straw removal rates: no removal NR, medium MR, high HR and total TR, leaving 12, 6, 3 and 0 Mg/ha on the soil surface, respectively. After rainfall, drainage water was collected and analysed for DOC content. Soil C stocks were determined after the 17‐month. Total DOC released at 1‐cm depth amounted to 606, 500, 441 and 157 kg/ha in NR, MR, HR and TR, respectively. Net‐DOC suggests straw as the main source of DOC. Most of DOC in NR (50%) was retained within the 1–20 cm layer, resulting in higher C stock (10 Mg/ha) in the topsoil. In HR and MR, DOC retention was higher within 20–50 cm, suggesting differences in DOC composition. DOC in TR was 40% higher at 20 cm than at 1 cm, indicating C losses from topsoil. Low concentrations of DOC were found at 100‐cm depth, but representing 30% in TR. Straw removal for bioenergy production is sustainable, but we should leave at least 3 Mg/ha of straw to ensure DOC production and soil C storage, taking account the DOC contribution to key soil functions.

Correction to: Sugarcane Straw Removal: Implications to Soil Fertility and Fertilizer Demand in Brazil

The original version of this article was published with a wrong heading by mistake. The correct heading is Original Paper.

Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy to Assess Decomposition Dynamics of Sugarcane Straw

Crop residue decomposition is an essential process that affects soil properties and ecosystem services related to nutrient recycling, soil organic matter, and aggregation. Therefore, understanding crop residue decomposition is recommended to evaluate soil changes and define a more sustainable sugarcane straw removal management for bioenergy production. The straw chemical composition may be an efficient indicator to investigate this complex process. Thus, we conducted a field study to assess the sugarcane straw composition changes over 1 year using DRIFT spectroscopy technique. The experimental design consisted of a split-plot and randomized block design, and straw removal rates provided the main plot factor with time of sampling as the split-plot factor. Three sugarcane straw removal rates were evaluated: no removal (~ 14.0 Mg ha−1 of dry mass), 50 (~ 7.0 Mg ha−1) and 75% (~ 3.5 Mg ha−1). Decreases in the absorbance associated with labile C (1200 to 1100 cm−1), and increases in more recalcitrant C (1800 to 1600 cm−1) were showed by the DRIFT spectra. In regional scale, the straw chemical composition was affected by environmental conditions, whereas at a local scale, the main driver was the decomposition time. Specific DRIFT peaks were correlated with the referential wet chemical method, commonly named Van Soest method. From the correlations between the Van Soest and DRIFT methods, we suggest using the absorbance of 896, 987, 1173, and 1447 cm−1 bands to assess cellulose and hemicellulose changes and 1500 cm−1 band for lignin changes in sugarcane straw. The application of DRIFT analysis to follow the decomposition process is a feasible and clean methodology to detect straw chemical changes induced by the environmental conditions and decomposition time.

Does Sugarcane Straw Removal Change the Abundance of Soil Microbes?

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.

Sugarcane Straw Removal: Implications to Soil Fertility and Fertilizer Demand in Brazil

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.

Changes in Soil Pest Populations Caused by Sugarcane Straw Removal in Brazil

The adoption of green cane harvesting system without burning has triggered the increase of pest infestations in sugarcane fields. This makes straw removal a promising alternative for pest control in sugarcane fields. However, further research is needed to understand the impacts of straw removal on pest infestation under field conditions. The objective of this study was to evaluate the incidence of root spittlebug (Mahanarva fimbriolata) and sphenophorus (Sphenophorus levis) under different rates of straw removal and contrasting edaphoclimatic conditions in the Sao Paulo state, southeastern Brazil. Eight field experiments were conducted throughout the sugarcane cropping cycle (two crop seasons) and were arranged in a randomized block design with four replicates. The experimental design included four straw removal rates: total removal, low removal, high removal, and no removal. The incidence of pests was evaluated during September and March (spring/summer seasons in the Southern Hemisphere) for root spittlebug and April to August (autumn/winter) for sphenophorus populations. Straw removal reduced root spittlebug infestation and this effect was more evident in clayey soils. The incidence of root spittlebug was three times higher in clayey soils compared to sandy soils. Straw removal did not influence sphenophorus population but reduced damages in sugarcane plants especially in clayey soils. The damage caused by sphenophorus was three times higher in clayey than in sandy soils. This study emphasizes that straw removal by itself is not a feasible strategy to control the root spittlebug and sphenophorus infestations in sugarcane cultivated areas of Brazil, thus an integrated management strategy should be a high priority.

Implications of Sugarcane Straw Removal for Soil Greenhouse Gas Emissions in S\xe3o Paulo State, Brazil

The use of sugarcane straw is a promising option to enhance bioenergy production. But the implications of straw removal for soil greenhouse gas (GHG) emission are not yet fully understood due to scarcity of studies under Brazilian conditions. Four field experiments were designed to assess soil N2O and CH4 emissions derived from nitrogen (N) fertilization under scenarios of sugarcane straw removal in São Paulo state, Brazil. Our focus was also to derive the direct N2O emission factor (EF) that represents the regional conditions of sugarcane production, taking into account the data obtained from this study and those from the literature. In each field study, four straw removal rates (no removal (NR); low removal (LR)—removal of 5 Mg ha−1; high removal (HR)—removal of 10 Mg ha−1; and total removal (TR)—removal of 15 Mg ha−1) were arranged in a randomized block design. This study shows a clear evidence that CH4 fluxes are very low for all assessed sites regardless of straw removal rates, indicating a predominance of CH4 consumption by the soil. Cumulative N2O emissions ranged from 0.20 to 4.09 kg ha−1 year−1 and were significantly affected by straw removal in two sites, indicating that straw removal reduces N2O emissions. The average direct N2O EFs obtained from this study and from the literature were 0.28, 0.44, 0.70, and 0.56% for TR, HR, LR, and NR treatments, respectively, which are consistently lower than the EF of 1% suggested by the IPCC. Based on our regional-specific EF (Tier 2), the direct N2O emissions derived from N fertilization under scenarios of straw removal showed a reduction of at least 50% in relation to IPCC approach. Our findings are a step forward in providing regional-specific data to reduce the high level of uncertainty concerning N2O emission assessments of sugarcane ethanol in Brazil, but further studies are needed to evaluate how straw removal for bioenergy production and the associated changes in soil organic carbon stocks affect the GHG balance of sugarcane.

Sugarcane straw management for bioenergy: effects of global warming on greenhouse gas emissions and soil carbon storage

Global warming can intensify the soil organic matter (SOM) turnover, damaging soil health. Crop residues left on the soil are important to maintain a positive SOM budget and nutrient cycling. But, sugarcane (Saccharum officinarum) straw has been removed from the field for bioenergy purposes. We hypothesize that global warming, together with straw removal, will negatively impact Brazil’s ethanol carbon footprint. Thus, we conducted an experiment under controlled conditions to evaluate the impacts of warming and straw removal on greenhouse gas (GHG) emissions, soil carbon and nitrogen storage, and nutrient cycling. Two soils (Rhodic Acrisol and Eutric Nitisol) were tested with three rates of sugarcane straw removal (no removal (NR): equivalent to 12 Mg ha−1; medium removal (MR): 6 Mg ha−1; and total removal (TR): bare soil) and submitted to two temperatures (24 °C and 30 °C) and soil moistures (30% and 50%). Straw decomposition was stimulated by lower rates of straw removal, resulting in increases on carbon dioxide (CO2) emissions between 5 to 14 times, and N2O between 25 and 40%. There were no significant methane (CH4) fluxes. Soil carbon and nitrogen did not change due to straw removal, yet labile carbon fractions (living and non-living) were highly impacted, causing reductions of 15 to 40% on the carbon management index (CMI). Furthermore, straw removal reduced nutrient cycling between 10 and 30%. Overall, in a scenario of warming, our findings point to an intensification of SOM dynamic, resulting in increases of 35% on the GHG emissions and a CMI reduction by 20%. In practical terms, at least 6 Mg ha−1 of straw should be left in the field, guaranteeing raw-material for bioenergy, without causing major impacts on the GHG emission and soil attributes.

Soil Bacterial Community Changes in Sugarcane Fields Under Straw Removal in Brazil

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 Sao 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.