Sugarcane residue and N-fertilization effects on soil GHG emissions in south-central, Brazil
Introduction
Globally, ethanol is the most widely used biofuel [1], with more than 60 countries promoting its use as a mainstream fuel [2]. Ethanol not only helps ensure energy security, supply, and stability in international petroleum prices, but also through support of North American, European Nation (EU), and Asian leaders, it provides a basis for achieving neutrality regarding to greenhouse gas (GHG) emission from fossil fuel by 2050 [3,4]. Therefore, global ethanol production is projected to increase 33% (from 100 to nearly 134.5 billion L) by 2028. Two-thirds of this increase is expected to be from Brazilian sugarcane-based ethanol [5].
Brazil is the world's largest sugarcane producer, responsible for 40% of global production [5]. Annual production has increased more than ten-fold in the past 50 years and doubled in the last ten [6]. Increased sugarcane production that began in the 1970's to ensure the country's energy security, is now also contributing to GHG mitigation (e.g., RenovaBio - Brazilian law number 13.576, 27.12.2017). Sugarcane-based ethanol is considered an extremely promising biofuels due to its economic and environmental sustainability [7] and carbon budget of ∼60% compared to gasoline [6].
Brazilian bioenergy production can be further increased by using crop residues to produce bioelectricity and/or cellulose-based ethanol (i.e., second-generation ethanol). The primary feedstock for these new bioenergy sources is post-harvest crop residue (sugarcane straw, green leaves and tops) that currently are left on the soil surface after stalks harvest. Depending on plant variety, the quantity of dry feedstock can vary from 7 to 24 Mg ha−1 [8].
However, it is not sustainable to harvest all available crop residue, since a portion of the straw is needed to maintain critical soil functions [8,9], including maintaining or increasing soil organic carbon (SOC) [10,11] enhancing nutrient cycling [12,13], stimulating biological activity [14], reducing soil compaction [12], buffering soil temperature and water balance [15,16], and mitigating soil erosion [17]. On the other hand, excessive amounts of sugarcane straw on the soil surface can immobilize N, delay plant regrowth [[18], [19], [20]], and increase soil GHG emissions [[21], [22], [23]].
The primary GHG source associated with agricultural crop production is N-fertilizer - manufacture and use [24], due to the associated N2O emission; this GHG has a global warm potential 265 times higher than CO2 [25]. Currently, N-fertilizer recommendations for sugarcane in Brazil range from 60 to 100 kg ha−1 [26] with most supplied by mineral fertilizer plus organic sources associated with sugarcane processing (e.g., vinasse and filter-cake). Due to the favorable growing conditions, the N rates applied in the Brazilian sugarcane plantation are less than half of those used in Australia, India and China, and even for other biofuel feedstocks - such as corn, resulting in a low C-footprint of Brazilian ethanol [27,28]. After all keeping a low emission in the field is one of the main concerns in crops used as feedstock for biofuels production, considering the fossil fuel replacement.
Vinasse is the main by-product of ethanol production, being produced in a ratio of 13-fold higher concerning hydrous ethanol [29]. Due to its high concentrations of short-chain organic compounds, it has great potential for the biogas production; however, the high cost of installation facilities and their supply seasonality due to the sugarcane ripening make most mills prefer to use vinasse directly as field fertigation [29,30].
The N-content of vinasse ranges from 0.2 to 0.6 kg m−3 and provide significant N amount to the plants [31]. Vinasse application rates in sugarcane fields vary between 100 and 200 m−3 ha−1, calculated not based on N, but on K content to avoid soil salinization. São Paulo state regulates vinasse applications K concentration [32], but guidelines in other states vary due to differences in soil properties, average nutrient content of vinasse, and legislation. Furthermore, rather than being applied simultaneously to N-fertilizer, vinasse is generally applied before or after mineral N due to differences in equipment what also reduce volatilization [28].
The synergic effects of straw, vinasse, and mineral N fertilization are evident in nearly all sugarcane fields, but the impact on GHG flux is highly variable [21,23,[34], [35], [36]]. In this sense, to better quantify those interaction and synergic effects, a field experiment was conducted during the first ratoon on a commercial sugarcane plantation. Our objective was to quantify the interaction between traditional N application (vinasse + fertilizer) and different sugarcane straw amounts left on the soil surface on GHG fluxes (CO2, CH4 and N2O) from soil in south-central Brazil. We hypothesized that: (i) applied N (from fertilizer + vinasse) + soil moisture (from vinasse) + surface (straw) mulch would significantly affect soil biological processes, and thus raise GHG emissions; while (ii) partial removal of sugarcane straw would achieve a ‘win-win situation’ by partially mitigating GHG emissions, providing C to offset soil organic matter turnover, and supplying feedstock for cellulosic bioenergy production.
Section snippets
Description of the study area
This study was conducted on a commercial production field, cultivated with sugarcane for more than 20 years in the municipality of the Piracicaba, São Paulo state, Brazil (22° 41′ 55″ S, 47° 33′ 33″ W). According to the Köppen classification, the regional climate is defined as humid subtropical with a dry winter (Cwa). Mean annual rainfall is approximately 1400 mm and average annual temperature is 22.9 °C (with the hottest summer month exceeding 23 °C and the coldest month below 18 °C). The
Results
The maximum amount of straw remaining on the soil surface after harvest was approximately 12 Mg ha−1 of dry matter (34% moisture) with total C and N content of 420 g and 8.2 g kg−1, respectively. With removal, approximately 9, 6, or essentially 0 Mg ha−1 remained on the soil surface. Mean air temperature during the 60 days experiment was 26.9 ± 1.5 °C and there were 31 days with precipitation totaling 404 mm. The first 222 mm fell during first 30 days (primarily between the 5th and 11th day)
Discussion
Brazil's goal of reducing GHG emissions by over 40% compared to 2005 levels by 2030 stimulated development of new technologies to produce bioenergy from by-products and sugarcane straw [41,42]. Coupled with environmental bans on pre-harvest burning, partial or total straw harvest from sugarcane fields became a reality. This study quantifies straw removal effects on GHG emissions for the most common sugarcane management practices (i.e., application of vinasse and mineral N fertilizer) to enhance
Conclusions
Replacing fossil fuels with renewable sources helps to mitigate GHG emissions and represents a challenge for Brazilian agribusiness. In fact, the success of the Brazilian sugarcane-based ethanol compared to other feedstocks crops to replacing fossil fuels is associated with three main factors: i) the adaptability of sugarcane to different edaphoclimatic conditions in Brazil; ii) productivity with longevity of sugarcane fields - avoiding costly soil preparation and replanting; and, iii) a broad
Funding
The Brazilian Development Bank (BNDES) and Raízen Energia S.A funded this project (Project #14.2.0773.1). ALSV thanks the Coordination for the Improvement of Higher Education Personnel for the scholarship.
Availability of data and material (data transparency)
Not applicable.
Code availability (statistical software)
Statistical Analysis System - SAS v. 9.3 software (SAS Inc., Cary, NC, USA).
Authors' contributions
CEPC, ALSV, MSN conceived and designed this study. ALSV. collected samples and performed sample preparation. ALSV estimated all fluxes. MSN and MRC helped with sample analysis and data interpretation. AFBR conducted the data analysis. ALSV wrote the first draft of the paper. All authors contributed to the discussion of ideas and commented on the manuscript.
Declaration of competing interest
The authors affirm that there are no conflicts of interest.
Acknowledgments
This work is dedicated to the memory of our eternal team leader Prof. Dr. Carlos Clemente Cerri. We are thankful to the Sugarcane Technology Center (CTC), Piracicaba, SP, for allowing us to conduct our experiment in their pe area. We also thank to Lilian Duarte, Admilson Margato, Dagmar Machesoni and Ralf Araújo for assistance in the experiment conduction.
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