Sugarcane residue and N-fertilization effects on soil GHG emissions in south-central, Brazil

https://doi.org/10.1016/j.biombioe.2022.106342Get rights and content

Highlights

  • 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

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 (NH4)2SO4]. GHG fluxes were measured for 60 days using static chambers. Our results showed that total straw removal reduced CO2 and N2O emissions by 15% and 25%, respectively. After vinasse and fertilizer additions, CO2 emissions were 2.5-fold higher, and N2O were 5-fold higher, regardless of straw rates. In additional, synergic effect of vinasse + fertilizer and straw removal decreased 60% CH4 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 CO2e (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.

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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