4.2 Straw removal impacts on SOC stocks
Sugarcane-based biofuels stand out as a good solution to reduce the dependence on fossil fuels, ensure energy security and mitigate greenhouse gas (GHG) emissions compared to other energy crops such as maize and sugar beet (Goldemberg & Guardabassi, 2010). In this sense, it is not clear thus far the extent to which the impact of crop residues removal on SOC stocks could be offset by the avoided GHG emissions of bioenergy production (e.g., cellulosic ethanol or electricity) in substitution of fossil fuel sources.
Covering ten experimental sites across the main growing sugarcane region in Brazil, which concentrates 90% of national and 36% of global production, this study provides the most complete and robust datasets about the impact of straw removal on SOC stock changes. For excessive straw removal rates (TR and HR), the SOC stocks (0-30 cm) were depleted in most areas, indicating decreases in SOC stock relative to the most common scenario of sugarcane production in Brazil (NR treatment). SOC losses were proportional to the increase of straw removal rates. Results of this study are consistent with previous long-term predictions obtained from modelling research conducted in Brazil (Carvalho et al., 2017b; Oliveira et al., 2017), and are in line with SOC declines reported in sites with corn stover removal around world (Johnson et al., 2014; Xu et al., 2019). Aligned with our findings, recent studies have shown that the maintenance of sugarcane straw on soil surface provides several soil ecosystem services, such as protection against soil erosion (Carvalho et al., 2017), favorable environment to microbiological processes (Pimentel et al., 2019; Tenelli et al., 2019), stabilization of aggregates (Castioni et al., 2019), nutrients cycling and reduction of fertilizer consumption (Cherubin et al., 2019), all of which having essential role to boost sugarcane crop yield (Carvalho et al., 2019).
By the way, what was the role of straw for each removal treatment? Considering all C added via straw, how much of this C was retained into the soil? Assuming that sugarcane straw has 440 g kg-1of C content in dry matter (Menandro et al., 2017), our results suggested that 19% and 25% of the C added via straw was retained into the soil of the sandy and clayey sites, respectively. These values are even higher than those found in the literature for sugarcane and other crop residues, which usually ranges from 6 to 15% (Bolinder et al., 1999; Robertson & Thorburn, 2007; Sousa Junior et al., 2018). For example, Robertson and Thorburn (2007) observed that 13% of C input via sugarcane straw was accumulated in the soil after five years of straw maintenance in Australia.
Based on the regression of cumulative straw returns against to measured SOC, clayey soils required 8.06 Mg ha-1yr-1 of straw to sustain SOC stocks, while an amount varying from 11.0 to 16.3 Mg ha-1yr-1 was necessary for sandy soils. But these results revealed that only one site of clayey areas and two of sandy areas presented a linear relationship between ΔSOC and straw additions (Fig. S1). Differently from our conditions, Johnson et al., (2014) observed a minimum corn stover amount of 6.38 Mg ha−1 yr−1 necessary to maintain SOC stocks in the soils of Corn Belt USA, where the soil was described to be close to C saturation. The conceptual approach to estimate SOC changes used by Johnson et al. (2014) did not fit very well for this study. For instance, the y-intercept of the regression equation was higher than zero in clayey areas, which means that soil C stocks were maintained even with complete removal of straw. This lack of negative effect of straw removal in clayey soils can be attributed to other sources of C inputs (e.g., roots, exudates), which may have been sufficient to sustain SOC stocks due to the protection of SOC by interactions with clay particles (Dignac et al., 2017). In sandy soils, the absence of response is likely related to the low capacity of these soils to accumulate C, showing that the minimum amount of sugarcane straw to sustain SOC was so high and far away from what those areas could potentially produce because of their limited conditions.
This study highlights that straw-derived bioenergy is not “zero impact” in terms of C budget, since it directly affects soil C stocks. Regardless of whether SOC stocks are increasing (clayey soils) or decreasing (sandy soils) in comparison with baseline as already discussed in section 4.1, our findings reinforce the role of straw as a primary source of C to the soil and indicate that straw removal tends to reduce SOC stocks. In order to have bioenergy production in an environmentally compatible manner, the benefits of biofuels produced from crop residues must compensate potential SOC losses.
Since sandy soils are more vulnerable environments and present difficult in accumulate or maintain SOC stocks, this study raises the following question: Can sandy areas really be subject of straw removal projects? The data clearly endorse that it would not be sustainable considering the current management system of sugarcane production. It is important the mention that the large-scale use of sandy soils for sugarcane production in south-central Brazil occurred mainly in the past 15 years, when a large expansion of sugarcane plantation occurred in the country. Based on that, the SOC stocks changes induced crop residues retention in these soils are poorly understood, and more comprehensive studies should be encouraged.
This study shows that the straw retention is crucial to reduce SOC losses in sandy soils managed conventionally in sugarcane cropping systems and alternative management such as no-tillage practices, crop rotation and organic amendments could offer a climate-smart solution to ensure food security and sustain soil productivity (Zhao et al., 2020). For example, Tenelli et al. (2019) concluded that the adoption of reduced tillage offsets C losses induced by straw removal, and consequently, greater amount of sugarcane straw can be sustainably removed from high productive fields without depleting SOC stocks. SOC stock changes are driven by a variety of processes that are interconnected, and therefore, determining how much straw is needed to maintain SOC stock levels for a sustainable bioenergy production using short- and medium-term empirical data is still challenging. The establishment of critical levels of straw removal at site/farm or regional scale should vary according to the site specificity of soil, climate and management strategies. In order to estimate the influence of each factor on SOC stocks, simulation models can be a useful approach to assess critical levels of straw mulching and predict these impacts on a long-term basis, which is a key aspect when it comes to SOC dynamics. Lastly, we advocate that the inclusion of SOC stocks changes in life‐cycle assessments is mandatory and should be encouraged considering scenarios of straw removal in areas of clayey and sandy soils for a more credible GHG balance of sugarcane straw-derived bioenergy.
Conclusions
The new biomass-based bioenergy context raises concerns about the effects of indiscriminate rates of straw removal on SOC stocks and sustainability of sugarcane production system. This study indicates that excessive rates of straw removal are impairing SOC stocks, suggesting that sustainable straw management must be adopted to prevent additional soil degradation and a GHG unbalance in the future for bioenergy production. Our findings showed strong SOC depletion in sandy soils regardless of the amount of straw left in the field. On the other hand, clayey soils exhibited SOC accumulation over time, even removing all the straw from the soil surface.
In this context, the removal of sugarcane straw for bioenergy production (i.e., cellulosic ethanol or bioelectricity) in Brazil may be advantageous from an energy security point of view, but should be avoided in sandy areas because it reduces SOC from a naturally infertile soil and decreases sugarcane yield, which is already low under these conditions. We advocate that the use of crop residues for bioenergy production in Brazil should not deplete SOC stocks, since tropical soils are characterized by low SOC levels and favorable environment for rapid decomposition processes of SOC.