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.