Introduction
Soil organic carbon (SOC) plays a crucial role for mitigating global
climate change (FAO, 2019; IPCC, 2019). However, the intensification of
monocropping has grown steadily and, when poorly managed, it has caused
serious soil degradation problems, leading to significant SOC losses to
the atmosphere (Sanderman et al., 2017). Because soil pool has the
potential to store three times as much C as the atmosphere (Lal, 2004;
Sanderman et al., 2017), public policy incentive for collective actions
has been proposed in applying management strategies to rebuild SOC
stocks (Vermeulen et al., 2019). Soils that are inadequately managed can
be source of CO2-C emissions, but sustainably managed
soils can considerably contribute to C sequestration, helping to reduce
global warming (FAO, 2019; IPCC, 2019) and providing key soil‐related
ecosystem services (Lorenz et al., 2019).
Globally, sugarcane (Saccharum spp.) stands out as a crop with a
sustainable potential to minimize the agricultural C footprint.
Sugarcane-derived bioethanol is one of the most promising renewable
energy alternatives to petroleum-based transport fuels and, is
recognized for its potential ability to emit less C in the life cycle
and avoid negative impacts on food security and biodiversity (Bordonal
et al., 2018). Brazil is the largest world producer of sugarcane, with a
production of 30 billion L of bioethanol from a cultivated area of 9.9
million hectares (Conab, 2019). In the last decades, concerns about the
sustainability of sugarcane cultivation under pre-harvest burning in
Brazil led to major changes in crop harvesting practices so that burned
manual harvesting has been gradually replaced by a green mechanized
system. This new harvest system resulted in a thick layer of straw
(10-20 Mg ha-1) returned to the soil, thus providing
several agro-environmental benefits such as SOC accretion, microbiota
support, nutrient cycling and improved soil structure (Carvalho et al.,
2017; Cerri et al., 2011).
More recently, the sugarcane sector has shown interest in the removal of
sugarcane straw for bioenergy production. The increased use of bioenergy
from sugarcane straw has been triggered by sectoral policies such as
Renovabio program driven by international commitments under the Paris
Convention of the Parties (COP 21) as a potential renewable substitute
for fossil fuels (IPCC, 2019). Nevertheless, recent studies have
confirmed that the indiscriminate removal of agricultural residues from
several crops leads to SOC depletion (Bordonal et al., 2018b; Cherubin
et al., 2018; Xu et al., 2019). In particular, special attention should
be given to the straw removal in coarse-textured soils, since these
soils usually are less resilient to SOC losses (Bordonal et al., 2018b).
There exists a strong interaction between clay fraction and SOC, so that
clay particles tend to form aggregates that physically protect SOC
against microbial and enzymatic attack (Dieckow et al. 2009). However,
there is a lack of comprehensive studies evaluating the effects of straw
removal on SOC stocks in different soil types and climate conditions in
Brazil to provide a robust scientific basis for public policy and
management decisions.
Based on this concern, we hypothesized that (i) high rates of sugarcane
straw removal for bioenergy production intensify SOC stocks depletion;
(ii) the removal of straw is more deleterious to SOC stocks in sandy
than in clayey soils in Brazilian conditions. To test these hypotheses,
we conducted a set of ten field experiments to evaluate the temporal
changes of SOC stocks in sugarcane fields under straw removal across
diverse edaphoclimatic conditions in central-southern Brazil, the
largest sugarcane-producing region in the country. Specific goals were
to evaluate SOC changes induced by sugarcane monocropping cultivation,
as well as to determine SOC responses to straw removal rates in sandy
and clayey soils. Additionally, we determined the minimum amount of
straw required to sustain the SOC levels for sandy and clayey soils.
Material and
methods
Description of the study
areas
Ten field experiments were conducted in southern-central region of
Brazil, covering areas under sugarcane production in Goiás and São
Paulo, the two largest sugarcane-producing states of the country (Fig.
1). The field experiments were setup under contrasting conditions of
climate and soil and conducted along the crop cycle within commercial
areas of sugarcane production. Descriptions of each experimental site
(e.g., municipalities, soil classification, texture, altitude,
precipitation and temperature), as well as details about the
characterization of the soil chemical and physical attributes at the
beginning of each field experiment are shown in Tables 1 and 2. In order
to simplify the data analyzes, the experiments were categorized in two
groups according to soil texture as follows: i) clayey soils = soils
with >33% of clay and ii) sandy soils = soils with
<23% of clay.
2.2. Experimental design and treatments
Each one of the ten field experiments were arranged in a randomized
block design with four replications. Each individual plot was
dimensioned with 10-m long by 12-m wide, containing eight sugarcane rows
spaced 1.5-m. The treatments of straw management were established after
the plant cane harvesting, when straw is available in sugarcane fields.
After sugarcane harvesting, the exact amount of straw produced in each
area was quantified through a metallic frame (0.25 m2)
randomly thrown in the field ten times. Straw moisture was measured
directly in the field using the sensor AL-104
Agrologic® with E-831 Electrode coupled. After
quantification of straw dry mass, the adjustment of each dose of straw
within each plot was performed manually using rakes and forks. The same
procedures of straw removal were repeated annually upon harvest of
subsequent ratoons. Four straw removal rates were established
(total–TR, high–HR, low–LR and no removal–NR) in seven experiments,
while three treatments (TR, HR and NR) were established in the other
three experiments. Information on the exact amount of straw maintained
on the field in each treatment, site and year are presented in Table 3.
Since the experimental groups correspond to distinct straw removal
rates, we have grouped the treatments for analysis purposes as follows:
NR (all straw left on soil surface), LR (from 25 to 33% removal), HR
(from 50 to 66% removal) and TR (no straw on the soil – bare soil). In
all field experiments, an annual sugarcane fertilization of 120 kg
ha-1 of nitrogen (ammonium nitrate) and 120 kg
ha-1 potassium (potassium chloride) were applied in
all plots and none of organic amendments such as filter cake and vinasse
were used. Fungicides, insecticides and herbicides were applied in all
plots uniformly according to the management strategies established by
each sugarcane mill.
Soil sampling and measurements
Sampling campaigns for SOC quantification were performed at the
beginning of the experimental period (baseline) in all experimental
sites and after two years of straw removal (sites 1, 5, 6 and 7) and
four years for the remainder areas (sites 2, 3, 4, 8, 9 and 10).
Composite soil samples were collected from the crop row and inter-row
positions at the 0-5, 5-10, 10-20 and 20-30 cm depths. Soil samples were
air-dried at 35 ºC for seven days. After air-drying and gentle grinding,
soil samples were sieved through a 2-mm sieve and 10 g of each sample
was finely ground and sieved through a 0.150-mm sieve for measurements
of the total C concentration (in duplicate) by dry combustion using the
Carbon Analyzer - LECO CN 628 (Nelson & Sommers, 1996).
2.4. Calculation of SOC stocks and annual rates of SOC loss/accumulation
SOC stocks were calculated for all soil depths (0-5, 5-10, 10-20 and
20-30 cm) by multiplying the C concentration by the bulk density and the
layer thickness. Additionally, SOC stocks for the 0-10 and 0-30 cm
layers were calculated as the sum of the stocks for each sampled depth.
Because samples were collected from fixed layers, the SOC stock was
adjusted for changes in bulk density that occurred after soil
management. Therefore, the methodology described in Ellert & Bettany
(1995) was used to adjust SOC stocks to an equivalent soil mass in
comparison with the baseline scenario (before experiment installation).
The annual rates of SOC accumulation or loss (Mg ha-1year-1) were calculated considering two times
horizons. First, it was calculated the difference between SOC stocks of
the treatments (NR, LR, HR and TR) relative to baseline, which
represents the effects of sugarcane cultivation on SOC stocks regardless
of straw removal treatments. Second, the annual rates of SOC
accumulation or loss were calculated considering the difference between
SOC stocks of NR treatment (business as usual scenario in Brazil) and
the treatments of straw removal (LR, HR and TR). In this last case, the
differences represent the isolated effect of straw removal on SOC
stocks.
Regression models using the relationship between ∆SOC
(SOCfinal – SOCinitial) as a function
of sugarcane straw biomass inputs were performed to estimate the minimum
amount of straw (Mg ha-1year-1)
needed to sustain SOC losses (when y=0). For the sites 7, 8 and 10, the
relationship between ∆SOC and C inputs was not significant, and
therefore, the minimum amount of straw to maintain SOC stock cannot be
determined. For sandy soils (sites 3, 4, 6), where negative slopes with
positive corresponding x-intercept were found, the minimum amount of
straw was much higher than the productive potential capacity of these
sites and, consequently, it was not possible to approximate a realistic
minimum amount of sugarcane straw to maintain SOC stock.
Data analyzes
Statistical analysis of data from each site was performed according to a
randomized block design, and analysis of variance (ANOVA) was used to
test the effects of straw removal on SOC stocks. Data normality was
confirmed by the Shapiro-Wilk test at 5% significance, and data
transformations were not necessary to meet ANOVA assumptions. When
statistically significant (F test; p<0.05), the average values
of SOC stocks were compared between treatments by Tukey’s test
(p <0.05) and by Dunnett’s test
(p <0.05) for comparison with baseline. Regression
analyzes were also performed to explore the relationships between SOC
stock changes and cumulative straw inputs for consecutive years. All
statistical analyzes were performed using R software (R Development Core
Team, 2019).
Results
3.1 Sugarcane cultivation effects on SOC stocks
Our findings indicate that sugarcane production, regardless soil type,
substantially affected SOC stocks (Table 4 and Fig. 2A). Compared with
baseline scenario, sugarcane production, on average, reduced SOC stocks
in the 30 cm by 3.9 Mg ha-1 in sandy soil and
increased in SOC stocks of 4.9 Mg ha-1 in clayey
soils. The data revealed that sandy soils under sugarcane cultivation
without straw management are losing SOC at mean rate of -1.8 Mg
ha-1 yr-1 in the 0-30 cm depth. So,
results showed that even with all straw maintained on the soil surface
(NR treatment), SOC stocks were significantly reduced by -1.4 Mg
ha-1 yr-1 in the sandy areas.
Different responses in SOC stocks were observed in clayey soils,
indicating a clear pattern of SOC accumulation for all areas with straw
maintenance in the field (Table 3, Fig. 2A). Our results showed that two
of the four clayey soils (sites 9 and 10) under sugarcane cultivation
without straw management had significant SOC accretion in the 0-10 cm
depth (mean of 4.5 Mg ha-1) relative to baseline
(Table 4). In the 0-30 cm depth, even though SOC changes were not
statistically significant (p<0.05), average SOC accumulation
rates were found by 0.9 Mg ha-1yr-1.
3.2 Straw removal effects on SOC stocks
Overall, straw removal induced significant SOC stock depletions and the
effects were more evident in the topsoil (0-10 cm) for sites 1, 2, 4, 8
and 9. In the 0-30 cm layer, SOC stocks were reduced for the sites 2, 4
and 9, but no changes were detected for the sites 3, 5, 6, 7 and 10
(Table 4).
Sandy soils were severely susceptible to C losses induced by straw
removal (Table S1, Table 4). In site 1, TR treatment (10.6 Mg
ha-1) depleted SOC stocks by 18% in the 0-10 cm layer
compared to NR (13 Mg ha-1), which in turn did not
differ from LR and HR. After four consecutive years of straw removal,
the effects were even more intense in site 2, where LR, HR and TR
induced SOC depletions (0-10 cm) ranging from 18 to 48% compared to NR.
Similarly, HR and TR treatments in site 4 reduced SOC stocks from 6% to
21% compared to NR. In the 0-30 cm layer, sites 2 and 4 presented SOC
reductions from 16 to 29% only for TR relative to NR treatment.
Considering only the sandy sites in which differences between treatments
were significant in the 0-30 cm (sites 1, 2 and 4), the data show that
straw removal resulted in SOC losses by -0.2 to -0.9 Mg
ha-1 year-1 compared to NR treatment
(Fig. 2B).
Sugarcane straw removal also depleted SOC stocks in clayey soils (Table
S1, Table 4). The impacts of straw removal on SOC stocks were also
significant for both layers after four years of evaluation (Table 4),
where SOC stock reductions were directly proportional to the increase in
straw removal rates (p<0.05). Increasing rates of straw
removal reduced SOC stocks in the 0-10 cm layer for the sites 8 and 9.
In site 8, decreases in SOC stock (13%) was observed in the TR (31.0 Mg
ha-1) compared to NR (39.3 Mg ha-1).
In site 9, the HR (27.2 Mg ha-1) and TR (26.8 Mg
ha-1) treatments reduced SOC stocks in the 0-10 cm by
about 16 % compared to NR (32.3 Mg ha-1). SOC
depletion in the 0-30 cm was up to 11% when all straw was removed from
the soil (TR–66 Mg ha-1) relative to NR treatment
(73.9 Mg ha-1). Annual SOC losses in the 0-30 cm
presented similar magnitudes to those values observed for sandy soils,
ranging from 0.5 to 1.0 Mg ha-1year-1 (Fig. 2B).
3.3 Relationship between cumulative straw inputs and SOC retention
Soil C stocks increased linearly as a function of the accumulated amount
of straw added to the soil over the evaluated period (Fig. 3). On
average, the data show that 85 kg C ha-1 were retained
in sandy soils for each megagram (Mg) of straw left in the field, but
the data ranged from 26 to 144 kg C ha-1. Clayey soils
showed averaged C retention of 109 kg C ha-1 for each
Mg of dry matter straw in the field, varying from 91 to 134 kg C
ha-1. Only in three out of ten evaluated sites was
possible to quantify the minimum amount of straw required to maintain
SOC stocks (y=0) (Fig. S1). The relationship between ∆SOC and straw
biomass inputs showed an estimated quantity of straw on the soil surface
of 16, 12 and 8 Mg ha-1year-1 for
sites 1, 2 and 9, respectively.
Discussion
4.1 Implications of sugarcane cultivation for SOC stocks
What happened with SOC stocks in sugarcane areas under green harvesting
system in south-central Brazil? Answer this question by robust data from
field experimentation is fundamental to provide scientific basis for
public and sectorial policies discussions related to the sustainability
of bioenergy production system. Experimental evidences from this study
showed a strong influence of soil texture on SOC changes over time. On
average, sugarcane cultivation resulted in C credit in clayey soils and
C debt in sandy soils (Fig. 2A). Regardless of straw management, the
data clearly showed that coarse-textured soils were highly susceptible
to SOC losses under sugarcane cultivation, indicating a mean rate of
-1.8 Mg C ha-1 year-1. Conversely,
clayey soils accumulated SOC over time independently of straw removal,
with a mean accumulation rate of 0.9 Mg C ha-1year-1.
Studies under tropical and subtropical conditions have reported that
fine-textured soils are less susceptible to SOC losses in cropping
systems (Dieckow et al., 2009). This pattern can be attributed to the
mechanisms that govern the stability of C, such as the high sorption
capacity of mineral surfaces in clayey soils. The strong interactions
with clay fractions stabilize organic-C compounds, preserving them
against decomposition (Dignac et al., 2017; Kopittke et al., 2020;
Spohn, 2020). Likewise, greater specific surface area of clayey soil
matrix and more complex pores network increase aggregate-protected C
substrates by physical inaccessibility to degradation (Kravchenko et
al., 2019). The organo-mineral interactions between C compounds and sand
fractions are recognized as weak (Dieckow et al., 2009; Neufeldt et al.,
2002), which may explain the significant SOC losses induced by sugarcane
cultivation in sandy soils. Our results indicate that the most common
scenario of sugarcane production in Brazil, based on green mechanized
harvesting (all straw maintained in the field), monoculture and
conventional tillage during the replanting periods, was not able to
sustain SOC stocks in sandy soils.
Additionally, the lower SOC stocks in sandy soils can be associated with
sugarcane productive potential of these areas. For example, Carvalho et
al. (2019) measured sugarcane yields in the same experimental areas and
concluded that sandy soils produce 40% less biomass than clayey soils.
The authors reported that the higher yields in clayey soils are linked
to greater water availability and soil fertility, thus providing proper
conditions for root growth and development. Since roots and exudates are
important inputs of C to the soil in sugarcane fields (Carvalho et al.,
2013), the contribution of root compartments to SOC stocks is likely to
be lower in sandy soils relative to clayey soils.
It is noteworthy that conventional tillage was carried out during
sugarcane renovation for all areas of this study. Intensive soil tillage
during sugarcane renovation exposes SOC that is protected by aggregates
and make it available for microbial use, thus causing SOC losses by
inducing CO2 emissions releases to the atmosphere
(Silva-Olaya et al., 2013). According to La Scala et al. (2006),
conventional tillage during sugarcane renovation increased soil
CO2 emissions by 8.4 Mg ha-1 relative
to no-tillage system. Similarly, Segnini et al. (2013) reported that
most part of the SOC accumulated along the sugarcane crop cycle could be
lost during the renovation period under conventional tillage, and Cerri
et al. (2011) mentioned that such C losses are higher in sandy soils.
Conversely, many studies have indicated that the adoption of reduced
tillage could be a feasible strategy to avoid not only SOC losses during
the renovation periods (Segnini et al., 2013; Tenelli et al., 2019), but
also to increase the capacity of sugarcane soils to accumulate C over
time. Alternative strategies to avoid SOC depletions in sugarcane fields
include the implementation of crop rotation in the sugarcane reform
period, as well as the application of organic amendments such as filter
cake, vinasse and biochar (Bordonal et al., 2018).
As important as implementing a set of management practices to avoid SOC
losses, the selection of the appropriate soil type is crucial to enhance
SOC sequestration in sugarcane cropping systems. In clayey soils, the
rates of SOC accumulation at the 0-30 cm ranged between 0.9 and 2.5%
per year, demonstrating that the cultivation of sugarcane in this soil
type is a realistic opportunity to reach values of C accretion even
above the global targets of 0.4% per year (“4 per 1000” Initiative –
www.4p1000.org) launched by the France government at the COP 21 held in
Paris (Minasny et al., 2017). On the other hand, the proportions of SOC
losses in sandy areas were quite contrasting relative to clayey-textured
soils, showing negative rates from -3.4% to -7.3%, and consequently
suggesting how challenging is to integrate sandy soils (marginal lands)
into a productive bioenergy system in a sustainable way. Over again, it
is imperative to establish guidelines for adopting sustainable soil
management in sandy soils under sugarcane land-use to reduce (even
partially) C losses over time. This study indicates that the SOC
accumulation found in clayey soils proves to be a sustainable strategy
to sustain C into the soil, thus helping the Brazilian sector to reduce
CO2 emissions to the atmosphere and comply with the
targets of the 2030 agenda for sustainable development proposed by the
United Nations (www.un.org/sustainabledevelopment).