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.
  1. 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).