Overall Implications of Straw Removal on Soil-Plant Systems
Sugarcane straw, a crop residue composed by top and dry leaves, has a dual purpose and can be maintained in the fields or be used to produce bioenergy . The maintenance of straw on the soil surface results in several well-known ecosystem services, such as nutrient recycling , carbon storage , erosion control , soil water retention , increases in soil biodiversity , reduction of soil compaction , and weed control . However, the maintenance of large amount of straw in the field may result in some drawbacks such as higher N2O emissions , higher pest infestation , and slower crop regrowth , especially in cooler regions . Therefore, complex interactions between straw and soil properties coupled with different management practices adopted by the farmers make crop responses to straw removal harder to be predicted by isolate/local studies. Based on this multilocation study, we seek to answer some questions that are fundamentally essential for guiding more sustainable straw management in Brazil. What is the direct impact of straw removal on sugarcane yields? What are the main drivers influencing sugarcane yield changes in south-central Brazil? How can we recommend straw removal rates without compromising sugarcane yields? In the following subsections, we discussed our data to provide scientific basis to answer these questions.
Regional Effects of Straw Removal on Sugarcane Yield
The south-central region is the largest core of sugarcane production in Brazil, representing 92% of national production of sugarcane . In that region, sugarcane cultivation extends for 9.1 million ha , including a wide range of climatic conditions . Several studies have indicated a direct relationship between crop yield and/or yield response to straw management with regional variations on climatic conditions in south-central Brazil [4, 5, 12, 33,34,35]. In our study, the sugarcane yield varied on a regional scale, indicating average yields of 129, 64, 82, and 99 Mg ha−1 of fresh stalk in southern Goiás and western, central-eastern, and northern São Paulo, respectively (Table 3). The higher sugarcane yields in southern Goiás relative to those in São Paulo regions can be associated with several factors including the high mean and minimal temperature and higher solar radiation that increases photosynthetic rates and sugarcane growth . Our results agree with those of Dias and Sentelhas , who pointed out that traditional areas under sugarcane plantation in São Paulo present lower yield potential compared with the south region of Goiás. Additionally, it is worth mentioning that Goiás experienced an intensive expansion (120%) of sugarcane area in the past decade, increasing from 0.495 (2007/08) to 1.107 million ha (2017/18) . Therefore, most of sugarcane areas in Goiás still present lower occurrence of pests and diseases, and the soils are less compacted, all of which are key factors that substantially impair crop yields in the most intensively cultivated sugarcane region in the São Paulo state. Several studies worldwide have indicated that the long-term sugarcane monoculture under mechanized conditions, such as those observed in São Paulo state, leads to soil degradation, which is considered the main cause of the sugarcane yield declines in traditional production areas [36,37,38].
The location of the sites within the four macroregions was one effective individual factor to explain the effects of straw removal on sugarcane yields. The higher yield response to straw removal was observed in southern Goiás and western São Paulo (Fig. 2 and Table 4). Conversely, the average crop responsiveness to straw removal was practically insignificant in central-eastern São Paulo (Fig. 5), which is slight cooler than other regions. It is important to highlight that, although the average daily minimal temperature ranges from 15.8 to 17.2 °C (Table 1), in this region, there are periods of the day in which the temperature is below 10 °C. The soil temperature for a proper sugarcane development ranges from 20 to 30 °C ; thus, crop growth is inhibited when plants are submitted to temperatures below 15 °C, while below 10 °C, some injuries may occur . In this case, especially in cooler regions such as central-eastern São Paulo, a moderate removal of straw from the soil surface may improve regrowth of sugarcane  and stalk yields (Fig. 2).
Climate effects on yield response to straw removal are associated with limiting and responsive factors. Low temperature is a limiting factor that can restrict sugarcane initial tillering and growth and eventually impair crop yield, while solar radiation and precipitation are responsive factors . The most accentuated reduction of crop yield induced by straw removal in southern Goiás can be associated with the best conditions of temperature and solar radiation (i.e., higher crop yield potential), and also because this region presents a higher water deficit in winter season. Straw mulching is also fundamental to reducing water losses and increasing soil moisture . Thus, the combination of these aspects makes this region more responsive to straw management, suggesting that recommendations for straw removal should be carefully analyzed. These findings are aligned with previous results showing that straw removal can be more detrimental to sugarcane yield in the central region of Brazil [12, 33].
Soil Texture Affecting Sugarcane Yield Response to Straw Removal
Soil texture is a key factor that regulates water and nutrient balance into soil profile, which are crucial characteristics defining the sugarcane yield capacity. Our results corroborate this statement with average sugarcane yields of 99, 80, and 66 Mg ha−1 for clayey, loamy, and sandy soils, respectively. Sandy soils have a predominance of macropores, promoting faster water drainage and reduced capacity of water retention relative to clay soils . Therefore, clay soils sustained greater sugarcane yields than sandy soils, as consistently reported in the literature [5, 12, 35].
Since sandy soils show more adverse conditions to sustaining higher crop production, we hypothesized that straw removal effects on sugarcane yield would be more detrimental in this soil type than in fine-textured soils. Nevertheless, contrary to our expectation, soils under distinct textures were responsive to straw removal, and the highest SYC indexes were observed in clayey soils in Goiás state. The higher sugarcane yield responses to straw removal in fine-textured soils (clayey) are associated with higher water storage capacity, which results in a high capacity to respond to management practices, especially when the water is the main limiting factor for sugarcane growth. The maintenance of straw also increases soil carbon stocks [12, 43] and consequently tends to improve water-use efficiency in sugarcane fields . Additionally, the benefits of straw mulching go beyond of soil water, since it contributes for soil organic matter formation, which plays an important role in improving cation exchange capacity in highly weathered tropical soils .
Although soil texture is an important controlling factor of sugarcane yield response, similar soils presented distinct patterns in different climate regions, suggesting that this factor in isolation does not explain the yield changes. In southern Goiás, most of SYC index values from clayey soils revealed yield losses due to straw removal, whereas yield gains were observed only in two sites (site 4—clayey soil under hydromorphic character and site 5—clayey soil under full irrigation), where water deficit was not a limiting factor to plant growth. In these conditions of adequate or excessive water supply, the removal of sugarcane straw tends to increase stalk yields. It reinforces that, at least in the short term, the main benefit of straw maintenance on soil surface is related to preserve soil moisture and reduce water stress to plants. Conversely, the sugarcane response to straw removal in less responsive regions shows no consistent results (Figs. 4 and 5), suggesting that crop yield response was likely dependent on other factors, such as local weather conditions, harvesting season, and pest and weed infestation.
Seasonal Effects of Straw Removal on Sugarcane Yield
Sugarcane harvesting seasons in south-central Brazil extend from March to November. The decision-making on the sugarcane harvest period is associated with the planting date and crop variety. Currently, there are more than one hundred commercial sugarcane varieties from different companies/research institutions and those are grouped within three maturation seasons, early (March–May), middle (June–August), and late (September–November). During each harvesting season, the weather conditions and management practices (including straw removal) can favor or delay sugarcane tillering, development, and growth. Therefore, crop yield responses to straw removal also depend on harvesting seasons .
In our study, when sites under similar soil and weather conditions were compared (sites 7, 8, and 9), there was a prominent effect of harvesting season in determining the response of straw removal on crop yield. For example, the sugarcane harvesting in site 7 occurred in early season (April), at the end of the rainy period, when the soil presents suitable moisture and temperature for a rapid sprouting, tillering, and initial growth of sugarcane. The dry season starts after this initial stage of crop development, and the straw layer has direct effects on preserving soil moisture to support the plant growth during the dry period. In general, the removal of straw in this site significantly reduced crop yields (Fig. 4). Conversely, when sugarcane is harvested in the middle season (i.e., winter), the soil presents lower moisture and temperature, which results in slow tillering and initial growth , especially in cooler regions of São Paulo. For this period, Gmach et al.  estimated that the complete maintenance of straw (12 Mg ha−1) or even partial maintenance (6 Mg ha−1) delays water-limiting condition to plants for approximately 1 month in relation to soil without straw (TR). Finally, when the harvesting and straw deposition occur in late season (beginning of the rainy and warmer seasons), sugarcane tillering occurs very fast and straw layer helps to preserve soil moisture for plant growth until the canopy closure , and after this period, the microclimate created by sugarcane crop can control soil water dynamics.
The seasonal effects of straw removal on sugarcane yields can also be observed between the areas located in Iracemápolis (sites 12, 13, and 14), which is a climatic region with low responsiveness to straw removal (Fig. 2). For this location, straw removal did not significantly change the yields in the early and late harvesting periods and induced a significant yield gain in the area harvested in the middle season (site 14). The maintenance of thick layers of straw in cooler seasons can hinder the sugarcane tillering and regrowth [5, 31], that, in some cases, may reduce sugarcane yield . However, it is important to highlight that the influence of the harvesting seasons on straw removal recommendations is relevant only in the regions with remarkable changes in weather conditions along the year. For example, in south Goiás, a region characterized by warmer temperatures along the year (Table 1), the sugarcane crop was responsive to straw removal independently of the harvesting season.
Crop Aging Effects of Straw Removal on Sugarcane Yield
The sugarcane crop cycle typically extends for 5–6 years, which includes the plant-cane cycle and the ratoons that are subsequently grown/harvested annually until the sugarcane yield becomes economically and technically unfeasible and a new planting is required . The impact of crop aging on sugarcane yield is well-known and in general, a significant yield reduction along the cycle is associated with several factors such as (i) crop damages caused by field operations ; (ii) increased soil compaction caused by machine traffic ; (iii) lower soil nutrient availability due to annual extraction by sugarcane stalks ; and (iv) higher proliferation of pests and diseases .
However, the effect of crop aging on sugarcane yield response to straw removal is not well understood. When some sites were analyzed separately, particularly those where several ratoons were evaluated, our results showed that the effects of straw removal on sugarcane yields tend to be higher in younger ratoons (Figs. 3, 4, 5, and 6), indicating that the crop is more responsive to management practices. This higher response of sugarcane crop in younger ratoons is associated with several factors, including high crop yield potential, high plant stand [51, 52], high soil fertility , low pest and weed infestation , and low soil compaction  at the beginning of the sugarcane cycle. Such pattern can be clearly observed in sites 1, 2, 3, 7, and 8 (Figs. 3 and 4), where the evaluation was more representative of the entire crop cycle and crop yield was responsive to straw removal.
Summary and Final Remarks
Straw represents one-third of the energy potential of the sugarcane crop  and until 15 years ago, this crop residue was burned prior to harvesting. With the widespread adoption of green cane harvesting in Brazil, the straw has been maintained in the field as soil amendment . While straw can benefit the long-term soil quality and crop yield, such residues also represent a valuable feedstock for bioenergy production and enable new opportunities for the Brazilian sugarcane industry . However, there exists a current lack of a guideline for recommendable amount of straw that can be sustainably removed without impairing the soil-plant system.
Our findings indicated that the impacts of straw removal on sugarcane yields are guided by a complex equation and dependent on numerous factors including local weather conditions, soil types, harvesting seasons, and crop aging. There is no single answer to recommend straw removal in south-central Brazil, so that the recommendation in a sustainably compatible manner should be prioritized as follows: (i) the first step is to identify the regional weather condition, since it was the most prominent factor that influenced the responses of sugarcane yield to straw removal management. Based on the minimal temperature, water deficit, and solar radiation of each specific region, we can predict how sugarcane crop is responsive to straw removal; (ii) soil type was an important factor but it should be analyzed within a specific region and generalizations about the straw removal according to soil type should be avoided. The magnitude of the changes was highly variable between soil types, where straw removal led to highest yield losses in soils with higher yield potential (i.e., clayey soils with high fertility) such as southern Goiás. Special attention should be taken for soils that exhibit water surplus under natural conditions (such as hydromorphic soils and shallow soils with water table near the surface) or full-irrigated system. In these environments, straw removal may increase sugarcane yields; (iii) harvesting season is another important factor, especially in regions where the minimal temperature is restrictive to sugarcane growth. Effects of minimal temperature on crop tillering, growth, and development are observed only when sugarcane is harvested in the early and middle seasons. This evidence is irrelevant when sugarcane is harvested after September (late season), since the minimal temperatures are suitable for crop growth in the most intensively cultivated sugarcane regions in south-central Brazil; (iv) lastly, the inclusion of crop aging in this complex equation can improve the recommendation of straw removal without significant yield losses. Our findings indicated that younger ratoons are more responsive to straw removal, resulting in higher yield losses compared with older ratoons.
Conclusions drawn from this study suggest that straw removal is a feasible reality for years to come, enabling new opportunities for the Brazilian sugarcane industry. However, straw management recommendations cannot be designed based on isolated factor but require holistic and integrated knowledge to ensure that straw amount left on field is enough to sustain multiple soil ecosystem services (e.g., protection against soil erosion, soil organic matter accretion, nutrient cycling, biodiversity maintenance) and crop yields. As far as the benefits of straw to the soil-plant system are integrally concerned, higher rates of straw removal can be performed in regions less responsive such as central-eastern and northeastern São Paulo, while only lower removal rates can be feasible in regions prone to yield losses (e.g., southern Goiás and western São Paulo) on a mid- and long-term basis. The final message of this paper reinforces that sugarcane straw should not be considered by researchers, managers, and farmers as “waste” and/or “trash” but should be viewed as a key component to increase the sustainability of sugarcane-based products.