ABSTRACT:The use of crop residues as a bioenergy feedstock is considered a potential strategy to mitigate greenhouse gas (GHG) emissions. However, indiscriminate harvesting of crop residues can induce deleterious effects on soil functioning, plant growth and other ecosystem services. Here, we have summarized the information available in the literature to identify and discuss the main trade-offs and synergisms involved in crop residue management for bioenergy production. The data consistently showed that crop residue harvest and the consequent lower input of organic matter into the soil led to C storage depletions over time, reducing cycling, supply and availability of soil nutrients, directly affecting the soil biota. Although the biota regulates key functions in the soil, crop residue can also cause proliferation of some important agricultural pests. In addition, crop residues act as physical barriers that protect the soil against raindrop impact and temperature variations. Therefore, intensive crop residue harvest can cause soil structure degradation, leading to soil compaction and increased risks of erosion. With regard to GHG emissions, there is no consensus about the potential impact of management of crop residue harvest. In general, residue harvest decreases CO 2 and N 2 O emissions from the decomposition process, but it has no significant effect on CH 4 emissions. Plant growth responses to soil and microclimate changes due to crop residue harvest are site and crop specific. Adoption of the best management practices can mitigate the adverse impacts of crop residue harvest. Longterm experiments within strategic production regions are essential to understand and monitor the impact of integrated agricultural systems and propose customized solutions for sustainable crop residue management in each region or landscape. Furthermore, private and public investments/cooperations are necessary for a better understanding of the potential environmental, economic and social implications of crop residue use for bioenergy production.

The sugarcane (Saccharum spp. L.) mechanical harvesting system leaves a large amount of straw mulch on the soil surface. The straw mulch may affect soil conditions, root regrowth, and sugarcane yield. Thus, this study assessed the response of sugarcane root system growth and stalk yield to different rates of straw removal. An experiment was conducted in a Rhodic Kandiudox with sand clay loam texture to test the impact of four rates of straw removal: no removal (18.9 Mg ha−1 of dry mass); moderate removal (8.7 Mg ha−1); high removal (4.2 Mg ha−1) and total removal on sugarcane root system and stalk yield. Higher concentrations of roots (60%) were found in the first 40 cm of soil. Moderate straw removal resulted in higher root mass (3.6 Mg ha−1) and stalk production (23 Mg ha−1 of dry mass). However, no straw removal reduced root mass by <40% (2099 kg ha−1) and reduced stalk yield by >20% (105 Mg ha−1). Through regression analysis, it was estimated that retaining between 8.5 and 13 Mg ha−1 of straw resulted in the highest root mass and stalk yield. Managing straw removal to retain a moderate amount enables producers to sustain suitable soil conditions for sugarcane root growth and stalk production while providing straw for industrial use.

Sugarcane straw is a potential feedstock to increase bioenergy production. However, leaving straw on the soil surface is important to sustaining soil organic matter (SOM). We conducted a field experiment to evaluate the impacts of two years of straw removal on SOM in a sugarcane area – on an Eutric Nitisol in south-eastern Brazil – with low straw production (7 Mg ha–1). The experiment was a randomised complete block design (four blocks) with four rates of straw removal: (i) no removal, (ii) medium removal, (iii) high removal and (iv) total removal. We evaluated straw decomposition, soil carbon (C) and nitrogen (N) stock (0–30 cm), labile C, and microbial biomass C and N (0–10 cm). Increases in the straw removal rates reduced decomposition, with 30–42% of the initial straw amount remaining on the soil after one year (first year) and 33–38% remaining at the end of the second year. During the first year, no effect was found on soil C (8 kg m–2) and N (0.7 kg m–2) stocks; however, during the second year, total straw removal reduced soil C stock by 10% (0–30 cm) compared with no straw removal. Straw removal rates negatively affected the labile C (6–10% and 20–25%, first and second year respectively) and microbial C (correspondingly 10–15% and 21–27%). Microbial N was not altered in the first year, but decreased 26% due to straw maintenance (no removal) during the second year. Total straw removal decreased the C management index (CMI) by 15% (first year) compared with no straw removal. However, during the second year, all rates of straw removal decreased the CMI: by 15% for medium, 22% for high and 26% for total removal. Our findings indicate that any straw removal in areas with low straw production negatively affects SOM, initially through reduction in labile C pools. Although slight impacts on soil C and N stock were detected, reduction in straw-derived inputs stimulates microbial utilisation of soil C pools, and consequently reduces SOM over time.

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