Quantifying above and belowground biomass carbon inputs for sugar-cane production in Brazil A. M. Silva-Olaya A,E , C. A. Davies B , C. E. P. Cerri C , D. J. Allen B , F. F. C. Mello D , and C. C. Cerri D A University of the Amazon, Street 17, Diagonal 17, Cr. 3F, Florencia 180002, Colombia. B Shell Technology Centre Houston, 3333 Highway 6 South, Houston, TX 77082, USA. C University of São Paulo ‘Luiz de Queiroz’ College of Agriculture, Av. Paduas Dias 11, Piracicaba 13400-970, Brazil. D University of São Paulo – Center for Nuclear Energy in Agriculture, Av. Centenário 303, Piracicaba 13416-000, Brazil. E Corresponding author. Email: amsolayaa@gmail.com Abstract. Expansion of sugarcane crop due to the increasing demand for sugar and ethanol can affect both existing soil carbon (C) stocks, and subsequent input of new C from above and belowground biomass, influencing the overall C intensity and C payback times due to the change of land use. We present above and belowground dry biomass production, shoot-to- root ratio (S : R) as well as the net annual C inputs to the soil for sugarcane in different ratoon stages. The selected areas were as follows: (1) recently planted sugarcane area (PC), (2) first year ratoon cane (RC1) and (3) 4-year ratoon cane (RC4), which were established under Typic Quartzipsamments located in north-eastern São Paulo State. The sugarcane S : R ratios ranged from 6.6 in PC to 3.4 in RC4, and total sugarcane C inputs from 29.6 to 30.8 Mg C ha –1 . The overall C balance for land use change requires effects on soil C and also C inputs from previous and future land uses. The sugarcane C input was between 3.7 and 4.0 Mg C ha –1 for each sugarcane cycle of 5 years. When accounting for soil C stock changes and aboveground biomass C losses from the prior land use, the payback times for sugarcane biofuel C debts are reduced by 3, 2 and 1 years for Cerrado wooded, Cerrado grassland and pasture conversions into sugarcane respectively. Additional keywords: ethanol, harvesting system without burning, land use change, roots, Saccharum spp., shoots, soil carbon. Received 6 April 2016, accepted 10 January 2017, published online 22 February 2017 Introduction Brazil is the second-largest producer of ethanol in the world, accounting for 32% of the market share (Baier et al. 2009). Its production is based primarily on sugar from sugarcane, with an energy and greenhouse gas (GHG) balance that makes it one of the most sustainable biofuels produced at commercial scales (Walter et al. 2008). Sugarcane (Saccharum spp.) is a C4 perennial crop that is very efficient at turning solar radiation into biomass, which is commercially cultivated in monoculture with an average full crop cycle ranging from 5 to 6 years, during which five harvests, four ratoon treatments, and one field renovation are performed (Macedo et al. 2008). Over the past 20 years there have been significant improvements introduced to the production of sugarcane ethanol, primarily associated with the use of bagasse (residue obtained after sugarcane is milled for juice extraction) for production of electricity, and recently the use of sugarcane trash (residual biomass following mechanised harvest) to increase this renewable energy production capacity (Pippo and Luengo 2013). Traditionally, the trash is burned before manual harvest of the cane to facilitate more efficient manual harvesting and transport. However, legal restrictions regarding the sugarcane pre-harvest burning have been implemented recently in Brazil because of economic and environmental concerns (Sao Paulo State 2002 – Law n 11.241/2002). According to the National Supply Co. (CONAB) 64% of the total harvested area in the 2011–2012 crop season involved mechanical harvest without burning. This unburnt biomass left in the field can enhance soil carbon (C) sequestration and provide other ecosystem benefits (Hartemink 2008). Despite improvements in the sustainability and efficiency of sugarcane production and the ethanol manufacturing process, other agricultural practices in the field can have a significant impact on the GHG balance mediated through changes in soil organic matter (SOM) dynamics and subsequently overall sustainability of a biofuel. The effects of land use change (LUC) during transition to sugarcane production and management can lead to decreases in C stocks (Lal and Kimble 1997; Six et al. 2002), and increases in atmospheric GHG emissions. It has been shown that increases in SOM within agroecosystems are positively correlated with the Journal compilation Ó CSIRO 2017 www.publish.csiro.au/journals/sr CSIRO PUBLISHING Soil Research http://dx.doi.org/10.1071/SR16090