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Brazil is recognized as a prominent renewable energy producer due to the production of ethanol from sugarcane. However, in order for this source of energy to be considered truly sustainable, conservation management practices, such as harvesting the cane green (without burning) and retaining the trash in the field, need to be adopted. This management practice affects mostly the nitrogen (N) cycle through the effect of trash on immobilization-mineralization of N by soil microorganisms. The aim of the experiments reported here was to evaluate N recovery from trash (trash-N) by sugarcane during three ratoon crop seasons : 2007, 2008 and 2009. Two field experiments were carried out, one in Jaboticabal and the other in Pradopolis, in the state of Sao Paulo, Brazil. The experiments were set up in a randomized block design with four replications. Within each plot, microplots were installed where the original trash was replaced by trash labelled with 15 N, and maintained up to the fourth crop cycle. Trash-N recovery was higher in the Jaboticabal site, the most productive one, than in the Prad opolis site. The average trash-N recovery across the two sites after three crop cycles was 7.6 kg ha À1 (or 16.2% of the initial N content in trash), with the remaining trash-N being incorporated into soil organic matter reserves. While these results indicate that the value of trash for sugarcane nutrition is limited in the short term, maintaining trash on the field will serve as a long-term source of N and C for the soil.

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This study manipulated the carbon-to-nitrogen ratio (C:N) of seaweed composts by varying the proportion of high N green seaweed (Ulva ohnoi) and high C sugarcane bagasse to assess their quality and suitability for use in agricultural crop production. Seaweed-bagasse mixes that had an initial C:N ratio greater than 18:1 (up to 50:1) could be transformed into a mature compost within 16 weeks. However, only composts with a high seaweed content and therefore low initial C:N (18 and 22:1) supported a consistently high rate of plant growth, even at low application rates. Sugarcane grown in these high seaweed composts had a 7-fold higher total above-ground biomass than low seaweed composts and a 4-fold higher total above-ground biomass than sugarcane grown in commercial compost that did not contain seaweed. Overall, the optimal initial C:N ratio for seaweed-based compost was 22:1 which corresponds to 82 % seaweed on a fresh weight basis. This ratio will produce a high quality mature compost whilst also ensuring that a high proportion of the nitrogen (>90 %) in the Ulva biomass is retained through the composting process.