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The area under mechanized sugarcane (Saccharum spp.) harvesting is expanding in Brazil, increasing the return of trash to the soil. The main questions regarding this management are: (i) after adopting unburned mechanical harvesting, how long will it take to observe decreases in fertilizer requirements, (ii) what will be the magnitude of this decrease and, (iii) the impact in the short run of removing trash for energy purposes in the nutrient cycling? This study aimed to build an N prediction model for long term assessment of the contribution of sugarcane crop residues to sugarcane nutrition and to evaluate the cycling of other nutrients derived from crop residues. Keeping crop residues over the soil will increase soil N stock and N recovery by sugarcane, reaching equilibrium after 40 years with recovery of approximately 40 kg ha -1 year -1 of N. Removing trash for energy production will decrease the potential reduction in N fertilizer requirement. Of the total nutrients in the trash, 75 % of the K 2 O (81 kg ha -1 year -1 ) and 50 % of the N (31 kg ha -1 year -1 ) are in the tops, indicating the importance of maintaining tops in the soil to sustain soil fertility. Because the input data employed in the simulations are representative of the conditions in Southeast Brazil, these results might not be definitive for situations not represented in the experiments used in the study, but the model produced is useful to forecast changes that occur in the soil under different trash management.

Sugarcane (Saccharum spp.) harvested without burning provides a substantial amount of remains (trash) on soil profiles which can be decomposed and release nutrients contributing to reduce fertilizer needs. The contribution of nitrogen (N) from sugarcane plant residues and fertilizer in sugarcane nutrition was assessed. Plant cane treatments were micro plots of 15 Nlabeled urea, sugarcane trash and root system; the last two to simulate the previous crop residues incorporated into the soil after crop renewal.

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.

-The objective of this work was to evaluate the agroindustrial production of sugarcane (millable stalks and sucrose yield) after successive nitrogen fertilizations of plant cane and ratoons in a reduced tillage system. The experiment was carried out at Jaboticabal, SP, Brazil, on a Rhodic Eutrustox soil, during four consecutive crop cycles (March 2005 to July 2009). Plant cane treatments consisted of N-urea levels (control, 40, 80, and 120 kg ha -1 N + 120 kg ha -1 P 2 O 5 and K 2 O in furrow application). In the first and second ratoons, the plant cane plots were subdivided in N-ammonium nitrate treatments (control, 50, 100, and 150 kg ha -1 N + 150 kg ha -1 K 2 O as top dressing over rows). In the third ratoon, N fertilization was leveled to 100 kg ha -1 in all plots, including controls, to detect residual effects of previous fertilizations on the last crop's cycle. Sugarcane ratoon was mechanically harvested. A weighing truck was used to evaluate stalk yield (TCH), and samples were collected in the field for analysis of sugar content (TSH). Increasing N doses and meteorological conditions promote significant responses in TCH and TSH in cane plant and ratoons, in the average and accumulated yield of the consecutive crop cycles.Index terms: Saccharum, agroindustrial production, biomass, crop residues, subsoiling, unburned cane. Produtividade de colmo e sacarose em resposta à adubação nitrogenada da cana-de-açúcar, sob preparo reduzidoResumo -O objetivo deste trabalho foi avaliar a produtividade agroindustrial de cana-de-açúcar (produtividades de colmos e sacarose) após sucessivas adubações nitrogenadas de cana-planta e soqueiras, em sistema de preparo reduzido. O experimento foi conduzido em Jaboticabal, SP, em um Latossolo Vermelho eutrófico durante quatro ciclos agrícolas consecutivos (março de 2005 a julho de 2009). Os tratamentos de cana-planta consistiram de doses de N-ureia (controle, 40, 80 e 120 kg ha -1 de N + 120 kg ha -1 de P 2 O 5 e K 2 O no sulco de plantio). Na primeira e na segunda soqueiras, as parcelas de cana-planta foram subdivididas em tratamentos de N-nitrato de amônio (controle, 50, 100 e 150 kg ha -1 de N + 150 kg ha -1 de K 2 O, sobre as fileiras de cana). Na terceira soqueira, a adubação com N foi de 100 kg ha -1 em todas as parcelas, inclusive nos controles, para detectar efeitos residuais das fertilizações anteriores na produtividade do último ciclo. Os colmos de cana foram colhidos mecanicamente. Utilizou-se caminhão instrumentado com balança para avaliação da produtividade de colmos (TCH), e amostras retiradas no campo para análise da quantidade de açúcar dos colmos (TSH). As doses crescentes de N e as condições meteorológicas proporcionam respostas significativas na TCH e na TSH da cana-planta e das soqueiras, na média e na produção acumulada dos ciclos agrícolas consecutivos.Termos para indexação: Saccharum, produtividade agroindustrial, biomassa, resíduos culturais, subsolagem, cana crua.

RESIDUES of green-harvested sugarcane contribute to nutrient recycling in production systems. Therefore, better understanding of trash decomposition dynamics can help crop fertilisation management. This study was conducted during the 2006-2008 seasons in Jaboticabal, north-eastern Sao Paulo State, Brazil and aimed to evaluate the nitrogen recovery rates from the previous crop residues or from urea applied on sugarcane planting in a minimum tillage system, thus without trash and rhizome incorporation in crop renewal. Previous crop residues consisted of 9 and 3 t/ha of sugarcane trash (dry tops ? leaves) and root system (roots ? rhizomes) enriched with 1.07 and 0.81% 15 N isotope, respectively. These contributed 51 and 33 kg/ ha of N. 15 N labelled trash laid on the soil surface and buried 15 N-root system attempted to simulate the original field residues disposal. The SP81-3250 variety was planted with 80 kg N/ha of a 5.17% 15 N-labelled urea. Recovery of sugarcane residues-N (trash-N and root system-N) or urea-N incorporated to the soil at planting were evaluated in distinct plant parts (stem, tops and dry leaves) during three consecutive harvest seasons. Recovery of urea-N was higher in the first harvest season (31% of initial N rate) and its uptake decreased in the second and third to 5 and 4%, respectively. In later harvested seasons, urea-N had probably been turned-over as soil organic matter and/or microbial biomass but remained in the soil N pool and available for plant recovery. Trash-N uptake closely resembled urea-N uptake, and only 13% of its N content was recovered in the first year, followed by 7 and 3% in the second and third seasons. Root system-N recovery was different since the second cut uptake was higher than the first followed by the third, 9, 6 and 2% respectively. Three year cumulative recovery of urea-N, trash-N and root system-N was 39, 23 and 17%, respectively. Most recovered N was found in stems followed by tops and dry leaves.

Resumo -O objetivo deste trabalho foi avaliar a recuperação, pela cana-planta, do nitrogênio ( 15 N) proveniente da ureia e de resíduos culturais da palhada e do sistema radicular da cultura da cana (Saccharum spp. Nitrogen derived from fertilization and straw for plant cane nutritionAbstract -The objective of this work was to evaluate the recovery, by plant cane, of the nitrogen ( N-SS and 15 N-RS by plant cane was 30.3±3.7%, 13.9±4.5% and 6.4±0.9%, respectively, representing 15.9, 4.7 and 1.4% of total nitrogen uptake by shoot.

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