Planting legume cover crop as a strategy to replace synthetic N fertilizer applied for sugarcane production

Abstract

Planting cover crops as a strategy to enhance soil quality has been long adopted in several agroecosystem worldwide. Among the cover crops, some legume species are must chosen due to their abilities to fix atmospheric nitrogen (N) into the soil, which increases soil N availability for the cash crop planted subsequently. Our hypothesis was that planting a legume within the sugarcane (Saccharum spp.) renewal period can be adopted as a strategy to supply the N required for the sugarcane at planting. The hypothesis was tested through an experiment set up within Southeast Brazil into two adjacent areas: in an area sunn hemp (Crotalaria junceae L.) was planted within the sugarcane renewal period (rotation system), whereas the another area was left in fallow (fallow system). Five N-fertilizer rates: 0, 130, 260, 390, and 520 kg N ha-1 were arranged in a randomized block design with four replications within both systems. The N amounts were divided throughout the plant cane, first and second ratoons cycles. Plant parameters evaluated included: tillering, stalk yield and stalk industrial quality. The N content on leaves as well as the inorganic N dynamic on soil were also evaluated. Overall, sunn hemp cropped within the sugarcane renewal period potentially replaces 60 kg ha-1 of N at sugarcane planting. Moreover, the rotation system enhanced stalk yield in 20 Mg ha-1 over both ratoon cycles, whereas the N amount required for each Mg of stalk harvested is reduced by 12.5%, indicating augmented the nitrogen recovering (NUE). The sucrose content and sugar yield are both enhanced under rotation system. However, soil N inorganic dynamic, the N content on leaves and amounts removed on plant components were little sensitive to the rotation system. Our findings highlight the rotation system as a feasible alternative to replace part of the N required by the sugarcane, which improves the NUE for the crop. Conclusions from this study indicate that legume rotation system adoption increases the sustainability of bioenergy from sugarcane by reducing the dependency on synthetic N fertilizers.