Crop Rotation Reduces Nitrogen Fertilizer Requirement for Sugarcane Production in Brazil.

Crop rotation with sugarcane has the potential to supply nitrogen (N) and increase sugarcane yield. This study hypothesized that sunn hemp rotation with sugarcane will increase soil N availability and reduces response to N fertilization during cane-plant cycle. The objective was to evaluate soil N dynamics in areas subjected or not to rotation, and the potential in reducing N fertilization at sugarcane planting. Two field trials were set up in Iracemápolis, SP, in a Rhodic Kandiudox, in a complete block experimental design with four repetitions. In the first trial, sunn hemp was sowed before sugarcane establishment, and in the second trial, the field was left without cover crops. At sugarcane establishment (April 2013) the treatments were applied: check plot control, 30, 60, 90 e 120 kg ha^-1 of N applied at borrow of the furrow. Soil sampling was performed bi-monthly to evaluate soil inorganic N content (N-NH4+, and N-NO3- + N-NO2-) and hydrolysable soil N (Illinois Soil Nitrogen Test – ISNT). Sugarcane sprouting was recorded monthly. At cane-plant harvest (July 2014), the stalk yield was recorded, as well as quality parameters (brix, pol, fiber, purity), and N uptake. Soil samples were collected do determine microbial biomass N and C, as well as respiration under incubation. Sunn hemp rotation increased sugarcane sprouting during initial growth stages. However, there was no effect of rotation or N rates in N uptake by sugarcane. Sugarcane yield increased with N fertilization only in the area without rotation. Besides soil inorganic N and ISNT content did not increases with rotation, the use of sunn hemp increased the availability of soil N to sugarcane, since the rotation reduced sugarcane response to N fertilization. Rotation of sunn hemp in sugarcane fields showed potential in increasing yield and reducing N fertilization at planting, contributing to the sustainability of sugarcane production.