Nutrient cycling in sugar cane: IV. Leaching losses of N in plant-cane fertilized with urea-15N

Abstract

Leaching losses of N from a field experiment in a sandy Red-Yellow Podzolic soil of Goiana, Pernambuco State, Brazil, were measured. Two of the treatments were monitored: a) 60 kg N/ha at planting; and b) 20 kg N/ha at planting plus 40 kg N/ha, three months later. The N was aplied as urea in plots 10 m x 12,5 m and as ¹⁵N-urea in subplots 1,25 m x 1,25 m in the middle furrow. Two tensiometers in duplicates at 1,0 m and 1,2 m and a neutron probe access tube were placed close to the subplots while a soil solution sampler was placed under the subplot at 1,0 m depth soil. Solution was analyzed for N-NO₃, N-NH₄ and ¹⁵N abundance. Water retention curves and water conductivity were determined, drainage was estimated from the conductivity and the potencial gradient between 1,0 m and 1,2 m and N losses calculated by multiplying flux of drainage by mineral N concentration in soil solution. In the first five months after planting (in October) rainfall was little and there was no drainage and N losses by leaching. Thereafter rainfall increased, N leaching losses started and in March occurred the only significant loss of N derived from the fertilizer, amounting to less than 1 kg N/ha in the treatment with a single N application. The highest N losses occurred from May to September, when 300 mm of water drained, in spite of mineral N concentrations being very low, 5 µgN/ml. Total N leaching loss during the 16 month cycle were 21 and 28 kg N/ha, for splitted and single urea applications, respectively.