NH3-N Losses From Urea Associated with Humic Acid Applied On Bare and Straw-Covered Soil.

The unburnt sugarcane harvesting in Brazil produces large amounts of trash on soil surface, hindering the incorporation of fertilizers into the soil. When urea is the source of N, the NH₃-N losses can reach 20 to 40 % of N applied over the trash blanket. This study aimed to quantify the NH₃-N losses from urea-N applied in two forms: soluble urea (SU) and soluble urea + humic acid (SU + HA), and under two conditions: with and without sugarcane straw on soil surface. The experiment was performed in laboratory conditions for 30 days. The NH₃-N losses were estimated by an open collector with acidified foam in order to capture the NH₃-N volatilized from the N fertilizer. The treatments were applied in a band on soil surface at a dose of 1050 mg collector^-1 N, corresponding to a dose of 50 kg ha^-1 N. The volatilization process was more intense in the first 5 days, when, on average, more than 45% of N was lost. The addition of SU and SU + HA over the trash showed NH₃-N cumulative loss of 471 (64%) and 395 mg collector^-1 (53%) respectively. On the other hand, when SU and SU + HA were applied on the bare soil, the NH₃-N losses achieved 249 (33%) and 362 mg collector^-1 (49%) respectively. Thus, the NH₃-N volatilization was higher in presence of crop residues than the in bare soil. On average, the NH₃-N losses were 305 mg collector^-1 (41%) in bare soil and 433 mg collector^-1 (59%) in soil covered with sugarcane straw. The higher NH₃-N volatilization in the treatments with straw must have been favored by the higher urease activity in plant tissues than in soil. The use of soluble urea associated with HA did not reduce the loss of NH₃-N.