Interação entre remoção de palha e adubação nitrogenada sobre a volatilização de NH3 e emissão de N2O na cultura da cana-de-açúcar

Abstract

Brazil has an area of 8.74 million hectares cultivated with sugarcane. Currently, 90% of this area is mechanically harvested without fire which results in large amounts of straw (7 to 25 Mg DM ha-1) remaining on the soil surface after harvest. The total or partial removal of the straw is a practice that has been studied due to the potential of straw as a raw material in the production of second generation ethanol and bioelectricity. However, little is known about the effects of this practice on nitrogen (N) loss by ammonia (NH3) volatilization after N fertilization and nitrous oxide (N2O) emissions from soil. The objective of the present work was to evaluate the interaction between different rates of straw removal and N fertilization on NH3 volatilization and N2O emission in the sugarcane cropping system. Two field experiments were carried out at the Federal University of Santa Maria, with a randomized block design, in a 4x2 factorial scheme, with four replications. The first factor: 0, 4, 8 and 12 Mg ha-1 of straw (100, 67, 33 and 0% removal). The second factor: 0 and 100 kg ha-1 of urea-N, which resulted in 8 treatments: 0S, 0S + N, 4S, 4S + N, 8S, 8S + N, 12S and 12S + N. Urea was applied in a single dose at 52 days in 2016 and 60 days in 2017 after cane harvest. In two years, during approximately two weeks after urea-N application were performed measurements of NH3 volatilization, soil inorganic N, water-soluble NH4 + and total N of straw. In the first year, were performed measurements of N2O emission, soil inorganic N, moisture and temperature, and remaining straw C and N from the addition of the straw to the soil until the cane harvest. NH3 volatilization were different according to rain pattern of each year and NH3-N losses reduced with increased straw removal levels (12S> 8S> 4S> 0S). This was related to the decrease of straw physical barrier that hinders urea-N infiltration in the soil. This hypothesis was confirmed by N dynamics in the top-soil and straw which suggested a higher retention of urea-N with increasing amounts of straw. In addition, part of NH3 volatilization occurs directly from the straw. Two “hot moments” (HM) for N2O emission were observed, the first one after the cane harvest and the second after nitrogen fertilization. In the first HM, N2O emissions were strongly related to the water-filled pore space (WFPS) and the amount of C availability and in the second HM, besides that, the NO3 - resulted from urea-N application influenced the emissions. Straw removal reduced cumulative N2O emissions in treatments with and without N. A similar effect was observed for the calculated emission factor for straw and urea-N. These results demonstrate that increase straw removal rate significantly reduce NH3 volatilization and N2O emission. However, indiscriminate straw removal is not a recommended practice and it is necessary to study different environmental and agronomic aspects to define the ideal quantity of straw to be removed.