Straw removal reduces the mulch physical barrier and ammonia volatilization after urea application in sugarcane

Crop residues left as mulch on the soil surface generally increase the risk of ammonia (NH3) volatilization from fertilizer-N, but the roles of mulch as a physical barrier and as a biological support of N transformation are not well known, particularly in response to straw removal. Thus, our goal was to assess how NH3 volatilization from urea-N was affected by different amounts of residues left on the soil surface after harvest in the sugarcane crop by mimicking different rates of straw removal. We ran a replicated field experiment during two consecutive years in the first ratoon sugarcane with 4 amounts of straw (0, 4, 8 and 12 Mg dry matter ha−1) and with 0 or 100 kg urea-N ha−1. Over approximately 2 weeks, we measured the NH3 emissions, levels of inorganic N in soil and water-soluble total N and NH4+ of straw. The dynamics of N in the soil and in straw supported the hypothesis that mulch acts as a physical barrier to the incorporation of urea in soil but also that NH3 emissions occurred directly from the mulch. The NH3 losses were different according to rain pattern of each year, but we showed that the emissions of NH3 were higher with higher amounts of straw (12 > 8 > 4 > 0) for both years (9.2-27.6% in 2016 and 30.7-60.8% in 2017). These results suggest that straw removal contributes to a reduction in NH3-N losses, but since total straw removal is not a recommended practice, N fertilization strategies should be implemented in systems with residue mulches on the soil surface to prevent N loss and to improve the environmental footprint of these cropping systems.