Nitrate leaching from the critical root zone of maize in two tropical highlands of Tanzania: Effects of fertilizer-nitrogen rate and straw incorporation

摘要

Nitrate (NO3-) leaching from agriculture is a growing environmental concern, but little attention has been given to the cropping systems in sub-Saharan Africa, where efforts are underway to increase fertilizer (especially nitrogen, N) use to secure food production. During 2015-2017, we monitored NO3- leaching from the critical root zone of maize in the tropical highlands of Tanzania using repacked soil monolith lysimeters. Four urea-N rates (0-150 kg N ha(-1)) and in combination with maize straw (similar to 2 Mg C ha(-1); C:N >= 60) were evaluated in two soil types (sandy Alfisols and clayey Andisols). The soil rewetting process, particularly at the onset of the rainy season and following N applications, was a critical driver of NO3- flux. Nitrate leaching increased exponentially with increasing N rates, yet inter-annual variation was observed. Relating cumulative NO3- flux to maize yield under increasing N rates revealed a tipping point occurrence depending on season above which yield increments were accompanied by substantial NO3- leaching. Such a tipping point occurred at the N rate of 62 and 50 kg N ha(-1) at the sandy and clayey site, respectively, in the second season. Straw incorporation induced net N immobilization in the early growing season, and reduced NO3- leaching by 3.3-6.3 kg N ha(-1), but no effect was observed on the cumulative NO3- fluxes or maize yields. The NO3- leaching reductions (equivalent to 1.2-2.7 kg N Mg-1 C) were far below the net N immobilization potential of the incorporated straw (18.0-38.1 kg N Mg-1 C; for C:N ratios of 60-206). This was likely caused by large pieces of straw (0.15 m) used in the field, which reduced the surface area exposed to soil and microbes; consequently, only limited N in the decomposition microsites could be immobilized. Our results showed the potential to enhance maize yield without inducing substantial N leaching by adopting the proper N rate in the tropical highlands of Tanzania, and highlighted that temporary immobilization of leachable N by using large pieces of straw in the field was inefficient for the improvement of N synchrony and benefits to yield.