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 (∼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.