Soil-atmosphere exchange of nitrous oxide in two Tanzanian croplands: Effects of nitrogen and stover management

Abstract

Cropland intensification is needed to meet the demand for food in sub-Saharan Africa (SSA). This process requires a dramatic increase in resource inputs, including fertilizer-nitrogen (N) and organic residues (e.g., stover), which alter the soil-atmosphere exchange of nitrous oxide (N2O). The dearth of N2O emission data for SSA croplands, however, limits our ability to define regional and global N2O flux and mitigation opportunities. In two soils planted with maize in Tanzania (Iringa, sandy Alfisols; Mbeya, clayey Andisols), we conducted year-round measurements for 2 consecutive years to quantify N2O emissions in response to increasing N rates and in combination with maize stover incorporation. Rainfall and the resulting soil moisture, rather than soil temperature, were important environmental drivers of N2O emissions in these fields. Applied N stimulated N2O fluxes across soil types but with different magnitudes—lower in Iringa because of the dominance of nitrification in N2O production and higher in Mbeya likely from promoted denitrification when the water-filled pore space was >47%. N2O emission increased exponentially or linearly with N rate, depending on the year. The direct N2O emission factors were well below the 1% of the IPCC Tier 1 method, ranging from 0.13% to 0.26% in Iringa and from 0.24% to 0.42% in Mbeya, for a N rate of 50–150 kg N ha−1 during the study. Compared with N application alone, stover plus N did not significantly alter maize yield, but did raise N2O emissions significantly (P ≤ 0.06). Consequently, stover incorporation markedly increased the emission factor (up to 0.46% in Iringa and 1.29% in Mbeya) as well as yield-scaled N2O emissions. Our results suggest that linear and exponential emission responses can occur in SSA croplands and challenge the promotion of combining stover with fertilizer-N as resource input management in this region.