Potential Soil N Depletion By Maize Hybrids Differing in N Use Efficiency in Sub-Saharan Africa.

Sub-Saharan Africa suffers from low soil fertility and poor grain yields creating a food security crisis for its growing population.  One proposal to address fertilizer cost problems and food security requirements is the use of hybrids with enhanced nitrogen‐use‐efficiency (NUE), which has the potential to increase the maize yield substantially.  Direct impacts of higher maize NUE systems on soil N are rarely assessed, but are very important to long-term productivity.  Our study centers on continuous maize field experiments established by CIMMYT scientists in 2010 under the Improved Maize for African Soils (IMAS) initiative. These trials compared three higher versus three lower NUE genotypes (HNUE vs. LNUE) at three locations (Harare, Zimbabwe; Embu, Kenya and Kiboko, Kenya) at four N rates ranging from 0 kg N/ha to 160 kg N/ha.  Intensive soil and plant sampling was conducted in 2015 (year 5) to determine if the HNUE hybrids are depleting the soil N pools and, if so, whether this depletion is significantly greater than that associated with the LNUE hybrids. Preliminary results of above-ground plant N removal over time have shown that all hybrids decreased the N balance (total plant N - N applied) and thereby potentially depleted the soil N pools more each year least 20 to 100 kg N/ha, but that the HNUE hybrids did not result in a more negative N balance than the LNUE hybrids.  Nitrogen recovery efficiency (NRE) at the Harare site was often greater than 1 for all but 1 hybrid, implying that more N is being taken up by the plant than is being applied.  Results of plant N uptake from the most recent growing season showed a significant decrease in the N balance at all sites averaged across all N rates and hybrids.