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

Land degradation and an inability to afford fertilizers has led to a depletion of soil N of 22kg/hectare annually in 38 African countries over the last 30 years.  One proposal to address this problem is the use of hybrids with enhanced nitrogen‐use‐efficiency (NUE), which has the potential to increase the maize yield by a substantial amount.  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. TRAD) 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, if this depletion is significantly greater than that associated with the TRAD hybrids.  Preliminary results have shown that all hybrids depleted the soil N pools at an increasing rate each year by at least 20 to 80 kgN/ha, but that the HNUE hybrids did not deplete oil nitrogen more than the TRAD hybrids.  Additional tests analyzing the hybrids’ impact on the individual soil N pools at different fertilizer rates are currently being conducted to confirm or reject this conclusion and to provide recommendations for remediation techniques.