Quantifying Net N Balance and Soil N Pools Following Maize Hybrids with High N Use Efficiency in Sub-Saharan Africa.

Land degradation and an inability to afford fertilizers has led to a depletion of soil N estimated at 22 kg/hectare annually in 38 African countries over the last 30 years. 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.  Soil N was quantified as inorganic, organic, and interlayer clay ammonium to see the impact of the hybrids and N rates on the different soil N pools.  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.  In Kiboko, the most recent growing season (2014LR) had a significantly lower N balance than the seasons prior to it.  N balance increased significantly and consistently with higher N rates, but there was no significant difference between the NUE groupings or the individual hybrids in 2014LR.