Long-Term Effect of Fertilizer Microdosing on Soil Fertility in Sahelian West Africa.

Soil nutrient status limits crop production in Sahelian West Africa, a region with decreasing production per capita and frequent food crises. Increasing soil fertility via inorganic fertilizer use is important for ensuring food security in the region. Inorganic fertilizer, however, is often inaccessible and unaffordable to smallholder farmers. Microdosing, the point-source application of a reduced rate of fertilizer, has potential to reduce fertilizer use constraints in Africa by capitalizing on more precise placement and root access compared to conventional broadcast applications. Short-term research across Sub-Saharan Africa shows microdosing improves fertilizer use efficiency and increases yields, however, little research has been done to determine the long-term effects of microdosing. The objective of this reasearch is to determine the long-term sustainability of microdosing in the Sahel, and is achieved by analyzing differences in yield, soil properties and carbon speciation at two long-term research sites in West Africa. These sites, located in Burkina Faso and Niger were established in 1965 and 1993 respectively, and compare microdosing to recommended and unfertilized treatments with varying rates and types of organic matter amendment and in multiple cropping systems.

Results show that initially broadcast fertilizer at higher rates resulted in greater yield over the unfertilized control compared to microdosing. Over time, however, the difference between the microdosing and recommended broadcast application was diminished, pointing to benefits of greater fertilizer efficiency and nutrient recycling in the microdose system, along with apparent reduced acidification. Compared to other treatments, microdosing does not have a greater yield decline or negative impacts on soil properties over time. In both systems, amendment with manure and crop residues helped to stabilize yield decline over time and buffer soil acidification caused by long-term fertilizer application. Cropping systems involving legumes stabilized long-term yield decline, however soil properties were not improved through incorporating legumes at either site. Changes in C composition under the varying treatments were determined using X-ray Absorption Near Edge Spectroscopy (XANES) at the C K-edge. No differences in carbon speciation between fertilization treatments were observed at either site, implying that microdosing does not greatly impact soil carbon forms in comparison to other treatments. Overall, microdosing appears to be a sustainable soil fertility management strategy in these regions of Africa.