Impacts of Increasing Chemical Fertilizer Use On Nitrous Oxide Emissions From a Smallholder Agricultural System in Western Kenya.

Impacts of increasing chemical fertilizer use on N2O emissions from a smallholder agricultural system in western Kenya. Over the last several decades, agricultural soils in many parts of sub-Saharan Africa have become depleted of nitrogen (N) and other nutrients, creating challenges to achieving food security in many countries. At only 8 kg N/ha/yr, average fertilizer application rates in the region are an order of magnitude lower than typical rates in the United States, and well below optimal levels. Increased use of nutrient inputs is a centerpiece of most African Green Revolution strategies, making it important to quantify the impacts of this change in practices. Increased N inputs are invariably accompanied by losses of N to the atmosphere as nitrogen oxides, including the greenhouse gas nitrous oxide (N2O). Several investigations of greenhouse gas emissions from sub-Saharan agricultural systems have been conducted over the last 20 years, but they typically include only two levels of fertilizer additions, and so are unable to identify potentially important thresholds in the response of trace gas emissions to fertilization rate. Here we examine the response function of N2O emissions to 5 different levels of inorganic fertilizer additions in a maize field in Maseno, Kenya during the 2010 long rainy season. We used an RCB design incorporating 5 levels of inorganic fertilizer additions (0, 50, 75, 100, and 200 kg/ha). We measured trace gas fluxes daily for one week starting the day before fertilizer application, followed by weekly measurements until trace gas emissions subsided to control levels. In order to identify thresholds in the N2O response, we use a stepwise backwards regression to identify departures from linearity in the mean chamber flux relative to the level of N fertilizer applied. Preliminary data suggest that N2O emissions may be slow to increase with increasing fertilizer additions, though important threshold effects may emerge. The identification of emission response thresholds combined with information to be collected on crop yield responses can provide insight into how to manage fertilizer use to optimize crop production per unit greenhouse gas emitted.