Hickman, J.
E., Tully, K. L., Groffman, P. M., Diru, W., and Palm, C. A. C. J. G.: A
potential tipping point in tropical agriculture: Avoiding rapid increases in
nitrous oxide fluxes from agricultural intensification in Kenya, Journal of
Geophysical Research: Biogeosciences, doi: 10.1002/2015jg002913, 10.1002/2015jg002913,
2015.
Abstract
There
are national and regional efforts aimed at increasing fertilizer use in
sub-Saharan Africa, where nitrogen (N) inputs must be increased by an
order of magnitude or more to reach recommended rates. Fertilizer inputs
increase N availability and cycling rates and subsequently emissions of
nitrous oxide (N2O), a powerful greenhouse gas and the primary catalyst of stratospheric ozone depletion. We established experimental maize (Zea mays L.) plots in western Kenya to quantify the relationship between N inputs and N2O emissions. Mean N2O
emissions were marginally, but not significantly, better described by
an exponential model relating emissions to N input rate in 2011; in
2012, an exponential relationship provided the best fit compared to
linear and other nonlinear models. Most N2O fluxes occurred during the 30 days following the second fertilizer application. Estimates of fertilizer N lost as N2O
annually were well below the 1% Intergovernmental Panel on Climate
Change default emission factor, ranging from 0.07% to 0.11% in 2011 and
from 0.01% to 0.09% in 2012. In both years, the largest impact on annual
N2O emissions occurred when inputs increased from 100 to 150 kg N ha−1: fluxes increased from 203 to 294 g N2O-N ha−1 yr−1 in 2011 and from 168 to 254 kg N ha−1
in 2012. Our results suggest that exponential emission responses are
present in tropical systems and that agricultural intensification in
western Kenya may be managed for increasing crop yields without
immediate large increases in N2O emissions if application rates remain at or below 100 kg N ha−1.
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