Abstract
Studies that quantify nitrous oxide (N2O) fluxes from African tropical forests and adjacent managed land uses are scarce. The expansion of smallholder agriculture and commercial agriculture into the Mau forest, the largest montane forest in Kenya, has caused large-scale land use change over the last decades. We measured annual soil N2O fluxes between August 2015 and July 2016 from natural forests and compared them to the N2O fluxes from land either managed by smallholder farmers for grazing and tea production, or commercial tea and eucalyptus plantations (n=18). Air samples from 5 pooled static chambers were collected between 8:00am and 11:30am and used within each plot to calculate the gas flux rates. Annual soil N2O fluxes ranged between 0.2 and 2.9kgNha−1yr−1 at smallholder sites and 0.6–1.7kgNha−1yr−1 at the commercial agriculture sites, with no difference between land uses (p=0.98 and p=0.18, respectively). There was marked variation within land uses and, in particular, within those managed by smallholder farmers where management was also highly variable. Plots receiving fertilizer applications and those with high densities of livestock showed the highest N2O fluxes (1.6±0.3kgN2O-Nha−1yr−1, n=7) followed by natural forests (1.1±0.1kgN2O-Nha−1yr−1, n=6); although these were not significantly different (p=0.19). Significantly lower fluxes (0.5±0.1kgNha−1yr−1, p<0.01, n=5) were found on plots that received little or no inputs. Daily soil N2O flux rates were not correlated with concurrent measurements of water filled pore space (WFPS), soil temperature or inorganic nitrogen (IN) concentrations. However, IN intensity, a measure of exposure of soil microbes (in both time and magnitude) to IN concentrations was strongly correlated with annual soil N2O fluxes.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas (GHG), estimated to contribute about 6%to anthropogenic climate forcing (Blanco et al 2014)
Soil texture was generally clay except for the grazing and forest land uses in the smallholder sites, which were clay loams and loams respectively
The highest concentrations of total soil nitrogen (TN) in the top soil was measured in the native forest soils, while lowest values were observed at the tea and grazing land at the smallholder site (Table 2)
Summary
Nitrous oxide (N2O) is a potent greenhouse gas (GHG), estimated to contribute about 6%to anthropogenic climate forcing (Blanco et al 2014). At higher levels of NO3--N (>0.4 μg NO3--N g-1) the N2O flux yield by denitrification often decreases (Gelfand et al 2016; Schelde et al 2012) as C substrate availability might become the rate limiting factor. Both nitrification and denitrification are influenced by the size of inorganic-N pools in the soil, and these pools depend on N turnover through mineralization and soil amendments such as fertilizers and livestock excreta
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