Abstract:
The smallholder cropping systems have an adverse effect on greenhouse gas (GHG)
emissions to the ecosystems due to varied contribution to the GHG budgets. Further, the
uncertainty on the contribution of an individual anthropogenic trace gas (CH4, CO2, and N2O)
to the GHG emissions is rising due to disproportionate concentration contribution. The
general objective was to quantify GHG emissions from selected cropping systems and asses
N2O dynamics. The specific objectives were to: i) quantify greenhouse gas fluxes (CH4, CO2
and N2O) from selected cropping systems in Tharaka-Nithi County ii) evaluate N2O emissions
from smallholders’ cropping systems, and ii) determine the environmental factors, climatic
conditions, farm management practices, and soil properties that influence N2O dynamics. The
field experiment was undertaken under on-farm conditions, fully managed by farmers for one
year (two cropping seasons). For the field experiment, five cropping systems were evaluated:
i) sole maize, ii) maize intercropped with beans, iii) coffee, iv) banana, and iv) agroforestry.
Gas was sampled using a static chamber arranged linearly in a randomized complete block
design replicated thrice per cropping system. Gases were analyzed using gas chromatography
(GC) fitted with a 63Ni-electron capture detector (ECD) for N2O and flame ionization detector
(FID) for CH4 and CO2 using N as carrier gas. The cumulative soil GHG fluxes ranged from
-1.34 kg CH4-N ha-1
yr-1
under agroforestry to -0.77 kg CH4-C ha-1
yr-1 under banana for CH4,
0.30 kg N2O-C ha-1
yr-1 to 1.23 kg N2O-C ha-1
yr-1 for N2O and 5949 kg CO2-C ha-1
yr-1 to
12954 kg CO2-C ha-1
yr-1 for CO2. The maize yields ranged from 0 to 3.38 Mg ha-1
. The
nitrous oxide yields scaled emissions ranged from 0.10 to 0.26 g kg-1 maize and 0.68 to 1.30
g kg-1 beans. The review indicated that coffee, tea, maize, and vegetables emit N2O ranging
from 1 to 1.9, 0.4 to 3.9, 0.1 to 4.26, and 48 to 113.4 kg N2O-N ha-1
yr-1
, respectively.
Precipitation was the key driver of both CO2 and N2O emissions in the study area. The yield scaled and N2O emissions factors ranged between 0.08 and 67 g N2O-N kg−1
and 0.01 and
4.1% across cropping systems. Soil characteristics, farm management practices, and climatic
and environmental conditions are significant drivers that influence N2O emissions across SSA
cropping systems. Smallholder farmers in Central Highland of Kenya also contribute to global
GHG emissions through cropping systems. These results are within the previous GHG fluxes
findings in SSA. Therefore, this forms the baseline for estimating GHG in the agro-ecosystem in Africa.
