Abstract. Field measurement data on greenhouse gas (GHG) emissions are still scarce
for many land-use types in Africa, causing a high level of uncertainty in GHG
budgets. To address this gap, we present in situ measurements of carbon dioxide
(CO2), nitrous oxide (N2O), and methane (CH4) emissions from
the lowlands of southern Kenya. We conducted eight chamber measurement
campaigns on gas exchange from four dominant land-use types (LUTs) comprising
(1) cropland, (2) bushland, (3) grazing land, and (4) conservation land
between 29 November 2017 and 3 November 2018, accounting for regional
seasonality (wet and dry seasons and transitions periods). Mean CO2
emissions for the whole observation period were the highest by a significant margin
(p value < 0.05) in the conservation land (75±6 mg CO2-C m−2 h−1) compared to the three other
sites, which ranged from 45±4 mg CO2-C m−2 h−1 (bushland) to 50±5 mg CO2-C m−2 h−1 (grazing land). Furthermore,
CO2 emissions varied between seasons, with significantly higher
emissions in the wet season than the dry season. Mean N2O emissions
were highest in cropland (2.7±0.6 µg N2O-N m−2 h−1) and lowest in bushland (1.2±0.4 µg N2O-N m−2 h−1) but did not vary with
season. In fact, N2O emissions were very low both in the wet and dry
seasons, with slightly elevated values during the early days of the wet
seasons in all LUTs. On the other hand, CH4 emissions did not show any
significant differences across LUTs and seasons. Most CH4 fluxes were
below the limit of detection (LOD, ±0.03 mg CH4-C m−2 h−1). We attributed the difference
in soil CO2 emissions between the four sites to soil C content, which
differed between the sites and was highest in the conservation land. In
addition, CO2 and N2O emissions positively correlated with soil
moisture, thus an increase in soil moisture led to an increase in emissions.
Furthermore, vegetation cover explained the seasonal variation in soil
CO2 emissions as depicted by a strong positive correlation between the normalized difference vegetation index (NDVI)
and CO2 emissions, most likely because, with more green (active)
vegetation cover, higher CO2 emissions occur due to enhanced root
respiration compared to drier periods. Soil temperature did not show a clear
correlation with either CO2 or N2O emissions, which is likely due
to the low variability in soil temperature between seasons and sites. Based
on our results, soil C, active vegetation cover, and soil moisture are key
drivers of soil GHG emissions in all the tested LUTs in southern Kenya. Our
results are within the range of previous GHG flux measurements from soils
from various LUTs in other parts of Kenya and contribute to more accurate
baseline GHG emission estimates from Africa, which are key to reducing
uncertainties in global GHG budgets as well as for informing policymakers
when discussing low-emission development strategies.
Nitrogen addition reduced carbon mineralization of aggregates in forest soils but enhanced in paddy soils in South China