Land-use change and Biogeochemical controls of soil CO₂, N₂O and CH₄ fluxes in Cameroonian forest landscapes

Abstract

Deforestation and land-use change are accelerating in the Congo Basin and elsewhere in the tropics affecting the soil-atmosphere exchange of greenhouse gases (GHG). There is a lack of data from Central Africa. We quantified fluxes of CO 2 , CH 4 , and N 2 O at the soilatmosphere interface in a secondary forest, a cocoa agroforest, and an unfertilized cropland. Soil respiration was highest in the secondary forest (15.37 ± 3.42 Mg C ha −1 y −1), intermediate in the cacao agroforest (12.26 ± 2.91 Mg C ha −1 y −1) and the lowest in the unfertilized cropland (8.74 ± 2.62 Mg C ha −1 y −1). Likewise, N 2 O fluxes were highest in the secondary forest (2.17 ± 0.20 kg N ha −1 y −1), intermediate in the cacao agroforest (1.40 ± 0.08 kg N ha −1 y −1) and lowest in the unfertilized cropland (1.04 ± 0.15 kg N ha −1 y −1). Soils were a sink for atmospheric CH 4 and sink strength was high in the secondary forest (−3.60 ± 1.83 kg CH 4 ha −1 y −1) and cacao agroforest (−3.61 ± 2.09 kg CH 4 ha −1 y −1) and low in the unfertilized cropland (−1.9 ± 1.59 kg CH 4 ha −1 y −1). Variation in soil water content rather than temperature was the dominant driver of seasonal variations of the fluxes at all study sites and N availability affected both N 2 O and CH 4 fluxes. Our results suggest that tropical land-use change is decreasing soil respiration, decreasing the strength of the soil CH 4 sink and decreasing N 2 O emissions, in landscapes that do not practice agriculture with chemical fertilization.

Substantial Stem Methane Emissions From Rainforest and Cacao Agroforest Partly Negate Soil Uptake in the Congo Basin

“…The latter study has higher clay contents compared to our study sites, which explains their correspondingly lower CH 4 uptake rate (Veldkamp et al., 2013). Compared to measurements conducted in sub‐Saharan Africa, our lowland forests had comparable soil CH 4 uptake rates as those reported for a tropical lowland forest in Cameroon (−41.1 to 51.6 μg C m −2 h −1 ; Tchiofo Lontsi et al., 2019; Verchot et al., 2020), and for tropical montane forests in Kenya (−35.4 to −66.2 μg C m −2 h −1 ; Wanyama et al., 2019) and in Tanzania (−31.0 to −44.6 μg C m −2 h −1 ; Gütlein et al., 2018). Conversely, our mean soil CH 4 uptake was lower than reported for tropical montane forests in Kenya (−56.4 μg C m −2 h −1 ; Werner et al., 2007), which had a sandy soil texture compared to our sites.…”

“…Forest conversion to CAF in our study sites generally lacked heavy land preparation activities and soil physical disturbance, and all our cacao sites were unfertilized, which may have resulted in the similar soil texture, bulk density and moisture contents between the two land uses at each site (Tables and ), and consequently, comparable CH 4 uptake. Our CAF sites had comparable soil CH 4 uptake rates (Table 2) as those reported for CAF in Cameroon (−41.2 μg C m −2 h −1 ; Verchot et al., 2020). The higher soil CH 4 uptake in our CAF sites compared to the jungle rubber agroforestry on highly weathered Acrisol soils in Indonesia (−20.8 to −26.9 μg C m −2 h −1 ; Hassler et al., 2015) may be the result of the higher soil NO 3 − contents in our CAF sites, which may have simulated CH 4 consumption and/or reduced CH 4 production in the soil (Matson et al., 2017; Veldkamp et al., 2013).…”

Nutrient limitations regulate soil greenhouse gas fluxes from tropical forests: evidence from an ecosystem-scale nutrient manipulation experiment in Uganda

“…Thailand (Hashimoto et al, 2004) and Hawaii (Townsend et al, 1995); within range to those from the Democratic Republic of Congo (Baumgartner et al, 2020), Panama (Koehler et al, 2009a;Pendall et al, 2010), Brazil (Sousa Neto 335 et al, 2011), and Cameroon (Verchot et al, 2020); and higher than those reported from Kenya (Wanyama et al, 2019), andIndonesia (van Straaten et al, 2011). The differences in soil CO2 fluxes between the control plots in this study and studies done in other tropical forest sites may be due to differences in soil environmental characteristics e.g.…”

Non-CO2 greenhouse gases: the underrepresented, complex side of the climate challenge

“…Two key findings suggest that in LUC in tropical regions decreases soil respiration and the strength of CH 4 soil storage. Verchot et al (2020) observed that in the absence of fertilizer use, N 2 O emissions decreased with LUC. They also found that soil water content, instead of temperature, was the dominant driver of variability; average fluxes in the wet season were more pronounced than in the dry season.…”

“…This Special Issue contains a selection of contributions to the symposium. It consists of this overview and seven topical papers as illustrative examples of the scientific results presented at the symposium (Wilson and Scholes 2020;O'neill et al 2020;Godwin and Ferenchiak 2020;Howarth 2020;Laban et al 2020;Verchot et al 2020;Zou and Osborne 2020). In the following section, we give an impression of the seven contributions.…”