Stem and soil nitrous oxide fluxes from rainforest and cacao agroforest on highly weathered soils in the Congo Basin

Abstract

Abstract. Although tree stems act as conduits for greenhouse gases (GHG) produced in the soil, the magnitudes of tree contributions to total (soil + stem) nitrous oxide (N2O) emissions from tropical rainforests on heavily weathered soils remain unknown. Moreover, soil GHG fluxes are largely understudied in African rainforests, and the effects of land-use change on these gases are identified as an important research gap in the global GHG budget. In this study, we quantified the changes in stem and soil N2O fluxes with forest conversion to cacao agroforestry. Stem and soil N2O fluxes were measured monthly for a year (2017–2018) in four replicate plots per land use at three sites across central and southern Cameroon. Tree stems consistently emitted N2O throughout the measurement period, and were positively correlated with soil N2O fluxes. 15N-isotope tracing from soil mineral N to stem-emitted 15N2O as well as correlations between temporal patterns of stem N2O emissions, soil-air N2O concentration, soil N2O emissions, and vapor pressure deficit suggest that N2O emitted by the stems originated predominantly from N2O produced in the soil. Forest conversion to extensively managed, mature (> 20 years old) cacao agroforestry had no effect on stem and soil N2O fluxes. The annual total N2O emissions were 1.55 ± 0.20 kg N ha−1 yr−1 from the forest and 1.15 ± 0.10 kg N ha−1 yr−1 from cacao agroforestry, with tree N2O emissions contributing 1 to 38 % for forests and 8 to 15 % for cacao agroforestry. These substantial contributions of tree stems to total N2O emissions highlight the importance of including tree-mediated fluxes in ecosystem GHG budgets. Taking into account that our study sites’ biophysical characteristics represented two-thirds of the humid rainforests in the Congo Basin, we estimated a total N2O source strength for this region of 0.18 ± 0.05 Tg N2O yr−1.

Conservation slows down emission increase from a tropical peatland in Indonesia

“…We did not measure the emissions from tree stems, the inclusion of which may increase the total annual N 2 O emissions by 11-38% (ref. 73 ). Notably, given the minor (<1%) contribution of N 2 O to the total GHG balance, our reported GHG emissions should be considered as representative.…”

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

“…To do this, we compared a land‐use independent soil characteristic, i.e., clay content at 30–50 cm depth, between the forest and CAF at each site. There was no difference in clay contents between the two land uses in each site (Table ), which suggests that both land uses at each site had comparable initial soil conditions prior to forest conversion (Iddris, Corre, et al., 2020; Iddris, Corre, & Veldkamp, 2020). Therefore, any measured differences in CH 4 fluxes and soil controlling factors can be attributed to land‐use change.…”

“…(d) The tree‐level annual CH 4 fluxes were then extrapolated on a ground–area basis for each replicate plot, following Iddris, Corre, et al. (2020) as: Where X 1‐24 and DBH 1‐24 are the corresponding annual tree‐level CH 4 flux (kg CH 4 ‐C yr −1 of each tree; step c) and DBH (cm) of each of the 24 measured trees (6 trees × 4 plots) per land use at each site, DBH n is the individual tree DBH (cm) measured for all trees (with ≥10 cm DBH) present within the inner 40 × 40 m area of each plot (Table 1), Σ is the sum of the annual CH 4 fluxes of all trees within each plot (kg CH 4 ‐C yr −1 ) and A is the plot area (0.16 ha).…”

“…The geological parent material of the study sites consists of metamorphic schists, phyllites, quartzites, inferior gneiss, and undifferentiated gneiss, formed on Precambrian basement rocks (Gwanfogbe et al., 1983). All the sites are situated on heavily weathered Ferralsol soils with acidic pH and low effective cation exchange capacity (ECEC) (Table ; Iddris, Corre, et al., 2020; Iddris, Corre, & Veldkamp, 2020). The study sites have a mean annual air temperature of 23.5°C and a mean annual (1982–2012) precipitation of 1,576 mm yr −1 in central Cameroon, increasing to 2,064 mm yr −1 in the south of Cameroon (Table 1).…”

“…Soil CH 4 fluxes were measured simultaneously with stem CH 4 fluxes using vented static chambers (0.04 m 2 area) that were permanently inserted about ∼0.02‐m into the soil at least one month before the start of soil CH 4 flux measurements (same method as described in our earlier studies, e.g., Iddris, Corre, et al., 2020; Matson et al., 2017). The chamber bases were covered with vented, static polyethylene hoods (resulting in 11 L total headspace volume) equipped with Luer lock sampling ports on each sampling day.…”