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 (GHGs) 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 and correlations between temporal patterns of stem N2O emissions, soil–air N2O concentration, soil N2O emissions and vapour 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 11 % 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-N yr−1.

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

“…The annual soil N 2 O fluxes from the control plots (Table 2) were at the higher end of those measured in (sub-)montane tropical forests (Iddris et al, 2020;Arias-Navarro et al, 2017;Gütlein et al, 2018) and at the lower end of those measured in lowland tropical forest sites (e.g., Koehler et al, 2009b). This may either be due to the differences in soil N cycling rates (Koehler et al, 2009b) or the differences in the spatial abundance of leguminous trees (Xu et al, 2020) at the respective sites.…”

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

“…The annual soil N2O fluxes from the control plots (Table 2) were at the higher end of those measured in (sub-) montane tropical forests (Iddris et al, 2020, Arias-Navarro et al, 2017, Gütlein et al, 2018, and at the lower end of those measured in lowland tropical forest sites (e.g. Koehler et al, 2009b).…”

Temporal variation in nitrous oxide (N2O) fluxes from an oil palm plantation in Indonesia: An ecosystem‐scale analysis

“…Such patterns have also been reported in other ecosystems: Das et al (2012) and Liang et al (2018) relate daytime N 2 O emission patterns of pastures to changes in soil temperature, while Keane et al (2017) found strong correlation of daytime N 2 O emissions with PAR for oilseed rape. It has also been shown from other environments that N 2 O produced in the soil is taken up by roots, transported into above‐ground plant tissues and finally released into the atmosphere via lenticels or stomata, or N 2 O is consumed along the stem of trees (Iddris et al, 2020; Machacova et al, 2016; Wen et al, 2017). Our analysis in the time‐frequency domain showed that correlations shifted over the course of the day, with variables related to photosynthesis, such as GPP, LE and PAR in , and TKE being associated with N 2 O fluxes at daytime (Figure 8a,c,d,m) and R eco and soil temperature being associated with N 2 O fluxes at night (Figure 8b,k).…”