In-depth analysis of N2O fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis

Abstract

Primary tropical forests generally exhibit large gaseous nitrogen (N) losses, occurring as nitric oxide (NO), nitrous oxide (N2O) or elemental nitrogen (N2). The release of N2O is of particular concern due to its high global warming potential and destruction of stratospheric ozone. Tropical forest soils are predicted to be among the largest natural sources of N2O; however, despite being the world’s second-largest rainforest, measurements of gaseous N-losses from forest soils of the Congo Basin are scarce. In addition, long-term studies investigating N2O fluxes from different forest ecosystem types (lowland and montane forests) are scarce. In this study we show that fluxes measured in the Congo Basin were lower than fluxes measured in the Neotropics, and in the tropical forests of Australia and South East Asia. In addition, we show that despite different climatic conditions, average annual N2O fluxes in the Congo Basin’s lowland forests (0.97 ± 0.53 kg N ha−1 year−1) were comparable to those in its montane forest (0.88 ± 0.97 kg N ha−1 year−1). Measurements of soil pore air N2O isotope data at multiple depths suggests that a microbial reduction of N2O to N2 within the soil may account for the observed low surface N2O fluxes and low soil pore N2O concentrations. The potential for microbial reduction is corroborated by a significant abundance and expression of the gene nosZ in soil samples from both study sites. Although isotopic and functional gene analyses indicate an enzymatic potential for complete denitrification, combined gaseous N-losses (N2O, N2) are unlikely to account for the missing N-sink in these forests. Other N-losses such as NO, N2 via Feammox or hydrological particulate organic nitrogen export could play an important role in soils of the Congo Basin and should be the focus of future research.

Using isotope pool dilution to understand how organic carbon additions affect N2O consumption in diverse soils

“…Soils that showed ICDE were OM‐limited, and the OC amendment induced heightened microbial activity, and altered the soil redox environment in a way that both drove net NO 3 − consumption and increased gross N 2 O consumption (Figures 1 and 2). This notion is supported by the PCA, which heavily weighted SOC, SON, microbial respiration, and nosZ:nirK with the soils that showed ICDE along PC1 (Figure 4), and other studies (Buchen et al, 2019; Gallarotti et al, 2021; Guo et al, 2020; Miller et al, 2009; Voigt et al, 2020). The significant role of pH in affecting N 2 O consumption, as revealed in PC1 and the pH one‐way ANOVA, is consistent with the effect of high pH being more favorable for nosZ enzyme activity (Figures 2 and 4; Blum et al, 2018; Richardson et al, 2009).…”

Increasing calcium scarcity along Afrotropical forest succession

“…3b), none of the leaching losses decreased to zero as stands were aging, as would be expected for a limiting nutrient during forest succession 32 . Important to note here is that our approach does not account for N inputs via BNF 18 , N losses via gaseous emissions 33 or P or cation inputs via soil weathering, the last probably very low in these highly weathered soils 34 . However, ferralsols are highly depleted of primary minerals, which reduces rock-derived nutrient input through weathering to near zero.…”

Conservative N cycling despite high atmospheric deposition in early successional African tropical lowland forests

“…The N 2 O emissions in the 60 years-old secondary forest was double the amount emitted in the lowland OG forests (Fig. 4, see also Gallarotti et al 2021;Barthel et al 2022) and above the global tropical forest N 2 O emissions mean (Werner et al 2007), but it was similar to the emissions in a secondary forest in Cameroun (Verchot et al 2020) and OG forests in Ghana (Castaldi et al 2013) and Congo (Serca et al 1994). The N 2 O emission peak in the 60 years-old secondary forest additionally indicates that the N cycle is recovering to its status in a mature forest, following shifting cultivation (Winbourne et al 2018).…”

“…In recent years, our understanding of N cycling in the central Congo basin has increased substantially (Bauters et al 2016(Bauters et al , 2018Baumgartner et al 2020;Gallarotti et al 2021). From these studies we know that the forests of the Congo basin receive high atmospheric N deposition (Bauters et al 2018) but, at the same time, show relatively low rates of N 2 O emission (Gallarotti et al 2021).…”

“…In recent years, our understanding of N cycling in the central Congo basin has increased substantially (Bauters et al 2016(Bauters et al , 2018Baumgartner et al 2020;Gallarotti et al 2021). From these studies we know that the forests of the Congo basin receive high atmospheric N deposition (Bauters et al 2018) but, at the same time, show relatively low rates of N 2 O emission (Gallarotti et al 2021). Especially the high N deposition loads further question if the N cycle also evolves from a conservative to a more open cycle as forest succession proceeds, as it is the case in the Neotropics (Davidson et al 2007;Figueiredo et al 2019).…”