Spatial and temporal variability of soil N2 O and CH4 fluxes along a degradation gradient in a palm swamp peat forest in the Peruvian Amazon.

Kristell Hergoualc’H,Nelda Dezzeo,Christopher Martius,Louis V Verchot,Jeffrey Van Lent,Mariela López Gonzales,Jhon Del Aguila‐Pasquel

doi:10.1111/gcb.15354

Abstract

Mauritia flexuosa palm swamp, the prevailing Peruvian Amazon peatland ecosystem, is extensively threatened by degradation. The unsustainable practice of cutting whole palms for fruit extraction modifies forest's structure and composition and eventually alters peat‐derived greenhouse gas (GHG) emissions. We evaluated the spatiotemporal variability of soil N2O and CH4 fluxes and environmental controls along a palm swamp degradation gradient formed by one undegraded site (Intact), one moderately degraded site (mDeg) and one heavily degraded site (hDeg). Microscale variability differentiated hummocks supporting live or cut palms from surrounding hollows. Macroscale analysis considered structural changes in vegetation and soil microtopography as impacted by degradation. Variables were monitored monthly over 3 years to evaluate intra‐ and inter‐annual variability. Degradation induced microscale changes in N2O and CH4 emission trends and controls. Site‐scale average annual CH4 emissions were similar along the degradation gradient (225.6 ± 50.7, 160.5 ± 65.9 and 169.4 ± 20.7 kg C ha−1 year−1 at the Intact, mDeg and hDeg sites, respectively). Site‐scale average annual N2O emissions (kg N ha−1 year−1) were lower at the mDeg site (0.5 ± 0.1) than at the Intact (1.3 ± 0.6) and hDeg sites (1.1 ± 0.4), but the difference seemed linked to heterogeneous fluctuations in soil water‐filled pore space (WFPS) along the forest complex rather than to degradation. Monthly and annual emissions were mainly controlled by variations in WFPS, water table level (WT) and net nitrification for N2O; WT, air temperature and net nitrification for CH4. Site‐scale N2O emissions remained steady over years, whereas CH4 emissions rose exponentially with increased precipitation. While the minor impact of degradation on palm swamp peatland N2O and CH4 fluxes should be tested elsewhere, the evidenced large and variable CH4 emissions and significant N2O emissions call for improved modeling of GHG dynamics in tropical peatlands to test their response to climate changes.

Highlights

We evaluated the spatiotemporal variability of soil N2O and CH4 fluxes and environmental controls along a palm swamp degradation gradient formed by one undegraded site (Intact), one moderately degraded site and one heavily degraded site
And annual emissions were mainly controlled by variations in water-filled pore space (WFPS), water table level (WT) and net nitrification for N2O; WT, air temperature and net nitrification for CH4
While the minor impact of degradation on palm swamp peatland N2O and CH4 fluxes should be tested elsewhere, the evidenced large and variable CH4 emissions and significant N2O emissions call for improved modeling of greenhouse gas (GHG) dynamics in tropical peatlands to test their response to climate changes

Summary

| MATERIALS AND METHODS

The study was carried out in the province of Loreto, southwest of Iquitos, in the Northern Peruvian Amazon. The peat was classified as minerotrophic (nutrient-rich; Lähteenoja, Ruokolainen, Schulman, Alvarez, et al, 2009; van Lent et al, 2019) and likely receives nutrients during flooding events, as well as during the annual flood pulses of the Amazon River. Along this swamp forest complex, we selected a site that was undegraded, thereafter referred to as “Intact” (S 03°49.949′ W 073°18.851′), a site moderately degraded (“mDeg,” S 03°50.364′ W 073°19.501′), and a heavily degraded site (“hDeg,” S 03°48.539′ W 073°18.428′). Soil flux measurements started in July 2014, October 2014 and April 2015 at the Intact, hDeg and mDeg sites, respectively, and (a)

Section 2 Plot 1

| DISCUSSION

Findings

| CONCLUSION