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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 - PubMed

. 2020 Dec;26(12):7198-7216.

doi: 10.1111/gcb.15354. Epub 2020 Oct 9.

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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 et al. Glob Chang Biol. 2020 Dec.

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 N2 O 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 N2 O 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 N2 O 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 N2 O; WT, air temperature and net nitrification for CH4 . Site-scale N2 O emissions remained steady over years, whereas CH4 emissions rose exponentially with increased precipitation. While the minor impact of degradation on palm swamp peatland N2 O and CH4 fluxes should be tested elsewhere, the evidenced large and variable CH4 emissions and significant N2 O emissions call for improved modeling of GHG dynamics in tropical peatlands to test their response to climate changes.

Keywords: GHG emissions; Mauritia flexuosa swamp forests; Peru; methane; nitrous oxide; peatland; tropical.

© 2020 The Authors. Global Change Biology published by John Wiley & Sons Ltd.

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Figures

FIGURE 1
FIGURE 1

Experimental design at the study sites. (a) Layout of sections and plots within each site. (b) Layout of the subplot within a plot at the Intact site. (c) Layout of the two subplots within a plot at the medium degraded (mDeg) and highly degraded (hDeg) sites, including a standing live palm and a cut palm. Chambers were permanently installed on hummocks and hollows to measure soil CH4 and N2O fluxes. Source of the Mauritia flexuosa scan in (b) and (c) is from Caballero (2017)

FIGURE 2
FIGURE 2

Monthly precipitation and annual precipitation per year of the monitoring period. Black bars show months with El Niño conditions. Gray bars show precipitations in months with weak La Niña conditions. White bars show El Niño/La Niña neutral months. Y, year

FIGURE 3
FIGURE 3

Monthly mean air (a) and soil (b) temperatures at the Intact, moderately (mDeg) and heavily (hDeg) degraded sites. Error bars are SE. Left charts display means per month (n = 18 at the Intact site, n = 36 at the degraded sites); right charts display 3‐year means (n = 648, 1,297 and 1,296, respectively at the Intact, mDeg and hDeg). Letters indicate significant differences in means between sites

FIGURE 4
FIGURE 4

Monthly and annual mean water table level in hummocks and hollows around live and cut palms at the Intact, moderately (mDeg) and heavily (hDeg) degraded sites. Error bars are SE. Left charts displays means per month (n = 9 per palm status by spatial position at each site) and site‐scale annual means per year. Right charts display 3‐year means per palm status by spatial position (n = 308–324) and at site scale. Letters indicate significant differences in means between microtopographies within a site and palm status. Numbers indicate significant differences in means between sites

FIGURE 5
FIGURE 5

Monthly and annual mean water‐filled pore space in hummocks and hollows around live and cut palms at the Intact, moderately (mDeg) and heavily (hDeg) degraded sites. Error bars are SE. Left charts displays means per month (n = 9 per palm status by spatial position at each site) and site‐scale annual means per year. Right charts display 3‐year means per palm status by spatial position (n = 241–260) and at site scale. Letters indicate significant differences in means between microtopographies within a site and palm status. Numbers indicate significant differences in means between sites

FIGURE 6
FIGURE 6

Monthly mean N2O emissions in hummocks and hollows around live and cut palms at the Intact, moderately (mDeg) and heavily (hDeg) degraded sites. Error bars are SE. Left charts displays means per month (n = 9 per palm status by spatial position); right charts display 3‐year means (n = 316–320 per palm status by spatial position). In left charts, note different scale between top (Intact), bottom (hDeg) and middle (mDeg) panels. Letters indicate significant differences in means between microtopographies within a site and palm status. Numbers indicate significant differences in means between sites within a microtopography and palm status

FIGURE 7
FIGURE 7

Relationship between monthly average (a, b) or annual average (c) N2O emissions and the water‐filled pore space (WFPS), the water table level (WT) and net nitrification rate. (a, b) present monthly emissions averaged within the respective WFPS (10% interval) and WT (10 cm interval) classes, disaggregated by site in left charts and all sites combined in right charts. In (c), annual averages were disaggregated by microtopography per site (excludes year 1 when mineral N data were not monitored). Black dots or bubbles and bars present average values and SE; dashed gray lines show the models. The models are presented with coefficients (SE) and their level of significance (*p < .05, **p < .01, ***p < .001). The size of the bubbles is relative to the sample size in (a) and (b); it is relative to net nitrification rate in (c) (range −0.3 to 1.8 mg N kg−1 day−1)

FIGURE 8
FIGURE 8

Monthly mean CH4 emissions in hummocks and hollows around live and cut palms at the Intact, moderately (mDeg) and heavily (hDeg) degraded sites. Error bars are SE. Left charts displays means per month (n = 9 per palm status by spatial position); right charts display 3‐year means (n = 312–318 per palm status by spatial position). In left charts, letters indicate significant differences in means between microtopographies within a site and palm status. Numbers indicate significant differences in means between sites within a microtopography and palm status

FIGURE 9
FIGURE 9

Relationship between monthly average (a, b), annual average (c) or site‐scale annual (d) CH4 fluxes and the water table level (WT), air temperature, net nitrification rate and annual precipitation. (a, b) present monthly emissions averaged within the respective WT (10 cm interval) and temperature (1°C interval) classes, disaggregated by site in left charts and all sites combined in right charts. In (c), annual averages were disaggregated by microtopography per site (excludes year 1 when mineral N data were not monitored). Black dots or bubbles present average values and SE; dashed gray lines show the models. In (a), as the SE of log‐transformed data can be misleading SE are not displayed. The models are presented with coefficients (SE) and their level of significance (*p < .05, **p < .01, ***p < .001). The size of the bubbles in (a) and (b) is relative to the sample size

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