Discussion
For the sampling period, the riparian forest soils examined in this
study were a net sink of CH4, and a net source of
CO2 and N2O. The reference sites had
significantly higher N2O emissions and significantly
higher CH4 uptake than the sites with no buffer.
CH4 uptake was significantly higher in the ND areas
compared to the DIS areas, reflecting the difference in soil moisture
and resulting aerobic conditions required for methanotrophy and
anaerobic conditions required for methanogenesis (Oertel and others
2016). Soil temperature, soil moisture, and depth to the groundwater
table were significant predictors of CO2 emissions. Soil
moisture and depth to the groundwater table were significant predictors
of CH4 fluxes.
Effects of forest harvest
Although riparian zones are widespread across the global landscape, we
found very few studies examining GHG fluxes from the riparian zones of
small streams (Goodrick and others 2016; Soosaar and others 2011), and
even fewer for headwater streams (Chang and others 2014; De Carlo and
others 2019a). Due to the distinct environmental conditions of riparian
zones (Vidon and others, 2018), the effects of forest harvest on GHG
fluxes are likely to differ here from upland soils. Riparian zones of
headwater streams in particular are unique due to their steep
geomorphology, high groundwater influence, and susceptibility to low
flows (Richardson and Danehy 2007). Therefore, this study fills the
knowledge gap of the effects of forest harvest on GHG fluxes from
riparian zones of headwater streams.
Clear-cutting has inconsistent effects on CO2 emission
rates (Striegl and Wickland 1998; Ullah and others 2009), with studies
reporting a decline (Striegl and Wickland 1998), an increase (Lavoie and
others 2013; Paul-Limoges and others 2015), or no change (Kähkönen and
others 2002) in CO2 emissions following harvest. In this
study, there was no significant difference in CO2emission rates between treatments, although CO2emissions were highest at the buffer sites. This lack of a significant
treatment difference may have been because of the distinct conditions in
riparian forests compared to upland forests. Riparian zone soils have
high moisture levels due to shallow water tables and a strong
groundwater influence (Goodrick and others 2016). Therefore, the rise in
temperature following forest harvest that can explain the rise in
CO2 following harvest in upland forests (Lavoie and
others 2013), may be buffered by the generally moister and cooler soils
of riparian forests (Clinton and others 2010), resulting in a less
distinct rise in CO2 emissions following harvest in
riparian forests compared to upland forests. Moreover, the mild mean
annual temperature and narrow range in annual temperature in our study
region may have meant limited temperature differences, resulting in a
small effect size of the treatments.
Soil temperature, soil moisture, and depth to the groundwater table were
significant predictors of CO2 fluxes. This is consistent
with the large body of literature on the drivers of soil respiration
(Luo and Zhou 2006). Soil respiration usually increases exponentially
with temperature, reaches a maximum, and then declines (Luo and Zhou
2006). Temperature controls many aspects of soil respiration from the
activity of cellular enzymes, to root growth and microbial activity (Luo
and Zhou 2006). Soil moisture is another well-established driver of soil
CO2 emissions, with the common conceptual relationship
where soil respiration is low under dry conditions, reaches a maximum at
intermediate soil levels, and decreases at high soil moisture content
where anaerobic conditions depress aerobic microbial activity (Luo and
Zhou 2006). The interactive effects of these two variables on soil
respiration is a key knowledge gap (Meyer and others 2018). Our research
provides valuable information to help understand the effects of these
factors on soil respiration in the unique riparian environment.
The results of our study were in line with other studies, which have
also found that clear-cutting increased CH4 efflux from
the soil (Kähkönen and others 2002; Wu and others 2011). This rise can
be attributed to higher average summer soil temperatures, greater soil
moisture, and higher dissolved organic carbon concentrations in
clear-cuts (Ullah and others 2009; Wu and others 2011). Forest harvest
can result in soil compaction, reduced transpiration, and a rise in the
groundwater table, all of which promote waterlogging and anaerobic
conditions (Christiansen and others 2017; Gundersen and others 2010). In
our study, soil moisture and depth to the groundwater table were
significant predictors of CH4 fluxes, while temperature
was not. In agreement with our results, CH4 uptake was
three times lower following clear-cutting in a temperate spruce forest
in southern Germany (Wu and others 2011). In another study,
clear-cutting turned a spruce forest soil in Finland from a sink to a
source of CH4, with a 40% decrease in
CH4 consumption rates (Kähkönen and others 2002). The
lower CH4 uptake following forest harvest may be
explained by the harmful impacts of soil disturbance on methanotrophic
bacteria, resulting in the inhibition of CH4-oxidation
(Le Mer and Roger 2001; Wu and others 2011). Alternatively, the
anaerobic conditions created by higher soil moisture concentrations
following forest harvest can promote the production of
CH4 (Wu and others 2011). A clear-cut wetland in Québec,
Canada produced 131 times more CH4 than the undisturbed
wetland soil, likely due to higher soil temperature and soil moisture in
the clear-cut (Ullah and others 2009). In our study, the no buffer sites
still were a net sink, albeit a weak sink, of CH4 over
the growing season. This is likely because unlike many wetlands, most
riparian soils do not have consistently anoxic soils, which promote
methanogenesis (Dalal and Allen 2008). Given that there was no
significant difference in CH4 flux rates between the
buffer and reference sites, it appears that riparian buffers may be
effective in preserving soil ecosystem conditions contributing to
CH4 fluxes. Consequently, riparian buffer zones may be
an effective strategy for forest managers interested in maintaining
CH4 balance in riparian zone soils.
The lower N2O emissions in the no buffer sites compared
to undisturbed riparian zones was a surprising result because many other
studies have reported an increase in N2O emissions
following forest harvest. Typically, forest harvest can increase soil
moisture and mobilize soil nitrogen, promoting N2O
emissions from logged forest sites (Kreutzweiser and others 2008).
Higher N2O emissions were seen following forest harvest
in the taiga region of eastern Siberia, Russia (Takakai and others
2008). Additionally, N2O emissions were 2.7 times higher
in clear-cut than in mature black spruce forest soil in Québec, Canada
(Ullah and others 2009). However, these studies were not conducted in
riparian forests, which have unique conditions such as shallow water
tables, high soil organic matter quality and quantity, and high soil
nitrogen availability, unlike most upland forests (Knoepp and Clinton
2009; Vidon and others 2018). Given that forests are typically sources
of N2O (Dalal and Allen 2008), the lower
N2O fluxes in the no buffer compared to the reference
sites is a departure from undisturbed ecosystem function. The
unexpectedly low N2O fluxes at the no buffer sites could
be explained by the mechanical soil disturbance in the riparian zone
caused by forest harvest. The disruption of the structure and function
of microbial communities responsible for nitrification and
denitrification could contribute to the comparatively low
N2O fluxes at the no buffer sites (Tan and others 2005).
Meanwhile, N2O fluxes at the buffer sites were not
significantly different from the reference sites, thus, riparian buffers
may be effective in preserving soil ecosystem conditions contributing to
N2O fluxes. A decline in soil CO2emissions was attributed to the disruption of the soil surface and death
of tree roots following the clear-cutting of a jack pine stand in
Saskatchewan (Striegl and Wickland 1998). Moreover, soil compaction as a
result of forest harvest has been found to reduce net nitrification
rates in the forest floor and mineral soil as well as reduce the soil
microbial biomass nitrogen (Tan and others 2005).
None of our measured environmental variables (i.e. soil temperature,
soil moisture, and depth to the groundwater table) were significant
drivers of N2O fluxes. Thus, perhaps some environmental
variables not measured in this study could explain some of the
unexplained treatment differences. For instance, soil nitrogen
concentrations are an important driver of N2O fluxes
(Christiansen and others 2012). A large proportion of nitrogen in upland
soils is transferred to riparian ecosystems, where it can be retained
via biological assimilation or removed via denitrification (Pinay and
others 2018). Denitrification rates were positively correlated with soil
nitrate content in a riparian forest along the Louge River in south-west
France (Pinay and others 1993). Moreover, streamwater nitrate
concentrations have been found to increase in the short term following
forest harvest in western North America due to enhanced soil
nitrification rates (Feller 2005). This increased flow of nitrate from
the soil into streams in the short term, subsequently returns to
pre-harvest levels due to uptake by rapidly growing biomass (Feller
2005). This phenomenon could explain the lower N2O
emissions we observed at the sites with rapidly re-growing vegetation,
harvested over three years earlier, compared to the reference sites.
In addition to the effects of external nutrient inputs, the potential
for nitrogen retention and removal can vary between climatic regions,
based on differences in soil moisture conditions and nitrogen supply. In
temperate regions with high humidity, such as our study region, high
soil moisture promotes nitrification and denitrification (Christiansen
and others 2012). In contrast, Mediterranean and arid regions have lower
denitrification rates due to low water availability and short residence
time of water and solutes (Pinay and others 2018).
In the context of other studies, the soils in our study area have a
moderate nutrient content and the GHG flux rates in this study were
comparable to those reported in other temperate riparian zones. The
soils in undisturbed stands at Malcolm Knapp Research Forest have
previously been categorized as medium-nutrient (Collins and others
2001), with reported mineral soil nitrate content of 2.5 kg
ha-1 and a C:N ratio of 23 (Turk and others 2008).
Mean gas flux rates at the reference sites (June to September) were 65.7
± 29.9 mg CO2 m-2h-1, -26.3 ± 17.7 µg CH4-C
m-2 h-1, and 3.6 ± 2.6 µg
N2O-N m-2 h-1. Since
we measured gas fluxes from June to September, our average flux rates
should be slightly higher than annual flux rates reported from temperate
studies that include winter months when microbial activity is lower, and
hence flux rates are lower. The average annual CO2 flux
rate in flood-prone riparian forest in an agricultural landscape along
the White River in Indiana, USA measured 48.8 mg CO2m-2 h-1 and was significantly
related to the C:N ratio (16.4) of soil organic matter (Jacinthe and
others 2015). At a temperate riparian forest in an agriculturally
dominated landscape in Ontario, Canada, the mean annual
CO2 emissions were 18 mg CO2m-2 h-1, with a soil organic carbon
content of 84.1 g C kg-1 and a total nitrogen content
of 12.5 g N kg-1 (De Carlo and others 2019a). At the
same location, mean annual N2O emissions were 5.93 µg
m-2 h-1, and were significantly
correlated with soil total nitrogen (12.5 g N kg-1)
(De Carlo and others 2019b). A riparian forest in New Jersey had a C:N
ratio of 21 and a N2O emission rate of 0.9 µg N
m-2 h-1 (Ullah and Zinati 2006).
Nevertheless, there is a lack of studies about GHG emissions from
riparian zones of headwaters streams, moreover most of the literature is
in agriculture-dominated landscapes and not in forested riparian areas
impacted by forest harvest practices.
Effects of local
groundwater conditions
Landscape features that dictate soil characteristics, such as local
microtopography, can be important for predicting riparian GHG emissions
as they may affect the spatial distribution of soil moisture, nutrients,
and organic matter, thus consequently affecting the intensity of GHG
emissions (Jacinthe and Vidon 2017; Soosaar and others 2011). Local
groundwater discharge conditions may create particularly important
microsite variation in the riparian zones of streams. Soil conditions in
DIS areas have been found to have higher base cations, soil moisture, pH
levels, and nitrogen concentrations when compared to surrounding soils
(Giesler and others 1998). These soil conditions may influence the
processes controlling soil GHG fluxes. However, it is unknown how forest
harvest in the riparian zone might influence conditions in DIS areas,
and how GHG fluxes may subsequently be influenced. We hypothesized that
due to the higher soil moisture at DIS areas, the emission of
anaerobically produced CH4 and N2O would
be higher than at ND areas. There were no significant differences
between the DIS and ND areas for CO2 and
N2O emissions, although ND areas generally had higher
CO2 emissions on average and DIS areas generally had
higher N2O emissions. However, CH4uptake was significantly lower in the DIS sites compared to the ND
sites. This means that DIS sites were more likely to be
CH4 sources, while ND sites were more likely to be
CH4 sinks. Similar results were found in riparian zones
in central Indiana, where a topographic depression in the riparian
forest accounted for 78% of annual CH4 emissions,
despite only covering <8% of the total land area (Jacinthe
and others 2015). Additionally, GHG fluxes from flowing stream waters
have been found to peak downstream of DIS areas, due to their lateral
gas inputs from riparian soils (Lupon and others 2019). Given that
CH4 fluxes were highest in DIS areas at no buffer sites,
the results of our study provide additional support for the use of
hydrologically adapted buffers, which provide more protection for wet
areas, such as DIS areas, in the riparian zone (Tiwari and others 2016).
The variable buffer width adapted to site-specific hydrological
conditions can protect biogeochemical and ecological functions as well
as provide economic savings when compared to fixed-width buffers (Tiwari
and others 2016).
Conclusions
In conclusion, our work shows that forest harvest and local groundwater
conditions influence GHG emissions from riparian forest soils alongside
headwater streams. Our results demonstrate that riparian buffers may be
effective in protecting soil ecosystem functions contributing to
CH4 and N2O fluxes. Nevertheless, our
findings should not diminish the importance of maintaining intact
riparian forests for other benefits, such as the unique biodiversity
riparian areas sustain. Given our finding that local groundwater
conditions play an important role in driving CH4 fluxes,
forest managers may choose to consider hydrologically adapted buffers to
reduce CH4 emissions. This new information about the
effects of forest harvest in the riparian zone on GHG flux rates should
be considered in local policy discussions regarding how forest harvest
in the riparian zone can contribute to climate change, and what can be
done to mitigate or diminish the impacts. Considering that many small
streams are left unprotected in British Columbia (Kuglerová and others
2020) and elsewhere, perhaps the leverage of climate change mitigation
may influence policy changes leading to increased retention of riparian
buffer zones in the region. In sum, this research may be useful to
forest managers interested in managing riparian buffer zones for GHG
balance and climate change mitigation.