1. Introduction
Methane (CH4) with the atmospheric concentration of approximately 1.8 mg/L is the second most abundant greenhouse gas in the atmosphere after carbon dioxide (CO2) (IPCC 2014). CH4 contributes to 32% of the current radiative forcing and its global warming potential is 25 times higher than CO2 (IPCC 2014). It explains approximately 18% of the recent increase in global temperature. Forest soils are recognized as an important sink for CH4 (Reeburgh 2003). Approximately 9 to 42 Tg of CH4 is oxidized in unsaturated soils worldwide per year (Kirschke et al., 2013). Temperate forest’s ecosystems contribute 30-50% of total soil-based CH4 sink worldwide (Dutaur and Verchot., 2007; Ojima et al., 1993).
CH4 is produced by methanogens under anaerobic condition in subsoil and oxidized by methanotrophs under aerobic condition in topsoil (Le Mer and Roger 2001). CH4 emission from the temperate forests is linked with biological, chemical, and physical changes in soil. It is mainly controlled by organic carbon substrate, soil temperature, soil water content, and so on (Smith et al., 2003; Von Fischer and Hedin., 2007). CH4 production depends on the availability of organic carbon for methanogens, which is produced under anaerobic decomposition of organic matter such as plant biomass, leaf litter, and fine roots in soil (Dalal et al., 2008). Soil temperature controls microbial growth rate and subsequently CH4 emission. Thirty degrees in Celcius is reported optimal temperature for microbial activity in soil (Gütlein et al., 2017; Moore et al., 2018). Soil water content in the forest soil controls diffusive transport of CH4 in soil (Wei et al.,2018).
Soil submersion in water triggers methanogenic activity due to the formation of an anaerobic condition and it decreases methanotrophic activity by reducing the oxidized zone. CH4 oxidation occurs at 20 to 60% of soil water content in dry season and it decreases at higher than 60% of soil water content in rainy reason (Castro et al., 1995). Temperate forest regions with lower precipitation are known for CH4 oxidation (Castro et al., 1995). CH4 oxidation is suppressed in wet summer due to the inhibition of oxygen diffusion and CH4 production in anoxic microsites (Itoh et al., 2009). Forest soils can emit CH4 in wet summer (e.g. Keller and Reiners., 1994; Weitz et al., 1999; Davidson et al., 2004; Vasconcelos et al., 2004; Teh et al., 2005). CH4 dynamics in forest soils may differ in regions with heavy summer precipitation. Itoh et al. (2009) reported CH4 oxidation -0.45 kg ha−1 y−1 in a dry season and CH4 emission 1.80 kg ha−1 y−1 in a rainy season. Wetting of dry soils generally increases the microbial activity within minutes (Borken et al., 2003; Lee et al., 2004; Sponseller 2007) or hours (Pulleman & Tietema., 1999; Prieme &Christensen., 2001). Diffusion of CH4 from the atmosphere into soil usually explained with Ficks first law (Ishizuka et al., 2000; Nakano et al., 2004; Wang et al., 2014). Soil water content controls CH4 uptake by regulating CH4 diffusion from the atmosphere into mineral soils (Castro etal., 1994; Czepiel et al., 1995; Whalen and Reeburgh., 1996). Soil wetting and drying experiments revealed significant reduction in CH4 uptake with wetting (Kim etal., 2012). Kessavalou et al., (1998) reported that CH4 uptake declined by about 60% after rewetting of dry soil. To best of our knowledge instant effect of variable intensity of precipitation on CH4 uptake has not been reported. Moreover, CH4 fluxes in forest soils are monitored weekly or biweekly using manually closed chamber method and then results are interpolated to estimate annual fluxes. Instant change in CH4 flux due to precipitation may mislead the total annual CH4 flux. Precipitation varies throughout a year and this variation affects soil moisture, which thereby affects CH4 uptake or emission. The objectives of this study were to investigate the instant effect of variable precipi- tation on CH4 uptake and to estimate the contribution of precipitation in reducing net CH4 uptake in temperate forest. We hypothesized that CH4 emission will occur when it starts to rain because rain water will replace CH4 present in subsoil. CH4 uptake may decrease after precipitation due to water-filled pore space and in result limited space for atmospheric CH4 uptake.
2. Materials and Methods
The experiment was conducted in a mature Platanus occidentalis forest on Hanyang University campus, Seoul, Republic of Korea (37°33'33''N, 127°02'47''E). The soil texture was sandy loam with sand, silt, and clay proportions of 55.1, 33.8, and 11.1%, respectively. Daily temperature and precipitation varied between -18.6 to 36.7oC and 0.1 to 260 mm, respectively (Korea Metrological Administration 2010-2017).
Three experimental plots were located as shown in Fig. 1. Each plot was 40 × 260 cm and distance between two adjacent plots was 10 cm. Each plot comprised with five treatments such as P-0, P-10, P-20, P-40, and P-80, where (P) is precipitation and the number followed by P is amount of the water equivalent to precipitation (mm day−1). The water 0.34, 0.67, 1.35, and 2.69 L was sprayed in P-10, P-20, P-40, and P-80, respectively. Water was sprayed inside the chamber bases on alternate gas sampling days. When water was not sprayed we assume no precipitation (NP), henceforth mentioned as (NP-0, NP-10, NP-20, NP-40, and NP-80). To minimize soil disturbance, one plot was exclusively dedicated for soil sampling and remaining two plots were used for gas sampling. P-0 was used as control treatment and water was not sprayed in this treatment. The volume of water for corresponding precipitation that was calculated by using guidelines of food and agriculture organization (Dastane 1978). The volume of water used in this experiment was within the range of average daily precipitation 0.1 to 260 mm in 2010-2017 (Korea Metrological Administration 2010-2017). The volume of water corresponded to precipitation below 10 mm was too low to spray on given surface area of chamber. Precipitation above 80 mm was much higher than the volume of closed chamber above ground. Therefore, treatments for precipitation below 10 mm and above 80 mm were not installed were not installed.
Fig. 1. On field experimental treatments to determine the effect of variable precipitation on CH4 uptake.
Five polyvinyl chloride (PVC) chamber bases of (20 cm diameter and 20 cm height) were randomly inserted 5 cm into the ground in each plot. An air-tight lid made of PVC was kept on the chamber base for one hour and a 30 mL gas sample was collected from the chamber at 0, 15, 30, 45 and 60 min after chamber closure. All gas samples were stored in 25 mL glass vials sealed with aluminum caps and gray butyl septa. Samplings were conducted between 09:00 to 10:00 between 14th September to 15th October in 2018
every third day. Gas samples were analyzed using a gas chromatograph (YL 6100, Young Lin Instrument Co., Korea) equipped with a flame ionization detector and GS-Alumina Agilent column (length, 50 m; inner diameter, 0.53 mm). The temperatures of the column, injector, and detector were 120, 250, and 250oC, respectively. Helium was used as the carrier gas at a flow rate of 30 ml min−1.
Hourly CH4 flux was calculated from the change in gas concentration over 60 min chamber closure for first experiment and 30 min closure for second experiment (Rolston 1986):
\(\begin{equation} F=\frac{V}{A} \times \frac{d c}{d t} \times\left(\frac{273}{273+T}\right) \end{equation}\) (1)
where F is the hourly CH4 flux (μg m−2 h−1), V is the gas volume (m3), A is the area of the chamber base (m2), and \(\begin{equation}\frac{d c}{d t}\end{equation}\) is the rate of CH4 concentration change over a 60 min period in the chamber (μg m−3 h−1).
Temperatures of ambient air, the air inside the chamber, and the soil were recorded at the time of CH4 sampling. Soil temperature and water content were monitored at 10, 20, and 30 cm depth of one plot on each sampling day. Soil samples were collected inside the chambers using a sampling tube with 2.5 cm internal diameter and 100 cm height. Soil gravimetric water content (θg) was determined using the oven drying method at the controlled temperature of 105oC for 24 h. Bulk density (ρb) of soil was measured before and after the experiment at 10, 20 and 30 cm depth using core sampler. Soil samples for bulk density were collected outside and inside of each chamber before and after experiment, respectively. The volumetric water content (θv) was calculated as:
\(\begin{equation} \theta_{v}=\rho_{b} \times \theta_{g} \end{equation}\) (2)
Volumetric water content was converted into absolute air- filled porosity (AFP, cm3 cm-3) knowing the bulk density (ρb) and the particle density of soil (ρs) with the equation (Epron et al., 2016):
\(\begin{equation} A F P=\left(1-\rho_{b} / \rho_{s}\right)-\theta_{v} \end{equation}\) (3)
Soil particle density (ρs) was assumed 2.65 g cm−3 of rock, sand grains and other soil mineral particles (Gao et al., 2018 ; Zhu et al., 2013). The water-filled pore space (WFPS) was calculated with the equation (Gao et al., 2018):
\(\begin{equation} W F P S=\theta_{\sqrt{V}} /\left(1-\rho_{b} / 2.65\right) \end{equation}\) (4)
Both AFP and WFPS were then converted in percent by multiplying with 100.
2.1. Statistical analysis
The SPSS 20 statistical software package was used for statistical analysis. Independent-sample t-test was used to test the significant difference between control and litter P-(0-80) and NP-(0-80) treatments. One-way ANOVA was used to test the significant difference between the results of CH4 emission in all treatments of P-(0-80) and NP-(0-80). The difference level was set at p<0.05. linear regression F analysis was performed to establish correlation between CH4 uptake and (soil moisture content, soil temperature, AFP, and WFPS).
3. Results and discussion
Average CH4 uptake in the entire experimental period was 30.6, 8.3, 5.6, 5.5, and 4.4 µg m−2 h−1 in P-0, P-10, P-20, P-40, and P-80, respectively. AverageCH4 uptake 29.3, 42.5, 44.4, 26.2, and 21.7 was observed in NP-0, NP-10, NP-20, NP-40, and NP-80, respectively (Fig. 2a). Average CH4 uptake in P-0, P-10, P-20, P-40, and P-80 was 5, 80, 87, 79, and 80%, respectively, lower than NP-0, NP-10, NP-20, NP-40, and NP-80, respectively. Average CH4 uptake in P-(10-80) was significantly lower than NP-(10-80) treatments (p=0.05). No significant difference was observed in control treatments P-0 and NP-0 (p=0.05). In all treatments, soil temperature decreased consistently throughout the experimental period (Fig. 2b). Maximal and minimal soil temperature was observed on September and October, respectively. Average soil temperature 17.9, 18.2, 18.5, 19.7, and 20.7oC was observed in P-0, P-10, P-20, P-40, and P-80, respectively. Relatively low temperature 16.4, 16.2, 16.1, 16.3, and 16.6 oC was observed in NP-0, NP-10, NP-20, NP-40, and NP-80, respectively. Soil temperature was not significantly different among the treatments in both P (0-80) and NP (0-80) (p=0.05). Soil temperature was positively correlated with CH4 uptake in P-0, P-10, P-20, P-40, and P-80 (R2=0.14, 0.49, 0.17, 0.44, and 0.12, respec-tively). Soil temperature was also positively correlated with CH4 uptake in NP-0, NP-10, NP-20, NP-40, and NP-80(R2=0.74, 0.65, 0.57, 0.03, and 0.29, respectively). Average soil water content in entire experimental period was 20.8, 26.2, 24.4, 24.4, and 27.2% in P-0, P-10, P-20, P-40, and P-80, respectively. Average soil water content in NP-0, NP-10, NP-20, NP-40, and NP-80 was 22.3, 21.7, 20.5, 20.7, and 24.5, respectively. Statistically there was no significant difference in P (0-80) and NP (0-80) (p=0.05). Average soil water content was positively correlated with average CH4 uptake in P-0, P-10, -20, P-40, and P-80 (R2=0.84, 0.61, 0.40, 0.64, and 0.13, respectively). Average soil water content was also positively correlated with average (CH4) uptake in NP-0, NP-10, NP-20, NP-40, and NP-80 (R2=0.40, 0.64, 0.10, 0.40, and 0.96, respectively).
Fig. 2. (a), CH4 flux; (b), soil temperature; and (c), soil water content with variable precipitation 0, 10, 20, 40, and 80 mm per day. Error bars represent ±1 standard error of mean. *water was applied on these dates.
Average soil air filled porosity in 0-10 cm soil depth was 48.1, 43.0, 36.2, 37.9, and 33.9%, in P-0, P-10, P-20, P-40, and P-80, respectively. AFP in 10-20 cm soil depth was 39.0, 23.3, 31.2, 25.9, and 21.2%, in P-0, P-10, P-20, P-40, and P-80, respectively. AFP in 20-30 cm soil depth was 40.7, 31.5, 33.6, 25.8, and 23.4%, in P-0, P-10, P-20, P-40, and P-80, respectively. Average AFP in all soil depths (0-30 cm) of P-10 treatment was not significantly different form P-0 (p=0.05). Average AFP in all soil depths (0-30 cm) of P-20, P-40, and P-80 treatment was significantly different form P-0 (p=0.05). CH4 uptake was positively correlated with AFP in all soil depths (0-30 cm) and all treatments (Fig. 3). CH4 uptake decrease significantly in P-80 due to lowest AFP. CH4 uptake was weakly correlated with AFP (P-80) in all soil depths (0-30 cm). Average AFP in soil depths 0-10, 10-20, and 20-30 cm in NP-0, NP-10, NP-20, NP-40, and NP-80 treatments was (50.2, 46.6, 41.78, 39.5, and 35.2%), (37.8, 36.0, 36.3, 31.3, 25.2%), and (35.6, 38.3, 34.8, 32.0, 27.9%), respectively. Average AFP soil depth 0-10 cm in treatments NP-40 and NP-80 were only significantly different from NP-0 (p=0.05). All other treatments NP (10-80) in all depths (0-30 cm) ware not significantly different from NP-0. This indicates soil water content was significantly evaporated form all treatments NP- (10-80) and all soil depths (0-30 cm). CH4 uptake increased significantly in NP-(10-80) as compare to P- (10-80) due to increase in AFP. CH4 uptake was positively correlated with AFP in NP-(0-40) and negatively correlated in NP-80 in all soil depths 0-30 cm. Negative correlation in NP-80 was due to lowest AFP as compared to NP-(0-40). Soil CH4 uptake significantly increased as AFP increased (Díaz et al., 2018).
Relationship between WFPS and CH4 uptake in all treatments was exactly opposite to relationship between AFP and (\CH4 uptake (Fig. 4). CH4 uptake was negatively correlated with WFPS in all treatments P-(0-80) and NP-(0-40). CH4 uptake was positively correlated with WFPS in all treatments NP-80). Positive correlation in NP-80 was due to highest WFPS as compared to NP-(0-40). WFPS in all soil depths (0-30 cm) of P-(10-80) and NP-(10-80) was not significantly different from P-0 and NP-80, respectively(p=0.05).
Fig. 3. Relationships (a-e, P (0-80) and a-e, NP (0-80)) between CH4 emission and soil air filled porosity in different soil depths (0-10, 10-20, and 20-30 cm).
Fig. 4. Relationships (a-e, P-(0-80) and NP-(0-80)) between CH4 emission and soil water filled pore space in different soil depths (0-10,10-20, and 20-30 cm).
Immediate effect of water application on CH4 reduction was prominent. CH4 uptake in P-(10-80) was extrapolated to actual precipitation in 2017 (Fig. 5). Hourly precipitation varied between 0.0042 to 6.021 mm m−2 h−1 (Korea Metrological Administration 2017). Estimated daily (CH4) uptake due to precipitation was calculated by extrapolation of field results. Since, average CH4 uptake in P-(20-80) was not significantly different from each other, CH4 uptake at P-80 was assumed same for precipitation above 80 mm m−2 h− 1.Minimal and maximal CH4 uptake 4.4 and 12.6 µg m−2 h−1 was observed at precipitation 80 and 0.1 mm m−2 h−1, respectively.
Fig. 5. Average daily precipitation, estimated average daily (CH4) uptake, South Korea.
Effect of CH4 sampling frequency (weekly and biweekly) on estimated total CH4 uptake 2017 was compared (Fig. 6). The most common CH4 sampling frequencies in temperate forests have been reported weekly and biweekly (Brumme and Borken., 1999; Kim et al., 2016; Kirschke et al., 2013). We assumed that CH4 uptake was measured on weekly or biweekly from the field. Weekly and biweekly data of daily CH4 uptake was subtracted from estimated CH4 uptake in 2017. After subtracting weekly and biweekly CH4 uptake, total estimated CH4 uptake was 18.2 and 19.7 mg m−2 y−1, respectively. Total estimated CH4 uptake in 2017 in this study was decreased by 11.3 and 4.2%. Total estimated CH4 uptake in this study was not significantly different from weekly and biweekly CH4 uptake subtracted data (p=0.05).
Fig. 6. Estimated total CH4 uptake (2017) in this study and its difference (%) with weekly and biweekly sampling frequencies.
CH4 uptake at variable intensity of precipitation was calculated by interapolation of CH4 uptake results in this study (Fig. 7). Maximal total precipitation in 2017 was 372.7, 370.1, 275, 112.5, and 39.2 mm in 48 h, 24 h, 6 h, 1 h, and 0.17 h, respectively (Korea Metrological Adminis- tration 2017). Precipitation intensity increased with decresing total precipitation time. Maximal and minimal precipitation was observed at 0.17 and 48 h, respectively. CH4 uptake decreased with increasing precipitation intensity. CH4 uptake in 24, 6, 1, and 0.17 h was 32.5, 47.2, 47.5, and 48.9% lower than CH4 uptake in 48 h, respectively. CH4 uptake in 6, 1 and 0.17 h was not significantely different from each other. CH4 uptake esponse to precipitation intensity was in agreement with CH4 uptake in this study.
Fig. 7. CH4 uptake at different intensities of precipitation.
Average annual CH4 uptake in temperate forests of different countries varied between 240 to 5890 µg m−2 day−1 as shown in Table 1. Annual CH4 uptake in the temperate forest of South Korea has been reported 1960 to 2920 µg m−2 day−1 with an average uptake 2440 µg m−2 day−1. Mininal and maximal daily CH4 uptake in this study was 186.2 and 957.0 µg m−2 day−1, respectively. This indicates that the experimental results from this research is not very different from the previous reports. It also confirms that the experimental procedure in this research is sound and comparable to the others. Average CH4 uptake in (P-0+NP-0) was compared with reduced CH4 uptake due to variable precipitation intensities as shown in Fig. 8. Hourly CH4 uptake was converted to daily uptake by multiplied with twenty four hours. Daily CH4 uptake decreased withincreasing precipitation intensity from 48 h to 6 h. CH4 uptake reduction in 6 h, 1 h, and 0.17 h was not significantely different from eachother. Maximal and minimal decreased in CH4 uptake was observed at 0.17h and 48h precipitation intensity, respectively.
Table 1. Summary of published CH4 uptake in temperate forests
Fig. 8. Comparison of CH4 uptake in this study with reduced CH4 uptake due to variable precipitation intensity. Error bar represent & plusmn; 1 standard error of mean.
4. Conclusion
We measured CH4 uptake in temperate plantation from different treatments of variable precipitation, i.e., 0, 10, 20, 40, and 80 mm m−2 day−1. CH4 flux was observed immediately after water application in P-(10-80) and observed after two days interval when water was not applied NP-(0-80). (CH4) uptake in P-(10-80) was significantly lower than NP-(10-80). In our first hypothesis we assumed CH4 emission may occur because rain water will replace CH4 present in subsoil. Throughout the experimental period temperate forest soil was CH4 sink rather than source. CH4 uptake decreased significantly due to increasing water application in P-(10-80). We also hypothesized that CH4 uptake will decrease with increasing WFPS, in P-80 WFPS was 53% higher than P-0 CH4 uptake decreased 85.6% in P-80. This decrease in CH4 uptake was positively correlated with air filled porosity and negatively correlated with water filled pore space. Soil texture at experimental site was sandy loam, which is relatively coarse texture further studies needed to establish if the relationship between variable precipitation to CH4 uptake holds true across different soil texture classes. Our results can be used as a reference for regions with similar conditions. This study demonstrated the effects of variable precipitation on net daily CH4 uptake and it may help in calculating more accurate net annual CH4 sink in temperate forests in the world.
Acknowledgements
This study was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, & Future Planning (NRF-2018R1A2A1A05023555).
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