Air temperature and precipitation
Over the thirteen years (2005-2017) of the recorded meteorological data (Source: Department of hydrology and meteorology, DHM, Lumle) representing the air temperature and precipitation of the R. arboreum forest area (Fig. 2) showed that the area mostly acquired higher air temperature in July and August and the lower during winter in December and January which was reached the highest at 22.3˚C in August and lowest at 7.9˚C in January 2012. The forest area received maximum precipitation as rainfall during the month of summer in July and August however some of the years June and September as well received the higher amount of precipitation due to early and delay rain events. The winter season received the least amount of rain and sometimes no precipitation occurred during that period so-called dry season. The highest rainfall was recorded at 1818.1mm in July 2010. The mean (2005-2017) annual-average air temperature of the forest area was recorded at 16.5˚C with the highest in July and August at 21.0 °C and the lowest was recorded in January at 9.9°C (Table 1). Similarly, the mean (thirteen years) annual-average precipitation of the forest area recorded was 434.7mm with the maximum and minimum monthly mean precipitations were 1523.9mm in July and 8.1mm in December, respectively. The mean (2005-2017) annual precipitation of R. arboreum forest was recorded at 5216.1mm y-1. ACA is located in central part of Nepal receives the highest amount of rainfall compared to all regions of Nepal.
Soil respiration (SR) and soil temperature (ST)
The temperature dependency of SR variations of this R. arboreum forest was assessed from the ST measured at 5 cm soil depth that was plotted against the SR (Eq. 1) with optimal regression using the exponential form of Equation (2) over the different periods and dates in October 2016 and 2017 for the entire experimental period (Fig. 3). The result exhibited increasing ST boosting the SR exponentially was explained positive statistically significant (p<0.05) curve (Day1: D1; R2=0.21, y=0.572e0.580x, Day2: D2; R2=0.13, y=0.567e0.585x, Day3: D3; R2=0.23, y=1.248e0.525x). Similar, exponential statistically significant (p<0.05; Day1: D1; R2=0.20, y=12.12e0.379x and Day2: D2; R2=0.24, y=3.789e0.500x), and statistically insignificant (p>0.05; Day3: D3; R2=0.19, y=6.688e0.448x and Day4: D4; R2=0.18, y=14.86e0.379x) curves were detected in 2017. The highly significant (p<0.05) exponential relation with the ST were derived when different dates in a season were integrated (R2=0.18, y=1.049e0.529x) in 2016 and (R2=0.17, y = 26.34e0.284x) in 2017 (Fig. 4 a1, b1). Within a short-range even at 2-3°C of the ST difference i.e. from 9.2°C to 11.3 °C in 2016 and 6.4 °C to 9.3 °C in 2017, the variations of the SR in this forest were well recognized to explain the ST effect on SR of the forest.
Soil respiration (SR) and soil water content (SWC)
The scattered relation was found for SWC dependence on SR variations in the R. arboretum forest in an experiment while assessing them in different periods of two consecutive years (Fig. 4 a2, b2) in experiment.
The SWC was ranged between 27.0% and 40.6% in 2016, and 22.3% and 44.8% in 2017. The peak values increase in SWC after the unexpected rain events that were very common in the study area. A comparatively much higher range of SWC was noticed in 2017 than the year 2016 as the measurements dates in 2016 were half a month earlier to the summer season i.e. much near to the rainy season than the year 2017.
Soil respiration (SR) and photosynthetic photon flux density (PPFD)
To elucidate the dependence of SR on the variability of light in the forest, SR was plotted against the PPFD in different periods, dates and years of the measurements showed that the light parameter, PPFD was not able to establish the relationship with SR variations to define independently the PPFD effect on SR in the R. arboreum forest (Fig. 43 a3, b3). The values obtained in measurements of PPFD were assessed much random in different chamber and even constitute in the same chamber at different points. The range of the PPFD observed was between 2.7 µ mol m-2 s-1 and 96.2 µ mol m-2 s-1 in 2016, and 1.3 µ mol m-2 s-1 and 761.0 µ mol m-2 s-1 in 2017. In two years, the values of the PPFD were most likely concentrated within 100 µ mol m-2 s-1 that were correctly measured in a different time period of a day for the consecutive 3-4 days in a year. The high and low values of the PPFD were most likely recorded in a different period sunny to the cloudiest weather.
Comparisons of soil respiration (SR) and soil temperature (ST), soil water content (SWC), photosynthetic photon flux density (PPFD) and litter biomass variations
The temporal, diurnal and inter-annual variations of the SR, ST, SWC and PPFD of the forest represented the degree of correlations between the SR and the climatic parameters. Comparatively higher SR (n=10) was observed in the afternoon (11:00 am – 1:00 pm) and the maximum value was recorded at 306.3 mg CO2 m-2 h-1 on 18th October 2016. But, the values became random in 2017 and the maximum value was observed during evening at 348.9 mg CO2 m-2 h-1 on 27th October (Fig. 5). The minimum SR (n=10) was observed in the morning on 17th October at 201.6 mg CO2 m-2 h-1 in 2016 and on 29th October at 232.5mg CO2 m-2 h-1 in 2017. With acquiring the temporal variations of the SR in each day of measurements, the values of SR were not compatible with each other. However, the difference value between the maximum and minimum SR was most likely compatible in both years. The ST (n=10) was recorded maximum at 10.7 °C on 16th October 2016 and 8.9 °C on 27th October 2017 during evening between 3:00 pm – 5:00 pm among different days of the measurements. The minimum ST was observed during morning at 9.6°C on 17th October 2016 and 7.1°C on 27th October 2017. The ST followed the similar trend of the SR and it was always lower during morning than that of the afternoon and evening.
The maximum SWC (n=10) was recorded during the evening at 36.7% on 16th October 2016 and 36.9% on 27th October 2017. The minimum SWC was observed during the afternoon at 31.8% and 31.4% on 16th October 2016 and 30th October 2017 respectively. The maximum and minimum SWC of the forest was very close and compatible for both years but it varied among the dates and the time of measurements in each year. The temporal and diurnal variations of the SWC were not compatible to the SR of the forest. The SR was increased during the afternoon when the SWC decreased and higher was observed while the SWC was decreased in the evening (Fig. 6).
Correlating to other climatic parameters, the PPFD of the forest was noticed much fluctuating in different dates and years in measurements. The maximum PPFD (n=10) were recorded at 41.0 µ mol m-2 s-1 during the morning on 18th October 2016 and 253.2 µ mol m-2 s-1 during the afternoon on 30th October 2017. Where, the minimum values of PPFD were recorded at 9.0 µ mol m-2 s-1 on 18th October 2016 and 7.3 µ mol m-2 s-1 on 27th October 2017 during the evening (Figure 7). The range of maximum and minimum values of the PPFD of this forest was found much higher than that of the other related parameters. Comparing the temporal variations of PPFD to the SR it was much similar to each other in the days of sunny weather; however the trend of diurnal changes in PPFD was not visible in cloudy day and they were not directly effects on the temporal changes in SR of the forest.
The diurnal variations of the SR, ST, SWC and PPFD were detected in both years (Table 2). The daily average SR was 278.2, 242.0 and 289.5 mg CO2 m-2 h-1 on 16th, 17th and 18th October in 2016 and it was 332.1, 301.6, 273.3 and 273.5 mg CO2 m-2 h-1 on 27th, 28th, 29th and 30th October in 2017. The rates of SR were varied among the days of measurements however the values were compatible with each other and not unexpectedly different. Similarly, the ST was 10.4, 10.0 and 10.1°C in 2016 and 8.5, 8.6, 8.0 and 7.4°C in 2017. The minimal diurnal variations of the ST were detected in both years (upto 0.4°C in 2016 and 1.2°C in 2017), to detect its effect. The SWC was 34.2, 34.2 and 34.5% in 2016 and 35.7, 32.8, 33.6 and 33.4% in 2017. The diurnal variations of SWC were minor (0.3%) in 2016 however it was higher (2.9%) in 2017. Much more variations between the days were observed for PPFD were 17.1, 14.1 and 23.7 µ mol m-2 s-1 in 2016 and 12.3, 39.0, 20.0 and 90.4 µ mol m-2 s-1 in 2017. Compared to the rest of climatic parameters, the PPFD varied more to the diurnal changes of season.
The measurements of SR, ST, SWC, PPFD, litter biomass, and air temperature in 2-years determined that the inter-annual variations of the soil C emission via. SR and the ecological parameters were much common in the R. arboreum forest; however the intensity of variance were incompatible for each component (Fig. 8). The SR over the entire measurements period each year averaged at 269.9 mg CO2 m-2 h-1 in 2016 and 295.1 mg CO2 m-2 h-1 in 2017, and lower the SR rate in 2016 than the year 2017 were even comparable. The 2-years averaged SR represented the seasonal SR of the R. arboreum forest was 282.5 mg CO2 m-2 h-1. The variations from seasons to the years in carbon sequestration and emission were adopted in multiple years of estimation were observed in a temperate ecosystem (Dhital et al. 2014). Similar variations of ST were observed in 2-years averaged throughout the measurements period was higher at 10.2°C in 2016 than at 8.1°C in 2017. The seasonal ST of the forest was calculated from two years records averaged were at 9.1°C. Comparing to the other climatic parameter of the forest, less variation was detected for SWC records between the years at 34.3% in 2016 and 33.9% in 2017, and slightly higher SWC was observed in 2016 than the year 2017, but the values were compatible to each other. The seasonal SWC value estimated from two years’ data of SWC averaged was 34.1%. Similarly, the PPFD of the forest in 2016 was 18.3 µ mol m-2 s-1 and in 2017 was 41.4 µ mol m-2 s-1 and the variance was noticeable between the years. The represented seasonal PPFD averaged both years’ measurement was 29.9 µ mol m-2 s-1. The dry weight of litter biomass collected from the forest floor was observed at 8.9 g d. w. m-2 in 2016 and 15.3 g d. w. m-2 in 2017 (Eq. 3). The higher litter was recorded in 2017 than the year 2016 as the collected litter in 2017 was from the late October and it was the season to leaf abscission of the seasonal trend of the forest. The amount of the seasonal litter of R. arboreum forest averaged 2-years was calculated at 12.1 g d. w. m-2. The air temperature of the forest was noticeably varied between the years and it was higher at 7.1 °C in 2016 than the year at 5.6 °C in 2017, and the average of 2-years was at 6.4 °C. Overall this study showed that, comparatively higher SR, PPFD and litter was detected in 2017 than the year 2016 that was across from the ST, SWC and the air temperature of the forest owed to the measurements in 2016 were carried half a month earlier in the season than the year 2017 that was not much far from the wet summer season with high temperature and precipitation.