Impact of abiotic factors on the decomposition of litter of peat-forming plants in the incubation experiment

Abstract

In bog ecosystems, the decomposition rate of plant residues is largely determined by the combined effect of abiotic (temperature and moisture of peat) and biotic (properties of peat-forming plants) factors. Model experiments under well controlled external conditions are a good tool to determine the contribution of each factor to the decomposition process. The aim of the research was to quantify the effect of temperature (T) and moisture (W) of peat on the decomposition rate of the main peat-forming plant residues of the oligotrophic wetland in the southern taiga subzone of Western Siberia. In a 3-month incubation experiment, we studied the initial stages of decomposition of 4 types of plant substrates at various temperatures (T = 2, 12, 22°C) and contrasting moisture levels corresponding to 30, 60 and 90% of their water holding capacity (WHC). We collected plant residues of Sphagnum fuscum Klinggr., Chamaedaphne calyculata Moench., and Eriophorum vaginatum L. in the Bakcharskoe oligotrophic bog (Bakcharsky district, Tomsk region, 56°58'N, 82°36'E) in September 2017. Besides the individual types of plant residues, a mixed sample of S. fuscum (60%) and Ch. calyculata (40%) was also studied. We determined the basic physicochemical characteristics in the original plant substrates: pH value in water and salt extracts (1M KCl) (litter: solution = 1:25), WHC, hygroscopic moisture, ash content, total carbon (C) and nitrogen (N) content, C to N ratio, and some organic compounds (alcohol-soluble compounds, cellulose, lignin and lignin-like substances) by the gravimetric method (See Table 1-2). Lignin and biogenetically related lignans and flavonoids were determined after removal of bituminous substances and treatment with 72 % sulfuric acid. Cellulose was extracted using a mixture of concentrated nitric acid and ethanol. For the experiment, we placed plant residues (1-3 g of air-dry mass) in 110-ml glass bottles and moistened to 30, 60 and 90% of their WHC with bog water, which contained native microflora. The bottles with moistened plant residues stayed at room temperature for 7 days (pre-incubation period) and, then, were placed in thermostats for a 3-month incubation at 2, 12, and 22°C. During the experiment, the moisture content of plant material was maintained at a constant level by adding bog water. The experiment was conducted in 3 replicates. CO2 emission rate (or decomposition rate, DecR) of the main peat-forming plant residues was measured using LI-820 (USA) infrared gas analyzer 3-5 times per week for the 1st month of the experiment and 2 times per week over the next 2 months. The decomposition constant (k) of plant residues was calculated based on the curves of cumulative C(CO2) losses over the entire experiment using an exponential regression model. DecR (μg C/g sample/hour) was calculated according to Kurganova I et al. (2018), Kurganova I et al. (2012). The impact of the temperature factor was estimated using Q10 temperature coefficient according to Kurganova I et al (2012), which was determined for two temperature intervals (2-12°C and 12-22°C). We demonstrated that the dynamics and intensity of C (CO2) release during the experiment were significantly influenced by all three factors: temperature, moisture content, and type of plant residues. In the initial stages of decomposition, we observed an enhanced release of C (CO2) for all plant samples caused by a surge in the activity of destructor microorganisms and the presence of readily available compounds in the litter composition. An increase in the decomposition rate at 22°C was recorded during the first 1-2 days of incubation and at 2°C after 1-2 weeks of experiment. At the same time, the decrease in the decomposition activity of plant residues at 2°C was much slower than at 22°C (See Fig. 1-3). The temperature coefficient Q for DecR depended on the type of plant residue and its moisture and varied from 0.97 to 1.53 in the low temperature range (2-12°C) and from 1.05 to 2.18 in the temperature range of 12-22°C (See Fig. 6). The highest total C (CO2) losses throughout the 3 months of the experiment (C ) were observed for Ch. calyculata and E. vaginatum. C value varied from 67 to 93 mg C/g at 22°C and decreased to 29-46 mg C/g at 2°C (See Fig. 4, A-C). Depending on temperature and moisture, S. fuscum lost only 3-5% of the initial amount of C over the 3 months of the experiment. The mixed sample lost 6-11% of the initial C content whereas the C(CO2) losses from Ch. valyculata and E. vaginatum varied from 6 to 18% (See Fig. 4, D-F). Based on the result of 3-way analysis of variance (ANOVA), we revealed that all the factors studied (type of plant substrate, temperature and moisture content) significantly influenced the variability of C value for 3 months of the experiment. The type of substrate is the main factor which affected the total C(CO2), explaining 61% of the total variance of C (See Table 3). Temperature and humidity accounted for 31 and 2% of the variance explained, respectively. The 2-way ANOVA was carried out individually for each plant substrate and showed that the temperature was the main factor that affected the total C(CO2) loss from the studied plant substrates explaining 75-90% of the C dispersion (See Table 4). The lowest values of the decomposition constant (k = 0.0003-0.0004 day-1) were attributed to the S. fuscum at 12 and 2°C. Decomposition constants for E. vaginatum and Ch. valyculata were the highest (k> 0.001 day-1) at all studied temperatures among all types of plant substrates (See Fig. 5). We can conclude that due to the slow decomposition rate, S. fuscum makes a major contribution to peat formation process.