Coupled effect of temperature and mineral additions facilitates decay of aspen bark

Publication date: 15 April 2018
Source:Geoderma, Volume 316
Author(s): Irina Kurganova, Valentin Lopes de Gerenyu, Natalia Galibina, Ekaterina Kapitsa, Ekaterina Shorohova
Tree bark represents a substantial component of coarse woody debris (CWD) in boreal forests. Estimating its decay rates improves our understanding of decomposition processes of woody debris and their effects on the forest carbon cycle. The atmospheric deposition of nitrogen (N) and phosphorus (P) from aerosols and gases has notably grown during the last century. We examined the effect of mineral N and P additions and temperature on the decay rate of aspen (Populus tremula) bark in a long-term incubation experiment at constant and sufficient moisture level. Fresh aspen bark with natural moisture was mixed with local soil (Haplic Phaeozems) in proportion of 2:1 by dry mass to prepare the soil-bark substrates (SBSs). Mineral elements (N and P) were added to the SBSs with a proportion of 1% of dry bark weight (for each element). The following treatments of SBS's were studied: (1) Bark+Soil, pure SBS (control); (2) Bark+Soil+N; (3) Bark+Soil+N+P and (4) pure soil, S . The SBSs were incubated in thermostats at 2, 12 and 22°С for 12months. The decay rate of SBSs (DecR, mgСkg⁻¹ h⁻¹) was measured by an infrared gas analyzer at least 1–2 times per week. Total carbon losses (TotL-C, gСkg⁻¹ of bark) were estimated using the accumulative curves of DecR throughout the experiment for each treatment. The decay constants (k) were calculated based on the single exponential model. The temperature sensitivity of the DecR was estimated using the temperature coefficient Q10. Mineral N and P additions decreased the C:N and increased the Lignin : Cellulose ratios during the long-term incubation of aspen bark and caused a considerable increase in the DecR, k values, TotL-C values, and consequently a decrease of the turnover time. The maximal values of TotL-C were attributed to the SBS-NP treatment at 22°С and comprised 72% of the initial C content in bark. The turnover time of soil-bark substrates varied from 2 to 7.3years depending on the treatment and temperature. The increase of the incubation temperature from 2 to 12°C and from 12 to 22°C caused a similar effect on the DecR over 12months of the experiment, and the Q10 in different treatments varied negligibly: from 1.21 to 1.37. The temperature effect was most significant only during the first 1–2months of experiment, explaining ca. 83% of the mean DecR variance whereas the ‘Treatment’ factor was attributed for 76–83% of the mean DecR variance in later stages (3–12months) of decay of aspen bark. A close relationship between the Lig:Cel ratio and DecR was observed during 1–2months of incubation. We conclude that the effect of mineral additions on the decay rate of aspen bark is more important than the effect of temperature. The coupled effect of N and P additions was more pronounced than the effect of N addition alone.