Mizue Ohashi

To clarify soil respiration (soil CO2 efflux, R-s) characteristics in a subtropical evergreen broad-leaved natural forest in Yambaru, Okinawa, Japan, we examined spatiotemporal variation in R-s and its determining factors. We then compared yearly R-s with the value in other forests. The spatial variation in R-s (coefficient of variation [CV] = 38.9%) was not significantly related to temperature or soil water content but was evidently dependent on ground surface litter coverage. R-s was greater in summer (ca. 7-10 mu mol m(-2) s(-1)), and its seasonal variation was exponentially related to soil temperature (Q(10) = 2.16). As a function of soil temperature, we estimated a yearly mean stand-scale R-s of 5.17 mu mol m(-2) s(-1), and a total carbon efflux from the soil of 1959 g C m(-2) year(-1) for 2014. Despite showing similar seasonal patterns as those in temporal forests, the R-s in this ecosystem is very high throughout the year, and the yearly value is much higher for natural mature forests. A mass balance approach suggests that the large amount of belowground carbon allocation of plants contributed to the high CO2 emissions from the soils.

Abstract. Dissolved organic matter (DOM) degradation in freshwater rivers and streams plays a major role in the global carbon cycle. However, little is known about how the source and composition of riverine DOM contribute to the production of greenhouse gases, especially in high-latitude areas with a large proportion of carbon-rich peatlands. Here, we conducted for the first time the combination of molecular-level characterization of terrestrially derived DOM and the potential carbon dioxide (CO2) production measurements in pristine subarctic rivers of Finnish Lapland. 21-day incubation studies were conducted with water samples taken from two rivers differing in DOM content during spring and fall 2018. The changes in the DOM concentration and molecular composition, as well as the CO2 production, were measured. The DOM molecular characterization was carried out using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Our results demonstrate efficient mineralization of dissolved organic carbon (DOC) into CO2 in mineral soil associated clearwater river during the incubation, while significantly lower CO2 production per DOC was observed in the brown-water river surrounded by peatlands. The limited degradability in the brown-water river was caused by a large number of terrestrial and aromatic compounds (i.e., highly unsaturated and phenolic compounds, condensed aromatics, and polyphenolics) from surrounding peatlands. In the clearwater river, the percentage of formulas assigned to aliphatics decreased over the incubation, indicating microbial utilization of biolabile DOM. This study highlights the importance of energy-rich, biolabile molecular compounds and the contribution of clearwater systems in the DOM degradation dynamics of subarctic catchments.

Tree resistance to uprooting can be estimated as the critical turning moment in tree-pulling experiments. The depth at the center point of rotation (Dcp) in tree-pulling experiments is measured as an indicator of below-ground traits and is related to this critical turning moment. However, few researchers have investigated the relationship between the Dcp and maximum root depth. Our objective in this study was to clarify whether the Dcp in tree-pulling experiments can be estimated as the maximum root depth of Pinus thunbergii Parl. in sandy soils. We also estimated which position of displacement of the center of rotation (Cp) can be applied as the Dcp. We conducted tree-pulling experiments, and compared the Dcp obtained from images with the measured maximum root depth. We found significant positive correlations between the Dcp and maximum root depth. The Cp displacement concentrated immediately below the stem when the maximum critical turning moment was reached. This position should be measured as the Dcp, which is related to the maximum root depth. We found that the Dcp can serve as a parameter, preventing the need for uprooting, when tree-pulling experiments are performed to obtain an important below-ground trait for understanding the critical turning moment.