Anomalous wet summers and rising atmospheric CO2 concentrations increase the CO2 sink in a poorly drained forest on permafrost.

At the northern high latitudes, rapid warming, associated changes in the hydrological cycle, and rising atmospheric CO2 concentrations, [CO2], are observed at present. Under rapid environmental changes, it is important to understand the current and future trajectories of the CO2 budget in high-latitude ecosystems. In this study, we present the importance of anomalous wet conditions and rising [CO2] on the long-term CO2 budget based on two decades (2003-2022) of quasicontinuous measurements of CO2 flux at a poorly drained black spruce forest on permafrost peat in interior Alaska. The long-term CO2 budget for the black spruce forest was a small sink of -53 ± 63 g C m-2 y-1. The CO2 sink increased from 49 g C m-2 y-1 for the first decade to 58 g C m-2 y-1 for the second decade. The increased CO2 sink was attributed to an 11.3% increase in gross primary productivity (GPP) among which 9% increase in GPP was explained by a recent increase in precipitation. Furthermore, a 3% increase in GPP in response to a 37-ppm increase in [CO2] was estimated from the data-model fusion. Our study shows that understanding the coupling between hydrological and carbon cycles and the CO2 fertilization effect is important for understanding the current and future carbon budgets of high-latitude ecosystems in permafrost regions.

Sequential loss-on-ignition as a simple method for evaluating the stability of soil organic matter under actual environmental conditions.

Soil organic matter (SOM) is one of the largest carbon (C) reservoirs on Earth, and therefore its stability attracts a great deal of interest from the perspective of the global C cycle. This study examined the applicability of loss-on-ignition with a stepwise increase in temperature (SIT-LOI) of soil to evaluate the stability of SOM using soil samples having different organic matter (OM) and mineral contents and different mean residence times (MRTs) for SOM. The responses of SOM to the SIT-LOI varied depending on the samples but were all successfully approximated by a liner regression model as a function of the temperature of LOI. The slope value in the liner model that determines the residual potential of carbon during the SIT-LOI highly correlated with MRT of SOM, suggesting that this value reflects the overall stability of SOM over a range of soil properties. This hypothesis was consistent with the observation that Δ14C values of SOM decreased with increasing LOI temperature and thus, older, slower-cycling SOM was preferentially left in the soil samples by SIT-LOI. Additionally, the hypothesis was also supported by the significant correlations (p < 0.01) between the slope value and OM and mineral contents in the samples because these components are considered to regulate SOM stability. In addition to the regression analysis of the SIT-LOI data, changes in carbon to nitrogen (C/N) and carbon to hydrogen (C/H) ratios and stable carbon isotope signatures (δ13C) of the samples were investigated. The results suggest that the mineral association of SOM is an important factor characterizing the response of SOM to LOI. Hence, it was concluded that SIT-LOI is a simple and useful method for evaluating the stability of SOM under actual environmental conditions.