Carbon and Nitrogen Turnover Times of South Korean Forests Estimated via Data‐Model Fusion

Abstract

Carbon (C) and nitrogen (N) are fundamental elements that support forest life. The assessment of C and N stocks has been a core focus since the inception of biogeochemistry in terms of the implications of C sequestration and N losses (Högberg et al., 2017;Howarth et al., 2006;Pan et al., 2011). At the ecosystem level, C and N stocks are the balance between (a) input fluxes to forests including photosynthesis, N deposition, and biological N fixation, (b) internal fluxes in forests such as litterfall, N mineralization, and N retranslocation, and (c) output fluxes from forests through respiration, N leaching, and denitrification (Agren & Andersson, 2012;Gundersen, 1991). This balance can be interpreted by turnover time, which represents the average time elapsed between the input of an element to forests and its output from forests in an autonomous Abstract Biogeochemical models use estimates of carbon (C) and nitrogen (N) turnover times for the future projection of global forest C and N stocks, but unexplained variation in the C and N turnover times is causing considerable uncertainty. This study aimed to estimate C and N turnover times of South Korean forests and explain their variation with forest type, temperature, precipitation, stand age, and ecosystem C:N ratio. We used the balance method and data-model fusion to estimate the C and N turnover times. Data-model fusion was used to integrate the National Forest Inventory data (2011-2020) with a biogeochemical model, Forest Biomass and Dead organic matter Carbon and Nitrogen (FBD-CAN). The N turnover time (376-499 years) was ∼45 times longer than the C turnover time (9-10 years). Forest type had no substantial effects on the C and N turnover times. However, the C and N turnover times were positively correlated with stand age and ecosystem C:N ratio, and negatively correlated with temperature. Overall, ecosystem C:N ratio, stand age, temperature, and precipitation explained 45%, 15%, 12%, and 3%, respectively, of the total variation of the C and N turnover times. These results contribute to the understanding and prediction of forest C and N changes in a changing world and highlight the importance of considering C:N ratio for reliable estimation of the C and N turnover times.Plain Language Summary Forests are major sinks of carbon and nitrogen that can cause global warming when present in gaseous forms. Turnover times, referring to the time an element remains in an ecosystem, are measures of the long-term stability of carbon and nitrogen. Models use estimates of turnover times for future projections of carbon and nitrogen cycles, but limited knowledge in turnover times and model uncertainty are causing trouble. Data-model fusion is a technic that combines data and model's output to decrease model uncertainty. We used data-model fusion to estimate carbon and nitrogen turnover times with a model and field-measured carbon and nitrogen data in the South Korean forests. We then explained how forest type, temperature, precipitation, forest age, and carbon:nitrogen ratio a...

Future Projection of CO2 Absorption and N2O Emissions of the South Korean Forests under Climate Change Scenarios: Toward Net-Zero CO2 Emissions by 2050 and Beyond

“…The FBD-CAN estimates carbon and nitrogen balances of tree biomass, litter, dead wood, and mineral soil pools in forests by simulating fluxes of carbon and nitrogen: (a) entering the forest through photosynthetic CO 2 uptake, nitrogen deposition, and biological nitrogen fixation; (b) circulating within the forest, such as litterfall, nitrogen mineralization, and nitrogen retranslocation; and (c) exiting the forest through respiratory CO 2 emissions (i.e., both autotrophic and heterotrophic respiration), nitrogen leaching, and denitrification (i.e., N 2 O emissions) [47][48][49].…”

“…Although air temperature affects photosynthetic CO 2 uptake [50], the photosynthetic CO 2 uptake does not change with the changes in air temperature in the FBD-CAN. This is because the impact of air temperature on photosynthesis was incorporated into the impact of air temperature on foliage respiration in autotrophic respiration to simplify non-linear responses of net photosynthesis to air temperature in the FBD-CAN [49].…”

“…The FBD-CAN requires neither soil temperature as input data nor a soil energy budget model, which calculates soil temperature from air temperature to calculate heterotrophic respiration from litter, dead wood, and mineral soil pools. This is because the parameters for the heterotrophic respiration function in the FBD-CAN were estimated from air temperature directly rather than from the recalculated soil temperature; in other words, the heterotrophic respiration function is adjusted to be used with air temperature [49].…”

“…The reliability and applicability of FBD-CAN to South Korean forests were validated by a pilot study in a Pinus densiflora Siebold & Zucc. forest in central Korea [47], a datamodel fusion study estimating the carbon and nitrogen turnover times in South Korean forests [49], and a study that quantified the impacts of human activities on the carbon and nitrogen dynamics of South Korean forests from 1973 to 2020 [48].…”