Significance Northern forests have sequestered a substantial amount of carbon dioxide from the atmosphere during the past several decades. This large carbon (C) sink usually is considered to be driven by tree regrowth after stand-replacing disturbance and growth enhancement due to environmental changes, but the relative contribution between these two processes remains unclear. In this study, we evaluate the contribution of growth enhancement induced by environmental changes to biomass C sink in Japan’s forests and demonstrate that this growth enhancement accounts for 8.4–21.6% of biomass C sink in four major plantations from 1980 to 2005. This finding highlights that global environmental changes can stimulate tree growth and thus enhance forest C sequestration over a broad geographical scale.

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme-kinetic hypothesis suggests that decomposition of low-quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high-quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme-substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low-density fraction (LF) which represents readily accessible, mineral-free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30-days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6-1.8 g cm(-3) ) and bulk soil was measured by solid-state (13) C-NMR. Decomposition Q10 was significantly correlated with the abundance of aromatic plus alkyl-C relative to O-alkyl-C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two- to three-fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C-use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme-kinetic interpretation of widely observed C quality-temperature relationship for short-term decomposition. Factors controlling long-term decomposition Q10 are more complex due to protective effect of mineral matrix and thus remain as a central question.

Stabilizing the global climate within safe bounds will require greenhouse gas (GHG) emissions to reach net zero within a few decades. Achieving this is expected to require removal of CO 2 from the atmosphere to offset some hard-to-eliminate emissions. There is, therefore, a clear need for GHG accounting protocols that quantify the mitigation impact of CO 2 removal practices, such as biochar sequestration, that have the potential to be deployed at scale. Here, we have developed a GHG accounting methodology for biochar application to mineral soils using simple parameterizations and readily accessible activity data that can be applied at a range of scales including farm, supply chain, national, or global. The method is grounded in a comprehensive analysis of current empirical data, making it a robust method that can be used for many applications including national inventories and voluntary and compliance carbon markets, among others. We show that the carbon content of biochar varies with feedstock and production conditions from as low as 7% (gasification of biosolids) to 79% (pyrolysis of wood at above 600 °C). Of this initial carbon, 63−82% will remain unmineralized in soil after 100 years at the global mean annual cropland-temperature of 14.9 °C. With this method, researchers and managers can address the long-term sequestration of C through biochar that is blended with soils through assessments such as GHG inventories and life cycle analyses.

An average O2:CO2 exchange ratio for net turbulent O2 and CO2 fluxes in a cool temperate deciduous forest in central Japan was obtained based on an aerodynamic method using continuous measurements of atmospheric O2/N2 ratio and CO2 concentration. The average daily mean O2:CO2 exchange ratio was 0.86 during summer, 2013, a value significantly lower than the 1.1 used as a globally averaged terrestrial biospheric O2:CO2 exchange ratio in a CO2 budget analysis. Using the value of 0.86, along with the O2:CO2 exchange ratio of 1.11 for the ecosystem respiration (RE) and 1.00 for the gross primary production (GPP), the net ecosystem production (NEP) measured with an eddy covariance method was separated into GPP and RE using a one‐box canopy O2/CO2 budget model. The estimated average daily‐mean GPP and RE values were consistent, within estimation errors, with those estimated from an empirical function of air temperature; the RE values were also comparable to the soil CO2 efflux observed using an open‐flow soil chamber method. These results suggest that the simultaneous observation of O2 and CO2 concentrations in a forest has potential as a new tool to evaluate the forest CO2 budget.

Plastic mulch films are widely used in agriculture, but most are not biodegradable in soil. Biodegradable mulch films are blends of different polymers whose composition-ratios vary notably from one product to another. Their degradation rates vary greatly according to the physio-chemical characteristics of the product and according to the properties of the soil and its microbial activity. The objective of this review is to provide an overview of the methods used to estimate the biodegradation performances of biodegradable plastics in the soil. In line with this objective, 80 papers were selected and systematically analyzed to extract information on the characteristics of the soil used in the experiments, the type of polymer analyzed, and the methods used to estimate biodegradation in soil environment. Our systematic analysis showed that studies were carried out under both laboratory-controlled and open-field conditions, with different approaches involving visual analysis, mass loss measurements, spectroscopy, and CO2 measurements. A linear estimation of biodegradation performance for four of the most common biodegradable polymers (i.e., polybutylene succinate, polybutylene succinate-co-adipate, polylactic acid, and polybutylene adipate-co-terephthalate), either pure or blended, showed a very wide range of results that appear only partially comparable. Many of the analyzed papers did not report soil characteristics at all, despite soil being one of the most important factors in the biodegradation process. Although methodologies for estimating biodegradation are well developed, at least under laboratory-controlled conditions, there is a need for a shared methodology to make results comparable among different experiments. Within such a shared methodology, visual analysis or mass loss measurements, despite not being able to scientifically prove the biodegradation of polymers, should not be discarded a priori as they might be useful indicators especially for open field experiments. When using indirect biodegradation indicators such as visual analysis or mass loss, it is necessary to couple them with CO2 measurements or to use materials whose biodegradability in the soil environment has already been tested.