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Chinese forests cover most of the representative forest types in the Northern Hemisphere and function as a large carbon (C) sink in the global C cycle. The availability of long-term C dynamics observations is key to evaluating and understanding C sequestration of these forests. The Chinese Ecosystem Research Network has conducted normalized and systematic monitoring of the soil-biology-atmosphere-water cycle in Chinese forests since 2000. For the first time, a reference dataset of the decadal C cycle dynamics was produced for 10 typical Chinese forests after strict quality control, including biomass, leaf area index, litterfall, soil organic C, and the corresponding meteorological data. Based on these basic but time-discrete C-cycle elements, an assimilated dataset of key C cycle parameters and time-continuous C sequestration functions was generated via model-data fusion, including C allocation, turnover, and soil, vegetation, and ecosystem C storage. These reference data could be used as a benchmark for model development, evaluation and C cycle research under global climate change for typical forests in the Northern Hemisphere.

Soil organic matter (SOM) mineralization represents one of the largest fluxes in the global carbon cycle. Numerous studies have shown that soil organic carbon decomposition was largely changed owing to the addition of litter, however very few studies have focused on the role of plant organs in the priming effects (PEs). Here, we studied the effects of different Pinus massoniana organs (fresh leaf, leaf litter, twigs, absorptive fine roots, and transport fine roots) on C4 soil respiration by applying the 13C isotopic natural abundance method. Results showed that the effects of plant organs on PEs were significantly different at the end of 210 days incubation, which can be ascribed to contrasting organs traits especially non-structural carbohydrates and water-soluble compounds. Transport fine roots and fresh leaf induced positive PE, whereas absorptive fine roots induced negative PE. Leaf litter did not change the native SOC decomposition. Plant organ addition can change the microbial community and result in the reduction of bacteria-to-fungi ratio. Our results suggest that with regard to determining the PE of the entire ecosystem, using fresh leaf to represent leaf litter and aboveground to represent underground is implausible., Competing Interests: The authors have declared that no competing interests exist.

It is critical to accurately estimate carbon (C) turnover time as it dominates the uncertainty in ecosystem C sinks and their response to future climate change. In the absence of direct observations of ecosystem C losses, C turnover times are commonly estimated under the steady state assumption (SSA), which has been applied across a large range of temporal and spatial scales including many at which the validity of the assumption is likely to be violated. However, the errors associated with improperly applying SSA to estimate C turnover time and its covariance with climate as well as ecosystem C sequestrations have yet to be fully quantified. Here, we developed a novel model-data fusion framework and systematically analyzed the SSA-induced biases using time-series data collected from 10 permanent forest plots in the eastern China monsoon region. The results showed that (a) the SSA significantly underestimated mean turnover times (MTTs) by 29%, thereby leading to a 4.83-fold underestimation of the net ecosystem productivity (NEP) in these forest ecosystems, a major C sink globally; (b) the SSA-induced bias in MTT and NEP correlates negatively with forest age, which provides a significant caveat for applying the SSA to young-aged ecosystems; and (c) the sensitivity of MTT to temperature and precipitation was 22% and 42% lower, respectively, under the SSA. Thus, under the expected climate change, spatiotemporal changes in MTT are likely to be underestimated, thereby resulting in large errors in the variability of predicted global NEP. With the development of observation technology and the accumulation of spatiotemporal data, we suggest estimating MTTs at the disequilibrium state via long-term data assimilation, thereby effectively reducing the uncertainty in ecosystem C sequestration estimations and providing a better understanding of regional or global C cycle dynamics and C-climate feedback., (© 2018 John Wiley & Sons Ltd.)

Background: Extensive studies have been conducted to evaluate the effect of external organic Carbon on native soil organic carbon (SOC) decomposition. However, the direction and extent of this effect reported by different authors is inconsistent. Objective: The objective was to provide a synthesis of existing data that comprehensively and quantitatively evaluates how the soil chemical properties and incubation conditions interact with additional external organic C to affect the native SOC decomposition. Data Source: A meta-analysis was conducted on previously published empirical studies that examined the effect of the addition of external organic carbon on the native SOC decomposition through isotopic techniques. Results and Conclusions: The addition of external organic C, when averaged across all studies, enhanced the native SOC decomposition by 26.5%. The soil with higher SOC content and fine texture showed significantly higher priming effects, whereas the soil with higher total nitrogen content showed an opposite trend. The soils with higher C:N ratios had significantly stronger priming effects than those with low C:N ratios. The decomposition of native SOC was significantly enhanced more at early stage of incubation (<15d) than at the later stages (>15d). In addition, the incubation temperature and the addition rate of organic matter significantly influenced the native SOC decomposition in response to the addition of external organic C. [ABSTRACT FROM AUTHOR]

Highlights • Girdling significantly affected stem CO 2 efflux by altered substrate availability. • Effects of girdling on stem CO 2 efflux differed in degree and timing with season. • Apparent Q 10 of the stem CO 2 efflux increased above the girdle. Abstract The effect of carbohydrate supply on stem CO 2 efflux and its temperature sensitivity is crucial in predicting forest carbon cycling under climate change and forest management but has been rarely reported. In this study, we altered carbohydrate supply via stem girdling and investigated the effect of nonstructural carbohydrate concentration on stem CO 2 efflux and its temperature sensitivity in Chinese fir and sweetgum trees in spring and winter. The stem CO 2 efflux of Chinese fir increased significantly above the girdle in spring and winter, but decreased below the girdle in winter. The stem CO 2 efflux of sweetgum increased above the girdle and decreased below the girdle in spring but not in winter. Seasonal variations in the response of stem CO 2 efflux to girdling were associated with the changes in stem soluble sugar and starch concentration, especially in sweetgum tree. Girdling altered Q 10 of the stem CO 2 efflux, and this effect was dependent on location, season and species. Compared with the Q 10 of the stem CO 2 efflux in non-girdled trees, that of CO 2 efflux in Chinese fir increased by 45% above the girdle but decreased by 16% below the girdle in winter. These values also exhibited 6% and 11% responses in spring. By contrast, the Q 10 of the stem CO 2 efflux in sweetgum increased by 95% above the girdle and decreased by 45% below the girdle in spring. However, these values were nearly constant in winter. We concluded that the effect of carbohydrate supply on stem CO 2 efflux and its temperature sensitivity depends on season and species. Therefore, the vital role of carbohydrate supply that regulates stem CO 2 efflux and its temperature sensitivity should be considered for the understanding and modeling of forest carbon cycling response to forest management, especially under global climate changes. [ABSTRACT FROM AUTHOR]

• Initial litter decomposition rates increased with litter quality. • Stable residue remaining after decay correlated negatively with litter quality. • Soil mesofauna had no effect on initial rates of litter decomposition. • Soil mesofauna decreased stable residue remaining regardless of litter quality. The regulation of factors like litter quality and soil mesofauna on litter decomposition in the early stage has been studied extensively in forest ecosystems. However, how litter decomposition was influenced by its quality and soil mesofauna in the late stage is largely unclear, especially in subtropical forest ecosystem. In the present study, litters from 16 species with contrasting chemical traits were incubated in a subtropical broadleaved forest for 2 years, and then the initial decomposition rate in early stage and stable residue remaining in late stage were calculated by using an asymptotic decomposition model. Litter bags with different mesh sizes (0.1 or 2 mm) were used to evaluate the role of soil mesofauna in regulating litter decomposition. Our results showed that litters with higher quality decomposed faster in early stage, and also had a larger fraction of stable residue remaining in the late stage. In addition, soil mesofauna exclusion had no influence on the decomposition rate at early stage, but significantly increased the stable residue remaining from 8.71% to 17.96%. We highlighted that the quality of plant litters play an important role in regulating the decomposition process in both early and late stage of decomposition, and thus the nutrient cycling and soil carbon (C) sink, in subtropical forest ecosystem. [ABSTRACT FROM AUTHOR]