Evaluating indices of soil organic carbon stability. A case study for forest restoration projects near Beijing, China

Acknowledgments This study was funded by Forestry scientific and technological achievements Popularization Project of State Forestry Administration ([2019] 3) and China State Construction Technology R&D Program Funding (CSECE-2020-Z-5). The authors gratefully acknowledge the Jiufeng National Forest Park (Haidian District, Beijing) and Daxing Forestry Centre (Daxing district, Beijing) for giving access to conduct fieldwork on their experimental sites.Afforestation of degraded lands close to mega-urban areas such as Beijing may help to restore some of the original soil carbon stocks and hold the potential for ameliorating the rate of increase in atmospheric CO2. However, the determinants of the stability of different soil carbon pools and the utility of indices of stability remain poorly characterized near these highly anthropogenic areas. In the current study, we compared metrics of soil organic carbon (SOC) stability taking into account different soil types and plantation forest combinations (Quartisamment soil-poplar plantation–QP, Eutrochrepts soil-Chinese pine plantation–ECP, Haplustepts soils-East-Liaoning oak plantation–HEO), in an experimental sub-humid area close to a mega-urban area (Beijing, China). We evaluated the following relative stability indices sequence: respired carbon from incubations (RI) for several incubation days to respire 5% of initial SOC (D), aggregate stability index (ASI), the ratio of SOC to total nitrogen (C: N), water-soluble carbon (WSC), particulate organic carbon (POC) and microbial biomass carbon (MBC). We examined the indices by three repeated measurements on soil samples from four soil layers (0–40 cm) in three soil-forest types in a forest area close to the peri-urban area of Beijing. Our results showed that there are inconsistencies among the six SOC stability indexes. The contribution rates of different indexes to the SOC in three plantations were different, for QP the highest contributor is WSC (54.73%), and for ECP and HEO the highest contributor is RI, contribution rates are 34.85% and 36.382%, respectively. Respired carbon from incubations registered the largest contribution rate to SOC (69.79%), and the correlation between RI and soil physical and chemical properties was the highest. We conclude that a combination of indices and knowledge of soil and vegetation types are needed for assessing SOC stability in restoration and reforestation projects close to mega-urban areas.Forestry scientific and technological achievements Popularization Project of State Forestry Administration ([2019] 3)China State Construction Technology R&D Program Funding (CSECE-2020-Z-5

Loading Capacity of Sewage Sludge for Forestry Application in Chinese Provincial Capital Cities

The application of sewage sludge (SS) in forestry is considered a viable option. However, the long-term application of SS potentially leads to metal accumulation, posing an environmental risk. Understanding the loading capacity of SS for forestry application is therefore of great significance. We used data from published studies and statistical bulletins across 31 provincial capital cities (PCCs) in China to calculate the loading capacity (LC) of SS for forestry application for each PCC. The results are as follows: (1) the mean value of the priority control threshold was 33 t·ha−1·y−1 in 31 PCCs, while the variations ranged from 7 to 91 t·ha−1·y−1 among different PCCs. The priority control thresholds (Smins) of 1/2 PCCs were higher than 30 t·ha−1·y−1 (CJ-T 362-2011). The Smin values of Lanzhou, Tianjin, Hohhot, Shanghai, and Yinchuan were above 55 t·ha−1·y−1, but Smin values of Kunming and Changsha were below 10 t·ha−1·y−1. (2) Cd was the priority control metal in most of the PCCs (27/31), with the exception of Shanghai and Guangzhou (Cu), Beijing (Hg), and Tianjin (Zn). (3) The total loading capacity was 507 million t·y−1, which was 125 times higher than the total quantity of the dry SS (404 × 104 t) for the 31 PCCs. Our results have important practical significance for the use of urban sludge forest land in China and suggest that SS disposal policies need to be tailored to specific regions. We provide a scientific basis to guide the development of national and provincial forestry policies

Response of transpiration to rain pulses for two tree species in a semiarid plantation

Responses of transpiration (E (c)) to rain pulses are presented for two semiarid tree species in a stand of Pinus tabulaeformis and Robinia pseudoacacia. Our objectives are to investigate (1) the environmental control over the stand transpiration after rainfall by analyzing the effect of vapor pressure deficit (VPD), soil water condition, and rainfall on the post-rainfall E (c) development and recovery rate, and (2) the species responses to rain pulses and implications on vegetation coverage under a changing rainfall regime. Results showed that the sensitivity of canopy conductance (G (c)) to VPD varied under different incident radiation and soil water conditions, and the two species exhibited the same hydraulic control (-dG (c)/dlnVPD to G (cref) ratio) over transpiration. Strengthened physiological control and low sapwood area of the stand contributed to low E (c). VPD after rainfall significantly influenced the magnitude and time series of post-rainfall stand E (c). The fluctuation of post-rainfall VPD in comparison with the pre-rainfall influenced the E (c) recovery. Further, the stand E (c) was significantly related to monthly rainfall, but the recovery was independent of the rainfall event size. E (c) enhanced with cumulative soil moisture change (∆VWC) within each dry-wet cycle, yet still was limited in large rainfall months. The two species had different response patterns of post-rainfall E (c) recovery. E (c) recovery of P. tabulaeformis was influenced by the pre- and post-rainfall VPD differences and the duration of rainless interval. R. pseudoacacia showed a larger immediate post-rainfall E (c) increase than P. tabulaeformis did. We, therefore, concluded that concentrated rainfall events do not trigger significant increase of transpiration unless large events penetrate the deep soil and the species differences of E (c) in response to pulses of rain may shape the composition of semiarid woodlands under future rainfall regimes.13 page(s

Sustainable effects of small hydropower substituting firewood program in Majiang County, Guizhou Province, China

Small hydropower substituting fuel (SHSF) is an ecological environment protection program to improve regional ecosystems and alleviate poverty. However, the sustainability of SHSF programs remains controversial due to lingering doubts about its potential for socioeconomic development and its environmental impacts. The sustainability of SHSF was examined based on field investigations and household questionnaire surveys. The results were as follows: (1) Biomass of SHSF protected mass on pine (Pinus massoniana) and weeping cypress (Platycladus orientalis) plantations were 11.06 t·ha−1 and 7.15 t·ha−1 higher than unprotected plantations, respectively. Furthermore, the differences in ecosystem biomass were mainly derived from arbor biomass. While the energy conversion efficiency based on field investigations was merely 1.28 kg (kWh)−1, which was only 64% of the empirical value and 54% of the guideline for accounting for the ecological benefit of small hydropower substituting fuel. (2) Households’ total income in SHSF villages was higher than in households with access to a hydropower plant but no substituting fuel or households with no hydropower plant. (3) Most of the households had a positive attitude towards SHSF because of its cheaper electricity and associated ecological environmental improvements. Overall, our results suggest optimistic and sustainable prospects for the SHSF program; however, continued education and policy communications are needed to sustain program success

Comparison of satellite-based evapotranspiration models over terrestrial ecosystems in China

Evapotranspiration (ET) is a key component of terrestrial ecosystems because it links the hydrological, energy, and carbon cycles. Several satellite-based ET models have been developed for extrapolating local observations to regional and global scales, but recent studies have shown large model uncertainties in ET simulations. In this study, we compared eight ET models, including five empirical and three process-based models, with the objective of providing a reference for choosing and improving methods. The results showed that the eight models explained between 61 and 80% of the variability in ET at 23 eddy covariance towers in China and adjacent regions. The mean annual ET for all of China varied from 535 to 852 mm yr− 1 among the models. The interannual variability of yearly ET varied significantly between models during 1982–2009 because of different model structures and the dominant environmental factors employed. Our evaluation results showed that the parameters of the empirical methods may have different combination because the environmental factors of ET are not independent. Although the three process-based models showed high model performance across the validation sites, there were substantial differences among them in the temporal and spatial patterns of ET, the dominant environment factors and the energy partitioning schemes. The disagreement among current ET models highlights the need for further improvements and validation, which can be achieved by investigating model structures and examining the ET component estimates and the critical model parameters