Carbon Sequestration in Two Alpine Soils on the Tibetan Plateau

Soil carbon sequestration was estimated in a conifer forest and an alpine meadow on the Tibetan Plateau using a carbon- 14 radioactive label provided by thermonuclear weapon tests （known as bomb-^14C）. Soil organic matter was physically separated into light and heavy fractions. The concentration spike of bomb-^14C occurred at a soil depth of 4 cm in both the forest soil and the alpine meadow soil. Based on the depth of the bomb-^14C spike, the carbon sequestration rate was determined to be 38.5 g C/m^2 per year for the forest soil and 27.1 g C/m^2 per year for the alpine meadow soil. Considering that more than 60% of soil organic carbon （SOC） is stored in the heavy fraction and the large area of alpine forests and meadows on the Tibetan Plateau, these alpine ecosystems might partially contribute to ＂the missing carbon sink＂.     

Present status and rate of carbon sequestration of forest vegetation in Jilin Province,Northeast China北大核心CSCD

Aims Forests represent the most important component of the terrestrial biological carbon pool and play an important role in the global carbon cycle. The regional scale estimation of carbon budgets of forest ecosystems, however, have high uncertainties because of the different data sources, estimation methods and so on. Our objective was to accurately estimate the carbon storage, density and sequestration rate in forest vegetation in Jilin Province of China, in order to understand the role of the carbon sink and to better manage forest ecosystems. Methods Vegetation survey data were used to determine forest distribution, size of area and vegetation types regionally. In our study, 561 plots were investigated to build volume-biomass models; 288 plots of shrubs and herbs were harvested to calculate the biomass of understory vegetation, and samples of trees, shrubs and herbs were collected to analyze carbon content. Carbon storage, density and sequestration rate were estimated by two forest inventory data (2009 and 2014), combined with volume-biomass models, the average biomass of understory vegetation and carbon content of vegetation. Finally, the distribution patterns of carbon pools were presented using ArcGIS soft ware. Important findings Understory vegetation biomass overall was less than 3% of the tree layer biomass, varying greatly among different forest types and even among the similar types. The carbon content of trees was between 45.80% 52.97%, and that of the coniferous forests was higher than that of the broadleaf forests. The carbon content of shrub and herb layers was about 39.79% 47.25% and 40%, respectively. Therefore, the vegetation carbon conversion coefficient was 0.47 or 0.48 in Jilin Province, and the conventional use of 0.50 or 0.45 would cause deviation of ±5.26%. The vegetation carbon pool of Jilin Province was at the upper range of regional carbon pool and had higher capacity of carbon sequestration. The value in 2009 and 2014 was 471.29 Tg C and 505.76 Tg C, respectively, and the total increase was 34.47 Tg C with average annual growth of 6.89 Tg C•a1. The corresponding carbon sequestration rate was 0.92 t•hm 2•a1. The carbon density rose from 64.58 t•hm 2 in 2009 to 66.68 t•hm2 in 2014, with an average increase of 2.10 t•hm2. In addition, the carbon storage of the Quercus mongolica forests and broadleaved mixed forests, accounted for 90.34% of that of all forests. The carbon increment followed the order of young > over-mature > near mature > middle-aged > mature forests. The carbon sequestration rate of followed the order of over-mature > young > near mature > middle-aged > mature forests. Both the carbon increment and the carbon sequestration rate of mature forests were negative. Furthermore, spatially the carbon storage and density were higher in the east than in the west of Jilin province, while the carbon increment was higher in northeast and middle east than in the west. The carbon sequestration rate was higher in Tonghua and Baishan in the south, followed by Jinlin in the middle and Yanbian in the east, while Baicheng and Songyuan, etc. in west showed negative values.     

Potential impacts of climate warming on active soil organic carbon contents along natural altitudinal forest transect of Changbai Mountain

Understanding soil carbon fractions and their responses to the global warming is important for improving soil carbon management of natural altitudinal forest ecosystem. In this study, the contents of soil total organic carbon (SOC), soil labile organic carbon (LOC), and microbial biomass carbon (MBC) in soil upper layers (0–20 cm) were measured along a natural altitudinal transect in the north slope of Changbai Mountain. The results showed that under natural conditions the contents of SOC and LOC were largest in Betula ermanii forest (altitude 1996 m), moderate in spruce-fir forest (altitude 1350 m), and smallest in Korean pine mixed broad-leaf tree forest (altitude 740 m). MBC contents in different forest ecosystems decreased in the order of Betula ermanii forest, Korean pine mixed broad-leaf tree forest, and dark coniferous forest. In addition, the responses of SOC, LOC, and MBC to soil warming were conducted by relocating intact soil cores from high- to low-elevation forests for one year. As expected, the soil core relocation caused significant increase in soil temperature but made no significant effect on soil moisture. After one year incubation, soil relocation significantly decreased SOC contents, whereas the contents of LOC, MBC, and the ratios of LOC to SOC and MBC to SOC increased.     

Carbon storage and potentials of the broad-leaved forest in alpine region of the Qinghai-Xizang Plateau,China北大核心CSCD

Aims Our objective was to explore the vegetation carbon storages and their variations in the broad-leaved forests in the alpine region of the Qinghai-Xizang Plateau that includes Qinghai Province and Xizang Autonomous Region. Methods Based on forest resource inventory data and field sampling, this paper studied the carbon storage, its sequestration rate, and the potentials in the broad-leaved forests in the alpine region of the Qinghai-Xizang Plateau. Important findings The vegetation carbon storage in the broad-leaved forest accounted for 310.70 Tg in 2011, with the highest value in the broad-leaved mixed forest and the lowest in Populus forest among the six broad-leaved forests that include Quercus, Betula, Populus, other hard broad-leaved species, other soft broad-leaved species, and the broadleaved mixed forest. The carbon density of the broad-leaved forest was 89.04 Mg•hm2, with the highest value in other hard broad-leaved species forest and the lowest in other soft broad-leaved species forest. The carbon storage and carbon density in different layers of the forests followed a sequence of overstory layer > understory layer > litter layer > grass layer > dead wood layer, which all increased with forest age. In addition, the carbon storage of broad-leaved forest increased from 304.26 Tg in 2001 to 310.70 Tg in 2011. The mean annual carbon sequestration and its rate were 0.64 Tg•a1 and 0.19 Mg•hm2•a1, respectively. The maximum and minimum of the carbon sequestration rate were respectively found in other soft broad-leaved species forest and other hard broad-leaved species forest, with the highest value in the mature forest and the lowest in the young forest. Moreover, the carbon sequestration potential in the tree layer of broad-leaved forest reached 19.09 Mg•hm2 in 2011, with the highest value found in Quercus forest and the lowest in Betula forest. The carbon storage increased gradually during three inventory periods, indicating that the broad-leaved forest was well protected to maintain a healthy growth by the forest protection project of Qinghai Province and Xizang Autonomous Region.     

Effects of grassland conversion to cropland and forest on soil organic carbon and dissolved organic carbon in the farming-pastoral ecotone of Inner Mongolia

Effects of grassland conversion to cropland and forest on soil organic carbon (SOC), dissolved organic carbon (DOC) in the farming-pastoral ecotone of Inner Mongolia were investigated by direct field sampling. SOC content and DOC content in soil decreased after grassland were shifted to forest or cropland, in the sequence of grassland soil > forest soil > cropland soil. SOC stock declined by 18% after grassland shifted from to forest. Reclamation of cropland for 10 years, 15 years and 20 years lost SOC in 0–30 cm soil layer, by 34%, 14% and 18%, respectively, compared with that of grassland. DOC in 3 soil layers was within 21.1–26.5 mg/L in grassland, 12.1–14.6 mg/L in forest soil, and 8.0–14.0 mg/L in cropland soil. Correlation analysis indicated that SOC content and DOC content were positively dependent on total nitrogen content (p < 0.05), but negatively on bulk density or land use type (p < 0.05). DOC was positively correlated SOC (p < 0.01). Moreover, SOC content could be quantitatively described by a linear combination of land use types (p = 0.000, r² = 0.712), and DOC content by a linear combination of two soil-related variables, land use types and SOC (p = 0.000, r² = 0.861).     

Effects of grassland conversion to cropland and forest on soil organic carbon and dissolved organic carbon in the farming-pastoral ecotone of Inner Mongolia

Effects of grassland conversion to cropland and forest on soil organic carbon (SOC), dissolved organic carbon (DOC) in the farming-pastoral ecotone of Inner Mongolia were investigated by direct field sampling. SOC content and DOC content in soil decreased after grassland were shifted to forest or cropland, in the sequence of grassland soil > forest soil > cropland soil. SOC stock declined by 18% after grassland shifted from to forest. Reclamation of cropland for 10 years, 15 years and 20 years lost SOC in 0–30 cm soil layer, by 34%, 14% and 18%, respectively, compared with that of grassland. DOC in 3 soil layers was within 21.1–26.5 mg/L in grassland, 12.1–14.6 mg/L in forest soil, and 8.0–14.0 mg/L in cropland soil. Correlation analysis indicated that SOC content and DOC content were positively dependent on total nitrogen content (p < 0.05), but negatively on bulk density or land use type (p < 0.05). DOC was positively correlated SOC (p < 0.01). Moreover, SOC content could be quantitatively described by a linear combination of land use types (p = 0.000, r² = 0.712), and DOC content by a linear combination of two soil-related variables, land use types and SOC (p = 0.000, r² = 0.861).     

Carbon cycle of larch plantation based on CO2FIX model北大核心CSCD

Aims Plantations play important roles in modifying regional carbon budget and maintaining regional carbon balance. In this study, we assessed larch plantation (Larix gmelinii var. principis-rupprechtii) carbon dynamics in Weichang County from a perspective of the forest biomass-soil-wood-products chain. Our objectives were to elucidate the carbon sink capacity of larch plantation and the influences of biomass, soil and wood product pools on carbon balance. Methods CO2FIX model was used to evaluate the carbon storage and flow of larch plantation over a time span of 120 years. Input data for model were derived from practical investigations and published papers. We validated the simulated results and found that this model was suitable in the region and the simulated results were reliable. Important findings (1) Soil was the largest carbon pool for larch plantation and the wood product pool had the smallest carbon storage. Meanwhile, carbon storage in wood products gradually increased with time. (2) In a rotation of 50 years from secondary poplar-birch forest to larch plantation, 250 t C•hm2 was sequestrated by the larch plantation. 70% of the carbon was transferred into soil in the form of litter and logging slash and the other 30% was transferred into wood products. (3) Larch plantation was a carbon sink during most of its growing period and turned to temporary carbon source when it was harvested. Larch plantation could sequestrate about 0.3 t C•hm2•a1 in the long term. Our results indicated the importance of wood product carbon pool in carbon dynamics of plantation, which facilitated our understanding in the carbon dynamics and capacity of plantation.     

Current stocks and potential of carbon sequestration of the forest tree layer in Qinghai Province,China北大核心CSCD

Aims Our objective was to estimate the carbon storage in the forest tree layer in Qinghai Province, China. Methods Based on forest resource inventory data and field investigation data, we estimated the carbon storage, sequestration rate and potentials in the forest tree layer in the Qinghai Province. Important findings The carbon density and total carbon storage of forest tree layer in Qinghai Province was 76.54 Mg·hm-2 and 27.38 Tg, respectively, of which four forest types (Picea spp. forest, Cupressus funebris forest, Betula spp. forest and Populus spp. forest) accounted for 86.67% while their areas were 96.23% of total forest areas in Qinghai. The carbon density and carbon storage of Picea spp. forest was 106.93 Mg·hm-2 and 14.78 Tg, respectively, which was the largest among all forest types. The carbon storage of the forest tree layer at different stand ages followed the sequence of over-mature forest > middle-aged forest > mature forest > near-mature forest > young forest. In addition, the carbon storage of forest tree layer in the province increased from 23.30 Tg in 2003 to 27.38 Tg in 2011. The average annual growth of carbon and carbon sequestration rate were 0.51 Tg and 1.06 Mg·hm-2·a-1, respectively. The maximum and minimum of carbon sequestration rate were respectively found in Cupressus funebris forest (0.44 Mg·hm-2·a-1) and Betula spp. forest (-1.06 Mg·hm-2·a-1). The mean carbon sequestration potential reached 8.50 Tg in 2011, with the highest value found in Picea spp. forest (3.40 Tg). These findings suggested high carbon sequestration potential of forest tree layer in Qinghai Province. Therefore, the carbon storage in Qinghai Province could be increased through better forest management and utilization. © 2018 Editorial Office of Chinese Journal of Plant Ecology. All rights reserved.     

Soil Organic Matter Dynamics Along a Vertical Vegetation Gradient in the Gongga Mountain on the Tibetan Plateau

Our knowledge about soil organic matter (SOM) dynamics is limited although this is an important issue in the study of responses of ecosystems to global climate changes. Twelve sampling plots were set up every 200 m from 1 700 to 3 900 m along the vertical vegetation gradient along the east slope of Gongga Mountain. Samples were taken from all 12 plots for SOM content measurement, although only 5 of the 12 plots were subjected to radiocarbon measurements. A radiocarbon isotope method and a time-dependent model were used to quantify the SOM dynamics and SOM turnover rates along the vertical vegetation gradient. The results showed that the SOM turnover rate decreased and turnover time increased with soil depth for all vegetation types. The litter layer turnover rates presented a clear trend along the gradient. The litter layer turnover rates decreased with an increase in elevation, except that the litter layer turnover rate of mixed forest was higher than that of evergreen forest. Climatic factors, such as temperature and precipitation, were the main factors influencing the surface soil carbon dynamics. The turnover rates of the subsoil (including the A, B, and C horizons in the soil profiles) along the vertical gradient had no clear trends. The SOM of subalpine shrub and meadow turned over more slowly than that of the forest types in almost all soil horizons. The characteristic of short roots distributing in the upper part of the soil profile leads to different SOM dynamics of shrub and meadow compared with the forest types. Coniferous and mixed forests were susceptible to carbon loss from the young carbon pool, but their long and big roots resulted in high △^14C values of the deep soil profiles and increased the input of young carbon to the deep soil. In evergreen forest, the carbon cumulative ability from the B horizon to the C horizon was weak. The different vegetation types, together with their different modes of nutrient and carbon intake, may be the mechanism conditioning the subsoil organic matter dynamics.     

Changes in annual CO2 fluxes estimated from inventory data in South Korea

Using a slightly modified IPCC method, we examined changes in annual fluxes of CO_2and contributions of energy consumption, limestone use, waste combustion, land-use change, and forest growth to the fluxes in South Korea from 1990 to 1997. Our method required less data and resulted in a larger estimate of CO_2 released by industrial processes, comparing with the originalIPCC guideline. However, net CO_2 emission is not substantially different from the estimates of IPCC and modified methods. Net CO_2 emission is intimately related to GDP as Korean economyhas heavily relied on energy consumption and industrial activities, which are major sources of CO_2.Total efflux of CO_2 was estimated to be 63.6 Tg C/a in 1990 and amounted to 112.9 Tg C/a in 1997. Land-use change contributed to annual budget of CO_2 in a relatively small portion. Carbon dioxide was sequestered by forest biomass at the rate of 6.5 Tg C/a in 1990 and 8.5 Tg C/a in 1997. Al-though CO_2 storage in the forests increased, the sink effect was overwhelmed by extensive energy consumption, suggesting that energy-saving strategies will be more effective in reducing CO_2 emission in Korea than any other practices. It is presumed that plant uptake of CO_2 is underesti-mated as carbon contained in plant detritus and belowground living biomass were not fully consid-ered. Furthermore, the soil organic carbon stored in forest decomposes in various ways in rugged mountains depending on their conditions, such as slope, aspect and elevation, which could havean effect on decomposition rate and carbon stores in soils. Thus, carbon sequestration of forests deserves further attention.     

东北地区两种主要造林树种生态系统固碳潜力

自从1980年,我国开展了一系列举世瞩目的造林工程,增加了森林面积3亿hm2。造林后生态系统有机碳库的微小变化都显著影响大气碳库,对全球碳素循环和平衡起着重要的作用。研究了退耕还林不同年限长白落叶松林的植被、凋落物和土壤碳库的变化规律,并且选择可比性较强的退耕还红松林、退耕还草和红松原始林作为参照,分析总结了退耕还林对生态系统储碳能力和碳循环的影响。结果表明,退耕还林后生态系统的植被、凋落物碳储量随退耕还林年限增加而增加:从退耕3a到33a,植被和凋落物碳储量分别从4.134、0Mg/hm2增加到74.11、11.31Mg/hm2。土壤碳储量则是先降低再增加:在还林初期的12a里,土壤碳密度降低到最小量75.87Mg/hm2,随后逐渐恢复和积累,21a后,土壤碳密度恢复到农田的水平84.28Mg/hm2,随后土壤碳密度出现净积累。在长白山地区,退耕3、12、22a和33a长白落叶松、33a红松生态系统的碳储量分别是81.778、114.488、130.004、187.255Mg/hm2和178.580Mg/hm2。长白落叶松的固碳能力随林龄而增加,两种主要造林树种(长白落叶松和红松)的生态系统的固碳潜力没有显著差异。长期来看(如250a),生态系统碳库存的能力非常大(269.57Mg/hm2)。这种状况表明,在长白山地区退耕还林后,生态系统长期来看是一个可观的碳汇。     

Afforestation effects on SOC in former cropland: oak and spruce chronosequences resampled after 13 years

Chronosequences are commonly used to assess soil organic carbon (SOC) sequestration after land‐use change, but SOC dynamics predicted by this space‐for‐time substitution approach have rarely been validated by resampling. We conducted a combined chronosequence/resampling study in a former cropland area (Vestskoven) afforested with oak (Quercus robur) and Norway spruce (Picea abies) over the past 40 years. The aims of this study were (i) to compare present and previous chronosequence trends in forest floor and top mineral soil (0–25 cm) C stocks; (ii) to compare chronosequence estimates with current rates of C stock change based on resampling at the stand level; (iii) to estimate SOC changes in the subsoil (25–50 cm); and (iv) to assess the influence of two tree species on SOC dynamics. The two chronosequence trajectories for forest floor C stocks revealed consistently higher rates of C sequestration in spruce than oak. The chronosequence trajectory was validated by resampling and current rates of forest floor C sequestration decreased with stand age. Chronosequence trends in topsoil SOC in 2011 did not differ significantly from those reported in 1998, however, there was a shift from a negative rate (1998: ?0.3 Mg C ha^?1 yr^?1) to no change in 2011. In contrast SOC stocks in the subsoil increased with stand age, however, not significantly (P = 0.1), suggesting different C dynamics in and below the former plough layer. Current rates of C change estimated by repeated sampling decreased with stand age in forest floors but increased in the topsoil. The contrasting temporal change in forest floor and mineral soil C sequestration rates indicate a shift in C source‐sink strength after approximately 40 years. We conclude that afforestation of former cropland within the temperate region may induce soil C loss during the first decades followed by a recovery phase of yet unknown duration.     

Stand age-related effects on soil respiration in a first rotation Sitka spruce chronosequence in central Ireland

The effect of stand age on soil respiration and its components was studied in a first rotation Sitka spruce chronosequence composed of 10‐, 15‐, 31‐, and 47‐year‐old stands established on wet mineral gley in central Ireland. For each stand age, three forest stands with similar characteristics of soil type and site preparation were used. There were no significant differences in total soil respiration among sites of the same age, except for the case of a 15‐year‐old stand that had lower soil respiration rates due to its higher productivity. Soil respiration initially decreased with stand age, but levelled out in the older stands. The youngest stands had significantly higher respiration rates than more mature sites. Annual soil respiration rates were modelled by means of temperature‐derived functions. The average Q ₁₀ value obtained treating all the stands together was 3.8. Annual soil respiration rates were 991, 686, 556, and 564 g C m^?2 for the 10‐, 15‐, 31‐, and 47‐year‐old stands, respectively. We used the trenching approach to separate soil respiration components. Heterotrophic respiration paralleled soil organic carbon dynamics over the chronosequence, decreasing with stand age to slightly increase in the oldest stand as a result of accumulated aboveground litter and root inputs. Root respiration showed a decreasing trend with stand age, which was explained by a decrease in fine root biomass over the chronosequence, but not by nitrogen concentration of fine roots. The decrease in the relative contribution of autotrophic respiration to total soil CO₂ efflux from 59.3% in the youngest stand to 49.7% in the oldest stand was explained by the higher activity of the root system in younger stands. Our results show that stand age should be considered if simple temperature‐based models to predict annual soil respiration in afforestation sites are to be used.     

黄土丘陵区三种典型退耕还林地土壤固碳效应差异

探讨了黄土丘陵区退耕种植10—40a的柠条、沙棘及刺槐林地土壤总有机碳库及其活性组分密度随退耕时间、土层分布及相对比例的变化差异。结果表明:100 cm深土壤碳库在退耕10a时仅柠条林地碳库未比坡耕地显著增加,但退耕20—40a3种林地比退耕10a时都已有显著增加,且增幅均为刺槐>沙棘>柠条,其中总有机碳的最大增幅分别达到90.92、27.87、14.89Mg/hm2,活性有机碳的分别达到30.28、10.51、9.67 Mg/hm2。各还林地碳库增加在退耕10a时主要来自0—40 cm浅层土,而40—100 cm深层土碳库到退耕20a起才开始显著增加。对比退耕10a时,到退耕40a时柠条、沙棘及刺槐林地0—20 cm表层土分别平均累积了35.4%、27.9%、27.1%的总有机碳,20.2%、45.1%、23.1%的活性有机碳,而20—100 cm各土层间对碳库累积比例大小变化无一致规律,平均每20 cm厚土层累积了17.4%的总有机碳和17.6%活性有机碳。并且相比坡耕地,各林地均使100 cm深土壤活性有机碳占总有机碳的比例提高,改良了碳库质量。综上分析,长期退耕下3种林地固碳效应有明显差异,以刺槐林地碳累积效应较强。     

天山北坡雪岭云杉林地开垦的土壤有机碳损失估算

在全球变暖的背景下,由土地利用变化导致的土壤碳库的变化已经受到越来越多的关注。首先采用物种分布模型预测了天山北坡雪岭云杉林的潜在分布,其次估计了与被开垦为农田的雪岭云杉林面积(PSC)以及由林地开垦为农田造成的有机碳损失。PSC分别由雪岭云杉林的现实分布、潜在分布和农田的现实分布确定。云杉林地和农田的土壤有机碳含量由野外采样和实验室分析获得。研究发现,PSC面积为2.68×10~6hm~2,被开垦为农田的雪岭云杉林土壤有机碳的损失为171.7 t/hm~2;研究区总有机碳的损失为459.70Tg。结果表明,研究区的林地恢复和重建项目将会使土壤有机碳储量有所增加,且土壤表层的增加量多于深层。     

Secondary forest growth deviation from chronosequence predictions in central Amazonia

Nearly all published rates of secondary forest (SF) regrowth for Amazonia are inferred from chronosequences. We examined SF regrowth on abandoned pastures over a 4‐year period to determine if measured rates of forest recovery differ from chronosequence predictions. We studied the emergence, development and death of over 1300 stems in 10 SFs representing three age classes (<1–5, 6–10 and 11–14 years old). Mean tree biomass accumulation in both the <1–5 and 6–10 years old (4.4 and 5.7 Mg ha⁻¹ yr⁻¹, respectively) abandoned pastures was lower than predicted and deviated significantly (57% and 41%) from rates estimated from the chronosequence. The older SFs, with a mean growth rate of 9.9 Mg ha⁻¹ yr⁻¹ followed the rate predicted by the chronosequence. Understocking was the primary cause of low biomass recovery rates in the youngest forests; although the youngest stands had a diameter at breast height increment three times the oldest stands, the youngest stands lacked sufficient density to cumulatively produce high biomass accumulation rates. Four years of measurement indicated that the youngest stands had developed 59% of the stems measured in the older stands during the same time period. The 6–10‐year‐old stands were rapidly self‐thinning and approached stem density values measured in the same aged stands at the onset of the study. Mortality was high for all stands, with 54% of the original stems remaining after 4 years in intermediate‐aged stands. The forests were dominated by the tree Vismia, which represented 55–66% of the biomass in all stands. The Vismia share of the biomass was decreasing over time, with other genera replacing the pioneer. Our measured rates of regrowth indicate that generalized estimates of forest regrowth through chronosequence studies will overestimate forest regrowth for the youngest forests that were under land use for longer time‐periods before abandonment. Certified Emission Reductions under the Clean Development Mechanism of the Kyoto protocol should consider these results when predicting and compensating for carbon sequestered under natural forest management.     

广西马山岩溶次生林群落生物量和碳储量

岩溶植被在岩溶生态系统碳循环和全球碳平衡中具有重要的作用。通过对马山县岩溶次生林年龄序列(幼龄林、中龄林和老龄林)3个演替阶段9个样地(20 m×50 m)的系统取样调查,研究了停止人为干扰后岩溶次生林生物量和碳储量的变化。结果表明:沿幼林、中林和老林群落的顺向演替发展,群落生物量显著增加(P0.05),从幼林群落的48.17 t/hm2、到中林群落113.47 t/hm2,再到老林群落242.59 t/hm2。老林生态系统的碳储量较高,平均为236.69 t/hm2,中林和幼林较低且非常相近,分别为225.17 t/hm2和224.76 t/hm2,各次生林生态系统的碳储量差异不显著(P0.05)。土壤碳储量的大小顺序为幼林(198.44 t/hm2)中林(167.39 t/hm2)老林(113.43 t/hm2)。沿群落正向演替,各次生林生态系统中植物碳储量和土壤碳储量的比例发生明显的变化。幼林的土壤碳储量占生态系统碳储量的88.29%,植物碳储量只占11.71%;中林相应为74.34%和25.66%;而老林为47.92%和52.08%。可见,随着岩溶植被的正向演替,土壤碳转变为植物碳的趋势十分明显,这是岩溶森林不同于酸性土森林的一个显著特征。     

1981-2019年华北平原农田土壤有机碳储量的时空变化及影响机制

农业土壤具有可观的固碳及减碳潜力,有助于减缓人类温室气体排放导致的气候变化。为了更好地了解华北平原土壤有机碳储量动态及其驱动因子,结合荟萃分析、随机森林机器学习模型和卫星遥感数据,研究了1981-2019年间中国华北平原农田土壤有机碳储量的时空变化及其驱动因子。结果表明,1981-2019年间华北平原0-20 cm农田土壤有机碳储量约为（523.10±79.36） Tg C （（14.56±1.66） Mg C/hm²）,并以5.94 Tg C/a （0.12 Mg C hm^-2 a^-1）的年固持速率稳步增长,占比约为中国农田每年新增土壤有机碳的23.3%。其中,常规农田管理措施,包括无机肥施用、有机肥施用和秸秆还田,对土壤有机碳增长的贡献平均为25.1%,即1.49 Tg C/a （0.03 Mg C hm^-2 a^-1）。相比对照组,氮磷钾无机肥施用可提高22.7%-26.0%的土壤有机碳固定速率,有机肥可提高48.3%,秸秆还田可提高23.4%。同时,上述常规农田管理措施对土壤有机碳的积累作用受到土壤本身理化性质的调控,在温度和降水较高的气候条件下更显著。值得注意的是,无论是无机肥施用、有机肥施用还是秸秆还田,当投入量超过农作物和土壤微生物对碳和养分的需求时,土壤有机碳累积速率会显著下降。这也导致2000年后土壤有机碳固持速率明显减缓,由9.4 Tg C/a下降为3.5 Tg C/a。总的来说,过去几十年农田管理措施的改进显著提高了华北平原农田土壤有机碳的增加速率,而未来华北平原农田系统固碳潜力仍然可观,但亟待明确在保证粮食产量的同时不同气候和土壤环境条件下最佳固碳所需的化肥、有机肥和秸秆投入量。     

Changes in soil properties after afforestation of former intensively managed soils with oak and Norway spruce

In many European countries, surplus agricultural production and ecological problems due to intensive soil cultivation have increased the interest in afforestation of arable soils. Many environmental consequences which might rise from this alternative land-use are only known from forest establishment on less intensively managed or marginal soils. The present study deals with changes in soil properties following afforestation of nutrient-rich arable soils. A chronosequence study was carried out comprising seven Norway spruce (Picea abies (Karst.) L.) and seven oak (Quercus robur L.) stands established from 1969 to 1997 on former horticultural and agricultural soils in the vicinity of Copenhagen, Denmark. For comparison, a permanent pasture and a ca. 200-year-old mixed deciduous forest were included. This paper reports on changes in pH values, base saturation (BS_eff), exchangeable calcium, soil N pools (N_min contents), and C/N ratios in the Ap-horizon (0–25 cm) and the accumulated forest floor. The results suggest that afforestation slowly modifies soil properties of former arable soils. Land-use history seems to influence soil properties more than the selected tree species. An effect of tree species was only found in the forest floor parameters. Soil acidification was the most apparent change along the chronosequence in terms of a pH decrease from 6 to 4 in the upper 5 cm soil. Forest floor pH varied only slightly around 5. Nitrogen storage in the Ap-horizon remained almost constant at 5.5 Mg N ha^–1. This was less than in the mineral soil of the ca. 200-year-old forest. In the permanent pasture, N storage was somewhat higher in 0–15 cm depth than in afforested stands of comparable age. Nitrogen storage in the forest floor of the 0–30-year-old stands increased in connection with the build-up of forest floor mass. The increase was approximately five times greater under spruce than oak. Mineral soil C/N ratios ranged from 10 to 15 in all stands and tended to increase in older stands only in 0–5 cm depth. Forest floor C/N ratios were higher in spruce stands (26.4) as compared to oak stands (22.7). All stands except the youngest within a single tree species had comparable C/N ratios.     

The legacy of harvest and fire on ecosystem carbon storage in a north temperate forest

Forest harvesting and wildfire were widespread in the upper Great Lakes region of North America during the early 20th century. We examined how long this legacy of disturbance constrains forest carbon (C) storage rates by quantifying C pools and fluxes after harvest and fire in a mixed deciduous forest chronosequence in northern lower Michigan, USA. Study plots ranged in age from 6 to 68 years and were created following experimental clear‐cut harvesting and fire disturbance. Annual C storage was estimated biometrically from measurements of wood, leaf, fine root, and woody debris mass, mass losses to herbivory, soil C content, and soil respiration. Maximum annual C storage in stands that were disturbed by harvest and fire twice was 26% less than a reference stand receiving the same disturbance only once. The mechanism for this reduction in annual C storage was a long‐lasting decrease in site quality that endured over the 62‐year timeframe examined. However, during regrowth the harvested and burned forest rapidly became a net C sink, storing 0.53 Mg C ha⁻¹ yr⁻¹ after 6 years. Maximum net ecosystem production (1.35 Mg C ha⁻¹ yr⁻¹) and annual C increment (0.95 Mg C ha⁻¹ yr⁻¹) were recorded in the 24‐ and 50‐year‐old stands, respectively. Net primary production averaged 5.19 Mg C ha⁻¹ yr⁻¹ in experimental stands, increasing by < 10% from 6 to 50 years. Soil heterotrophic respiration was more variable across stand ages, ranging from 3.85 Mg C ha⁻¹ yr⁻¹ in the 6‐year‐old stand to 4.56 Mg C ha⁻¹ yr⁻¹ in the 68‐year‐old stand. These results suggest that harvesting and fire disturbances broadly distributed across the region decades ago caused changes in site quality and successional status that continue to limit forest C storage rates.