Soil organic carbon stock and chemical composition in four plantations of indigenous tree species in subtropical China

Subtropical China has more than 60% of the total plantation area in China and over 70% of these subtropical plantations are composed of pure coniferous species. In view of low ecosystem services and ecological instability of pure coniferous plantations, indigenous broadleaf plantations are being advocated as a prospective silvicultural management for substituting in place of large coniferous plantations in subtropical China. However, little information is known about the effects of tree species conversion on stock and stability of soil organic carbon (SOC). The four adjacent monospecific plantations were selected to examine the effects of tree species on the stock and chemical composition of SOC using elemental analysis and solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy. One coniferous plantation was composed of Pinus massoniana (PM), and the three broadleaf plantations were Castanopsis hystrix (CH), Michelia macclurei (MM), and Mytilaria laosensis (ML). We found that SOC stock differed significantly among the four plantations in the upper (0–10 cm) layer, but not in the underneath (10–30 cm) layer. SOC stocks in the upper (0–10 cm) layer were 11, 19, and 18% higher in the CH, MM, and ML plantations than in the PM plantation. The differences in SOC stock among the four plantations were largely attributed to fine root rather than aboveground litterfall input. However, the soils in the broadleaf plantations contained more decomposable C proportion, indicated by lower percentage of alkyl C, higher percentage of O-alkyl C and lower alkyl C/O-alkyl C ratio compared to those in the PM plantation. Our findings highlight that future strategy of tree species selection for substituting in place of large coniferous plantations in subtropical China needs to consider the potential effects of tree species on the chemical composition in addition to the quantity of SOC stock.     

Accumulation of soil organic carbon after cropland conversion to short‐rotation willow and poplar

The demand for bioenergy has increased the interest in short‐rotation woody crops (SRWCs) in temperate zones. With increased litter input and ceased annual soil cultivation, SRWC plantations may become soil carbon sinks for climate change mitigation. A chronosequence of 26 paired plots was used to study the potential for increasing soil organic carbon (SOC) under SRWC willow and poplar after conversion from cropland (CR) on well‐drained soils. We estimated SOC stocks in SRWC stands and adjacent CR and related the difference to time since conversion, energy crop species, SOC stock of the adjacent CR (proxy for initial SOC of SRWC) and the fine soil percentage (<63 μm) (FS). Soil cores to 40 cm depth were sampled and separated by layers of fixed depths (0–5, 5–10, 10–15, 15–25 and 25–40 cm). Additionally, soils were sampled from soil pits by genetic horizons to 100 cm depth. Comparisons of SOC stocks by equivalent soil masses showed that mean SOC stocks in SRWC were 1.7 times higher than those of CR in the top 5 cm of the soil (P < 0.001). The differences between SRWC and CR remained significant for the plough layer (0–25 cm) by a factor of 1.2 (P = 0.003), while no changes were detectable for the 0–40 cm (P = 0.32), or for the entire 0–100 cm soil layer (P = 0.29). The SOC stock ratio, that is the ratio of SOC stock in SRWC relative to CR, did not change significantly with time since conversion, although there was a tendency to an increase over time for the top 40 cm (P = 0.09). The SOC stock ratio was negatively correlated to SOC in CR and FS percentage, but there was no significant difference between willow and poplar at any depth. Our results suggest that SOC stocks in the plough layer increase after conversion to SRWC.     

城市沿江土地覆被变化对土壤有机碳和轻组有机碳的影响

采用Vario EL III型元素分析仪,2007年7月分析了福州城市沿江3种土地覆被类型（芦苇湿地以及草坪和片林）土壤有机碳（SOC）与轻组有机碳（LFOC）的垂直分布特征.结果表明：3种土地覆被类型SOC和LFOC含量均表现为在土壤表层富集并向下层递减的趋势,且人工绿地在土壤剖面（0～60 cm）不同层次的SOC和LFOC含量均显著高于沿江芦苇湿地,其中,草坪0～20 cm土层的SOC含量显著高于片林;沿江芦苇湿地转为草坪和片林后,其SOC储量分别增加了94.8%和72.0%,LFOC储量分别增加了225%和93%;城市沿江湿地转变为城市人工绿地后,因植物物种、密度以及周期性人工管理等变化,使土壤质量得以改善、SOC和LFOC储量增加,其中LFOC对土地覆被变化的响应较SOC敏感,以表层(0～20 cm)土壤LFOC受到土地利用/覆地变化的影响最大.     

Controls on Soil Organic Carbon Stocks and Turnover Among North American Ecosystems

Despite efforts to understand the factors that determine soil organic carbon (SOC) stocks in terrestrial ecosystems, there remains little information on how SOC turnover time varies among ecosystems, and how SOC turnover time and C input, via plant production, differentially contribute to regional patterns of SOC stocks. In this study, we determined SOC stocks (gC m⁻²) and used soil radiocarbon measurements to derive mean SOC turnover time (years) for 0–10 cm mineral soil at ten sites across North America that included arctic tundra, northern boreal, northern and southern hardwood, subtropical, and tropical forests, tallgrass and shortgrass prairie, mountain grassland, and desert. SOC turnover time ranged 36-fold among ecosystems, and was much longer for cold tundra and northern boreal forest and dry desert (1277–2151 years) compared to other warmer and wetter habitats (59–353 years). Two measures of C input, net aboveground production (NAP), determined from the literature, and a radiocarbon-derived measure of C flowing to the 0–10 cm mineral pool, I, were positively and SOC turnover time was negatively associated with mean annual evapotranspiration (ET) among ecosystems. The best fit model generated from the independent variables NAP, I, annual mean temperature and precipitation, ET, and clay content revealed that SOC stock was best explained by the single variable I. Overall, these findings indicate the primary role that C input and the secondary role that C stabilization play in determining SOC stocks at large regional spatial scales and highlight the large vulnerability of the global SOC pool to climate change.     

Carbon Sequestration in Bamboo Plantation Soil with Heavy Winter Organic Mulching Management

Carbon sequestration in soils is considered to be an important option for the mitigation of increasing atmospheric CO₂ concentrations as a result of climate change. High carbon accumulation was observed in Lei bamboo (Phyllostachys praecox) soils when using large amounts of organic material in a mulching technique. Soil samples were collected from Lei bamboo fields in a chronosequence. The composition and stability of soil organic carbon (SOC) in the bamboo soils was investigated by a combination of ¹³C CPMAS NMR analysis and with a decomposition incubation experiment in the laboratory. SOC content decreased in the first 5 years after planting of Lei bamboo from the original paddy soil and increased strongly subsequently. The stability of SOC after application of the winter mulch was higher as compared to the original paddy soil with no mulching, indicating that SOC can be stored effectively within Lei bamboo fields under intensive management.     

林地覆盖经营对雷竹生物量及土壤肥力的影响

以不同覆盖栽培年限(0、3、6、9和12年)雷竹林为研究对象,分析雷竹林退化过程中林分生长与土壤养分的变化趋势.结果表明:随着覆盖年限的增加,竹林地上及地下生物量均在覆盖3年时达到最大值,与对照相比增幅分别为14.6%和146.6%,差异显著;土壤养分含量受覆盖年限和土层深度的影响而出现差异性,并逐渐表现出上层富集的现象,土壤有机碳和全氮含量随覆盖年限的增加呈上升趋势,全磷含量在不同土层中均呈先降低后升高的变化趋势,在表层(0~20 cm)和底层(40~60 cm)土壤中覆盖6年时达到最低,亚表层(20~40 cm)土壤中覆盖3年时达到最低,全钾含量在表层土壤中持续增大,在亚表层和底层土壤中则表现为在覆盖0~3年下降、3~12年上升的变化趋势,试验雷竹林间差异显著.覆盖9年后,土壤肥力综合指数总体得到较大幅度的提高,亚表层土壤肥力优于表层和底层土,但不同覆盖年限间土壤肥力综合指数与雷竹各器官生物量均没有显著的相关性,而在亚表层中,土壤氮含量与竹叶生物量,以及钾含量与竹叶和鞭根生物量呈显著负相关.这表明长期覆盖及大量施用化肥导致的土壤养分过量积累已经对雷竹林的扩繁和生物量积累产生严重的抑制作用,加剧了竹林衰退趋势.     

Transport of Carbon and Nitrogen Between Litter and Soil Organic Matter in a Northern Hardwood Forest

We used sugar maple litter double-labeled with ¹³C and ¹⁵N to quantify fluxes of carbon (C) and nitrogen (N) between litter and soil in a northern hardwood forest and the retention of litter C and N in soil. Two cohorts of litter were compared, one in which the label was preferentially incorporated into non-structural tissue and the other structural tissue. Loss of ¹³C from this litter generally followed dry mass and total C loss whereas loss of ¹⁵N (20–30% in 1 year) was accompanied by large increases of total N content of this decaying litter (26–32%). Enrichment of ¹³C and ¹⁵N was detected in soil down to 10–15 cm depth. After 6 months of decay (November–May) 36–43% of the ¹³C released from the litter was recovered in the soil, with no differences between the structural and non-structural labeled litter. By October the percentage recovery of litter ¹³C in soil was much lower (16%). The C released from litter and remaining in soil organic matter (SOM) after 1 year represented over 30 g C m⁻² y⁻¹ of SOM accumulation. Recovery of litter ¹⁵N in soil was much higher than for C (over 90%) and in May ¹⁵N was mostly in organic horizons whereas by October it was mostly in 0–10 cm mineral soil. A small proportion of this N was recovered as inorganic N (2–6%). Recovery of ¹⁵N in microbial biomass was higher in May (13–15%) than in October (about 5%). The C:N ratio of the SOM and microbial biomass derived from the labeled litter was much higher for the structural than the non-structural litter and for the forest floor than mineral SOM, illustrating the interactive role of substrates and microbial activity in regulating the C:N stoichiometry of forest SOM formation. These results for a forest ecosystem long exposed to chronically high atmospheric N deposition (ca. 10 kg N ha⁻¹ y⁻¹) suggest possible mechanisms of N retention in soil: increased organic N leaching from fresh litter and reduced fungal transport of N from soil to decaying litter may promote N stabilization in mineral SOM even at a relatively low C:N ratio.     

Soil carbon dynamics following land‐use change varied with temperature and precipitation gradients: evidence from stable isotopes

Knowledge of soil organic matter (SOM) dynamics following deforestation or reforestation is essential for evaluating carbon (C) budgets and cycle at regional or global scales. Worldwide land‐use changes involving conversion of vegetation with different photosynthetic pathways (e.g. C₃ and C₄) offer a unique opportunity to quantify SOM decomposition rate and its response to climatic conditions using stable isotope techniques. We synthesized the results from 131 sites (including 87 deforestation observations and 44 reforestation observations) which were compiled from 36 published papers in the literatures as well as our observations in China's Qinling Mountains. Based on the ¹³C natural abundance analysis, we evaluated the dynamics of new and old C in top soil (0–20 cm) following land‐use change and analyzed the relationships between soil organic C (SOC) decomposition rates and climatic factors. We found that SOC decomposition rates increased significantly with mean annual temperature and precipitation in the reforestation sites, and they were not related to any climatic factor in deforestation sites. The mean annual temperature explained 56% of variation in SOC decomposition rates by exponential model (y = 0.0014e^0.1395x) in the reforestation sites. The proportion of new soil C increased following deforestation and reforestation, whereas the old soil C showed an opposite trend. The proportion of new soil C exceeded the proportion of old soil C after 45.4 years' reforestation and 43.4 years' deforestation, respectively. The rates of new soil C accumulation increased significantly with mean annual precipitation and temperature in the reforestation sites, yet only significantly increased with mean annual precipitation in the deforestation sites. Overall, our study provides evidence that SOC decomposition rates vary with temperature and precipitation, and thereby implies that global warming may accelerate SOM decomposition.     

Impact of moisture dynamic and sun light on anthracene removal from soil

In a previous study, remediation of anthracene from soil was faster in the top 0–2 cm layer than in the lower soil layers. It was not clear whether this faster decrease was due to biotic or abiotic processes. Anthracene-contaminated soil columns were covered with black or transparent perforated polyethylene so that aeration occurred but that fluctuations in water content were minimal and light could reach (LIGHT treatment) or not reach the soil surface (DARK treatment), or left uncovered so that soil water content fluctuate and light reached the soil surface (OPEN treatment). The amount of anthracene, microbial biomass C, and microbial activity as reflected by the amount of CO₂ produced within 3 days were determined in the 0–2 cm, 2–8 cm, and 8–15 cm layer after 0, 3, 7, 14, and 28 days. In the 0–2 cm layer of the OPEN treatment, 17% anthracene remained, 48% in the LIGHT treatment and 61% in the DARK treatment after 28 days. In the 2–8 cm and 8–15 cm layer, treatment had no significant effect on the dissipation of anthracene from soil after 14 and 28 days. It was found that light and fluctuations in water content stimulated the removal of anthracene from the top 0–2 cm soil layer, but not from the lower soil layers. It can be speculated that covering contaminated soil or pilling it up will inhibit the dissipation of the contaminant.     

Evolution of soil carbon stocks under Miscanthus × giganteus and Miscanthus sinensis across contrasting environmental conditions

Miscanthus is a C4 bioenergy perennial crop characterized by its high potential yield. Our study aimed to compare the carbon storage capacities of Miscanthus sinensis (M. sinensis) with that of Miscanthus × giganteus (M. × giganteus) in field conditions in different types of soils in France. We set up a multi‐environment experimental network. On each trial, we tested two treatments: M. × giganteus established from rhizomes (Gr) and M. sinensis transplanted seedlings (Sp). We quantified the soil organic carbon (SOC) stock at equivalent soil mass for both genotypes in 2014 and 2019 and for two sampling depths: L1 (ca. 0–5 cm) and L1‐2 (ca. 0–30 cm). We also calculated the total and annual variation of the SOC stock and investigated factors that could explain the variation and the initial state of the SOC stock. ANOVAs were performed to compare the SOC stock, as well as the SOC stock variation rates across treatments and soil layers. Results showed that the soil bulk density did not vary significantly between 2014 and 2019 for both treatments (Gr and Sp). The SOC concentration (i.e. SOC expressed in g/kg) increased significantly between 2014 and 2019 in L1, whereas no significant evolution was found in L2 (ca. 5–30 cm). The SOC stock (i.e. SOC expressed in t/ha) increased significantly in the superficial layer L1 for M. × giganteus and M. sinensis, by 0.48 ± 0.41 and 0.54 ± 0.25 t ha^?1 year^?1 on average, respectively, although no significant change was detected in the layer L1‐2 for both genotypes. Moreover, SOC stocks in 2019 did not differ significantly between M. × giganteus and M. sinensis in the soil layers L1 and L1‐2. Lastly, our results showed that the initial SOC stock was significantly higher when miscanthus was grown after set‐aside than after annual crops.     

Extracellular Enzyme Activities and Soil Organic Matter Dynamics for Northern Hardwood Forests receiving Simulated Nitrogen Deposition

Anthropogenic nitrogen enrichment alters decomposition processes that control the flux of carbon (C) and nitrogen (N) from soil organic matter (SOM) pools. To link N-driven changes in SOM to microbial responses, we measured the potential activity of several extracellular enzymes involved in SOM degradation at nine experimental sites located in northern Michigan. Each site has three treatment plots (ambient, +30 and +80 kg N ha⁻¹ y⁻¹). Litter and soil samples were collected on five dates over the third growing season of N treatment. Phenol oxidase, peroxidase and cellobiohydrolase activities showed significant responses to N additions. In the Acer saccharum–Tilia americana ecosystem, oxidative activity was 38% higher in the litter horizon of high N treatment plots, relative to ambient plots, while oxidative activity in mineral soil showed little change. In the A. saccharum–Quercus rubra and Q. velutina–Q. alba ecosystems, oxidative activities declined in both litter (15 and 23%, respectively) and soil (29 and 38%, respectively) in response to high N treatment while cellobiohydrolase activity increased (6 and 39% for litter, 29 and 18% for soil, respectively). Over 3 years, SOM content in the high N plots has decreased in the Acer–Tilia ecosystem and increased in the two Quercus ecosystems, relative to ambient plots. For all three ecosystems, differences in SOM content in relation to N treatment were directly related (r² = 0.92) to an enzyme activity factor that included both oxidative and hydrolytic enzyme responses.     

湖南省森林土壤有机碳密度及碳库储量动态

基于2000—2014年文献和著作资料中的湖南省森林土壤剖面有机碳含量数据,湖南会同杉木林生态系统国家野外科学观测研究站近15年的实测数据,分析了湖南省主要森林类型土壤有机碳密度,结合1983年至2009年湖南省4次森林资源清查数据,研究了湖南省森林土壤有机碳库储量的动态特征。结果表明:湖南省主要森林类型土壤有机碳算术平均含量在9.53—22.86g/kg之间,灌木林最高,土壤有机碳含量的分异主要发生在0—40 cm土层,0—80 cm土壤层有机碳密度在95.44—181.30 t C/hm2之间,平均为137.15 t C/hm2,主要分布在0—40 cm土层中,随土壤深度增加,各森林类型土壤有机碳密度的差异下降,受森林类型的影响减弱。从1983—1987年到2009年,湖南省乔木林土壤层(0—80 cm)有机碳库储量净增加了414.86×106t C,面积加权平均有机碳密度提高了10.98 t C/hm2,不同乔木林土壤层(0—80 cm)有机碳库储量的差异随着时间进程逐渐增大,主要分布在杉木林、松木林、阔叶林。天然林是湖南省乔木林土壤有机碳库储量的主要贡献者,人工林土壤有机碳储量正逐步提高,经济林、竹林、灌木林对湖南省森林土壤层(0—80 cm)有机碳库储量贡献不同,且动态变化趋势也不同。森林土壤层有机碳库储量的变化与各森林类型面积的变化密切相关,而各森林类型面积的增减,与各项林业政策的实施密切相关。因此,人类活动深刻影响森林土壤的碳汇功能。     

The soil organic carbon in particle-size separates under different regrowth forest stands of north eastern Costa Rica

Despite the importance of the secondary forest (SF) in tropical areas, few studies have quantified the soil organic carbon (SOC) pool in Costa Rica. Most of the studies conducted to date in this country have focused mainly on changes in the soil C pool following conversion of forests to pastures, which is the predominant land use in the tropics. The aim of this study was to measure SOC concentration and pool in particle-size fractions down to 50 cm depth in four SF stands regenerating from different intensities of prior land use in loamy sand and sandy loam soils of northeast Costa Rica: (i) a gallery forest (GF), (ii) a 15-year-old SF enriched with commercially planted native trees (15SF), (iii) a 25-year-old SF (25SF), and (iv) an abandoned Theobromma cacao plantation >60 years old (60SF). Additional objectives were (1) to determine the relationship of SOC concentration with selected physical and chemical soil properties, and (2) to establish the key determinants of the depth distribution of SOC in order to identify meaningful trends in the SOC pool. The SOC pool was highest under the 60SF (221.4 Mg C ha⁻¹) followed by the 15SF (212.1 Mg C ha⁻¹), the 25SF (195.9 Mg C ha⁻¹) and the lowest in the GF (183.5 Mg C ha⁻¹). The SOC concentration decreased significantly from 59.7 to 94.1 g kg⁻¹ in the 0–10 cm layer down to 31.0 to 45.5 g kg⁻¹ in the 40–50 cm layer in all forest stands. The fine silt + clay fraction contained the highest values of SOC concentration in all forest stands. Soil texture and the age of the SF were identified as the main factors that explained the variability in SOC. The age of SF stand influenced the distribution of size class aggregates and SOC.     

Impact of long-term nitrogen addition on carbon stocks in trees and soils in northern Europe

The aim of this study was to quantify the effects of fertiliser N on C stocks in trees (stems, stumps, branches, needles, and coarse roots) and soils (organic layer +0–10 cm mineral soil) by analysing data from 15 long-term (14–30 years) experiments in Picea abies and Pinus sylvestris stands in Sweden and Finland. Low application rates (30–50 kg N ha⁻¹ year⁻¹) were always more efficient per unit of N than high application rates (50–200 kg N ha⁻¹ year⁻¹). Addition of a cumulative amount of N of 600–1800 kg N ha⁻¹ resulted in a mean increase in tree and soil C stock of 25 and 11 kg (C sequestered) kg⁻¹ (N added) (“N-use efficiency”), respectively. The corresponding estimates for NPK addition were 38 and 11 kg (C) kg⁻¹ (N). N-use efficiency for C sequestration in trees strongly depended on soil N status and increased from close to zero at C/N 25 in the humus layer up to 40 kg (C) kg⁻¹ (N) at C/N 35 and decreased again to about 20 kg (C) kg⁻¹ (N) at C/N 50 when N only was added. In contrast, addition of NPK resulted in high (40–50 kg (C) kg⁻¹ (N)) N-use efficiency also at N-rich (C/N 25) sites. The great difference in N-use efficiency between addition of NPK and N at N-rich sites reflects a limitation of P and K for tree growth at these sites. N-use efficiency for soil organic carbon (SOC) sequestration was, on average, 3–4 times lower than for tree C sequestration. However, SOC sequestration was about twice as high at P. abies as at P. sylvestris sites and averaged 13 and 7 kg (C) kg⁻¹ (N), respectively. The strong relation between N-use efficiency and humus C/N ratio was used to evaluate the impact of N deposition on C sequestration. The data imply that the 10 kg N ha⁻¹ year⁻¹ higher deposition in southern Sweden than in northern Sweden for a whole century should have resulted in 2.0 ± 1.0 (95% confidence interval) kg m⁻² more tree C and 1.3 ± 0.5 kg m⁻² more SOC at P. abies sites in the south than in the north for a 100-year period. These estimates are consistent with differences between south and north in tree C and SOC found by other studies, and 70–80% of the difference in SOC can be explained by different N deposition.     

Bomb <Superscript>14</Superscript>C enrichment indicates decadal C pool in deep soil?

Studies of changes in soil organic carbon (SOC) stocks normally limit their focus to the upper 20–30 cm of soil, yet 0–20 cm SOC stocks are only ∼40% of 0–1 m SOC. Accounting for only the upper 20–30 cm of SOC has been justifiable assuming that deeper SOC is unreactive since it displays ¹⁴C-derived mean residence times of hundreds or thousands of years. The dramatic increase in the ¹⁴C content of the atmosphere resulting from thermonuclear testing circa 1963 allows the unreactivity of deep SOC to be tested by examining whether deep soils show evidence of ‘bomb-¹⁴C’ incorporation. At depths of 40–100 cm, a well-studied New Zealand soil under stable pastoral management displays progressive enrichment of over 200‰ across samplings in 1959, 1974 and 2002, indicating substantial incorporation of bomb ¹⁴C. This pattern of deep ¹⁴C enrichment—previously observed in 2 well-drained California grassland soils—leads to the hypothesis that roots and/or dissolved organic C transport contribute to a decadally-reactive SOC pool comprising ∼10–40% of SOC below 50 cm. Deep reactive SOC may be important in the global C cycle because it can react to land-use or vegetation change and may respond to different processes than the reactive SOC in the upper 20–30 cm of soil.     

A global meta‐analysis of soil organic carbon response to corn stover removal

Corn (Zea mays L.) stover is a global resource used for livestock, fuel, and bioenergy feedstock, but excessive stover removal can decrease soil organic C (SOC) stocks and deteriorate soil health. Many site‐specific stover removal experiments report accrual rates and SOC stock effects, but a quantitative, global synthesis is needed to provide a scientific base for long‐term energy policy decisions. We used 409 data points from 74 stover harvest experiments conducted around the world for a meta‐analysis and meta‐regression to quantify removal rate, tillage, soil texture, and soil sampling depth effects on SOC. Changes were quantified by: (a) comparing final SOC stock differences after at least 3 years with and without stover removal and (b) calculating SOC accrual rates for both treatments. Stover removal generally reduced final SOC stocks by 8% in the upper 0–15 or 0–30 cm, compared to stover retained, irrespective of soil properties and tillage practices. A more sensitive meta‐regression analysis showed that retention increased SOC stocks within the 30–150 cm depth by another 5%. Compared to baseline values, stover retention increased average SOC stocks temporally at a rate of 0.41 Mg C ha^?1 year^?1 (statistically significant at p < 0.01 when averaged across all soil layers). Although SOC sequestration rates were lower with stover removal, with moderate (<50%) removal they can be positive, thus emphasizing the importance of site‐specific management. Our results also showed that tillage effects on SOC stocks were inconsistent due to the high variability in practices used among the experimental sites. Finally, we conclude that research and technological efforts should continue to be given high priority because of the importance in providing science‐based policy recommendations for long‐term global carbon management.     

不同植茶年限土壤团聚体及其有机碳分布特征

作为土壤结构的基本单元和土壤肥力的重要组成部分,土壤团聚体对土壤的物理、化学和生物特性均有重要影响。试验选取了雅安市名山区中峰乡生态茶园区12—15a、20—22a、30—33a和50a的茶园,研究其土壤团聚体及其有机碳总量、储量和活性组分的分布特征,探究植茶年限对土壤团聚体及其有机碳分布的影响。结果表明:(1)研究区土壤以2 mm粒级团聚体为主,约为70%—80%,且在0—20 cm土层植茶20—22a土壤团聚体含量最高;(2)茶园土壤团聚体有机碳含量随团聚体粒级的减小而增加,最大值出现在0.25 mm粒级团聚体,且在植茶50a时达最高值,0—20 cm土层团聚体有机碳含量均高于20—40 cm,土壤团聚体水溶性有机碳和微生物生物量碳随植茶年限的延长呈先增加后降低的变化趋势,植茶30—33a时含量最高,且小粒级团聚体水溶性有机碳含量较高而微生物量碳较低;(3)土壤团聚体对有机碳的贡献率约有70%来自2 mm粒级团聚体,团聚体有机碳储量随植茶年限延长呈增加的趋势,不同植茶年限0—20 cm土层各粒级团聚体有机碳储量均高于20—40 cm土层,且以0.25 mm粒级团聚体有机碳储量最高。研究结果在一定程度上揭示了不同植茶年限土壤团聚体及其有机碳的分布特征,可为改善区域土壤质量及实施退耕还茶工程提供理论指导。     

Vertical distribution and seasonal pattern of fine-root dynamics in a cool–temperate forest in northern Japan: implication of the understory vegetation, <Emphasis Type="Italic">Sasa</Emphasis> dwarf bamboo

We measured the vertical distribution and seasonal patterns of fine-root production and mortality using minirhizotrons in a cool–temperate forest in northern Japan mainly dominated by Mongolian oak (Quercus crispula) and covered with a dense understory of dwarf bamboo (Sasa senanensis). We also investigated the vertical distribution of the fine-root biomass using soil coring. We also measured environmental factors such as air and soil temperature, soil moisture and leaf area indices (LAI) of trees and the understory Sasa canopy for comparison with the fine-root dynamics. Fine-root biomass to a depth of 60 cm in September 2003 totaled 774 g m⁻², of which 71% was accounted for by Sasa and 60% was concentrated in the surface soil layer (0–15 cm), indicating that understory Sasa was an important component of the fine-root biomass in this ecosystem. Fine-root production increased in late summer (August) when soil temperatures were high, suggesting that temperature partially controls the seasonality of fine-root production. In addition, monthly fine-root production was significantly related to Sasa LAI (P<0.001), suggesting that fine-root production was also affected by the specific phenology of Sasa. Fine-root mortality was relatively constant throughout the year. Fine-root production, mortality, and turnover rates were highest in the surface soil (0–15 cm) and decreased with increasing soil depth. Turnover rates of production and mortality in the surface soil were 1.7 year⁻¹ and 1.1 year⁻¹, respectively.     

Effects of forest conversion into grassland on soil aggregate structure and carbon storage in Panama: evidence from soil carbon fractionation and stable isotopes

Land-use and land-cover strongly influence soil properties such as the amount of soil organic carbon (SOC), aggregate structure and SOC turnover processes. We studied the effects of a vegetation shift from forest to grassland 90 years ago in soils derived from andesite material on Barro Colorado Island (BCI), Panama. We quantified the amount of carbon (C) and nitrogen (N) and determined the turnover of C in bulk soil, water stable aggregates (WSA) of different size classes (<53 μm, 53–250 μm, 250–2000 μm and 2000–8000 μm) and density fractions (free light fraction, intra-aggregate particulate organic matter and mineral associated soil organic C). Total SOC stocks (0–50 cm) under forest (84 Mg C ha⁻¹) and grassland (64 Mg C ha⁻¹) did not differ significantly. Our results revealed that vegetation type did not have an effect on aggregate structure and stability. The investigated soils at BCI did not show higher C and N concentrations in larger aggregates, indicating that organic material is not the major binding agent in these soils to form aggregates. Based on δ¹³C values and treating bulk soil as a single, homogenous C pool we estimated a mean residence time (MRT) of 69 years for the surface layer (0–5 cm). The MRT varied among the different SOC fractions and among depth. In 0–5 cm, MRT of intra-aggregate particulate organic matter (iPOM) was 29 years; whereas mineral associated soil organic C (mSOC) had a MRT of 124 years. These soils have substantial resilience to C and N losses because the >90% of C and N is associated with mSOC, which has a comparatively long MRT.     

In situ separation of root hydraulic redistribution of soil water from liquid and vapor transport

Nocturnal increases in water potential (ψ) and water content (θ) in the upper soil profile are often attributed to root water efflux, a process termed hydraulic redistribution (HR). However, unsaturated liquid or vapor flux of water between soil layers independent of roots also contributes to the daily recovery in θ (Δθ), confounding efforts to determine the actual magnitude of HR. We estimated liquid (J _l) and vapor (J _v) soil water fluxes and their impacts on quantifying HR in a seasonally dry ponderosa pine (Pinus ponderosa) forest by applying existing datasets of ψ, θ and temperature (T) to soil water transport equations. As soil drying progressed, unsaturated hydraulic conductivity declined rapidly such that J _l was irrelevant (<2E−05 mm h⁻¹ at 0–60 cm depths) to total water flux by early August. Vapor flux was estimated to be the highest in upper soil (0–15 cm), driven by large T fluctuations, and confounded the role of HR, if any, in nocturnal θ dynamics. Within the 15–35 cm layer, J _v contributed up to 40% of hourly increases in nocturnal soil moisture. While both HR and net soil water flux between adjacent layers contribute to θ in the 15–65 cm soil layer, HR was the dominant process and accounted for at least 80% of the daily recovery in θ. The absolute magnitude of HR is not easily quantified, yet total diurnal fluctuations in upper soil water content can be quantified and modeled, and remain highly applicable for establishing the magnitude and temporal dynamics of total ecosystem water flux.