[Effects of water vapor source and local environmental factors on the hydrogen and oxygen isotopic compositions of precipitation in Huitong, Hunan Province, China].

Deuterium (D) and oxygen-18 (18O) are common environmental tracers in water. Understanding the isotopic compositions of precipitation is necessary for further studies on local and global water cycling processes. To reveal the mechanism of isotopic compositions of precipitation in subtropical monsoon region in response to environmental changes, we collected 49 precipitation samples and recorded related environmental factors from May 2017 to August 2019 in Huitong field station of Chinese Academy of Sciences in Hunan Province. We analyzed the temporal variations in D and 18O values in precipitation and analyzed the influence of water vapor source and local environmental factor on stable isotopic compositions of precipitation. The local meteoric water line was established as δD=(7.45±0.17)δ18O+(10.10±1.25) (R2=0.93, P<0.01), the slope of which was slightly lower than China's meteoric water line and the global meteoric line. The D and 18O values of precipitation samples were closely coupled with local meteorological conditions and dominant moisture sources. The 18O and D contents were depleted during summer monsoon season but enriched during winter monsoon season. During the summer monsoon and post-monsoon seasons, precipitation in this area came mainly from the Bay of Bengal, the South China Sea, and the West Pacific at three different types of air pressure, which lead to the similarity of the D-excess value of the precipitation to global average. The lower intercept of meteoric water line and the higher D-excess value for precipitation during the winter monsoon season resulted from moisture from remote westerly air masses, degenerated tropical marine air masses from the Bay of Bengal, and inland moisture in the pre-monsoon period, which were also affected by local environmental factors.

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Selecting the optimal sample size accurately is important for improving the measurement precision of soil physicochemical properties. In this study, we examined the within-site heterogeneity of soil physical and chemical properties in an evergreen broad-leaved forest and a Chinese fir (Cunninghamia lanceolata) plantation in subtropical China. We applied both traditional and bootstrapping technique to determine the optimal sample size, the number of observation (n), for predicting soil nutrient content with an allowable error of 10% at a 95% confidence level for both forest types. The results showed that soil pH and C/N had low variation, while the soil nutrients had a moderate variation in both forests. The coefficients of variation (CV) of total element concentration in the evergreen broad-leaved forest, such as total carbon and total nitrogen, were significantly larger than those in the Chinese fir plantation, while the CVs of available nutrients were similar between two forests. The evaluation error of all soil properties in two forests showed a stable trend after a sharp decline with the increase of sample size. The mean values also changed from violent fluctuation to stable, and extravagant sample could not effectively improve the accuracy of mean value measurement. The sample sizes of soil water content, total carbon, total nitrogen, nitrate (NO3--N), total potassium, total calcium, and total magnesium in the evergreen broad-leaved forest were signifi-cantly larger than those in the Chinese fir plantation, while the sample size for ammonium (NH4+-N) showed opposite pattern, and the others had no significant difference. In addition, the sample sizes of different soil physicochemical properties varied significantly in the same forest. The available elements, such as NH4+-N and NO3--N, needed a larger sample size than total elements in both forests. Accordingly, we proposed that the heterogeneity and sample size of soil physicochemical properties should be considered before soil survey. An adequate sample size may be varied according to the purpose of sample and the site-specific variation of soil characters.

Recovery time of soil carbon pools of conversional Chinese fir plantations from broadleaved forests in subtropical regions, China.

The conversion from natural forest to plantation has been widely applied, with consequences on ecosystem carbon pool. The experimental results of changes of soil carbon stocks after forest conversion are often contradictory. Moreover, the recovery time of soil carbon stocks after forest conversion varies among different sites. To examine the changes of soil carbon stocks following the forest conversions in the long-term and to estimate the recovery time, we selected 116 subtropical forests, including 29 pair-wise replicates for evergreen broadleaved forests (EBF, 40-100-year-old), young Chinese fir plantations (Cunninghamia lanceolata) (YCP, 4-8-year-old), middle-aged Chinese fir plantations (MACP, 13-20-year-old), and mature Chinese fir plantations (MCP, 23-32-year-old), and estimated soil carbon stocks. Soil carbon stocks of YCP and MACP decreased in average 12.5 and 28.7Mgha-1 compared with EBF, and showed no variation between MCP and EBF. Soil carbon stocks were positively correlated to soil total nitrogen stocks and C:N ratio. Our results showed that the forest conversions didn't cause a variation of soil carbon stocks in the long-term, although there was a short-term decline after conversion. The recovery time of soil carbon stock is 27years. These results indicate that the conversion from evergreen broadleaved forests to Chinese fir plantations in subtropical region of China causes soil carbon release in early stage, but has no effect on soil carbon stocks in the long-term. Prolonging the rotation period (>27years) would offset the adverse effects of the forest conversion on soil carbon stocks, and be critical in alleviating global climate change.

Thinning effect on photosynthesis depends on needle ages in a Chinese fir (Cunninghamia lanceolata) plantation.

Canopies in evergreen coniferous plantations often consist of various-aged needles. However, the effect of needle age on the photosynthetic responses to thinning remains ambiguous. Photosynthetic responses of different-aged needles to thinning were investigated in a Chinese fir (Cunninghamia lanceolata) plantation. A dual isotope approach [simultaneous measurements of stable carbon (δ13C) and oxygen (δ18O) isotopes] was employed to distinguish between biochemical and stomatal limitations to photosynthesis. Our results showed that increases in net photosynthesis rates upon thinning only occurred in the current-year and one-year-old needles, and not in the two- to four-year-old needles. The increased δ13C and declined δ18O in current year needles of trees from thinned stands indicated that both the photosynthetic capacity and stomatal conductance resulted in increasing photosynthesis. In one-year-old needles of trees from thinned stands, an increased needle δ13C and a constant needle δ18O were observed, indicating the photosynthetic capacity rather than stomatal conductance contributed to the increasing photosynthesis. The higher water-soluble nitrogen content in current-year and one-year-old needles in thinned trees also supported that the photosynthetic capacity plays an important role in the enhancement of photosynthesis. In contrast, the δ13C, δ18O and water-soluble nitrogen in the two- to four-year-old needles were not significantly different between the control and thinned trees. Thus, the thinning effect on photosynthesis depends on needle age in a Chinese fir plantation. Our results highlight that the different responses of different-aged needles to thinning have to be taken into account for understanding and modelling ecosystem responses to management, especially under the expected environmental changes in future.

A broadleaf species enhances an autotoxic conifers growth through belowground chemical interactions.

Plants may affect the performance of neighboring plants either positively or negatively through interspecific and intraspecific interactions. Productivity of mixed-species systems is ultimately the net result of positive and negative interactions among the component species. Despite increasing knowledge of positive interactions occurring in mixed-species tree systems, relatively little is known about the mechanisms underlying such interactions. Based on data from 25-year-old experimental stands in situ and a series of controlled experiments, we test the hypothesis that a broadleaf, non-N fixing species, Michelia macclurei, facilitates the performance of an autotoxic conifer Chinese fir (Cunninghamia lanceolata) through belowground chemical interactions. Chinese fir roots released the allelochemical cyclic dipeptide (6-hydroxy-1,3-dimethyl-8-nonadecyl-[1,4]-diazocane- 2,5-diketone) into the soil environment, resulting in self-growth inhibition, and deterioration of soil microorganisms that improve P availability. However, when grown with M. macclurei the growth of Chinese fir was consistently enhanced. In particular, Chinese fir enhanced root growth and distribution in deep soil layers. When compared with monocultures of Chinese fir, the presence of M. macclurei reduced release and increased degradation of cyclic dipeptide in the soil, resulting in a shift from self-inhibition to chemical facilitation. This association also improved the soil microbial community by increasing arbuscular mycorrhizal fungi, and induced the production of Chinese fir roots. We conclude that interspecific interactions are less negative than intraspecific ones between non-N fixing broadleaf and autotoxic conifer species. The impacts are generated by reducing allelochemical levels, enhancing belowground mutualisms, improving soil properties, and changing root distributions as well as the net effects of all the processes within the soil. In particular, allelochemical context alters the consequences of the belowground ecological interactions with a novel mechanism: reduction of self-inhibition through reduced release and increased degradation of an autotoxic compound in the mixed-species plantations. Such a mechanism would be useful in reforestation programs undertaken to rehabilitate forest plantations that suffer from problems associated with autotoxicity.

[Effects of Chinese fir litter on soil organic carbon decomposition and microbial biomass carbon].

By using 13C stable isotope tracer technique, this paper studied the effects of Chinese fir litter addition on the soil organic carbon (SOC) decomposition, microbial biomass carbon, and dissolved organic carbon in 0-5 cm and 40-45 cm layers. The decomposition rate of SOC in 40-45 cm layer was significantly lower than that in 0-5 cm layer, but the priming effect induced by the Chinese fir litter addition showed an opposite trend. The Chinese fir litter addition increased the soil total microbial biomass carbon and the microbial biomass carbon derived from native soil significantly, but had less effects on the soil dissolved organic carbon. Turning over the subsoil to the surface of the woodland could accelerate the soil carbon loss in Chinese fir plantation due to the priming effect induced by the litters.

[Characteristics of soil microbial community structure in Cunninghamia lanceolata plantation].

By using dilution plate, fumigation extraction, and phospholipid fatty acid (PLFA) methods, this paper studied the quantities of soil microbial populations and the characteristics of soil microbial community structure in a Chinese fir (Cunninghamia lanceolata) plantation converted from an evergreen broadleaved forest. The results showed that, during the vegetation change from evergreen broadleaved forest to Chinese fir plantation, the microbial biomass carbon and the quantities of culturable bacteria and actinomyces were decreased. The total PLFAs, bacterial PLFAs, and fungi PLFAs in the woodland soil from Chinese fir plantation were decreased by 49.4%, 52.4%, 46.6%, simultaneously. And G+ and G- bacterial PLFAs in Chinese fir plantation were lower than in evergreen broadleaved forest. As compared with those in rhizosphere soil from Chinese fir plantation, the microbial biomass carbon and the quantities of culturable bacteria and actinomyces in bulk soil were decreased. The total PLFAs, bacterial PLFAs, and G+ and G- bacterial PLFAs in the rhizosphere soil were increased, while the ratio of fungal to bacterial PLFAs was lowered. The principal component analysis of the soil microbial community structure indicated that the first principal component (PC1) and the second principal component (PC2) together accounted for 78.2% of total variation of soil microbial community structure. This study showed there were some differences in the soil microbial community structure between evergreen broadleaved forest and Chinese fir plantation.

[A new assessment method for the quality of ecological monitoring data: taking CERN's tree growth dataset as a case].

This paper presented a new and simple assessment method for the quality of ecological monitoring data. This method theorized the associations between the data reliability as an ordinal variable with different number of classes and the data sources such as natural main ecological processes, secondary ecological processes, and extraneous and exotic processes, and offered a new data quality index to estimate the quality of the whole dataset by using the reasonableness ratio of observations. The assessment results provided the reliability class of each dataset, good explanations for outlier (or error data) flagging decisions, and quality value of the whole dataset. The method was applied to assess two tree growth datasets from Chinese Ecosystem Research Network (CERN), and the results demonstrated that the new data quality index could quantitatively evaluate the quality of the tree growth datasets. The new method would facilitate the development of corresponding software.

[Application of indicator species in predicting forest management effect on forest species diversity and community composition].

By using indicator species analysis (ISA) method, this paper studied the feasibility of using indicator species to reflect the responses of species diversity and community composition of subtropical forests in Huitong of China to forest management. Ninety-four significant indicator species from 357 understory species were identified, and a new indicator species dataset (community level) was constructed to examine the association between indicator species dataset and original community dataset, and to evaluate the predictive potential of indicator species in reflecting forest management effect. There existed a strong association between the two datasets (Mantel r = 0.898). The indicator species dataset could well predict the management effect on species diversity (regression analysis, R2 > 0.74) and community composition (ANOVA, F >16.79). When the two datasets were applied to Nonmetric Multi-Dimensional Scaling (NMDS) ordination and K-mean cluster analysis, the indicator species dataset could well identify the forest types with different management treatments, as the original community dataset did. Also, the indicator species dataset nearly played the same role as the original community dataset in identifying the species diversity, community composition, and forest type. It was suggested that for saving costs in overall investigation of forest ecosystem, indicator species could be used as a surrogate of full community to predict forest management effect.

[Short-term effects of understory vegetation removal on nutrient cycling in litter layer of Chinese fir plantation].

A short-term understory vegetation removal experiment was conducted at three experimental sites of Chinese fir (Cunninghamia lanceolata) plantation in Huitong of Hunan Province, South-central China to examine the short-term effects of the absence of understory vegetation on litter decomposition. In the plantation, the standing stock of the litter in L layer (un-decomposed litter), F layer (semi-decomposed litter), and H layer (decomposed litter) under understory intact condition was (123.7 +/- 46.3) g m(-2), (204.2 +/- 79.1) g m(-2), and (187.1 +/- 94.8) g m(-2), and the corresponding nutrient storage was (447.51 +/- 16.75) g kg(-1), (418.89 +/- 35.75) g kg(-1), and (376.11 +/- 47.33) g kg(-1) for carbon, (4.87 +/- 1.24) g kg(-1), (6.4 +/- 2.38) g kg(-1), and (4.66 +/- 2.64) g kg(-1) for nitrogen, and (0.14 +/- 0.01) g kg(-1), (0.16 +/- 0.03) g kg(-1), and (0.16 +/- 0.02) g kg(-1) for phosphorus, respectively. At sites DL87 and MS90, understory vegetation removal had significant effects on the standing stock of litter and the corresponding storage of carbon, nitrogen, and phosphorus in F layer, though the effects varied with sites (e. g., the storage of litter carbon, nitrogen, and phosphorus decreased by more than 55% at DL87 while increased by about 1 time at MS90). Across the three sites, understory vegetation removal had little effects on the standing stock and nutrient storage of the litter in L layer and H layer. In addition, the removal had little effects on the litter decomposition at its early stage, but showed significant inhibitory effects after 150 days elapsed, with the decomposition rate decreased by 20.8%. These results suggested that the short-term effects of understory vegetation removal on the litter nutrient cycling could be depended on the forest site condition and the duration after the understory vegetation removal.

[Dynamics of nutrients in an age sequence of Pinus massoniana plantation].

By using a chronosequence approach, this paper studied the nutrients accumulation, allocation, and cycling in 7-, 17-, 31-, and 51-year-old Masson pine plantations, aimed to analyze the dynamics of nutrients in Masson pine plantation at its different developmental stages. The results showed that 1) nutrient accumulation was not synchronized with biomass accumulation, with the accumulation rate of biomass being 3.3, 5.4, 3.3, 3.7, and 9.8 times of that of N, P, K, Ca, and Mg from age 17 to 51, respectively, 2) nutrient allocation to a specific component was related to the growth rate of the nutrient pool in the component, and the nutrient allocation to stem as well as the ratio of nutrient allocation to root/shoot increased with plantation age, and 3) the biomass production per unit nutrient, i. e., nutrient use efficiency, and the nutrient cycling coefficient also increased with plantation age, being higher for 51-year-old plantation than for younger plantations, suggesting that shortening the rotation length could reduce the possibility to raise the nutrient use efficiency. It was suggested that both extending the current rotation length to as long as 50 years and reducing the harvest intensity could be helpful to conserve the nutrients and to maintain long-term productivity.

[Dynamics of biomass- and nutrient accumulation in a Chinese-fir plantation].

Based on the over 30 years observation data in Huitong National Research Station of Forest Ecosystem under Chinese Academy of Sciences and related literatures, this paper analyzed the dynamics of the biomass- and nutrient accumulation and allocation in a Chinese-fir (Cunninghamia lanceolata) plantation, and the effects of rotation length and harvest intensity on the nutrient input/output. The results showed that stand age was the main factor affecting the biomass- and nutrient accumulation and allocation, and different organs had significant differences in their biomass- and nutrient storage. Stand age and different organs explained 37.1% and 40.3% of the variations of biomass- and nutrient storage, respectively. With the increase of stand age, the proportions of biomass and nutrients allocated to stem increased, while those allocated to foliage decreased. The accumulation rates of biomass and nutrients peaked at the age of 17 and 13, respectively, indicating that the nutrients reached their maximum accumulation rate ahead of the biomass. The nutrient requirement per unit dry biomass production decreased rapidly from the stand age 3 to 20 and reached a steady state after then, suggesting that the nutrient use efficiency increased with stand age. The analysis of different harvest scenarios showed that to prolong rotation length and to lower utilization intensity could reduce nutrient output. For example, if the rotation length was prolonged from 28 years to 56 years and only stem was harvested, the harvested biomass would be decreased by 31.57%, and the output of N, P, K, Ca, and Mg would be decreased by 42.02%, 58.93%, 27.70%, 31.07%, and 45.26%, respectively.

[Impact of heavy snow storm and freezing rain disasters on soil fauna in Chinese fir plantation in southern China].

In January 2008, southern China suffered an unusual heavy snowstorm and freezing rain over a large area for almost a month long. This catastrophic event was the worst one in past 50 years, which brought the area a serious impact on the infrastructure, ecology, and environment. To understand the long-term impact of this catastrophic event on the forest ecosystems in this area, a field investigation was conducted on the soil fauna in a pure Chinese fir plantation and a mixed Chinese fir plantation-alder plantation in Huitong County of Hunan Province on March 23, 2008, the date 40 days after the heavy snowstorm and freezing rain. With the abundance and community composition as the main parameters and the monitoring data from the two plantations on March 23, 2007 as the reference, the flexibility and resistance of soil fauna to the disturbances of the catastrophic event was preliminarily evaluated. The results showed that there was a significant deviation of soil fauna communities in the two plantations from the reference. An outbreak increase in microfauna nematode abundance was found from 12216.9 ind x m(-2) to 118343.9 ind x m(-2) in pure Chinese fir plantation and from 25435.9 ind x m(-2) to 84573.0 ind x m(-2) in mixed Chinese fir plantation-alder plantation, while a 27.0% and 85.6% decrease of macrofauna abundance was found in the two plantations, respectively, compared with the reference. Mesofauna abundance also had a significant decrease in litter layer but not in soil. The abundance recovery displayed a trend from quick rate for microfauna to slow rate for macrofauna, which indicated that the soil fauna functional groups, in terms of body size, could be used as a vulnerable indicator in evaluating disturbance event and post-disturbance recovery. By using community ordinations, no shift in soil fauna community composition was detected 40 days after the catastrophic event, suggesting that the community composition of soil invertebrate had a high resistance to catastrophic snowstorm and freezing rain disturbances.

[Effects of root system and litter of Chinese fir on soil microbial properties].

A simulation test was conducted to study the effects of the root system and litter of Chinese fir (Cunninghamia lancceolata) on soil microbial properties. After the amendment with C. lancceolata root system, the soil microbial biomass C, basal respiration, total organic C, and microbial quotient increased significantly (P < 0.05), while soil metabolic quotient (qCO2) presented an opposite trend. In the treatment amended with C. lancceolata litter, the soil basal respiration and qCO2 decreased significantly (P < 0.05), whereas soil microbial biomass C, total organic C, and microbial quotient had less change. A significant interaction between C. lancceolata root system and litter was observed on soil basal respiration and qCO2. The qCO2 had significant positive correlations with soil total organic C (R2 = 0.209) and dissolved organic C (R2 = 0.325), suggesting that the C use efficiency of soil microbes decreased with increasing soil organic C content. Comparing with litter, the root system of C. lancceolata played more important roles in soil ecological processes.

[Nutrient contents and resorption characteristics in needles of different age Pinus massoniana (Lamb.) before and after withering].

Taking the 10-, 23-, and 45-year-old Pinus massoniana plantations in Huitong of Hunnan Province as test objects and with the parameters nutrient content per unit needle mass, nutrient content per unit needle length, and nutrient content of single needle, the nutrient contents and resorption characteristics in the needles of different age P. massoniana before and after withering were studied. The results showed that after withering, the mass of single needle, length of single needle, and mass per unit needle length were decreased by 15.9%, 4.6% and 13.9%, respectively. The nutrient contents in the needles differed with the age of P. massoniana, and the mean resorption of N, P and K in the needles was all higher than 50%. With the increase of tree age, the nutrient resorption efficiency and proficiency increased first and declined then. According to the calculations with the parameters mentioned above, there was no significant difference in the resorption efficiency of the nutrients except N.

[Effects of Cunninghamia lanceolata-broadleaved tree species mixed leaf litters on active soil organic matter].

With incubation test, this paper studied the effects of Cunninghamia lanceolata leaf litter and its mixture with the litters of main broadleaved tree species in subtropical China, such as Alnus cremastogyne, Kalopanax septemlobus and Michelia macclurei on active soil organic matter. The results showed that adding leaf litters into soil could significantly increase soil microbial biomass C and N, respiration rate and dissolved organic C, and mixed leaf litters were more effective than C. lanceolata leaf litter in increasing soil dissolved organic C. By the end of the incubation, the increment of soil microbial biomass C and N, respiration rate, and dissolved organic C in treatments C. lanceolata leaf litter and C. lanceolata-broadleaved tree species mixed leaf litters was 49% and 63%, 35% and 75%, 65% and 100%, and 66% and 108%, respectively, compared with control. The addition of leaf litters had no significant effects on soil microbial quotient and microbial biomass C/N ratio.

[Effects of tree species fine root decomposition on soil active organic carbon].

With incubation test, this paper studied the effects of fine root decomposition of Alnus cremastogyne, Cunninghamia lanceolata and Michelia macclurei on the content of soil active organic carbon at 9 degrees C , 14 degrees C , 24 degrees C and 28 degrees C. The results showed that the decomposition rate of fine root differed significantly with test tree species, which was decreased in the order of M. macclurei > A. cremastogyne > C. lanceolata. The decomposition rate was increased with increasing temperature, but declined with prolonged incubation time. Fine root source, incubation temperature, and incubation time all affected the contents of soil microbial biomass carbon and water-soluble organic carbon. The decomposition of fine root increased soil microbial biomass carbon and water-soluble organic carbon significantly, and the effect decreased in the order of M. macclurei > A. cremastogyne > C. lanceolata. Higher contents of soil microbial biomass carbon and water-soluble organic carbon were observed at medium temperature and middle incubation stage. Fine root decomposition had less effect on the content of soil readily oxidized organic carbon.