Qingqiang Chen

Organic matter turnover rates and CO2 flux from organic matter decomposition of mountain soil profiles in the subtropical area, south China

Abstract Two different Ferralsol profiles from the forest and shrub meadow zones in Dinghushan Biosphere Reserve, south China, were sampled at narrow vertical intervals to study soil carbon dynamics in the subtropical area. Soil organic matter (SOM) turnover rates ( m ) were calculated using numerical models based on SOM Δ 14 C. The model results indicate that values of m are high in the upper 10 cm of the profiles and decrease quickly downwards. They suggest that the SOM is composed of compartments with different turnover times. A compartment with rapid turnover rates is predominant in the upper part of the profiles (0–10/12 cm), compartments with slower turnover rates occur in the middle part (10/12–20O/70 cm) and only stable compartments occur in the lower part (>20/70 cm). The CO 2 flux derived from SOM decomposition (CFSD) was calculated from values of m , SOM content, soil bulk density and thickness of various soil layers of the profiles. The results indicate that 98% of the CFSD is from the upper 11 cmm of the profiles. The values of m and CFSD for the forest profile are greater than those for the shrub meadow profile. The study suggests that above-ground vegetation is the main factor controlling m and CFSD within the same climate zone, which supports the idea that growth of green crops can intensify the activity of soils as sinks for atmospheric carbon.

Spatial and temporal distribution of carbon isotopes in soil organic matter at the Dinghushan Biosphere Reserve, South China

The spatial and temporal distribution of carbon isotopes (13C, 14C) in soil organic matter (SOM) were studied based on SOM content, SOM Δ14C and SOM δ13C of thinly layered soil samples for six soil profiles with different elevations at the Dinghushan Biosphere Reserve (DHSBR), South China. The results indicate that variations of SOM δ13C with depth of the soil profiles at different elevations are controlled by soil development, and correlate well with SOM composition in terms of SOM compartments with different turnover rates, and SOM turnover processes at the DHSBR. The effect of carbon isotope fractionation was obvious during transformation of organic matter (OM) from plant debris to SOM in topsoil and SOM turnover processes after the topsoil was buried, which resulted in great increments of OM δ13C, respectively. Increments of SOM δ13C of topsoil from δ13C of plant debris were controlled by SOM turnover rates. Both topsoil SOM δ13C and plant debris δ13C increase with elevation, indicating regular changes in vegetation species and composition with elevation, which is consistent with the vertical distribution of vegetation at the DHSBR. The six soil profiles at different elevations had similar characteristics in variations of SOM δ13C with depth, alterations of SOM contents with depth and that SOM 14C apparent ages increasing with depth, respectively. These are presumably attributed to the regular distribution of different SOM compartments with depth because of their regular turnover during soil development. Depth with the maximal SOM δ13C value is different in mechanism and magnitude with penetrating depth of 14C produced by nuclear explosion into atmosphere from 1952 to 1962, and both indicate controls of topography and vegetation on the distribution of SOM carbon isotopes with depth. Elevation exerts indirect controls on the spatial and temporal distribution of SOM carbon isotopes of the studied mountainous soil profiles at the DHSBR. This study shows that mountainous soil profiles at different elevations and with distinctive aboveground vegetation are presumably ideal sites for studies on soil carbon dynamics in different climatic-vegetation zones.

Soil organic matter turnover in the subtropical mountainous region of South China

Studies on soil organic matter (SOM) cycling in different climate zones are an important basis for further understanding of the feedback mechanism of terrestrial carbon storage to global climatic changes and are crucial for accurate projections of future concentrations of CO2 in the atmosphere. Using thin-layer methods, six soil profiles in the Dinghushan Biosphere Reserve (DHSBR), South China, and the Xiaoliang Ecological Station of the South China Institute of Botany, Chinese Academy of Sciences (CAS) were excavated and sampled for studies on the dynamics of SOM in the southern subtropical areas, based on SOM δ13C, Δ14C, soil grain size characteristics, and soil organic carbon (SOC) contents. Results indicate that the turnover of SOM occurs in three stages: (i) Rapid turnover of SOM occurs within 100 years, with SOC content decreasing sharply downwards from the ground surface and δ13C values becoming correspondingly enriched in 13C as a result of carbon isotope fractionation in the process of SOM turnover; maximum till is reached at about 260–270 years; (ii) from about 260–270 years to 800–1400 years, SOM turnover rates lessen, SOC content decreases slowly downwards, and δ13C values become gradually depleted in 13C due to the decomposition of SOM compartments with higher δ13C values; (iii) after about 1500 years, SOC content approaches the minimum, with slight fluctuations, and δ13C values become stable. Comparison analyses suggest that soil clay materials control existing forms and turnover processes of SOM directly, SOM in the soil sections with high clay content at DHSBR are not easily decomposed and have longer turnover periods, and soil textures are an important factor controlling SOM dynamics. Fall leaf litter generally has more negative δ13C value than the topsoil samples, which may be a result of isotope fractionation caused by rapid carbon decay prior to the penetration of carbon from the litter into the topsoils. Above-ground vegetation species and composition impact SOM turnover processes directly; δ13C analysis may be an important tool for determination of the improvement in soil quality during the restoration of degraded ecosystems. Vegetation occupation history also influences the SOM dynamics of soil profiles at different sites in one area with similar vegetation species, as shown by the soil profiles at the Xiaoliang Ecological Station of the South China Institute of Botany, CAS.

DETERMINATION OF SULFATE IN COASTAL SALT MARSH SEDIMENTS WITH HIGH-CHLORIDE CONCENTRATION BY ION CHROMATOGRAPHY: A REVISED METHOD

In order to elucidate the relationship between sulfate reduction and mineralization of organic matter in coastal sediments, the content of sulfate in sediment samples should be measured. Chloride, sulfate, bromide, nitrate, and phosphate are the standard anions in coastal sediment samples. Ion chromatography (IC) is one of the popularly used techniques for determinations of these anions. However, the peaks of the five anions in the IC system coincide with each other under high chloride conditions when using the normal method. In this article, an improved, highly sensitive method for simultaneous separation and determination of sulfate, bromide, nitrate, and phosphate in the samples with high chloride concentration is established and evaluated by IC. In high concentrations of chloride samples, the effects of eluent concentration on the retention time and separation of other anions are studied. The peaks of these anions can be well separated on an AS11 ion-exchange column by a low concentration of NaOH eluent (10 mM). in one injection. Gradient elution is helpful to shorten retention time, especially for phosphate. The method detection limit of sulfate is 0.091 mg · L−1. The recovery of sulfate ranged from 100.5% to 106.8%, when the concentrations of standard solution are close to those of the samples. This technique was validated by determining the content of sulfate and chloride in coastal salt marsh sediment samples.

Stage characteristics for the development of late Quaternary paleosols in the Yangtze Delta area

The late Quaternary paleosols in the Yangtze Delta area developed in the period of 25 000–12 000 aBP. Phytolith ratios (value A) of contents of fan, square and rectangle types indicating the warm and humid climate to those of bar, hat and point types indicating cold and dry climates decrease upwards overall on profiles of the paleosols, suggesting that the climates turned from warm and humid to cold and dry. So, the paleosols developed mainly during the regression before the last glacial maximum. Changes of value A indicate no trend of warming, suggesting that the transgression was rapid after the last glacial maximum. The development of the paleosols exhibited obvious stages, which were controlled remarkably by transgression and regression: I. Period of alternation of deposition and pedogenesis, which was from the occurrence of regresson before the last glacial maximum to the time when sea levels fell close to the lowest; II. Exposure period with continual pedogenesis, which was terminated when the paleosols were drown in the post-glacial transgression; III. Period of being drown and buried, the paleosols were buried by marine sediments, and reworked by early diagenesis.

Relationship between magnetic fabric parameters and dynamic characteristics of deposits in different environments

Comparison of magnetic fabric parameters of deposits in different modern sedimentary environments indicates that values ofP, F andL of eolian deposits are lower obviously than those of hydrogenic ones respectively. On relationship diagrams of magnetic fabric parameters, data points of eolian samples concentrated in limited area near origin of coordinates indicate that dynamic intensity is lower with less variation; while those of hydrogenic ones depart from origin of coordinates suggest higher intensity of sedimentary dynamics. Analyses of dynamic characteristics of different sedimentary environments indicate that values of magnetic fabric parameters of deposits are determined by dynamic types and characteristics of depositional processes. Magnetic fabric analyses of deposits are effective in determination of sedimentary environments.

Paleovalley in the Yannan-Sancang area of Jiangsu Province and its geological significance

IN the southern Yellow Sea there exists a large radial sand ridge system,which covers about 20 000 km2. Sand ridges and troughs occur alternately within the system, and single sandridges are tens of kilometers, even more than one hundred kilometers long and ten to fifteenkilometers wide. Due to the distribution pattern of the radial sand ridge system being like finer...