Weixi Yi

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.

14C measurement of forest soils in Dinghushan Biosphere Reserve

Organic carbon in forest soils of Qingyunsi and Wukesong profiles can be divided into fast and slow components. Δ14C values of these profiles decrease with increasing of depth. The Δ14C values in 30–40 cm depth interval of Wukesong profile are decreasing sharply until a very low value, showing that a strong geological environment change occurred about 1 560 years ago. The14C apparent ages of Wukesong profile show that the coniferous and broad-leaf mixed forests around Wukesong profile have been developing since 425 a BP, which is consistent with historical documents. The penetrating depths of “bomb14C” in Qingyunsi and Wukesong profiles are 10 and 20 cm, respectively.

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.

The elemental carbon record in Weinan loess section since the last 21 ka

We studied the records of elemental carbon (EC) of the last 21 ka in the Weinan loess section, Shanxi Province. The variations of EC abundance and δ13CEC value along with depth (or age) were presented. There are four large peaks of EC abundance around the following years: 20.16 ka, 17.76 ka, 11.97 ka and 4.49 ka. Climatic situation was changed rapidly during these periods. The peaks around 11.97 ka and 20.16 ka are particularly sharp, occurring over intervals of tens to hundreds of years, which could represent short-duration intense events. δ13CEC values in the upper 4 m of the Weinan loess section vary between −11.71‰ and −21.34‰, which suggests that the vegetation pattern of the last 21 ka on the Loess Plateau is C4-dominated grasses.

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.

Distribution Of 14c And 13c In Forest Soils Of The Dinghushan Biosphere Reserve

We report here first results on the bulk soil organic carbon (SOC), apparent radiocarbon ages and δ 13 C characteristics of the tropical and subtropical forest soil in Dinghushan Biosphere Reserve (DHSBR). The forest oxisol in Dinghushan has developed during the Holocene. The δ 13 C variation curves in all three profiles may be divided into two sections. The upper section from 0 to 40 cm has δ 13 C values varying from -27.4 to -24.1‰, -27.5 to -22.2‰, and -24.4 to -20.1‰. in the Wukesong, Qingyunsi and Kengkou profiles, respectively. The lower section, including the 40-160 cm horizons, has a uniform δ 13 C. The mean δ 13 C values of the soil organic carbon could be used not only to discriminate between C 3 and C 4 plants, but also to distinguish between coniferous and broad-leaf plants.

Late Pliocene–Pleistocene expansion of C4 vegetation in semiarid East Asia linked to increased burning

Plants using the C 4 photosynthetic pathway, commonly tropical and subtropical grasses, increased in abundance in East Asia dur- ing the late Cenozoic. Determining the exact timing and likely fac- tors leading to this major vegetation change requires region-specific studies. Here variations in pyrogenic carbon mass accumulation rate (PyC-MAR) and isotope composition (δ 13 C PyC ) from an ~7-m.y.- long depositional sequence from the central Loess Plateau, China, suggest increased biomass burning and an increased contribution to combusted material from C 4 taxa from 2.6 Ma. Changes in the composition of PyC after 0.6 Ma likely reflect the effects of lower temperatures, particularly during glacial periods, and changes in seasonality of precipitation. Increased PyC-MAR without concomi- tant changes in δ 13 C PyC at ca. 0.15 Ma appears to indicate a decou- pling of feedbacks between changes in climate, fire regime, and veg- etation, and may mark the onset of anthropogenic burning in the region. These new data suggest that C 4 taxa were present on the Loess Plateau from at least the late Miocene, rising to prominence at ca. 2.6 Ma following changes in climate and, critically, an increase in biomass fires.

Turnover rate of soil organic matter and origin of soil 14CO2 in deep soil from a subtropical forest in Dinghushan biosphere reserve, south China.

This paper examines the carbon isotopes (13C, 14C) of soil organic carbon (SOC) and soil CO2 from an evergreen broadleaf forest in southern China during the rainy season. The distribution of SOC δ13C, and SOC content with depth, exhibits a regular decomposition of SOC compartments with different turnover rates. Labile carbon is the main component in the topsoil (0-12 cm) and has a turnover rate between 0.1 and 0.01 yr-1. In the middle section (12-35 cm), SOC was mainly comprised of mediate carbon with turnover rates ranging between 0.01 and 0.025. Below 35 cm depth (underlayer section), the SOC turnover rate is slower than 0.001 yr-1, indicating that passive carbon is the main component of SOC in this section. The total production of humus-derived CO2 is 123.84 g C m-2 yr-1, from which 88% originated in the topsoil. The middle and underlayer sections contribute only 10% and 2% to the total humus-derived CO2 production, respectively. Soil CO2 δ13C varies from -24.7‰ to -24.0‰, showing a slight isotopic depth gradient. Similar to soil CO2 δ13C, ∆14C values, which range from 100.0‰ to 107.2‰, are obviously higher than that of atmospheric CO2 (60-70‰) and SOC in the middle and underlayer section, suggesting that soil CO2 in the profile most likely originates mainly from SOC decomposition in the topsoil. A model of soil CO2 ∆14C indicates that the humus-derived CO2 from the topsoil contributes about 65-78% to soil CO2 in each soil gas sampling layer. In addition, the humus-derived CO2 contributes ~81% on average to total soil CO2 in the profile, in good agreement with the field observation. The distribution and origin of soil 14CO2 imply that soil CO2 will be an important source of atmospheric 14CO2 well into the future.

10BE, 14C distribution, and soil production rate in a soil profile of a grassland slope at Heshan Hilly Land, Guangdong

Concentrations of organic carbon, carbon isotopes ( (super 13) C and (super 14) C), atmospheric (super 10) Be in soil, and in situ (super 10) Be in bedrock and weathering rock were determined in a study of a profile of a grassland slope at the Heshan Hilly Land Interdisciplinary Experimental Station, Chinese Academy of Sciences, in Guangdong Province, China. A good linear relationship between depth and the (super 14) C apparent age of the organic carbon demonstrates that the rock weathering process and the accumulation process of organic matter in the slope are relatively stable. Both (super 14) C and (super 10) Be results show that about 34% of soil in the grassland slope has been eroded during the past 3800 yr. The (super 10) Be results for interstitial soil from weathered rocks show that the 90-cm-thick weathering rock layer above the bedrock has evolved over a period of 1.36 Myr. The concentrations of in situ (super 10) Be in the weathered rock and bedrock are 10.7X10 (super 4) atoms/g and 8.31X10 (super -4) atoms/g, respectively. The weathering rate of the bedrock, equivalent to the soil production rate, was estimated at 8.8X10 (super -4) cm/yr, and the exposure ages of the weathered rock and the bedrock were 72 kyr and 230 kyr, respectively.

Improved application of bomb carbon in teeth for forensic investigation

While radiocarbon is widely applied in dating ancient samples, recent studies reveal that 14C concentrations in modern samples can also yield precise ages due to the atmospheric testing of thermonuclear devices between 1950 and 1963. 14C concentrations in both enamel and organic matter of 13 teeth from 2 areas in China were examined to evaluate and improve this method of forensic investigation. Choosing enamel near the cervix of the tooth can reduce the error caused by the difference between the sample formation time and whole enamel formation time because tooth enamel formations take a long time to complete. A proper regional data set will be helpful to get an accurate result when calculating the age of the sample (T1) by the CALIBomb program. By subtracting the enamel formation time (t), the birth date of an individual (T2) can be confirmed by enamel F14C from 2 teeth formed at different ages. Calculated enamel formation dates by 14C concentration are basically consistent with corresponding actual values, with a mean error of 1.9 yr for all results and 0.2 yr for the samples formed after AD 1960. This method is more effective for dating samples completed after AD 1960. We also found that 14C concentrations in organic matter of tooth roots are much lower than atmospheric concentrations in root formation years, suggesting that the organic material keeps turning over even after tooth formation is complete. This might be a potential tool for identification of death age to extract a proper component for 14C dating. We also observed that ?13C values between hydroxyapatite and organic matter indicate that isotopic fractionation during the biomineralization is 8?9? more positive in mineral fractions than in organic matter.