Haibing Xiao

Tracing the source of sedimentary organic carbon in the Loess Plateau of China: An integrated elemental ratio, stable carbon signatures, and radioactive isotopes approach

Soil erosion, which will induce the redistribution of soil and associated soil organic carbon (SOC) on the Earths surface, is of critically importance for biogeochemical cycling of essential elements and terrestrial carbon sequestration. Despite the importance of soil erosion, surprisingly few studies have evaluated the sources of eroded carbon (C). This study used natural abundance levels of the stable isotope signature (13C) and radioactive isotopes (137Cs and 210Pbex), along with elements ratio (C/N) based on a two end member mixing model to qualitatively and quantitatively identify the sources of sedimentary OC retained by check dam in the Qiaozigou small watershed in the Loess Plateau, China. Sediment profiles (0-200xa0cm) captured at natural depositional area of the basin was compared to possible source materials, which included: superficial Loess mineral soils (0-20xa0cm) from three land use types [i.e., grassland (Medicago sativa), forestland (Robinia pseudoacacia.), shrubland (Prunus sibirica), and gully land (Loess parent material.)]. The results demonstrated that SOC in sediments showed significantly negative correlation with pH (Pxa0<xa00.01), and positive correlation with soil water content (SWC) (Pxa0<xa00.05). The sedimentary OC was not derived from grasslands or gullies. Forestland and shrubland were two main sources of eroded organic carbon within the surface sediment (0-60xa0cm deep), except for that in the 20-40xa0cm soil layer. Radionuclides analyses also implied that the surface sediments retained by check-dams mainly originated from soils of forestland and shrubland. Results of the two end-member mixing model demonstrated that more than 50% SOC (mean probability estimate (MPE) 50.13% via 13C and 60.53% via C/N) in surface sediment (0-20xa0cm deep) derived from forestland, whereas subsurface sedimentary SOC (20-200xa0cm) mainly resulted from shrubland (MPExa0>xa050%). Although uncertainties on the sources of SOC in deep soils exist, the soil organic δ13C and C/N is still an effective indicator for sources of sedimentary organic carbon in the deposition zone in the short term (<10 years).

Apportioning source of erosion-induced organic matter in the hilly-gully region of loess plateau in China: Insight from lipid biomarker and isotopic signature analysis

Understanding the dynamics of organic matter (OM) at global and local scales is one of the challenges in the environmental sciences and i.e. terrestrial biogeochemistry. The accurate identification of OM is an essential element to achieve this goal. In our study, a novel application for quantitatively apportioning sources of eroded sedimentary OM from an eco-geomorphologic perspective was shown successfully via a coupled molecular n-alkane biomarkers and stable isotopic signatures (13C and 15N) along with elemental compositions (TOC and TN) using a Bayesian mixing model (SIAR). Soil source samples were collected from different land use types (i.e., forests, grassland, cropland, and fallow) and gully, which were probably transported downstream along the steep terrain. Meanwhile, three soil profiles with a total of 90 sediment samples were also sampled in check dam. The results indicated that cropland was the main sedimentary OM source in this catchment, contributing 29.5%, whereas the forests, grassland, fallow and gully contributed 12.17%, 15.39%, 21.53% and 21.85%, respectively. Although the molecular biomarker as a tracer was not valid solely, the combined approaches of n-alkanes biomarker and bulk parameters were efficient complements in tracing OM source in a hilly-gully region on the Loess Plateau of China.

Microbial CO 2 assimilation is not limited by the decrease in autotrophic bacterial abundance and diversity in eroded watershed

The impacts of soil erosion on soil structure, nutrient, and microflora have been extensively studied but little is known about the responses of autotrophic bacterial community and associated carbon (C)-fixing potential to soil erosion. In this study, three abandoned croplands (ES1, ES2, and ES3) and three check dams (DS1, DS2, and DS3) in the Qiaozi watershed of Chinese Loess Plateau were selected as eroding sites and depositional sites, respectively, to evaluate the impacts of soil erosion on autotrophic bacterial community and associated C-fixing potential. Lower abundance and diversity of autotrophic bacteria were observed in nutrient-poor depositional sites compared with nutrient-rich eroding sites. However, the relative abundances of obligate autotrophic bacteria, such as Thiobacillus and Synechococcus, were significantly enhanced in depositional sites. Deposition of nutrient-poor soil contributed to the growth of obligate autotrophic bacteria. The maximum microbial C-fixing rate was observed in DS1 site (5.568u2009±u20091.503xa0Mgxa0Cxa0km−2xa0year−1), followed by DS3 site (5.306u2009±u20092.130xa0Mgxa0Cxa0km−2xa0year−1), and the minimum was observed in ES2 site (0.839u2009±u20090.558xa0Mgxa0Cxa0km−2xa0year−1). Soil deposition significantly enhanced microbial C-fixing rate. Assuming a total erosion area of 1.09u2009×u2009107xa0km2, microbial C-fixing potential in eroded landscape can range from 0.01 to 0.06xa0Pgxa0Cxa0year−1. But its effect on the C pool recovery of degraded soil is limited. Dissolved organic C (DOC) was the main explanatory factor for the variation in soil microbial C-fixing rate (72.0%, Pu2009=u20090.000).