Xiaozeng Han

Adsorption of antibiotic ciprofloxacin on carbon nanotubes: pH dependence and thermodynamics.

The environmental risks of antibiotics have attracted increasing research attention, but their environmental behaviors remain unclear. In this study, functionalized carbon nanotubes (CNTs), namely, hydroxylized (MH), carboxylized (MC), graphitized multi-walled CNTs (MG) and single-walled CNTs (SW) were used as adsorbents and ciprofloxacin (CIP) as an adsorbate to investigate the effect of pH and temperature on sorption and desorption processes. Sorption isotherms of CIP were fitted well by Freundlich and Dubinin-Ashtakhov models. Highly nonlinear isotherms of CIP were observed, indicating the highly heterogeneous site energy distribution on CNTs. At all pHs, SW had the highest sorption for CIP due to its largest surface area among all CNTs. Sorption distinction between MH and MC was explained by π-π electron donor-acceptor interactions. For SW, CIP sorption was thermodynamically favorable and endothermic associated with an entropy driven process, while the reverse process occurred for MC and MG. The rearrangement of CNTs bundles/aggregates and covalent bond formation may be responsible for CIP desorption hysteresis on CNTs. Desorption of antibiotics from CNTs may lead to potential exposure, particularly under changing environmental conditions such as temperature and pH.

Soil Organic Carbon Dynamics in Black Soils of China Under Different Agricultural Management Systems

Cultivation can reduce soil organic carbon (SOC) content and lead to soil deterioration, but some agricultural management systems may increase SOC content and soil productivity. This research examined the SOC dynamics during a 50-year cultivation and how long-term agricultural management practices influenced SOC content in a typical black soil (Mollisol) region of P.R. China. The experiments selected four areas with different cultivation periods: uncultivated, five years, fourteen years, and fifty years. In addition, four long-term agricultural managements were initiated in 1992: conventional wheat–soybean rotation, wheat–sweet clover rotation, wheat–soybean rotation with addition of pig manure, and wheat–soybean rotation with addition of wheat straw. The SOC content declined rapidly at early years of cultivation and gradually afterwards. Wheat–soybean rotation with addition of wheat straw or pig manure resulted in a substantial increase in SOC content in 9 years. Thus, proper soil management can improve soil quality and health by increasing SOM content, and mitigate the greenhouse effect by sequestrating carbon dioxide from the atmosphere as indicated by the significant increase of organic carbon content in soil.

Physical and Chemical Characteristics of a Typical Mollisol in China

Abstract Mollisols (called Black soils) in China are distributed primarily in the northeast region, which is one of the three largest Mollisol areas in the world. Black soils are of major agricultural importance in China. This study reports the physical and chemical properties of this soil with use of a typical Black soil profile selected in Heilongjiang Province, China, and standard soil analytical procedures. The soil is characterized with a thick (60 cm) mollic epipedon. The upper layer of the epipedon contains 5.8% organic carbon, and its CEC is 43.7 cmol(+)/kg. The macroaggregate (>0.25 mm) stability of the epipedon is high (between 63 and 68% of the soil sample weight), which provides favorable soil structure and conditions for plant growth. Soil texture is clay loam for all horizons except the upper layer of the mollic epipedon, which is sandy clay loam. Bulk density increases with depth, and total porosity declines with depth, due most likely to the profile distribution of organic carbon (decreasing with depth). Total nitrogen (N) and phosphorus (P) and available water are also larger in the upper epipedon than the lower horizons. Overall characteristics make this soil fertile and productive. The results of this study can be used as baseline data for examining any change in soil properties of the same soils resulting from agricultural management and practices and for comparisons of pedogenic and carbon cycling studies of Mollisols in China or worldwide.

Effects of Long‐Term Continuous Cropping, Tillage, and Fertilization on Soil Organic Carbon and Nitrogen of Black Soils in China

Abstract Cultivation and tillage practices alter soil properties and often lead to decline of soil quality. Adoption of appropriate agricultural management systems, however, may maintain soil productivity. This research examined the effects of long‐term continuous cropping, tillage, and fertilization on soil organic carbon (C) and nitrogen (N) contents of black soils in China. Soil samples from 11‐year tillage, 11‐year continuous cropping, and 16‐year fertilization experiments were analyzed. Soil organic carbon (SOC) and N declined with depth in all treatments. Compared with a wheat‐corn‐soybean rotation, continuous cropping of wheat, corn, or soybean reduced SOC and N contents, particularly SOC content. Continuous cropping of corn reduced SOC more than that of soybean or wheat in topsoil layers. Moldboard plowing significantly reduced SOC and N contents, whereas integrated tillage (i.e., moldboard plow for wheat, deep tillage (subsoiling) for soybean, and rotary tillage for corn) increased SOC and N relative to conventional tillage. Use of chemical fertilizers [N, phosphorus (P), and potassium (K)] along with return of crop residues resulted in a substantial increase in SOC and N in topsoil layers. It is proposed that the best management for maintaining soil productivity in the study area would be crop rotation along with the integrated tillage and addition of crop residues and chemical fertilizers.

Effect of Long-Term Application of Chemical Fertilizers on Microbial Biomass and Functional Diversity of a Black Soil

National Natural Science Foundation of China [40321101]; Ministry of Science and Technology of China [2005CB121105]; Chinese Aca

Nodule Formation and Development in Soybeans (Glycine max L.) in Response to Phosphorus Supply in Solution Culture

Phosphorus (P) is necessary for growth and nitrogen fixation, and thus its deficiency is a major factor limiting legume production in most agricultural soils. The effect of phosphorus supply on nodule development and its role in soybeans (Glycine max L.) was studied in a nutrient solution. Plants were inoculated with Bradyrhizobium japonicum and grown for 35 days in a glasshouse at a day and night temperature of 25℃ and 15℃, respectively. Although increasing P supply increased the concentrations of P and N in the shoots and roots, the external P supply did not significantly affect the P concentration in the nodules, and the N fixed per unit nodule biomass decreased with increasing P supply. The nitrogen content in the shoots correlated well with the P content (r=0.92(superscript **)). At an inoculation level of 102 cells mL^(-1), the P supply did not affect the number of nodules; however, at inoculation levels of 10^(3.5) and 10^5 cells mL^(-1), increasing P supply increased both the number and size of nodules. Irrespective of the inoculation level, increasing P supply increased the nodule biomass relative to the biomass of the host plant. It is suggested that the P deficiency specifically inhibited the nodule development and thereby the total N2 fixation.

Impact of Soil Management on Organic Carbon Content and Aggregate Stability

Abstract Soil cultivation influences organic carbon storage and soil structures. To evaluate the impact of different soil‐management practices on soil organic carbon (SOC) pools and aggregate stability in black soils, SOC in whole soil, various size aggregates, and density‐separated fractions from three long‐term experiments (20 years) was examined. The three soil‐management systems were grassland (GL), bare land (BL), and croplands. The croplands had two treatments: nitrogen and phosphorus fertilizer application (NP) and NP together with organic manure (NPM). The SOC in the 0‐ to 10‐cm layer decreased in the order NPM>GL>NP>BL and also declined with the soil depth. The SOC of GL increased by 9.7% as compared to NP after 20 years of natural vegetation restoration. The SOC of NPM increased by 11% over NP after 13 years of organic manure application. The percentages of water‐stable aggregate (>0.25 mm) (WSA>0.25mm) decreased in the order GL>BL>NPM>NP in the top 0‐ to 20‐cm horizon. WSA>2mm, the most important fraction for carbon (C) storage in GL and NPM, accounted for 33 and 45% of the whole soil for GL in the depths of 0–10 and 10–20 cm, respectively, and 25 and 18% for NPM in the same soil layers. A significant positive correlation was found between the C stored in WSA>2mm and total SOC (r=0.81, P<0.05) and between the mean weight diameters (MWD) of aggregates and total SOC (r=0.78, P<0.05). Water‐stable aggregate0.25–2mm was the largest fraction of WSA>0.25mm, ranging from 54 to 72% for the 0‐ to 10‐cm layer and 46 to 71% for the 10‐ to 20‐cm layer; thus these aggregates would play a major role in soil sustainability as well as the resistance to soil erosion. The organic carbon (OC) of heavy fraction (HF) accounted for 94–99% of the OC in the WSA0.25–2mm, whereas free particulate organic matter (fPOM) and occluded particulate organic matter (oPOM) contributed a minor fraction of the OC in the WSA0.25–2mm, suggesting that C sequestration in HF could enhance the stability of aggregates and C pools in black soil.

Phosphorus Deficiency Delays the Onset of Nodule Function in Soybean

ABSTRACT Effects of phosphorus (P) deficiency on nodulation were examined in soybean grown in nutrient solution for 7 weeks. Increasing P supply increased shoot growth of nitrogen (N2)-fixing plants from week 5 and that of nitrate-fed plant from week 4 after treatment. Nitrogen (N2)-fixing plants had a greater P requirement for maximum growth at week 5. Increasing P supply from 1 to 16 μ M increased N concentration in N2-fixing plants at week 4 but did not affect it from week 5. By contrast, P deficiency increased N concentration in nitrate-fed plants. Increasing P supply improved nodule formation from week 3. Nodule mass was affected more by P supply than nodule number, which, in turn, was affected more than plant growth. However, P supply did not decrease nodule specific N2 fixation from week 5. The results suggest that P deficiency impaired symbiotic N2 fixation through delaying onset of nodule function and decreasing nodule development.

Carbon dioxide emission from black soil as influenced by land-use change and long-term fertilization.

Land‐use change and soil management play a vital role in influencing losses of soil carbon (C) by respiration. The aim of this experiment was to examine the impact of natural vegetation restoration and long‐term fertilization on the seasonal pattern of soil respiration and cumulative carbon dioxide (CO2) emission from a black soil of northeast China. Soil respiration rate fluctuated greatly during the growing season in grassland (GL), ranging from 278 to 1030 mg CO2 m−2 h−1 with an average of 606 mg CO2 m−2 h−1. By contrast, soil CO2 emission did not change in bareland (BL) as much as in GL. For cropland (CL), including three treatments [CK (no fertilizer application), nitrogen, phosphorus and potassium application (NPK), and NPK together with organic manure (OM)], soil CO2 emission gradually increased with the growth of maize after seedling with an increasing order of CK < NPM < OM, reaching a maximum on 17 August and declining thereafter. A highly significant exponential correlation was observed between soil temperature and soil CO2 emission for GL during the late growing season (from 3 August to 28 September) with Q10 = 2.46, which accounted for approximately 75% of emission variability. However, no correlation was found between the two parameters for BL and CL. Seasonal CO2 emission from rhizosphere soil changed in line with the overall soil respiration, which averaged 184, 407, and 584 mg CO2 m−2 h−1, with peaks at 614, 1260, and 1770 mg CO2 m−2 h−1 for CK, NPK, and OM, respectively. SOM‐derived CO2 emission of root free‐soil, including basal soil respiration and plant residue–derived microbial decomposition, averaged 132, 132, and 136 mg CO2 m−2 h−1, respectively, showing no difference for the three CL treatments. Cumulative soil CO2 emissions decreased in the order OM > GL > NPK > CK > BL. The cumulative rhizosphere‐derived CO2 emissions during the growing season of maize in cropland accounted for about 67, 74, and 80% of the overall CO2 emissions for CK, NPK, and OM, respectively. Cumulative CO2 emissions were found to significantly correlate with SOC stocks (r = 0.92, n = 5, P < 0.05) as well as with SOC concentration (r = 0.97, n = 5, P < 0.01). We concluded that natural vegetation restoration and long‐term application of organic manure substantially increased C sequestration into soil rather than C losses for the black soil. These results are of great significance to properly manage black soil as a large C pool in northeast China.

Genotypic variation in phosphorus utilisation of soybean [Glycine max (L.) Murr.] grown in various sparingly soluble P sources

This study compared the utilisation of sparingly soluble P among soybean genotypes selected from 3 geographical regions with contrasting soil pH. Plants of 5 genotypes from each region were grown for 72 days in soil columns supplied with hydroxyapetite (Ca-P), AlPO4 (Al-P), or FePO4 (Fe-P) at a rate of 25 mg P/kg soil. NaH2PO4 (Na-P) was used as control at the same rate. Shoot weights ranged from 2.4 to 5.9 g/plant. On average, the genotypes selected from calcareous soils of north-west China produced the highest shoot biomass whereas those from neutral soils of the north-east region the least. Root biomass and root surface area followed the same trend. In contrast, the root-to-shoot weight ratio was highest in Fe-P and lowest in Na-P while root length in Na-P and Fe-P was greater than in the other P forms. The genotypes from the north-east region had higher P concentration in the shoot than those from the other 2 regions. Total P uptake ranged from 9.0 to 15.9 mg/plant for Na-P, and from 6.3 to 12.4 for the sparingly soluble P. Average total P uptake was the highest for the genotypes from the north-west region, and was greatest in Na-P and lowest in Fe-P. The genotypes from the north-east region displayed the greatest ability to use sparingly soluble P, and those from the north-west the least relative to Na-P. Total P uptake from the sparingly soluble P correlated highly with plant biomass production, N2 fixation and nodulation, and seed P, while the relative P uptake correlated highly with P concentration in shoots but neither correlated with root carboxylate release nor leachate pH. The results suggest that there is a substantial genotypic variation in utilisation of sparingly soluble P, which is related to early vigour, nodulation, and seed P reserve but not to origin site pH or root exudation.