Chuanhui Gu

Riparian Biogeochemical Hot Moments Induced By Stream Fluctuations

Hyporheic exchanges in riparian zones induced by stream stage fluctuations, referred to as bank storage, can influence contaminant transport and transformation when mixing of groundwater and surface waters with distinct chemical signatures occur, which might lead to a high biochemical activity. The effect of bank storage on nutrient transport was analyzed here using a two-dimensional, variably saturated and multi-species reactive transport model, which accounted for the water flow and solute transport and reactions within riparian zones. After verification with field observations, our model demonstrated that high biogeochemical activities occurred at the near-stream riparian zone during stage fluctuation, a process referred to as bank storage hot moment (BSHM). We used Monte Carlo simulations to study the uncertainty of BSHM and related nutrient dynamics to biogeochemical and hydrological factors. The results indicated that stream fluctuations can lead to maximum bank storage volume ranging from 0 to 259 m3 m1 of stream linear length (median ¼ 9.7 m3 and SD ¼ 53.2 m3). Taking denitrification as an example, BSHM can lead to considerable NO3 removal with a median removal rate of 2.1 g d1 and SD of 17.2 g d1 per meter of stream linear length. The NO3 uptake velocity (median ¼ 2.7 105 and SD ¼ 2.4 104 mmin1) was comparable to that of in-stream transient storage from the literature. This result suggests that BSHM may be a significant process contributing to the nutrient budget at the ecosystem level. Finally, a theoretical framework representing the coupled hydrobiogeochemical controls on riparian hot spots was developed to help predicting when BSHM can become important in a particular stream.

Influence of stream‐groundwater interactions in the streambed sediments on NO3− flux to a low‐relief coastal stream

[1] Water-saturated, organic-rich sediments immediately surrounding a stream channel can provide a protective buffer between streams and adjacent land-based activities by removing plant nutrients from shallow groundwater flowing through them, but the hydrological factors that influence the effectiveness of nitrate removal are not well characterized. A two-dimensional, fully distributed, variably saturated flow and transport model was evaluated for its success in using mechanisms of biological reaction in streambed sediments to quantify nitrate flux into the stream under base flow conditions. The model was used to interpret the observed hydrological dynamics during storms at Cobb Mill Creek, Virginia. During base flow conditions, relatively deep groundwater flow paths carrying water containing high nitrate concentrations discharged through the streambed sediments, and high denitrification rates were observed along with a substantial reduction in the nitrate concentration. During storm events, reduced discharge of groundwater in the face of a diminished hydraulic gradient during passage of a flood wave led to longer residence times for water in the biologically active sediments underlying the stream channel, thus providing an opportunity for enhanced denitrification to further reduce nitrate loads to the stream. We conclude that in cases of low-relief streams with substantial hillslopes adjacent to the stream combined with transmissive sediments, storm events can actually contribute to enhanced removal of nitrate locally (at the hillslope scale) as opposed to a simple lowering of concentration due to dilution.

Effect of freshets on the flux of groundwater nitrate through streambed sediments

The passage of a flood wave over sandy stream sediments can cause changes in the discharge of groundwater to the stream that range from a transient reduction in the discharge rate to a reversal of flow and temporary storage of stream water in the stream sediments. These phenomena were examined in laboratory studies with an intact core and by numerical modeling to assess the effect of both physical and biological processes on the nitrate flux from nitrate-contaminated groundwater to a stream. The balance between the rates of groundwater flow through organic-rich sediments and of denitrification ultimately determines the efflux of nitrate to the stream. The transient impact on nitrate loading suggested by results from the numerical model indicates that high flood stage and low-sediment hydraulic diffusivity can result in excess nitrate loading to surface water following storm events, whereas large-sediment hydraulic diffusivity, regardless of the size of stream stage changes, facilitates greater removal of NO- by denitrification by allowing for a longer residence time of water in the subsurface.

Combined effects of short term rainfall patterns and soil texture on soil nitrogen cycling - a modeling analysis.

Precipitation variability and magnitude are expected to change in many parts of the world over the 21st century. We examined the potential effects of intra-annual rainfall patterns on soil nitrogen (N) transport and transformation in the unsaturated soil zone using a deterministic dynamic modeling approach. The model based on TOUGHREACT [corrected], which has been tested and applied in several experimental and observational systems, mechanistically accounts for microbial activity, soil moisture dynamics that respond to precipitation variability, and gaseous and aqueous tracer transport in the soil. Here, we further tested and calibrated the model against data from a precipitation variability experiment in a tropical system in Costa Rica. The model was then used to simulate responses of soil moisture, microbial dynamics, N leaching, and N trace-gas emissions to changes in rainfall patterns; the effect of soil texture was also examined. The temporal variability of nitrate leaching and NO, NH(3), and N(2)O effluxes were significantly influenced by rainfall dynamics. Soil texture combined with rainfall dynamics altered soil moisture dynamics, and consequently regulated soil N responses to precipitation changes. The clay loam soil more effectively buffered water stress during relatively long intervals between precipitation events, particularly after a large rainfall event. Subsequent soil N aqueous and gaseous losses showed either increases or decreases in response to increasing precipitation variability due to complex soil moisture dynamics. For a high rainfall scenario, high precipitation variability resulted in as high as 2.4-, 2.4-, 1.2-, and 13-fold increases in NH(3), NO, N(2)O and NO(3)(-) fluxes, respectively, in clay loam soil. In sandy loam soil, however, NO and N(2)O fluxes decreased by 15% and 28%, respectively, in response to high precipitation variability. Our results demonstrate that soil N cycling responses to increasing precipitation variability depends on precipitation amount and soil texture, and that accurate prediction of future N cycling and gas effluxes requires models with relatively sophisticated representation of the relevant processes.

Sewage sludge as an initial fertility driver for rapid improvement of mudflat salt-soils

Sewage sludge is by-product in the process of centralized wastewater treatment. Land application of sewage sludge is one of the important disposal alternatives. Mudflats in the interaction zone between land and sea can be important alternative sources for arable lands if amended by large amount of organic fertilizers. Rich in organic matter and other nutrients, sewage sludge has been considered as the economic choice for an initial fertility driver. However, sewage sludge amendment has been greatly hampered due to availability of potential toxic metals. Using sewage sludge in compliance with the national standards for agricultural usage could avoid the accumulation of heavy metals. Nevertheless, it is not clear whether massive input of sewage sludge would increase heavy metals concentration in crops. The objective of this study was to investigate impact of sewage sludge amendment (SSA) as an initial fertility driver by one-time input, with the rates of 0, 30, 75, 150, and 300tha-1, on biomass of green manures, soil chemical properties, and growth and heavy metals uptake of maize (Zea mays L.) grown in mudflat soil. Results showed that one-time sewage sludge amendment promoted an initial fertility for infertile mudflat soil, supported growth of ryegrass as the first season green manure. By tilled ryegrass, it modified the chemical properties of mudflat soil by increasing soil organic carbon, total and available N and P, and decreasing soil salinity and pH, which promoted subsequent growth of two green manures for sesbania and ryegrass. The sewage sludge as an initial fertility driver combined with planting and tilling green manures, increased dry matter of the aerial part and grain yield of maize grown in mudflat soil. Cd and Ni concentrations in grain of maize were positively correlated with sewage sludge amendment rates. Importantly, heavy metal concentrations in grain of maize at all SSA rates did not exceed the safety standard for food in China (GB 2762-2012). The study suggests that sewage sludge can be applied as an initial fertility driver for mudflat soil amendment, which provides an innovative solution for arable land resources and solid waste disposal.

Mudflat soil amendment by sewage sludge: Soil physicochemical properties, perennial ryegrass growth, and metal uptake

Abstract The fast pace of cropland loss in China is causing alarm over food security and China’s ability to remain self-reliant in crop production. Mudflats after organic amendment can be an important alternative cropland in China. Land application of sewage sludge has become a popular organic amendment to croplands in many countries. Nevertheless, the land application of sludge to mudflats has received little attention. Therefore, the objective of the present work was to investigate the impact of sewage sludge amendment (SSA) at 0, 30, 75, 150 and 300 t ha−1 rates on soil physicochemical properties, perennial ryegrass (Lolium perenne L.) growth and heavy metal accumulation in mudflat soil. The results showed that the application of sewage sludge increased organic matter (OM) content by 3.5-fold while reducing salinity by 76.3% at the 300 t ha−1 rate as compared to unamended soil. The SSA reduced pH, electric conductivity (EC) and bulk density in mudflat soil, increased porosity, cation exchange capacity (CEC) and contents of nitrogen (N), phosphorus (P), exchangeable potassium ions (K+), sodium ions (Na+), calcium ions (Ca2+) and magnesium ions (Mg2+) in comparison to unamended soil. There were 98.0, 146.6, 291.4 and 429.2% increases in fresh weight and 92.5, 132.4, 258.6 and 418.9% increases in dry weight of perennial ryegrass at 30, 75, 150, and 300 t ha−1, respectively, relative to unamended soil. The SSA increased metal concentrations of aboveground and root parts of perennial ryegrass (p < 0.05). The metal concentrations in perennial ryegrass were Zn > Cr > Mn > Cu > Cd > Ni, and the metal concentrations in roots were significantly higher than aboveground parts. The metal accumulation in perennial ryegrass correlated positively with sludge application rates and available metal concentrations in mudflat soil. Land application of sewage sludge was proved to be an effective soil amendment that improved soil fertility and promoted perennial ryegrass growth in mudflat soil. However, heavy metal accumulation in plants may cause food safety concern.

Environmental controls on nitrogen and sulfur cycles in surficial aquatic sediments

Enhanced anthropogenic inputs of nitrogen (N) and sulfur (S) have disturbed their biogeochemical cycling in aquatic and terrestrial ecosystems. The N and S cycles interact with one another through competition for labile forms of organic carbon between nitrate-reducing and sulfate-reducing bacteria. Furthermore, the N and S cycles could interact through nitrate (NO3-) reduction coupled to S oxidation, consuming NO3-, and producing sulfate (SO42-). The research questions of this study were: (1) what are the environmental factors explaining variability in N and S biogeochemical reaction rates in a wide range of surficial aquatic sediments when NO3- and SO42- are present separately or simultaneously, (2) how the N and S cycles could interact through S oxidation coupled to NO3- reduction, and (3) what is the extent of sulfate reduction inhibition by nitrate, and vice versa. The N and S biogeochemical reaction rates were measured on intact surface sediment slices using flow-through reactors. The two terminal electron acceptors NO3- and SO42- were added either separately or simultaneously and NO3- and SO42- reduction rates as well as NO3- reduction linked to S oxidation were determined. We used redundancy analysis, to assess how environmental variables were related to these rates. Our analysis showed that overlying water pH and salinity were two dominant environmental factors that explain 58% of the variance in the N and S biogeochemical reaction rates when NO3- and SO42- were both present. When NO3- and SO42- were added separately, however, sediment N content in addition to pH and salinity accounted for 62% of total variance of the biogeochemical reaction rates. The SO42- addition had little effect on NO3- reduction; neither did the NO3- addition inhibit SO42- reduction. The presence of NO3- led to SO42- production most likely due to the oxidation of sulfur. Our observations suggest that metal-bound S, instead of free sulfide produced by SO42- reduction, was responsible for the S oxidation.

Growth characteristics, nutrient uptake, and metal accumulation of ryegrass (Lolium perenne L.) in sludge-amended mudflats

Abstract The objective of this study was to investigate the impact of sewage sludge amendment (SSA), with the rates of 0, 30, 75, 150, and 300 t ha−1, on soil chemical properties and biomass, aboveground and root growth, and nitrogen and phosphorus uptake of ryegrass (Lolium perenne L.) seedling grown on a mudflat soil. Results showed that SSA modified the chemical properties of mudflat soil by decreasing soil salinity and pH, increasing soil organic matter, total and available N and P, and heavy metals. The biomass of root and aboveground parts of ryegrass showed increments with increasing SSA rates. Maximum increases of 5.6- and 1.3-fold in fresh weight of aboveground parts and root of ryegrass were observed at 300 t ha−1 rate. Root grown at all SSA rates showed positive response to higher nutrient availability in the mudflat soil with increasing root average diameter and volume. The root length and surface area at 30 t ha−1 SSA rate was significantly higher than those in other treatments. The sewage sludge amendment increased N and P content in both aboveground and root parts of ryegrass. Cd, Cr, Cu, Mn, Ni, and Zn accumulation in ryegrass was positively correlated with SSA rates. However, heavy metal concentrations in ryegrass did not exceed the safety standard for feed in China (GB 13078-1999), except for Cd at 300 t ha−1 SSA rate. The study suggests that sewage sludge can be applied for mudflat soil amendment, which provides an innovative solution pertaining to new arable land resources and solid waste disposal. The optimum SSA rate is 150 t ha−1 with maximum plant yield yet permissible metal accumulation.

The Effects Of Household Management Practices On The Global Warming Potential Of Urban Lawns

Nitrous oxide (N2O) emissions are an important component of the greenhouse gas (GHG) budget for urban turfgrasses. A biogeochemical model DNDC successfully captured the magnitudes and patterns of N2O emissions observed at an urban turfgrass system at the Richland Creek Watershed in Nashville, TN. The model was then used to study the long-term (i.e. 75 years) impacts of lawn management practice (LMP) on soil organic carbon sequestration rate (dSOC), soil N2O emissions, and net Global Warming Potentials (net GWPs). The model simulated N2O emissions and net GWP from the three management intensity levels over 75 years ranged from 0.75 to 3.57 kg N ha(-1)yr(-1) and 697 to 2443 kg CO2-eq ha(-1)yr(-1), respectively, which suggested that turfgrasses act as a net carbon emitter. Reduction of fertilization is most effective to mitigate the global warming potentials of turfgrasses. Compared to the baseline scenario, halving fertilization rate and clipping recycle as an alternative to synthetic fertilizer can reduce net GWPs by 17% and 12%, respectively. In addition, reducing irrigation and mowing are also effective in lowering net GWPs. The minimum-maintenance LMP without irrigation and fertilization can reduce annual N2O emissions and net GWPs by approximately 53% and 70%, respectively, with the price of gradual depletion of soil organic carbon, when compared to the intensive-maintenance LMP. A lawn age-dependent best management practice is recommended: a high dose fertilizer input at the initial stage of lawn establishment to enhance SOC sequestration, followed by decreasing fertilization rate when the lawn ages to minimize N2O emissions. A minimum-maintained LMP with clipping recycling, and minimum irrigation and mowing, is recommended to mitigate global warming effects from urban turfgrass systems. Among all practices, clipping recycle may be a relatively malleable behavior and, therefore, a good target for interventions seeking to reduce the environmental impacts of lawn management through public education. Our results suggest that a long-term or a chronosequence study of turfgrasses with varying ages is warranted to capture the complete dynamics of contribution of turfgrasses to global warming.

Straw incorporation increases solubility and uptake of cadmium by rice plants

Abstract Straw incorporation is a widespread agricultural practice to enhance soil fertility and to recycle crop residues. However, how straw incorporation affects mobility and plant uptake of heavy metals such as cadmium (Cd) still remains unclear. The effect of straw incorporation on Cd solubility and uptake by rice was investigated in a pot experiment. A Cd-contaminated paddy soil was incorporated with wheat straw at different rates (0, 0.25%, 0.5%, and 1% w/w). Cd and dissolved organic carbon (DOC) concentration in soil solution sampled during the experiment and Cd content in rice plant at maturity stage were assessed for each treatment. The results showed that the straw incorporation decreased pH and increased DOC and soluble Cd in the soil solutions, while rice growth was significantly inhibited by straw incorporation. Straw incorporation enhanced the Cd uptake by rice from plant tissue Cd concentration of 15.41 mg kg−1 (without straw) to 45.1 mg kg−1 (at 1% incorporation rate). Cd accumulated in the rice plant increased by 120% compared to the control. The Cd uptake by rice showed a plateau response, which cannot be explained by soluble Cd alone. Inhibited rice growth at high straw incorporation might decrease Cd uptake. Thus, we conclude that straw incorporation in heavy metal-contaminated rice paddy soil should not be recommended to avoid heavy metal contamination through food chain.