Minimisation of N2O emissions from a plant-soil system under landfill leachate irrigation

Photosynthesis of alfalfa (Medicago sativa) in response to landfill leachate contamination

Thousands of unlined landfills and open dumpsites have put great threat on the security of soil and ground water due to leachate leakage. Alfalfa is believed potential as a phytoremediation plant for leachate contamination based on strong root system and the excellent capacity of removing various kinds of pollutants. A lab-scale investigation was conducted to figure out the sensitiveness of alfalfa photosynthesis in response to leachate contamination. The results demonstrated that both of the maximum photosynthetic efficiency (F_v/F_m) and net photosynthetic rate (P_n) were slightly inhibited in the high-dosage group. Based on statistical analysis, higher sensitivity of P_n to leaching parameters than F_v/F_m was observed. There were significant correlations between most of leaching parameters (pH, ammonium and COD) and P_n with correlation coefficients of 0.530, -0.580 and -0.578 (p < 0.01), respectively. Therefore, P_n is potential for acting as an effective indicator for staple leaching characteristics of leachate contaminated soils.

Contributions of nitrification and denitrification to N2O emissions from aged refuse bioreactor at different feeding loads of ammonia substrates

Nitrous oxide (N₂O) is a strong greenhouse gas, and its emissions from microbial nitrification (NF) and denitrification (DNF) are a threat to the environment. In the present study, a combined approach consisting of ¹⁵N stable isotope and molecular biology (qPCR) was used to determine the contributions of autotrophic nitrification (ANF), heterotrophic nitrification (HNF), and DNF to N₂O emissions in laboratory incubations of aged refuse for different ammonia (NH₄⁺-N) loads (200, 400, and 800mg·NH₄⁺-N/kg·aged refuse) and incubation times (2-144h). Experimental results showed that the N₂O emissions increased with the increase in applied amount of NH₄⁺-N substrates. Simultaneous nitrification and denitrification (SND) were demonstrated to be present in the incubations of aged refuse. The results of ¹⁵N stable isotope labelling experiment indicated that NF (54.60%-68.8%) and DNF (83.38%-85.90%) contributed to majority of N₂O emissions in the incubations of 24h and 72h, respectively. The results of functional genes (amoA and nosZ) quantification experiments indicated that the high gene copies of amoA and nosZ were present at 24h and 72h, respectively. The study also demonstrated the utility of a combined stable isotope and molecular biology approach. The approaches not only provide similar inferences about the N₂O emissions, but also enable the determination of relative contributions of ANF, HNF, and DNF to N₂O emissions. The results of the study are important in providing guidance to artificially optimize the operating conditions for alleviating N₂O emissions in aged refuse bioreactors.

A comparison of CH4, N2O and CO2 emissions from three different cover types in a municipal solid waste landfill

High-density polyethylene (HDPE) membranes are commonly used as a cover component in sanitary landfills, although only limited evaluations of its effect on greenhouse gas (GHG) emissions have been completed. In this study, field GHG emission were investigated at the Dongbu landfill, using three different cover systems: HDPE covering; no covering, on the working face; and a novel material-Oreezyme Waste Cover (OWC) material as a trial material. Results showed that the HDPE membrane achieved a high CH₄ retention, 99.8% (CH₄ mean flux of 12 mg C m^-2 h^-1) compared with the air-permeable OWC surface (CH4 mean flux of 5933 mg C m^-2 h^-1) of the same landfill age. Fresh waste at the working face emitted a large fraction of N₂O, with average fluxes of 10 mg N m^-2 h^-2, while N₂O emissions were small at both the HDPE and the OWC sections. At the OWC section, CH₄ emissions were elevated under high air temperatures but decreased as landfill age increased. N₂O emissions from the working face had a significant negative correlation with air temperature, with peak values in winter. A massive presence of CO₂ was observed at both the working face and the OWC sections. Most importantly, the annual GHG emissions were 4.9 Gg yr^-1 in CO₂ equivalents for the landfill site, of which the OWC-covered section contributed the most CH₄ (41.9%), while the working face contributed the most N₂O (97.2%). HDPE membrane is therefore, a recommended cover material for GHG control.

IMPLICATIONS

Monitoring of GHG emissions at three different cover types in a municipal solid waste landfill during a 1-year period showed that the working face was a hotspot of N₂O, which should draw attention. High CH₄ fluxes occurred on the permeable surface covering a 1- to 2-year-old landfill. In contrast, the high-density polyethylene (HDPE) membrane achieved high CH₄ retention, and therefore is a recommended cover material for GHG control.

N₂O emission from a combined ex-situ nitrification and in-situ denitrification bioreactor landfill

A combined process comprised of ex-situ nitrification in an aged refuse bioreactor (designated as A bioreactor) and in-situ denitrification in a fresh refuse bioreactor (designated as F bioreactor) was constructed for investigating N2O emission during the stabilization of municipal solid waste (MSW). The results showed that N2O concentration in the F bioreactor varied from undetectable to about 130 ppm, while it was much higher in the A bioreactor with the concentration varying from undetectable to about 900 ppm. The greatly differences of continuous monitoring of N2O emission after leachate cross recirculation in each period were primarily attributed to the stabilization degree of MSW. Moreover, the variation of N2O concentration was closely related to the leachate quality in both bioreactors and it was mainly affected by the COD and COD/TN ratio of leachate from the F bioreactor, as well as the DO, ORP, and NO3(-)-N of leachate from the A bioreactor.

Ammonia volatilization, N(2)O and CO(2) emissions from landfill leachate-irrigated soils

Effects of leachate addition on ammonia volatilization and N(2)O and CO(2) emissions from two different soils were investigated using the 10-day laboratory incubation method at two levels of moisture content. Ammonia volatilization was dominated by soil pH and only occurred in alkaline clay soil, where 0.26-0.32% of soil ammonia could be lost. The N(2)O emission from the alkaline clay soil was one order of magnitude greater than that from the acidic sandy soil, when either water or leachate was irrigated. Increasing the moisture content from 46% water-filled pore space (WFPS) to 70% WFPS in the alkaline clay soil or the acidic sandy soil by either water or leachate irrigation increased the N(2)O emission by over twofold. The CO(2) emission from each soil sample at the two WFPSs was almost the same. The CO(2) emission from the alkaline clay soil with leachate addition was 72% lower than that from the acidic sandy soil with leachate addition, and 6.7 times higher than that from the alkaline clay soil with distilled water addition. Ammonia volatilization and N(2)O emission under leachate irrigation could be minimized by avoiding the excessively wet condition and by selecting the acidic sandy soil with low organic carbon and total nitrogen content.