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This study investigated organic matter and nitrogen reduction and transformation mechanisms within a field-scale hybrid natural purification system. The system included an oxidation pond, two serial surface-flow wetlands with a cascade in between, and a subsurfaceflow wetland receiving secondary treated dormitory sewage. The average biochemical oxygen demand (BOD) and chemical oxygen demand (COD) removal was 81 and 48%, respectively. Microbial degradation was the primary process contributing to organic reduction. Total Kjeldahl nitrogen (TKN) and ammonium decreased from 7.1 to 3.9 and 5.58 to 3.25 mg/L, respectively, within the surface-flow wetlands. The results indicated that nitrification occurred within the aerobic compartments. The nitrate levels continued to decrease from 1.26 to 1.07 mg/L, indicating nitrate reduction occurred in the surface-flow wetland. Total nitrogen decreased from 8.61 to 5.12 mg/L, equivalent to a 41% reduction, within the surface-flow wetlands. Results revealed that denitrification might concurrently occur in the compartment of surface-flow wetland. Total nitrogen continued to decrease from 5.12 to 3.99 mg/L within the anoxic subsurface-flow wetlands through denitrification transformation. The significant total nitrogen reduction observed was 65%. The predominant reduction of total nitrogen might take place within the sediment of surface flow and the subsurface-flow wetland where denitrification occurred. The microbial identification results also indicated that nitrification/denitrification might occur concurrently within the sediments of surface-flow wetlands.The results of this study show that hybrid wetland systems are a viable option for organic matter and nitrogen transformation and removal in tropical regions where tertiary wastewater systems are too costly or unable to operate. Treated water from these systems can comply with local surface water criteria rendering water for reuse and groundwater recharge. Water Environ. Res., 82, 27 (2010).

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A significant increase in reactive nitrogen (N) added to terrestrial ecosystems through agricultural fertilization or atmospheric deposition is considered to be one of the most widespread drivers of global change. Modifying biomass allocation is one primary strategy for maximizing plant growth rate, survival, and adaptability to various biotic and abiotic stresses. However, there is much uncertainty as to whether and how plant biomass allocation strategies change in response to increased N inputs in terrestrial ecosystems. Here, we synthesized 3516 paired observations of plant biomass and their components related to N additions across terrestrial ecosystems worldwide.Our meta-analysis reveals that N addition (ranging from 1.08 to 113.81 g m −2 year −1 ) increased terrestrial plant biomass by 55.6% on average. N addition has increased plant stem mass fraction, shoot mass fraction, and leaf mass fraction by 13.8%, 12.9%, and 13.4%, respectively, but with an associated decrease in plant reproductive mass (including flower and fruit biomass) fraction by 3.4%. We further documented a reduction in plant root-shoot ratio and root mass fraction by 27% (21.8%-32.1%) and 14.7% (11.6%-17.8%), respectively, in response to N addition. Meta-regression resultsshowed that N addition effects on plant biomass were positively correlated with mean annual temperature, soil available phosphorus, soil total potassium, specific leaf area, and leaf area per plant. Nevertheless, they were negatively correlated with soil total N, leaf carbon/N ratio, leaf carbon and N content per leaf area, as well as the amount and duration of N addition. In summary, our meta-analysis suggests that N addition may alter terrestrial plant biomass allocation strategies, leading to more biomass being allocated to aboveground organs than belowground organs and growth versus reproductive trade-offs. At the global scale, leaf functional traits may dictate how plant species change their biomass allocation pattern in response to N addition.