Wenxu Dong

Methane, Carbon Dioxide and Nitrous Oxide Fluxes in Soil Profile under a Winter Wheat-Summer Maize Rotation in the North China Plain

The production and consumption of the greenhouse gases (GHGs) methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) in soil profile are poorly understood. This work sought to quantify the GHG production and consumption at seven depths (0–30, 30–60, 60–90, 90–150, 150–200, 200–250 and 250–300 cm) in a long-term field experiment with a winter wheat-summer maize rotation system, and four N application rates (0; 200; 400 and 600 kg N ha−1 year−1) in the North China Plain. The gas samples were taken twice a week and analyzed by gas chromatography. GHG production and consumption in soil layers were inferred using Fick’s law. Results showed nitrogen application significantly increased N2O fluxes in soil down to 90 cm but did not affect CH4 and CO2 fluxes. Soil moisture played an important role in soil profile GHG fluxes; both CH4 consumption and CO2 fluxes in and from soil tended to decrease with increasing soil water filled pore space (WFPS). The top 0–60 cm of soil was a sink of atmospheric CH4, and a source of both CO2 and N2O, more than 90% of the annual cumulative GHG fluxes originated at depths shallower than 90 cm; the subsoil (>90 cm) was not a major source or sink of GHG, rather it acted as a ‘reservoir’. This study provides quantitative evidence for the production and consumption of CH4, CO2 and N2O in the soil profile.

Denitrification Rates and Controlling Factors for Accumulated Nitrate in the 0-12 m Intensive Farmlands: a Case Study in the North China Plain

Abstract Denitrification in subsoil (to a depth of 12 m) is an important mechanism to reduce nitrate ( NO 3 − ) leaching into groundwater. However, regulating mechanisms of subsoil denitrification, especially those in the deep subsoil beneath the crop root zone, have not been well documented. In this study, soil columns of 0–12 m depth were collected from intensively farmed fields in the North China Plain. The fields had received long-term nitrogen (N) fertilizer inputs at 0 (N0), 200 (N200) and 600 (N600) kg N ha−1 year−1. Main soil properties related to denitrification, i.e., soil water content, NO 3 − , dissolved organic carbon (DOC), soil organic carbon (SOC), pH, denitrifying enzyme activity (DEA), and anaerobic denitrification rate (ADR), were determined. Statistical comparisons among the treatments were performed. The results showed that NO 3 − was more heavily accumulated in the entire soil profile of the N600 treatment, compared to the N0 and N200 treatments. The SOC, DOC, and ADR decreased with increasing soil depth in all treatments, whereas considerable DEA was observed throughout the subsoil. The long-term fertilizer rates affected ADR only in the upper 4 m soil layers. The ADRs in the N200 and N600 treatments were significantly correlated with DOC. Multiple regression analysis indicated that DOC rather than DEA was the key factor regulating denitrification beneath the root zone. Additional research is required to determine if carbon addition into subsoil can be a promising approach to enhance NO 3 − denitrification in the subsoil and consequently to mitigate groundwater NO 3 − contamination in the intensive farmlands.

Reassessing carbon sequestration in the North China Plain via addition of nitrogen.

Soil inorganic carbon (SIC) exerts a strong influence on the carbon (C) sequestered in response to nitrogen (N) additions in arid and semi-arid ecosystems, but limited information is available on in situ SIC storage and dissolution at the field level. This study determined the soil organic/inorganic carbon storage in the soil profile at 0-100cm depths and the concentration of dissolved inorganic carbon (DIC) in soil leachate in 4N application treatments (0, 200, 400, and 600kgNha(-1)yr(-)(1)) for 15years in the North China Plain. The objectives were to evaluate the effect of nitrogen fertilizer on total amount of carbon sequestration and the uptake of atmospheric CO2 in an agricultural system. Results showed that after 15years of N fertilizer application the SOC contents at depths of 0-100cm significantly increased, whereas the SIC contents significantly decreased at depths of 0-60cm. However, the actual measured loss of carbonate was far higher than the theoretical maximum values of dissolution via protons from nitrification. Furthermore, the amount of HCO3(-) and the HCO3(-)/(Ca(2+)+Mg(2+)) ratio in soil leachate were higher in the N application treatments than no fertilizer input (CK) for the 0-80cm depth. The result suggested that the dissolution of carbonate was mainly enhanced by soil carbonic acid, a process which can absorb soil or atmosphere CO2 and less influenced by protons through the nitrification which would release CO2. To accurately evaluate soil C sequestration under N input scenarios in semi-arid regions, future studies should include both changes in SIC storage as well as the fractions of dissolution with different sources of acids in soil profiles.

Irrigation reduces the negative effect of global warming on winter wheat yield and greenhouse gas intensity

Global warming may exacerbate drought, decrease crop yield and affect greenhouse gas (GHG) emissions in semi-arid regions. However, the interactive effects of increases in temperature and water availability on winter wheat yield and GHG emissions in semi-arid climates are not well-understood. Here, we report on a two-year field experiment that examined the effects of a mean soil temperature increase of ~2 °C (at 5 cm depth) with and without additional irrigation on wheat yield and GHG emissions. Infrared heaters were placed above the crop canopy at a height of 1.8 m to simulate warming. Fluxes of CH4, CO2 and N2O were measured using closed static chamber technique once per week during the wheat growing seasons. Warming decreased wheat yield by 28% in the relatively dry year of 2015, while supplemental irrigation nullified the warming effect completely. Warming did not alter the wheat yield significantly in the relatively wet year of 2016, but supplemental irrigation with no warming decreased the wheat yield by 25%. Warming increased CO2 emissions by 28% and CH4 uptake by 24% and tended to decrease N2O emissions. Supplemental irrigation increased N2O emissions but had little effect on CO2 emissions and CH4 uptake. Evidently, warming and supplemental irrigation had interactive effects on wheat yield, GHG emissions and GHG emissions intensity. Precision irrigation appears to be a means of simultaneously increasing wheat yield and reducing GHG emissions under warming conditions in semi-arid areas.