The regulatory role of endogenous iron on greenhouse gas emissions under intensive nitrogen fertilization in subtropical soils of China.

Anaerobic batch experiments were conducted to study the regulatory role of endogenous iron in greenhouse gas emissions under intensive nitrogen fertilization in subtropical soils of China. Fe2+, Fe3+, and NO3--N dynamics and N2O, CH4, and CO2 emissions, as well as the relationships between N fertilizer, endogenous iron, and greenhouse gas emissions were investigated. The emissions of N2O increased to different extents from all the test soils by N1 (260 mg N kg-1) application compared with N0. After 24 days of anaerobic incubation, the cumulative emissions of N2O from red soils in De'an (DR) were significantly higher than that from paddy soils in De'an (DP) and Qujialing (QP) under N1. However, N application enhanced CH4 and CO2 emissions from the red soils slightly but inhibited the emissions from paddy soils. The maximal CH4 and CO2 emission fluxes occurred in DP soil without N input. Pearson's correlation analysis showed that there were significant correlations (P < 0.01) between Fe2+ and Fe3+, NO3--N, (N2O + N2)-N concentrations in DP soil, implying that Fe2+ oxidation was coupled with nitrate reduction accompanied by (N2O + N2)-N emissions and the endogenous iron played a regulatory role in greenhouse gas emissions mainly through the involvement in denitrification. The proportion of the electrons donated by Fe2+ used for N2O production in denitrification in DP soil was approximately 37.53%. Moreover, positive correlations between Fe2+ and CH4, CO2 were found in both DR and QP soils, suggesting that endogenous iron might regulate the anaerobic decomposition of organic carbon to CH4 and CO2 in the two soils. Soil pH was also an important factor controlling greenhouse gas emissions by affecting endogenous iron availability and C and N transformation processes.

Effects of nitrogen fertilization on the acidity and salinity of greenhouse soils.

A greenhouse pot experiment was conducted to study the effects of conventional nitrogen fertilization on soil acidity and salinity. Three N rates (urea; N0, 0 kg N ha(-1); N1, 600 kg N ha(-1); and N2, 1,200 kg N ha(-1)) were applied in five soils with different greenhouse cultivation years to evaluate soil acidification and salinization rate induced by nitrogen fertilizer in lettuce production. Both soil acidity and salinity increased significantly as N input increased after one season, with pH decrease ranging from 0.45 to 1.06 units and electrolytic conductivity increase from 0.24 to 0.68 mS cm(-1). An estimated 0.92 mol H(+) was produced for 1 mol (NO2 (-) + NO3 (-))-N accumulation in soil. The proton loading from nitrification was 14.3-27.3 and 12.1-58.2 kmol H(+) ha(-1) in the center of Shandong Province under N1 and N2 rate, respectively. However, the proton loading from the uptake of excess bases by lettuces was only 0.3-4.5 % of that from nitrification. Moreover, the release of protons induced the direct release of base cations and accelerated soil salinization. The increase of soil acidity and salinity was attributed to the nitrification of excess N fertilizer. Compared to the proton loading by lettuce, nitrification contributed more to soil acidification in greenhouse soils.

Nitrate source apportionment in a subtropical watershed using Bayesian model.

Nitrate (NO3-) pollution in aquatic system is a worldwide problem. The temporal distribution pattern and sources of nitrate are of great concern for water quality. The nitrogen (N) cycling processes in a subtropical watershed located in Changxing County, Zhejiang Province, China were greatly influenced by the temporal variations of precipitation and temperature during the study period (September 2011 to July 2012). The highest NO3- concentration in water was in May (wet season, mean±SD=17.45±9.50 mg L(-1)) and the lowest concentration occurred in December (dry season, mean±SD=10.54±6.28 mg L(-1)). Nevertheless, no water sample in the study area exceeds the WHO drinking water limit of 50 mg L(-1) NO3-. Four sources of NO3(-) (atmospheric deposition, AD; soil N, SN; synthetic fertilizer, SF; manure & sewage, M&S) were identified using both hydrochemical characteristics [Cl-, NO3-, HCO3-, SO42-, Ca2+, K+, Mg2+, Na+, dissolved oxygen (DO)] and dual isotope approach (δ15N-NO3- and δ(18)O-NO3-). Both chemical and isotopic characteristics indicated that denitrification was not the main N cycling process in the study area. Using a Bayesian model (stable isotope analysis in R, SIAR), the contribution of each source was apportioned. Source apportionment results showed that source contributions differed significantly between the dry and wet season, AD and M&S contributed more in December than in May. In contrast, SN and SF contributed more NO3- to water in May than that in December. M&S and SF were the major contributors in December and May, respectively. Moreover, the shortcomings and uncertainties of SIAR were discussed to provide implications for future works. With the assessment of temporal variation and sources of NO3-, better agricultural management practices and sewage disposal programs can be implemented to sustain water quality in subtropical watersheds.