For a long time, farmers in the red soil region of southern China have returned crop residues to the soil, but how various crop residues influence nitrous oxide (N 2 O) emissions is not well understood. We compared the influence of returning different crop residues [rapeseed cake (RC), maize straw, rice straw and wheat straw (WS)] in combination with different levels of nitrogen (N) fertilizer (nil, low and high) on red soil N 2 O emissions. Results confirmed the inverse relationship between cumulative N 2 O emissions and residue C:N ratio in red soil under different levels of N fertilizer. However, Nfertilizer application did not significantly influence N 2 O emissions in the WS (which had the highest C:N ratio) and corresponding control treatments, while it enhanced N 2 O emissions in the RC (which had the lowest C:N ratio) treatment and displayed significantly higher cumulative N 2 O emissions with low N fertilizer application. This phenomenon may be attributed to the poor nutrient content in red soil, which leads to 'Liebig's Law of the Minimum' on available C. N fertilizer application provided sufficient available N, while the readily available C, which was mainly dependent on the degradability of the residue, became the crucial factor influencing N 2 O emissions. Additional experiments, which showed that the addition of glucose and sucrose could increase N 2 O emissions when N (NH þ 4 À N) was sufficient, confirmed this hypothesis. Thus, to reduce N 2 O emissions when returning residues to red soil, we suggest that both the residue C:N ratio and the quality should be considered when deciding whether to apply N fertilizer.

A dual color fluorescence quantitative detection method for Hg 2+ in soil has been developed by synchronous fluorescence analysis based on graphene oxide (GO) and dye-labeled nucleic acids. In this strategy, two complementary dye-labeled single-stranded nucleic acids (ssDNAs) with thymine-thymine (T-T) mismatches and GO were employed. Two dyes 6-carboxyfluorescein (FAM) and 6-carboxy-xrhodamine (ROX) were labeled on 3 0 ends of two different ssDNAs, respectively. In the absence of Hg 2+ , dye-labeled ssDNAs were adsorbed on GO, then the fluorescence of dyes was quenched by GO and the fluorescence signals were very weak. But in the presence of Hg 2+ , two complementary ssDNAs with T-T mismatches formed double-stranded DNA (dsDNA) via the T-Hg 2+ -T coordination structure. The formation of dsDNA resulted in the release of dye-labeled ssDNA from GO, and the fluorescence of dyes was restored. The wavelength intervals between the maximum excitation and emission of FAM and ROX are very close, so the fluorescence signals of two dyes can be obtained simultaneously using synchronous fluorescence analysis. Thus, dual color fluorescence quantitative detection of Hg 2+ can be realized by simultaneously measuring the fluorescence signals of FAM and ROX, respectively. Under optimum conditions, the total fluorescence intensity of FAM and ROX exhibited a good linear dependence on the Hg 2+ concentration in the range from 8 Â 10 À10 to 8 Â 10 À8 mol L À1 . The fitted regression equation was obtained with a correlation coefficient (R 2 ) of 0.9952 and the detection limit (3s) of Hg 2+ was estimated to be 5 Â 10 À10 mol L À1 . This proposed method can use the total fluorescence intensity of FAM and ROX to realize the quantitative detection of Hg 2+ , so the detection sensitivity of the analytical method can be greatly improved.

Transgenic Bacillus thuringiensis (Bt)‐modified plants release Bt toxins into soil and, as a result of worldwide adoption of this technology, concern about their environmental effects has arisen. The sorption of Bt toxin has been studied on four contrasting soils: latosol (Ferralsol), latosolic‐red soil (Ferralsol), red soil (Acrisol) and paddy soil (Anthrosol). Sorption of Bt toxin was also measured on residues of these soils after chemical treatment to remove free (Fed and Ald), amorphous (Feo and Alo) and exchangeable Fe and Al (FeEx and AlEx). The results indicated that the specific surface area (SSA) of soils decreased after the removal of Fe and Al, especially after the removal of Fed and Ald. However, the absence of these three species of Fe and Al did not have a significant effect on the organic matter (OM) content of soil residues when compared with their intact soils. The presence of Fed and Ald, Feo and Alo and FeEx and AlEx increased the sorption of Bt toxin by soil, and the influence of Fed and Ald (with about 46% decrease) was greater than the effect of the other two species of Fe and Al.

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