RESUMEN
In paddy soils, arsenic (As) stress influences nitrogen (N) transformation while application of N fertilizers during rice cropping affects As transformation. However, specific interactive effects between As and N in flooded paddy soils on As mobility and N availability were unclear. Here, we examined N and As dynamics in flooded paddy soils treated with four As levels (0, 30, 80 and 150 mg kg-1) and three urea additions (0, 4 and 8 mmol N kg-1). Arsenic contamination inhibited diazotrophs (nifH) and fungi but promoted AOA and denitrification genes (narG, nirK, nirS), decreasing dissolved organic N, NH4+-N and NO3--N. Besides, urea application stimulated As- and Fe-reducing bacteria (arrA and Geo) coupled with anammox. On Day 28, the addition of 8 mmol N kg-1 increased total As concentrations in solutions of soils treated with 30 and 80 mg As kg-1 by 2.4 and 1.8 times compared with the nil-N control. In contrast, at 150 mg As kg-1, it decreased the total As concentration in soil solution by 63 % through facilitating As(III) oxidation coupled with NO3--N reduction. These results indicate that As contamination decreases N availability, but urea application affects As mobility, depending on As contamination level.
RESUMEN
Contamination of soil with cadmium (Cd) threatens food safety and human health. In general, crop straws from contaminated soils could accumulate considerable amounts of Cd. The addition of Cd-containing rice straw can have negative effects on soil environment. In this study, straws varying in Cd concentration were added to soil at a rate of 5% (w/w) to investigate the effects of Cd-containing straw on soil Cd dynamics and soil microbial communities. Results showed that large amounts of Cd, especially bioavailable Cd, were released into soil during the decomposition of Cd-containing straws. The addition of straws with 10, 20 and 40 mg kg-1 Cd increased total Cd in soils from 0.31 mg kg-1 to 0.89, 1.39 and 2.09 mg kg-1, respectively, exceeding the screening value of total Cd < 0.4 mg kg-1 for paddy soils of pH 5.5-6.5 according to Chinese Soil Environmental Quality Standards. Moreover, the addition of Cd-containing straw decreased alpha-diversity of bacterial and fungal communities compared to the clean straw. Indeed, changes in soil factors including pH, Eh, dissolved organic C and Cd level jointly reconstructed soil microbial communities. The addition of Cd-containing straw increased the relative abundance of bacterial species Acidobacteria and Proteobacteria but decreased that of Firmicutes. Meanwhile, it increased the relative abundance of fungal species Basidiomycota and Fusarium which were considered Cd-tolerant. This study revealed the potential environmental risk and the variation of microbial communities caused by increasing soil Cd bioavailability after direct application of Cd-containing rice straw to the field.
