ABSTRACT
Carbon and nitrogen in soils play an important role in the global carbon and nitrogen cycle. The enhancement of ultraviolet radiation (predominantly UV-B) resulting from the depletion of stratospheric ozone has raised significant concern. The effects of UV-B radiation on soil carbon and nitrogen transformation is connected directly to the physical and chemical properties of the soil. In order to clearly understand the effects of soil moisture on UV-B radiation, this study collected soil samples from two paddy fields with different levels of organic matter in a subtropical region of China. The response of the total organic carbon (TOC), dissolved organic carbon (DOC), ammonia nitrogen (NH4+ -N), nitrate nitrogen (NO3- -N) and cumulative net nitrogen mineralization to UV-B radiation under three different moisture gradients (W1=25%, W2=50%, and W3=100%) were monitored in laboratory for 120 h. After this period, the results were compared with a control treatment (CK) and it was found that:the TOC content had significantly decreased under UV-B radiation (p<0.05). From low to high moisture content (W1, W2 and W3), the TOC decreased by 9.9%, 4.5% and 6.3%, respectively for soil with low organic matter (L), and by 10.9%, 5.6% and 6.3%, respectively for soil with high organic matter (H), under UV-B radiation. However, UV-B radiation was found to enhance the DOC content in the soil compared with the CK. Furthermore, the DOC for soil moisture contents under 100% (W3) was higher than for other moisture contents (W1, W2). The measured DOC increased by 21.5% (W1), 9.4% (W2), and 26.3% (W3) for soil with L. In addition, the measured DOC increased by 26.7% (W1), 14.2% (W2) and 33.8% (W3) for soil with H under UV-B radiation after 120 h. Compared with control treatment (CK), UV-B radiation decreased the NH4+ -N content significantly, but there was an increased NO3- -N content. The decrease of the NH4+ -N content was largest for W3 and smallest for W1. The increase in NO3- -N content was largest for W2 and smallest for W1 for the two soil samples under UV-B radiation. UV-B radiation demonstrated an obvious effect on the cumulative net nitrogen mineralization (p<0.05) after 24 h compared with the CK and the effect of different soil moisture treatment was also significant (p<0.05). Overall, light degradation played a major role in the stabilization of soil organic matter, soil moisture, and UV-B radiation could accelerate the loss of soil organic carbon and has a major impact on the transformation of mineral nitrogen in the soil. Therefore, in agricultural production systems, completely bare surfaces should be avoided. For example, paddy rice-upland crop rotation systems could reduce the use of fallow periods.
