Effects Of Solar Radiation On Soil Organic Matter Stability

Soils are the largest carbon reservoir in terrestrial ecosystems. Its dynamic balance has significant influence on soil fertility and greenhouse gases emissions. Sunlight has been shown to enhance the losses of organic matter from aquatic sediments and terrestrial plant litter, whether sunlight has effects on soil organic matter still remains unknown. Meanwhile, NO3- has strong photo-chemical activity, whether NO3- has a promoting effect on the photolysis of soil organic matter is unclear. Here, we evaluated the influence of solar radiation and nitrate on soil organic matter using the results of experiments carried out under simulated and natural sunlight. We examined the effect of irradiation time, moisture, different soil types and nitrate on the photolysis of soil organic matter. Soils were collected from Xianning(XH, XS), Yangting(YH, YS), Haikou(HLJ) and Qiongshan(HH). The main results are as follows:1) Under 120 h continuous radiation incubation, DOC content in YH, YS soils increased significantly. The production rates of DOC in dry soils were significantly lower than wet soils. The production rates of DTN in dry soils were significantly lower than wet soils. DOM aromaticity and relative molecular weight were significantly decreased under irradiation and moisture further facilitated the process.2) Under 8 d light-dark incubation with different moisture levels, DOC content in soils first increased and then decreased with increasing moisture. DOC content increased most with W2 moisture(W:S was 1:5) and increased least with W1 moisture. DTN content increased with increasing moisture.3) Under 8 d light-dark incubation, soil TC and TN contents decreased with time. The TC decrease rates in dry soils were 168.75 、 17.51 mg kg-1 d-1, which were significantly lower than wet soils(258.75、66.25 mg kg-1 d-1). The decrease rates of TN were 4.06、7.18 mg kg-1 d-1, which were significantly lower than wet soils(8.59、12.7 mg kg-1 d-1). Soil TC/TN ratio decreased and C, N solubility increased with time. Moisture promoted the above process.4) Under 8 d light-dark incubation, increase rates of soil DOC were 2.94~23.96 mg kg-1 d-1. The sequence was HLJ > HH > YS > YH > XH > XS. Generally soils in the Northern China were more likely to photo-produce DOC than in the Southern. The increase rates of soil DOC were 0.36~1.76 mg kg-1 d-1. The NH4+、NO3- contents in soils increased with time and moisture facilitated the production of NH4+. while NO3-、NO2-content decreased with time in wet soils. Under irradiation, WEP, AEP, Fe2+ and Fe3+contents in dry soils increased with time and moisture accelerated the process. Correlation analysis showed that soil Fe2+, Fe3+, NO3- contents had positive correlation with soil DOC and DTN contents.5) Under 8 d light-dark incubation with different nitrate levels, DOC、NH4+ contents in soils increased with increasing levels of nitrate, and moisture promoted the above process. NO2- contents increased most under N1-D、N2-D treatments and least under N3-D、N4-D treatment. WEP contents increased most under N4-D treatment in dry soils. And in wet soils WEP content increased most under N2-W treatment.6) Under outdoor natural solar irradiation, soil DOC increased linearly with time and adding nitrate further promoted the increase of DOC. DOM aromaticity and the relative molecular weight were reduced under solar irradiation and nitrate promoted the process.To sum up, soil TC, TN significantly decreased and DOC, DTN content increased, DOM aromaticity and relative molecular weight significantly decreased under irridiation. Adding moisture and certain amounts of nitrate to soils, the above changes were further promoted. The production of water soluble small molecule organic matter increased the possibility of loss due to leaching and biological utilization. Therefore, increasing vegetation coverage to prevent soil from direct sunlight, avoiding irrigation under strong sunlight and appropriate nitrate fertilizer usage, are likely to be a practical method to stabilize soil carbon.