Effects of short-term global warming and precipitation reduction on the quantity and spectral characteristics of soil DOM in Cunninghamia lanceolata plantation
As a small but reactive soil organic matter (SOM) pool, dissolved organic matter (DOM) is considered to be a sensitive indicator of the dynamic of SOM and an important component of terrestrial biogeochemical cycle. The quality and quantity of DOM are crucial in the dynamics of C, N and other nutrients in forest soils because these attributes are closely related to carbon accumulation in soil and nutrient availability to microorganisms and plants. Global warming has been widely recognized and has induced drastic changes in global precipitation patterns. Because temperature and precipitation are the two most important environmental drivers regulating forest SOM cycle, it is critical that we have a solid understanding of the response of soil DOM to such climatic changes. In this study, we conducted a short-term experiment in a Cunninghamia lanceolata plantation in subtropical China that stimulated soil warming and precipitation decline. The objective of the study was to address the effects of the two factors (temperature and precipitation) on the quantity and composition of DOM. The experimental design was a randomized complete block factorial design, with warming and precipitation as fixed factors. The four treatments (each replicated three time) were set up in the experiment included no warming and natural precipitation (CT), warming and natural precipitation (W), no warming and reduced precipitation (P), warming with reduced precipitation (WP). We found that: 1) W treatment increased the quantity of labile soil dissolved organic carbon (DOC), but decreased the aromaticity and humification degree of DOM. The 0-10 cm soil layer contained more alkanes and little esters, while the 10-20 cm soil layer had more carbohydrates. 2) P treatment resulted in a smaller soil water content, which limited DOC production. In the 0-10 cm soil layer, soil DOM was apparently less humified and less condensed, with much alkanes present. However, in the 10-20 cm soil layer, aromaticity and humification degree were enhanced, while the quantity of carbohydrates decreased. In addition, precipitation decline also improved the quantity of DON. 3) The combination of warming climate and declining precipitation increased the concentrations of DOC and DON, and reduced the aromaticity index and humification degree. The 0-10 cm soil layer had relatively more carbohydrates than the 10-20 cm soil layer. 4) In the 0-10 cm soil layer, warming climate was the only factor that significantly influenced the quantity and quality of DOM. With increasing soil depth, precipitation decline became the second significant factor influencing soil DOM structure. Our results provided a deeper insight into the dynamic changes of soil DOM under global warming and declining precipitation, which was helpful in more accurately predicting soil C and N cycle in response to future global climate change.