Abstract: We explored the effects of C:N ratio in root exudates of Robinia pseudoacacia plantations on soil nutrient cycling and microbial activity on the Loess Plateau. We collected in-situ soil from the R. pseudoacacia plantations with essentially identical habitat conditions and growing time of 15, 25, 35, and 45 years. By adding root exudates with different C:N ratios (N only, C:N=10, C:N=50, C:N=100, C only) to the soil and using deionized water as a control, we analyzed the effects of C:N ratio of root exudates on the physicochemical properties of elements such as carbon, nitrogen and phosphorus, soil pH, and soil respiration. The results showed that: 1) Organic carbon content was positively correlated with the C:N ratio of root exudates. Soil organic carbon (SOC) decomposition was faster when root exudates C:N=10. Higher C:N ratio of root exudates (C:N=100) could inhibit SOC decomposition, but only C addition had no significant effect on SOC. 2) Different root exudate C:N produced no significant influence on the total nitrogen. The addition of carbon promoted microbial uptake of ammonium nitrogen, while the addition of nitrogen promoted the nitrification of ammonium nitrogen. As the C:N ratio of root exudates increased, soil ammonium nitrogen content decreased. 3) The addition of nitrogen would reduce soil pH and increase soil total phosphorus content. 4) Soil respiration of R. pseudoacacia plantations was positively correlated with the C:N ratio of root exudates. With the increases of C:N ratio, the promoting effect of root exudates on soil respiration at 25 and 35 years R. pseudoacacia plantations was stronger. In conclusion, higher C:N ratio of root exudates will significantly promote the effect on soil respiration of R. pseudoacacia plantations. Our results improved the understan-ding of the root-soil-microbial interactions in forests.

Key words: root exudate C:N, soil property, soil respiration, Robinia acacia plantation, Loess Plateau