Effects of Different Exogenous C/N Ratios on Nutrient Release Rate during Litter Decomposition in Cunninghamia lanceolata Forests with Different Understory Vegetation Management Measures

The aim of this study was to study the nutrient elements dynamics during litter decomposition in Cunninghamia lanceolata forests with different C/N ratios, so as to provide reference for scientific management of Cunninghamia lanceolata forest. The decomposition bag method was used to observe Cunninghamia lanceolata litter decomposition for 300 days. Four kinds of exogenous C/N treatments were set up: C/N ratio 60.86 (CK), C/N ratio 40.57 (N1), C/N ratio 30.45 (N2), C/N ratio 20.25 (N3). The nutrient contents in the decomposition process of Cunninghamia lanceolata litter in different stands were determined and analyzed. The results were as follows: (1) During the decomposition of Chinese fir litter, the C/N and C/P ratios were lower than the initial value, while the N/P ratio was higher than the initial value; (2) The concentrations of N, P, Ca and Mn were higher in CK treatment, and there were significant differences in the concentrations of P, Ca and Mn among different treatments; (3) During the litter decomposition, C, K, Ca and Mg all showed nutrient release, N showed nutrient enrichment, and P and Mn showed enrichment phenomenon during 180-300 days of decomposition, but on the whole showed nutrient release; (4) Decomposition time had great influence on seven elements (N, C, P, K, Ca, Mg, Mn) and three element ratios (C/N, C/P, N/P). There was a significant negative correlation between N and stand, a significant positive correlation between C/N and stand, a very significant negative correlation between different C/N treatments and Mn, and no significant correlation between other elements and stand and C/N treatments. (5) The retention of understory vegetation was useful for the release of C, K and Mg, while removal of understory vegetation is useful for the enrichment of N, Ca, P and Mn. During the 300-day litter decomposition process, exogenous nitrogen had a low promoting and high inhibiting effect on Cunninghamia lanceolata litter decomposition of , and with the extension of decomposition time, the promoting effect weakened and the inhibiting effect enhanced. Cunninghamia lanceolata litter decomposition required a low C/N ratio and a large demand for phosphorus. The research results could provide scientific basis for the management of Cunninghamia lanceolata forest.