Simulated Warming Increases Litter Decomposition and Release Rates of Some Metallic Elements and Recalcitrant Components in Different-Aged Chinese Fir Plantations

With global warming, understanding the effect of elevated temperature on the decomposition of Chinese fir needle litter has significant implications for nutrient cycling, yield, and management of economically important Chinese fir plantations. We conducted simulated warming decomposition experiments in incubators at 25 °C, 30 °C, and 35 °C on Chinese fir needle litter from middle-aged, mature, and overmature stands. Changes in litter mass and concentrations of some metallic elements and recalcitrant components were measured in litter sampled at different decomposition time-steps up to 264 days (d). Warming to 35 °C significantly increased the mass loss rate of needle litter from overmature stands throughout the experiment (except at 72 d). The effect of warming on litter mass loss rate for middle-aged and mature stands was lower and is attributed to higher litter quality in these stands. Compared to 25 °C, warming to 30 °C and 35 °C increased the needle litter decomposition rate across all developmental stages by 17.3% and 48.3%, respectively. Potassium (K), calcium (Ca), and magnesium (Mg) were mostly released during needle litter decomposition in all Chinese fir developmental stages. Lignin, condensed tannins, total phenols, and cellulose were enriched in needle litter, while the release of hemicellulose from near the start of the decomposition experiment was attributed to its lower molecular weight compared with other carbohydrates in litter. Compared with 25 °C, warming to 35 °C increased the release rates from litter of K, Ca, and Mg by 14.7%, 24.6%, and 21.5%, and the release rates of lignin, total phenols, cellulose, and hemicellulose by 7.5%, 8.8%, 10.4%, and 13.7%. Needle litter iron (Fe), aluminum (Al), and sodium (Na) in different development stages and manganese (Mn) in the overmature stands were mostly enriched during the experiment. Warming significantly promoted the enrichment of Fe, Al (except for mature stands), and Na, and reduced the enrichment of Mn. In summary, the sensitivity of needle litter to temperature in overmature stands is higher than that in middle-aged and mature stands, suggesting that forest managers can extend the rotation length of Chinese fir plantations to increase the yield of large-diameter timber, litter decomposition, and ecosystem nutrient return.