Litterfall mass,nutrient contents,and nutrient release characteristics of typical forests in Jinyun Mountains of China under the background of acid rain

A field observation was made on the litterfall mass and nutrient release of four types of forest stand(mixed conifer and broad-leaved forest,evergreen broad-leaved forest,bamboo forest,and shrub) in the Jinyun Mountains of China,and a simulated acid rain leaching experiment with the litters of the four forest stands was conducted in laboratory.The thickness of the existing litters in the four forest stands was 1.4-4.5 cm,and had an obvious hierarchical structure.Across the four forest stands,the litter reserve in un-decomposed layer(U layer),semi-decomposed layer(S layer),and decomposed layer(D layer) was 1.97-8.59,2.84-10.18,and 3.58-17.43 t·hm-2,respectively,the annual litterfall mass was 2.17-9.86 t·hm-2·a-1,the total litter reserve was 14.21-32.42 t·hm-2,and the annual litter decomposition rate was 0.15-0.31.It needed more than 10 years to decompose 95% of the litters in mixed conifer and broad-leaved forest and bamboo forest,showing that the litter decomposition rate in the two forest stands was relatively slow.The main elements in litter layer were Ca and N,followed by Fe,K,and Mg.The release rate of nutrient elements in the litters of different forest stands was in the sequence of evergreen broad-leaved forest(0.80) shrub(0.72) mixed conifer and broadleaved forest(0.50) bamboo forest(-0.17).As compared with those in leaves,the N,K,and Mn contents in litter layer had an obvious decrease.In order to approach the action mechanisms of acid rain on the nutrient cycling,the ion contents in simulated acid rain and the base ion contents in litter leachates were analyzed,which showed that the correlation coefficient between the contents of different elements in the simulated acid rain and in the litter leachates was in the order of bamboo forest(r = 0.895) mixed conifer and broad-leaved forest(0.826) shrub(0.700) evergreen broad-leaved forest(0.699),suggesting that acid rain had the greatest effects on the leaching of nutrient elements in bamboo forest litters,and the weakest effects on the leaching of nutrient elements in evergreen broad-leaved forest litters.It could be speculated that the main function of the leaching process was the nutrient adsorption by litter.