Effect of Acid Production from Forest Litter on the Availability of Heavy Metals in Soil

Abstract

Forest leaf litter is an important source of soil nutrients, but how its decomposition products affect the availability of soil heavy metals is not totally clear. In order to understand the effects of leaf litter decomposition on soil heavy metal availability in a forest ecosystem, leaf litter samples were collected from Daluoshan in Wenzhou and the Wuyanling National Nature Reserve of Zhejiang and subjected to analysis. The leaf litter was produced by the forests’ dominant tree species, such as Myrica ruba, Bambusa cerosissima, Pinus sylvestris, Machilus thunbergia, Cunninghamia lanceolata, and Quercus acutissima. Decomposition of the leaf litter samples at room temperature and leaching decomposition of the samples were carried out to analyze the acid production characteristics of the litter samples and their impact on soil heavy metal availabilities. Oxalic acid, lactic acid, fumaric acid, succinic acid, malic acid, and citric acid were the dominant organic acids in the leaf litter decomposer, and the sum of these six organic acids accounted for more than 50% of the total organic acid. During a 30-day litter decomposition, the levels of organic acids basically increased, with significant increases appearing in the early stage. After leaching, the available contents of Cr, Mn, Ni, Cu, Zn, As, Cd, and Pb increased by a maximum of 11.95, 2.33, 12.00, 0.80, 0.83, 0.54, 51.50, and 0.28-fold, respectively, compared with those of the original soil sample. During leaching, the higher the content of low molecular weight organic acids in the leaf litter, the longer the leaching time required and the more obvious the influence on the level of available heavy metals. A stronger leaching effect was found for the soil samples taken from a depth of 0–20 and 20–40 cm accompanied by a more obvious increase in the available heavy metals in the soil. PLSPM results showed that there were significant differences between the soil acidity index and the available heavy metals in Daluoshan (p < 0.05) and that there were significant differences between the soil acidity index and the available heavy metals in Wuyanling (p < 0.01). The decomposition of litter produced low molecular weight organic acids, which caused a decrease in soil pH and an increase in exchangeable H+ and Al3+. Both changes had an impact on soil organic matter and further led to an increase in the activity of heavy metals in the soil. This will further lead to the increase of ecological and environmental risks to forest soil.