Substrate quality and soil environmental conditions predict litter decomposition and drive soil nutrient dynamics following afforestation on the Loess Plateau of China

Abstract

To elucidate the effects of land-use types and afforestation age on litter decomposition, soil nutrient dynamics, and their relationship with substrate quality and soil environmental conditions, soil and litter samples were collected from farmland as well as four afforested land-use types (Robinia pseudoacacia: Rps; Caragana korshinskii: Cko; Pinus tabulaeformis: Pta; and Armeniaca sibirica: Asi), with each land-use type having three succession chronosequences (10, 25, and 40 a) in the Gaoxigou catchment area. These twelve afforested lands were converted from similar farmlands, and litterbag experiments were conducted in each land to determine litter decomposition rate (LDR). In addition, the carbon, nitrogen, and phosphorus contents and stoichiometry in soil and litter, soil properties, and litter biomass were determined. The results showed that soil environmental conditions such as soil water content, bulk density, pH, and temperature improved with afforestation age. Soil nutrient contents were higher in afforested lands and increased with afforestation age. The soil organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) were positively correlated with the litter biomass, soil microbial carbon, and soil water content, but were negatively correlated with the soil bulk density, pH, and temperature. The litter OC, TN, and TP contents were mainly affected by the land-use types without being influenced by afforestation age. LDR is the main litter factor affecting soil nutrients, and is significantly influenced by substrate quality and environmental conditions, especially litter TN and N:P ratio, soil water content, and pH. The annual rates of increase for soil OC and TN during the initial (farmland–10a) and middle (10–25 a) periods were significantly higher than those during the later period (25–40 a) in Rps and Cko, but the Pta forest showed a completely opposite trend, which can be explained by a synchronous change in the LDR driven by the soil water content. In addition, the soil OC, TN, and TP contents were positively correlated with the litter TN, TP, and biomass, but had no correlation with the litter OC. The N:P ratio can be used as an indicator to reveal a tight coupling among soil and litter nutrients. Overall, these results provide evidence that litter decomposition in afforestation systems is linked to soil nutrient dynamics, and is mainly limited by substrate quality and environmental conditions.