Soil Nutrient Dynamics in Age Sequence of Mixed Plantations of Alder and Cypress in the Hilly Area of Central Sichuan Basin, China

Soil nutrients are essential to ecosystem processes. A field study was conducted in 2004 to study the soil nutrient dynamics in the mixed plantations of alder (Aluns cremastogyne) and cypress (Cupressus funebris) (MPAC) in the hilly area of central Sichuan Basin (HACSB), China. 25 samples in an age sequence of 10-, 15-, 20-, and 25-year-old MPAC and 30-year-old pure cypress plantation (PCP) were selected by the method of spatial-temporal transition and 150 soil samples were collected at 0-15 and 15-30 cm layers to study the soil nutrient dynamics. The concentrations of soil organic carbon (SOC), total nitrogen (N-tot) and available nitrogen (AN) in purplish soil under MPAC increased significantly within initial 15 years (p<0.01 or 0.05) and then declined gradually from 15- to 30-year-old. The concentrations of total phosphorus (P-tot) and total potassium (K-tot) in 0-15 cm layer didn't significantly change (p>0.05) during 30 years, while those of the 15-30 cm layer changed significantly (p<0.05). The concentrations of available phosphorus (AP) and available potassium (AK) in two layers changed with no significance (p>0.05) among five age stages. But the concentrations of SOC, N-tot, AN, P-tot, AP and AK in each age stand significantly declined (p<0.01 or 0.05) with soil depth increase, but the K-tot concentrations declined with on significance (p>0.05) except for 20-year-old stands (p<0.05) because purplish soil is originally rich in potassium. The amounts of alder litter input and it's fixation-N-2 function are the principle reasons for the SOC, N-tot and AN dynamics related to stand ages and soil depths, but the ratios of uptake by the cypress root system and the return by its litters are the main reasons for P-tot dynamics. The results indicated that the plantation age and the change of dominant species composition of MPAC are the basic reasons for soil nutrient dynamics.