Soil organic phosphorus fraction in pine–oak forest stands in Northeastern Germany

Abstract

Pines ( Pinus sylvestris L.) have been widely used for monoculture forest stands in north-eastern Germany. However, several studies have discussed the need to convert monoculture into mixed forest stands such as pine–oak forest. In this study, we evaluated the impact of 102 years-old pine ( P. sylvestris L.) in monoculture and mixed forest stands of P. sylvestris + Quercus petraea (Matt.) Liebl. of different ages (10, 35, 106 and 124 years-old) on the dynamics of soil organic phosphorus (Po) pools. The study was carried out in the Northern German Lowlands of Brandenburg. Soil samples were taken from forest floor and two mineral soil layers at the depths of 0–10 and 10–20 cm. Different P forms of the sandy soil were obtained by sequential P fractionation, using acid and alkaline extractants. The soil total P (STP) ranged from 100 to 183 mg kg − 1 whereas total organic P (TPo) ranged from 77 to 148 mg kg − 1 . STP was higher in mixed forest stand than in monoculture and decreased with soil depth. The TPo and labile-P in both soil layers increased significantly with increase in age of oak trees. In addition, TPo content was lower in mineral soil compared to the forest floor and accounted for more than 50% of soil total P in the forest stands. The most available-P fraction–labile-P predominated in the oldest pine–oak forest stand (P + O124), accounting for 29% of STP at the 0–10 cm soil depth. The largest P fraction comprised NaOH–Po and represent 62% of STP. Results showed that forest transformation from pure pine monoculture forest into pine-oak mixed forest stands promoted an increase in the TPo and P available. Furthermore, the forms of labile available P increased with age of oak trees, which are capable of maintaining larger fractions of available P under mixed forest stands.