Long-term phosphorus fertilization effects on the litter dynamics of an age sequence of Pinus elliottii plantations in the southern Cape of South Africa

Abstract

The effects of phosphorus fertilization and its duration of impact on litter dynamics were investigated in an 8-, 20- and 25-year-old Pinus elliottii (Engelm.) plantation age sequence. Each stand contained plots fertilized with 30–60 kg ha −1 superphosphate at establishment and an equal number of control (unfertilized) plots. At the oldest stand there were additional variables, namely timing of fertilizer application and fertilization frequency. Annual litterfall rates and the mass of pine litter accumulated on the forest floor were significantly higher in the fertilized plots. Fertilization also increased the age of peak litterfall rates. The significance of litterfall and estimated decomposition constants in controlling litter accumulation was tested. At the youngest stand (8 years old) only litterfall contributed to litter accumulation, whereas in the 25-year-old stand, both litterfall and decomposition were significant factors. Contrary to predictions that phosphorus fertilization increases decomposition rates, lower decay constants were found in the fertilized plots in spite of the higher phosphorus content of the litter. These lower decay constants were ascribed to the lower nitrogen concentration of the fertilized litter which in turn was due to the greater extent of nitrogen retranslocation from needles of the fertilized trees. The extent of phosphorus retranslocation decreased with phosphorus fertilization which also meant reduced phosphorus use efficiency in the fertilized stands. The aspects of litter dynamics shown in this study to be affected by phosphorus fertilization (especially where there are adverse effects) should be considered when recommending a fertilization regime aimed at maintaining long-term productivity in pine plantations.