Seasonal changes in soil phosphorus and associated microbial properties under adjacent grassland and forest in New Zealand

Abstract

Land-use change from grassland to short rotation plantation forest can have significant impacts on soil nutrient dynamics and microbial processes. Seasonal dynamics of soil phosphorus (P) and associated microbial properties were investigated in upper (0-5 cm) soils under adjacent unimproved grassland and a 19-year-old forest stand (mixture of Pinus ponderosa and Pinus nigra). Afforestation of grassland ameliorated upper soil moisture and temperature regimes, resulting in reduced but less variable soil moisture beneath forest, and reduced temperature extremes, with soil being cooler under forest in summer, but warmer in winter. Results from this study showed that levels of soil organic carbon (C), total nitrogen (N) and organic P fractions under grassland were consistently higher, but levels of inorganic P fractions (bicarbonate extractable Pi and total Pi), microbial biomass C and P, and phosphatase enzyme activities were lower compared with forest over all seasons. Similar seasonal patterns of soil P fractions under grassland and forest demonstrated that labile organic P was mineralized by increasing microbial activity to meet increasing plant demand in spring and summer, but accumulated as a result of increased organic inputs, slower plant growth and low microbial activity in late autumn and winter. It was concluded that P recycling was mainly driven by plant P demand and sustained by root litter inputs in the grassland ecosystem and leaf litter inputs in the forest ecosystem. Seasonal changes in environmental conditions (rainfall, soil moisture and temperature) influenced microbial processes involved in P cycling. Microbial biomass plays a pivotal role in P cycling. Annual release of P through microbial biomass was higher in the forest soil (16.1 kg ha-1) than in the grassland soil (13.9 kg ha-1). The turnover rate of biomass P was also higher in the forest soil (1.28 per year) than in the grassland soil (0.80 per year). In addition, abundant C and P (particularly labile forms) and high microbial and enzyme activities found in the forest floor highlight the importance of the forest floor in P cycling.