Potential nitrification and factors influencing nitrification in pine forest and agricultural soils in Central Java']Java, Indonesia

Potential nitrification and factors controlling nitrifying activity in tropical pine forest and agricultural soils in Central Java, Indonesia were studied. Soils from three different pine forest sites and a fertilized coffee plantation were sampled monthly during the dry and wet period of 1997-1998. Both organic material (litter [L], and fragmented litter [F]) and mineral soil (M) up to 15 cm depth showed a high potential N-NO3- production. In the growth-medium used, the production of NO3- increased exponentially and was substantially higher than the net NO3- production in laboratory soil incubation experiments. Nitrifying populations assessed using the most-probable-number method were highly abundant in all soil layers, without significant differences between layers. However, net NO3- production in the L and F layers was significantly higher than that found in the M layers. High seasonal and temporal variations were observed for net NO3- production and nitrifying populations. This was most likely explained by soil moisture content, N-NH4+ content and soil pH. Denitrification might also be one of the major factors causing the high temporal nitrification variation due to high potential NO3- reduction and the fact that denitrifying populations were abundant in these soils. The fertilized coffee plantation soil showed nitrifying populations, potential nitrification and nitrifying activity that were significantly higher than that of the three pine forest soils, whereas litter removal activities at one site in a pine forest substantially reduced NO3- production. It is concluded that tropical pine forests and fertilized agricultural soils in Indonesia have a high potential for carrying out nitrification, which is due to a high abundance of nitrifying populations. Soil moisture content, pH and N-NH4+ availability are the major factors controlling nitrifying activity in the soil.