Biomass and nutrient accumulation of Piper aduncum and Imperata cylindrica fallows in the humid lowlands of Papua New Guinea

Shifting cultivation with short fallow periods (<3 years), is an important form of land use in the humid lowlands of Papua New Guinea. The secondary forest vegetation is dominated by the shrub Piper aduncum which originates from South America, and Imperata cylindrica grasslands in areas where annual bush fires an common. No information is available on the rate of biomass and nutrient accumulation of these two fallow types. Plots with I! aduncum and I. cylindrica were planted on a Typic Eutropepts and sampled every 3 months for 23 months to assess above ground biomass and nutrient content. Total biomass of imperata was slightly higher than that of piper during the first year, but remained around 23 Mg dry matter (DM) ha(-1) in the second year. Above ground biomass of piper increased linearly, and reached 48 Mg DM ha(-1) at 23 months when three-quarter of the biomass consisted of wood. Growth rates of piper were on average 69 kg DM ha(-1) per day, and increased with higher rainfall. Nutrient content of imperata was 100 kg N, 12 kg P, 62 kg K, 63 kg Ca, 40 kg Mg and 9 kg S ha(-1) at 23 months. The concentration of K and Ca was high in piper leaves but declined over time. At 23 months, piper had accumulated 222 kg N, 50 kg P, 686 kg K, 255 kg Ca, 75 kg Mg, and 24 kg S ha(-1). More than half of the P,K, Ca and Mg was found in the stem (wood) which is removed from the field and used as firewood when farmers slash the fallow. piper biomass (excluding wood) returned about three times more K to the soil than imperata, but differences between total P and S contents were small. For the accumulation of biomass and nutrients, imperata fallows should not exceed 1 year. Piper accumulated large amounts of biomass and nutrients, particular K, which is an important nutrient for root crops that dominate the cropping phase in the shifting cultivation systems of the humid lowlands. <(c)> 2001 Elsevier Science B.V. Ah rights reserved.