Nitrogen mineralization of Hevea leaf litter in three rubber growing soils (proceedings

Abstract

Mineralization of annual leaf fall is considered as an important source of N in Hevea plantations. The objectives of this study were to quantify the N pool in the leaf litter of two Hevea clones with different canopy architectures, and to determine the rates and pattern of N release from Hevea leaf litter in three major rubber growing soils. Two //eveaclones (RRIC 121 and RRISL 203) planted in 1992 were selected to quantify the N pool in the leaf litter. Leaf litters under each clone were collected manually and the dry mater and total nitrogen contents were determined. Soils from three rubber growing areas were used for N mineralization study. Soils (air-dried and 2-mm sieved) from each group were packed into cores to a bulk density of 1.3 Mg m'3. Half of the cores of a given soil group were mulched with 90 mg of ground leaf litter. The soil cores were incubated under controlled conditions and NH4+-N and N 0 3'-N contents were determined at the end o f 2, 4, 8 and 12 weeks of incubation. The N pool in the leaf litter were significantly different between the two clones (25.7 and 13.6 Kg N ha''for RRISL 203 and RRIC 121, respectively). The SMN contents in all three soils increased during the first eight weeks of incubation, reached a maximum and then the rates of increase were slowed down. However, the highest mineralization was observed in Kegalle soils and the lowest in Monaragala soils through out the incubation period, possibly due to the differences in clay content in the three soils. The SMN contents were significantly higher in residue added soils throughout the incubation period. The largest residue effect was observed at about 8 weeks after incubation. On the average, SMN content was higher in residue added soils by about 9.0 mg kg'1 during the entire period. The N release from the decomposition o f added residue was not influenced by the soil type.