Study of Nitrogen mineralization and organic matter content from green manure in saline soil   Yuthchai Anuluxtipun, Somsri Arunin, Manop Tantathemeya and Arunee Yuvaniyama   ABSTRACT        The field experiment was conducted at Ban Thabo, Roi-Et province, Northeast Thailand during 1989-1990. The soil was infertile acid saline. Five species of legume namely Sesbania aculeata, S. speciosa, S. cannabina, S. rostrata and Aeschynomene afraspera were planted as green manure before rice in randomized complete block design with 3 replications at the spacing of 20x10 cm, with application of P and K fertilizer of 22 and 42 kg/ha to green manure crops, plants were incorporated at the age of 60 days, biomass and N accumulation were determined. Rice of KDML 105 was transplanted at the spacing of 50 and 25 kg/ha to rice after transplanting. Soil samples at the depth of 15 cm in open and close system were periodically taken for analysis of NH4+- N.      The results showed that S. rostrata gave highest biomass of 7 t/ha and highest N accumulation of 199 kg N/ha while S. aculeata gave lowest biomass and N accumulation of 1.3 t/ha and 36.54 kg N/ha, respectively. Although the percentage of N content of A. afraspera (3.38%) was highest but the biomass was much lower than that of S. rostrata resulting in low total N accumulation. The total N accumulation of green nanure rang from high to low were S. rostrata, A. afraspera S. cannabina, S. speciosa and S. aculeata, respectively.      Rice yield of S. rostrata as green manure was highest (2.18 t/ha) and significantly different from S. aculeata, S. cannabina, S. speciosa, and the N fertilizer treatments. The lowest rice yield of 1.21 t/ha was obtained from the treatment of without N fertilizer.      Mineralization peak of NH4+- N in S. rostrata were 14.71 mg N/kg soil at 7 days after incorporation in open system, and 27.32 mg N/kg soil 28 days after incorporation in close system. While the NH4+-N from nitrogen fertilizer were 21.81 and 169.74 mg N/kg soil at 6 and 14 days after incorporation in open and close system, respectively. However, in the presence of rice plants, S. rostrata and A. afraspera showed suitable pattern and release adequate of N to rice while NH4+- N from N fertilizer rapidly decreased to a very low level after 28 days of N application in open system resulting in low rice yield compared to those of S. rostrata and A. afraspera. However, both total and distribution of rainfall, and optimum planting date of Sesbania spp. are to be considered to compatible for rainfall pattern are the main factors that limit the biomass and rice yield.        Keywords : N accumulation, mineralization, S. rostrata, green manure