Organic matter management is believed to solve many of the chemical and physical problems of coarse-textured, low fertility soils of Northeast Thailand. We tested the influence of different plant residues available in this area on soil C and N dynamics in upland (Oxic Paleustult) and lowland (Aerie Paleaquult) soils. Residues included groundnut (upland) or Sesbania rostrata stover (lowland), rice straw, Tamarindus indica and Dipterocarpus tuberculatus leaves applied at 10 t ha-1 (dry matter). For the former three residues additional application rates of 20 t ha-1 were included as well as a mixture (50:50) of groundnut/Sesbania -rice straw treatment. Groundnut stover and Sesbania had C/N ratios <28: I and low lignin, and polyphenol contents whereas rice straw had the highest C/N ratio of 79: I. Dipterocarp and tamarind leaves were characterized by high lignin (> 17%) and polyphenol (>4.5%) contents. These latter residues, despite slow decomposition, apparently resulted in only moderate soil C «I mm) build-up after one year due to the fact that a large proportion of their residues remained in particulate form (> I mm). Thus the mixture of groundnut/Sesbania with straw was among those residue treatments that led to the highest soil C «I mm) buildup under both upland and lowland conditions.Groundnut stover under upland condition resulted in immediate net N mineralisation but also an early decline in soil mineral N presumably due to leaching. By mixing groundnut or Sesbania with rice straw with a high C/N ratio residue N mineralisation could be delayed and prolonged, improving potentially the synchrony ofN release and plant demand. Additions of dipterocarp and tamarind resulted in an initial N immobilization phase and net mineral N release remained low thereafter. Dynamics of microbial biomass N were closely related to N mineralisation and immobilization cycles in both upland and lowland experiments. Residue N concentration was the most significant factor controlling N release in both systems. While extractable polyphenols exhibited a significant influence on N release in upland conditions their effect was not evident in the lowland.