Soil organic matter and nitrogen transformation mediated by plant residues of different qualities in sandy acid upland and paddy soils

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 (Aeric 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 CIN ratios <28:1 and low lignin, and polyphenol contents whereas rice straw had the highest C/N ratio of 79:1. 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 (<1 mm) build-up after one year due to the fact that a large proportion of their residues remained in particulate form (>1 mm). Thus the mixture of groundnut/Sesbania with straw was among those residue treatments that led to the highest soil C (<1 mm) build-up 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 of N 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.