Influence of decomposition of roots of tropical forage species on the availability of soil nitrogen

Immobilization of mineral N induced by decomposition of roots of four tropical forage species (Stylosanthes guianensis, Centrosema sp., Andropogon gayanus and Brachiaria decumbens) in an Oxisol was studied under laboratory conditions. Root materials had a high lignin content (12–20%) but total polyphenol content was small (<0.8%). Roots, at 2.5 and 5.0 g kg−1, and 10 mg N kg−1 of 15N-labelled ammonium sulphate (20.3 at.%) were thoroughly mixed with the soil which was maintained at field capacity for 117 d. Decomposition of the roots (as monitored by CO2 evolution) was initially rapid and the legume materials (S. guianensis, Centrosema sp.) with their lower C-to-N ratio and lignin content, decomposed more quickly than the grass roots (A. gayanus, B. decumbens). After 8 d of incubation the rate of CO2 evolution decreased and was similar for all root materials. CO2 evolution from the decomposing roots in all cases fitted closely (R2>0.99) a double exponential equation defining two compartments of root carbon of differing susceptibility to decomposition. The equation predicted that between 43% (Centrosema) and 62% (Brachiaria) of root carbon would not be decomposed even at infinite time under incubation conditions. Mineral N in the soil was immobilized rapidly at the start of the incubation, and the immobilization was greatest with the higher rate of application of root material. Although the C-to-N ratio of legume roots was narrower their higher degradability stimulated greater immobilization of soil mineral N than the grass roots. The results are discussed with reference to N immobilization and carbon sequestration in planted pastures of tropical South America.