INFLUENCE OF DECOMPOSITION OF ROOTS OF TROPICAL FORAGE SPECIES ON THEAVAILABILITY OF SOIL-NITROGEN

Immobilization of mineral N induced by decomposition of roots of four tropical forage species (Stylosanthes guianensis, Centrosema sp., Andr opogon gayanus and Brachiaria decumbens) in an Oxisol was studied unde r laboratory conditions. Root materials had a high lignin content (12- 20%) but total polyphenol content was small (< 0.8%). Roots, at 2.5 an d 5.0 g kg(-1)and 10 mg N kg(-1) of N-15-labelled ammonium sulphate (2 0.3 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. guiane nsis, Centrosema sp.) with their lower C-to-N ratio and lignin content , decomposed more quickly than the grass roots (A. gayanus, B. decumbe ns). After 8 d of incubation the rate of CO2 evolution decreased and w as similar for all root materials. CO2 evolution from the decomposing roots in all cases fitted closely (R-2 > 0.99) a double exponential eq uation defining two compartments of root carbon of differing susceptib ility to decomposition. The equation predicted that between 43% (Centr osema) and 62% (Brachiaria) of root carbon would not be decomposed eve n at infinite time under incubation conditions. Mineral N in the soil was immobilized rapidly at the start of the incubation, and the immobi lization was greatest with the higher rate of application of root mate rial. Although the C-to-N ratio of legume roots was narrower their hig her degradability stimulated greater immobilization of soil mineral N than the grass roots. The results are discussed with reference to N im mobilization and carbon sequestration in planted pastures of tropical South America. (C) 1998 Elsevier Science Ltd. All rights reserved.