Nitrogen and phosphorus release from decomposing leaves under sub-humid tropical conditions

For many soils of the tropics, inputs of organic materials are essential to sustain soil fertility and crop production. Research in the quality of org anic inputs, a key factor controlling rates of decomposition and nutrient r elease, continues to guide selection and use of organic materials as nutrie nt sources. The relationship between decomposition patterns and the quality parameters of the fresh leaves of six agroforestry species: Sesbania sesba n, Croton megalocarpus, Calliandra calothyrsus, Tithonia diversifolia, Lant ana camara, and Senna spectabilis, was investigated in a litterbag study ov er a period of 77 days in the highlands of western Kenya. The litterbags we re buried 1 cm below the soil surface and covered with soil of ca 1 cm thic kness. Percent leaf mass and total N and P that remained with time strongly correlated with total P and C/P ratio (R-2 = 0.60-0.90) during the first 3 5 days of study; bur afterwards, correlation was stronger with the initial soluble polyphenolics (Pp)/P ratio (R-2 = 0.69-0.92) than with coral P and CIP ratio. Loss of leaf mass and release of N and P followed the exponentia l function, y(t) = y(o) * e(-kt), from which the specific decay rare consta nts (k) were calculated for loss of leaf mass (k(B)) and release of N (k(N) ) and P (k(p)). Among the plant species, the k values were lowest in Callia ndra with k(B) = 0.012/d, k(N) = 0.017/d and k(p) = 0.044/d. Lantana had th e highest k values with k(B) = 0.067/d and k(p) = 0.119/d, but the highest k(N) value of 0.109/d occurred in Tithonia. The k(B) values for all organic materials were lower than their corresponding k(N) and k(p) values, sugges ting that leaching of N and P from litters may have augmented the microbial mineralization of N and P. There was a strong correlation between the k(B) , k(N), and k(p) values and coral P (r = 0.82-0.36; 1)< 0.01), bur not tota l N, lignin (LIG), or Pp. Rates of N and P release followed the general tre nd: Tithonia > Senna > Lantana > Sesbania > Croton > Calliandra. The result s indicated that, among the quality parameters studied, total P is the most important factor controlling race of decomposition and N and P release fro m organic inputs in the area of study.