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 organic inputs, a key factor controlling rates of decomposition and nutrient release, continues to guide selection and use of organic materials as nutrient sources. The relationship between decomposition patterns and the quality parameters of the fresh leaves of six agroforestry species: Sesbania sesban, Croton megalocarpus, Calliandra calothyrsus, Tithonia diversifolia, Lantana camara, and Senna spectabilis, was investigated in a litterbag study over a period of 77 days in the highlands of western Kenya. The litterbags were buried 1 cm below the soil surface and covered with soil of ca 1 cm thickness. 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 35 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 exponential function, y(t) = y(o) * e(-kt), from which the specific decay rare constants (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 Calliandra with k(B) = 0.012/d, k(N) = 0.017/d and k(p) = 0.044/d. Lantana had the 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, suggesting 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 total N, lignin (LIG), or Pp. Rates of N and P release followed the general trend: Tithonia > Senna > Lantana > Sesbania > Croton > Calliandra. The results indicated that, among the quality parameters studied, total P is the most important factor controlling race of decomposition and N and P release from organic inputs in the area of study.