Decomposition and nutrient release by green manures in a tropical hillside agroecosystem

The decomposition and nutrient release of 12 plant materials were assessed in a 20-week litterbag field study in hillsides from Cauca, Colombia. Leaves of Tithonia diversifolia (TTH) and Indigofera constricta (IND) decomposed quickly (k=0.035+/-0.002 d(-1)), while those of Cratylia argentea (CRA) and the stems evaluated decomposed slowly ( k=0.007+/-0.002 d(-1)). Potassium presented the highest release rates (k>0.085 d(-1)). Rates of N and P release were high for all leaf materials evaluated (k>0.028 d(-1)) with the exception of CRA (N and P), TTH and IND (P). While Mg release rates ranged from 0.013 to 0.122 d(-1), Ca release was generally slower (k=0.008-0.041 d(-1)). Initial quality parameters that best correlated with decomposition ( P>0.001) were neutral detergent fibre, NDF (r=-0.96) and in vitro dry matter digestibility, IVDMD (r=0.87). It is argued that NDF or IVDMD could be useful lab-based tests during screening of plant materials as green manures. Significant correlations (P>0.05) were also found for initial quality parameters and nutrient release, being most important the lignin/N ratio (r=-0.71) and (lignin+polyphenol)/N ratios ( r=-0.70) for N release, the C/N (r=0.70) and N/P ratios ( r=-0.66) for P release, the hemicellulose content (r=-0.75) for K release, the Ca content (r=0.82) for Ca release, and the C/P ratio ( r=0.65) for Mg release. After 20 weeks, the leaves of Mucuna deerengianum released the highest amounts of N and P (144.5 and 11.4 kg ha(-1), respectively), while TTH released the highest amounts of K, Ca and Mg (129.3, 112.6 and 25.9 kg ha(-1), respectively). These results show the potential of some plant materials studied as sources of nutrients in tropical hillside agroecosystems.