Decomposition and Nutrient Release of Lablab (Lablab purpureus cv. Rongai) and Mucuna Pruriens Leaves During the Dry and Wet Seasons in Puerto Rico.

There is renewed interest in the use of the tropical annual legumes Lablab purpureus cv. Rongai and Mucuna pruriens (dwarf-type) as green manures (environmentally sound N sources) for sorghum baleage.  There is limited information on benefits of the legumes as surfaced applied mulch or disked in Puerto Rico. A study was conducted at the Agricultural Experimental Station of Isabela, University of Puerto Rico. Rongai and dwarf Mucuna were seeded on an Oxisol (Coto series) at 10 kg ha^-1. The objective was to assess seasons (dry and wet) for leaf tissue decomposition and residual N release placed on litterbags on soil surface or incorporated (<15-cm) into the soil.  Biomass ranged from 3785 to 5535 kg ha^-1, with the highest level found for Rongai during the dry season. Decomposition rate (K values) was similar for Rongai and Mucuna (0.10) during the dry seson, but K values were higher for Rongai (0.26) than Mucuna (0.17) during the wet season, indicating a faster decomposition rate for Rongai. N concentration was higher (P<0.05) in the wet season (2.70%) than the dry (2.28%), and higher in mucuna (2.59%) than Rongai (2.38%). N released was highest for Mucuna during the dry season (65 μg g^-1).  Position and season were important for determining the amount of N remaining regardless of the cover crop species, with higher mean value found on surface placed litterbags during the wet season (77 μg g^-1).  Percentage organic matter was also higher for surfaced placed liiterbags during the wet season (75%). This suggest that less N is lost during leaf decomposition on surface placed litterbags during the wet season.  Both legumes offer potential for use as green manures under wet and dry season, and best if surface mulch applied during the wet season to be followed by sorghum planting during the dry season.