Seasonal patterns in decomposition and nutrient release from East African savanna grasses grown under contrasting nutrient conditions

Litter decomposition and nutrient release is one of the key biogeochemical processes that regulate plant productivity and nutrient cycling in African savanna ecosystems. We examined the influence of nitrogen and phosphorus additions on grass decomposition and nutrient release rates in an Acacia savanna ecosystem in central Kenya. Grass was clipped from a factorial nitrogen×phosphorus experiment and decomposed in a common plot that had not received fertilizer. After 20 weeks, including one dry season and one wet season, 50-65% of carbon, 68-75% of nitrogen and 73-83% of phosphorus had been released from the litter. Decomposition was slow in the dry season (mass loss 1-2% wk^-1) compared to the wet season (7-11% wk^-1). Wet season decomposition was more rapid for grasses that had been fertilized with nitrogen, even though tissue nitrogen was not significantly different from the control grass, indicating that factors other than litter nitrogen concentration influenced decomposition rates under nitrogen enrichment. Surprisingly, nutrient loss from decomposing litter was relatively high during the dry season, suggesting a role for dew in leaching nutrients from dry litter. We conclude that seasonal rain and nitrogen addition (but not phosphorus addition) accelerate decomposition of grass litter, but that nutrient leaching during the dry season can be considerable.