Tree Pruning Residues Increase Soil Carbon and Nitrogen in Shade and Full-Sun Coffee Agroecosystems in Hawaii

Agroforestry can increase carbon sequestration in agroecosystems through the accumulation of woody biomass and higher inputs of organic material to the soil. Tree pruning to maintain high crop productivity reduces woody biomass accumulation but may increase soil organic matter through decay and humification of pruning residues, especially with N-fixing trees. Currently, there is no information on carbon and nutrient cycling in shaded or open-grown coffee agroecosystems in Hawaii, although both are common. Our objective was to study tree mulch decay and effects on soil C and N in shaded and open-grown coffee agroecosystems. We measured decomposition, nitrogen mineralization, and changes in soil carbon and nitrogen over one year after additions of chipped tree pruning residues (mulch) added to coffee plots in full-sun or shaded with the leucaena hybrid KX2. Mass loss was 80% in the shade system and 63.5% in the full-sun system. Both followed first-order decay dynamics. Net N mineralization was positive throughout the entire period. There was significant loss of all major biochemical components during the mulch decomposition process, but soluble constituents and Soil C and N increased significantly due to mulch additions in shade and full-sun systems. Without mulch addition, there was no change in soil C and N in the full-sun plots, but shaded plots did show significant increases. Even without mulching, the addition of N-fixing trees in coffee agroecosystems can increase both plant and soil C sequestration. Mulching of tree pruning residues may provide an important mechanism to additionally enrich soil organic matter and conserve C and N accumulated as tree biomass. This may also result in improved soil physical properties and nutrient-holding capacity.