Agroforestry C Sequestration

Leucaena KX2 stand

Summary

Agroforestry systems have a much greater inherent C sequestration potential than agronomic systems without trees. Increased plant, litter, and soil C can be sequestered in components with short to long turnover times. This may be enhanced through the incorporation of N-fixing trees, since soil organic matter has a highly conserved C:N ratio. Optimizing crop productivity with C sequestration requires the appropriate selection of crop and tree species and management practices.

Leucaena root system

We have chosen to investigate the C sequestration and productivity potential of a shade-coffee agroforestry system. The tree shade is provided by a hybrid of Leucaena leucocephala developed at the University of Hawaii. The hybrid grows fast and responds well to pollarding, resprouting vigorously after cutting. We have underplanted coffee seedlings within an existing leucaena stand and out in the open in an adjacent field. Pruning frequency is also being manipulated to determine the optimal trade-off between coffee growth and net ecosystem C sequestration. Leucaena prunings are being chipped and mulched back into the shade plots and in half of the open-grown coffee plots to investigate a "cut-and-carry" system for weed control, soil improvement, and C sequestration. Measurements are being taken of C pools and flux rates from plants and soils.

Effects of leucaena mulch

Early results indicate much higher soil C and nutrients underneath the leucaena stands, but mulch additions to open-grown plots can also improve soil C and N. This translates into improved growth and nutrition of the coffee plants. Under shade, soil temperature is lower, mulch decay is faster, and soil respiration rates are higher. Pollarding of the trees leads to a temporary "shock" to the coffee seedling leaves that developed under shade.

Publications

Youkhana A and Idol TW. Tree pruning residues increase soil C and N in shade and full sun coffee agroecosystems in Hawaii. in preparation.