Carbon sequestration, growth and yield in a Coffea-Leucaena agroecosystem in Hawaii.

摘要

To explore the C sequestration and production potential of shaded Coffea-Leucaena agroforestry systems in Hawaii, different shade levels (full-sun, low and medium) under various pruning frequencies and mulching treatments were applied. Allometric models were developed for Leucaena-KX2 trees to predict above- and belowground biomass and the regrowth potential of pollarded trees, with and without mulch addition. Stem and stump diameter were good predictors of aboveground, belowground, total biomass and regrowth after pollarding. Generalized allometric models tended to over- and/or under predict biomass compared to the site-specific models. Leucaena-KX2 mulch decayed rapidly, followed first-order decay dynamics and provided significant quantities of (N) and organic matter to the soil on an annual basis. There was significant loss from all major biochemical components. Mulch additions from 2006 to 2008 significantly increased soil C and N in the top 20 cm of mineral soil. Carbon sequestration in soil organic matter was half of the C added as mulch over the same period. To estimate the relative stability of soil C pools after mulch addition, soil samples were fractionated by microaggregate isolation, acid hydrolysis and density flotation and dispersion, resulting in five distinct fractions that differed in relative stability. With mulch addition, the fine particulate organic matter fraction increased, and there was a shift in the proportion of soil C to more stable silt+clay fractions. In the no-mulch treatment, there was no significant change in soil C fractions. Given that the turnover time of slow and passive pools is on the order of decades to centuries, the potential benefits of active shade management and mulching compensated for the loss of C sequestration in tree biomass associated by pruning. Active management of Leucaena -KX2 shade in a coffee agroecosystem reduced yield but increased the proportion of larger and thus higher-value beans. For the low-shade treatment, the total value of the coffee yield was similar with the full-sun treatment. Within mechanized management, the low shade treatment may be an economically feasible trade-off between yield and bean size while providing benefits of soil cover, reduced stress, and lower demand for fertilizer or other inputs.