Nitrogen-Fixing Tree Mulch Increases Labile and Stable Soil Carbon Fractions in an Agroforestry System.

Pruning of shade trees in agroforestry systems provides an opportunity to recycle organic matter and nutrients. We investigated changes in soil organic carbon (C) fractions due to additions of chipped pruning residues (mulch) of the multipurpose nitrogen (N)-fixing tree Leucaena var. KX2 in a coffee agroforestry system in Hawaii, USA. Annual mulch additions averaged 1 kg C m^-2 and 17 g N m^-2 over 3 years. Coarse and fine particulate organic matter (POM) C initially comprised ~60% of the total soil C, and resistant silt+clay C made up 22%. After 3 years of mulch addition, all soil C fractions except coarse POM increased significantly. There was a significant decrease in the fraction of soil C in the coarse POM fraction and a significant increase in the proportion in the fine POM fraction. There were no significant changes in soil C fractions in the no-mulch treatment. This study demonstrates the capacity for significant increases in soil C sequestration with management of shade trees in agroforestry systems after only 2-3 years. Increases in more stable soil C fractions and a lack of change in the no-mulch treatment suggest gains can be sustained, even if management pratices change, such as use of pruning residues for animal fodder, fueldwood, or use as mulch in adjacent fields.