College of Tropical Agriculture and Human Resources

About: College of Tropical Agriculture and Human Resources is a based out in . It is known for research contribution in the topics: Population & Soil water. The organization has 513 authors who have published 604 publications receiving 15798 citations.

Tree pruning mulch increases soil C and N in a shaded coffee agroecosystem in Hawaii

Abstract: Agroforestry can increase the sequestration of carbon (C) in soils of tropical agroecosystems through increased litter and tree pruning inputs. Decomposition of these inputs is a key process in the formation of soil organic matter and in nutrient cycling. Our objectives were to study decay of tree pruning mulch and effects on soil C and N in a shaded coffee agroecosystem in Hawaii. Chipped tree pruning residues (mulch) were added to coffee plots shaded with the Leucaena hybrid KX2 over three years. We measured mulch decomposition and nitrogen loss over one year and changes in soil carbon and nitrogen (N) over two years. Mass loss of mulch was 80% over one year and followed first-order decay dynamics. There was significant loss from all major biochemical components. Net N loss from the mulch was positive throughout the entire period. The C:N and lignin:N ratios of the mulch declined significantly over the decomposition period. Mulch additions significantly increased soil C and N in the top 20 cm by 10.8 and 2.12 Mg ha −1 , respectively. In the no-mulch treatment, there was no significant change in soil C or N concentration, but a decline in soil bulk density led to a significant decline in total soil C. Leucaena mulch can provide an important source of organic C and N to coffee agroecosystems and can help sequester C lost as plant biomass during shade tree management.

Manganese Solubility and Phytotoxicity Affected by Soil Moisture, Oxygen Levels, and Green Manure Additions

Abstract: Manganese (Mn) toxicity is common in tropical acid soils, second only to aluminum toxicity. Changes in soil pH and redox potential (Eh) caused by soil moisture conditions and organic amendments certainly affect Mn solubility and toxicity to plants grown in soils with high Mn reserves. Laboratory incubation and greenhouse experiments were conducted to quantify such effects using a high-Mn Oxisol and a moderate-Mn Mollisol from Hawaii, with soybean (Glycine max L. cv. Kahala) being a test crop. The soils were mixed with a green manure (leucaena leaf) at 0, 10, and 20 g kg−1, and were incubated at 24 ± 1°C at 80% of field water holding capacity—equilibrated with air (FC), or N2 gas (N2)—or submerged under 8 cm of water. Soil pH, Eh, and soluble Mn as measured in saturated paste extract, Mehlich-3 and hydroxylamine hydrochloride (NH2OH) solutions were periodically determined over 56 days of incubation. The submergence and N2 treatments increased soil pH towards 7.0 and decreased soil Eh from 600 mV t...