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SOIL & WATER MANAGEMENT & CONSERVATION

Soil Organic Carbon and Carbon Dioxide Emission from an Organically Amended Hawaiian Tropical Soil

Dep. of Natural Resources and Environmental Management, Univ. of Hawaii at Manoa, 1910 East West Rd., Honolulu, HI 96822

^* Corresponding author (Farhat{at}hawaii.edu).

Application of organic manure (OM) to arable lands improves soil tilth. The objectives of this study were to: (i) simulate the effect of OM application rates (0, 168, 336, and 672 kg total N ha^–1) and types (chicken [Gallus domesticus] and dairy manures) on soil organic C (SOC) and CO₂ emissions from a Hawaiian highly weathered tropical soil; and (ii) correlate SOC, CO₂ emissions, and two major soil properties: bulk density (_b) and saturated hydraulic conductivity (K_sat). Measurements of SOC and _b were conducted on samples collected from the top 10 cm of soil tilled before and after manure application, cultivated with sweet corn (‘Super Sweet 10,’ Zea mays L. ssp. mays), and drip irrigated for two consecutive growing seasons. The K_sat values were calculated from infiltration data measured with a tension infiltrometer. The Rothamsted C turnover model was used to simulate SOC and CO₂ emissions. The simulated and measured SOC agreed reasonably during model calibration (r² = 0.93) and validation (r² = 0.90). Results revealed that SOC, CO₂ emissions, and K_sat increased while _b decreased with increasing OM rates. There was no significant effect of OM type. There was a highly significant (P < 0.01) correlation between the measured and simulated SOC and between the measured SOC and the simulated CO₂ emissions. The K_sat values significantly correlated (P < 0.05) with the measured and simulated SOC and the simulated CO₂ emissions. A significant inverse relationship between _b and K_sat was observed. We concluded that, in addition to improving soil aggregation, decreasing _b, and increasing K_sat, OM application to this tropical soil increases SOC pools that contribute to atmospheric CO₂ following tillage and other agricultural practices.

Abbreviations: BIO, microbial biomass • DPM, decomposable plant material • HUM, humified organic matter • IOM, inert organic matter • OM, organic manure • RothC, Rothamsted carbon model • RPM, resistant plant material • SOC, soil organic carbon • SOM, soil organic matter