Soil organic carbon and carbon dioxide emission from an organically amended hawaiian tropical soil

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 CO ₂ emissions from a Hawaiian highly weathered tropical soil; and (ii) correlate SOC, CO ₂ emissions, and two major soil properties: bulk density (ρ _b) and saturated hydraulic conductivity (K _sat). Measurements of SOC and ρ _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 CO ₂ emissions. The simulated and measured SOC agreed reasonably during model calibration (r ² = 0.93) and validation (r ² = 0.90). Results revealed that SOC, CO ₂ emissions, and K _sat increased while ρ _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 CO ₂ emissions. The K _sat values significantly correlated (P < 0.05) with the measured and simulated SOC and the simulated CO ₂ emissions. A significant inverse relationship between ρ _b and K _sat was observed. We concluded that, in addition to improving soil aggregation, decreasing ρ _b, and increasing K _sat;, OM application to this tropical soil increases SOC pools that contribute to atmospheric CO ₂ following tillage and other agricultural practices.