Carbon dioxide emissions from selected soils in Zambia : effects of nitrogenous fertilizer and agricultural lime.

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Ngoma, Astridah
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The University of Zambia
Soils can serve as sources or sinks of atmospheric carbon dioxide (CO2) depending on how they are managed. As atmospheric CO2 concentrations continue to rise there is need to find ways of reducing CO2 emissions from soils. This study was conducted to assess the effects of applying agricultural lime and an NPK fertilizer on CO2 emissions from soils and to identify soil properties that have a significant influence on soil organic carbon (SOC) contents and on soil organic carbon mineralization. Soil samples collected from surface horizons of cultivated fields from four Zambia Agricultural Research Stations were used. They were characterized for their particle size distribution, pH, total nitrogen, SOC, plant available P, effective cation exchange capacity (ECEC), and lime requirements. A 14-week laboratory incubation study was conducted to measure effects of treatments on CO2 emissions from the soils. The treatments were: (i) soil alone, (ii) soil with fertilizer (iii) soil with agricultural lime and (iv) soil with fertilizer and agricultural lime. The samples were incubated in 1 dm3 plastic jars at room temperature and CO2 emissions were measured and recorded weekly. Seasonal CO2-carbon emissions across the soils ranged from 378 to 543 mg C/kg soil with a mean of 473 mg C/kg soil. The mean seasonal carbon mineralization rate across soils was 0.043 or 4.3 %. Non statistically significant differences were observed between mean seasonal CO2-C emissions of soils with amendments and those of the control. However, soils with combined inorganic fertilizer and agricultural lime had significantly lower CO2 emissions than soils with inorganic fertilizer alone with seasonal C emission rates of 3.6 % and 4.9 % respectively, and mean residence times (MRTs) of 28 and 20 seasons respectively. The resultssuggest that on acid soils, applying inorganic fertilizer with agricultural lime may be a better soil management practice for reducing CO2 emissions from soils than applying inorganic fertilizer alone. The SOC content was found to be significantly (p < 0.001) and strongly positively correlated (r = 0.93) with the clay content, and moderately positively correlated (r = 0.60) with soil pH. Carbon mineralization rates were highly significantly (p < 0.001) positively correlated (r = 0.88) with the soil C:N ratio. The mean residence time (MRT) of SOC was significantly (p < 0.001) negatively correlated (r =-0.88) with the soil C:N ratio, indicating that soils with high C:N ratios emit more CO2 compared to soils with low C:N ratios. Therefore, applying inorganic N fertilizer to high C:N ratio residues is likely to result in lowering CO2 emission and promote C sequestration in the soil. Key words: Soil organic carbon, CO2 emission, carbon mineralization rate, Mean residence time, inorganic fertilizer, agricultural lime
Soil science. , Inorganic fertilizer--Effect of. , Inorganic fertilizer. , Agriculture lime.