Purpose The most important factor affecting soil quality is soil humic fraction. Effects of adding carbon in soil humic compounds in arid environments have only been slightly investigated. The change in soil humic acids' structures in relation with manure and palm compost application and the evolution of humic acids' structures during palm composting have not been sufficiently investigated. The purpose of this study is to evaluate if date palm compost is suitable for the sustainable increase of soil carbon content with increasing soil humic compounds. Methods Soil was amended with date palm compost and sheep manure, which represented two treatments, in addition to a control represented by unamended soil. Soil samples were taken from depth layers of 0-20 cm and 20-40 cm. Physicochemical and spectroscopic analyses were used to study humic acids extracted from organic amendment and soil. Results The neutral pH and the C/N ratio of 11.9 revealed that the palm/manure compost was mature. Date palm compost application on soil showed its effectiveness in improving soil quality. Spectroscopic studies of humic compounds extracted from date palm compost and manure, showed the abundance of aromatic structures, confirmed by the low E4/E6 ratio associated with an increase in polycondensation and polymerization. Infrared spectroscopy of humic acids extracted from amended soil revealed its enrichment with aromatic structures. Conclusions Using date palm compost for a short term suggests that this compost may be useful as an alternative to increase the soil fertility by increasing the humic acids quantity with enhancing aromaticity degree and functional groups.

The changes of soil organic matter (SOM) humification induced by long-term combination of tillage and olive mill wastewater (OMW) application compared to natural and cultivated soil have been little investigated. This study aimed to compare effects of no cultivation with natural vegetation soil (NC), tillage (CT1) for 80 years and combination of tillage with OMW application (CT2) for 20 years on SOM humification degree. Fluorescence spectroscopy and UV-visible ratios (E4/E6 and CHA/CFA) were used to study soil humic acids (HAs). The SOM and humification distribution was determined for the whole field area using the Inverse Distance Weighting method. Results showed that SOM content, fluorescence emission area and E4/E6 and CHA/CFA ratios were higher in NC. Tillage reduced SOM amount, molecular size, aromatic condensation and humification degree as shown by the strong correlation between fluorescence area and CHA/CFA ratio in CT1 conversely to E4/E6. Contradictory results between fluorescence emission area and E4/E6 ratio found in NC and CT1 indicated that E4/E6 ratio was not a reliable indicator of SOM humification degree. The SOM amount, CHA/CFA ratio and emission fluorescence area increased conversely to E4/E6 ratio in CT2. This revealed a greatly humified organic matter and aromatic structure condensation with tillage and OMW application. Spatial distribution showed a progressive increase of SOM and CHA/CFA from north-west to south-east linked to the positive relationship between CHA/CFA ratio and SOM amount independent of soil management practices. Soil amended with OMW provided a favourable environment for the development of HAs which improved soil quality. The UV-visible ratio CHA/CFA with fluorescence emission area can be used as parameters to investigate SOM humification degree.

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The application of organic amendments is an important technique used to enhance carbon storage in soils, thereby reducing CO2 emissions and mitigating climate change. The role of the composition of organic amendment in CO2 emissions is not well understood, however. The aim of this study was to understand the effect of the biochemical composition of organic amendments on CO2 emission and carbon stabilization. To do this, the application to the soil of five organic amendments with different biochemical compositions was considered. The effects of two types of compost (CH and CP), green manure (GM), olive mill waste water (OMW) and manure (M) on the rate of mineralization and humification of organic carbon were studied under controlled conditions. A single dose of 350 mg of organic carbon/100 g of soil was tested. These treatments, and the control soil, were incubated for 56 days. Soil respiration, organic carbon evolution, and the polymerization index were measured during the incubation period. The results showed that CO2 emission was strongly associated with the soluble fraction and the lignin content. Olive mill water, with the highest soluble fraction, induced the highest rate of mineralization. Composts CH and CP, with the greatest lignin content, were the most resistant to short‐term decomposition, and permitted the highest amount of carbon to be stored in the soil. Overall, this study showed that composts were the best form of organic C application. Indeed, with the composts, low direct mineralization was detected, improving the humidification processes and resulting in enhanced C storage in the soil. © 2018 Society of Chemical Industry and John Wiley & Sons, Ltd.

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