The effect of salinity and soil texture on the degradation and humification of soil organic matter

Abstract

Soil organic matter (SOM) is one of the most critical components of soil, owing to its essential roles in influencing physical, chemical, and biological properties. It is an essential source of energy and carbon for many soil organisms that are highly dependent on its availability. The SOM content and its fractionation vary significantly due to factors such as climate, land management practices, soil properties, redox status and salinity, with the latter emerging as a widespread concern in irrigated agricultural land. The aim of this study was to investigate the effects of soil texture, redox status, and increasing salinity on SOM content and its humification. The study was divided into two phases. In the first phase, a survey of 138 locally collected soil samples was conducted to analyse the relationships between SOM and pH, carbonate content, soil texture, and salinity. In the second phase, three soils; a Luvisol, a Calcisol, and a Vertisol; were amended with 4 % peat and NaCl to achieve electrical conductivity (EC 1:5) levels of 1000 and 3000 µS cm⁻¹, along with an unamended control. The soils were then incubated under both aerobic and anaerobic conditions for 150 days. The results from the 1st phase indicated that SOM dynamics may be influenced by several soil factors, including soil texture, pH, carbonate content, and salinity. However, the observed correlations were generally weak. In contrast, the 2 nd phase demonstrated that SOM content, humification, and the FA:HA ratio are regulated by a complex interaction among salinity, redox conditions, and soil texture. Clay-rich soils under aerobic conditions tend to promote humification, whereas elevated salinity and anaerobic conditions suppress humification, with outcomes varying depending on the specific mineralogical characteristics of the soil.