Carbon mineralization and biochemical effects of short-term wheat straw in crude oil contaminated sandy soil

The aim of this study was to investigate the influences of crude oil (CO), wheat straw (WS) and their interaction (CO + WS) on pH, EC, microbial respiration (CO₂–C evolution), microbial biomass C, crude oil biodegradation, and DTPA-extactable trace metals of sandy calcareous soil in an incubation experiment lasting 60 days. The treatments consisted of crude oil at 50 ml kg⁻¹ (CO50), 100 ml kg⁻¹ (CO100), 150 ml kg⁻¹ (CO150) and 250 ml kg⁻¹ (CO250), wheat straw (WS) at 5 g kg⁻¹, and their interaction (CO50 + WS, CO100 + WS, CO150 + WS and CO250 + WS) along with the untreated soil (CK). The results showed that contamination of sandy soil with crude oil caused significant changes in the biochemical properties of sandy calcareous soil. Soil contamination with CO led to an increase in the cumulative CO₂–C respiration. On the contrary, the highest contamination level of crude oil (CO250) resulted in a significant reduction in the cumulative CO₂–C respiration by 19.7% compared to CK. Applying WS over the oil contamination levels recorded a rise in the cumulative CO₂–C respiration, microbial biomass C and crude oil biodegradation. The highest percentage of oil biodegradation (36.6% and 38.4%, respectively) was observed in CO100 + WS and CO150 + WS treatments, while the lowest percentage (4.8%) was found in soil contaminated with CO250. For DTPA-extractable trace metals, soil available Al, Fe, Zn, and Cu decreased significantly due to the contamination with crude oil, whereas significant increases in soil available Mn were noticed. It could be generally concluded that applying wheat straw as biostimulator may mitigate the negative impacts of the contamination with crude oil on soil organic carbon mineralization and crude oil degradation.