Inappropriate Mixed Addition of Plant Residues Weakens the Remediation Effect of Crude Oil-Contaminated Soil

The mixed application of plant residues to treat petroleum-contaminated soil may increase remediation efficiency by leveraging their mixed decomposition effects. However, it remains unclear whether this approach may also lead to opposite effects. In this study, plant residues of 7 common plant species, namely, Bothriochloa ischaemum (Bi), Lespedeza davurica (Ld), Artemisia gmelinii (Ag), Heteropappus altaicus (Ha), Artemisia annua (Aa), Sophora davidii (Sd), and Agropyron cristatum (Ac), from the petroleum-producing area of northern Shaanxi and their 9 mixtures were used to treat petroleum-contaminated soil with a crude oil content of 15.00 g kg−1. The samples were incubated at 25 °C under constant humidity for 120 days to determine the potential effects of mixed plant residues on stimulating contaminant degradation and restoring soil biological and chemical properties. The results revealed that (1) among all mixed plant residue treatments, combinations of BiLdAg, LdSd and HaAa significantly weakened the overall contaminant degradation efficiency. In particular, the LdSd and HaAa mixtures simultaneously strongly inhibited the degradation of saturated hydrocarbons and aromatic hydrocarbons; compared with the expected theoretical values, their degradation rates decreased by 12.36–35.25% and 9.91–19.82%, respectively (p < 0.05). (2) Mixtures including LdAgHa, BiLdAg, LdSd, LdAgHaAa, LdHa and HaAgAcSd significantly impaired the capacity of plant residues to replenish soil available nutrients and activate soil enzyme activities. The LdAgHa mixture exhibited the strongest antagonistic effect: its improvement efficiency in terms of soil nitrate nitrogen content and sucrase, dehydrogenase and polyphenol oxidase activities (the increase relative to that in contaminated soil) decreased by 30.60–76.89% relative to the theoretical expectations (p < 0.05). Overall, the effects of mixed plant residue addition on contaminant degradation and the restoration of damaged soil biochemical properties were negatively correlated. (3) Increased chemical specialization of mixed residues and higher mass proportions of Ld and Bi residues in mixtures tended to aggravate the above antagonistic inhibitory effects. When all the remediation indicators were comprehensively evaluated, the mixed application of plant residues failed to universally improve the remediation performance of crude oil-contaminated soil. Under experimental conditions, BiLdAg and LdSd residue combinations may lead to remarkable antagonistic interactions and lower the efficiency of necrophytoremediation, which should be avoided in practical field applications of this remediation technology.