Is it permissible to leave a found item lying on the ground and not pick it up due to the difficulties of identifying and preserving it?

There is no blame on the questioner for not picking up the lost item, because it is not obligatory in the first place. Rather, some scholars even recommended leaving it, based on the Prophet's (peace be upon him) saying: "The lost property of a Muslim is a burning fire." Moreover, picking it up might expose oneself to consuming unlawful wealth and neglecting the obligation of publicizing it. However, the Malikis made it obligatory to pick it up if there was fear of a betrayer and the picker felt secure from betraying it himself. The majority of scholars, who hold the view of it being merely recommended, disagreed with them.