The gemara uses the pesukim in parshas ki tavo, in the context of viduy ma'aser as a source for making a bracha. לא עברתי ממצותך ולא שכחתי - לא עברתי מלברכך ולא שכחתי מלהזכיר שמך עליו. Rashi on chumash quotes this and implies that the requirement to make a bracha is d'oraysa. The meforshim on Rashi starting with the mizrachi are very troubled with this since we know that the requirement to make brachos on the performance of mitzvos such as teruma and ma'aser are only d'rabonon. Therefore, the Mizrachi explains that the bracha being referred to is not the technical birchas hamitzvah, rather it is the appreciation we show Hashem for giving us the produce from which we can separate teruma and ma'aser. The problem with the Mizrachi is that Rashi on our daf writes explicitly that the gemara is really referring to the Bracha of אשר קדשנו במצותיו וצוונו להפריש תרומה ומעשר. The problem is that we always assume that birchos ha'mitzvos are d'rabonon so how can the pasuk be referring to this bracha?
The Tzlach has an approach where he says that there are two sources, a mishna and a braisa. The mishna is listing things that are d'oraysa which is what rashi quotes in chumash, therefore in that context the bracha refers to showing hakaras ha'tov, however, in our gemara we are citing a braisa which uses the pasuk as an esmachta, therefore can refer to the specific birchas hamitzvah..
It seems to me that the pasuk is teaching us that we have a Torah obligation to show appreciation for what Hashem has given us. Hakaras HaTov is d'oraysa. However, this appreciation can be fulfilled through the specific birchas hamitzah on teruma and ma'aser that was instituted by chazal. Therefore, it is correct to say that birchos hamitzvos are only d'rabonon, yet we can still use this pasuk as a source for the requirement to make a birchas hamitzah because by doing so we fulfill the d'oraysa obligation to express appreciation to Hashem in some form.