Jamieson, Fausset and Brown on Revelation 8:7

7. The common feature of the first four trumpets is, the judgments under them affect natural objects, the accessories of life, the earth, trees, grass, the sea, rivers, fountains, the light of the sun, moon, and stars. The last three, the woe-trumpets (Re 8:13), affect men's life with pain, death, and hell. The language is evidently drawn from the plagues of Egypt, five or six out of the ten exactly corresponding: the hail, the fire (Ex 9:24), the WATER turned to blood (Ex 7:19), the darkness (Ex 10:21), the locusts (Ex 10:12), and perhaps the death (Re 9:18). Judicial retribution in kind characterizes the inflictions of the first four, those elements which had been abused punishing their abusers.

mingled with--A, B, and Vulgate read, Greek, "... IN blood." So in the case of the second and third vials (Re 16:3, 4).

upon the earth--Greek, "unto the earth." A, B, Vulgate, and Syriac add, "And the third of the earth was burnt up." So under the third trumpet, the third of the rivers is affected: also, under the sixth trumpet, the third part of men are killed. In Zec 13:8, 9 this tripartite division appears, but the proportions reversed, two parts killed, only a third preserved. Here, vice versa, two-thirds escape, one-third is smitten. The fire was the predominant element.

all green grass--no longer a third, but all is burnt up.