Here is my roman numeral analysis for measures 137 - 148:
i / i (with a passing run) i64 i6 / i / iv (passing run) iv64 iv6 (Note: same as measure 138, but just a iv rather than a i) / iv then passing note in bass and then V / iv6 IV passing note iv64 / N6 / viio42/bII / bII (pedal tone) / bII6 / vii/V V / i
The end of the piece takes a very dark turn as the man returns to his home, finding his son dead in his arms, and Schubert emphasizes this distressing matter in the nature of the music and the modal mixtures and augmented sixths he employs.
In measure 143, Schubert introduces an N6, not with correct proper voice leading, but almost as a surprise. It hits the listener's ear and surprises us. Not a good surprise, though. It sounds foreboding and bad. But its purpose is to catch our attention and inform us of the bad news in the music, even before the words declare it. And we do not truly leave the N6, indeed, the chord is cemented through toncization in measures 144 through 145. This allows the reader to know that the bad news is not only terrible, but also permanent, as is the change in the music.
Schubert uses scalewise motion in measures 132 and 134. This motion feels, to me, like the horses footsteps as the man races toward his home. The motion is emphasized by the contrast between the eighth-note movement and the quarter beats of the bass in the rest of the work. And when he reaches his home, the footsteps stop and all that is left is the beating of the quarter note in the bass...no more movement, as the boy is dead. The movement helped to propel the piece forward and emphasize the importance of the man's horse-ride and when it stops so abruptly, it emphasizes the sinking feeling of death, and that movement is no longer necessary, or possible, in the father's emotional state.
The entire piece is a wonderful example of text painting. The emotions of the boy and son are vividly portrayed through the changing harmonies and the movement of the horse is illustrated through the moving eighth-note runs in the final measures 130, 132, and 134 of the piece.