What does Walt Whitman mean by the last two lines in his poem, "The Voice of the Rain"?

Question

Here's Walt Whitman's poem "The Voice of the Rain"

And who art thou? said I to the soft-falling shower,
Which, strange to tell, gave me an answer, as here translated:
I am the Poem of Earth, said the voice of the rain,
Eternal I rise impalpable out of the land and the bottomless sea,
Upward to heaven, whence, vaguely form'd, altogether changed, and yet the same,
I descend to lave the drouths, atomies, dust-layers of the globe,
And all that in them without me were seeds only, latent, unborn;
And forever, by day and night, I give back life to my own origin, and make pure and beautify it;
(For song, issuing from its birth-place, after fulfilment, wandering
Reck'd or unreck'd. duly with love returns.)

What does Whitman mean by the last two lines?

Answer 1

The last two lines compare the composition of poetry to the hydrological cycle. This is the converse of the third line, in which the rain compares itself to poetry.

The first eight lines are a poetic description of the hydrological cycle. Water, personified as the “voice of the rain”, evaporates “out of the land and the bottomless sea”, becoming “impalpable” vapour, and rises “upward to heaven” whence it returns to the land as a “soft-falling shower” of rain.

Then the last two lines describe an analogous process for “song” (which can be understood in the sense of “lyrics” or “poetry”). Song “issues from its birth-place” (is composed by musicians or written by poets), “wanders reck’d or unreck’d” (is communicated by singers and poets to their audience, whether or not the audience pays attention), and “duly with love returns” (inspires other musicians and poets to write new songs in their turn).

For “fulfilment” we need the sense “accomplishment, performance, completion” (OED) and for “reck” the sense “heed, regard, take notice of” (OED).