How is the reaper's concentration on her work reflected in "The Solitary Reaper" by William Wordsworth?

In this poem, Wordsworth describes the experience of watching a solitary reaper who has no idea she is being watched. She is very much in "tune" with nature. Wordsworth was always interested in the artist's connection with nature. Also, his Romanticism and his poetry often concern the theme of the individual in nature. So, when he watches and listens to the "solitary" reaper, we can relate her art (singing) to Wordsworth's own art (poetry). In the first stanza, her work is linked with her song.

Alone she cuts and binds the grain, 

And sings a melancholy strain; 

O listen! for the Vale profound 

Is overflowing with the sound.

Wordsworth suggests that being in nature and/or working with it (in solitude) produces an atmosphere conducive to expressive feelings and emotions. Just as Wordsworth would often find inspiration in his solitary walks in nature, the reaper finds inspiration in her own solitude.

In the final stanza, she is seen/heard "singing at her work": 

Whate'er the theme, the Maiden sang 

As if her song could have no ending; 

I saw her singing at her work . . .

To say that she sings "at" her work suggests that she is singing "to" her work. And since her work is the grain, we might say that she is singing to nature itself. So, it is more accurate to say that it is her concentration on nature that inspires her beautiful song.