I've been reading Orr, "Common Slavic Nominal Morphology".
Needless to say, I do not agree at all with his rejection of
the Slavic Auslautgesetze.
On p. 181, however, he has an interesting chapter heading:
"Syncretism between Present and Past Active Participle",
under which he mentions that Trubetskoj (followed by Arumaa
and Otkups^c^ikov) noticed a relation between u-adjectives
with active meaning and verbal roots.
If there was a degree of confusion between the ptc. praes.
act. ("carrying") and the ptc. praet. act. ("having
carried") at a stage where the latter form had already
acquired its analogic ending N. *-us^ (~ *-uh), A. *-us^iN,
the same phenomenon may already have taken place at an
earlier stage.
This might explain the enigmatic N.sg. (m/n) of the ptc.
praes. act. in Russian and Czech: <nesa>.
The South Slavic form <nesy> is completely regular according
to my Slavic Auslautgesetze[*]: PIE *nek^onts, PBS
*nes'an(t)s, with raising to *nesuNh by final -N and -s,
lengthening to *nesu:Nh before -Rh, and denasalization of
-u:N to -u:. Soft roots are also regular, e.g. PBS
z'no:jan(t)s => zno:ju:Nh => zno:je:Nh => znajeN. But the
form nesa, as if from *nek^o:, is unexplicable within the
paradigm of the ptc.praes.act.
The nominative of the ptc.praet.act. *nek^wo:t(s), on the
other hand, just might provide the right input: if -s was
dropped (as in Lith. me:nuo < *meh1no:ts), the ending -o:t
would regularly develop into Slavic -a. I don't think the
loss of -w- constitutes much of a problem here.
In other words, the ptc.praet.act. split into a phonetically
regular form *neso:t > nesa, which was reassigned to the
present active ptc. (but only in some Slavic dialects,
elsewhere it was lost), and a new analogical formation
*nesus^ > nesU.
[*] Which are:
(1) circumflex raising: -e~, -a~ and -o~ in final syllables
become -i:, -o: and -u:
(2) -n and -s raising: back vowels a, a: and o: are raised
to u, o: and u: before final -s > -h and -N (and twice
before -ns > -Nh)
(3) shortening of V:R to VR (a:R > aR, e:R > eR, i:R > iR,
o:R > aR, u:R > uR) and lengthening of -V(:)Rh to -V:Rh
[occurs in acc.pl./ptc.praes. -Nh, o-stem ins.pl. *-ujh]
(-a(:)Rh > -a:Rh, -e(:)Rh > -e:Rh, -i(:)Rh > -i:Rh, -u(:)Rh
> -u:Rh).
(4) umlaut of a > e (with two exceptions) and of u(:) > i(:)
(with two exceptions) after j. The exceptions are: -jau =
-jau > -ju, -jaN = -jaN > -joN; -jui > -jô > -ju and -ju:N >
-je:N [not -ji:N] > -jeN/-jê.
=======================
Miguel Carrasquer Vidal
mcv@...