This answer is going to be based off Orlando Figes’s account of the Russian Revolutionary period as expounded on in “A People’s Tragedy” since I just finished it and it’s fresh in my head (and within arms reach on my bookshelf for quotes).

1916 is a tricky year for determining the class origins of the Russian Imperial Army’s officer corps.

The pre-war officer corps, especially commissioned officers and the highest positions in the General Staff, was predominantly made up of the nobility since officers had to go through a sort of cursus honorum which involved graduating from Junker schools reserved for noblemen (p. 61) and for the highest positions receive commissions from the Tsar who would only bestow these honors on the loyal aristocracy (p. 58). Similarly, lower tier and most non-commissioned officers were drawn from petty nobility and the non noble but affluent artisan class (p. 264) who had the ability to receive higher levels of education at cadet corps schools and increasingly at the Junker schools themselves as they were progressively opened up after military reverses in the 19th Century (p. 58). Thus, despite this latter point, at the outbreak of the Great War the officer corps was “dominat[ed] [...] by the court and the aristocracy” (p. 61).

This situation changed dramatically over the course of the war. The mix of heavy mobilization (9 million in the first 12 mo. of the conflict) and heavy officer casualties (60k) (p. 263) led to increased promotion of the lower and peasant classes into officer positions, especially NCOs. Therefore, while ranks above major remained largely in the hands of the nobility (notably the Stavka central command in charge of grand strategy decisions (p. 258-259)), by 1916 (after 2 years of warfare with massive human toll) the make up of the Russian Imperial officer corps shifted dramatically away from the nobility. Figes argues that this phenomenon contributed to the willingness of Russian officers to support the revolutions in 1917, a direct consequence of which was the issuing of Order No. One in February which formally invested soldiers with the right to elect their own officers which, as one can image, further dismantled the (now ex-) noble monopoly on military command (p. 330-331).