Thanks, the Japanese losses seem to have been much heavier than I have thought! I wonder if there is breakdown on those numbers about the cause - are the numbers reflecting total losses including accidents or only combat losses?
Anatoli Demin´s article series Soviet Fighters in the Sky of China (Part I) mention that officially Japanese admitted the total losses of only 39 aircraft during the period of 14.8.-10.10.1937 in operations over China.
By the late of 1937 most of China´s original air force with Hawk III´s and other American/European planes had been eliminated. From that point Chinese re-equipped their air forces mainly with Soviet aircraft. Most of the Chinese air victories during 1938 - 1940 were achieved with Soviet-built Polikarpov fighters. According to Demin the success was rather limited - the combined efforts of Soviet volunteer pilots and Chinese pilots with I-15bis/I-16 are mentioned to be only 81 destroyed Japanese planes during the period of January 1938 to the May 1940 - although this is "incomplete tally". Considerable number of Japanes planes were also destroyed on the ground by couple of succesfull Chinese (Soviet) surprise air raids against Japanese airfields during the same period (most succesfull attack was against IJNAF airbase in Hankou on 3.10.1939 - "according to Japanese records" about 50 IJNAF machines destroyed - this attack is mentioned also in Saburo Sakai book Samurai).
According to same source (Demin) the aircraft losses of Japanese would have been quite minimal on 1941 - small contingent of Zero´s terrorizing the skies of China and making Chinese Air Force to avoid combat. During this most difficult period the Chinese Air Force would have been able to shoot down only 1 Japanese bomber during the first half of 1941 (the sole Chinese air victory scored over Lazhou on 21.5.1941). The start of Pacific War in Dec. 1941 saved Chinese Air Force and probably Nationalist China from the complete annihilation.
The above is based on these sources - see the title China Air War:
www.j-aircraft.com/research/otherres.htm.