Why according to the Chinuch are women obligated in the mitzvas aseh of shemitah (shnas shabason…) when it is zman gerama? As we once discussed (link), some Rishonim (see Tosfos Kiddushin 34a, Ramban) treat the case of an aseh which coincides with a lav as different than a regular aseh. Since women are obligated to keep the lav, the aseh tags along even though it is zman gerama. With respect to shemita, since women are obligated to keep the lav of not working the field, they are also obligated in the aseh as well.
The Ritva distinguishes between two types of mitzvos. The exemption of zman gerama applies where the mitzvah demands some act on the part of the individual, a chovas hagavra. However, where the mitzvah relates to the object instead of the person there is no zman gerama exemption. According to the Ritva the gemara does not classify milah as zman gerama mitzvah (See Tos Kiddushin 29) because the purpose of the mitzvah is to bring the child into the state of being nimol, not the actual act of cutting off the orlah.
The Minchas Chinuch applies the logic of the Ritva to shemita. The aseh of shemita is not a chovas hagavra on the individual, but is din in the land, i.e. the land must be given a year of rest. Therefore, women as well as men are obligated.
A number of other interesting conclusions follow from this chiddush. 1) If one asks an aku”m to work the land during shemita, one is not merely in violation of an issur derabbanan of amira l’aku”m, as on Shabbos, but rather one is in violation of the issur d’oraysa of shabason. Since the focus of the mitzvah is the land, not the farmer, causing the land to be disturned by work violates this aseh. 2) The M.C. further debates whether the aseh would be violated if one planted seed just before shemita and that seed took root on shemita. While no actual work was done during shemita in this case, there was an effect produced on the land.
My son asked me recently why we need a gezeiras hakasuv (see Shavuos 15b) to teach us that the beis hamikdash cannot be built on Shabbos or Yom Tov – since binyan mikdash is an aseh and yom yov is an aseh and a lav, the standard rule that ain aseh doche lo ta’aseh v’aseh should apply. A simple answer is that without the gezeiras hakasuv you might have thought that just like korbanos are doche shabbos and yom tov, the building of the mikdash itself should be doche yom tov. Also, it also does not seem from the gemara that all the amoraim agree that there is in fact an aseh associated with yom tov. I prefer a more lomdish answer. Perhaps the chiddush here is that the issur of building is not just a din in shabbos or yom tov, but is a din in the mitzvah of binyan mikdash. A nafka minah would be in a case where an aku”m is asked to do the building: if the issur were a regular hilchos shabbos din based on ain aseh doche lo ta’aseh v’aseh, then we would be dealing with a standard issur derabbanan of amira l’aku”m; however, if the issur is a din in binyan mikdash, meaning the mikdahs is precluded from being built on yom tov or shabbos, then the work of the aku”m would be an issur d’oraysa, similar to the sevara of the Minchas Chinuch with respect to shemita.