I think it's fairly safe to assume that Mars has plenty of nitrogen locked in its mineral deposits, since it's one of the most abundant elements of the Solar system and the planet formed out of the same protoplanetary disk as Earth has. Some nitrogen rich minerals are a safe bet, especially the ones of magmatic origins that might be easily accessible in top layers of the Martian soil in regions that were volcanically more active. Since we know of many such regions, this shouldn't be a problem. For example, silicate minerals are known to lock substantial amounts of environmental nitrogen when superheated (lava / magma), and chondrites can lock up to 27.96 % of nitrogen per weight (e.g. sinoite Si2N2O).
Such nitrogen rich mineral deposits could be used either directly as a fertilizer and let the plants and bacteria slowly enrich the atmosphere with it through their nitrogen cycle, or extract it chemically, with superheating, or other processes, possibly as a byproduct of extracting other sought after minerals and ores, perhaps titanium mineral osbornite (TiN) that consists of 22.63 % of nitrogen per weight.
So this extraction of nitrogen depends on how much of it you'd need. As an atmospheric gas, it's not really essential and could be substituted with other non-toxic inert gases (perhaps argon that's already roughly 2% of the Martian atmosphere?) to, e.g. increase the atmospheric pressure. Plants naturally don't take nitrogen from air (although we now have the technology to enable that for them), so it isn't essential for that, and the industry needs could be satisfied through already mentioned nitrogen gas byproduct of other processes, such as ore mining.