PERONALLY I would recommend decent hiking boots as a priority over alternative lenses! We ride where mountain goats fear to tread! CAN be a bit tricky getting to where the action is! And plastic bags... its either dry and dusty and everything gets dirty, or its wet and muddy and everything gets dirty!
But start at the top and tell us what Nikon it's for. No good recommending you DX lenses for crop sensor models if you have a Full-Frame FX or vica versa.
Then... what you got now? And what's your budget?
Trials? Photographically speaking the sports not THAT challenging. Though I have to confess I haven't shot much... usually been riding! Though when co-opted into observing, I have done pretty good with a fixed 35mm lens, 35mm film compact! One beauty of the sport you may get up close and personal with the subject.
The difficult bit IS getting in and finding a decent vantage point, AND knowing what the 'obstacle' being tackled is and getting that in frame, along with the tension and concentration of the rider... its not a particularly dramatic sport, and there's only ever one rider on the course at one time; I have never photo's golf... but I suspect that is similarly challenging, trying to find some 'interest' in the (lack of) action!
However; camera wise; even in woodland, where it may be a bit shaddowed, things don't tend to be moving so fast, and I have got nice clear snaps on 100 or 200ASA film...
So.. as a starting point, If I were going out to snap some of my old trialzin buddies... I'd pick up my D3200... with kit 18-55 and just go for it!
Camera has ISO up to 6400 or something daft, that's plenty high enough to cope with low light.... I shoot rock bands in dark & dingy little venues; if it can cope with that, it'll cope with a bit of tree-shade in half decent day-light, believe me!
Kit 18-55? Covers fair range from wide to mild telephoto; you would have to get up close for frame filling shots, but then standing back, trying to get the angle from a distance with a longer tele, you are more than likely going to end up with stuff getting in the way.... OK, let me re-phrase that... you are more than likely going to end up with PEOPLE, other riders mostly, 'walking the section' watching thier fellow compatitors tyre tracks, wispering incantations they get a 'three'... poking thier head between lens and rider on course!
For some, frame filling far shots, kit 55-200 or kit 55-300 may be helpful; but the 18-55 is likely to be mother-of-all-work lens.
For some dramatic VERY close-ups, if I was taking my film camera.. I would probably try and get in close and low, with my fish-eye... but that is really just for 'fun', and you have to be VERY close for it to really 'work'. Not a lens I would recommend you rush out and get as 'essential'.
