Generally the burden is on you to prove onward travel. Whether that be air or land.
Some case examples:
I flew into Canada, and was going to bus to the USA. I had to prove I had a bus ticket to leave
I flew into Argentina, with the intention of busing around South America. I had a return ticket to the UK. This was fortunate, as it at least showed I planned on leaving at some point. I was still then able to bus out. I was required to prove this even before getting on the plane TO Argentina - airlines can be fined if they let you onboard without proof of onward travel AND a valid visa / entry permit.
Therefore, you will likely need either a plane ticket out of Brazil which you will then 'decide' not to use, or will need to purchase a bus ticket in advance.
At a push, having a hotel booked in Argentina might be enough, as it's evidence you're leaving. I've been told by people that Canada is satisfied enough with such proof, given you could go to Niagara Falls and then walk across, although I've not put that to the test.