Hi
I'm 23 year old and I started working out about a month ago. I only have two dumbbells of five kilo each.
The dumbbells are not heavy enough for a 'Dumbbell Bent-over Row'. I can't replace the weigths in order to make one of ten kilo (old dumbbell).
Is there an exercise to strength my back without equipment?

pullups!

I'm a student and live in a small studio at the univ, so I don't have a lot of space, and certainly no bar. Is there an other alternative?

Looks like your just going to have to be creative.

I'm sure there is some ledge, somewhere where you live or near it that will let you pull yourself up, you can use that as a pull up bar.

Or look around, and look for any heavy objects. Start deadlifting some heavy stuff around where you live.

If you're scraping the bottom of the barrel, you can always hitch a towel round a railing or pole and then (standing) lean back and pull yourself so you have some sort of standing pull-up row.

ok, thanks for the tips, looks like I will have to watch Rocky IV all over again .

you could do HIIT training on a kayak

I know this is kind of an old thread but here's what I've come up with:

Door Pullups - open a strong, heavy door and use something to keep it from moving like a towel or doorstop. Take another towel and fold it in half and drape it over the top of the door. Do pullups on the doortop. The door actually prevents you from "kipping" or otherwise cheating.

Pullup Rows - (like tribal said) Using just your arms or a towel, grab onto a vertical pole or other sturdy structure that you can grab on to or get a towel around. Place your feet as close to the structure as possible and lean back until your arms are fully extended. Pull yourself forward and lower yourself back to the starting position. As you get stronger you can go to a single-arm version for more resistance. This is also a killer on your grip.

Suitcase Rows - If you have a suitcase (the 22" roll-a-board kind) like they'll let you take on an airplane you can fill it with whatever (both your dumbbells and then some) and use the handle to do both One-Arm, Bent-Over Rows and Upright Rows. Don't worry if it's not perfectly balanced, it will bring your stabilizer muscles into play more.

Table Rows - Lay on the floor underneath a table or desk. Reach up and wrap your hands around the edge and pull yourself up. Kind of like doing a reverse pushup.

Superman Extensions - Lay on the floor, putting your hands straight out in front of you. Simultaneously lift your arms and feet off the ground, arching at your lower back, hold for a count of one and slowly lower. Repeat. For added resistance, do it on a soft surface like a mattress. You can also do this alternating left leg/right arm, right leg/left arm together.