Salts

The salts in water, soil, and dog urine causes discoloration and even death to lawns and plants. Salt damage happen along the seashore, in places where there is low rain fall, poor drainage, and areas where salt is use to melt snow and ice. Too much salt in the soil causes the water to accumulate in the leaf tips and edges where it kills the tissues. The salt also interferes with how much water the plant gets. The dog urine has nitrogen that sometimes causes the grass to grow rapidly that cause a dark green and vigorously growing ring.

Here are some sites that have more info. http://web1.msue.msu.edu/imp/mods1/00000089.html

http://www.hilltopanimalhospital.com/urine%20lawn%20burn.htm