Positive nitrogen balance is when nitrogen intake is greater than nitrogen output. a good example of this is

The answer is muscle growth. Positive nitrogen balance in the body means the anabolism of proteins exceeds its catabolism processes (since only proteins have nitrogen in their structure). This occurs, especially,when the person is eating well-balanced food and is not starving. Therefore positive nitrogen balance results to muscle growth since muscles demand a lot of proteins to build the actin and myosin

Alright, i'm gonna try to explain this as good as possible. the law of conservation is like a collection overtime. it never changes though. no matter how you change the constituent parts.