Hi, Belching is way of body to expel out air from stomach whether it is due to excess swallowed air or produced because of food digestion or acid reflux or gastritis etc. During deep breathing some amount of air get swallowed in stomach through esophagus. Your deep breathing exercises is good for your lung but whenever you take a deep breath, it must be taken with your nose only because even slight open mouth will allow the air to get swallowed and lead to belching. In addition,you take out breath through nose very slowly while keeping lips pursed. Eat high protein and easily digestive food rather than carbohydrates or fatty foods. Drink much of water will reduce belching. Consult gastroenterologist if there is any underlying stomach problems like hiatus hernia,reflux esophagitis, gastritis, gastric ulcer. Take care.