(21)The gross nutrient balance is one of the most widely used agri-environmental indicators. It is described in the Eurostat/OECD common methodology as the calculated difference between the total quantity of nutrient inputs entering an agricultural system and the quantity of nutrient outputs leaving that agricultural system. Despite its importance, not all Member States provide the data on gross nutrient balance voluntarily to the Commission (Eurostat). It is therefore essential for the gross nutrient balance to be incorporated into this Regulation.