We use a multi-equation model of polluting technologies to evaluate excess nitrogen's marginal abatement cost (MAC). The MAC is estimated using the convex non-parametric quantile regression. The empirical application is conducted at the plot level for wheat production in France in 2017. Results show a median shadow price for excess nitrogen of about €21 per kilogram. If the current European Union's Nitrates Directive (which sets a 170-kg constraint on organic nitrogen per hectare) were extended to mineral nitrogen, this would allow a reduction of total excess nitrogen by 9.5%, but this would be accompanied by a 3.1% decrease in wheat revenue.