Soil nitrogen environments are highly heterogeneous, containing microsites that differ in relative and absolute ammonium and nitrate contents. We investigated whether black birch (Betula lenta L.) seedlings can maximize growth by foraging preferentially for a particular form of nitrogen, and whether supply rate of N and ammonium: nitrate in a patch affects preferences. Seedlings were transplanted into 5 1 split pots with roots partitioned evenly, and assigned to 1 of 4 nitrogen form treatments: nitrate (NO3−) both sides, ammonium (NH4+) both sides, NH4NO3, ‘homogeneous choice, and NO3− one side/NH4+ on the other, ‘patchy choice, crossed with two rates of nitrogen supply. Use of 15N–labelled nitrate revealed the proportion of seedling N from nitrate. Seedlings offered a patchy choice did not significantly out–perform seedlings in homogeneous ammonium and nitrate treatments, whereas seedlings offered a homogeneous choice did out–perform homogeneous nitrate and patchy choice treatment plants. In choice treatments, approx. 35 % of seedling N came from a nitrate source. This selectivity was unaffected by rate of nitrogen application and ammonium: nitrate ratio. That patchy choice seedlings integrated patches to achieve this ammonium: nitrate uptake suggests that black birch has a preferred balance. The seedlings offered a homogeneous choice were significantly heavier than the seedlings offered a patchy choice, perhaps reflecting an additional cost of foraging for ammonium and nitrate separately.