SOIL MINERAL NITROGEN CHANGES FOLLOWING PRESCRIBED BURNING IN PONDEROSA PINE

In a ponderosa pine (Pinus ponderosa Laws.) forest near Flagstaff, AZ, USA, there was a striking pattern in forest characteristics, consisting of substands of old-growth trees (28-120 cm diameter at breast height (dbh) and 200-500 years old) and of saplings ( 1.5-10 cm dbh and 60-70 years old) in a matrix of pole-sized trees ( 10-28 cm dbh and 60-100 years old). Tree size and forest floor mass decreased from old-growth to pole to sapling substands, but soluble N (as KCl extractable inorganic N) in the mineral soil did not vary substantially among substands before prescribed burning. Prescribed burning had little immediate impact on the overstory but substantially decreased the forest floor mass, with the greatest absolute and relative decreases being in the old-growth substands. Immediately after burning, ammonium-nitrogen (NH4-N) increased from 2.3 to 45.1 mg kg-1 in the old-growth substands, from 1.3 to 26.7 mg kg-1 in the pole substands, and from 1.3 to 8.3 mg kg-1 in the sapling substands. These differences in NH4-N among the substand types are directly correlated with differences in the initial forest floor weight and the amount of forest floor burned. Nitrate-nitrogen (NO3-N) was not changed immediately after burning. By 1 year after burning, NH4-N had declined substantially with only the old-growth substands being significantly higher than controls (30.0 versus 2.56 mg kg-1). Much of this decline showed up as an increase in NO3-N, particularly for the old-growth substands where NO3-N was 18.6 mg kg on burned plots compared with 0.03 mg kg-1 on controls. These results support the conclusion that while increases in NH4-N are an immediate effect of burning (most likely from NH4-N produced by pyrolysis of forest floor material), nitrate increases, which occur later, are most likely caused by nitrification of the high NH4-N produced immediately after burning. A preliminary budgetary analysis for the mineral soil showed an immediate increase in inorganic N of 34.3 kg ha-1 in old-growth substands, 14.7 kg ha-1 in pole substands, and 4.0 kg ha-1 in sapling substands. Because these increases appear to have been transferred directly to the mineral soil from the forest floor, previous studies which have not accounted for this phenomenon may have somewhat overestimated N volatization losses from burning.