Conversion of Ammonium to Nitrate In a Pine Tree Substrate

Pine tree substrate (PTS), a relatively new substrate for container-grown plants, is manufactured by chipping, then milling, pine (P. taeda) logs.  Nitrification in PTS, pine bark (PB), and a peat-based substrate (PL) was studied by measuring nitrate accumulation biweekly for 12 weeks.  Treatments were: 1) PTS without lime (PTS), 2) PTS with 1.8 kg·m^-3 lime (PTSL), 3) PTS amended with PB (PTSPB) in a ratio of 3PTS:1PB, and 1.8 kg·m³ lime, 4) PTS amended with peat moss (PTSP) in a ratio of 4PTS:1 peat moss, and 1.8 kg·m^-3 lime, 5) PB amended with 3.6 kg·m^-3 lime, and 6) PL amended with 5.4 kg·m^-3 lime.  Substrates were placed in 1 L containers and fertilized with a 300 mg·L^-1 NH₄ – N fertilizer solution weekly to biweekly.  Substrate solutions were extracted from container substrates by the saturated medium extract method, and subsequently analyzed for ammonium-N and nitrate-N.  From weeks 2 to 12, substrate pH decreased by 0.3, 0.5, 0.8, 0.8., 1.1, and 1.2 units for the PTS, PTSL, PTSPB, PTSP, PB, and PL substrates, respectively.  After 12 weeks, nitrate was not present in any of the PTS substrates.  In contrast, at week 4, nitrate concentrations for PB and PL were 5 and 3 mg·L^-1, respectively, and week 12 nitrate concentrations were 82 and 78 mg·L^-1, respectively.  In all weeks (except week 8), PTSL had lower NH₄–N concentrations than PTS, indicating 1) greater adsorption to substrate particles, 2) possible oxidation by nitrifying microorganisms, without measurable nitrate due to subsequent immediate immobilization, and 3) consumption by a potentially greater microbe population (enhanced by the relatively high substrate pH in this limed substrate).  The apparent lack of nitrification in PTS (inferred by no nitrate detection) will significantly influence the choice of nitrogen fertilizers for growing crops in PTS.