Development of 15N enrichment in a nitrogen-fertilized forest soil-plant system

Abstract

The 15N abundance of the current needles of Scots pine ( Pinus sylvestris L.) collected during 19 yr of a long-term forest N fertilization experiment was measured. Unlabelled urea and NH in4NO 3 had been added each year at four rates, NO (0 kg N ha −1), N1 ( x ̄ = 36 kg N ha −1 , N2 ( x ̄ = 73 kg N ha −1) and N3 ( x ̄ = 109 kgN ha −1 . Hence, the total amount of N added to N3 plots exceeded 2000 kgN ha −1. N budget studies had shown that 75% of this N had been lost from the soil-plant system. In the N1 treatments there was an initial decline in 15N abundance, while in the N3 treatments 15N increased throughout. The decline in the N 1 treatments should reflect a change in 15N abundance of needle N towards that of fertilizer N (— discrimination during uptake), which in turn should indicate retention of added N. Processes leading to N losses (e.g. NN 3 volatilization, denitrification, nitrification followed by leaching or denitrification) have been documented to discriminate against 15N. The 15N enrichment of needles in the N3 treatments thus confirmed that the system retained 15N more strongly than 14N. Differences between doses of N were larger in the urea treatments than in the NH 4NO 3 treatments. This was due to isotope fractionation during NH 3 volatilization from urea pellets and the higher rates of nitrification on urea plots. Changes in 15N abundance of NO (control) plots showed that these had become contaminated. Studies of changes in 15N abundance of ecosystem components in experiments like this offer possible ways to estimate critical loads of N.