Open access publications in the SLU publication database

Abstract

Long-term addition of nitrogen (N) reduces the decomposition rate of humified soil organic matter (SOM) in forest ecosystems causing an increase in SOM storage. This could be an effect of (1) suppressed enzyme activity among white-rot fungi degrading lignin (2) formation of recalcitrant phenolic compounds or (3) altered microbial composition. The aims were to study the long-term effects of N addition on CO2 evolution, production of dissolved organic matter (DOM), lignin degradation and structural changes of the C and N composition. CuO oxidation, solid-state and dipolar-dephasing CPMAS 13C and 15N NMR, Klason lignin and acid detergent lignin (ADL) were used as methods. Fresh and decomposed needle litter, Oi, Oe and Oa layers, and fresh and decomposed root litter were investigated. The needle litter was decomposed under laboratory conditions, whereas the root litter was decomposed in litterbags under field conditions. To estimate CO2 evolution and DOM production, Oe and Oa layers were laboratory incubated in columns. The samples originated from the long-term N fertilization experiment Skogaby in southwestern Sweden (plots denoted NS receiving 100 kg N ha-1 yr-1 as (NH4)2SO4). The long-term N fertilization experiment Stråsan in south-central Sweden (plots denoted N1, N2 and N3 receiving 35, 73 and 108 kg ha-1 yr-1 as NH4NO3) was also used in the column incubation. The unfertilized and the N-fertilized plots at both sites were compared with each other for each layer or decomposition period. The CO2 evolution rate was significantly lower in the NS Oe layer than in the control Oe layer at Skogaby. The N3 Oe layer at Stråsan showed a similar trend. No significant treatments effects on the degree of lignin degradation (expressed as Ac/Al)V) could be found except for 853-day decomposed root litter. The lignin degradation products (VSC) obtained by CuO oxidation did not show any significant treatment effects. No heterocyclic N compounds could be detected by 15N NMR in the forest floor at Skogaby. The dipolar-dephasing technique showed that tannin was present. Tannin might have caused the slight increase in phenolic C with depth. In conclusion, no major qualitative change in the SOM due to N was found and no heterocyclic N compounds were observed. Thus, the common explanations for decreased decomposition due to N additions did not seem to be valid.