Root and root-lignin degradation in a Norway spruce stand: Effects of long-term nitrogen addition

Abstract

Mass loss, degradation of lignin and the qualitative change of the organic C structures of spruce root litter (2 – 5 mm in diameter) in O-horizon were studied for a period of 6 years (1995 – 2001) in a Norway spruce stand with a current deposition of 13 kg N and 12 kg S ha−1 yr−1. The stand was fertilized annually by addition of 100 kg N and 114 kg S ha−1 (NS). Litterbags, acid detergent lignin (ADL), CuO-oxidation as well as 13C-NMR were used for measurements of mass loss, lignin concentration, degradation of lignin and changes of the organic C structures, respectively. The roots originating from the NS-treated plots lost 20% of their mass in the first year while in control (CON) plots the corresponding value was 10%. After 1879 days of decomposition the fertilized roots had a cumulative mass loss of 54% compared with the CON roots of 44%. The C/N ratios were significantly lower in the NS roots (35) than in the CON roots (59) after 1879 days of decomposition. The initial concentrations of ADL were 34.7 and 36.6 in CON and NS roots and increased to 50 and 56%, respectively, after 1879 days. Using CuO-oxidation method the degree of lignin degradation was significantly higher in the NS than CON roots after 853 days while 13C NMR method showed no change. Our results indicate that CuO-oxidation and solid-state 13C NMR methods give a qualitative measure of lignin decomposition, while the litterbag and ADL methods allow us to quantify mass loss and lignin concentration, respectively. It is concluded that the mass loss of root litter in fertilized plots is higher than needle litter decomposition in the same stand and the higher nitrogen concentration increases the lignin degradation.