Altered biochemical composition of organic matter under N addition in temperate forest ecosystems

Background/Question/Methods

Elevated nitrogen (N) deposition has the potential to increase soil carbon (C) storage. Nonetheless, how N fertilization would influence biochemical composition of organic  matter is still not well understood. It has been reported variable responses, indicating that N addition might suppress, increase, or have no significant effect on the degradation of lignin-rich detritus and secondary humic compounds. In this study, according to the fact that a few studies indicate a decline in activity of lignin-degrading enzymes, we hypothesized that N addition should inhibit lignin oxidation and other recalcitrant materials such as aliphatic components to increase soil carbon storage. The research was conducted in a broad-leaved Korean pine mixed forest in Changbai Mountain, northeastern China. Soil is clay loam. Litter (L), decomposed litter (LD), O horizon (O), and mineral soil (0-15 cm) (MS) samples were separately collected in N addition plots (50 Kg N ha^-1 a^-1) and control after 6 years treatment. Samples were processed for elemental C and N abundance. Also lignin syringyl (S), cinnamyl (Ci), vanillyl (V) phenols as well as substituted fatty acids (SFA) derived from leaf cutin, root and bark suberin were extracted with alkaline CuO method and quantified by gas chromatography mass spectrometry.

Results/Conclusions

The relative abundance of vanillic acid (Ad) to vanillin (Al) increased in the order L<LD<O<MS indicating progressive lignin decay while the amount of lignin decreased in this order. Also, the Ad/Al was lower, and sum of SVCi-lignin was higher for LD N addition plots than LD control indicating N deposition inhibited oxidation of lignin. N addition had little effect on lignin in the O horizon and MS. However, cutin SFA, total SFA, and the proportion of cutin were significantly higher, and there were no differences in suberin SFA in the O horizon. This implies N addition slowed degradation of leaf cutins to accumulate SFAs. The Ci/V phenol ratio increased in mineral soil, which could be attributed to leaching of highly oxidised Ci from Korean pine litter into mineral horizons where stabilized with clay soil. Our results support the hypotheses that oxidation of lignin and cutin aliphatic materials were suppressed by N addition which should contribute to organic matter accumulation, but the effect was different at different layers. Lignin significantly accumulated in decomposed litter, while cutin accumulated in O horizon. Mineral soils did not show any difference of OC between treatment and control, probably due to the short treatment time.