Contrasting short-term vs. long-term effects of warming and nitrogen addition on soil organic matter density fractions in a temperate grass-dominated system

Background and aims Climate warming and atmospheric nitrogen deposition are both expected to exert strong influences on soil organic matter in northern temperate ecosystems over the next century, but it is unclear to what extent these effects may be revealed by short-term field experiments. We compared the short-term (1–2 year) versus long-term (14–15 year) effects of nitrogen and warming on soil organic matter, while controlling for the effects of interannual environmental variability. Methods We added the new nitrogen addition and warming plots (short-term) to a pre-existing nitrogen and warming field experiment (long-term) in a grass-dominated field. We used soil density fractionation and size fractionation to separate the soil organic matter into the fine free light fraction, the coarse free light fraction and the occluded light fraction. We analyzed the quality of each soil organic matter fraction using Fourier-transform infrared (FTIR) spectroscopy. Results While the free light fraction decreased by 14% with nitrogen addition in the short-term plots, contrary to our prediction, it increased by 12% in the long-term plots. Likewise, in the long-term plots, the occluded light fraction increased by 58% with nitrogen addition when combined with warming. Organic matter quality generally did not differ among treatments for the different density and size fractions. Conclusion Our results demonstrate a divergence in the directions of long-term nitrogen addition and warming responses of soil organic matter fractions from those of short-term responses.