Short-term vs. long-term effects of climate warming and nitrogen deposition on soil carbon fractions

Background/Question/Methods

Climate warming and atmospheric nitrogen deposition are two global change drivers expected to have strong effects on the productivity and soil carbon cycling of grasslands in northern temperate regions over the next century. Numerous studies investigating the effects of climate warming and nitrogen additions on soil carbon in grass-dominated systems have been performed over a short time period (< 5 years). In contrast, long-term experiments (>10 years) have been less common but may reveal important cumulative effects on soil carbon. We used a warming and nitrogen deposition field experiment located at a grass-dominated site in London, Ontario, Canada to investigate the extent to which the soil carbon responses in short-term global change experiments can be extrapolated to the longer term. The experiment featured an array of 1 m2 plots that had been receiving warming and/or nitrogen fertilization treatments for 15 years, with overhead infrared heaters used to administer warming (2-3 °C increase at the soil surface). Nitrogen was applied at a rate of 6 g per year. New warming and nitrogen addition plots were established in 2019 to directly compare the short-term and long-term effects, with the effects of interannual variation in weather controlled for. Soil samples were fractionated by density using sodium polytungstate to isolate the free light fraction (FLF) from the heavy fraction of soil organic matter. Following subsequent dispersion of the soil aggregates in the heavy fraction using sodium hexametaphosphate, the occluded light fraction (OLF) also was isolated from the heavy fraction using density fractionation.

Results/Conclusions

In the ambient temperature plots, there was an interaction between plot age and nitrogen addition for the FLF, with the latter increasing in the long-term nitrogen addition plots. There was no long-term effect of nitrogen addition alone on the OLF. In the plots that did not receive nitrogen, there were no effects of warming on the FLF or OLF, even over the longer term. However, for the long-term plots there was an interaction between warming and nitrogen addition, whereby their combined effect increased the OLF. Overall, our results confirm the emergence of significant treatment effects on soil carbon fractions after 15 years of nitrogen addition, which is consistent with the effects of nitrogen addition on plant productivity in the experiment. While warming alone did not affect the soil carbon fraction over the long term, likewise, there were no consistent effects of warming on plant productivity over that time.