Stimulation of litterfall, soil CO2 flux, and C and N sequestration after shrub expansion in a mesic environment

Background/Question/Methods

Expansion of woody vegetation in grasslands is a worldwide phenomenon with implications for C and N cycling at local, regional and global scales.  Although woody encroachment is often accompanied by increased ANPP and increased litterfall, mesic ecosystems with high antecedent levels of soil organic matter (SOM) may become sources for C after woody encroachment because of concurrent stimulation of soil CO₂ flux.  Our objective was to determine if barrier islands, where young, sandy soils are extremely low in organic matter, became sources for C after shrub encroachment or if higher litterfall resulted in increased pools of soil organic carbon (SOC) and soil total nitrogen (TN).  We measured variations in litterfall in thickets of the nitrogen-fixing shrub Morella cerifera for five years across a chronosequence of shrub expansion on a barrier island.  In the final year, we measured soil CO₂ flux, standing litter C and N pools and SOM, SOC and TN in shrub thickets and adjacent relic grasslands. 

Results/Conclusions

Litterfall in shrub thickets exceeded total aboveground ANPP previously reported for adjacent grasslands at all sites, although litterfall declined with increasing thicket age.  High levels of litterfall resulted in a heavy, N-rich litter layer storing ~8.0 Mg C ha^-1 and ~0.35 Mg N ha^-1.  Variations in soil CO₂ flux during the growing season were primarily a result of differences in soil moisture.  Soil CO₂ flux was stimulated by shrub encroachment in younger sites but did not vary between shrub thickets and grasslands at the oldest sites suggesting that differences were transient.  Increases in CO₂ flux in young shrub thickets were not enough to offset contributions of litterfall to SOC.   SOC was ~4 times higher in soils beneath shrub thickets than in grassland soils.  SOM and TN were also 4-6 times higher under shrub thickets.  Expansion of shrub thickets on barrier islands and in other mesic systems with low nutrient soils may substantially increase C sequestration and this increase may be further enhanced if shrubs are nitrogen fixers.