From CO2Science: Covering an estimated 30% of the world’s land surface, arid and semi-arid ecosystems represent a significant component of the global terrestrial carbon cycle. And according to Koyama et al., they may play a more important role in the future as “these ecosystems can be more responsive to elevated CO2 than others because net primary productivity is mostly limited by water availability.” Thus there is a need to investigate carbon dynamics of these regions under changing atmospheric CO2 concentrations.
Paper reviewed: Koyama, A., Harlow, B. and Evans, R.D. 2019. Greater soil carbon and nitrogen in a Mojave Desert ecosystem after 10 years exposure to elevated CO2. Geoderma 355: 113915, doi.org/10.1016/j.geoderma.2019.113915.
Hoping to shed some light on the topic, the three scientists investigated patterns in soil organic carbon and total nitrogen in a desert ecosystem following 10 years of exposure to elevated CO2. The CO2 enrichment experiment was conducted at the Nevada Desert FACE Facility located 15 km north of Mercury, Nevada, USA. The arid site is home to six cover types, five of which were based on the dominant species (an evergreen shrub, three deciduous shrubs and a C4 grass) and the sixth representing plant interspace (i.e., no vegetation), the latter of which type held the greatest percent cover at 83.4%. During the ten years of CO2 enrichment the elevated portion of the study area was an average of 138 ppm higher than that observed in the ambient portion. At the end of the ten-year CO2 enrichment period the authors collected and analyzed soil samples for organic carbon and total nitrogen contents.
Results of the study revealed elevated CO2 stimulated soil organic carbon (SOC) contents from 9.4% to 38.0% in five of the six cover types (SOC in the C4 grass cover type declined by 9%). It also had a positive effect on total soil nitrogen (N), which varied from 6% to 42% depending once again on soil cover type.
In commenting on these observations Koyama et al. say the highest SOC and total N contents were “most evident under dominant shrubs, suggesting greater C and N input resulting from shrub growth stimulated by elevated CO2.” Furthermore, they say “the greater SOC under elevated CO2 was evident in the surface as well as deeper soils, suggesting contributions of root litter and/or exudation as well as aboveground litter, [whereas] the greater total N in the top soil profile under elevated CO2 was most likely a result of both increased N2-fixation and reduced N loss.” Consequently, the authors’ work “suggests that, in arid ecosystems, elevated CO2 can stimulate soil organic matter formation under dominant shrubs through production of aboveground litter as well as root litter exudates.” And because of the corresponding CO2-induced increase in total soil N, which “sustained net N supply,” Koyama et al. conclude that “N limitation for vascular plants is less likely to occur under elevated CO2.” And that strongly implies arid ecosystems will gain (store) greater amounts of carbon in the future as the air’s CO2 content rises, which will most certainly benefit these ecosystems.