Plants in the sun: linking controls of photodegradation and plant species on carbon turnover in aridland ecosystems of Patagonia

Background/Question/Methods

Water availability is one of the primary regional controls on ecosystem processes in terrestrial ecosystems, and ultimately determines the limits of vegetation and soil development in aridland ecosystems. Nevertheless, alternative controls on carbon turnover have been demonstrated to be important as vectors of carbon loss in semiarid ecosystems. Photodegradation, the photochemical mineralization of organic matter due to interception by solar radiation, has recently emerged as an important control on aboveground litter decomposition in semiarid ecosystems, although the impact of variation in plant species composition on this control is relatively unknown.  We conducted a range of litterbag experiments in natural and modified aridland ecosystems in Patagonia, which provided an opportunity to examine ways in which plant species modulate the importance of photodegradation and their interactive effects on carbon turnover. Litterbags of dominant species of natural and modified aridland ecosystems (Mulinum spinosum, Pappostipa speciosa, Nothofagus antarctica, Pinus ponderosa) or a common substrate (Populus nigra) were incubated in field conditions in spatially explicit positions (open areas, below vegetation, or buried).  Organic matter loss and changes in litter lignin and soil characteristics were monitored over a two year period.  

Results/Conclusions

Plant species modulated the effects of photodegradation on litter decomposition in three different ways: through their alteration of radiation interception at the soil surface; though their effects on lignin chemistry; and through the effects on carbon lability in the decomposing substrate. All litter types had significantly higher rates of decomposition when exposed to solar radiation, but as expected, there was substantial variation among species, with lower rates of decomposition in pine litter independently of spatial location. The pine plantations reduced decomposition by nearly 60% in the semiarid steppe and was highly correlated with incident solar radiation at the soil surface, while native shrub cover reduced surface litter decomposition ten-fold. Surprisingly, marked changes in the rates of aboveground litter decomposition did not translate to changes in soil characteristics or belowground decomposition. Changes in plant species that result in increased cover, decreased incident solar radiation and altered litter chemistry may be much more important than changes in precipitation for determining litter decomposition in these Patagonian aridland ecosystems.