Biocrust modulation of dryland climate change impacts: Synthesizing and contrasting two recent papers

We are already accumulating evidence that biocrusts and soil microbial abundance and diversity are sensitive to climate change, in a string of high-profile papers (Reed et al. 2012, Ferrenberg et al. 2015, Maestre et al. 2015). Decreases in biocrust or microbial abundance or diversity might be expected to lead to reduced ecosystem function in drylands as they warm. Warming in and of itself may affect biota and their functions, but it is important to note that warming is also drying due the the effect of temperature on evapotranspiration, and this may be an even more important climate change impact than temperature per se.

Two other recent papers are suggesting either a buffering of negative climate change impacts on dryland soils by biocrusts, or a pronounced warming increase in soils induced by biocrust succession. The papers do not necessarily conflict, in fact they are very different in their goals and the data collected, but they do make a fascinating contrast that leads to some cool questions.

Biocrusts to the rescue?

In Delgado-Baquerizo et al. 2015, dryland soils were collected along aridity gradients in three continents: North America (USA), Europe (Spain), and Australia. Within each site sampled, biocrusts dominated by mosses were present, so the surveyors sampled microsites underneath the moss biocrusts and in uncrusted areas in each site. They measured indicators of C, N, and P cycling, and also aggregated all of these indicators into a multi functionality index. In semi-arid to arid ecosystems, they found that climate does negatively impact nutrient cycling indicators and multi functionality, supporting the general expectation of negative impacts of warming (and drying) on dryland function. The cool thing was that the presence of moss-dominated biocrusts dampened the negative effects of warmer, drier climate. This was especially so in the driest sites sampled where mosses appear to substantially increase ecosystem function relative to uncrusted areas. Also noteworthy was that the abundance of microbes was greater underneath the biocrusts, and this was linkable to the boost in ecosystem function.

Do biocrusts exacerbate global warming effects?

Couradeau et al. 2016 just last week published their results regarding biocrust effects on albedo and soil warming. They focused on cyanobacterial biocrusts from a single site on the Colorado Plateau, and explicitly focused on the successional progression from "light crust" (highly dominated by Microcoleus vaginatus) to "dark crust" (with a visible presence of Nostoc, Scytonema, and Tolypothrix). This latter group of genera is important because, in contrast to Microcoleus which can actually move underneath soil particles, they are surface-bound and unable to shelter from intense light exposure on the soil surface. Thus, they invest heavily in sunscreen pigments which are responsible for the darkening appearance of biocrusts. The outcomes of the darkening are impressive. The authors report that dark biocrusts are up to 10 degrees C warmer than light biocrusts! This alteration of the soil climate goes on to induce a shift in the dominant species of the biocrust from Microcoleus vaginatus to the more thermotolerant Microcoleus steenstrupii. The authors make no statements about nutrient cycling or multifunctionality.

Gradient of light to dark biocrusts (source: soil crust.org)

Comparing these two studies brings up many interesting questions:
1. Are moss biocrusts warmer, cooler, or the same temperature as uncrusted microsites? Could they be buffering underlying soil from climate fluctuations?

2. Is cyanobacterial succession decreasing soil function through its effect on warming? We generally perceive dark cyanobacterial biocrusts to be more functional than light cyanobacterial biocrusts - greater stability, greater N-fixation, greater biological activity in general. Is this because of, or in spite of, their warming effect on soil? What would the Couradeau et al. have seen if they measured the same nutrient cycling indices as Delgado-Baquerizo et al.?

3. What might we gain from combining what both groups of authors have done? A multi-continent study along aridity gradients comparing the microbial composition and functionality in soils under biocrusts ranging from incipient cyanobacterial biocrusts to successionally more mature moss and lichen assemblages. Seems like a worthy target for a network approach!

What do you think?