Assessing the contribution of microbial primary production to the SOC pools of arid lands.

Primary production by photosynthetic soil microbes can contribute to SOC, and this
contribution can take on additional significance in regions where plant productivity is
restricted, as is the case of arid lands. However, a quantitative rate assessment of these
contributions anywhere is still lacking. We used a combination of direct determinations
and meta-analyses of published literature on a large survey of biological soil crust
communities (biocrusts) dominated by cyanobacteria in US arid regions to obtain
relevant estimates. Directly measured SOC accumulation rates in a single site during a
five-year period yielded an estimate of 3.5 ± 2.2 g C m -2 y -1 in the top 1 cm of soil. Indirect
estimates from a multisite (n= 127) but single time-point survey using Chl a content as a
proxy for biocrust development, which was translated to actual time units using rates of
Chl a accumulation obtained from meta-analyses of the literature, yielded an estimate of
6.2 ± 2.0 g C m -2 y -1 for the same parameter. In order to obtain values integrated
through the soil profile, we used a generalized depth decay relationship in SOC under
biocrusts obtained from a survey of soil cores (n = 93). This resulted in contributions to
the overall SOC pool of 0.0630.039 Kg C m -2 yr -1 (single site) and 0.112 ± 0.036 Kg C
m -2 yr -1 (multi-site). Biocrust sit atop SOC stocks ranging from 1.6 to 5.5 Kg m -2 , not
significantly different from those typical of arid and semiarid pedons. Their average
contribution to these pools through a biocrust’s lifetime (by subtraction of the pools
under crustless soils) is estimated at 1.26 Kg C m -2 Kg m -2 . Not having resisted the
temptation to scale up, and based on published global assessments of biocrust cover,
some 22-33 PgC globally, or some 6-8 % of the global SOC pool in arid lands, may be
attributable to microbial photosynthate.