Microbial decomposition of 13-C labeled substrates across a gradient of root density, Marcell Experimental Forest, Minnesota, USA, 2014

Roots influence microbial decomposition of organic matter. We
investigated how roots influence decomposition of a simple and complex
carbon (C) substrate in mesocosms in a field experiment. Mesocosms
were PVC lined with mesh that varied in size, which manipulated root
access to soil and produced a gradient of root density. Mesocosms
either received an injection of water (control), 13C labeled starch or
13C labeled leaf material. We collected gas samples from the mesocosms
and analyzed 13C-CO2, microbial biomass, and enzyme activity. Our
empirical data set was compared to a rhizosphere simulation model,
CORPSE, to evaluate two alternative model hypotheses: 1) microbes are
generalist decomposers, and 2) microbes have different affinities for
substrates and may be influenced by roots differently. Our field
experiment was conducted in 2014 at Marcell Experimental Forest, MN,
USA. We installed mesocosms in May and after six weeks we injected
water or substrates into soil. Gas samples were collected 1, 2, 3, 4,
5, 10, 20, and 40 days after injections. Mesocosms that received the
starch substrate were harvested from the field 5 days after injections
and those receiving leaf material were harvested 40 days after
injections. Half of the water-control mesocosm were harvested 5 and 40
days after injections to pair with the substrate mesocosm harvests.
Our results suggested decomposition of leaf material was more
sensitive to root density than starch. The CORPSE model simulations
with microbe-substrate affinity (hypothesis 2) showed a similar
pattern to the field experiment. One way that roots influence
microbial decomposition is through alleviating C limitation via
exudates. Overall, our results suggest that microbial decomposition of
starch is not a C limited process and that root density does not alter
the rate of starch decomposition. On the other hand, decomposition of
more complex substrates such as leaf tissue, which contains cellulose,
hemi cellulose, and lignin, may be more C limiting and therefore more
sensitive to roots and root exudates.