Figure 2: Spatial distributions of microbes and substrate in litter decomposition.

Distributions with and without microbial cheaters. ‘Community distribution’: each blue or green pixel depicts a model microsite occupied by microbes of a certain functional group, while black pixels depict empty microsites. ‘Substrate distribution’: spatial distributions of the two complex substrates considered in the model (remaining plant material and microbial necromass); the colours of pixels indicate relative substrate concentrations according to the colour bar on the right. Snapshots of these distributions are shown for model runs in three different scenarios. (a) In the first scenario, without cheaters, all microbes produce extracellular enzymes at the same rate (here the microbes depicted in blue and green have identical functional traits; the community distribution is functionally homogeneous and the shown pattern just illustrates colony growth of initially randomly distributed microbes depicted in blue and green). (b) The second scenario considers a mixed community of enzyme producers (blue, with an enzyme production rate of 0.12) and cheaters (green, with an enzyme production rate of 0), only differing in their enzyme production rates. (c) In the third scenario, cheaters additionally have a faster maximum growth rate and a higher N demand compared with enzyme producers. All model runs start with microbes being randomly distributed across the grid. Each panel shows the model grid of 100 × 100 microsites at the time when 60% of the initial C is decomposed, that is, respired (a: 287 days, b: 556 days, c: 5,625 days). Consequently, the total amount of C stored across the two shown compound substrate pools, ‘plant carbon’ and ‘microbial necromass carbon’, is the same for all three scenarios at the time these snapshots are taken; only spatial distributions and relative allocations to these substrate pools differ. The presence of cheaters generally leads to a higher heterogeneity of substrate and to a higher relative allocation of C to the microbial necromass pool, which in turn leads to a higher proportion of N kept in the system (see main text). Fast-growing cheaters strongly constrain the dispersal of enzyme producer patches, leading to a different spatiotemporal dynamics of microbes and substrate.