Fig. 3

Model of synthetic spike addition. Number of sequencing reads of synthetic spike per 1000 total reads (y-axis) from 16S rRNA, 18S rRNA and ITS following addition to soil of different levels of P, E and F synthetic spikes (x-axis). Experimental results are shown by solid symbols while model data is presented with lines of corresponding colours. The model shows the expected spike contribution in the sequencing output for each spike level using the averaged gene abundance for a specific soil (as in Table 2). Dotted green lines indicate the region with 200–800 synthetic reads per 1000 reads. Spike levels on the x-axis correspond to the synthetic spike levels in Table 1