Multi-isotope labelling (&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C, &amp;lt;sup&amp;gt;18&amp;lt;/sup&amp;gt;O, &amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H) of fresh assimilates to trace organic matter dynamics in the plant-soil system