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Life on earth drives a continuous exchange of carbon between soils and the atmosphere. Some forms of soil carbon, or organic matter, are more stable and have a longer residence time in soil than others. Relative differences in stability have often been derived from shifts in δ¹³C (which is bound to a vegetation change from C3 to C4 type) or through ¹⁴C-dating (which is bound to small sample numbers because of high measurement costs). Here, we propose a new concept based on the increase in δ¹⁵N and the decrease in C:N ratio with increasing stability. We tested the concept on grasslands at different elevations in the Swiss Alps. Depending on elevation and soil depth, it predicted mineral-associated organic carbon to be 3 to 73 times more stable than particulate organic carbon. Analysis of ¹⁴C-ages generally endorsed these predictions.