Table S2.

Ionization energies for fragments associated with most prominent mass peaks and their evolution over 6 y of litter decomposition

m/z	340	368	396	399	400	406	414
Sum peak intensity	∑ar+	∑ar+	∑ar−	∑ar−	∑ar−	∑ar−	∑ar−
Ionization energy	8.5	8.2	8.1	8.5	8.2	8.0	7.3
Correlation coefficient (R2)	0.94	0.82	0.94	0.96	0.92	0.86	0.92

∑ar⁻ and ∑ar⁺ denote the sums of peak intensities of masses identified in LDPI spectra shown in Fig. 2C. ∑ar⁺ is the sum of normalized intensities of peaks that increase with litter age (m/z = 340–368) whereas ∑ar⁻ represents the sum of normalized intensities of peaks decreasing over time (m/z = 396–414).