Tracing litter C and N fate
To trace the fate of 13C and 15N litter amendments, the soil fractions were analyzed for %C, %N, δ13C, and δ 15N using a Thermo Fisher Delta V+ isotope ratio mass spectrometer interfaced with a Carlo Erba NC2500 Elemental Analyzer. First, the proportion of litter-derived C or N in each soil fraction (flitter ) was determined with two endmember mixing models (eq. 1) (Derrien & Amelung, 2011; Poeplau et al., 2018). Here, the C and N isotope signatures were measured from the enriched litter substrate and each of the 3 SOM fractions from control, high N, and organic soils.
eq. 1: flitter =\(\ \frac{sample\ isotope\ signature-SOM\ isotope\ signature}{litter\ isotope\ signature-SOM\ isotope\ signature}\)
Next, the litter C and N recovered in each SOM pool (shown in Fig.2 , Fig. 5 ) was determined for each ingrowth core (eq. 2). Here, the mass proportion of each SOM fraction was determined from lab fractionation and the %C, %N, δ 13C, and δ15N were measured on an elemental analyzer as described above. The distribution of litter C between the SOM fractions (shown in Fig. 3 ) was calculated as the litter mass in each SOM fraction out of the total litter mass remaining in the ingrowth core soil.
eq. 2: litter mass = dry soil mass in each core \(\times\) SOM fraction mass proportion \(\times\) SOM fraction %C/100 or %N/100 \(\times\)flitter