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Abstract Lignin is an abundant and complex plant polymer that may limit litter decomposition, yet lignin is sometimes a minor constituent of soil organic carbon (SOC). Accounting for diversity in soil characteristics might reconcile this apparent contradiction. Tracking decomposition of a lignin/litter mixture and SOC across different North American mineral soils using lab and field incubations, here we show that cumulative lignin decomposition varies 18-fold among soils and is strongly correlated with bulk litter decomposition, but not SOC decomposition. Climate legacy predicts decomposition in the lab, and impacts of nitrogen availability are minor compared with geochemical and microbial properties. Lignin decomposition increases with some metals and fungal taxa, whereas SOC decomposition decreases with metals and is weakly related with fungi. Decoupling of lignin and SOC decomposition and their contrasting biogeochemical drivers indicate that lignin is not necessarily a bottleneck for SOC decomposition and can explain variable contributions of lignin to SOC among ecosystems. 

We used incubations of soil and stable isotope measurements to measure lignin, litter, and SOC decomposition over an 18-month lab incubation and assessed their relationships with geochemical, microbial, N-related and climatic factors across 156 mineral soils collected from 20 National Ecological Observatory Network (NEON) sites, which span broad biophysical gradients (climate, soil, and vegetation type) across North America. The soils were collected in 2019. Lignin decomposition and biogeochemical variables were also measured in an approximately 12-month field incubation. 

We incubated 10 forest soils (collected from sites across North America, including the Luquillo LTER/CZO) in the laboratory for over two years to quantify the decomposition of carbon derived from added litter and lignin, as well as from extant soil organic matter. Each soil was subjected to two substrate addition treatments: a) litter derived from a C4 grass precipitated with 13C-enriched lignin, or the same C4 grass litter was precipitated with natural-abundance lignin. The concentrations and delta13C composition of carbon dioxide produced from each soil were measured periodically over time and partitioned into sources (soil organic matter, litter, and added lignin) using isotope mixing models. The methods and results are described in detail by a manuscript in Ecology (Hall et al., 2020).