Where and why do particulate organic matter (POM) and mineral-associated organic matter (MAOM) differ among diverse soils?

Abstract

Soil organic matter (SOM) has often been separated into operational physical fractions, such as particulate organic matter (POM) and mineral-associated organic matter (MAOM), to improve our understanding of SOM persistence. While it is generally assumed that POM and MAOM have distinct biogeochemical characteristics, it remains unresolved where and why POM and MAOM differ in their composition and relationships to total SOM decomposition among heterogenous soils. We analyzed elemental, isotopic, and chemical composition, including diffuse reflectance infrared Fourier transform (DRIFT) spectra, of POM and MAOM in 156 soil samples collected from 20 National Ecological Observatory Network (NEON) sites spanning diverse ecosystems (tundra to tropics) across North America. We used a classic size separation method for POM (53–2000 μm) and MAOM (<53 μm) following chemical dispersion. Values of C/N, δ13C, and DRIFT spectra for C–H (aliphatic)/CO were correlated and often similar in POM and MAOM fractions across diverse soils; DRIFT spectra for CC (aromatic)/CO were often similar but uncorrelated between fractions. A prevalent hypothesis holds that MAOM is dominated by microbial-derived OM, yet our findings suggest that plant-derived OM can also contribute substantially to MAOM, especially in wet forests receiving >1200 mm annual precipitation (with MAOM C/N > 15). Multiple statistical analyses showed that C quantity and chemical composition of MAOM could as effectively predict soil C decomposition during an 18-month incubation as measures of POM. Thus, POM and MAOM both likely contributed significantly to decomposition over timescales of months, possibly because characteristics of POM and MAOM were often related and/or a large pool size of MAOM could compensate for its lower decomposition rate relative to POM. Further, we found that soil geochemical composition (such as silt and clay, calcium, oxalate-extractable iron and aluminum), along with climate and ecosystem type, could partly predict differences in quantity and composition between POM and MAOM. Overall, relative coupling vs. decoupling between POM and MAOM among soils was predictable based on geochemistry, and these similarities/differences provide insight into variation in the plant-derived sources of MAOM across diverse ecosystems. The importance of MAOM to short-term soil C decomposition has probably been underappreciated.