Evidence for the primacy of living root inputs, not root or shoot litter, in forming soil organic carbon

• Published in: The New phytologist Vol. 221; no. 1; pp. 233 - 246
• Main Authors: Sokol, Noah W., Kuebbing, Sara. E., Karlsen‐Ayala, Elena, Bradford, Mark A.
• Format: Journal Article
• Language: English
• Published: England New Phytologist Trust 01.01.2019; Wiley Subscription Services, Inc
• Subjects: Animals; C4 plants; Carbon; Carbon Cycle; Connecticut; Cycles; field experimentation; Food Chain; Food chains; Food webs; forest litter; Forests; grasses; Introduced Species; Litter; litter inputs; living roots; microbial biomass; Microorganisms; Minerals; natural‐abundance 13C tracer; Organic carbon; Organic soils; Plant Roots - chemistry; Plant Roots - metabolism; Plant Shoots - chemistry; Plant Shoots - metabolism; Poaceae; rhizodeposition; roots; shoots; Soil; Soil - chemistry; soil carbon formation; soil food webs; Soil Microbiology; Soil microorganisms; soil organic carbon; soil organic matter; Soils; Tracking; Trees; Understory; Connecticut; natural-abundance 13C tracer; rhizodeposition; litter inputs; microbial biomass; soil carbon formation; living roots; carbon cycle; soil organic matter
• ISSN: 0028-646X; 1469-8137; 1469-8137
• DOI: 10.1111/nph.15361

Soil organic carbon (SOC) is primarily formed from plant inputs, but the relative carbon (C) contributions from living root inputs (i.e. rhizodeposits) vs litter inputs (i.e. root + shoot litter) are poorly understood. Recent theory suggests that living root inputs exert a disproportionate influence on SOC formation, but few field studies have explicitly tested this by separately tracking living root vs litter inputs as they move through the soil food web and into distinct SOC pools. We used a manipulative field experiment with an annual C4 grass in a forest understory to differentially track its living root vs litter inputs into the soil and to assess net SOC formation over multiple years. We show that living root inputs are 2–13 times more efficient than litter inputs in forming both slow-cycling, mineral-associated SOC as well as fast-cycling, particulate organic C. Furthermore, we demonstrate that living root inputs are more efficiently anabolized by the soil microbial community en route to the mineral-associated SOC pool (dubbed ‘the in vivo microbial turnover pathway’). Overall, our findings provide support for the primacy of living root inputs in forming SOC. However, we also highlight the possibility of nonadditive effects of living root and litter inputs, which may deplete SOC pools despite greater SOC formation rates.