Microbial and abiotic controls on mineral-associated organic matter in soil profiles along an ecosystem gradient

Robert Mikutta,Sandra Meyer-Stüve,Sarah J Richardson,Norman Gentsch,Günter Hempel,Thimo Klotzbücher,Duane A Peltzer,Axel Schippers,Georg Guggenberger,Leo M Condron,Andre Eger,Stephanie Turner,Klaus Kaiser

doi:10.1038/s41598-019-46501-4

Abstract

Formation of mineral-organic associations is a key process in the global carbon cycle. Recent concepts propose litter quality-controlled microbial assimilation and direct sorption processes as main factors in transferring carbon from plant litter into mineral-organic associations. We explored the pathways of the formation of mineral-associated organic matter (MOM) in soil profiles along a 120-ky ecosystem gradient that developed under humid climate from the retreating Franz Josef Glacier in New Zealand. We determined the stocks of particulate and mineral-associated carbon, the isotope signature and microbial decomposability of organic matter, and plant and microbial biomarkers (lignin phenols, amino sugars and acids) in MOM. Results revealed that litter quality had little effect on the accumulation of mineral-associated carbon and that plant-derived carbon bypassed microbial assimilation at all soil depths. Seemingly, MOM forms by sorption of microbial as well as plant-derived compounds to minerals. The MOM in carbon-saturated topsoil was characterized by the steady exchange of older for recent carbon, while subsoil MOM arises from retention of organic matter transported with percolating water. Overall, MOM formation is not monocausal but involves various mechanisms and processes, with reactive minerals being effective filters capable of erasing chemical differences in organic matter inputs.

Highlights

Formation of mineral-organic associations is a key process in the global carbon cycle
Knowledge on processes involved in the formation of mineral-associated organic matter (MOM) has been identified as key in understanding soil carbon sequestration and climate change mitigation[2]
The framework assumes that the microbial community processes plant litter and microbial assimilates become sorbed by minerals (‘microbial filter’ of MOM formation; Fig. 1)

Summary

Introduction

Formation of mineral-organic associations is a key process in the global carbon cycle. Direct sorption processes without preceding microbial assimilation of plant carbon may be more important in free-draining soils, supporting the vertical transport of dissolved organic matter. The influence of litter quality and related microbial assimilation on MOM formation throughout entire soil profiles is poorly understood, as is the contribution of direct sorption processes, mineral surface saturation, and the downward migration of organic matter (Fig. 1).

Results

Conclusion