Physical, chemical, and biochemical mechanisms of soil organic matter stabilization under conservation tillage systems: A central role for microbes and microbial by-products in C sequestration

Abstract Conservation tillage practices that entail no or reduced soil disturbance are known to help preserve or accumulate soil organic matter (OM). However, the underlying mechanisms especially at the molecular level are not well understood. In this study soil samples from 25-year-old experimental plots continuously cropped with barley ( Hordeum vulgare L.) under no-tillage (NT) and chisel tillage (CT) were subjected to a new physical fractionation method to isolate dissolved OM, mineral-free particulate OM located outside aggregates (physically and chemically unprotected), OM occluded within both macroaggregates and microaggregates (weakly and strongly protected by physical mechanisms, respectively), and OM in intimate association with minerals (protected by chemical mechanisms). The whole soils and OM fractions were analyzed for organic C and N content and by modern nuclear magnetic resonance (NMR) techniques. The soil under NT stored 16% more organic C and 5% more N than the soil under CT. Compared to CT, NT increased free organic C content by 7%, intra-macroaggregate organic C content by 20%, intra-microaggregate organic C content by 63%, and mineral-associated organic C content by 16% and decreased dissolved organic C content by 11%. The mineral-associated OM pool accounted for 65% of the difference in total organic C content between NT and CT, whereas the intra-microaggregate OM only explained 18%, intra-macroaggregate OM 14%, and free OM 11%. The NMR experiments revealed that the free and intra-aggregate OM fractions were dominated by crop-derived materials at different stages of decomposition, whereas the mineral-associated OM pool was predominately of microbial origin. Overall, our results indicate that microbes and microbial by-products associated with mineral surfaces and likely physically protected by entrapment within very small microaggregates constitute the most important pool of OM stabilization and C sequestration in soils under NT. Most probably the slower macroaggregate turnover in NT relative to CT boosts not only the formation of microaggregates and thereby the physical protection of crop-derived particulate OM, but more importantly the interaction between mineral particles and microbial material that results in the formation of very stable organo-mineral complexes.

[1]  A. Piccolo.  THE SUPRAMOLECULAR STRUCTURE OF HUMIC SUBSTANCES , 2001 .

[2]  M. Simpson,et al.  Assessing the fate and transformation of plant residues in the terrestrial environment using HR-MAS NMR spectroscopy , 2006 .

[3]  I. Kögel‐Knabner.  Analytical approaches for characterizing soil organic matter , 2000 .

[4]  David S. Powlson,et al.  A Procedure for Isolating Soil Organic Matter Fractions Suitable for Modeling , 2001 .

[5]  M. Simpson,et al.  Microbially derived inputs to soil organic matter: are current estimates too low? , 2007, Environmental science & technology.

[6]  Johan Six,et al.  Aggregate and Soil Organic Matter Dynamics under Conventional and No-Tillage Systems , 1999 .

[7]  Johan Six,et al.  The potential to mitigate global warming with no‐tillage management is only realized when practised in the long term , 2004 .

[8]  I. Kögel‐Knabner,et al.  Improvement of 13C and 15N CPMAS NMR spectra of bulk soils, particle size fractions and organic material by treatment with 10% hydrofluoric acid , 1997 .

[9]  R. Lal,et al.  Soil Carbon Sequestration Impacts on Global Climate Change and Food Security , 2004, Science.

[10]  C. Plaza,et al.  A Comprehensive Method for Fractionating Soil Organic Matter Not Protected and Protected from Decomposition by Physical and Chemical Mechanisms , 2012 .

[11]  K. Paustian,et al.  Measuring and understanding carbon storage in afforested soils by physical fractionation , 2002 .

[12]  K. Ekschmitt,et al.  Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions – a review , 2006 .

[13]  D. McNally,et al.  NMR spectroscopy in environmental research: from molecular interactions to global processes. , 2011, Progress in nuclear magnetic resonance spectroscopy.

[14]  A. Simpson,et al.  Purge NMR: effective and easy solvent suppression. , 2005, Journal of magnetic resonance.

[15]  E. Torres,et al.  Microbiological parameters as indicators of soil quality under various soil management and crop rotation systems in southern Brazil , 2007 .

[16]  T. Ochsner,et al.  Tillage and soil carbon sequestration—What do we really know? , 2007 .

[17]  C. Chenu,et al.  Clay‐sized organo‐mineral complexes in a cultivation chronosequence: revisiting the concept of the ‘primary organo‐mineral complex’ , 2006 .

[18]  F. J. Stevenson.  HUmus Chemistry Genesis, Composition, Reactions , 1982 .

[19]  Jin He,et al.  Soil chemical properties and microbial biomass after 16 years of no-tillage farming on the Loess Plateau, China , 2008 .

[20]  M. Simpson,et al.  Nuclear Magnetic Resonance Analysis of Natural Organic Matter , 2009 .

[21]  J. Marinissen,et al.  Physical protection of mineralizable C in aggregates from long-term pasture and arable soil , 2004 .

[22]  Johan Six,et al.  Aggregation and soil organic matter accumulation in cultivated and native grassland soils , 1998 .

[23]  W. Post,et al.  Soil organic carbon sequestration rates by tillage and crop rotation : A global data analysis , 2002 .

[24]  Andrew P. Whitmore,et al.  The potential to increase soil carbon stocks through reduced tillage or organic material additions in England and Wales: a case study , 2012 .

[25]  Johan Six,et al.  Soil macroaggregate turnover and microaggregate formation: a mechanism for C sequestration under no-tillage agriculture , 2000 .

[26]  C. Chenu,et al.  Turnover of intra- and extra-aggregate organic matter at the silt-size scale , 2010 .

[27]  K. Paustian,et al.  Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils , 2002, Plant and Soil.

[28]  H. W. Hunt,et al.  Management options for reducing CO2 emissions from agricultural soils , 2000 .

[29]  K. Paustian,et al.  Soil Aggregation and Soil Organic Carbon Stabilization: Effects of Management in Semiarid Mediterranean Agroecosystems , 2009 .

[30]  D. McNally,et al.  The degradation characteristics of microbial biomass in soil , 2011 .

[31]  Johan Six,et al.  Carbon Sequestration in Microaggregates of No‐Tillage Soils with Different Clay Mineralogy , 2004 .

[32]  M. Schnitzer,et al.  Chapter three - Quo vadis Soil Organic Matter Research?: A Biological Link to the Chemistry of Humification , 2011 .

[33]  Anònim Anònim.  Keys to Soil Taxonomy , 2010 .

[34]  K. Timmis,et al.  'Clay hutches': a novel interaction between bacteria and clay minerals. , 2000, Environmental microbiology.

[35]  P. Buurman,et al.  Effect of land use on the composition of soil organic matter in density and aggregate fractions as revealed by solid-state 13C NMR spectroscopy , 2006 .

[36]  M. Simpson,et al.  Association of specific organic matter compounds in size fractions of soils under different environmental controls , 2011 .

[37]  P. Barré,et al.  Microaggregation and organic matter storage at the silt-size scale , 2008 .

[38]  D. Manning,et al.  Persistence of soil organic matter as an ecosystem property , 2011, Nature.

[39]  W. Schlesinger.  Biogeochemistry: An Analysis of Global Change , 1991 .

[40]  J. Baldock,et al.  Role of the soil matrix and minerals in protecting natural organic materials against biological attack , 2000 .

[41]  R. Dick,et al.  Long-term tillage and crop rotation effects on microbial biomass and C and N mineralization in a Brazilian Oxisol , 2004 .

[42]  P. Brookes,et al.  Microbial biomass and activity at various soil depths in a Brazilian oxisol after two decades of no-tillage and conventional tillage , 2010 .

[43]  R. Allmaras,et al.  Soil organic carbon and nitrogen in a Minnesota soil as related to tillage, residue and nitrogen management , 2006 .

[44]  K. Ekschmitt,et al.  Stabilization mechanisms of organic matter in four temperate soils: Development and application of a conceptual model , 2008 .

[45]  I. Kögel‐Knabner.  13C and 15N NMR spectroscopy as a tool in soil organic matter studies , 1997 .

[46]  M. Hayes,et al.  Unraveling the structural components of soil humin by use of solution-state nuclear magnetic resonance spectroscopy. , 2007, Environmental science & technology.

[47]  Zhongkui Luo,et al.  Can no-tillage stimulate carbon sequestration in agricultural soils? A meta-analysis of paired experiments , 2010 .

[48]  C. Bayer,et al.  Tillage Effects on Particulate and Mineral‐Associated Organic Matter in Two Tropical Brazilian Soils , 2006 .

[49]  Julie D. Jastrow,et al.  Soil aggregate formation and the accrual of particulate and mineral-associated organic matter , 1996 .

[50]  W. Horwath,et al.  Acid fumigation of soils to remove carbonates prior to total organic carbon or CARBON‐13 isotopic analysis , 2001 .

[51]  Rattan Lal,et al.  Challenges and opportunities in soil organic matter research , 2009 .

[52]  J. Skjemstad,et al.  Soil structure and carbon cycling , 1994 .

[53]  M. Kästner,et al.  SOM genesis: microbial biomass as a significant source , 2012, Biogeochemistry.