Lecture 20: Soil Organic Matter; Soil Aeration

Nature of Soil OM

Major components are: Biomass, detritus, humus, and char (from fires; often considered part of humus) A subset is rapidly degraded (labile) because it is either fresh biomass or identifiable small molecules Humus is the residual material that degrades slowly Physical or chemical protection can prevent compounds from being degraded, even if they normally turnover quickly Char is partially-burned remnants of plant matter

Compound Structure May Not Control Long-Term Decomposition Rates From: Schmidt et al. (2011) Nature

Traditional View of Soil Organic Matter was Based on Harsh Chemical Extractions

Modern View Shows that Compounds Obtained by Extractions are Artifacts: OM in Soils is Largely Identifiable Molecules From: Schmidt et al. (2011) Nature

OM is Largely Derived from Fire and Rhizosphere Inputs, not Leaf Litter, and Its Cycling is Highly Susceptible to Disturbance From: Schmidt et al. (2011) Nature

Char is a Resistant Component of Soil Organic Matter

Organic Matter Undergoes Extensive Cycling in Soils

Mineralized Fate of Plant Residue in Soil Only 10-30% ends up in humus Most becomes CO 2 A small portion becomes microbial and fungal biomass

Variations in OM Contents

Variation in Soil OM Across Soil Orders OM always highest in surface horizons, as this is where most OM originates

Effect of Climate on Soil OM

Effect of T and O 2 on OM Accumulation Aerobic Conditions Anaerobic Conditions Decomposition is slower in cool climates and under anaerobic conditions These conditions favor the buildup of OM

Effect of Cultivation on Soil OM

Key Concepts in Soil OM Soil OM consists of biomass (including bacteria!), detritus, humus, and char 60-80% of plant residues added to soil end up as CO 2, 10-40% as humus, and a few % as new biomass Emerging view: OM is composed of many identifiable biological compounds plus fire-derived materials OM content varies among soil orders, but is generally highest near the surface and rarely exceeds a few % in mineral soils (even Mollisols) Cool and wet conditions favor OM accumulation Human activity can destabilize soil OM

Soil Aeration

Soil Aeration is Important Plant roots and soil organisms need a steady supply of O 2 to carry out respiration Good soil ventilation prevents the buildup of potentially toxic gases [carbon dioxide (CO 2 ), methane (CH 4 ), ethylene (C 2 H 4 )] O 2 availability in soils affected by: Soil macroporosity Soil water content O 2 consumption by respiring organisms Poor soil aeration refers to conditions where O 2 availability does not support plant roots and aerobic organisms Occurs from flooding (>80% pore space filled with water) and soil compaction

Plants Affected by Poor Soil Aeration Root suffocation plus sever nitrogen limitations caused by enhanced denitrification (more details on nitrogen in November!)

Plants Affected by Poor Soil Aeration

Plant Adaptations to Waterlogged Soils Plants adapted to waterlogged soils are hydrophytes Many marsh grass species have aerenchyma tissues Transport O 2 to their roots Rice, spartina Mangroves and other hydrophytic trees produce aerial roots

Aerenchyma Tissue in Marsh Plants

Composition of Soil Air Atmosphere: 78% N 2, 21% O 2, and 0.0405% CO 2 (today) Soil air lower in O 2 and higher in CO 2 Soil O 2 varies from 20% near surface of well aerated soil to 0% in flooded soils Soil is considered anaerobic if O 2 is 0% Soil CO 2 increases as O 2 decreases Often CO 2 is at least 0.35% in soils May reach 10%, toxic to some plants Anaerobic soils also contain methane (CH 4 ), hydrogen sulfide (H 2 S), and ethylene (C 2 H 4 )

Soil Gas-Atmosphere Exchange is Largely Controlled by Diffusion Driven by partial pressure gradients

Factors Affecting Soil Aeration Drainage of excess water Respiration rates of organisms Location in the soil profile Soil heterogeneity Tillage/plowing Pore size Plant roots Seasonal changes in biological activity

Water Limits O 2 Diffusion into Soil

Compaction Reduces Aeration

Improving Soil Aeration

Key Concepts in Soil Aeration Soil aeration is critical Root and soil organisms need O 2 from the atmosphere and for toxic soil gases to be vented High water content leads to poor soil aeration Some plants are adapted to waterlogged soils: aerial roots or tissue to pump O 2 to roots Soil air is lower in O 2, higher in CO 2 than atmosphere CO 2 may reach 10% if decomposition rates are high Gas exchange with the atmosphere is controlled primarily by diffusion

CO 2 in Soil

Soil CO 2 Content Varies with Biological Example for an Agricultural Soil Activity CO 2 flux is a measure of CO 2 released by a soil: This is proportional to CO 2 content Respiration by roots increases substantially in summer Production of root exudates also increases in summer Results in increased microbial respiration Introduction of plant residues also increases microbial respiration

Seasonal Variations in Soil CO 2 Air = 0.04% CO 2 Today Data from a Missouri corn field Note that atmospheric CO 2 content was ~0.035% when study was conducted

Slow Diffusion Causes CO 2 to Buildup in Subsoil

Key Concepts in CO 2 in Soil CO 2 content is soil pores is often 10-100x atmospheric concentrations Soil CO 2 content varies with plant activity High respiration by roots and soil microorganisms produces high CO 2 levels CO 2 concentrations buildup in soil in summer Highest levels in root zone Buildup caused by the slow rate of gas diffusion, especially in the subsoil