Soil Organic Matter

Keith R. Baldwin

NC A&T State University

Soil Organic Matter

• SOM consists of a broad spectrum of chemical classes, including amino acids, lignin, polysaccharides, proteins, cutins, chitins, melanins, suberins, and paraffinic macromolecules, as well as organic chemicals produced by humans.

Benefits of Organic Matter

• Increases soil CEC

• Stabilizes nutrients

• Builds soil friability and tilth

• Reduces soil splash

Benefits of Organic Matter

• Reduces compaction and bulk density

• Provides a food source for microorganisms

• Increases activities of earthworms and other soil critters

Carbon Sequestration

• C cycling in agroecosystems has a significant impact at the global scale because agriculture occupies approximately 11% of the land surface area of the earth.

The Morrow Plots

• Continuous corn

• Corn-oat or Corn-soybean

• Corn-oat-red clover

Sanborn Field

• After 100 years, all topsoil was eroded in continuous corn and 50% of topsoil was eroded from a 6-year rotation.

• Shorter rotations general provide greater economic returns but longer rotations are more effective in maintaining soil productivity over the long run.

Sanborn Field

• The decline in soil N was less rapid with longer periods of forages in the rotation and with less frequent soil cultivations.

• Where erosion has decreased topsoil depth, reduced water-holding capacity is the most limiting factor and additions of nutrients have limited value in restoring soil productivity.

Old Rotation107 years

• Continuous cotton with 0 N, 134 kg N, and crimson clover or vetch: 16% without N

• 2-yr cotton corn rotation with winter legume and winter legume with 134 kg N: 160% w/ legume and 188% w/leg. + N

• 3-yr cotton-winter legume-corn-winter cereal-soybean: 203%

Old Rotation107 years

• SOC was substantially reduced under continuous cotton in the absence of legumes or N.

• A winter legume cover crop greatly increased SOC compared to cotton with or without N.

• Rotations with N increased biomass and C inputs and further increased SOC.

Old Rotation107 years

• In this highly weathered soil, the 3-yr rotation, with copious residue addition to soil, resulted in the lowest bulk density, penetration resistance, and greatest hydraulic conductivity.

• Importantly, water-stable aggregates also increased.

Southern Conditions

• The need for crop residues-manures and conservation tillage practices to sustain SOC and consequently effect changes in soil quality is greater for warmer more humid climates.

• In Georgia, 12 Mg ha-1 crop residues left to decompose on the soil surface were required to sustain soil quality commensurate with the inherent soil and climatic resources.

Southern Conditions

• Without significant inputs of C from crop residues and/or manures, conservation tillage alone can only slow the loss of SOC, not halt or reverse it.

• One year with conventionally tilled soybean destroyed the benefits achieved after 4 years of sustainable, no-till cropping.

Carbon Inputs to Soil

• Crop residues

• Cover crops

• Compost

• Manures

Carbon Substrate

• The majority of C enters the soil in the form of complex organic matter containing highly reduced, polymeric substances.

• During decomposition, energy is obtained from oxidation of the C-H bonds in the organic material.

Soil Carbon Equilibrium

• Input primarily as plant products

• Output mediated by activity of decomposers

• It is common that from 40 to 60% of the C taken up by microorganisms is immediately released as CO2.

The Soil Food Web

In 1 teaspoon of soil there In 1 teaspoon of soil there areare…… Bacteria 100 million to 1 billion

Fungi 6-9 ft fungal strands put end to end

Protozoa Several thousand flagellates & amoebaOne to several hundred ciliates

Nematodes 10 to 20 bacterial feeders and a few fungal feeders

Arthropods Up to 100

Earthworms 5 or more

Travis & Gugino - PSU

Classical C Pools

• Nonhumic substances—carbohydrates, lipids, proteins

• Humic substances—humic acid, fulvic acid, humin

The Light Fraction

• The light fraction (LF) with a density of ~1.6 gm cm-3 is relatively mineral free and consists of partially decomposed plant material, fine roots and microbial biomass with a rapid turnover time.

• The LF is a source of readily mineralizable C and N, accounts for ~50% of total soil C and declines rapidly under cultivation.

The Heavy Fraction

• The heavy fraction (HF) is organic matter adsorbed onto mineral surfaces and sequestered within organomineral aggregates.

• The HF is less sensitive to disturbance an chemically more resistant than the LF.

Bacteria vs. Fungi

• Bacteria are smaller than fungi and can occupy smaller pores and thus potentially have greater access to material contained within these pores.

• Bacteria are less disrupted than are fungi by tillage practices commonly used in agriculture.

MicroorganismsMicroorganisms

BacteriaBacteria

UBC EM facility

Ed Basgall

CIMC

Pseudomonas

ArthrobacterBacillus

Travis & Gugino - PSU

Bacteria vs. Fungi

• Fungi tend to be selected for by plant residues with high C/N ratios.

• Fungi have a greater influence on decomposition in no-till systems in which surface residues select for organisms that can withstand low water potentials and obtain nutrients from the underlying soil profile.

MicroorganismsMicroorganisms

FungiFungi

PSU Em facility

Trichoderma

Aspergillus

FusariumD.C. Straney

K.J. Kwon-Chung

Travis & Gugino - PSU

Bacteria vs. Fungi

• Fungi often produce more cell wall than cytoplasmic material when starved for N, and thus can extend into new regions of the soil without requiring balanced growth conditions.

• The filamentous growth structure of a fungus permits it to access C in one location and nutrients in another.

MycorrhizaeMycorrhizae

The Microbial Biomass

• The C contained in microbial biomass ranges from 1 to 5% of the total organic C in the soil.

• Being one of the most labile pools of soil organic matter, microbial biomass is an important reservoir of plant nutrients.

• Because process rates are strongly dependent on the size of microbial populations, quantification of total microbial population is important in estimating the rates of C turnover.

MicroorganismsMicroorganisms

ActinomycetesActinomycetes

SSSA

Univ of Iowa

Paul R. August

StreptomycesTravis & Gugino - PSU

NematodesNematodes

The Microbial Biomass

• Biomass is influenced by rainfall or irrigation.

• Microbial biomass is positively correlated to an estimate of the organic N available to crops in no-tillage surface soil.

Microbial Biomass Pool

• Low C/N and labile

• 1 to 5% of SOC is in microbial biomass and 2 to 6% of soil organic N.

• 1 to 4% of soil organic N is mineralized annually.

• Microbial biomass represents a significant amount of potentially mineralizable N.

Many beneficial effects from activities of microorganisms

• Microorganisms produce:– Plant growth

hormones– Stimulate plant growth

hormones– Compete with disease

organisms

How Microorganisms Eat

• Large, complex organic molecules are digested by enzymes outside the cell wall and converted into small compounds that are easy for microorganisms to take up.

C Utilization by Microorganisms

• Respiration

• Assimilation

• Maintenance

Crops Use Biomass N

• Even when N fertilizers are added in amounts that are sufficient to satisfy the crop’s demand, the crop recovery of fertilizer-derived N is no more than 60% of that which was added, with the remainder being made up from N released from organic matter pools and small amounts from atmospheric inputs.

Soil Activity Hotspots

Mineralization

• The C/N ration is often used to predict mineralization but one must remember that both mineralization and immobilization are taking place at the same time.

• The negative impact of a high C/N ratio may be short-lived and harmful only if plant demand for N is high during periods of substantial immobilization.

Building C Stores

• There is interest in increasing organic carbon to improve soil structure and reduce erosion.

• Increasing SOM storage involves increasing C inputs and/or decreasing rates of C decomposition.

Increasing SOM Storage

• Increasing crop productivity

• Increasing fertilizer where yields are far below maximum attainable yields

• Decreasing fallow

• Improving management

Increasing SOM Storage

• Conservation tillage can sustain or actually increase SOC when coupled with intensive cropping systems.

• Controlling rates of decomposition by manipulating organic matter quality

• Physically protecting residues

Decreasing Decomposition

• Chemically adsorbed to clay

• Biochemical stabilization—OM “quality”

• Physical protection within stable macroaggregates and isolated in micropores

Importance of Clay

• Preserves microbial biomass

• Provides an environment for close interactions between microorganisms and heir metabolites

• Sequesters what would otherwise be readily decomposable OM

• Preserves C in the presence of physical disruption (tillage).

Root Exudates

• Root exudates (including other losses) can account for 10 to 33% of the net plant photosynthate.

• Total exudation and grazing loss by soil predators may account for as much as 150% of C in the root biomass at harvest.

Glue

• Organisms not limited by carbon can undergo uncoupled growth, resulting in energy-spilling reactions that result in the synthesis of storage polymers and excretion of extracellular polymeric substances.

Aggregate Formation

• Soil extracellular polymers are most often associated with the clay fraction.

• Polysaccharides are protected from decomposition through their interaction with clay, metal ions, and tannins.

• Intact bacterial colonies and fungal hyphae in soil are commonly coated with fine clay.

Failure Zones

• The presence of readily mineralizable C in POM will result in a growth in the biomass of microbes and their predators and the production of extracellular materials including polysaccharides.

• Polysaccharides are strongly adsorbed on mineral materials and are effective in strengthening failure zones, accounting for improved aggregate stability.

Benefits of Benefits of Microorganisms in SoilMicroorganisms in Soil

Improved soil physical properties

eg. structure and texture Air

flow

Water retention

Soil porosity

Aggregation

• A matrix of plant residues, microbial biomass, and extracellular material that becomes encrusted by mineral material dominated by small pores may become the center of water stable aggregates.

• The contribution to plant-soil water relations depends on pore continuity between aggregates and soil pores.

Benefits of Aggregation

• Available water holding capacity

• Improved soil structure and tilth

• Improved infiltration

• Improved hydraulic conductivity

• Improved oxygen diffusion

Stability of Aggregates

• Amounts of fine roots and fungal hyphae which strengthen failure zones through physical entanglement, and which also act as sources of carbon for bacteria, thereby contributing to increased production of microbial cementing materials.

Staying Power

• Polysaccharides are readily mineralized and their effects on water adsorption (increased available water holding capacity) and aggregate stability is transient unless the residue is continually renewed or the polysaccharides are physically protected from attack by microorganisms or extracellular enzymes.

Aggregates Conserve SOC

• Compartmentalization of substrate and microbial biomass and grazers

• Reduced diffusion of oxygen

• Inaccessibility of substrates within aggregates is due to pore size exclusion and related to water-filled porosity.

Macroaggregates

• Macroaggregates are more transient than microaggregates because their organic binding agents, roots and hyphae, are more rapidly degradable than the older humified material making up some of the mineral—organic complexes binding the microaggregates.

Soil As HomeEcosystem Engineers

Failure Zones

• Freeze/Thaw and Wetting/Drying

• Provide sites for root development

• Once roots penetrate the failure zones, the deposition of carbon will subsequently lead to stabilization of the root channels and the adjacent matrix.

• Pores created by soil fauna may be stabilized in the same way.

Soil as HomeNematode

Mite

Pillbug

Springtail

Symphylan

Other Arthropods

Other ArthropodsThrips

Wireworm Rootworm

White grubs

Diversity promotes stability

Abundance alone cannot explain impacts of soil organisms on soil quality