Box 14.4: Soil Engineers and Greenhouse Gas Emissions
Alain Brauman and Eric Blanchart
Is Soil a Desert or an Oasis for Microorganisms?
One gram of soil encompasses more than one billion
of microorganisms and shelter between 3,000 and
10,000 different species; despite this putative richness,
soil from a bacterial point of view, is more a desert
than an oasis. In fact, microorganisms occupy only a
tiny portion of the soil: less than 6 % of the pore
volume. The limited accessibility to organic resources
(mineral material represents up to 95 % of soil)
involves that only a small portion of the microbial
biomass is active, and most of the microorganisms
are dormant. Therefore, the microbial functions
involved in the biogeochemical cycles are mostly
performed in oasis (or microbial patches) generally
associated with a higher availability in carbon
substrates. These oases are multiple (organic fractions,
aggregates, litter, soil pores). However, the formation,
distribution, and maintenance of these microbial
habitats are mainly under the dependence of soil
organisms like soil macrofauna.
Soil Engineers and Soil’s Microbial Compartment
(Lavelle 2002)
Soil invertebrates play a key role in soil functioning:
(1) they decompose the litter which is further
incorporated in the soil; (2) thanks to their building
activities (galleries, nest etc.), they deeply influence
the soil structure and its aggregation pattern; (3) they
influence the diversity and activities of the soil
microbiota; (4) they contribute to plant protection
against diseases and pathogens. These organisms,
like earthworm, ants, and termites (in tropical soils)
regulate the microbial activity in a non-trophic manner
by modifying the physical environment and the availability of nutrients for microorganisms. These regulation properties of the soil microbial compartment are at
the origins of the soil engineer’s concept (Lavelle et al.
1997). The soil volumes affected by the engineer’s
activity constitute some major soil functional domains
such as the drilosphere for earthworms and the
termitosphere for termites. These functional domains
are in constant interactions. The formation of galleries
and cast (dejections) by earthworms influences the root
dynamics (rhizosphere) and modify the litter distribution (residusphere) in the soil. In these functional
domains, the microbial activity is not obligatory
stimulated, but regulated in time and space. In fresh
A. Brauman (*) E. Blanchart
IRD, UMR 210 ECO&SOLS, Montpellier, France
(continued)
Box 14.4 (continued)
earthworms casts (¼dejections), microbial activities
are deeply stimulated, resulting in a strong mineralization of the organic material, which leads to mineral
nitrogen liberalization while within the “old” and dry
earthworm casts, the microbial activity is reduced
resulting in a weak mineralization. Soil engineers are
affecting all microbial parameters, as density, diversity, and/or the genetic structure. However, the intensity of this modification varies depending on the
organism and the microbial function studied. To illustrate this point, we will now discuss the regulation by
earthworms and termites of an important ecosystemic
microbial function: greenhouse gas emission (methane
and nitrous oxide).
Earthworms and N 2 O (Drake and Horn 2007;
Majeed et al. 2013)
Earthworms are present in the majority of soils where
they represent the principal soil fauna in terms of
biomass. They process between 500 and 1,000 t of
soil per hectare and per year. Thanks to this huge
activity, the major part of the surface layer of the soil
is transiting at least once in their digestive tube. During
this intestinal transit, the soil microbial community
will find more appropriate conditions for its activity
(neutral pH, rich substrates and labile form of mucus,
weak pO 2 etc.). This gut environment will particularly
stimulate the facultative anaerobic bacterial
communities like the denitrifiers, rather than the strict
anaerobic one such as methanogens. This explains the
relative important impact of earthworms on the biogenic N 2 O emissions rate. In the laboratory, this emission rate varies according to species, but is between 5
and 225 ng N 2 O h
À1 g
À1 (fresh weight of earthworms).
This leads to a potential annual estimate of 3 Â 10
8 kg
N 2 O
À1 year
À1 (1 % of the total N 2 O annual emissions).
Surface casting is also one of the major aspects of
earthworm activities with a production rate varying
from few tens to several hundreds of tons dry soil.
Within the fresh casts, the microbial community
behave mostly the same than in the gut (stimulation
of the denitrifying activity without selection of the
microbial community). However, in the older and
dryer casts, the scenario becomes more complex: diminution of the microbial biomass but increase of the
metabolic activities. However, whatever the location
(gut or cast), the intensity of these N 2 O emission rates
vary according to the different feeding behaviour (epigeic, endogeic, and anecic) of worms. Globally, epigeic earthworms emitted much more N 2 O than
endogeic species.
(continued)
14 Biogeochemical Cycles
545
Alain Brauman and Eric Blanchart
Is Soil a Desert or an Oasis for Microorganisms?
One gram of soil encompasses more than one billion
of microorganisms and shelter between 3,000 and
10,000 different species; despite this putative richness,
soil from a bacterial point of view, is more a desert
than an oasis. In fact, microorganisms occupy only a
tiny portion of the soil: less than 6 % of the pore
volume. The limited accessibility to organic resources
(mineral material represents up to 95 % of soil)
involves that only a small portion of the microbial
biomass is active, and most of the microorganisms
are dormant. Therefore, the microbial functions
involved in the biogeochemical cycles are mostly
performed in oasis (or microbial patches) generally
associated with a higher availability in carbon
substrates. These oases are multiple (organic fractions,
aggregates, litter, soil pores). However, the formation,
distribution, and maintenance of these microbial
habitats are mainly under the dependence of soil
organisms like soil macrofauna.
Soil Engineers and Soil’s Microbial Compartment
(Lavelle 2002)
Soil invertebrates play a key role in soil functioning:
(1) they decompose the litter which is further
incorporated in the soil; (2) thanks to their building
activities (galleries, nest etc.), they deeply influence
the soil structure and its aggregation pattern; (3) they
influence the diversity and activities of the soil
microbiota; (4) they contribute to plant protection
against diseases and pathogens. These organisms,
like earthworm, ants, and termites (in tropical soils)
regulate the microbial activity in a non-trophic manner
by modifying the physical environment and the availability of nutrients for microorganisms. These regulation properties of the soil microbial compartment are at
the origins of the soil engineer’s concept (Lavelle et al.
1997). The soil volumes affected by the engineer’s
activity constitute some major soil functional domains
such as the drilosphere for earthworms and the
termitosphere for termites. These functional domains
are in constant interactions. The formation of galleries
and cast (dejections) by earthworms influences the root
dynamics (rhizosphere) and modify the litter distribution (residusphere) in the soil. In these functional
domains, the microbial activity is not obligatory
stimulated, but regulated in time and space. In fresh
A. Brauman (*) E. Blanchart
IRD, UMR 210 ECO&SOLS, Montpellier, France
(continued)
Box 14.4 (continued)
earthworms casts (¼dejections), microbial activities
are deeply stimulated, resulting in a strong mineralization of the organic material, which leads to mineral
nitrogen liberalization while within the “old” and dry
earthworm casts, the microbial activity is reduced
resulting in a weak mineralization. Soil engineers are
affecting all microbial parameters, as density, diversity, and/or the genetic structure. However, the intensity of this modification varies depending on the
organism and the microbial function studied. To illustrate this point, we will now discuss the regulation by
earthworms and termites of an important ecosystemic
microbial function: greenhouse gas emission (methane
and nitrous oxide).
Earthworms and N 2 O (Drake and Horn 2007;
Majeed et al. 2013)
Earthworms are present in the majority of soils where
they represent the principal soil fauna in terms of
biomass. They process between 500 and 1,000 t of
soil per hectare and per year. Thanks to this huge
activity, the major part of the surface layer of the soil
is transiting at least once in their digestive tube. During
this intestinal transit, the soil microbial community
will find more appropriate conditions for its activity
(neutral pH, rich substrates and labile form of mucus,
weak pO 2 etc.). This gut environment will particularly
stimulate the facultative anaerobic bacterial
communities like the denitrifiers, rather than the strict
anaerobic one such as methanogens. This explains the
relative important impact of earthworms on the biogenic N 2 O emissions rate. In the laboratory, this emission rate varies according to species, but is between 5
and 225 ng N 2 O h
À1 g
À1 (fresh weight of earthworms).
This leads to a potential annual estimate of 3 Â 10
8 kg
N 2 O
À1 year
À1 (1 % of the total N 2 O annual emissions).
Surface casting is also one of the major aspects of
earthworm activities with a production rate varying
from few tens to several hundreds of tons dry soil.
Within the fresh casts, the microbial community
behave mostly the same than in the gut (stimulation
of the denitrifying activity without selection of the
microbial community). However, in the older and
dryer casts, the scenario becomes more complex: diminution of the microbial biomass but increase of the
metabolic activities. However, whatever the location
(gut or cast), the intensity of these N 2 O emission rates
vary according to the different feeding behaviour (epigeic, endogeic, and anecic) of worms. Globally, epigeic earthworms emitted much more N 2 O than
endogeic species.
(continued)
14 Biogeochemical Cycles
545