Soil ecology and agroecosystem studies - Microbial Ecology in Sustainable Agroecosystems - page 5

The organic by-products, death, and turnover of primary producers enter the system as
different forms of detritus for consumption by microbes and invertebrates. Direct con-
sumption of primary producers by consumers leads to consumer by-products and corpses
that enter the soil milieu as detritus. Outside or allochthonous inputs of organic mate-
rial enter systems largely as detritus. Specific examples include algal wracks from marine
habitats washing up onto the terrestrial realm, as well as wind- or water-driven materials
transported from one terrestrial or aquatic habitat to another.
At this point, it is essential to note that the most significant proportion of the carbon
inputs to soils coming from the plant litter (leafy and woody materials) on the soil sur-
face and the plant root materials that enter the soils are all subject to direct consumption
by invertebrates and degradation by microbes. The direct contribution by herbivores and
predators is modest; for most terrestrial systems, less than 10% of total plant biomass is
consumed by herbivores (Coleman et al., 2004). The remaining 90% of the aboveground
and belowground plant biomass enters soils as structural and metabolic forms, with the
contribution from roots dominating the SOM development. From 16% to 33% of the total
carbon assimilated by plants is released directly into the soil in the form of root exudates,
mucigels, and cells, contributing from 30% to 60% of the organic pool in soil ( Table 1.2 ;
Andrén et al., 1990; Heal et al., 1997). The amounts of persistent organic matter compounds
in ecosystems have been the subject of intensive study by many researchers. For a syn-
optic review of studies making use of radiocarbon in soil systems and profiles, see the
work of Trumbore (2009). Trumbore noted that there are four principal mechanisms for
stabilizing soil carbon: (1) In climatic stabilization, SOM can be preserved due to freezing
temperatures, low O 2 content, or high moisture content in a range of ecosystems. This type
of preservation occurs in high-latitude soils and water-saturated soils in temperate and
tropical environments. (2) For intrinsic recalcitrance, while all organic carbon is ultimately
thermodynamically unstable, soils contain a number of compounds that may be difficult
to decompose owing to their specific chemical structure. Pyrolized carbon formed in fire
can be one example (Preston and Schmidt, 2006); some lipid compounds are also extremely
long-lived in soil (Mikutta et al., 2006). (3) Regarding physical stabilization, there is clear
evidence that the association of organic compounds with aggregates and mineral sur-
faces through a range of interactions (inclusion, sorption, etc.) slows their decomposition
(Oades, 1984; Jastrow, 1996; Six et al., 2004; Mikutta et al., 2006). (4) For inhibition of micro-
bial activity/inaccessibility, until recently, links between the microbial community and
the amount and age distribution of SOM have received less attention than physical and

Table 1.2 Biomass Inputs into Agricultural and Agroforestry Systems
(Agroecosystem, Kjettslinge, Sweden)

Experimental treatment NPP Net additions to soil

Barley 0 a 484 250

Barley 120 b 963 300

Grass ley 1464 400

Lucerne ley 1580 400

Source: Modified from Andrén, O., T. Lindberg, K. Paustian, and T. Rosswall,
eds. 1990. Ecology of Arable Land. Organisms, Carbon and Nitrogen Cycling .
Ecological Bulletins 40. Copenhagen: Munksgaard.
Note: All input means in grams of dry matter per square meter per year (gdm m −2
yr −1 ). Net additions include roots and crop residues.
a 0 = no fertilizer.
b 120 = 120 kg ha −1 yr −1 nitrogen fertilizer.