Further Reading

be it leaves, needles, or stems, some vascular plants produce woody tissue (i.e., twigs, branches, and logs). Although initially terrestrial, vascular plants grow in, and produce detritus into, the terrestrial, the freshwater, and the marine environment (seagrasses and salt marsh vegetation). Macroalgae solely occur in the marine environment, but some of their detritus is exported into the semiterrestrial environment of the littoral and supralittoral coastal area. Under particular conditions, such as in flood plain forests, initially terrestrial detritus that has been deposited in fresh-waters may, along with small amounts of detritus of aquatic origin, occasionally be relocated into the terrestrial system (e.g., upon seasonal inundation or, as in the case of the marine-terrestrial transition zone, upon extreme tide events). On the other hand, inundation of flood plains may import additional terrestrial material from the terrestrial into the aquatic system.

Considering the overwhelming amount of dead plant material that is annually supplied into the detrital food chain, another contributor that has rarely been investigated should be mentioned here. In systems characterized by vascular plants, the belowground biomass (i.e., roots) equals or even exceeds aboveground biomass. The input of died-off roots into the total detritus pools, as well as how and how fast belowground detritus is degraded, however, is largely unknown. More detailed studies on this topic are urgently needed.

Even though some sources of detritus provide detrital material continuously, the release of detritus exerts some quantitative (and qualitative) seasonality. Leaf litter input into terrestrial and aquatic (mostly freshwater) systems (i.e., senesced leaves of broad-leaved deciduous trees) peaks during fall, but some leaves are shed while still green and photosynthetically active, owing to physical disruption, be it through animal action or wind. For the same reasons - mostly physical disruption but also senescence - twigs and smaller branches or needles of coniferous trees fall onto the forest floor or into forested water bodies. By contrast, bigger branches and logs become detritus through anthropogenic action (silviculture and logging) or upon death of the tree. All these events are more or less evenly distributed seasonally. Aquatic vascular plants - freshwater macrophytes, marine seagrasses, and intertidal salt marsh vegetation - exhibit strong seasonality with increased release of detritus during fall, in some cases accompanied by a total die-off of aboveground biomass. This seasonal pattern gets enforced by storm-driven wave action that predominates in fall and, to a lesser extent, in spring. Marine macroalgae, finally, show a pattern of detritus production that is similar to, but less marked than in, aquatic macrophytes.

In addition to providing an important autochthonous source of nutrients in most ecosystems (see Table 1), detritus also acts in linking adjacent habitats by means of an allochthonous subsidy (Figure 1). Thus, huge amounts of up to several kilograms of marine macrophyte detritus (often called wrack) is deposited per kilometer shoreline at the upper margin of the intertidal zone of rocky shores and soft sediment beaches with each incoming high tide, summing up to hundreds of tons per kilometer shoreline per year. Owing to the strong influence of tidal water movements and coastal winds as well as fast decomposition rates of some but not all marine detritus, the resulting

Table 1 Estimated average production of detritus in selected generalized types of ecosystems (based on different sources)

Production (g m 2yr 1)

Freshwater macrophytes

300

Freshwater microalgae

Phytoplankton

20

Phytobenthos

20

Marine microalgae

Phytoplankton

20

Phytobenthos

30

Marine macrophytes

Seagrass beds

500

Marine kelp forests

800

Macroalgae

300

Terrestrial macrophytes

Saltmarshes

800

Mangrove forests

900

Tropical forests

600

Deciduous forests

300

Boreal forests

50

Grasslands

75

Tundras

25

Note that variation around these averages may be high when comparing, e.g., temperate and tropic systems, or seasonal effects.

Note that variation around these averages may be high when comparing, e.g., temperate and tropic systems, or seasonal effects.

Figure 1 Possibilities of exchange of detritus between, and detrital connectivity of, ecological systems. The strength of an arrow indicates the significance and probability of the depicted connection and its direction.

high-intertidal wrack line is subject to enormous temporal variability in both quantity and quality of detritus. On the other hand, tropical mangrove forests and temperate intertidal salt marshes export an annual amount of up to roughly 11 angiosperm detritus per kilometer coastline into the coastal area, adjacent estuaries, and the open sea. Detritus of terrestrial origin is also the major energy source to many lentic freshwaters and low-order streams in forested areas, making up several hundreds of grams per meter square per year. The contribution of terrestrial detritus to freshwaters is mostly significant during winter (i.e., after autumnal leaf fall and under conditions of low productivity of planktonic or benthic aquatic plants).

Composition

The species composition of detritus in different ecosystems reflects the relative importance of different sources to the respective habitat. Within a given system, it is necessary to distinguish between terrestrial angiosperms, aquatic macrophytes, and algae (cf.Figure 1). Terrestrial plant detritus has a substantially wider C:N (and C:nutrient) ratio than most aquatic organic matter, and is thus more difficult to decompose. An even narrower C:N ratio than in aquatic vegetal detritus is characteristic of detritus of animal origin (see below), the significance of which varies temporally and between distinct habitats.

Wrack deposits in the high intertidal can be rich in nitrogen and poor in structural defense compounds, if they mainly consist of macroalgal material and animal necromass, or they are rich in cellulose and lignins but poor in nutrients, if mostly angiosperms, be it subtidal seagrasses, intertidal salt marsh vegetation, or terrestrial vegetation, contributed to the wrack. Similarly, it is essential to freshwaters whether the detrital source is aquatic macrophytes and planktonic algae or terrestrial angios-perms. In the terrestrial realm, we distinguish between monocot detritus in grasslands and dicot detritus in forests, the latter, in turn, originating from either shrubs or woody plants, being either deciduous or coniferous trees.

All these types of detritus remarkably differ from each other in terms of chemical composition (Table 2) that, in turn, controls decomposition processes of detritus (see Table 3). Upon decay, the C:N ratio of the detritus-microbe conglomerate usually significantly drops, owing to the developing dense microbial biomass that accumulates nitrogen from the surrounding environment as well as leaching of carbonic but retention of nitrogenous detrital compounds.

Decomposition

Detritus of whatever origin is degraded through leaching of water-soluble compounds of mostly low molecular mass, the action of microbial and fungal decomposers, and feeding by animals, named detritivores (mostly in aquatic ecology; Latin: detritus: scree, rock debris; vordre.

Table 2 Comparison of terrestrial and aquatic sources of detritus with respect to estimated ranges of contents (%) of significant determinants of detrital quality to decomposing organisms (based on different sources)

Limnetic

Marine

Terrestrial angiosperms

Angiosperms

Algae

Angiosperms

Algae

Cellulose

10-30

0-25a

20-30

0-25a

20 50

Hemicellulose

10-20

_b

10-15

-b

10 30

Lignin

10-20

c

15-20

c

10 30

Phenolics

1-15

0-5

1-10

0-10

2 20

Lipids

1-5

1-5

1-5

0.5-5

1-10

C:N

10-30

10-15

20-30

5 25

20 25

Nitrogen

1-5

1-5

2-5

1-5

0.1 5

aAmong algae, it is only green algae (Chlorophyta) that contain cellulose in their cell walls.

bBrown (Phaeophyta) and red algae (Rhodophyta) contain alginate, carrageenan, or agar, being structurally and functionally similar to hemicelluloses, in concentrations of up to 30%, but hemicelluloses are restricted to angiosperms.

cSome green algae (i.e., Charaphyceae) contain lignin-like precursors, but lignins are restricted to angiosperms.

aAmong algae, it is only green algae (Chlorophyta) that contain cellulose in their cell walls.

bBrown (Phaeophyta) and red algae (Rhodophyta) contain alginate, carrageenan, or agar, being structurally and functionally similar to hemicelluloses, in concentrations of up to 30%, but hemicelluloses are restricted to angiosperms.

cSome green algae (i.e., Charaphyceae) contain lignin-like precursors, but lignins are restricted to angiosperms.

Table 3 Ranges (based on values reported in the literature) of decay rates (-k d-1) for selected types of detritus decomposing in different habitats

-k (d-1)

Decomposition habitat

Freshwater detritus

Macrophytes

0.01-0.04

Aquatic

Marine detritus

Macroalgae

0.01-0.07

Subtidal

0.005-1

Intertidal

Seagrass

0.007 0.02

Subtidal

0.005 0.01

Intertidal

Terrestrial detritus

Wood

0.0001 0.007

Aquatic

0.0001 0.003

Terrestrial

Leaf and needle

litter

Narrow C:N

0.01 0.07

Aquatic

0.01 0.2

Terrestrial

Wide C:N

0.001 0.01

Aquatic

0.001 0.01

Terrestrial

to gobble, to gulp), being detritivorous, or saprophages (mostly in terrestrial ecology; Greek: sapro's. rotten, foul; phagein: to feed), being saprophagous. Besides the purely physical process of leaching, biochemical (degradation by microbes and digestion by detritivores) and biological processes (dislocation, fragmentation, and numerous animal-microbe interactions) play major roles in decomposing detritus (Figure 2). Some authors consider leaching and microbial degradation of detrital compounds as decay and distinguish it from decomposition that in addition includes the action of detritivores. Nonetheless, the most widespread parameter that describes detrital mass loss over time is called decay rate:

with Mt, the mass of detritus at time t; M0, the initial mass of detritus (at time 0); t, time (in days); k, the decay rate. Decay rates are usually expressed per day (d-1), but may also be given per year. Wood decay (see below) depends on the diameter of the twig, branch, or stem, and is usually measured by means of tissue toughness or density.

Because of having received most of scientific interest in decomposition processes, leaf litter (i.e., annually shed leaves of deciduous trees) will be used as a model to exemplify decomposition processes. While the outline presented here can be generalized in first approximation, decomposition of other types of detritus may differ in detail to varying degrees. Mass loss rates that are generally used as a measure of decomposition rates not only depend upon the type of detritus (see Table 3), but also on numerous environmental factors, such as moisture (in terrestrial habitats), temperature, or the presence and activity of detritivorous animals. Detritus can be classified as decomposing fast (-k>0.01; >1% d-1), medium (0.001 < -k<0.005; 0.1-0.5% d-1), or slowly (-k<0.005; <0.1% d-1) (see Table 3).

Leaf litter

When freshly fallen plant litter is exposed to water, water-soluble compounds (e.g., amino acids, simple sugars, or phenolics) are rapidly lost as DOM. This early-stage leaching accelerates litter degradation through the promotion of microbial activity and increased palatability of the litter to detritivorous animals. On the other hand, excessive leading results in low-quality litter remnants that decompose slowly (see below). The amount of mass lost through leaching varies according to both the leaf species and environmental conditions such as moisture. Once dissolved in water, be it an aquatic environment or interstitial soil water, DOM is prone to chemical degradation through microbial activity (decay), eventually resulting in the formation of humic substances of varying chemical

Detritus Subsystem

Figure 2 Chemical fluxes between, and biotic interactions of, different compartments of the detrital subsystem in aquatic and terrestrial habitats. Broken arrows between detritivores and microbiota depict the diverse trophic, symbiotic and synergistic interactions of these central players in decomposition processes.

Figure 2 Chemical fluxes between, and biotic interactions of, different compartments of the detrital subsystem in aquatic and terrestrial habitats. Broken arrows between detritivores and microbiota depict the diverse trophic, symbiotic and synergistic interactions of these central players in decomposition processes.

Figure 3 Simplified scheme of changes upon decomposition in detrital mass (broken line) and contents of selected compounds (plain lines) (in % of initial value), as well as microbial activity against lignocellulose (dotted line) (arbitrary units).

composition. The other significant contributor to humic substances is the degradation of recalcitrant lignocellulose.

While leaching results in initial mass loss of detritus, the relative content of recalcitrant detrital compounds increases during the initial phase of decomposition (Figure 3). At the same time, rapid microbial colonization leads to both a spatial accumulation of nitrogen and increasing degrading activity on the detrital surface. Owing to a successive shift from microbes that utilize easily accessible low-molecular-mass compounds present at the detrital surface upon leaching to those that degrade lignocellulosic compounds, lignocellulolytic activity peaks after considerable mass loss through leaching and early degradation. Throughout late decomposition stages the formation of diverse humic substances due to the degradation of proteins, cellulose, and phenolic compounds (including lignins) stabilizes the remaining detrital mass through inhibiting degradative microbial activity.

The decomposition rate, determining the release of nutrients to be available for growing plants and microorganisms, depends on both physical and chemical detritus characteristics (see Table 3) and the activity of organisms involved in decomposition. Accordingly, some types of leaf litter (e.g., alder, elder, or chestnut) decompose within a couple of months, while others (e.g., oak, beech, or eucalypt) may last for years. Needle litter decomposes even more slowly. Microbial colonizers of detritus act in processing their substrate, rendering it an acceptable food source for detritivores. Most of the detri-tal biomass, particularly recalcitrant compounds such as polyphenols and lignocellulosic cell wall compounds, require microbial activity for its degradation. Thus, microbial litter colonizers serve detritivorous animals twofold. They precondition the detritivores' food source and they are utilized as readily available source of (partly essential) nutrients. On the other hand, microorganisms are supported in degrading detrital matter through fragmentation by animals (see below), resulting in a significantly increased detrital surface. Further, microbial cells and propagules that survive the passage through a detritivore gut find themselves on a favorable substrate, detritivore feces, spread on a larger scale than microbial propagules themselves could achieve, and relocation of detrital particles by detritivores into favorable microhabitats facilitates microbial activity.

Consumption of detritus by detritivorous animals is the most obvious mode of mass loss. Detritivores belongs to numerous diverse taxa, including mollusks, crustaceans, myriapods, insects, mites, nematodes, and annelids. There is good evidence for general preferences of detritivores for some species of plant litter over others. Some consumers meet their decision based on the micro-bial community that inhabits the litter, but since microorganisms colonize different litter types at different species compositions (and densities), the outcome of this feeding behavior is similar. Typically litter of Betulaceae, Ulmaceae, Oleaceae, and Aceraceae is preferred over that of Fagaceae and gymnosperm trees, and litter of dicotyledonous plants over that of monocots. Toughness of plant tissue, and contents of nutrients and repellents, respectively, are probably major reasons for the observed feeding preferences. Generally, high nitrogen contents promote decomposition processes, while phenolics deter feeding by detritivores, impair enzymatic degradation processes, and thus, slow down decomposition. Thus, the loss of phenolics through leaching comprises an important initial step of decomposition that determines subsequent processes, and overwintered litter which is low in phenolics is a preferred food source to detritivores and decomposes faster than fresh litter.

Similarly, most detritivores exhibit a marked preference for litter that carries high microbial biomass, exerting high microbial activity. This may proximately be due to phagostimulating compounds present in the microbial biomass. Further, detritivores obviously gain from feeding on microbially inoculated litter, but the ultimate reasons for this have not been unambiguously identified. Microbial biomass may significantly contribute to nutrition by providing essential nutrients, and/or ingested microbiota may assist in digestive processes, through the release of digestive enzymes that can even break recalcitrant compounds such as lignocellulose. Alternatively, or in addition, microbial processing of the litter prior to detritivore consumption may be the determinant for this feeding preference.

Feeding by detritivores results in fragmentation ofthose detrital particles that are not ingested. Fragmentation of detritus may increase leaching through increased surface area, but at the same time, nutrient release is reduced, owing to immobilized nutrients in increasingly produced microbial biomass. In the long term, however, fragmentation increases nutrient availability by stimulating microbial activity. Grazing on litter-colonizing microbiota by detri-tivores may be expected to reduce microbial biomass but, in part due to selective grazing on inactive or senescent cells and inducing nutrient release and compensatory regrowth, increase microbial activity and thus possibly even microbial biomass in the long run.

Since detritus has, on one hand, to be processed by microbiota before detritivores feed on it, on the other hand, microbiota are supported in decomposing the litter through fragmentation by soil animals, the interactions of the soil fauna and microbiota are, at least in part, considered truly mutualistic (Figure 2).

Leaf litter decomposition on land

Early estimates suggested the soil fauna to contribute less than 5% to overall decomposition. Thus, indirect contributions to decomposition processes were assumed to prevail. These estimates, however, were based on respiration data and did not account for the large amounts of fragmented, ingested, and digestively utilized litter. Currently, most students of decomposition processes agree that the contributions ofmicroorganisms and animals are roughly equivalent. On average, soil animal activity increases decomposition by almost 25% through indirect promotion of microbial detritus degradation. In addition to those interactions of microorganisms and detritivores that are common to all habitats where decomposition is mediated by animals, saprophagous soil animals control microbial activity during decomposition processes through the creation of favorable conditions for microbial decomposers and dislocation of leaf litter fragments into these conditions, as well as through dispersal of microbial pro-pagules and inoculation of leaf litter.

Mechanical breakdown of lignocellulosic material, remaining in leaf litter after leaching, by chewing detriti-vores (shredders in aquatic ecology, macrofauna in terrestrial ecology) facilitates its enzymatic degradation. Fragmentation of detritus positively affects the biomass and/or activity of detritus-colonizing microbiota. Microbial biomass and activity increases when fragment size drops, but fragmentation may also inhibit fungal growth when particle size drops below a certain level of about 0.3 mm. Thus, the ratio of detritus-colonizing bacteria and fungi, and hence further decomposition processes, may in part be determined by particle size. Both a detritivore's ability to fragment litter and the fragment size resulting from fragmentation strongly depend on the litter type and, in turn, in part determines its significance for decomposition processes.

Upon decomposition and subsequent mineralization brought about by microbial activity, nutrients derived from the decaying and disintegrating leaf litter material are eventually incorporated into the soil, where they are either taken up by the vegetation and soil microorganisms or may be incorporated as humic substances into clay— humic complexes that are recalcitrant toward further degradation and hence act as long-term nutrient store. In flood plains, leachates from litter remnants and nutrients released from detritivore feces or microbial cells that are dissolved in the interstitial soil water may eventually reach the hyporheic zone of the adjacent freshwater, providing input to the aquatic pool of DOM. It is already during these fluxes that degradation of DOM proceeds.

Decomposition in water

Much less is known about decomposition processes in the aquatic environment; particularly marine processing of detrital matter has been studied in detail only recently. Except for the fact that leaching of water-soluble compounds proceeds much faster in water than on land, the basic processes are considered similar. However, there is recent evidence that soluble compounds are mainly degraded in the water column after leaching early during decay rather than inside the particulate detritus. While forested freshwaters, shaded by canopies, are fed by the energy input from terrestrial plant litter, open water bodies without (or with little) shoreline canopy exhibit higher significance of autochthonous primary production by phytoplankton, macrophytes, and their attached algae. In these systems, the significance of allochthonous detritus for nutrient cycles is mainly during winter, when in situ primary production is negligible. Overall, however, freshwaters must be considered heterotrophic (i.e., they are fueled from outside). By contrast, marine systems may be net producers or net consumers, depending on both adjacent habitat types (and the corresponding amounts and types of detritus) and the prevailing water currents.

Owing to water movements, in contrast to the situation in the terrestrial environment with mostly negligible effects of wind, detritus accumulates at sites with little or no flow, rendering the in situ detritus distribution patchy (see above). While these trapped accumulations provide valuable shelters and refugia for benthic animals, detritus accumulation at large scales may result in temporal creation of anoxic conditions underneath the patches that, in turn, will reduce decomposition processes, because many of these are oxygen-dependent, especially those that involve animal activity.

In aquatic habitats, particularly in freshwaters, it is mostly fungi (aquatic hyphomycetes) that are involved in breakdown of POM, but benthic shredders interact with them in similar manners to terrestrial detritivores. Considering DOM a contributor to detrital matter (see above) that makes up 10-25 times the annual amount of POM, planktonic bacteria are just as important in decomposition, or even more important (see below), as benthic microorganisms acting on POM are. Thus, the dominance of fungi that prevail the benthic decomposition of angios-perm detritus diminishes when suspended detritus of algal or microbial origin or DOM, being mostly decomposed by pelagic bacteria, is considered. Upon eventual sedimentation of suspended POM, both benthic bacteria and fungi act together.

In both cases, it is essentially only DOM (and some fine-POM) that can be transported to, and enter, adjacent habitats. Through this transport, remarkable amounts of c. 0.4 x 109tC of terrestrial detritus reach the open sea annually worldwide. By contrast, POM sediments and is decomposed by benthic organisms. Upon seasonal flood events, part of the benthic detritus may be relocated into terrestrial floodplains, but the major portion of detritus is eventually incorporated into sediments and hyporheic waters, where oxygen consumption through aerobic degradation processes may result in anoxic conditions.

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