Proceedings of the Association of Applied Biologists 2‘5 LEES, A. H. (1917). ‘Reversion’ of black currants. Ann. Rep. Long Ashton Res. Stn for 1916,

LEES, A. H. (1922). Leaf characters in reverted black currants. Ann. appl. Biol. 9, I . MASSEE, A. M. (1952). Transmission of reversion in black currants. Rep. E. Mulling Res. Stn

SMITH, B. D. (1960). The behaviour of the black currant gall mite (Phytoptus ribis Nal.) during

P- 31-

for 1951, p. 162.

the free-living phase of its life cycle. Ann. Re$. Long Ashton Res. Stn for 1959, p. 130.

Ann. appl. Biol. (1961), 49, 215-218

Metabolism of soil invertebrates in relation to soil fertility

BY AMYAN MACFADYEN University College of Swansea

It is useful, if not quite accurate, to distinguish between the mechanical and chemical activities of the soil fauna. Other papers have been concerned with mechanical activities and I do not propose to consider them now except to point out that in most fertile soils the greater Rart of dead plant matter is processed by the guts of the larger invertebrates. When we consider moorland and other poor soils, however, these larger forms are usually the first to disappear- with marked effects on the structure and constitution of the soil.

I propose to consider the chemical effects of the fauna under two headings: first, effects resulting directly from the animals’ metabolism and secondly, those in which the animals’ role is more in the nature of a catalytic one.

The difficulty of comprehending the interacting effects of animal metabolism on soil can be appreciated when it is remembered that most soils contain up to 1000 species of animals. The complexity of the ‘food web’ in such a situation is enormous and the best way to start to study such a system is to make measurements and try to ascertain which of many possible links are the most important.

Unfortunately this is not yet possible for any single environment. However, I have, by taking many liberties with the data and combining figures from a variety of sources, produced a diagram (Fig. I ) which illustrates approximately the way in which the energy of photosynthesis is transferred through a typical meadow under temperate conditions. Only about 0-5 % of solar energy striking the earth‘s surface is utilized by photosynthesis and the ‘channels’ on the diagram (whose width is logarithmically proportional to the energy flow in small calories per square metre per day) illustrate the fate of the photosynthesized energy.

The main points to notice are that about one-sixth of the whole is immediately dissipated by plant respiration, that although one-quarter is consumed by herbivores about half this is returned to the soil as faeces, dead bodies and other waste products, so that finally nearly three- quarters of the whole amount passes to the soil as dead organic matter. In other words when we exploit pasture by cattle grazing less than one-sixteenth of the products of photosynthesis is potentially available to the cattle and less than 0.05 % is in fact likely to contribute to human food.

The ‘boxes ’ representing the calorific content of the standing crops of the different groups of organisms are also drawn on a logarithmic scale. It should be noticed that the arbitrarily fixed ratio of standing crop to respiration per day decreases through the food chain ; it requires about 1000 calories of plant protoplasm to process I calorie per day in respiration but the ratio for cattle and for man are about zoo and 1 5 0 to I .

The second figure (Fig. 2) shows what happens to the energy reaching the soil as organic waste. Here data are even more difficult to come by and I have not even managed to complete the picture; especially since the relations of the predators to their prey are uncertain and the proportion of energy metabolized to food attacked is rarely known. I have therefore given only

216 Proceedings of the Association of Applied Biologists

\ \ 2,400 j 10.000

I I I Grasses f 10 scale 2.000.000

10 Flow 100 scale m . / M z / d a y 24.ooo

-

Fig. I . Transfer of energy of photosynthesis through a typical meadow.

29,000 +, Nematoda 90 j Woodlice i

26,000

- - - . . . .

12,686

1

Fig. z. Fate of energy reaching soil as organic wastes.

Proceedings of the Association of Applied Biologists 217 net energy utilization figures for these organisms and it should be appreciated that much of the food ‘consumed’ is returned to the reserve of dead organic matter as material killed but not eaten and as faeces.

It will be seen from the flow figures that the microbes liberate about 85 yo of the energy, protozoa perhaps 8 O/,-although this is a particularly variable and unsatisfactory figure-the larger invertebrates about 4 % and the small arthropods and nematodes about 4 % between them. The ratio of standing crop to flow is about 400: I for the micro-organisms, 200 : I for the ‘meso fauna’, and varies up to nearly 1000 : I for the large invertebrates.

Looked at from the point of view of the plants, dead plant matter once used contains valuable nutrients now out of circulation and the longer these remain bound up the less available will the nutrients be. Thus ‘ bottle-necks ’ in the energy-flow picture represent a threat to fertility, while anything which increases the flow from the large ‘boxes ’ in the diagram-the reserves of dead organic matter and to a lesser extent the micro-organisms and large invertebrates-will increase the availability of nutrients.

If we are to understand more of the role of animals in clearing these bottle-necks we need to know more about the numbers and food preferences of the fauna and microflora, we require metabolic life tables of the different species showing how much whole populations respire, we need knowledge such as Phillipson (1960) has recently provided for Mitopus morio on the relations between feeding and metabolism and we also require much more information on particular soil types, especially the poorer ones.

T H E C A T A L Y T I C A C T I O N O F A N I M A L S O N S O I L M E T A B O L I S M

It is only recently that we have begun to realize that animals may have effects on the meta- bolism of the soil out of proportion to their own respiration rates. This conclusion is to some extent conjectural and I should like first to list a number of observations and facts which point towards it.

(I) The numbers of living micro-organisms visible in soil are usually low in comparison with those which become apparent when small pieces of humic material are incubated in humid conditions in the absence of animals.

(2) The census of soil fauna shows that there are numbers of the order of I O ~ microbe-feeding arthropods and 1o6 to 10’ microbe-feeding nematodes per square metre. Rough calculations based on Witkamp’s (1960) work indicate that the arthropods may consume up to ten times their respiration value of such forms-in other words, that they are inefficient but prolific browsers of micro-organisms.

(3) Hinshelwood (1951) was perhaps the first to show that bacterial colonies, after initial phases of growth and peak activity, become senescent. This has since been confirmed and also shown to apply to fungi. This means that micro-organisms in the absence of grazing become ‘bottle-necks’ in which metabolism is slow in relation to energy content and it suggests that grazing by animals may accelerate the energy flow. (4) The widespread occurrence of mycostasis and bacteriostasis resulting-it is suggested-

from the presence of antibiotic substances in soil means that much potential microbial activity is held in abeyance. Stasis of this kind can be abolished by some plating techniques used to estimate micro-organisms. It has also been shown recently (Witkamp, 1960) that the introduc- tion of single terrestrial Trichoptera or Glomeris marginata to soil cultures can cause hundred- fold increases in spore germination.

The doubling of total soil respiration by the introduction of small numbers of earthworms reported by Satchell (1960) possibly illustrates a different aspect of the same phenomenon.

( 5 ) The demonstration by Witkamp (1960) that soil oribatid mites can rapidly cause the re-infection of soil samples by transporting micro-organisms on and inside their bodies illustrates another way in which microbial metabolism may be enhanced due to insemination in more favourable places.

(6) It seems likely that the corpses, faeces and exuviae of animals will provide enriched sites from which soil micro-organisms can spread into surrounding soil. Satchell (1960) has given a figure of z cwt. per acre as the equivalent in sodium nitrate to the nitrogen content of earth-

218 Proceedigs of the ,hociation of *4pplied Biologists worms. The small forms often have very short generation times-a few days for the smallest nematodes, a few weeks for many arthropods-and must produce great numbers of corpses containing high concentration of nutrients. Some fungi, of course, do not wait for the deaths of their food organisms and capture them alive.

These points seem to me to show that we should take seriously the possibility that soil animals promote metabolism on a vastly greater scale than that which results from their own internal physiology: by grazing, by breakdown of 'stasis', by transport of spores and by producing foci of high nutrient status. This working hypothesis is open to investigation by simple laboratory experiments, by direct observation, bv exclusion experiments such as those pioneered by Bocock & Gilbert (1957) and by the measurement of respiration rates of systems of varying complexity. I Tvould advocate this as a field of research in which the problem can now be defined, the techniques exist, the theoretical interest is great and the practical significance with respect to our understanding of the measuring and control of fertility cannot be doubted.

R E F E R E K C E S

BOCOCK, I(. I,. & GILBERT, 0. J. W. (1957). The disappearance of leaf litter under different

HINSHELWOOD, C. (195 I). Decline and death of bacterial populations. Nature, Lond., 167,666. PHILLIPSOX, J. (1960). A contribution to the feeding biology of Mitopus movio (F.) (Phalangida).

J. Anim. Ecol. 29, 35. SATCHELL, J. F. (1960). Earthworms and soil fertility. Nezc Scientist, 7, 79. WITKAMP, M. (1960). Seasonal fluctuations of the fungus flora in mull and mor of an oak forest.

woodland conditions. Plant &' Soil, 9, 179.

Inst. toeg. bioi. Ondem. Nat . Med. 46, I .