The influence of earthworms, Allolobophora caliginosa Savigny, and grass grubs, Costelytra zealandica White, on the availability of pasture soil nitrogen

Abstract

The importance of organisms in soil formation has long been recognised (Kubiena, 1955, 1965; Jacks, 1963a). In the early stages of soil formation, a restricted pioneering flora and fauna invade the new area. After the new habitat is developed by organic weathering (Syers, 1964), characteristic successions of flora and fauna developed accordingly. In primitive soil developed from bare rocks through a succession of lichens and mosses the following sequence of soil animals occur; acarids, mites (particularly Oribatei), collembola and other insects (Stebaev, 1963; Van der Drift, 1965), In well developed arable soils in temperate regions, earthworms and scarabaeid larvae, among other soil animals, are important in their evolution. Soil, which includes an associated flora and fauna is continuously changing and hence, is in a dynamic state. Without organisms, soil is not fertile, and is said to be static.

An important part of the organic cycle is the living soil. In the soil system, intrinsic energy is continually in a dynamic state. This concept includes the addition to soil of organic materials, which supply nitrogen and other plant nutrients. Soil organisms keep this cycle in operation, and hence keep soil fertile (Taylor and Pohlen, 1962; Jacks, 1963b).

Soil nitrogen is low in the early stages of soil formation, but in well developed soils, nitrogen reserves are much higher as a result of the greater accumulation of organic matter (Walker, Orchiston and Adams, 1958). This suggests that reserves of soil nitrogen are built up and conserved. Nitrogen accumulates mainly the form of organic matter which is subjected to the following important soil biological processes;

(i) breakdown of organic matter, (ii) humification, (iii) mineralization.

Mineralization is essentially a biological process which transfers organic nitrogen into inorganic, available forms, namely ammonium and nitrate forms. Mineralization of organically fixed nitrogen has long been recognised as essentially a microbiological process (Waksman, 1922; Broadbent, 1962). The importance, however, of the initial breakdown of bulky organic matter by saprophagous soil fauna cannot be over emphasized. Kevan (1962) and McGarity (1958) point out that soil animals, especially earthworms, prepare soil organic matter to a suitable physical state for micro-organisms to work on it.

Harmen and Schraven (1957) have exhaustively reviewed all factors associated with mineralization of soil nitrogen, but they have barely touched upon the probable significance of soil fauna. More recently, other workers such as Barley and Jennings (1959) and Heath (1962, 1963), have found that soil fauna, especially earthworms, have some influence on the availability of nitrogen in soil.

In New Zealand, nitrogen is rarely applied art artificially to pasture soil and levels are maintained by reliance on rhizobia, associated with clovers, fixing atmospheric nitrogen and eventually making it available to plants. While many aspects of soil nitrogen availability are known New Zealand (Doak, 1952,1954; Robinson, 1962; and Ross, 1966a, 1966b), virtually nothing known of the possible role of soil animals the general nitrogen cycle in this country.

The work of Stockdill (1949, 1966) and Neilson (1951, 1965) suggests that earthworms may be important in this cycle. Pasture growth responses, after insecticide application, suggest a response to nitrogen in addition to the release from feeding pressure pasture pests. The widespread occurrence of large populations of the earthworm Allolobophora caliginosa Savigny, (Waters, 1951, 1955), grassgrub, Costelytra Zealandica (White, Kelsey, 1951), and porina caterpillar, Wiseana cervinata Waker (Doull, 1951), suggest that these animals could play an important role in the nitrogen cycle. With these points in mind, the following objectives were formulated as a basis for this study:

(1) To study the influence or the most commonly occurring macro soil fauna, earthworms and grass grubs, on the availability or soil nitrogen. (2) To study the influence ir micro-organisms associated with those two soil animals on the availability or soil nitrogen. (3) To study the problems associated with the use or electrolytic respirometers y in a soil nitrogen study or the kind proposed.