Soil fauna, nitrogen dynamics and plant growth

Summary
The achievements of this sub-project was the result of extended collaboration with other DARCOF projects.

Identification of interactions between soil organisms and the effect on nitrogen dynamics
Comparison of earthworm fauna in agricultural soils from different localities in Denmark showed that the lowest mean population densities were found in conventionally managed fields based on grain cultivation (c. 30 individuals per m2), while organic farms with ley-rich crop rotations had much denser earthworm populations, but depending on soil type. The earthworm population increased by a factor 20 during a period of 10 years following conversion of a field at Reseach Centre Foulum from conventional to organic farming.

A biannual field experiment included a control and an earthworm manupulation treatment where it was attempted to reduce the earthworm population by means of intensive electro-extraction at the beginning of each growth season. The treatment resulted in relatively modest and short-lived reductions of earthworm density and biomass as compared with the control treatment. No effect of the earthworm reduction was observed on the population size of the other soil fauna group included in the study, Enchytraeidae, Collembola and Acari. Also the soil nitrogen pools, the mineralisation capacity of the soil and the plant biomass, including C- and N-content, at harvest were unaffected of the earthworm manipulation.

Density and biomass of all fauna groups were strongly reduced between the April and the May sample in 1997 when the 2-year old grass/clover field was ploughed and grown with spring barley. During that period the N-content of the soil increased strongly. Another fauna reduction occurred between September and November in 1997 when the soil was tilled and sown with winter wheat. During the periods after soil tillage the fauna populations increased and the nitrogen pools decreased. The population density and biomass of all fauna groups were high at most sampling occasions compared with most data available in the literature for cultivated soils. Estimates for the direct contribution of the fauna to nitrogen mobilisation were calculated. Earthworms contributed in 1997 with 69.2 kg N ha-1. The majority was due to earthworms killed during the tillage periods. Enchytraeidae contributed with 9 kg N ha-1 while the contribution by Collembola was insignificant (183 g N ha-1 in 1997, 311 g N ha-1 in 1998).

A mesocosm experiment comprising 6 different combinations of soil mesofauna species (Collembola, Enchytraeidae, Gamasida) forming a rank of increasing complexity showed no direct effect of the fauna on the uptake by the barley plants of 15N from tagged clover leaves, but most fauna combinations resulted in significant increases in shoot biomass at harvest compared to the control treatment.

The role of microfauna for nitrogen supply to crops in organic farming systems
Field and laboratory experiments explored the role of the microfauna in decomposition of litter of catch crops and made it possible to evaluate the relative significance of microfungi and bacteria based on the structure of the nematode community. A marked increase of bacterivorous and fungivorous nematodes and microarthropods was characteristic of the plant substrate, but was not influenced by the kind of the previous crop in the field. The results from the field experiment were confirmed by incubation experiments in the laboratory. Omnivorous nematodes seem to be able to regulate the importance of different bacteriovorous groups depending on the ressource. In summary, the microfauna studies have provided detailed knowledge about the dynamics of microbial activity, which could not have been obtained from isolated studies of microorganisms alone.

Adaptation of a crop rotation simulation model to organic farming
The FASSET model has been extended with a model for the competition between several plant species. This model is used to simulate multi-species crops, including grass/clover and pea/barley. The grass/clover model has been designed to include a model of the spatial heterogeneity in deposition of urine and dung under grazing. This implies that the model in future may be used to simulate nitrogen turnover and losses in systems where grazing livestock is an important component.

The quality of the existing data for intercropped species is not sufficiently high to permit model calibration. A measurement programme was therefore initiated for both grass/clover and pea/barley. The collected data for growth of pea/barley at different levels of soil nitrogen fertility showed that the competition between species started early in their development. The nitrogen level also affected the above-ground competition mainly through effects on green leaf area index, that was relatively higher for spring barley compared with pea at high nitrogen fertility.

Nitrogen is one of the main factors limiting production in organic farming. It is therefore essential that simula-tion models can realistically reproduce the yield response of different nitrogen levels, whether this is caused by manure or precrop effects. A comparison of methods for describing the growth response. The simulation results have been compared with data from Denmark and USA for time course of nitrogen uptake, leaf area and biomass at different levels of N application, and with data from Denmark for yield response to increasing nitrogen rates. The results show that the FASSET model is better than most other models for representing the effect of nitrogen level on crop production.

The effect of nitrogen on crop production is caused by effects on both light interception by the crop canopy and by the efficiency of use of the intercepted radiation. Under the high nitrogen rates in coventional farming the effects on radiation use efficiency are often insignificant, but an analysis of data from experiments under organic farming shows that this parameter is essential in organic farming. The FASSET model describes the effects on both parameters satisfactorily and the measured effects on radiation use efficiency are in accordance with the model parameters. The FASSET model can therefore with some confidence be used also for organic farming. The model, however, still needs to be validated against data from whole crop rotations. This part of the project has been postponed because of increased experimental effort in obtaining essential data for model calibration.

Abundance of species of Collembola and epigeic insect predators in organic farming
The aim of this sub-project was to acquire knowledge to and to develop a simulation model, which could be used as a tool to assess the occurrence of springtails and surface living insect predators in organic crop rotations. The model development was combined with the model development on DARCOF project IV.3. This made it possible to develop a model that includes a complete soil food web. Furthermore, this food web model has been combined with the soil and crop modules of FASSET (sub-project 3). Thus a very compre-hensive model has been achieved. The model has been programmed and is ready for parameterisation, validation and use in scenario analyses.

In the empirical part of this subproject the food quality of the most common springtails for two epigeic preda-tors, the carabid Bembidion lampros and the money spider Erigone atra was investigated. The results dem-onstrated that most Collembola were of limited, but acceptable, quality for B. lampros, and that I. anglicana was of extraordinarily high quality for E. atra. This means that high densities of Collembola will enhance the population of B. lampros, and that high densities of I. anglicana will boost the population density of E. atra considerably. Increased densities of these two predators will improve the natural pest control, especially the control of aphids.

In order to investigate how to enhance the springtail species the food preferences of the most common spe-cies was investigated in the laboratory. The results showed a preference for the three micro fungi Alternaria, Cladosporium and Fusarium. In field investigations we found correlation between the Collembola community and several parameters, e.g. time from sowing (autumn sown or spring sown), cellulolytic fungi, actinomy-cetes, yeast, porosity K+ and by the end of the summer the sand content.

Finally we also investigated the occurrence of springtails and mites in green manured research plots. This revealed extremely high densities of springtails (up to 120.000 m-2) and mites (up to 90.000 m-2).