Nematode community structure and function as a bio-indicator of the effects of soil amendments on soil health in deciduous fruit orchards

The nematode community structure and function was determined in various localities, under different environmental conditions and different treatments to determine the effectiveness as a bio-indicator of soil health. Through the series of treatments, it was determined that the soil nematode community and inferred soil health is strongly influenced by time and various factors including soil moisture

Soil samples were collected from the root zone of each treatment, in the work and tree row when required. The nematodes were extracted by means of the Cobb’s decanting and sieving method followed by 48 hours in a modified Baermann funnel. Nematodes were counted and identified to genus level, categorised into feeding groups according to Yeates et al. (1993) and assigned cp-values as defined by Bongers (1990).

The results were used to determine the nematode community structure and function as well as the associated soil health status of these orchards. The nematode faunal analysis was used as a means of evaluating the function and structure of the soil food web.

Nematode communities present in stone and pome fruit on an organic farm differed in structure and function and not so much due to the ‘mow and blow’ treatment that was applied. The pome fruit (quince) revealed a unique nematode community structure compared to that of the stone fruit (plum, peach and apricot).

Nematode communities under net were influenced by soil preparation techniques as well as root stock. Low numbers of plant-parasitic nematodes, especially ring nematode (Criconemoides xenoplax) and lesion nematode (Pratylenchus sp.), were present in the orchard. The G222 soils were more enriched than the M7 and M9 soil, it also indicated the highest level of soil food web structure. The Maturity Index indicated that all the entire orchard is disturbed and/or enriched.

The nematode community structure and function determined over a four-year period in a site where five different cover crop treatments were applied revealed that the enrichment and structure of the work and tree row for each of the five treatments deteriorated over the four years to such an extent that all data points indicated stressed conditions by the time the trial was suspended. This indicates that nematodes are effective as a bio-indicator to determine soil health.

Nematode feeding groups in an orchard inundated with EPNs, indicated a similar pattern in their distribution, indicating that changes in population could be due to external effects rather than EPN treatment. Multiple regression analyses using specific indicator nematode families, showed no significant differences in the nematode populations for the duration of the experiment. No succession in the soil food web could be determined. The faunal analysis for each treatment indicated the most significant effect when 5 x10⁶ IJ’s per m² is applied.

Sampling should occur at least twice a year for more than two years, preferably for 5 years to accurately determine the effect of management practices on soil health. Climate change will influence nematodes and the soil food web, therefore management practices such as the utilisation of cover crops and mulches need to be implemented to buffer the soil environment against such harsh climatic conditions. The host status of cover crops needs to be evaluated for susceptibility to pant parasitic nematodes which negatively influence the cash crop. Weather station, including soil temperature readings, would give valuable information regarding the soil environment and possible outbreaks of plant-parasitic nematodes and other pests and diseases. Information regarding the management practices followed in the orchard is likely to form a crucial part of the interpretation of the sampling analysis.