CHAPTER 4: EFFECTS OF A NEEM-AMENDED PRODUCT AND A SUCCESSIVE
4.4 DISCUSSION
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correlates with the results obtained by Gruzdeva et al. (2007), who found that changes in the nematode community composition, as well as population densities of different genera, depended on the type of fertilisers applied in a long-term study on a meadow planted with timothy grass (Phleum pratense L). In a long-term biodiversity study conducted by Dietrich et al. (2021) in Germany, focusing on nine different plant species, including five different types of grass, it was found that two grass species viz. bulbous oat grass (Arhenatherum elatius (L.) P.Beauv. ex J.Presl & C.Presl, 1819) and Cock’s-foot grass (Dactylis glomerata L.), had an overall positive effect on nematode diversity. Their study further found that nematode communities were greatly impacted by plant species diversity. Bacterivores especially were more diverse and abundant in plots with more plant species mixtures than in monocropped plots. Their study also showed that PPN abundance was higher in monocropped plots than in grass and mixed plant species plots.
This supports the findings in the present study, where bacterivore abundance increased, while PPN abundance decreased slightly after the 27-month White Buffalo grass fallow period.
Regarding M. javanica, the predominant PPN species that occurred in the Deerpark trial site where this project was done in particular, this correlates with a report by Lenné (1981) that White Buffalo grass is a poor host to this RKN species in Brazil.
From the results of the present study, it would seem that bacterivore nematodes were more resilient against environmental changes and increased with the increase of available organic matter as shown in the multivariate PCA results. No information in this regard could be found for potato studies, but this trend partially coincides with the findings of Liu et al. (2016), where the addition of amendments (straw and mineral fertilisers) showed an increase both in bacterivore (Eucephalobus) and fungivore (Filenchus) nematode population densities in a rice (Oryza sativa L.)-wheat (Triticum aestivum L.) agroecosystem. The results from this study showed an increase in only bacterivore nematode abundance similar to that observed in the study conducted by Liu et al. (2016), while fungivore nematode abundance did not correlate with the increase in organic material. The study of Herren et al. (2020) also did not find a correlation between increased organic material and fungivore nematode abundance and supports the findings of this study. The article stated that compost effects on soil nematodes in various crops can be variable and this is supported by various other studies (Herren et al., 2020; Renčo et al., 2010; Steel et al., 2012;
Thoden et al., 2011). The effects of neem-amendments have not been as widely studied as the effect of composts, however, a study by Abbasi et al. (2005) found that the addition of different neem seed-cake treatments to soil increased or had no significant effect on the beneficial soil nematode population densities in a tomato (Solanum lycopersicum L.) study in Canada, which correlates with what was observed in this study. Another study by Langat et al. (2008), however, contradicts this, as their findings showed that neem significantly reduced beneficial soil
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nematodes to a level comparable to that obtained with fenamiphos (Nemacur®) in carnation (Dianthus caryophyllus L.) in Kenya.
Coinertia analysis identified correlations between soil chemical parameters and nematode communities. However, no obvious correlations with specific parameters were apparent. At 10 WAE, pH and Ca (mg/kg) were correlated with elevated levels of RKN in the roots and stubby root nematodes in soil and roots while at harvest, RKN was positively related with Al (mg/kg) and
% clay. This corroborates the findings by Nielsen et al. (2014), who sampled 12 environmentally diverse regions globally and found that nematode family abundance and diversity showed no significant correlations with local environmental properties. A more recent study by Nguyen et al.
(2020) found that the abundance of PPN was closely related to soil pH, while beneficial soil nematodes were related to the soil electrical conductivity (EC). These kinds of correlations were not observed in this trial.
The effect of a natural fallow period, with the natural growth of White Buffalo grass, did not seem to improve the soil health nor did it significantly impact on the overall beneficial soil nematode community composition over time. However, PPN abundance decreased (by 72%), substantiating results by Lenné (1981) specific for M. javanica that this grass is a poor host of this RKN species, while bacterivore abundance increased (by 186%) over time in the untreated plots (purely the effect of the grass fallow). Adediran et al. (2005) showed that PPN population densities were suppressed by 30-49% with natural bush regrowth fallow over 12, 18, 24 and 30 months in different agroecological systems in south-western Nigeria, which supports the findings in this trial.
The natural bush regrowth fallow showed the lowest percentage suppression of PPN than other cover crops tested in the trial by Adediran et al. (2005). The findings in this trial were also supported by that of Nguyen et al. (2020), who showed that, although the abundance of different nematode trophic groups changes in the Mekong Delta in Vietnam during seasons, the overall nematode community composition remained similar over three climatic cultivation seasons between 2017 and 2018, in a multi-year rice cropping system.
During the extreme drought conditions experienced under high temperature conditions from the onset of this trial, 627 mm rainfall experienced in the 2018/2019 season and 744 mm in the 2019/2020 season compared to an average of 1148 mm during previous years and the subsequent stress to the crop are suggested to have played an extensive role in the inconclusive nematode data obtained and poor crop yield. The drought started in the 2014/2015 season, where only 443 mm rainfall was experienced in this area (38% of the average rainfall from before the onset of drought in this area). Persisting drought conditions could be a result of climate change and pose a great challenge to agricultural production and research such as this study. Climate change is known to result in irregular weather patterns, drought, extreme fluctuations in
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precipitation and temperature (Rapp, 2014), all of which were experienced during the duration of this trial. However, there is very little research available regarding the long-term effects of climate change on beneficial soil nematodes and in this respect, the present study represents some baseline data.
Besides the extreme drought conditions experienced during the duration of this study, the length of the grass fallow period may have been too short to result in substantial changes in the beneficial nematode community. The lack of plant species variation and abundance during the fallow period may also have played a role. The study conducted by Dietrich et al. 2021 showed that over 15 years, PPN abundance was higher in monocrop (single fallow plant species) treatments than in treatments that contained a mixture of plants, including various grasses. Moreover, the potentially insufficient content of both neem and manure in the Kalahari 3:1 product could also have contributed to the lack of impact experienced on the beneficial community. This was mentioned as a potential contributing factor to the lack of reduction in PPN densities in Chapter 3, paragraph 3.4.
The insignificant change in MI over time would suggest that just leaving a field after cultivation would not necessarily increase the overall soil health and additional management practices are needed to improve this specific field’s health/fertility. This suggests that the soil was stressed before planting activities commenced and very little changed over time. The c-p triangle indicated that the soil was and remained stressed over time. This is supported by the minimal changes in omnivore abundance in the field over time, indicating that the soil remained disturbed from before the trial was initiated and that this did not improve during the fallow period. As stated by Gomes et al. (2003), omnivores from the Dorylaimidae, which was the only family of omnivores found in both the soil and root samples taken from the field, are very sensitive to soil disturbances caused by agricultural practices.