The short term influence on the soil food web base of regenerative pastoral systems in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Agricultural Science at Massey University, Manawatū, New Zealand

Abstract

Food production systems are coming under increasing societal pressure to improve sustainability and reduce environmental impacts. This has led to the development of alternative agricultural practices, such as regenerative agriculture (RA). New Zealand’s economy relies heavily on its pastoral agriculture production, while also being an important contributor to global food production. Healthy soils are essential for productive and sustainable agriculture and are a key part of the RA concept. Soil biota is a crucial component of soil health. Monitoring soil health is important for improving the sustainability of agriculture and for testing whether RA achieves its objectives. The current study is part of the Whenua Haumanu research program. This involves comprehensive farmlet studies investigating the effects of RA management and diverse pastures within New Zealand’s pastoral farming system on a wide range of outcomes, including soil health. Treatments on the farmlets consist of combinations of contemporary New Zealand farming management practices vs RA management and standard New Zealand pasture vs diverse pasture. This is implemented across two farm systems: drystock sheep farmlets and dairy farmlets. These are both located near Palmerston North, New Zealand, but are on different soil types. The current study investigates the effects of RA management and diverse pasture treatments on three different components of soil biota, focusing on the soil microbiome and the base of the soil food chain. The physiological state of soil bacteria and the community structure of the soil microbiome was investigated. The effects of the treatments on the soil food web were monitored by an analysis of soil nematodes. Soil moisture and temperature were measured for each sample, as these factors are known to have an influence on soil biota. Analysis of the physiological states of soil bacteria was carried out using epifluorescent microscopy in combinations with differential stains that allowed the active, potentially active, dormant and dead pools of soil bacteria to be enumerated. The microbial community composition was investigated by analysis of soil phospholipid fatty acid (PLFA) biomarkers. This allowed the proportions of major microbial groups in the soil to be determined, as well as provided a proxy of total microbial biomass. Analysis of the 16:1ω5 biomarker in the neutral lipid fraction, via neutral lipid fatty acid (NLFA) analysis was used to measure arbuscular mycorrhizal fungi (AMF). Total nematode abundance and the relative abundance of each trophic group was analysed. Soil temperature was measured in the paddock at the time of sampling while soil moisture was determined by oven drying and calculating the gravimetric water content. On the sheep farmlets, diverse pasture under contemporary management (Div-Con) had higher dead bacteria than the other treatments, while on the dairy farmlets, standard pasture under contemporary management (Std-Con) had higher active bacteria compared to diverse pasture under regenerative management (Div-Reg). The sheep farmlets had higher total bacteria counts. The results from the epifluorescent microscopy indicated that the staining procedure used to enumerate total bacteria was unreliable and, by comparison with published results and the PLFA analysis results of the current study, was shown to underestimate the total bacteria present by an order of magnitude. This limits the conclusions that can be drawn from these results, especially regarding the proportions of active and dead bacteria. Due to the methodological limitations and lack of consistent treatment effects between the sheep and dairy farmlets, generalised conclusions on the effects of the different treatments on soil bacteria physiological state cannot be made. The PLFA analysis results showed that there was no difference in the structure of the microbial communities, the NLFA 16:1ω5 biomarker concentration, or total PLFA concentration between treatments. However, on the dairy farmlets, the fungi to bacteria ratio was highest on the Div-Reg treatment. The total PLFA concentration was higher on the sheep farmlets compared to the dairy farmlets, and there was a difference in PLFA concentration between the soil types on the dairy farmlets. The community composition also differed between the soil types on the dairy farmlets and between the sheep and the dairy farmlets, which are also on different soil types. This indicates the importance of accounting for differences in soil type when measuring soil biota. Total nematode abundance was higher on the sheep farmlets compared to the dairy farmlets, and the trophic group composition differed between the farmlet systems. The total abundance of nematodes was higher under contemporary management than under regenerative management on the sheep farmlets, although the same trend was not observed on the dairy farmlets. The nematode community was dominated by bacterivorous and herbivorous nematodes. No differences in the trophic group composition were observed on the sheep farmlets, however, on the dairy farmlets the Div-Reg treatment had a higher portion of bacterivorous nematodes than the Std Con treatment. The observed differences in nematode abundance and community composition between the contemporary and regenerative managed treatments on the sheep and the dairy farmlets are likely due to differences in the grazing management between contemporary and regenerative practices which changes nematode food source availability. As the nematode community contains a large portion of herbivorous nematodes, their high abundance is likely detrimental to plant growth. The results from the analysis of soil moisture and soil temperature indicated that both diverse pastures and regenerative management reduce soil temperature, potentially buffering it from fluctuations. Similarly, there was evidence that diverse pastures and regenerative management increase soil moisture. However, more data is needed to draw definitive conclusions. Overall, the results show that the soil food webs across all treatments are dominated by the bacterial decomposition pathway and the bacterial and herbivore energy channels. As differences in the soil biota measurements were inconsistent between treatments and across the farmlet systems, drawing general conclusions on the effects of diverse pasture and regenerative management is difficult. The largest and most consistent differences where between soil types and the farmlet systems. As the two farmlets systems were on different soil types, the differences between them could also be due to soil type differences. This has important implications for correctly controlling for soil type differences in studies on soil biology. However, both the soil type and the farmlet systems were fully confounded with different sampling dates, and therefore the possibility that this contributed to observed differences cannot be eliminated. At the time of this study, the soils had only been under their respective treatments for two and a half years. As soils often take a long time to adapt to changes, it is possible that differences between them will become more apparent as the research program continues. It is also important to acknowledge that all farmlet treatments are comparing stable pasture systems which have had minimal historic soil cultivation. These conditions are known to promote healthy soil and soil biota, so it is possible that against this back drop, subtle changes in management and the addition of more diverse pasture species are not resulting in clear differences between treatments. Therefore, although there is some evidence to suggest that diverse pastures and RA management is influencing soil biology in the current study, the nature of this effect is difficult to determine. Continued monitoring is required to confirm whether the treatments have an effect on soil biology and to determine what the impacts are and the possible mechanisms.