Permaculture management of arable soil increases soil microbial abundance and diversity, nutrient and carbon stocks compared to conventional arable agriculture

Abstract

A third of the world’s arable land has been lost since the 1950’s largely attributed to the rise of industrialised conventional agriculture, since its practices severely deplete the soil of essential organic matter, nutrients, and crop diversity, increasing its vulnerability to disease, drought, and flooding. Therefore, there is a pressing need to develop regenerative methods of cultivation to enrich soil fertility. Permaculture is a form of agroecology adopting holistic management to create a set of principles and design frameworks enriching soil based on a whole ecosystem approach. To date, there is little scientific evidence on the influence permaculture management has on soil fertility and subsequently microbial abundance and diversity. This study investigates the effect of permaculture management on soil fertility by comparing two mature permaculture managed allotment soils with a conventional arable soil. Soil fertility was assessed by microbial biomass and diversity (measured by phospholipid fatty acid analysis), soil nutrient (nitrate, ammonium and phosphate) and soil organic carbon contents. The greenhouse gas emission potential of soils was also measured with an in vitro incubation and gas chromatography analysis. Both bacterial and fungal abundance were 3-4 times higher under permaculture managed soils (with the more mature site showing higher fungal abundance) compared to conventionally managed soils. Furthermore, the bacterial/fungal ratio significantly varied between sites, with the arable soil showing a much lower abundance of fungi compared to its bacterial biomass. The greater soil microbial abundance and diversity under permaculture management was attributed to the use of organic amendments, crop rotation and diversity and no till practices promoting symbiotic relationships between the soil microbes and crop, exchanging essential nutrients and minerals. Consequently, permaculture soils had significantly higher organic matter, organic carbon, and nutrient contents as well as soil moisture compared to the arable soil. Regarding greenhouse gas emissions, the permaculture soils had 2-3 times higher soil respiration rate measured as carbon dioxide, which was explained by a multiple linear regression combination of soil nitrogen, organic carbon and moisture (77.34 % variance explained). Nitrous oxide (N2O) and methane (CH4) emissions were not statistically different between soil types due to high variability between replicates. However, N2O from permaculture soils was marginally higher representing soil conditions at the time of sampling (October), which did not include fertilisation effects of the arable soil. This study found permaculture management of soils leads to increased fertility compared to conventionally managed arable soil, as expressed by the soils’ higher microbial abundance, nutrient, and organic carbon contents. The management of permaculture focused on mimicking the natural recycling of an ecosystem with addition of organic amendments, little disturbance to the soil using no dig raised beds, and crop diversity and rotation to aid microbial activity and synergy with the plant, creating a dense network of hyphae within the soil that contributes to enriched carbon and nutrient content.