Permaculture practices effect on soil fertility and organic matter distribution in aggregate-size classes

Abstract

The agricultural transition must address the need for a sustainable agricultural productivity. In this regard, the role played by soil organic matter (SOM) is key. Here we aimed to study the impact of permaculture and biointensive micro-gardening practices, mainly characterized by (1) intensive cultivation (2) the use of large and localized organic inputs and (3) the non-use of mineral fertilizers and pesticides, on soil fertility parameters and SOM distribution in aggregate-size fractions. In identical geopedoclimatic conditions, we compared SOM allocation in a pasture that evolved towards cultivation under permaculture practices for seven years and with a soil under conventional agriculture practices. For this purpose, soils were separated into three aggregate-size fractions (250-2000, 50-250 and <50 µm) by wet sieving. Then, the 250-2000 µm fraction (macroaggregates + free coarse particulate organic matter (POM)) were separated into coarse POM (free + occluded), occluded 50-250 µm and occluded <50 µm fractions. Organic carbon (OC) concentrations were measured in the 250-2000 µm, 250-50 µm, <50 µm and occluded 50-250 µm fractions and soil fertility parameters on the bulk soils. Our dataset shows that permaculture practices increased the concentrations of bioavailable nutrients Ca, Mg, K and P extracted with EDTA-ammonium, total nitrogen concentrations and OC stocks in bulk soils, which was explained by the very large manure inputs characteristics of this practice. Permaculture practices had only little effect on soil aggregation, except an increase of coarse POM proportion at the expense of the 50-250 µm fraction. Despite higher OC concentrations in the occluded 50-250 µm fraction, its contribution to the increase of OC stocks was not significant. Indeed, the increase of OC stocks was mainly attributable to coarse POM. We hypothesized that time since cultivation is not sufficient to allow the complete decomposition of coarse POM in fine inter-POM, more associated with mineral particles. We can argue that permaculture/biointensive micro-gardening practices could enhance soil fertility parameters and SOM storage. However, further temporal research is needed to study the dynamic of the macroaggregates and their ability to form protected microaggregates through the important coarse POM content. In addition, a C balance should be performed to determine if the additional OC storage correspond to a net sink of CO2