Influence of reduced tillage on earthworm and microbial communities under organic arable farming

Summary

Although reduced tillage is an agricultural practice reported to decrease soil erosion and external inputs while enhancing soil fertility it has still rarely been adopted by European organic farmers. The objective of this study was to assess the long-term interactive effects of tillage [conventional (CT) versus reduced (RT)] and fertilization [slurry (S) versus composted manure/slurry (MCS)] on earthworms and microbial communities in a clay soil under spelt in an organic 6-year crop rotation. Earthworm populations (species, density and biomass, cocoons) were investigated by handsorting the soil nine years after initial implementation of the treatments. Soil microbial carbon (Cmic) and nitrogen (Nmic) were measured by chloroform-fumigation extraction and a simplified phospholipid fatty acid (PLFA) analysis was used to separate for populations of bacteria, fungi and protozoa. Total earthworm density was significantly increased in RT plots mainly being related to increased numbers of juveniles. Moreover, we found five times more cocoons with RT. Species richness was not affected by the treatments, but tillage treatments have differentially altered populations on species-level. In addition, cluster analysis on community level revealed two distinct groups of plots in relation to tillage treatments. In RT plots Cmic increased in the 0-10 cm and 10-20 cm soil layers, while PLFA concentrations indicative of Gram-negative bacteria, fungi and protozoa only increased in the topsoil. Lower bacteria-to-fungi ratios in the upper soil layer of RT plots indicated a shift to fungalbased decomposition of organic matter whereas a higher Cmic-to-Corg ratio pointed towards enhanced substrate availability. Slurry application decreased microbial biomass and enhanced density of juvenile anecic earthworms but overall fertilization effect was weak and no interactions with tillage were found. In conclusion, tillage is a major driver in altering communities of earthworms and microorganisms in arable soils. The use of reduced tillage provides an approach for eco-intensification by enhancing inherent soil biota functions under organic arable farming.