Wettability and biogeochemical properties of the drilosphere and casts of endogeic earthworms in pear orchard

The aim of this work was to evaluate the effect of endogeic earthworms on the burrow system (burrow wall 0-3 mm, transitional zone 3-7 mm, bulk soil >= 20 mm from the burrow wall) and cast aggregates of a loess soil under pear orchard. This effect was tested by determining wettability and selected biogeochemical properties (organic carbon, potential redox, respiration, denitrification and methanogenic potentials, and ammonium and nitrate N) using soil from each compartment immediately after sampling. Wetting was measured with an apparatus consisting of a sponge connected with a graduated capillary tube and biogeochemical soil properties using standard methods. The mean wetting rate was the lowest in the cast aggregates and burrow walls (0.154 and 0.542 mm(3) s(-1)) and the highest in bulk soil (1.016 mm(3) s(-1)). Total organic carbon, C-org (range 1.59-2.40%), microbial carbon, C-mic (range 0.567-1.85 mg g(-1)) and C-mic:C-org ratio (range 3.57-7.69%) followed the ranking series: cast aggregates > burrow walls > transition zone > bulk soil with the lowest relative differences between the cast aggregates and burrow walls. The same compartment arrangement was observed for denitrification potential (range 34.7-98.0 mg N2O-N kg(-1) day(-1)), respiration rate (range 11.3-56.0 mg CO2-C kg(-1) day(-1)), metabolic quotient qCO(2) (respiration to microbial biomass ratio) (range 0.83-1.56 mu g CO2-C mg(-1) C-mic h(-1)), and methane production (range 4.00-429.9 mg CH4-C kg(-1)). Foregoing indicates that C-enriched burrows and cast aggregates are the least wettable and encouraging habitats for the growth and activity of microorganisms. Ammonium N concentrations were not influenced by earthworms, but concentrations of nitrate N and N-tot were significantly greater in the earthworm affected compartments than in the bulk soil. Principal component analysis (PCA) showed high sensitivity of wettability to concentrations of C-org and C-mic and C-rnic:C-org ratio. The quantitative description of the spatial distribution of the wettability and the biogeochemical properties in the earthworm system may have implications to the understanding of lateral movement of water in the earthworm inhabited soils. (C) 2014 Published by Elsevier B.V.