Impact of Small-Scaled Differences in Micro-Aggregation on Physico-Chemical Parameters of Macroscopic Biopore Walls

Christoph Haas,Rainer Horn

doi:10.3389/fenvs.2018.00090

Abstract

Macroscopic biopores, like earthworm burrows or channels which remain after a root decayed, act as preferential flow paths for water, gas, and heat transport processes, and viewing at agricultural production, as preferential elongation paths for plant roots. These processes result in intense alterations of the soil volume and its composition that surrounds the pores. The effects of these processes were analyzed at small-scale and physico-chemical soil parameters, i.e. relative oxygen diffusion coefficient (Ds/DO), oxygen partial pressure pO2, Eh and pH of biopore walls, were measured. The analyses were carried out on undisturbed soil samples with different colonization history, excavated from a haplic Luvisol derived from loess. Soil resistance to penetration was determined simultaneously with Ds/DO and pO2 using a coupled, self-developed approach, and four matric potentials (namely, ᴪ_m = -1 kPa; -3 kPa; -6 kPa or -30 kPa) were considered. We hypothesized that physico-chemical soil parameters in biopore walls were altered due to differing influences on the soil aggregation. Aggregation was visualized with scanning electron microscopy, classified and used to explain differences in - soil properties. Plant roots and earthworms altered aggregation next to biopore surfaces in a contrasted way, either by enhancing aggregates diversity or homogenizing it. Roots led to the formation of subpolyeders while earthworms formed subplates. Pore functions of microaggregates were comparable to those of larger scale, and subpolyeders showed much more favorable soil properties in terms of soil aeration (Ds/Do, pO2). Replicates of all parameters scattered intensely and showed deviations up to several orders of magnitude in case of Ds/DO underlining the large variability of soil properties in biopore walls.

Highlights

IntroductionPreferential elongation paths for plant roots (Passioura, 2002; McKenzie et al, 2009) and preferential flow paths like earthworm burrows and root channels (Jarvis, 2007) are hot spots of soil microorganisms (Hoang et al, 2016; Banfield et al, 2017), for nutrient turnover (Hoang et al, 2016) and exchange processes within the plant-soil-atmosphere-continuum
The objective of this paper is to quantify the alteration of physicochemical parameters like oxygen diffusivity, pO2, Eh, and pH of biopore walls to better understand exchange and mass transport processes in soils
Because of differences in water-saturation we found 30 s to be long enough for stable and reliable measurement readings

Summary

Introduction

Preferential elongation paths for plant roots (Passioura, 2002; McKenzie et al, 2009) and preferential flow paths like earthworm burrows and root channels (Jarvis, 2007) are hot spots of soil microorganisms (Hoang et al, 2016; Banfield et al, 2017), for nutrient turnover (Hoang et al, 2016) and exchange processes within the plant-soil-atmosphere-continuum. This results in a decrease in pH because of the reaction of CO2 with water to carbonic acid (H2CO3) Beside these biologically and chemically alterations, roots and earthworms release and incorporate organic matter into the soil volume that surrounds the cavities. Thereby they alter e.g., the chemical composition in terms of the ratio of hydrophobic and hydrophilic functional groups (C–H/C=O ratio), even after the root decayed completely (Haas et al, 2018) or the replenishment of earthworm casts stopped. A decreased hydraulic and air conductivity or a decreased biological activity is known due to the loss of pore volumes (for more details see Horn and Fleige, 2003 or Haas et al, 2016)

Objectives

Methods

Results