Stability of Aggregates Made by Earthworms in Soils with Organic Additives

Agnieszka Józefowska,Andrzej Bieganowski,Agata Sochan,Magdalena Ryżak,Justyna Sokołowska,Karolina Woźnica,Tomasz Zaleski

doi:10.3390/agronomy11030421

Abstract

Earthworm activity is a key factor in creating soil aggregates, but introduced organic matter and abiotic factors are also equally important. The purpose of this study was to investigate the stability of aggregates made by earthworms in soils with organic additives. Additionally, the two aggregate stability measurement methods were compared: (i) the wet-sieve method and (ii) the laser diffraction method. A six-month container experiment containing sixteen treatments and controls were made. Each treatment included one of four types of soil texture: sand, loam, silty loam and clay, and one of four additives: straw, peat, compost and compost with added microorganisms. To each treatment, six earthworms were added, two each of species commonly occurring in Polish soils: Dendrodrilus rubidus, Aporrectodea caliginosa and A. rosea. This study confirmed that earthworm activity was the factor favoring aggregate formation. In terms of the investigated organic additives, the efficiency of aggregate creation was as follows: compost with active bacteria, compost, peat and straw. Nevertheless, earthworms alone, without the addition of an organic additive, did not form permanent aggregates. The wet sieving and laser diffractometry methods of measuring aggregate stability were comparable for silty, clayey and loamy soils.

Highlights

The formation of soil aggregates is complicated and depends on the parent material as well as physical and biochemical soil-forming processes [1]
Chenu et al [5], in their preliminary observations of the clay fraction, noticed that the organic constituents and organic matter were strongly associated with clay particles, which increased the hydrophobicity of clay minerals
This study aims to verify three hypotheses: H1: aggregate stability is related to soil texture and organic additives applied to the soil H2: aggregate stability is connected with carbon stabilization and microbial activity H3: two methods of measuring aggregate stability, namely (i) the wet-sieve method and (ii) the laser diffraction method are comparable

Summary

Introduction

The formation of soil aggregates is complicated and depends on the parent material as well as physical and biochemical soil-forming processes [1]. The aggregate structure is closely associated with the presence of clay minerals and organic matter [2,3]. In soils with the dominance of sand fraction, the stability of aggregates is low [6,7,8] This is because the sand has a large size and low surface area compared with clay, so its bonding capacity, e.g., with metal cations or organic molecules, is very low [9]. The differences in surface area, charge of clay and swelling behavior influence the forming process and aggregate stability [4,10,11,12]. Aggregate formation is a comprehensive interaction between soil minerals and soil organic matter [13]. As opposed to sandy soil dominated by the coarse fractions (sand), soils with fine fractions (clay, silt) show a positive correlation with total organic carbon [21,22], which indicates that the soil rich in clay minerals can create a stable organo-mineral association with a significantly large amount of organic substances [23]

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