What is the best way to measure earthworm-processed soil? A comparison of common water stable aggregates, the smartphone app MOULDER, and a novel SlakeLight method

Abstract

Soil aggregates are important for soil fertility and earthworms can support aggregate formation and stability during gut passage, burrowing activity and secretion of polysaccharides. To determine these effects in different soil tillage systems, soil properties, and the earthworm community associated with Lumbricus terrestris (Linnaeus, 1758) were evaluated in two long-term field experiments using plough, cultivator, or no-till. The aim was to investigate effects of earthworm populations on soil aggregate stability and to evaluate three methods, namely: water stable aggregate (WSA) index, MOULDER (formerly SLAKES) and SlakeLight. The WSA method wet-sieves aggregates for 3 min in distilled water and records mass of the dispersed aggregates. MOULDER uses a smartphone application to measure an increase in area of dispersed soil submerged in distilled water to measure slaking, while SlakeLight analyses light transmission corresponding to the area covered by the soil material. Soil samples were collected in autumn at three levels: i) middens, ii) 5 cm radius around burrows of L. terrestris (burrow-midden-complex), and iii) in bulk soil without burrows of L. terrestris. All samples were hand-searched for earthworms or air-dried for soil aggregate stability analyses. The hypothesis tested was that there is a gradient of earthworm worked soil, such that the level of earthworm activity away from a L. terrestris burrow decreases so that: midden > burrow-midden-complex > bulk soil. Total earthworm abundance (individuals level−1 ± standard deviation), mostly endogeic earthworms, was 3-times higher in the burrow-midden-complex (4.3 ± 2.7) than bulk soil for cultivator and doubled for burrow-midden-complex under a ploughing regime (3 ± 2.1), while no-till was only slightly increased (4.6 ± 2). With rising earthworm numbers, aggregate stability increased, with a higher effect for bulk soil than for burrow-midden-complex. At both sites, MOULDER identified a more stable soil in middens than in bulk soil, while other methods were not so discriminating in their outcomes of middens. However, WSA was more sensitive to interactions of soil tillage × earthworm abundance than SlakeLight or MOULDER and showed that ploughed soil and bulk soil aggregates stabilised the most with increasing earthworm activity. Comparison of the three methods showed that all can be used for earthworm-processed soil, but that selection of the method should depend on the research questions and on resource availability.