Fate of 13 C labelled root and shoot residues in soil and anecic earthworm casts: A mesocosm experiment

Abstract

Earthworms are known to have a major impact on organic matter dynamics in soils. The precise dynamics of carbon incorporation and/or decomposition in soil under the influence of earthworms still need to be investigated. In a mesocosm experiment, the fate of Ryegrass root and shoot litter was monitored in the soil, in the presence and absence of anecic earthworms Lumbricus terrestris L. Residues were 13 C labelled and deposited onto the soil surface. Incorporation of 13 C in surface casts and in the 0-20 and 40-60 cm soil layers was monitored 1, 2, 4, 8, 24 and 54 weeks after adding labelled litter. Organic carbon content and δ 13 C values were obtained for all samples, allowing the determination of the percentage of carbon derived from labelled litter (Clab). Roots and shoots were incorporated in the 0-20 cm soil layer during the year of experiment, Clab reaching 11.4 % of the soil organic carbon after 54 weeks. On the contrary, no significant contribution from labelled residues was observed in the 40-60 cm layer. Roots decomposed at a slower rate compared to shoots. Litter incorporation was observed in casts from the very first weeks of experiment (Clab from 34.8 to 51.4 % after 2 weeks). In the soil, a significant effect of earthworms on the Clab was detected after 24 weeks. Earthworms accelerated root and shoot decomposition in the soil. They also enhanced, in the presence of shoot residues, the decomposition of the organic matter originally present in the soil. However, after one year, earthworms smoothed the difference between residue types in casts and to a lesser extent in soil, revealing their capacity to enhance the decomposition of both roots and shoots.

Earthworm Cast Formation and Development: A Shift From Plant Litter to Mineral Associated Organic Matter

“…Three mesocosms were filled with ∼75 L of a loamy-sand soil (clay, 19%; silt, 25%; sand, 56%) collected on permanent grassland in North of France (Oise, France). The soil characteristics are described in Table 1 and available in Vidal et al (2017) and Vidal (2016). Mesocosms were placed in a greenhouse where soil humidity and temperature were maintained at 23% and 13 • C, respectively.…”

“…As we combined all used techniques on one sample per treatment each, no replication could be achieved due to time concern. The time points of sampling were selected according to the contrasted isotopic and molecular composition measured on bulk samples in previous works (Vidal et al, 2016a(Vidal et al, , 2017. For example, the shift from more than 50-12% of litter-derived carbon in casts from 8 to 54 weeks (Table 1) showed a clear differentiation into a first and second decomposition phase which led to the two chosen sampling dates.…”

“…So as to follow the decomposition processes in intact cast samples at the microscale, we combined elemental and isotopic information obtained with nano-scale secondary ion mass spectrometry (NanoSIMS) with high-resolution information on the arrangement of organic and mineral constituents obtained with transmission electron microscopy (TEM). In previous works, we focused on the method development and technical requirements of the micro-scale analyses (Vidal et al, 2016b) and demonstrated soil alteration due to earthworm activity using bulk measurements and molecular analyses (Vidal et al, 2016a(Vidal et al, , 2017. We now use the developed methods and bulk analyses in addition to fractionation and NMR analyses to demonstrate the fine scale mechanisms of litter degradation through time.…”

The Role of Different Earthworm Species (Metaphire Hilgendorfi and Eisenia Fetida) on CO2 Emissions and Microbial Biomass during Barley Decomposition

“…The weight of each bottle was measured every day and distilled water was added to replace the evaporated water. Before the application of the barley and the earthworms, the bottles with soils were pre-incubated for one week to allow passing the bacterial flush induced by drying-rewetting of soils and/or soil physical disturbance by sieving [23][24][25][26].…”

“…The earthworm density was determined in accordance to the review by Fründ et al [27], showing that the typical earthworm density (all species concerned) in temperate region was 10-1000 individuals m −2 . Vidal et al [26] determined 25 mg of dry matter g −1 fresh weight day −1 as a good compromise to favour earthworm survival. Thus, we applied the same quantity; 5 g of barley (2.25 g C kg −1 soil) was added to each bottle.…”