What happens to earthworm casts in the soil? A field study of carbon and nitrogen dynamics in Neotropical savannahs
Introduction
At present there is increasing evidence that soil macroinvertebrates play a key role in soil organic matter (SOM) transformations and nutrient dynamics at different spatial and temporal scales (Lavelle and Spain, 2001). Earthworms, through their burrowing activities, produce aggregates and pores (i.e. biostructures) in the soil and/or at the soil surface, thus affecting its physical properties, nutrient cycling and plant growth (Lal, 1991; Scheu, 2003). They affect some important soil ecological processes at the scale of these biostructures within their “functional domain” (Lavelle, 1997) where they concentrate nutrients and resources that are further exploited by soil microorganism communities (Scheu, 1987; Marinissen and De Ruiter, 1993). The biogenic structures constitute assemblages of organo-mineral aggregates where the stability and the concentration of organic matter may have an impact on soil physical properties and SOM dynamics (Blanchart et al., 1997).
The effects of earthworms on SOM are expressed at different spatial and temporal scales (Lavelle, 1997). In the short-term, large quantities of nutrients easily assimilable by plants are found in fresh depositions (Blair et al., 1995). Most of these nutrients derive from earthworm excretion of urine and secretion of mucus (Barois and Lavelle, 1986). Nitrogen mineralization by microflora is also quite intense in the earthworm gut and continues for several hours in fresh casts (Barois and Lavelle, 1986). Over longer periods of time, OM is protected within aggregates from further decomposition in the ageing earthworm casts and might become accessible again for the microflora once these are degraded into small fragments (Blanchart et al., 1997; Decaëns, 2000; Bossuyt et al., 2005). At larger-scale domains (soil profile to ecosystem), earthworms seem to accelerate the mineralization and the turnover of SOM (Villenave et al., 1999). However, the available information to date has mainly focused on those endogeic species of pantropical distribution, e.g. Pontoscolex corethrurus (Glossoscolecidae) Muller, whereas little attention has been paid to other ecological groups or to endemic species.
Although some studies have been conducted to assess the role of earthworm casting in nutrient cycling and soil structure, only a few have dealt with casts deposited under field conditions (Lavelle and Spain, 2001). In the savannahs of Colombia, important regulation of soil processes has been described at the scale of the surface casts deposited by the native anecic species Martiodrilus sp. (Decaëns et al., 2003; Jiménez et al., 2003; Jiménez and Decaëns, 2004). However, laboratory rearing showed that 36–53% of the total production of casts occurs below-ground (Decaëns, 2000) which represents between 5 and 60 tonnes of dry material per hectare each year (Decaëns et al., 1999). Whether the same processes occur in the casts deposited within the soil profile remains unanswered. The objective of this study was to describe the dynamics of Ctot, Navail and the distribution in size of aggregates in ageing below-ground casts of Martiodrilus sp. in order to better understand the impact of anecic earthworms on soil functioning. Since it was not possible to determine and monitor the ages of below-ground casts, fresh casts were collected at the soil surface and were injected into vertical burrows that were artificially created in the 0–10 cm layer, the main zone of activity of the species (Jiménez et al., 1998a). The fresh material deposited by Martiodrilus sp. inside the soil is very likely not different from the fresh material deposited at the surface because the earthworm emits at the soil surface material from all the depths of its burrow. However, our artificial burrows lacked the impregnation with urine and mucus of the real earthworm burrow walls (Jégou et al., 2001). We took this into account in the interpretation of our results.
Section snippets
Study site
The study was carried out in the CIAT-CORPOICA1 Research Station of Carimagua, in the well-drained isohyperthermic savannahs of the Eastern Plains of Colombia (4°37′N and 71°19′W and 175 m a.s.l.). Average annual rainfall and temperature are 2,280 mm and 26 °C, respectively (1972–1995, CIAT unpublished data), with a dry season from December to March. Soils at the study site are Oxisols
Below-ground casts and burrows of Martiodrilus sp.
A total of 971 dry casts were found on the 120 m2 sampled area, 53.4% of which were located at the opening of a burrow filled with below-ground cast. The length distribution of the burrows located under the other 46.6% followed a normal distribution (Kolmogorov–Smirnov–Lilliefor test d=0.0835, p<0.05) with 67.6% of the lengths ranging from 10 to 25 cm (Fig. 1). Burrow diameter was measured below 24 and 59 surface casts in the savannah and the pasture, respectively (Table 1). The median of the
Natural versus artificial below-ground casts and burrows of Martiodrilus sp.
The diameter and direction of the artificial burrows of our experiment was quite similar to those created by Martiodrilus sp. in natural conditions. The diameter of 1 cm corresponds to the maximum diameter observed in the savannah and surpasses the values obtained in the pasture by 20%.
Our results show that 53.4% of the casts deposited by Martiodrilus sp. at the soil surface had a below-ground cast underneath. Thus the injected casts of our experiment are representative of a frequent
Acknowledgements
We thank the staff at Carimagua station for their friendship and invaluable help during field work, especially Luis Soto and Nixon Betancourt. We thank CIAT for technical support through project PE-2 (“Overcoming Soil Degradation”) and the CGIAR system-wide Soil, Water and Nutrient Management Program for financial support. Thanks to Drs. Pierre Legendre (Université de Montréal, Canada) and Marti J. Anderson (University of Auckland, New Zealand) for help with statistical analysis, Dr. Bellier
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2015, Soil and Tillage ResearchCitation Excerpt :The overall influence of soil engineers on soil structure result on the balance between their ability to incorporate surface residues in soil, to produce biogenic aggregates and galleries, and the lifetime of these structures in soil. If information is available on the production dynamic of earthworm casts and galleries (Lee, 1985; Capowiez, 2000; Capowiez and Belzunces, 2001; Capowiez et al., 2001, 2014; Felten and Emmerling, 2009), very few studies measure the evolution of these structures in field submitted to pedoclimatic or anthropic forces or from ingestion by other soil organisms (Ligthart and Peek, 1997; Capowiez et al., 1998; Le Bayon and Binet, 1999; Mariani et al., 2007; Jouquet et al., 2010a). We assume that one of the main obstacles is our difficulty to determine the origin of soil aggregates and pores that have been produced by soil engineers.
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