Assimilation dynamics of soil carbon and nitrogen by wheat roots and Collembola

Abstract

It has been demonstrated that plant roots can take up small amounts of low-molecular weight (LMW) compounds from the surrounding soil. Root uptake of LMW compounds have been investigated by applying isotopically labelled sugars or amino acids but not labelled organic matter. We tested whether wheat roots took up LMW compounds released from dual-labelled ( 13 C and 15 N) green manure by analysing for excess 13 C in roots. To estimate the fraction of green manure C that potentially was available for root uptake, excess 13 C and 15 N in the primary decomposers was estimated by analysing soil dwelling Collembola that primarily feed on fungi or microfauna. The experimental setup consisted of soil microcosm with wheat and duallabelled green manure additions. Plant growth, plant N and recoveries of 13 C and 15 N in soil, roots, shoots and Collembola were measured at 27, 56 and 84 days. We found a small (<1%) but significant uptake of green manure derived 13 C in roots at the first but not the two last samplings. About 50% of green manure C was not recovered from the soil-plant system at 27 days and additional 8% was not recovered at 84 days. Up to 23% of C in collembolans derived from the green manure at 56 days (the 27 days sampling was lost). Using a linear mixing model we estimated that roots or root effluxes provided the main C source for collembolans (54À79%). We conclude that there is no solid support for claiming that roots assimilated green manure derived C due to very small or no recoveries of excess 13 C in wheat roots. During the incubation the pool of green manure derived C available for root uptake decreased due to decomposition. However, the isotopic composition in Collembola indicated that there was a considerable fraction of green manure derived C in the decomposer system at 56 days thus supporting the premise that LMW compounds containing C from the green manure was released throughout the incubation.

Rhizosphere fauna: the functional and structural diversity of intimate interactions of soil fauna with plant roots

“…This is convincingly exemplified for the best studied group of soil fungivores, the Collembola (Larsen et al 2007;Ostle et al 2007). By tracing 13 C-signatures of grasses in grassland soil Jonas et al (2007) have shown that depending on soil type and plant species, saprophytic fungi made up between 40-80%, while arbuscular mycorrhizal fungi constituted up to 60% of the collembolan diet, respectively.…”

“…In contrast to conventional wisdom, recent evidence suggest that a major part of the soil animal food web strongly relies on the C-inputs from plant roots and less so on the carbon and nutrient inputs via leaf litter (Albers et al 2006;Larsen et al 2007;Pollierer et al 2007), but see Elfstrand et al (2008). Therefore special attention must be given to the consumers of microorganisms in the rhizosphere, because they are at the base of the soil food web channeling the energy to the higher trophic levels via two distinct routes, the fungal and the bacterial energy channel (Moore and Hunt 1988) and determine the rates of nutrient cycling and the availability of mineral nutrients to plants (Clarholm 1985;Kuikman et al 1990;Ekelund and Rønn 1994;Laakso and Setälä 1999;Bonkowski 2004).…”

Stable isotope fingerprinting: a novel method for identifying plant, fungal, or bacterial origins of amino acids

“…Amino acid exchange may also occur in symbiotic associations between plants and fungi, a situation in which each partner has the ability to synthesize amino acids but fungi may also absorb amino acids from the substrate (Read andPerez-Moreno 2003, Brundrett 2004). A marker of amino acid biosynthetic origin would be a powerful tool for investigations of amino acid exchange in symbiotic and trophic relationships, as well as a way to monitor diet in organisms that are otherwise intractable for studies of foraging behavior such as soil microarthropods (Chahartaghi et al 2005, Larsen et al 2007). Here we present a new approach to tracing the origins of amino acids: using unique patterns of carbon isotope signatures generated by amino acid synthesis in plants, fungi, and bacteria (here termed a '' 13 C fingerprint'').…”

Biotic interactions and trait-based ecosystem functioning in soil

“…Our understanding of collembolan feeding habits at the species or functional group level is steadily increasing (Chamberlain et al, 2006;Ngosong et al, 2011;Ruess et al, 2007;Sechi et al, 2014a), but several gaps still exist. It is known that Collembola are closely associated with the rhizosphere food web being nutritionally supported by root-derived resources (Endlweber et al, 2009;Larsen et al, 2007;Sabais et al, 2012Larsen et al 2016b), but the path through which they obtain these resources is not yet clear. While euedaphic and hemiedaphic species have been shown to incorporate recent photosynthate carbon (C) from crops (Larsen et al, 2007;Ostle et al, 2007), A.M. recently demonstrated that only a fraction of these plant derived resources derives from direct foraging on roots.…”

“…It is known that Collembola are closely associated with the rhizosphere food web being nutritionally supported by root-derived resources (Endlweber et al, 2009;Larsen et al, 2007;Sabais et al, 2012Larsen et al 2016b), but the path through which they obtain these resources is not yet clear. While euedaphic and hemiedaphic species have been shown to incorporate recent photosynthate carbon (C) from crops (Larsen et al, 2007;Ostle et al, 2007), A.M. recently demonstrated that only a fraction of these plant derived resources derives from direct foraging on roots. Hence, Collembola are influenced by plant-related changes that affect availability, quality and palatability of microbial derived food sources.…”