Root exudates of a legume tree as a nitrogen source for a tropical fodder grass

Abstract

Exudation of nitrogenous compounds from the roots of dinitrogen-fixing plants is a potential source of nitrogen for adjacent plants in intercropping systems. We studied (1) the extent of N exudation from the roots of a tropical legume tree Gliricidia sepium (Jacq.) Kunth ex Walp., and (2) the ability of a C4 fodder grass Dichantium aristatum (Poir) C.E. Hubbard and its mycorrhizal symbionts to absorb N from tree exudates in a glasshouse experiment. Root exudates of 15 N-labelled trees were collected in hydroponic culture and applied with irrigation water on grass grown in separate pots. During the 10-week experiment, the trees exuded 34.1 ± 5.0 mg of N, which represented 1.7 ± 0.2% of their total N by the end of the experiment. The total amount exuded would have been enough to supply 16% of grass N content by the end of the experiment. The grass, however, absorbed only 3.8-7.5% of 15 N in exudates and gained 0.8-1.1% of its N from exudates. The low absorption of exudate N by grass was explained by probable soil microbial immobilisation and by the dilution of exuded N in the substantially larger pool of soil mineral N. A close contact between the root systems of N donor and recipient plants directly or via their mycorrhizal symbionts seems to be a precondition of the apparently direct N transfer earlier observed in field studies of the same soil-plant system.

Detection of nitrogen transfer from N2-fixing shade trees to cacao saplings in 15N labelled soil: ecological and experimental considerations

“…Indications of direct transfer of fixed N from a legume to an associated grass have been found in both pot (Sierra et al 2007;Ta and Faris 1987;Vallis et al 1967) and field studies (Jayasundara et al 1997). Direct transfer of symbiotically-fixed N may occur when the legume and non-legume crop are interconnected via a common mycelial network (CMN; He et al 2003) or when the receiver plant absorbs root exudates of the donor before they undergo modifications by soil microorganisms (Jalonen et al 2009a;Paynel et al 2001). In the nature, the latter may occur if the rhizospheres of the two species overlap.…”

Nitrogen Fixation and Transfer in Agricultural Production Systems

“…Since N uptake is along with concentration gradients [47], close proximity between N 2 -fixing legumes and non-N 2 -fixing crops reduces the distance of travel for dissolved N compounds [1]. Close proximity is achieved either through direct root contact or mycorrhizal hyphae connections [61]. However, Issah et al [62] reported that maximum oat productivity was obtained when grown 4 m from caragana shelterbelt compared to 2 m from the shelterbelt.…”

Temporal Variation of N Isotopic Composition of Decomposing Legume Roots and Its Implications to N Cycling Estimates inN15Tracer Studies in Agroforestry Systems

“…The physicochemical properties of the very clayey soil in the experimental studies restrict solute flow [38,39]. Therefore, it can be assumed that spots of high N concentration emerged in soil as a result of root decay, and that the N recipient plant absorbed proportionally more N from these spots.…”