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

Abstract

Theobroma cacao seedlings were grown alone (TCA) or associated with saplings of N 2 -fixing shade trees Gliricidia sepium and Inga edulis in 200 l of 15 N labelled soil within a physical root barrier for studying direct nitrogen transfer between the trees and cacao. Root:shoot partitioning ratio for sapling total N was lower than biomass root:shoot ratio in all species. Sapling total 15 N was partitioned between root and shoot in about the same ratio as total N in cacao and inga but in gliricidia much higher proportion of 15 N than total N was found in roots. Thus, whole plant harvesting should be used in 15 N studies whenever possible. Average percentage of fixed N out of total tree N was 74 and 81% for inga estimated by a yield-independent and yield-dependent method, respectively, and 85% for gliricidia independently of estimation method. Strong isotopic evidence on direct N transfer from trees to cacao was observed in two cases out of ten with both tree species. Direct N transfer was not correlated with mycorrhizal colonisation of either donor or receiver plant roots. Direct N transfer from inga and gliricidia to cacao is conceivable but its prevalence and the transfer pathway via mycorrhizal connections or via reabsorption of N-rich legume root exudates by cacao require further study. Competition in the restricted soil space may also have limited the apparent transfer in this study because the trees accumulated more soil-derived N than cacao in spite of active N 2 fixation.

Nitrogen symbiotically fixed by cowpea and gliricidia in traditional and agroforestry systems under semiarid conditions

“…This lower signal can indicate the occurrence of transference of some of the N fixed by cowpea to the associated trees, as reported by Kurppa et al (2010).…”

“…The 15 N abundance of the tree species of the agroforestry system intercropped with maize + cowpea indicated no apparent fixation in gliricidia, as previously mentioned. The reason for this absence of fixation is not known and it is intriguing because gliricidia is considered a species with high N 2 -fixing capacity (Paulino et al, 2009;Kurppa et al, 2010), and the trees fixed abundantly when intercropped with buffel grass and prickly-pear cactus.…”

“…Agroforestry systems with trees or shrub alleys within annual crop fields can be an option to reduce the fragility of certain agricultural production systems (Wick & Tiessen, 2008;Foroughbakhch et al, 2009;Li et al, 2009;Kurppa et al, 2010;Sousa et al, 2010;Lima et al, 2011). Those systems produce a considerable amount of biomass that can be incorporated into the soil, transferring nutrients from the trees to the crops (Bertalot et al, 2010;Munroe & Isaac, 2014).…”

Cacao Nutrition and Fertilization

“…For example, in a legume-cacao association, Kurppa et al ( 2010 ) found limited net N transfer from associated legume tree species to cacao in spite of active N 2 fi xation. No response to N application has generally been found in fertilizer trials, particularly in Côte d'Ivoire (Lotodé and Jadin 1981 ), Ghana , or Sri-Lanka (Heenkende and Gunarantne 2000 ).…”

Growth, herbivore distribution, and herbivore damage of timber trees in a tropical silvopastoral reforestation system

“…However, as nitrogen concentrations and δ 15 N values of mature leaves of TGli trees were not significantly different from TGua trees and TSol trees, our findings suggest that even 4 years after plantation establishment atmospheric nitrogen fixed by G. sepium is not bioavailable in quantities that enhance nitrogen content of T. rosea leaves. The lack of detectable nitrogen transfer may be related to missing direct belowground interactions through root exudates or interconnected mycelial networks (Sierra and Nygren 2006) or to an absence of indirect interactions actually driven by the decomposition of nitrogen-enriched G. sepium biomass (Kurppa et al 2010).…”