Symbiotic dinitrogen fixation by trees: an underestimated resource in agroforestry systems?

Abstract

We compiled quantitative estimates on symbiotic N2 fixation by trees in agroforestry systems (AFS) in order to evaluate the critical environmental and management factors that affect the benefit from N2 fixation to system N economy. The so-called ''N2-fixing tree'' is a tripartite symbiotic system composed of the plant, N2-fixing bacteria, and mycorrhizae-forming fungi. Almost 100 recognised rhizobial species associated with legumes do not form an evolutionary homologous clade and are functionally diverse. The global bacterial diversity is still unknown. Actinorrhizal symbioses in AFS remain almost unstudied.Dinitrogen fixation in AFS should be quantified using N isotopic methods or long-term system N balances. The general average ± standard deviation of tree dependency on N2 fixation (%Ndfa) in 38 cases using N isotopic analyses was 59 ± 16.6 %. Under humid and sub-humid conditions, the percentage was higher in young (69 ± 10.7 %) and periodically pruned trees (63 ± 11.8 %) than in freegrowing trees (54 ± 11.7 %). High variability was observed in drylands (range 10-84 %) indicating need for careful species and provenance selection in these areas. Annual N2 fixation was the highest in improved fallow and protein bank systems, 300-650 kg [N] ha-1. General average for 16 very variable AFS was 246 kg [N] ha-1, which is enough for fulfilling crop N needs for sustained or increasing yield in low-input agriculture and reducing N-fertiliser use in large-scale agribusiness. Leaf litter and green mulch applications release N slowly to the soil and mostly benefit the crop through long-term soil improvement. Root and nodule turnover and N rhizodeposition from N2-fixing trees are sources of easily available N for the crop yet they have been largely ignored in agroforestry research. There is also increasing evidence on direct N transfer from N2- fixing trees to crops, e.g. via common mycelial networks of mycorrhizal fungi or absorption of tree root exudates by the crop. Research on the below-ground tree-cropmicrobia interactions is needed for fully understanding and managing N2 fixation in AFS. (Résumé d'auteur

Agroforestry: Fertilizer Trees

“…These values are much higher than the global estimates (23-176 kg N ha −1 year −1 ) for food legumes and herbaceous pasture and fodder legumes (Herridge et al, 2008). Among the nonlegume−Frankia associations, N-fixation has been estimated at 40-320 kg N ha −1 year −1 for alder (Russo, 2005) and casuarina 73 kg N ha −1 year −1 (Nygren et al, 2012). Although the annual N inputs vary widely between species and even among provenances of the same species across different locations and stand age, the amounts reported are enough to fulfill crop N needs for sustained yield (Nygren et al, 2012).…”

“…N availability and uptake The percentage of N derived from the atmosphere (NDFA) is more than 59% according to a recent analysis of 38 cases using N isotopic analyses (Nygren et al, 2012). Under humid and sub-humid conditions, on average NDFA was 69% in Table 2 Vegetable yield a response to biomass transfer using fertilizer tree prunings from sub-Saharan Africa young trees and 63% in periodically pruned trees compared to 54% in free-growing (nonpruned) trees (Nygren et al, 2012).…”

“…Under humid and sub-humid conditions, on average NDFA was 69% in Table 2 Vegetable yield a response to biomass transfer using fertilizer tree prunings from sub-Saharan Africa young trees and 63% in periodically pruned trees compared to 54% in free-growing (nonpruned) trees (Nygren et al, 2012). The average N-fixation for 16 variable fertilizer tree species was found to be 246 kg N ha −1 year −1 , but much higher (300-650 kg N ha −1 year −1 ) in improved fallows and protein banks where trees are managed by pruning or coppicing (Nygren et al, 2012). These values are much higher than the global estimates (23-176 kg N ha −1 year −1 ) for food legumes and herbaceous pasture and fodder legumes (Herridge et al, 2008).…”

“…Among the nonlegume−Frankia associations, N-fixation has been estimated at 40-320 kg N ha −1 year −1 for alder (Russo, 2005) and casuarina 73 kg N ha −1 year −1 (Nygren et al, 2012). Although the annual N inputs vary widely between species and even among provenances of the same species across different locations and stand age, the amounts reported are enough to fulfill crop N needs for sustained yield (Nygren et al, 2012). In Latin American coffee and cacao agroforestry, pruning residues and litter fall from shade trees have been shown to contribute 60-340 kg N ha -1 year -1 (Beer et al, 1998).…”

Maize Crop N Uptake from Organic Material of <i>Gliricidia sepium </i>Coinoculated with <i>Rhizobium</i> and Arbuscular Mycorrhizal Fungus in Sub-Saharian Africa Sandy Soil

“…Results of Douxchamps et al (2011) revealed that most of the amendment N remained in the soil for maize cultivation under in-corporation of canavalia residues which allowed N recovery of 12% [26]. Nygren et al (2012) reported that leaf litter and green mulch applications release N slowly to the soil and mostly benefit the crop through long-term soil improvement [27]. In this way, Ambrosano et al (2013) suggested that legume residue decomposition provided long-term supply of N for the subsequent crops, by not supplying the nutrient as an immediate source [28].…”