Differences in nitrogen cycling and soil mineralisation between a eucalypt plantation and a mixed eucalypt and Acacia mangium plantation on a sandy tropical soil

Abstract

International audienceSustainable wood production requires appropriate management of commercial forest plantations. Establishment of industrial eucalypt plantations on poor sandy soils leads to a high loss of nutrients including nitrogen (N) after wood harvesting. An ecological intensification of eucalypt plantations was tested with the replacement of half of the Eucalyptus urophylla x E. grandis by Acacia mangium in the eucalypt monoculture to sustain soil fertility through enhancement of the N biological cycle. A randomised block design was set up on ferralitic arenosol in the Congolese coastal plains to assess differences in soil N mineralisation, N fluxes in litterfall, and N stocks in forest floor litter and soil between pure acacia (100A), pure eucalypt (100E) and mixed-species treatments (50A50E). Soil N mineralisation was enhanced under acacia, reaching on average 0.17 and 0.15 mg kg(-1) soil d(-1) in 100A and 50A50E, respectively, compared with 0.09 mg kg(-1) soil d(-1) in 100E. Higher amounts of N returning to the soil through harvest residues and litterfall were observed under acacia than under eucalypt. However, N stock in mineral soil was not increased in 100A and exhibited a limited increase only in the top soil layer of 50A50E. Our results suggest a much faster N turnover under acacia than under eucalypt. Although A. mangium is an exotic N-2-fixing tree in central Africa, it appears to be well adapted to the climatic and edaphic conditions of the Congo, showing an efficient growth strategy. Eucalypt trees could benefit from the increase in soil N availability in mixed-species stands

Contribution of Harvest Residues to Nutrient Cycling in a Tropical Acacia mangium Willd. Plantation

“…By comparison, in E. dunnii, the C/N ratios of leaf, branch, and bark were much higher at 36, 134, and 174, respectively [17] due to 13−47% lower N concentration in the eucalypt harvest residues. These lower C/N ratios in acacias than eucalypts were associated with a much faster release of N from A. mangium than E. hybrid (E. urophylla × E. grandis) harvest residues [34] and higher rates of N turnover in topsoil under A. mangium than E. grandis [58]. By the end of the 1.5-year study period, the total amount of N released from decomposing A. mangium harvest residues was 137 kg ha −1 , 35% of which released from bark.…”

“…Given that decomposition and nutrient release from plant materials are largely influenced by environment [28] and substrate quality [29][30][31], materials of lower nutrient content generally decompose more slowly, immobilize more nutrients during decomposition, and have slower rates of net mineralization than nutrient-rich materials [32]. Acacias are a N-fixing species, and therefore the concentration of N in a given material is generally higher in acacias than in other genera, such as eucalypts [33,34]. This may lead to different activities of microbial decomposers, and thus rates of decomposition and nutrient release.…”

“…This may lead to different activities of microbial decomposers, and thus rates of decomposition and nutrient release. As little is known about the dynamics of nutrient release from the decomposition of acacia harvest residues, particularly bark residue [33][34][35][36], it is crucial to measure rates of decomposition and nutrient release from its various components, especially in tropical environments where the area of acacia plantation estates has grown rapidly.…”

Acacia mangium Willd: benefits and threats associated with its increasing use around the world

“…Both species were also planted for agroforestry and forestry in the two countries. A. auriculiformis is valued by farmers for both agricultural and wood-energy production in agroforestry or forestry systems in DR Congo (Kasongo et al 2009;Shure et al 2010;Dubliez et al 2018), while A. mangium is mainly used to sustain eucalypt plantations to provide pulp, fuel and wood energy in the Congolese coastal plains of the Republic of the Congo (Shure et al 2010;Bouillet et al 2013;Epron et al 2013;Tchichelle et al 2017).…”

“…mangium, one of the Australian acacias that is widely planted in many parts of the world, has clear benefits in agricultural, agroforestry and forestry ecosystems (Bernhard-Reversat 1993;Franco et al 1994;Parrotta and Knowles, 1999;Richardson et al 2004;Kull et al 2011;Epron et al 2013). Key reasons for the widespread planting of A. mangium, in commercial monoculture plantations or in mixed plantings with other tree species or crops in areas with infertile soils, are its capacity to improve soil fertility (Wang et al 2010;Forrester et al 2013;Koutika et al 2014;Machado et al 2017;Tchichelle et al 2017), change the soil faunal, microbial and bacterial communities (Bernhard-Reversat 1993;Bini et al 2012Bini et al , 2013Huang et al 2014;Pereira et al 2017), and to stimulate crop or tree growth and forest productivity Epron et al 2013;Paula et al 2015). The species is considered useful for these purposes due to the enhanced nutrient cycling, higher nutrient availability and microbial activities that accrue from its presence (Khanna 1998;Bini et al 2012;Rachid et al 2013;Santos et al, 2017a, b).…”