Abstract
BackgroundAfforestation of savannas in the Congolese coastal plains with eucalypt has provided wood pulp for industry and fuel energy for the local population. Typically, following afforestation, Acacia mangium are introduced to improve soil fertility and sustain productivity. Through investigations of particulate organic matter (POM), potential soil organic matter (SOM) quality was assessed in acacia and eucalypt plantations along rotations.MethodsNutrients in POM (4000–50 μm) in the 0–5 cm soil layer were measured after five years into the second rotation (R2Y5) in relation to soil pH and P availability. Data were compared to those at the end of the first 7-year-rotation (R1Y7) and after two years into the second rotation (R2Y2) to evaluate overall SOM quality in the topsoil.ResultsAt R2Y5, soil pH was higher in the pure eucalypt stands (100E) than in stands containing acacia, either in monoculture (100A) or evenly mixed with eucalypt (50A50E). Coarse POM (cPOM, 4000–250 μm) beneath 100A had the highest N concentration (1.71%), followed by those beneath 50A50E (1.42%) and 100E (1.30%). Higher N was always found in the stands containing acacia. Lower sulphur (S) concentrations and P availabilities were observed in cPOM (50A50E). The greatest amount of coarse (414.7 g) and fine (214.5 g) forest floor litter were found in 100A stands, whereas higher C concentrations were found in the 100E stands for coarse forest floor litter (36.5%) and in the 50A50E stands for fine forest floor litter (38.7%). The decrease in cPOM N and C concentrations were lower than 20% (R1Y7) and 26% (R2Y5) relative to the younger stage (R2Y2). This tendency was more pronounced in fine POM (250–50 μm) and organo-mineral fraction (< 50 μm).ConclusionsThe main changes occurred in cPOM beneath stands containing acacia while higher weight of forest floor litter was found in 100A. Soil pH decreased in stands containing acacia. Overall N and C dynamics was enhanced in older stands (R2Y5) than in the younger stands (R2Y2). This may reveal a creation of more labile SOM with lower N and C concentrations in POM fractions in the surface layer, i.e., an ecosystem with a lower potential to mitigate climate change along rotations.
Highlights
Afforestation of savannas in the Congolese coastal plains with eucalypt has provided wood pulp for industry and fuel energy for the local population
Water and KCl pH At year 5 of the second rotation (R2Y5), both afforested stands with acacia, i.e., pure acacia or 100% acacia (100A) and 50% of acacia and 50% of eucalypt (50A50E) had significantly lower soil pH (H2O) than pure eucalypt (100E) stands (4.2) (Table 1)
I.e., from the end of the first 7-year-rotation (R1Y7) to year 2 (R2Y2) and 5 (R2Y5) of the second rotation, soil pH-H2O was higher in pure eucalypt or 100% eucalypt (100E), and lower in 100A at year of the first rotation (R1Y7) and year 2 of the second rotation (R2Y2)
Summary
Afforestation of savannas in the Congolese coastal plains with eucalypt has provided wood pulp for industry and fuel energy for the local population. Fluxes of C from soil to atmosphere are partially regulated by the physical, and chemical characteristics of this SOM (Stewart et al 2009; Kirkby et al 2013), and is linked to other nutrients such as N, P and S (Binkley 1992; Sang et al 2013; Kopittke et al 2017) Factors such as soil texture, climate and relief have an undeniable impact on SOC status, i.e., on SOM quality and quantity (Pieri 1989; Hassink et al 1993; Koutika et al 1999; Galantini et al 2004). Land-use changes such as afforestation of unmanaged lands, pastures, savannas or cerrados alter SOM i.e., SOC inputs and decomposition, and, impact on C, N and P dynamics (Gonçalves et al 2008; Pérez-Cruzado et al 2012; Forrester et al 2013; Epron et al 2015; Dou et al 2016; Tchichelle et al 2017; Koutika 2019)
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