Abstract
The indigenous agroforestry systems practised by smallholders in south-eastern Ethiopia have high biodiversity and productivity. However, little is known about their carbon (C) inputs and outputs. We carried out a 1-year litterbag study to determine leaf litter decomposition k constants for six woody species common to these agroforestry systems. The k values were then used to calculate the decomposition C losses from measured litterfall C fluxes and the results compared to modelled soil respiration (Rs) C losses. Litterbag weight loss at the end of the year was 100% or nearly so, k values 2.582–6.108 (yr−1) and half-life 41–112 days. k values were significantly (p = 0.023) correlated with litter N contents, nearly so with C/N ratios (p = 0.053), but not with other nutrients (Ca, Mg and K), and negatively correlated with temperature (p = 0.080). Using species, farm elevation, temperature and litter quality as predictors, partial least squares regression explained 48% of the variation in k. Depending on species, estimated decomposition C losses from litterfall were 18 to 58% lower than annual litterfall C inputs. Using a heterotrophic respiration (Rh) to Rs ratio of 0.5, modelled Rh C losses were 89 to 238% of litterfall decomposition C losses estimated using k values. However, using an Rh/Rs ratio of 0.27, which is appropriate for tropical humid forests, Rh C losses were 11 to 138% of estimated litterfall decomposition C losses. Our decomposition and soil respiration estimates indicate that litterfall is sufficient to maintain soil organic C contents and thereby the soil fertility of these unique agroforestry systems.
Highlights
Interest in agroforestry has greatly increased recently because of food security issues and because of its potential for carbon (C) sequestration and climate change mitigation (Thangata and Hildebrand 2012; Mbow et al 2014a, b)
The rapid decomposition of the leaf litter and resulting high k values we found are typical for tropical forests (Bernhard-Reversat 1982; Powers et al 2009) and agroforestry systems (Hossain et al 2011; PetitAldana et al 2019; Jairo et al 2017)
The high k value for M. ferruginea recorded at farm 13, and the reason for its deviation from the PLS-R predicted value was the result of the litterbag weight loss recorded at 3 months
Summary
Interest in agroforestry has greatly increased recently because of food security issues and because of its potential for carbon (C) sequestration and climate change mitigation (Thangata and Hildebrand 2012; Mbow et al 2014a, b). Litterfall production and quality in agroforestry systems, as in general, varies with many factors including tree species, size and age, climate and season, and soil and management practice (Isaac et al 2005; Isaac and Nair 2006; Hairiah et al 2006; Das and Das 2010; Dawoe et al 2010; Murovhi et al 2012). Studies on litter decomposition in tropical agroforestry systems in Asia and South America have reported high but highly variable decomposition rates that are related to litter quality and climatic factors (Hairiah et al 2006; Hossain et al 2011; Jairo et al 2017; Petit-Aldana et al 2019). Agroforestry related litter decomposition studies carried out in eastern Africa have shown that leaf litter decomposition greatly differs among species, whether the trees are leguminous or not, the leaves are green or senesced, and on the season (Gindaba et al 2004; Teklay 2004, 2007; Teklay and Malmer 2004; Mahari 2014; Abay 2018)
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