Contribution of tree and crop roots to soil carbon stocks in a Sub-Sahelian agroforestry parkland in Senegal

Highlights

• Tree and crop roots in an agroforestry parkland confirmed complementarity theory.

• Equivalent crop root biomass density found under the tree crown and far from it.

• Annual crop root litter-derived C inputs represent up to 3.7 % of the soil C stocks.

• F. albida root biomass density was 5 times lower than crops but reached 5–6 m depth.

• F. albida roots highly contribute to soil C stocks, especially below 30 cm depth.

Abstract

In the Sub-Sahelian region (Senegal), Faidherbia albida trees have been maintained for years in agroforestry parklands for their numerous services offered to people and to crops. The aerial crop biomass and yield are notably greater under the tree crown, but most of this biomass is exported and does not contribute to soil carbon (C) inputs. Belowground litter inputs are crucial, but little is known about the root system distribution, productivity and contribution to soil C inputs. This study aims to assess the field heterogeneity of F. albida and crop root systems along the soil profile based on the distance to the tree to evaluate the root contribution to soil C stocks.

During two successive rotations of pearl millet in 2020 and groundnut in 2021, soils and roots were sampled from two locations (Under and Far from the tree), in 5 soil layers to a depth of 150 cm and with three tree replicates. The aerial biomass was also sampled. The annual relative contribution of roots to soil C stocks was estimated as the ratio between root litter-derived C inputs (considering hypothetical root turnover only for trees and the whole root biomass for crops) and soil C stocks.

We found a similar crop root biomass density Under and Far from the tree, but the higher root:shoot ratio Far from than Under the tree confirmed the beneficial effect of trees on the aerial crop biomass. Unexpectedly, the tree root biomass density was higher at + 30 m from the trunk than Under the tree in the 30–100 cm soil layer, suggesting a long-distance soil exploration by the trees. Down to 150 cm of depth, the tree root litter contribution to soil C stocks represented 6.1 % and 7.1 % of the total root litter-derived C inputs (Under and Far from the tree, respectively). In this type of Sub-Sahelian parklands with low soil C stocks, increasing the tree density or selecting deep and highly rooted crop varieties should be favoured.

Introduction

Lands in tropical climates are particularly affected by soil degradation (Lal, 1984, Islam and Weil, 2000, Wang et al., 2005). In these regions, soil carbon (C) sequestration could be a key strategy to improve food security and sustainable agriculture management, in addition to reducing the concentration of atmospheric greenhouse gases (Chenu et al., 2019). Compared to temperate soils, tropical soils have high vegetal biomass production (Nair et al., 2009) and a low soil C content, which should be enhanced to improve soil properties and food security. Agroforestry, which is recognized for fostering soil C sequestration (Albrecht and Kandji, 2003, Nair et al., 2009, Atangana et al., 2014), is an ancient practice in tropical climates. More specifically, in the Sub-Sahelian area, agroforestry is mostly based on cultivated tree parklands (Bayala et al., 2014) with a natural dispersion of trees. The tree Faidherbia albida is a common species that has been maintained by farmers due to the numerous services it provides for crops (Barnes and Fagg, 2003) and its reverse phenology, offering fodder during the dry season (Roupsard et al., 1999). While the effects of F. albida pruning (Toib et al., 2021) and aerial litter (Gnankambary et al., 2008) on pedo-climatic conditions (Ong et al., 2015, Sileshi, 2016), crop yield, soil fertility and soil C stocks have been widely studied (Charreau and Vidal, 1965, Dancette and Poulain, 1968, Rao et al., 1997, Diallo et al., 2021), few studies have included roots, despite their main role in deep soil water extraction (Bayala and Prieto, 2020) and nutrient uptake (Roupsard et al., 1999, Danthu et al., 2002). Studying the root interspecific interactions in Faidherbia parklands is needed to understand the competition or complementarity with the associated plant root systems and their contribution to soil C sequestration as an important source of C.

In contrast to crop aerial biomass, crop root biomass density has been observed to be equivalent regardless of distance to trees (Defrenet et al., 2016, Roupsard et al., 2020). Up to 200 cm, crop root biomass density was twice the tree fine root biomass. Moreover, F. albida fine root biomass was absent at 30 m from the tree trunk (Siegwart et al., 2022). In addition, the soil profile distributions of tree and crop root systems in agroforestry parklands are different. Crop roots are mostly concentrated in the topsoil with a maximum rooting depth of 80–100 cm and are exposed to moisture variations (rainfall, topsoil evaporation, plant water use, etc.) (Van Noordwijk et al., 2015). Due to competition in agroforestry systems (Cardinael et al., 2015) and their internal genetic program, trees explore deeper soil layers than annual crops (Van Noordwijk et al., 2015), sometimes reaching the water table (Akinnifesi et al., 2004), as is the case for F. albida (Roupsard et al., 1999, Toib et al., 2021). Indeed, the deepest rooting patterns are found in semi-arid ecosystems (Canadell et al., 1996), and tree rooting under the crop root zone is a key factor in agroforestry according to Van Noordwijk et al. (1996). ‘Complementarity theory’ (Van Noordwijk and Purnomosidhi, 1995, Das and Chaturvedi, 2008) suggests that such tree rooting favours efficient soil resource use thanks to the association of different species with different belowground strategies. By contrast in monocultures (Schroth, 1998), soil acts as a buffer that stores water and nutrients for crop needs (Van Noordwijk et al., 1996), especially in poor tropical soils. Such soil layer division between crop and tree roots is desired but is not always reached in plurispecific systems (Battie-Laclau et al., 2020), and more precise evaluation of root systems in relation to soil properties is needed to validate and understand the complementarity theory in Sub-Sahelian agroforestry parkland dominated by F. albida.

In addition to root biomass and distribution differences between trees and crops, root properties, including root functional traits and chemical composition, are expected to be different between different species associated with the agroforestry system. For example, functional traits indicate an acquisition strategy for crop roots and a conservation strategy for tree fine roots (Roumet et al., 2006). Moreover, crop roots generally have a higher specific root length and root N content than tree fine roots (Roumet et al., 2006), but F. albida is a N-fixing tree, and its root litter is enriched in N (Stephen et al., 2020, Siegwart et al., 2022). Not only genetics but also the environment can impact root functional traits (Schroth, 1998, Van Noordwijk et al., 2015). First, root functional traits can inform the distribution strategy in response to belowground competition in agroforestry (Cardinael et al., 2015). Second, strategies responding to soil nutrient shortages can involve root functional trait changes (Pan et al., 2018). Indeed, under water and nutrient shortages and in infertile or stressed conditions, such as those found far from trees in tropical parklands (Traore et al., 2004, Bayala et al., 2015), roots are expected to have higher specific root lengths to maximize soil exploration (Fitter, 1985, Eissenstat, 1991, López-Bucio et al., 2003). Last, soil depth is a major factor controlling root C stabilization (Lorenz and Lal, 2005, Cotrufo et al., 2013, Poirier et al., 2018) due to different root decomposition kinetics according to depth (Pries et al., 2018, Siegwart et al., 2022).

In agroforestry parklands, the heterogeneity of trees and crop root systems along the soil profile and according to the distance to the tree directly impacts the quantity, distribution and quality of root litter-derived C inputs to the soil (at harvest for crops and during root turnover for trees), which could contribute to heterogeneity in the soil C stocks. This study aimed to evaluate the impact of the association of trees and crops in a Sub-Sahelian agroforestry parkland (Senegal) dominated by F. albida on crop and tree root distribution, biomass and properties according to soil depth, and distance from the tree (under or far from the tree). Furthermore, this study aimed to investigate the contribution of tree and crop root litter-derived C inputs to soil C stocks along the soil profile.

We first hypothesized that the heterogeneity of the root systems (biomass, distribution and properties) in the agroforestry parkland would be the result of the interactions of trees and crop species, with deeper, vertically oriented and N-richer root systems of the N-fixing tree than the crops. Second, we hypothesized that the root systems would be influenced by variations in soil chemical properties, with a higher root:shoot ratio in poor soils (far from the trees, no interspecific competition) despite equivalent root mass than in zones with higher nutrient levels (under trees, with interspecific competition). Last, the crop root contribution to soil C stocks was expected to be higher in the topsoil but equivalent at all distances from the tree (similar crop root biomass density), while the tree roots would contribute to C stocks mainly in the subsoils and near the tree trunk.