Agroforestry enhances the capacity of deep tropical soils to act as nutrient sinks and biogeochemical buffers, thereby improving internal nutrient recycling – insights from incubation experiments

Abstract. The understanding of the interactions between the lower and upper compartments of the Critical Zone (CZ) is of major importance to the maintenance of life across continental landscapes. The extent to which deep root uptake supplies nutrients from depth and redistributes them to the entire ecosystem remains unknown. Identifying the processes at stake and quantifying their effects on nutrients cycles is of paramount importance for assessing ecosystem resilience and designing management practices enhancing biologically mediated CZ processes in agrosystems. The impact of depth and land-use on the decomposition of soil organic matter (SOM; roots free soil) and fine roots (<2 mm) was investigated for a tropical conventional irrigated agriculture plot and an agroforestry one, at two depths (0–15 cm and between 8 and 10 m), through soil incubations in small lysimeters over a period of 115 days. Dynamics of CO₂ and major soluble species (NO₃^-, SO₄^2-, PO₄^3-, Na⁺, Ca²⁺, Mg²⁺, K⁺, DOC and DIC) were monitored over the whole incubation period using sodium hydroxide traps and free pore water extraction at each time step, respectively. Soil initial and final C, N and exchangeable stocks were also characterized. During the 115 days that lasted this experiment, more than 90 % of the C fluxes were released as CO₂ for all treatments. However, the final cumulative fluxes varied depending on soil depth and land use, both for the decomposition of SOM and for that of fine roots, with a potential priming in soils under conventional agriculture and a legacy effect in agroforestry soils. Cation and anion fluxes varied primarily with depth and could be classified into three groups based on their extraction dynamics: (1) Na and K, preferentially leached from the CEC; (2) nitrate and sulfate, mostly related to OM biological processes i.e. either production of microbial necromass (roots free soils) or immobilization in microbes’ biomass (soils with root OM added), and were mainly compensated in terms of charges by Ca and Mg release; (3) phosphate with a significant impact of the sampling interval on its extraction due to the interaction between phosphates and soil mineral surfaces. However, at equivalent rates of root decomposition, subsoils retained N, S, P, K, and Ca more effectively than topsoils, and this retention was generally stronger under agroforestry than under conventional irrigated agriculture, without perennial deep rooting. Overall, this study supports the view that agroforestry enhances the capacity of deep tropical soils to act as nutrient sinks and biogeochemical buffers, thereby improving internal nutrient recycling and potentially increasing the resilience of the agrosystem. It also highlights the importance of taking into account the multiple correlations among the various major elements found in interaction in soil pore solutions into modeling for estimating soil carbon stocks in these tropical soils.