Unexpected phenology and lifespan of shallow and deep fine roots of walnut trees grown in a silvoarable Mediterranean agroforestry system
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Background and Aims
Fine roots play a major role in the global carbon cycle through respiration, exudation and decomposition processes, but their dynamics are poorly understood. Current estimates of root dynamics have principally been observed in shallow soil horizons (<1 m), and mainly in forest systems. We studied walnut (Juglans regia × nigra L.) fine root dynamics in an agroforestry system in a Mediterranean climate, with a focus on deep soils (down to 5 m), and root dynamics throughout the year.
Sixteen minirhizotron tubes were installed in a soil pit, at depths of 0.0–0.7, 1.0–1.7, 2.5–3.2 and 4.0–4.7 m and at two distances from the nearest trees (2 and 5 m). Fine root (diameter ≤ 2 mm) dynamics were recorded across three diameter classes every 3 weeks for 1 year to determine their phenology and turnover in relation to soil depth, root diameter and distance from the tree row.
Deep (>2.5 m) root growth occurred at two distinct periods, at bud break in spring and throughout the winter i.e., after leaf fall. In contrast, shallow roots grew mainly during the spring-summer period. Maximum root elongation rates ranged from 1 to 2 cm day−1 depending on soil depth. Most root mortality occurred in upper soil layers whereas only 10 % of fine roots below 4 m died over the study period. Fine root lifespan was longer in thicker and in deeper roots with the lifespan of the thinnest roots (0.0–0.5 mm) increasing from 129 days in the topsoil to 190 at depths > 2.5 m.
The unexpected growth of very deep fine roots during the winter months, which is unusual for a deciduous tree species, suggests that deep and shallow roots share different physiological strategies and that current estimates based on the shortest root growth periods (i.e., during spring and summer) may be underestimating root production. Although high fine root turnover rates might partially result from the minirhizotron approach used, our results help gain insight into some of the factors driving soil organic carbon content.
KeywordsAlley cropping Juglans Deep root growth Root elongation rate Root mortality Root turnover
This study was financed by the French ANR funded project ECOSFIX (Ecosystem Services of Roots - Hydraulic Redistribution, Carbon Sequestration and Soil Fixation, ANR-2010-STRA-003-01), by the ADEME funded project AGRIPSOL (Agroforestry for soil protection) and by la Fondation de France. We thank the farmers Mr. Henri and Alain Breton, for their authorization to open the deep pit. We are very grateful to our INRA colleagues Jean-François Bourdoncle, Lydie Dufour, Alain Sellier and Didier Arnal for their help with field and laboratory work and logistics. The Restinclières farm is the property of the Conseil Départemental de l’Hérault, which provides financial support to INRA since 1995 for the monitoring of agroforestry systems, and their support is warmly appreciated.
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