Nitrogen leaching and soil nutrient supply vary spatially within a temperate tree-based intercropping system

Well-designed temperate tree-based intercropping (TBI) systems can enhance soil nutrient cycling compared to conventional agricultural systems. To improve the TBI designs and their subsequent wide-scale adoption, greater understanding is required regarding the extent to which widely-spaced tree rows and tree management practices influence spatio–temporal dynamics of soil nutrients. Our 2-year study (2021 and 2022) assessed N-leaching and soil nutrient supply at increasing distances from tree rows (0, 4, 12, 20 m); the 10-year-old TBI system (50 trees ha−1) together with agricultural controls was established in southern Québec (Canada). The TBI included hybrid poplars (Populus deltoides × P. nigra) planted alternately with high-value hardwoods in the rows. In each experimental block (n = 3), the TBI system and control were divided into two treatments: without root-pruning versus with (0.75 m depth using a sub-soiler). In 2022, NO3− supply rates near tree rows (0 and 4 m; 0.28 ± 0.04 [mean ± SE] and 0.37 ± 0.05 µg cm−2 d−1, respectively) were lower than alley centres (12 and 20 m) and controls (0.62 ± 0.07, 0.52 ± 0.07 and 0.82 ± 0.07 µg cm−2 d−1, respectively). A first structural equation modelling (SEM) analysis revealed that NO3− supply rates were mostly modulated by indirect effects of tree row distance and soil clay content through volumetric water content (VWC). Nitrate leaching (400-mm depth) at 0 and 4 m from the tree row was respectively 8.8 × and 7.5 × lower than that in the control. A second SEM analysis showed direct and indirect (through soil VWC affecting NO3− supply rates) effects of distance from tree rows on NO3− leaching rates. Within TBI, greater tree leaf litter dry-mass was trapped at 0 and 4 m versus 12 and 20 m. Phosphorus and K availability under tree rows was higher than all other distances within cultivated alleys and control plots. Phosphorus, K, Ca and Mg supplies within cultivated alleys were generally similar among distances (4, 12, and 20 m) and did not differ from controls. An unexpected lack of effect of tree root pruning was observed regarding soil nutrient supply and N leaching. Clay content was a major driver of soil nutrient supply and N leaching. The role of TBI systems in determining soil nutrient dynamics depended upon the soil nutrient and sampling period that was measured, with greater effects beneath the trees and at the tree-crop interface.