Tree–wheat vertical fine root distribution in a 4-year-old temperate alley-cropping system

Competition or complementarity between associated plants due to belowground interactions has been observed in alley-cropping systems (ACs), but the initialization of these processes remains poorly investigated. Here, we used the core-break and soil coring method to quantify the vertical tree and wheat fine root abundance and biomass down to 120 cm in a 4-year-old temperate AC. Fine roots were measured at 2 m from a reference tree (hornbeam, wild cherry or willow) in tree–wheat AC, pure-forest associated with ryegrass (FC) and wheat sole-crop (CC) plots at the Ramecourt experimental site. The mean wheat fine root abundance (WFRA) was twice as high in the CC plot (874 ± 152 m⁻²) as in the AC plot (437 ± 47 m⁻²). It was significantly higher for wheat associated with hornbeam than for willow, particularly at the 10 cm depth. Tree fine root abundance (TFRA) was linearly correlated with tree fine root biomass (TFRB) for hornbeam (R² = 0.79***), willow (R² = 0.77***) and wild cherry (R² = 0.54***). Using TFRA, the van Noordwijk's equation gave a better prediction of the TFRB for willow and wild cherry than for hornbeam. The mean value of the TFRA was seven times higher in the FC plot (1116 ± 97 m⁻²) than in the AC plot (146 ± 24 m⁻²) for all soil depths and all tree species due to the lack of nutrients from the absence of fertilization. At 4 years old, willow and hornbeam fine roots cohabited with wheat in the upper soil layer, whereas wild cherry had already developed deep fine roots under the crop rooting zone.