Response of fine roots of apple to plastic film mulching in the dry tableland of eastern Gansu

This study investigated the annual growth dynamics of apple tree roots in the dry plateau of Longdong and the temporal and spatial differences in the number, morphology, branching characteristics of the roots, and soil physical and chemical properties in vertical soil layers under film mulching and soil moisture conservation measures. Eighteenth-year-old apple trees (‘ Nagano Fuji No.2’) were assessed three times in the rooting peak times of apple tree: from spring sprouting to vigorous growth of new shoots (Ⅰ), shoots stopped growing (Ⅱ), and from fruit harvest to defoliation (Ⅲ). Using the soil profile and stratified sampling method, different treatments (conventional tillage CK, film-mulching for two years 2Y, film-mulching for four years 4Y, and film-mulching for six years 6Y) were investigated to analyze the spatial distribution of biomass, root length, surface area, specific root length, and the specific branch (branch number/dry matter weigh) of roots. Regression analysis was used to assess the fine root growth strategy for apple trees with plastic film mulching. The results showed that the rooting peak Ⅲ was the most important stage of the annual growth cycle of apple roots. The fine roots biomass at rooting peak Ⅲ under each treatment was 73.55%–84.85% of the total biomass at the three rooting peaks. The number of fine root branches at rooting peak Ⅰ in the surface soil (0–20 cm) was 130.67%, 100.53%, 156.63%, and 238.63% of that at rooting peak Ⅲ, which effectively improved the utilization of the soil resources in situ. At rooting peak Ⅲ, CK promoted the distribution of fine root length and root surface area in the surface soil, which were 275.64% and 248.96% of those at rooting peak Ⅰ, respectively. The number of branches and specific branches were only 76.53% and 14.68% of those at rooting peak Ⅰ, which expanded the effective nutrient space and reduced the internal competition of the root system. The soil water content in the short-term mulching (2Y) treatment in each soil layer were 112.39% (0−20 cm), 118.04% (20−40 cm), 124.06% (40−60 cm), 133.59% (60−80 cm), and 114.49% (80−100 cm) of CK; and the fine root biomass was 116.72%, 232.35%, and 112.09% of CK at the three rooting peak times. Compared with CK, the specific root length of the surface fine roots increased by 47.1% and 62.92% at rooting peaks Ⅰ and Ⅲ, and the root surface area increased by 67.21% and 56.88% in 2Y treatment. The number of fine root branches in the deep soil (80–100 cm) increased by 282.22% and 7.27%, respectively, compared with CK. The 2Y treatment promoted fine root morphological trait expression at the surface soil and branch structure establishment in the deep soil. Fine roots were evenly distributed in the 0–100 cm vertical soil layer and 0–120 cm horizontally from tree. Compared with CK, the 6Y treatment increased the number of fine root branches and specific branches by 6.11% and 34.6%, respectively, in the early growth stage, but by 58.1% and 19.56% in the late growth stage. These results demonstrate the characteristics of complex branches in the early growth stage and simplified branches in the late growth stage significantly inhibit the growth of fine roots at rooting peak Ⅲ. The fine root length, root surface area, and specific root length in the deep soil were 35.19%, 40.43%, and 82.67% of those of CK, respectively, in 6Y treatment. Fine root growth was affected by the phenological period and the turnover of tree nutrients; the “conservatively obtaining resources” growth strategy was applied in the early growth stage, and the “rapidly obtaining resources” growth strategy was adopted in the late growth stage. Short-term film mulching (2Y) can improve the physical and chemical properties of the soil and promote fine root extension. Damage from long-term plastic film mulching (6Y) to the subsurface soil (20–40 cm) prevented the fine roots from settling down and they became concentrated in the surface layer.