Impact of mixed plantation strategies on the nutrient concentrations of green and senescent leaves and their nutrient resorption efficiencies in temperate forests of the Loess Hilly Region

Mixed-species reforestation/afforestation has been a common practice for improving ecosystem nutrient sustainability. In the Loess Hilly Region, however, it remains unclear whether the mixture of dinitrogen (N₂)-fixing species <i>Robinia pseudoacacia</i> with economic species <i>Amygdalus davidiana</i> and <i>Armeniaca sibirica</i> can improve nutrient concentrations of green and senescent leaves and their nutrient resorption. In 2022, we analyzed N and phosphorus (P) concentrations in green (GLNC, GLPC) and senescent leaves (SLNC, SLPC), resorption efficiencies (NRE and PRE), and relative resorption efficiency (RRE) in mixtures of <i>R. pseudoacacia</i> × <i>A. davidiana</i> and <i>R. pseudoacacia</i> × <i>A. sibirica</i> and monocultures of three species, considering tree structure and topsoil (0-20 cm) properties. One-way ANOVA followed by Tukey's test showed that senescent leaves nutrient concentrations for <i>R. pseudoacacia</i> and Student's t test that green and senescent leaves of <i>A. davidiana</i> and <i>A. sibirica</i> were significantly greater in mixed stands than monocultures. Based on the same statistical methods, NRE and PRE of <i>R. pseudoacacia</i> varied; those of <i>A. davidiana</i> and <i>A. sibirica</i> were significantly lower in mixed stands than monocultures. Additionally, RRE was < 100% for <i>R. pseudoacacia</i>, but > 100% for <i>A. davidiana</i> and <i>A. sibirica</i>; NRE and PRE was significantly positively associated in <i>A. sibirica</i> and all species. The correlation analysis demonstrated that crown area were significantly positively correlated with N and P concentrations of green and senescent leaves but negatively correlated with NRE and PRE for <i>A. davidiana</i> and <i>A. sibirica</i>, however, they were unrelated for <i>R. pseudoacacia</i> and all species. Further, a random forest model analysis showed that soil available P was main factor driving GLNC for individual rather than all species. Within each species, linear regression analysis revealed that NRE was significantly negatively correlated with SLNC; for <i>A. davidiana</i> and <i>A. sibirica</i>, NRE and PRE were significantly negatively correlated with SLNC and SLPC, respectively. In contrast, the PRE of all species was significantly negatively correlated with GLPC but positively correlated with SLPC. Overall, the results suggested that tree mixture increased N and P cycling more in <i>A. davidiana</i> and <i>A. sibirica</i> than in <i>R. pseudoacacia</i>, in the plant-soil system.