Ecological stoichiometric comparison of plant-litter-soil system in mixed-species and monoculture plantations of Robinia pseudoacacia, Amygdalus davidiana, and Armeniaca sibirica in the Loess Hilly Region of China

We examined how afforestation patterns impact carbon (C), nitrogen (N), and phosphorus (P) stoichiometry in the plant-litter-soil system. Plant leaf, branch, stem, and root, litter, and soil samples were collected from mixed-species plantations of Robinia pseudoacacia with Amygdalus davidiana (RPAD), R. pseudoacacia with Armeniaca sibirica (RPAS), and monocultures of R. pseudoacacia (RP), A. davidiana (AD), and A. sibirica (AS) in the Loess Hilly Region. The results showed that in mixed-species plantations, R. pseudoacacia had lower leaf N and P concentrations than in monocultures, while both A. davidiana and A. sibirica had higher leaf N and P concentrations. Soil P limited tree growth in both afforestation models. Mixing R. pseudoacacia with A. davidiana or A. sibirica reduced N-limitation during litter decomposition. Average soil total N and P concentrations were higher in RPAS than in RPAD, and both were higher than the corresponding monocultures. The average soil C:N ratio was the smallest in RPAS, while the average soil C:P ratio was larger in RPAS than in RP. A positive correlation between N and P concentrations, and between C:N and C:P ratios, was found in litter and all plant organs of mono- and mixed-stands. Alternatively, for N concentration and C:N ratio, the correlations between plant (i.e., leaf, branch, root) and litter and between plant and soil were inverse between plantation types. RPAD has an increased litter decomposition rate to release N and P, while RPAS has a faster rate of soil N mineralization. RPAD was the best plantation (mixed) to improve biogeochemical cycling, as soil nutrient restrictions, particularly for P-limitation, on trees growth were alleviated. This study thus provides insights into suitable tree selection and management by revealing C:N:P stoichiometry in the plant-litter-soil system under different afforestation patterns.