Effect of straw-bentonite-polyacrylamide composites on nitrogen adsorption of sandy soil

In order to improve soil adsorption ability and decrease nitrogen loss, soil conditioners composited with cron straw, bentonite and polyacrylamide(PAM) were applied in sandy soil and incubated for 90 d. The effects of incubation time, proportion of PAM and rate of composite soil conditioners on soil nitrogen adsorption characteristics were investigated using isothermal adsorption experiments. The results showed that the quantity of NH4+-N adsorpted by composite soil increased with incubation time but then decreased, which reached the maximum level at 60 d. The curves of isothermal adsorptions could be well fitted by Langmuir equation models. The maximum amount of NH4+-N absorption (Qmax) increased with increasing composite proportion, but the absorption constant (k) decreased, which implied that the soil conditioners could improve the absorption capacity but reduce the strength of NH4+-N adsorption by soils. Meanwhile, the adsorption amount of NH4+-N increased then decreased as the proportion of PAM in the soil conditioner increased, with the highest Qmax value and the lowest k value being observed at the proportion of 0.99% for PAM. The maximum buffer capacity (MBC) for NH4+-N of soils decreased significantly with the increasing rate of soil conditioners, but remained constant as the proportion of PAM changed. From the present study, it is concluded that straw-bentonite-PAM composite soil conditioner can improve the ability of nitrogen fixation, consequently decrease the loss of nitrogen from soils by increasing the NH4+-N adsorption capacity of soils. The best proportion of PAM in the composite soil conditioner was 0.99%.