Vertical distribution of soil total nitrogen and soil total phosphorus in the critical zone on the Loess Plateau, China

It is important to determine the vertical distributions of soil total nitrogen (STN) and soil total phosphorus (STP) as well as the factors that influence them in the Earth's critical zone (CZ) in order to understand the N/P cycle in the CZ. However, few data are available regarding the vertical distributions of STN and STP in deep soil profiles in the CZ. Thus, this study investigated the vertical distributions of the STN and STP as well as related factors in a deep soil profile on the Loess Plateau, China. Soil drilling was used to collect 703 soil samples throughout the soil profile from five sites. The STN decreased initially at all of the sampling sites (except at Shenmu), before fluctuating with increasing depth throughout the profile, whereas the STP exhibited a fluctuating trend at all sites. The mean STN and STP concentrations ranged from 0.12 g kg−1 to 0.25 g kg−1 and from 0.45 g kg−1 to 0.58 g kg−1, respectively. The adjusted r2 values based on stepwise multiple linear regressions for STN and STP ranged from 35% to 83% and from 4% to 17%, respectively, and thus, STN was explained better by the measured soil variables compared with STP. The mean STN and STP stocks ranged from 1.87 to 3.90 mg ha−1 and from 6.75 to 9.09 mg ha−1, respectively. The results of this study facilitate evaluations of STN and STP stocks and studies of the N/P cycle in the CZ on the Loess Plateau.