The application of excessive amounts of nitrogen fertilizer in dryland habitats continues to result in an increase in the amount of residual nitrate found in dryland soil profiles in the southern Loess Plateau of north central China. Efforts to use this resource have attracted considerable attention. A two-year field experiment was conducted in the southern Loess Plateau to investigate and compare how residual soil nitrate is used in winter wheat and alfalfa fields. Crops of alfalfa and winter wheat were grown in fields with six levels of residual soil nitrate, measured as the accumulated nitrate at soil depths of 0-3 m, with continuous alfalfa and wheat-summer fallow-wheat systems. Both crops exhibited similar yet different changes in the levels of residual soil nitrate during the two year study without the application of additional nitrogen fertilizer.
During the first wheat growing season, soil nitrate at a depth of 0-3 m decreased by 42%-66% in the wheat fields while it decreased by 41%-73% in the alfalfa fields, compared with N levels before sowing. N uptake (63.9-130.3 kg/hm²), the proportion of N uptake from the total residual soil nitrate prior to crop sowing (18%-27%), and the proportion of N uptake from soil nitrate reduction (29%-62%) was much higher for winter wheat than for alfalfa. The apparent reduction of soil nitrate in the upper 0-3 m of soil in the wheat field (62.1-317.1 kg/hm²) was lower than that in the alfalfa field (95.0-614.1 kg/hm²). During the second wheat growing season, N uptake of alfalfa was 593.8 kg/hm², six times higher than the N uptake of wheat. However, residual soil nitrate in the alfalfa treatments increased as a result of strong N fixation by alfalfa. Compared with the N levels prior to crop sowing, the residual soil nitrate in the alfalfa field declined by 72.4 kg/hm² while it declined by 158.3 kg/hm² in the wheat field after two growing seasons. These results indicate that winter wheat used more residual soil nitrate than alfalfa during the two year study. If the goal is to reduce residual soil nitrate levels, winter wheat has the advantage of reducing nitrate levels faster than does alfalfa in a short period of time.
During the first wheat growth season, N uptake by wheat had a significant positive correlation to the levels of residual soil nitrate at a depth of 0-2 m (r=0.920^**) and 0-3 m (r=0.857^*) while it had no significant correlation to residual soil nitrate at depths of 0-1 m. On the contrary, N uptake by alfalfa had a significant positive correlation to residual soil nitrate at a depth of 0-1 m (r=0.846^*) prior to alfalfa sowing while it was not significantly correlated to residual soil nitrate at depths of 0-2 m and 0-3 m. These results indicate that residual nitrate in deeper soil contributed more to winter wheat N uptake when compared to N uptake by alfalfa.
The uptake of N by both wheat and alfalfa was significantly positively correlated to the levels of residual nitrate in the soil at depths of 0-3 m prior to the first season of wheat sowing. However, with higher initial soil nitrate levels, the apparent loss of N also increased. These results suggest that with increased levels of residual nitrate in the soil, crops were able to increase their uptake of soil N, resulting in the removal of higher amounts of N by the crops while higher amounts of N are lost from soil profile.