Effect of water and nitrogen interaction on plant species diversity and biomass in a desert grassland

To demonstrate the effects of atmospheric nitrogen deposition and precipitation on species diversity and biomass in desert grassland, we examined water and nitrogen interactions, using three levels of water treatment natural precipitation (CK), increased precipitation 30% (W) and reduced precipitation 30% (R) and four nitrogen (NH4NO3) levels: 0 (N0), 30 (N30), 50 (N50), and 100 (N100) kg·(hm2·a)-1 (which does not include atmospheric nitrogen deposition). The following results were obtained: 1) The species diversity of desert grassland under CK×N and R×N treatments increased with an increase in nitrogen application rate, showing an initial increase trend and a subsequent decrease, with maximum diversity being attained with the N30 treatment. Under the W×N treatment, the species diversity was significantly reduced (P<0.05) with an increase of nitrogen application rate. 2) The aboveground biomass obtained with CK×N0 and R×N0 treatments was significantly increased (P<0.05) compared with the W×N0 treatment. Under the water and nitrogen interaction, the aboveground biomass of CK×N and W×N treatments was significantly increased (P<0.05) with an increase in nitrogen application rate. The water and nitrogen interaction had a significant effect on annual and biennial plants. The overall aboveground biomass trend was as follows: (W×N) > (CK×N) > (R×N). 3) Under the water and nitrogen interactions, the underground biomass decreased gradually with the depth of soil, and was mainly concentrated in the 0-30 cm. The W×N treatment promoted root extension into deeper soil. The biomass of desert grassland under CK×N and R×N treatments showed an initial increase trend and then a subsequent decrease with an increase of nitrogen application rate, and the underground biomass obtained with the W×N treatment showed a significant increase (P<0.05). 4) The treatments of CK×N30 and R×N30 significantly reduced the root/shoot ratio (P<0.05). Collectively, the results of this study showed that the plant species diversity and biomass in desert grassland are closely related to water and nutrients.