Effects of different nitrogen application rates on soil water stable aggregates and N2O emission in winter wheat field.

Excessive nitrogen application would deteriorate soil structure and increase greenhouse gas emission. We set up six treatments, i.e., N0, N120, N180, N240, N300and N360(nitrogen application rates of 0, 120, 180, 240, 300 and 360 kg·hm-2, all straws returned into the field in situ) in the nitrogen fertilizer experimental site to investigate the effects of different nitrogen application rates on soil N2O emission, soil water-filled porosity (WFPS), soil temperature, nitrate and ammonium contents, composition and stability of water stable aggregates in winter wheat filed in 2018-2020. The results showed that there was a significant positive correlation between soil N2O emission and nitrogen application rate. There was no correlation between WFPS and nitrogen application rate. Soil temperature in the 0-10 cm layer decreased significantly with the increases of nitrogen application rates. There was a significant positive correlation between nitrate and ammonium contents and nitrogen application rate. With the increases of nitrogen application rates, the content of water stable aggregates with diameter >2 mm decreased, while that of water-stable aggregates with diameter <0.5 mm increased. The particle size of soil water-stable aggregates also decreased gradually. There was a significant negative correlation between nitrogen application rate with mean weight diameter (MWD) and geometric mean diameter, while no correlation with fractal dimension. The fitting equation between MWD and N2O emission flux was y=3928.3e-2.171x (R2=0.55, P<0.001), indicating that N2O emission increased markedly as MWD decreasing. The increases in nitrogen application rate reduced soil temperature in the 0-10 cm layer, increased nitrate and ammonium contents, decreased the average particle size of soil water stable aggregates, and the stability of soil aggregates, and increased soil N2O emission.

[Examining effects of salt stress on leaf photosynthesis of cotton based on the FvCB model]. / 基于FvCBæ¨¡åž‹åˆ†æžç›åˆ†èƒè¿«å¯¹æ£‰èŠ±å¶ç‰‡å ‰åˆä½œç”¨çš„å½±å“.

To understand the responsive mechanism of leaf photosynthesis of cotton to salinity stress, we investigated the effects of salt stress on leaf photosynthetic characteristics of cotton seedlings with the FvCB model under five levels of salt concentration, i.e., 0 (CK), 50, 100, 150 and 200 mmol·L-1. Results showed that, compared with CK, the salt concentrations of 50 and 100 mmol·L-1 increased the maximum carboxylation rate (Vc max) and the maximum electron transport rate (Jmax), while the salt concentrations of 150 and 200 mmol·L-1 significantly decreased Vc max and Jmax. The net photosynthetic rate (Pn), mesophyll conductance (gm) and dark respiration rate (Rd) gradually decreased with the increases of salt concentration. Compared with CK, the salt concentrations of 50 and 100 mmol·L-1 did not affect gm, but significantly decreased Pn and Rd. The salt concentrations of 150 and 200 mmol·L-1 significantly decreased Pn, gm and Rd, which were significantly different from the salt concentrations of 0, 50 and 100 mmol·L-1. Pn of cotton seedlings under different salt concentrations was simulated by the FvCB model. Compared with the results from the FvCB model without considering gm, the FvCB model with gm improved the determination coefficient between the simulated and measured values and decreased the mean absolute error. The salinity threshold of cotton seedlings ranged between 100 and 150 mmol·L-1. With the increases of salt concentration, the limiting factor of leaf photosynthesis changed from mesophyll conductance to impaired components of photosynthetic apparatus. The FvCB model combined gm could improve the accuracy of photosynthesis simulation.