Moderate nitrogen addition can enhance plant growth performance under salt stress. However, the regulatory effects of nitrogen addition on the growth of the leguminous halophyte medicinal plant, Sophora alopecuroides, under salt stress remain unclear. In this study, a two-factor pot experiment with different NaCl levels (1 g/kg, 2 g/kg, 4 g/kg) and NH4NO3 levels (0 mg/kg, 32 mg/kg, 64 mg/kg, 128 mg/kg) was set up to systematically study the response of S. alopecuroides plant phenotype, nodulation and nitrogen fixation characteristics, nitrogen (N), phosphorus (P), potassium (K) nutrient absorption and utilization efficiency, plant biomass and nutrient accumulation to nitrogen addition under salt stress. The results demonstrated that under mild (1 g/kg NaCl) and moderate (2 g/kg NaCl) salt stress, S. alopecuroides exhibited a relatively low nitrogen demand. Specifically, low (32 mg/kg N) and medium (64 mg/kg N) nitrogen levels significantly enhanced nodule nitrogenase activity and nitrogen fixation capacity. Furthermore, the uptake of essential nutrients, including N, P, and K, in the aboveground biomass was markedly increased, which in turn promoted the accumulation of major nutrients such as crude protein, crude fat, and alkaloids, as well as overall biomass production. However, under severe (4 g/kg NaCl) salt stress, S. alopecuroides exhibited a preference for low nitrogen levels (32 mg/kg N). Under S3 conditions, excessive nitrogen application (e.g., 64 mg/kg and 128 mg/kg N) exacerbated the damage caused by salt stress, leading to significant inhibition of nitrogen fixation and nutrient uptake. Consequently, this resulted in a substantial reduction in biomass. This study provides a theoretical basis for nitrogen nutrition management of S. alopecuroides under salt stress conditions and offers valuable insights for optimizing fertilization and nutrient management strategies in saline-alkali agricultural production.

Drought and soil nitrogen (N) deficiency are the limiting factors for poplar plantation productivity improvement in semi-arid regions. N addition could alleviate the growth decline of trees caused by drought; however, the effectiveness under severe drought and the underlying ecophysiological understanding remains uncertain. To further clarify the mechanisms of N addition in regulating tree biomass accumulation under different drought levels, we investigated the effects of 6 g NH4NO3 per plant addition on the carbon and N assimilation and biomass accumulation of potted poplar seedlings under moderate or severe drought (40 % or 20 % of field capacity) conditions, with a particular emphasis on carbon and N interactions. We found that under moderate drought, N addition markedly promoted the activities of antioxidases, nitrate reductase (39 %), and N concentration (56 %) in leaves, significantly alleviated the damages of the membranes and photosystem II, and increased both leaf area (69 %) and chlorophyll content per unit leaf area, along with net photosynthesis rate (34 %), thereby significantly alleviating growth restrictions. However, under severe drought, although N addition increased the accumulation of both soluble sugars and N of the whole plant, it did not ameliorate the damage to membranes and photosystem II, nor did it improve chlorophyll content, leaf area, or biomass accumulation. Therefore, N addition could increase leaf area, enhance antioxidants, and positively influence leaf carbon assimilation (0.60, p < 0.001) in poplar seedlings under moderate drought. The restrictions on leaf area and carbon assimilation were exacerbated during severe drought, which mitigated the positive effects of N addition on carbon assimilation and biomass accumulation. The findings of this study suggest that the growth of hybrid poplar can be enhanced by applying N fertilizer under mild drought conditions. In contrast, N fertilization has no significant effect in severe drought conditions.

Information regarding the interactive effects of global warming and increasing nitrogen (N) deposition on CO2 emissions in the alpine grassland ecosystem is scarce, especially in the permafrost region of the Qinghai-Tibetan Plateau. We conducted a manipulative field experiment in an alpine swamp meadow to investigate the responses of ecosystem respiration (ER) to simulated warming and N addition. Results showed that the interaction between warming and N addition significantly increased ER by 41.3-239.6%, which might be related to the enhancements in plant autotrophic respiration and soil microbial biomass and activity. The correlations between ER and a single hydrothermic factor tended to be weakened with the increasing complexity of the treatment method. The drought stress on CO2 emissions was not found due to the thawing of the permafrost and the upward diffusion of soil moisture, thus air temperature combined with soil temperature explained 80% of the ER fluctuations. Meanwhile, warming increased the aboveground biomass (AGB) and belowground biomass (BGB) by 44.2-68.1% and 48.1-82.6%, respectively, suggesting that more biomass was allocated to the belowground components. N addition increased AGB by 21.2-30.3%, while there was no significant effect on BGB. Warming combined with N addition strongly increased AGB and BGB by 52.0-159.5% and 59.0-102.1%, respectively. These results indicated that plant production and allocation pattern might also be important factors affecting CO2 emissions. In addition, warming alone and warming combined with N addition increased soil microbial biomass carbon (MBC) by 19.1-90.7% and 28.1-80.4%, respectively, and the enhancement in soil microbial biomass and activity might promote the release of soil carbon.

Uncertainties in the seasonal changes of greenhouse gases (GHG) fluxes in wetlands limit our accurate understanding of the responses of permafrost ecosystems to future warming and increased nitrogen (N) deposition. Therefore, in an alpine swamp meadow in the hinterland of the Qinghai-Tibet Plateau, a simulated warming with N fertilization experiment was conducted to investigate the key GHG fluxes (ecosystem respiration [Re], CH4 and N2O) in the early (EG), mid (MG) and late (LG) growing seasons. Results showed that warming (6.2 degrees C) increased the average seasonal Re by 30.9% and transformed the alpine swamp meadow from a N2O sink to a source, whereas CH4 flux was not significantly affected. N fertilization (4 g N m(-2) a(-1)) alone had no significant effect on the fluxes of GHGs. The interaction of warming and N fertilization increased CH4 uptake by 69.6% and N2O emissions by 26.2% compared with warming, whereas the Re was not significantly affected. During the EG, although the soil temperature sensitivity of the Re was the highest, the effect of warming on the Re was the weakest. The primary driving factor for Re was soil surface temperature, whereas soil moisture controlled CH4 flux, and the N2O flux was primarily affected by rain events. The results indicated: (i) increasing N deposition has both positive and negative feedbacks on GHG fluxes in response to climate warming; (ii) during soil thawing process at active layer, low temperature of deep frozen soils have a negative contribution to Re in alpine ecosystems; and (iii) although these alpine wetland ecosystems are buffers against increased temperature, their feedbacks on climate change cannot be ignored because of the large soil organic carbon pool and high temperature sensitivity of the Re. (C) 2017 Elsevier B.V. All rights reserved.

The limited number of in situ measurements of greenhouse gas (GHG) flux during soil freeze-thaw cycles in permafrost regions limits our ability to accurately predict how the alpine ecosystem carbon sink or source function will vary under future warming and increased nitrogen (N) deposition. An alpine meadow in the permafrost region of the Qinghai-Tibet Plateau was selected, and a simulated warming with N fertilization experiment was carried out to investigate the key GHG fluxes (ecosystem respiration [Re], CH4 and N2O) in the early (EG), mid (MG) and late (LG) growing seasons. The results showed that: (i) warming (4.5 degrees C) increased the average seasonal Re, CH4 uptake and N2O emission by 73.5%, 65.9% and 431.6%, respectively. N fertilization (4 g N m(-2)) alone had no significant effect on GHG flux; the interaction of warming and N fertilization enhanced CH4 uptake by 10.3% and N2O emissions by 27.2% than warming, while there was no significant effect on the Re; (ii) the average seasonal fluxes of Re, CH4 and N2O were MG > LG > EG, and Re and CH4 uptake were most sensitive to the soil freezing process instead of soil thawing process; (iii) surface soil temperature was the main driving factor of the Re and CH4 fluxes, and the N2O flux was mainly affected by daily rainfall; (iv) in the growing season, warming increased greenhouse warming potential (GWP) of the alpine meadow by 74.5%, the N fertilization decreased GWP of the warming plots by 13.9% but it was not statistically significant. These results indicate that (i) relative to future climate warming (or permafrost thawing), there could be a hysteresis of GHG flux in the alpine meadow of permafrost region; (ii) under the scenario of climate warming, increasing N deposition has limited impacts on the feedback of GHG flux of the alpine meadow. (C) 2017 Elsevier Ltd. All rights reserved.