Responses of soil N2 O emissions and their abiotic and biotic drivers to altered rainfall regimes and co-occurring wet N deposition in a semi-arid grassland.

Global change factors such as changed rainfall regimes and nitrogen (N) deposition contribute to increases in the emission of the greenhouse gas nitrous oxide (N2 O) from the soil. In previous research, N deposition has often been simulated by using a single or a series of N addition events over the course of a year, but wet N deposition actually co-occurs with rainfall. How soil N2 O emissions respond to altered rainfall amount and frequency, wet N deposition, and their interactions is still not fully understood. We designed a three-factor, fully factorial experiment with factors of rainfall amounts (ambient, -30%) rainfall frequency (ambient, ±50%) and wet N deposition (with/without) co-occurring with rainfall in semi-arid grassland mesocosms, and measured N2 O emissions and their possible biotic and abiotic drivers. Across all treatments, reduced rainfall amount and N deposition increased soil N2 O emissions by 35% and 28%, respectively. A significant interactive effect was observed between rainfall amount and N deposition, and to a lesser extent between rainfall frequency and N deposition. Without N deposition, reduced rainfall amount and altered rainfall frequency indirectly affected soil N2 O emissions by changing the abundance of nirK and soil net N mineralization, and the changes in nirK abundance were indirectly driven by soil N availability rather than directly by soil moisture. With N deposition, both the abundance of nirK and the level of soil water-filled pore space contributed to changes in N2 O emissions in response to altered rainfall regimes, and the changes in the abundance of nirK were indirectly driven by plant N uptake and nitrifier (ammonia-oxidizing bacteria) abundance. Our results imply that unlike wetter grassland ecosystems, reduced precipitation may increase N2 O emissions, and N deposition may only slightly increase N2 O emissions in arid and semi-arid N-limited ecosystems that are dominated by grasses with high soil N uptake capacity.

Effects of seed size and bract of Ceratoides arborescens on germination and seedling growth under drought stress.

Drought is a crucial factor affecting seed germination and seedling growth of desert plants. In the study, we examined the effects of seed size (large, small) and bract (without bracts, with bracts) on seed germination and seedling growth of Ceratoides arborescens, a perennial super xerophyte semi-shrub, under different drought levels (0, 100, 200, 300 and 400 g·L-1 PEG6000). The results showed that drought stress significantly inhibited seed germination and reduced shoot length of seedlings. Under the 100 and 200 g·L-1 PEG6000 stress conditions, root length of seedlings were significantly increased, whereas it was significantly reduced under the 300 and 400 g·L-1 PEG6000 stress intensities. The presence of bract significantly reduced seed germination percentage by 12%, germination index by 50.5%, and shoot length by 20.8%, but increased root length by 6.3%. Seed size did not affect seed germination index. Compared with small seeds, germination percentage of large seeds was 3% higher, shoot length and root length of seedlings were 20.5% and 33.0% higher, respectively. In summary, seed bract in C. arborescens through delaying seed germination and seed size through influencing the survival of the early state of seedling were the most important strategies to adapt extremely arid environment.