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Global change factors such as changed rainfall regimes and nitrogen (N) deposition contribute to increases in the emission of the greenhouse gas nitrous oxide (N2O) 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 N2O 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 N2O emissions and their possible biotic and abiotic drivers. Across all treatments, reduced rainfall amount and N deposition increased soil N2O 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 N2O 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 N2O 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 N2O emissions, and N deposition may only slightly increase N2O emissions in arid and semi‐arid N‐limited ecosystems that are dominated by grasses with high soil N uptake capacity. [ABSTRACT FROM AUTHOR]

Ecosystem functions, such as soil nitrogen cycling, are being altered by climate change. The responses of soil (de)nitrification to climate change are different and sometimes opposite across global grasslands. However, it remains unclear how the local environmental conditions and the duration and magnitude of climate change experiments influence these responses. We synthesized the results of 49 studies corresponding to 518 observations on the effects of warming, elevated CO 2 , and elevated precipitation, and unfolded the reasons for the divergent responses in (de)nitrification to these climate change factors. We found that the responses of (de)nitrification are mainly related to annual precipitation and temperature, and less to the duration and magnitude of experimental treatment. In contrast, soil variables such as pH and organic carbon had no significant effects on these responses. Specifically, the effect of warming on the abundance of ammonia-oxidizing archaea (AOA) shifted from negative for mean annual precipitation (MAP) < 600 mm to positive for MAP>700 mm, whereas the effects on denitrification decreased with mean annual temperature (MAT). The effects of elevated CO 2 on nitrification, denitrification and soil nitrate all decreased with MAP, with negative effects observed for MAP>700 mm. The effects of elevated P on nitrification and soil ammonium decreased with MAP, with negative effects observed for MAP>300 mm. The results suggested that in dry regions, altered water availability governs climate change effects, while changed nitrogen availability is likely the main determinant in wet regions. This shows how biochemical models should include local climatic conditions when predicting nitrogen dynamics across the grassland biome under future climate change scenarios. [Display omitted] • The responses of grassland (de)nitrification are mainly related to MAP and MAT. • The duration/magnitude of experimental treatment is less vital for these responses. • These determinants affect the responses by altering the relative limitations of water/nutrients. • The altered water availability governs climate change effects in dry regions. • The changed nitrogen availability is the main determinant in wet regions. [ABSTRACT FROM AUTHOR]

• The relationship between soil pH and plant-diversity is unimodal. • The correlation between soil pH and vegetation nitrogen storage is unimodal. • Slight increases in soil pH induced high plant-diversity then enhanced SOC and SNC storage. Grassland soil organic carbon and nitrogen storage (SOC storage and SNC storage) have been regarded as indicators to evaluate impacts of global climate change on ecosystem functions due to their significant impact on atmospheric carbon (C) concentration. The variations in vegetation and soil properties in diverse vegetation patch types may change soil C and nitrogen (N) sequestration capacity. However, the quantities of SOC and SNC stored and the mechanisms behind the variation in this storage under diverse vegetation patches in grassland ecosystems are still unclear. Here we conducted a field experiment to measure the variations in vegetation composition, soil properties and SOC storage and SNC storage among five common vegetation patch types in the Eastern Eurasian steppe. Then we investigated the link between vegetation variation and the SOC and SNC storage. The results showed that (1) the combined effects of competitive ability and physiological stress drove the unimodal relationship between species diversity and soil pH. (2) Biomass production did not reach its maximum in patch types with the greatest plant diversity due to saline-alkaline stress, but the live vegetation N storage reached its maximum in these highly diverse patches due to complementary resource utilization effects. (3) Although biomass production of patches with the greatest biodiversity did not reach a maximum due to a slight increase in soil pH, the largest SOC and SNC storage values were found in the highly diverse patches. Our study implies that in natural grassland, high levels of species diversity may accelerate the decomposition rate, resulting in more recalcitrant organic C and N are released into the soil. At the same time, our observation that reductions in the area of the originally dominant patch type due to the expansion of other species during grassland degradation suggests that comprehensive measurements of SOC and SNC storage in different vegetation patches should be undertaken for accurate evaluation of the C and N sequestering capacity of grasslands. Our results can also help policy makers determine how to achieve sustainable development of grasslands based on C and N sequestration. [ABSTRACT FROM AUTHOR]