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The nature of the relationship between water limitation and facilitation has been one of the most contentious debates surrounding the stress‐gradient hypothesis (SGH), which states that plant‐plant interactions shift from competition to facilitation with increasing environmental stress. We take a closer look at the potential role of soil moisture in mediating plant‐plant interaction outcomes by assessing effects of climate and soil texture on plant modulation of soil moisture. Using an empirically‐parameterized soil moisture model, we simulated soil moisture dynamics beneath shrubs and in un‐vegetated coarse and fine soils for 1000 sites in the Western United States with <700 mm mean annual precipitation. This threshold reflects the transition from dryland (<600 mm precipitation) to mesic ecosystems. Positive effects of shrubs on shallow soil moisture (i.e. the difference between shrub and interspace soil moisture) decreased along the aridity gradient when long‐term average conditions were considered, contrary to expectations based on the SGH. Negative effects of shrubs on deeper soil moisture also increased with aridity. However, when extreme years were considered, positive effects of shrub on soil moisture were greatest at intermediate points along the spatial aridity gradient, consistent with a hump‐backed model of plant‐plant interactions. When viewed through time within a site, shrub effects on shallow soil moisture were positively related to precipitation, with more complex relationships exhibited in deeper soils Taken together, the results of this simulation study suggest that plant effects on soil moisture are predictable based on relatively general relationships between precipitation inputs and differential evaporation and transpiration rates between plant and interspace microsites that are largely driven by temperature. In particular, this study highlights the importance of differentiating between temporal and spatial variation in weather and climate, respectively, in determining plant effects on available soil moisture. Rather than focusing on the somewhat coarse‐scale predictions of the SGH, it may be more beneficial to explicitly incorporate plant effects on soil moisture into predictive models of plant‐plant interaction outcomes in drylands.

In the context of global change, the frequency of precipitation pulses is expected to decrease while nitrogen (N) addition is expected to increase, which will have a crucial effect on soil C cycling processes as well as methane (CH 4 ) fluxes. The interactive effects of precipitation pulses and N addition on ecosystem CH 4 fluxes, however, remain largely unknown in grassland. In this study, a series of precipitation pulses (0, 5, 10, 20, and 50 mm) and long-term N addition (0 and 10 g N m -2 yr -1 , 10 years) was simulated to investigate their effects on CH 4 fluxes in a semi-arid grassland. The results showed that large precipitation pulses (10 mm, 20 mm, and 50 mm) had a negative pulsing effect on CH 4 fluxes and relatively decreased the peak CH 4 fluxes by 203-362% compared with 0 mm precipitation pulse. The large precipitation pulses significantly inhibited CH 4 absorption and decreased the cumulative CH 4 fluxes by 68-88%, but small precipitation pulses (5 mm) did not significantly alter it. For the first time, we found that precipitation pulse size increased cumulative CH 4 fluxes quadratically in both control and N addition treatments. The increased soil moisture caused by precipitation pulses inhibited CH 4 absorption by suppressing CH 4 uptake and promoting CH 4 release. Nitrogen addition significantly decreased the absorption of CH 4 by increasing NH 4 + -N content and NO 3 – -N content and increased the production of CH 4 by increasing aboveground biomass, ultimately suppressing CH 4 uptake. Surprisingly, precipitation pulses and N addition did not interact to affect CH 4 uptake because precipitation pulses and N addition had an offset effect on pH and affected CH 4 fluxes through different pathways. In summary, precipitation pulses and N addition were able to suppress the absorption of CH 4 from the atmosphere by soil, reducing the CH 4 sink capacity of grassland ecosystems.

Background and aims Changes in climate and land-use are altering soil respiration patterns and thus affecting C sequestration rates globally. This study aims to understand the effect of revegetation induced land-use change on the response of soil respiration to precipitation pulses during an extreme-drying-and-rewetting period. Methods Soil respiration (SR) in cropland, grassland, shrubland, and orchard were intensively monitored along with environmental variables during an extreme drought period with precipitation pulse on China's Loess Plateau. Results SR was strongly correlated to soil water content for all land-uses. However, the relationship was highly dependent on land-use types: SR was only strongly suppressed in cropland and orchard when moisture content exceeded 10.8 and 13.7%, respectively, whereas no clear suppression was observed under other land-uses. As a result, the C loss in grassland and shrubland was 49.1–78.9% higher than in cropland following significant precipitation events. In addition, SR was negatively and weakly correlated with soil temperature, indicating the change in the dominant control on SR due to extreme drought. Conclusions Land-use change alters the response of soil respiration to soil moisture during extreme-drying-and-rewetting periods in this revegetated ecosystem. Its effect on respiration pulses will amplify as extreme climate events increase in the future, which may potentially alter the existing C balance.

Moisture inputs drive soil respiration (SR) dynamics in semi-arid and arid ecosystems. However, determining the contributions of root and microbial respiration to SR, and their separate temporal responses to periodic drought and water pulses, remains poorly understood. This study was conducted in a pine forest ecosystem with a Mediterranean-type climate that receives seasonally varying precipitation inputs from both rainfall (in the winter) and fog-drip (primarily in the summer). We used automated SR measurements, radiocarbon SR source partitioning, and a water addition experiment to understand how SR, and its separate root and microbial sources, respond to seasonal and episodic changes in moisture. Seasonal changes in SR were driven by surface soil water content and large changes in root respiration contributions. Superimposed on these seasonal patterns were episodic pulses of precipitation that determined the short-term SR patterns. Warm season precipitation pulses derived from fog-drip, and rainfall following extended dry periods, stimulated the largest SR responses. Microbial respiration dominated these SR responses, increasing within hours, whereas root respiration responded more slowly over days. We conclude that root and microbial respiration sources respond differently in timing and magnitude to both seasonal and episodic moisture inputs. These findings have important implications for the mechanistic representation of SR in models and the response of dry ecosystems to changes in precipitation patterns.

PREMISE OF THE STUDY: Mediterranean‐type climate ecosystems experience significant variability in precipitation within and across years and may be characterized by periods of extreme drought followed by a brief, high‐intensity precipitation pulse. Rapid root growth could be a key factor in effective utilization of precipitation pulses, leading to higher rates of seedling establishment. Changes in root growth rate are rarely studied, however, and patterns in seedling root traits are not well explored. We investigated the influence of an extreme postdrought precipitation event on seedlings that occur in southern California coastal sage scrub. METHODS: We measured root elongation rate, root tip appearance rate, new leaf appearance rate, and canopy growth rate on 18 mediterranean species from three growth forms. KEY RESULTS: Root elongation rate responded more strongly to the precipitation pulse than did root tip appearance rate and either metric of aboveground growth. The majority of species exhibited a significant change in root growth rate within 1 week of the pulse. Responses varied in rapidity and magnitude across species, however, and were not generally predictable based on growth form. CONCLUSIONS: While the majority of species exhibited shifts in belowground growth following the pulse, the direction and magnitude of these morphological responses were highly variable within growth form. Understanding the implications of these different response strategies for plant fitness is a crucial next step to forecasting community dynamics within ecosystems characterized by resource pulses.

Gravel mulching is a widely employed strategy for water conservation in arid agricultural regions, with potential implications for soil carbon (C) sequestration and greenhouse gas emissions. However, soil respiration and CO2-C emissions remain uncertain owing to less consideration of the influence of precipitation patterns and planting age. In this study, we investigated the soil respiration rate (Rsoil) and cumulative CO2-C emission (Ccum), both measured over a period of 72 h, along with soil properties and enzyme activities under different precipitation conditions based on gravel mulching with different planting ages. We analyzed the effects of planting ages on Rsoil and Ccum and revealed the underlying mechanisms driving changes in environmental factors on Rsoil and Ccum. The results demonstrated that the Rsoil reached the maximum value at about 1 h, 0.5 h, and 0.25 h after rewetting in 1, 10, and 20 years of gravel mulching under the condition with 1 mm, 5 mm, and 10 mm of precipitation, respectively, whereas the Rsoil exhibited its maximum at about 8 h after soil rewetting under precipitation of 30 mm. The Ccum induced by precipitation pulses tends to decrease with increasing years of gravel mulching. The Ccum was 0.0061 t ha−1 in the 20-year gravel-mulched soil, representing a 53.79% reduction compared to the 1-year gravel-mulched soil. Soil organic matter (SOM), planting ages, and alkaline phosphatase (ALP) were the primary factors influencing the Rsoil and Ccum in 0–20 cm, while SOM, planting ages, and soil porosity (AirP) were the key factors affecting the Rsoil and Ccum in 20–40 cm. The Rsoil and Ccum in the 0–20 cm soil were regulated by soil enzyme activities, while those of 20–40 cm soil were controlled by soil properties. This indicates that the decrease in Rsoil and Ccum is caused by soil degradation, characterized by a decrease in SOM and ALP. This study offers a novel insight into the long-term environmental impact of gravel mulching measures in arid areas, which is helpful in providing a theoretical basis for dryland agricultural management. It is imperative to consider the duration of gravel mulching when predicting the potential for C sequestration in arid agricultural areas.

The niche differentiation of resources among coexisting plants commonly reflects the fundamental functions of plant coexistence in water-limited ecosystems. However, the dynamics of water use patterns by coexisting plants that respond to soil moisture pulses are scarcely known in the semiarid alpine ecosystems of the Qinghai–Tibetan Plateau, particularly for deep-rooted grasses such as Achnatherum splendens in the Qinghai Lake watershed. Hence, we used the stable deuterium isotope method to detect the potential water sources for A. splendens during two growing seasons of 2013 and 2014. Our results indicate that SWC and δD values of shallow soil layer (0–10 cm) showed the highest variations in comparison with those in other soil layers during the growing seasons due to the combined effect of evaporation and precipitation inputs. A. splendens depended largely on shallow soil water availability at the early growing season. At the peak of the growing season, this deep-rooted grass A. splendens and shallow-rooted grass Leymus chinensis showed a high degree of response in the water use source to the changes in soil moisture pulses and shifted their water source from shallow to deep soil layer because water in the shallow soil layer became less available due to a long-time rainless days with strong evaporation effect on the shallow soil layer. In contrast, shallow soil water was utilized by all coexisting plants owing to an abrupt increase in shallow SWC with large events or long-lasting small events. At the late growing season, A. splendens used water from the shallow (0–10 cm) and middle soil layer (10–30 cm), while L. chinensis mainly relied on the shallow soil layer water. Comparatively, shallow-rooted herbs ( Heteropappus altaicus and Allium tanguticum ) predominantly used water from the shallow soil layer (0–10 cm) over the entire growing seasons regardless of soil water availability. Overall, the contrasting water use patterns by coexisting plants demonstrate their adaptations to the fluctuations of soil moisture pulses in water-limited ecosystems.

The hypothesis that drought intensity constrains the recovery of photosynthesis from drought was tested in the C₃ woody legume Prosopis velutina, and the mechanisms underlying this constraint examined. Hydraulic status and gas exchange were measured the day before a 39 mm precipitation pulse, and up to 7 d afterwards. The experiment was conducted under rainout shelters, established on contrasting soil textures and with different vegetation cover at the Santa Rita Experimental Range in southeastern Arizona, USA. Rates of photosynthesis and stomatal conductance after re-watering, as well as the number of days necessary for photosynthesis to recover after re-watering, were negatively correlated with predawn water potential, a measure of drought intensity (R² = 0.83, 0.64 and 0.92, respectively). Photosynthetic recovery was incomplete when the vascular capacity for water transport had been severely impaired (percentage loss of hydraulic conductance > 80%) during the drought, which largely increased stomatal limitations. However, changes in biochemical capacity or in mesophyll conductance did not explain the observed pattern of photosynthesis recovery. Although the control that hydraulic limitations impodson photosynthesis recovery had been previously inferred, the first empirical test of this concept is reported here.

Arid ecosystems receive precipitation pulses of different sizes that may differentially affect nitrogen (N) losses and N turnover during the growing season. We designed a rainfall manipulation experiment in the Patagonian steppe, southern Argentina, where we simulated different precipitation patterns by adding the same amount of water in evenly spaced three-small rainfall events or in one-single large rainfall event, three times during a growing season. We measured the effect of the size of rainfall pulses on N mineralization and N losses by denitrification, ammonia volatilization, and nitrate and ammonia leaching. Irrigation pulses stimulated N mineralization (P < 0.05), with small and frequent pulses showing higher responses than large pulses (P < 0.10). Irrigation effects were transient and did not result in changes in seasonal net N mineralization suggesting a long-term substrate limitation. Water pulses stimulated gaseous N losses by denitrification, with large pulses showing higher responses than small pulses (P < 0.05), but did not stimulate ammonia volatilization. Nitrate leaching also was higher after large than after small precipitation events (P < 0.05). Small events produced higher N transformations and lower N losses by denitrification and nitrate leaching than large events, which would produce higher N availability for plant growth. Climate change is expected to increase the frequency of extreme precipitation events and the proportion of large to small rainfall events. Our results suggest that these changes would result in reduced N availability and a competitive advantage for deep-rooted species that prefer nitrate over ammonia. Similarly, the ammonium:nitrate ratio might decrease because large events foster nitrate losses but not ammonium losses.

Observations of the temporal and spatial distribution of poststorm soil moisture in open shrublands and savannas are limited, yet they are critical to understanding the interaction and feedback between moisture distribution and canopies. The objective of this analysis was to study the hydrologic impacts of precipitation pulses on the upper layer of soils under and between shrubs. The study was based on measurements of precipitation, runoff, and under‐ and between‐shrub soil moisture over a period of 20 years (1990–2009) at a shrub‐dominated site in the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona. Within much of the root zone (to 30 cm depth), infiltration was not significantly different under versus between shrubs, and the under:between infiltration ratio was not related to pulse size or intensity. However, root‐zone soil moisture was significantly higher between shrubs than under shrubs. The soil moisture measured at the surface (at 5 cm depth) was not consistently different under and between shrubs, but the soil moisture measured at depths of 15 and 30 cm were both significantly higher between shrubs than under shrubs. Considering mechanisms that explain the interaction between plants and soil moisture, we found no differences in infiltration, evaporative losses, and surface soil moisture in locations under and between shrubs. This led to the conclusion that lower root‐zone soil moisture under shrubs was due largely to greater root density under shrubs than between shrubs. This study adds to the understanding of the impact of precipitation patterns on infiltration and soil moisture in shrub‐dominated sites and the potential for vegetation change in arid and semiarid lands.