Precipitation pulses and soil CO2 flux in a Sonoran Desert ecosystem

Precipitation is a major driver of biological processes in arid and semiarid ecosystems. Soil biogeochemical processes in these water‐limited systems are closely linked to episodic rainfall events, and the relationship between microbial activity and the amount and timing of rainfall has implications for the whole‐system carbon (C) balance. Here, the influences of storm size and time between events on pulses of soil respiration were explored in an upper Sonoran Desert ecosystem. Immediately following experimental rewetting in the field, CO₂ efflux increased up to 30‐fold, but generally returned to background levels within 48 h. CO₂ production integrated over 48 h ranged from 2.5 to 19.3 g C m⁻² and was greater beneath shrubs than in interplant spaces. When water was applied on sequential days, postwetting losses of CO₂ were only half a large as initial fluxes, and the size of the second pulse increased with time between consecutive events. Soil respiration was more closely linked to the organic matter content of surface soils than to rainfall amount. Beneath shrubs, rates increased nonlinearly with storm size, reaching an asymptote at approximately 0.5 cm simulated storms. This nonlinear relationship stems from (1) resource limitation of microbial activity that is manifest at small time scales, and (2) greatly reduced process rates in deeper soil strata. Thus, beyond some threshold in storm size, increasing the duration or depth of soil moisture has little consequence for short‐term losses of CO₂. In addition, laboratory rewetting across a broad range in soil water content suggest that microbial activity and CO₂ efflux following rainfall may be further modified by the routing and redistribution of water along hillslopes. Finally, analysis of long‐term precipitation data suggests that half the monsoon storms in this system are sufficient to induce soil heterotrophic activity and C losses, but are not large enough to elicit autotrophic activity and C accrual by desert shrubs.     

Hierarchy of responses to resource pulses in arid and semi-arid ecosystems

In arid/semi-arid ecosystems, biological resources, such as water, soil nutrients, and plant biomass, typically go through periods of high and low abundance. Short periods of high resource abundance are usually triggered by rainfall events, which, despite of the overall scarcity of rain, can saturate the resource demand of some biological processes for a time. This review develops the idea that there exists a hierarchy of soil moisture pulse events with a corresponding hierarchy of ecological responses, such that small pulses only trigger a small number of relatively minor ecological events, and larger pulses trigger a more inclusive set and some larger ecological events. This framework hinges on the observation that many biological state changes, where organisms transition from a state of lower to higher physiological activity, require a minimal triggering event size. Response thresholds are often determined by the ability of organisms to utilize soil moisture pulses of different infiltration depth or duration. For example, brief, shallow pulses can only affect surface dwelling organisms with fast response times and high tolerance for low resource levels, such as some species of the soil micro-fauna and -flora, while it takes more water and deeper infiltration to affect the physiology, growth or reproduction of higher plants. This review first discusses how precipitation, climate and site factors translate into soil moisture pulses of varying magnitude and duration. Next, the idea of the response hierarchy for ecosystem processes is developed, followed by an exploration of the possible evolutionary background for the existence of response thresholds to resource pulses. The review concludes with an outlook on global change: does the hierarchical view of precipitation effects in ecosystems provide new perspectives on the future of arid/semiarid lands?     

Spatial and temporal litterfall heterogeneity generated by woody species in the Central Monte desert

In arid and semiarid environments, the presence of woody species generates a series of environmental gradients that increase spatial heterogeneity and modify the pattern of distribution of the other species. We postulate that the temporal and spatial variability in litter input generated by woody species is a relevant factor in the generation of edaphic heterogeneity by redistribution of nutrients and the physical effects of litter. The objective of this study was to determine the temporal and spatial variability in the amount of litter input under the canopy of dominant woody plants (Prosopis flexuosa and Larrea divaricata) and in exposed areas at the Ñacuñán Reserve, in the central zone of the Monte desert. Litterfall was collected during 2 years from 30-cm-diameter litter traps distributed at three microsites: under P. flexuosa canopy, under L. divaricata canopy, and in exposed areas. Microhabitats beneath Prosopis showed the highest litter input per m² (between 320 and 527 g/m²), and, consequently, more than 50% of it fell to the soil beneath the canopy of P. flexuosa. Only 10% fell on exposed areas, which exhibited an annual input rate per m² of a lower order of magnitude than the sites under Prosopis. Litterfall presented a peak in summer as a consequence of convective storms, and a second one in autumn due to phenological shedding. Our results suggest that woody species have a central importance in the dynamics of nutrients in arid lands by both the increase of total productivity and litterfall, and the spatial and temporal regulation of litter input.     

Pulse additions of soil carbon and nitrogen affect soil nitrogen dynamics in an arid Colorado Plateau shrubland

Biogeochemical cycles in arid and semi-arid ecosystems depend upon the ability of soil microbes to use pulses of resources. Brief periods of high activity generally occur after precipitation events that provide access to energy and nutrients (carbon and nitrogen) for soil organisms. To better understand pulse-driven dynamics of microbial soil nitrogen (N) cycling in an arid Colorado Plateau ecosystem, we simulated a pulsed addition of labile carbon (C) and N in the field under the canopies of the major plant species in plant interspaces. Soil microbial activity and N cycling responded positively to added C while NH₄⁺–N additions resulted in an accumulation of soil NO₃⁻. Increases in microbial activity were reflected in higher rates of respiration and N immobilization with C addition. When both C and N were added to soils, N losses via NH₃ volatilization decreased. There was no effect of soil C or N availability on microbial biomass N suggesting that the level of microbial activity (respiration) may be more important than population size (biomass) in controlling short-term dynamics of inorganic and labile organic N. The effects of C and N pulses on soil microbial function and pools of NH₄⁺–N and labile organic N were observed to last only for the duration of the moisture pulse created by treatment addition, while the effect on the NO₃⁻–N pool persisted after soils dried to pre-pulse moisture levels. We observed that increases in available C lead to greater ecosystem immobilization and retention of N in soil microbial biomass and also lowered rates of gaseous N loss. With the exception of trace gas N losses, the lack of interaction between available C and N on controlling N dynamics, and the subsequent reduction in plant available N with C addition has implications for the competitive relationships between plants species, plants and microbes, or both.     

Water pulses and biogeochemical cycles in arid and semiarid ecosystems

The episodic nature of water availability in arid and semiarid ecosystems has significant consequences on belowground carbon and nutrient cycling. Pulsed water events directly control belowground processes through soil wet-dry cycles. Rapid soil microbial response to incident moisture availability often results in almost instantaneous C and N mineralization, followed by shifts in C/N of microbially available substrate, and an offset in the balance between nutrient immobilization and mineralization. Nitrogen inputs from biological soil crusts are also highly sensitive to pulsed rain events, and nitrogen losses, particularly gaseous losses due to denitrification and nitrate leaching, are tightly linked to pulses of water availability. The magnitude of the effect of water pulses on carbon and nutrient pools, however, depends on the distribution of resource availability and soil organisms, both of which are strongly affected by the spatial and temporal heterogeneity of vegetation cover, topographic position and soil texture. The inverse texture hypothesis for net primary production in water-limited ecosystems suggests that coarse-textured soils have higher NPP than fine-textured soils in very arid zones due to reduced evaporative losses, while NPP is greater in fine-textured soils in higher rainfall ecosystems due to increased water-holding capacity. With respect to belowground processes, fine-textured soils tend to have higher water-holding capacity and labile C and N pools than coarse-textured soils, and often show a much greater flush of N mineralization. The result of the interaction of texture and pulsed rainfall events suggests a corollary hypothesis for nutrient turnover in arid and semiarid ecosystems with a linear increase of N mineralization in coarse-textured soils, but a saturating response for fine-textured soils due to the importance of soil C and N pools. Seasonal distribution of water pulses can lead to the accumulation of mineral N in the dry season, decoupling resource supply and microbial and plant demand, and resulting in increased losses via other pathways and reduction in overall soil nutrient pools. The asynchrony of resource availability, particularly nitrogen versus water due to pulsed water events, may be central to understanding the consequences for ecosystem nutrient retention and long-term effects on carbon and nutrient pools. Finally, global change effects due to changes in the nature and size of pulsed water events and increased asynchrony of water availability and growing season will likely have impacts on biogeochemical cycling in water-limited ecosystems.     

Precipitation pulse use by an invasive woody legume: the role of soil texture and pulse size

Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or “pulses”. The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (Ψ_pd), the isotopic abundance of deuterium in stem water (δD), the abundance of ¹³C in soluble leaf sugar (δ¹³C), and percent volumetric soil water content (θ_v) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between Ψ_pd and δ¹³C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting–drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.     

Mind the gap among patches in arid plant communities: rapid root proliferation in response to N addition

Aims In arid communities, it has been proposed that individual plants can extend their roots beyond their canopy exploring neighbouring bare ground areas. This becomes relevant in systems where the vegetation is distributed in patches surrounded by bare soil. However, whether roots of different species may be overlapping under bare ground areas is still controversial. The factors controlling root responses when no plants appear to be directly influencing the gap among patches are still unclear. The aim of our study was to detect perennial grasses responses to an N enrichment pulse.     

模拟降水对古尔班通古特沙漠生物结皮表观土壤碳通量的影响

水分是控制干旱区生态过程的重要环境因素,在水分受限制的生态系统中,降水通过改变土壤的干湿状态直接控制地下生物过程。生物结皮作为干旱区主要的地表覆盖物,能利用空气中有限的水分进行光合作用,其自身的碳交换是干旱区土壤碳通量的重要组成部分。通过模拟0(对照)、2、5 mm和15 mm 4个降水梯度,利用红外气体分析仪,对古尔班通古特沙漠中部生物结皮以及裸地表观土壤碳通量进行测量,探讨不同强度降水条件下生物结皮对表观土壤碳通量的影响,结果表明:(1)降水增加了生物结皮表观土壤碳释放量,2、5 mm和15 mm 3种降水处理累积碳释放量分别是对照的151.48%、274.97%、306.44%,并且随着降水后时间的延长,表观土壤碳通量逐渐减小直至达到降水前的水平;(2)生物结皮与裸地的表观土壤碳通量对降水的响应不同,对照和最大降水量下,生物结皮表观土壤碳通量大于裸地,但是2 mm和5 mm降水后,生物结皮表观土壤碳通量小于裸地,并且二者在2 mm降水时差异显著(P<0.05),而在其它降水处理下无显著差异;(3)连续两次降水事件,活性碳在初级降水后的大量释放使得二次降水后释放量下降,其中裸地碳释放量下降速率与降水强度正相关。本研究说明,在探求荒漠地区土壤碳交换对降水的响应规律时,应该考虑生物结皮的影响以及连续降水事件的差异。     

荒漠结皮层藓类植物死亡对表层土壤水分蒸发和入渗的影响

水分是荒漠植物生长最主要的限制因子,藓类结皮作为荒漠土壤表层重要覆被物,对土壤水分蒸发入渗具有重要影响。研究表明,在全球气候变化背景下,不确定的降水格局变化导致结皮层藓类植物出现集群死亡现象,但这一过程对荒漠地表土壤水分蒸发与入渗过程的影响及其机理尚不清楚。以古尔班通古特沙漠齿肋赤藓结皮为研究对象,利用便携式渗透计和蒸发仪,研究了结皮层藓类植物死亡对土壤水分蒸发与入渗的影响。结果表明,与裸沙相比,藓类结皮的存在显著抑制了水分入渗,而藓类植物死亡的结皮层抑制作用最大,其初渗速率、稳渗速率和累积入渗量分别是活藓类结皮的39.89%、85.91%及64.48%,仅为裸沙的5.96%、13.13%及20.42%。在水分蒸发初期,裸沙的水分蒸发速率明显高于活藓类结皮和藓类植物死亡的结皮层,但藓类植物死亡的结皮层维持相对稳定的蒸发速率的时间长于裸沙和活藓类结皮,这也导致最终累计蒸发量以藓类植物死亡的结皮层最高、裸沙最低。可见,荒漠生物土壤结皮中藓类植物死亡会明显减少土壤水分入渗、增大水分蒸发,进一步影响荒漠表层土壤水分格局,从而影响生物土壤结皮与维管植物的水分利用关系。