Impacts of Biological Soil Crust Disturbance and Composition on C and N Loss from Water Erosion

In this study, we conducted rainfall simulation experiments in a cool desert ecosystem to examine the role of biological soil crust disturbance and composition on dissolved and sediment C and N losses. We compared runoff and sediment C and N losses from intact late-successional dark cyanolichen crusts (intact) to both trampled dark crusts (trampled) and dark crusts where the top 1 cm of the soil surface was removed (scraped). In a second experiment, we compared C and N losses in runoff and sediments in early-successional light cyanobacterial crusts (light) to that of intact late-successional dark cyanolichen crusts (dark). A relatively high rainfall intensity of approximately 38 mm per 10-min period was used to ensure that at least some runoff was generated from all plots. Losses of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and ammonium (NH₄⁺ ) were significantly higher from trampled plots as compared to scraped and intact plots. Sediment C and N losses, which made up more than 98% of total nutrient losses in all treatments, were more than 4-fold higher from trampled plots relative to intact plots (sediment C g/m², intact = 0.74, trampled = 3.47; sediment N g/m², intact = 0.06, trampled = 0.28). In light crusts, DOC loss was higher relative to dark crusts, but no differences were observed in dissolved N. Higher sediment loss in light crusts relative to dark crusts resulted in 5-fold higher loss of sediment-bound C and N. Total C flux (sediment + dissolved) was on the order of 0.9 and 7.9 g/m² for dark and light crusts, respectively. Sediment N concentration in the first minutes after runoff from light crusts was 3-fold higher than the percent N of the top 1 cm of soil, suggesting that even short-term runoff events may have a high potential for N loss due to the movement of sediments highly enriched in N. Total N loss from dark crusts was an order of magnitude lower than light crusts (dark = 0.06 g N/m², light = 0.63 g/m²). Overall, our results from the small plot scale (0.5 m²) suggest that C and N losses are much lower from intact late-successional cyanolichen crusts as compared to recently disturbed or early-successional light cyanobacterial crusts.     

高寒沙区生物土壤结皮覆盖土壤碳通量对模拟降水的响应

生物土壤结皮作为干旱半干旱地区重要的地表覆盖类型和景观特征之一,其自身具备的光合与呼吸能力对荒漠生态系统地表与大气界面中的碳交换与循环产生了重要影响。水分是干旱半干旱地区许多生态过程中的主要限制因子,能够影响生物土壤结皮的光合与呼吸过程,进而影响生物土壤结皮覆盖土壤的碳通量规律。针对高寒沙区藓类和藻类结皮为主的生物土壤结皮覆盖土壤,设置了1、2、5、10mm以及0(对照)的模拟降水梯度,利用LI-8100土壤碳通量测定系统,对模拟降水后结皮覆盖土壤的碳通量进行测定,以探讨不同结皮种类和不同强度降水对碳通量的影响。结果表明:(1)降水对生物土壤结皮覆盖土壤的净碳通量、暗呼吸均有激发作用,使碳通量在极短时间内到达峰值且与对照差异显著,但各降水量之间差异不显著。两种不同结皮覆盖类型相比,藓类结皮覆盖土壤在降水后的碳通量峰值和受降水激发的有效时间均显著高于藻类结皮。(2)两种结皮覆盖土壤在模拟降水后的48h累积碳释放随降水量的增加而增加且与对照差异显著。同时藓类结皮覆盖土壤累计碳释放显著高于藻类结皮覆盖土壤。(3)两种生物土壤结皮覆盖土壤的碳通量和土壤水分体现出显著的相关性,净碳通量和暗呼吸均随水分的增加而增加。因此,在降水条件下生物土壤结皮覆盖土壤表现出明显的碳源效应,其碳通量以及碳释放量都有显著的改变,在研究干旱半干旱地区碳交换规律时应该考虑不同生物结皮的覆盖和降水事件的影响。     

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。     

The Effect of Biological Soil Crusts on Throughput of Rainwater and N into Lake Michigan Sand Dune Soils

Biological soil crusts composed of cyanobacteria, green algae, bryophytes, and lichens colonize soils in arid and semiarid ecosystems worldwide and are responsible for significant N input to the soils of these ecosystems. Soil crusts also colonize active sand dunes in more humid regions, but studies of structure and function of such sand dune crusts are lacking. We identified the cyanobacterial, algal, and bryophytic constituents and N production and leachates of biological soil crusts that colonize beach dunes at the Indiana Dunes National Lakeshore along southern Lake Michigan in Indiana, USA. To determine the role of these crusts in this system, we conducted a greenhouse experiment in which intact soil cores with biological crusts were subjected to artificial rainfall over a full growing season. The volume and N content of leachate from the cores were quantified in relation to degree of crust development, taxonomic composition, rainfall volume and intensity, light intensity, and the presence of plant litter. Net N throughput significantly exceeded N inputs to cores in rainwater. Net N outputs from crusts to subsurface soil ranged from 0. 01 to 0.19 g NH ₄ ⁺ -N m⁻² yr⁻¹ and 0.01 to 0.61 g NO ₃ ^– N m⁻² yr⁻¹. Thus, total inorganic N inputs associated with biological soil crusts ranged from 0.02 g N m⁻² yr⁻¹ to 0.8 g N m⁻² yr⁻¹. High volume (≥2 cm) rainfall resulted in more N leaching than low volume events, and plant litter added over the surface of crusted soil cores significantly increased the amount of N in leachate. Exploratory path analysis revealed direct and indirect linkages among environmental factors, crust development, and crust composition in regulating the throughput of H₂O and N from these intact soil cores. Biological soil crusts at this site, combined with other properties of the soil surface, substantially increase N inputs to this water- and nutrient-limited sand dune ecosystem.     

Controls over nitric oxide and ammonia emissions from Mojave Desert soils

Emissions of reactive N compounds produced during terrestrial N cycling can be an important N loss pathway from ecosystems. Most measurements of this process focus on NO and N(2)O efflux; however, in alkaline soils such as those in the Mojave Desert, NH(3) production can be an important component of N gas loss. We investigated patterns of NO and NH(3) emissions in the Mojave Desert and identified seasonal changes in temperature, precipitation and spatial heterogeneity in soil nutrients as primary controllers of soil efflux. Across all seasons, NH(3) dominated reactive N gas emissions with fluxes ranging from 0.9 to 10 ng N m(-2) s(-1) as compared to NO fluxes of 0.08-1.9 ng N m(-2) s(-1). Fluxes were higher in April and July than in October; however, a fall precipitation event yielded large increases in both NO and NH(3) efflux. To explore the mechanisms driving field observations, we combined NO and NH(3) soil flux measurements with laboratory manipulations of temperature, water and nutrient conditions. These experiments showed a large transient NH(3) pulse (~70-100 ng N m(-2) s(-1)) following water addition, presumably driven by an increase in soil NH(4) (+) concentrations. This was followed by an increase in NO production, with maximum NO flux rates of 34 ng N m(-2) s(-1). Our study suggests that immediately following water addition NH(3) volatilization proceeds at high rates due to the absence of microbial competition for NH(4) (+); during this period N gas loss is insensitive to changes in temperature and soil nutrients. Subsequently, NO emission increases and rates of both NO and NH(3) emission are sensitive to temperature and nutrient constraints on microbial activity. Addition of labile C reduces gaseous N losses, presumably by increasing microbial immobilization, whereas addition of NO(3) (-) stimulates NO and NH(3) efflux.     

Nitrogen Fixation and Leaching of Biological Soil Crust Communities in Mesic Temperate Soils

Biological soil crust is composed of lichens, cyanobacteria, green algae, mosses, and fungi. Although crusts are a dominant source of nitrogen (N) in arid ecosystems, this study is among the first to demonstrate their contribution to N availability in xeric temperate habitats. The study site is located in Lucas County of Northwest Ohio. Using an acetylene reduction technique, we demonstrated potential N fixation for these crusts covering sandy, acidic, low N soil. Similar fixation rates were observed for crust whether dominated by moss, lichen, or bare soil. N inputs from biological crusts in northwestern Ohio are comparable to those in arid regions, but contribute substantially less N than by atmospheric deposition. Nitrate and ammonium leaching from the crust layer were quantified using ion exchange resin bags inserted within intact soil cores at 4 cm depth. Leaching of ammonium was greater and nitrate less in lichen than moss crusts or bare soil, and was less than that deposited from atmospheric sources. Therefore, biological crusts in these mesic, temperate soils may be immobilizing excess ammonium and nitrate that would otherwise be leached through the sandy soil. Moreover, automated monitoring of microclimate in the surface 7 cm of soil suggests that moisture and temperature fluctuations in soil are moderated under crust compared to bare soil without crust. We conclude that biological crusts in northwestern Ohio contribute potential N fixation, reduce N leaching, and moderate soil microclimate.     

Carbon exchange in biological soil crust communities under differential temperatures and soil water contents: implications for global change

Biological soil crusts (biocrusts) are an integral part of the soil system in arid regions worldwide, stabilizing soil surfaces, aiding vascular plant establishment, and are significant sources of ecosystem nitrogen and carbon. Hydration and temperature primarily control ecosystem CO₂ flux in these systems. Using constructed mesocosms for incubations under controlled laboratory conditions, we examined the effect of temperature (5–35 °C) and water content (WC, 20–100%) on CO₂ exchange in light (cyanobacterially dominated) and dark (cyanobacteria/lichen and moss dominated) biocrusts of the cool Colorado Plateau Desert in Utah and the hot Chihuahuan Desert in New Mexico. In light crusts from both Utah and New Mexico, net photosynthesis was highest at temperatures >30 °C. Net photosynthesis in light crusts from Utah was relatively insensitive to changes in soil moisture. In contrast, light crusts from New Mexico tended to exhibit higher rates of net photosynthesis at higher soil moisture. Dark crusts originating from both sites exhibited the greatest net photosynthesis at intermediate soil water content (40–60%). Declines in net photosynthesis were observed in dark crusts with crusts from Utah showing declines at temperatures >25 °C and those originating from New Mexico showing declines at temperatures >35 °C. Maximum net photosynthesis in all crust types from all locations were strongly influenced by offsets in the optimal temperature and water content for gross photosynthesis compared with dark respiration. Gross photosynthesis tended to be maximized at some intermediate value of temperature and water content and dark respiration tended to increase linearly. The results of this study suggest biocrusts are capable of CO₂ exchange under a wide range of conditions. However, significant changes in the magnitude of this exchange should be expected for the temperature and precipitation changes suggested by current climate models.     

Substrate, climate, and land use controls over soil N dynamics and N-oxide emissions in Borneo

Nitrogen (N) enrichment of tropical ecosystems is likely to increase with rapid industrial and agricultural development, but the ecological consequences of N additions in these systems are not well understood. We measured soil N- oxide emissions and N transformations in primary rain forest ecosystems at four elevations and across two substrate types on Mt. Kinabalu, Borneo, before and after short-term experimental N additions. We also measured N pools and fluxes across a land use gradient of primary forest, burned secondary forest, and fertilized agriculture. Background soil N₂O and NO emissions in primary forest decreased with elevation, and soils derived from sedimentary substrates had larger pools of inorganic N, rates of nitrification, and N-oxide fluxes than ultrabasic soils when there were significant differences between substrate types. N-oxide emissions after N additions and background rates of nitrification were low in all soils derived from ultrabasic substrates compared to sedimentary substrates, even at lowland sites supporting, diverse Dipterocarp forests growing on morphologically similar Oxisols. Rates of potential nitrification were good predictors of N-oxide emissions after N additions. N₂O and NO fluxes were largest at low elevations and on sedimentary-derived soils compared to ultrabasic-derived soils, even at the smallest addition of N, 15kgNha^–1. Because current methods of soil classification do not explicitly characterize a number of soil chemical properties important to nutrient cycling, the use of soil maps to extrapolate biogeochemical processes to the region or globe may be limited in its accuracy and usefulness. In agricultural systems, management practices were more important than substrate type in controlling N-oxide emissions and soil N cycling. N-oxide fluxes from agricultural fields were more than an order of magnitude greater than from primary forests on the same substrate type and at the same elevation. As primary forests are cleared for intensive agriculture, soil N₂O and NO emissions are likely to far exceed those from the most N-saturated tropical forest ecosystems. This study highlights the inter-dependence of climate, substrate age, N deposition, and land-use practices determining N cycling and N-oxide emissions in humid tropical regions.     

Consequence of salinity and excess boron on growth, evapotranspiration and ion uptake in date palm (Phoenix dactylifera L., cv. Medjool)

Patches of common juniper (Juniperus communis L.) shrubs potentially facilitate the formation of fertile islands in heath tundra ecosystems thereby influencing the long-term resilience of these ecosystems. Although the role of juniper in the formation of such ‘islands of fertility’ has been studied in semiarid landscapes, there has been little attention paid to the importance of juniper in other ecosystems. In this study we contrast the soil fertility and rates of N fixation under juniper shrubs with that in open heath tundra in northern Sweden. Plots were established at several individual sites in alpine heath tundra in Northern Sweden and mineral soils to a depth of 10 cm were characterized for available N and P and total C, N, P, Ca, Mg, K, Fe, Mn, Zn, and Cu. Nitrogen fixation rates were measured by acetylene reduction in feather mosses under juniper canopies and contrasted with N fixation in both feather mosses and surface soils in the open heath. Soils under juniper had concentrations of total P greatly in excess of P in open heath, furthermore, juniper islands had the highest concentrations of bioavailable P. Nitrogen fixation rates in the feather moss Pleurozium schreberi (Bird.) Mitt were approximately 150 μmol acetylene reduced m⁻² d⁻¹ under the juniper canopy compared to less than 10 μmol acetylene reduced m⁻² d⁻¹ in the open heath. Feather mosses under the juniper canopy also fixed N at a significantly higher rate (on an aerial basis) than that of surface cores from the open heath that included lichen, mosses, and soil crusts. Juniper facilitates the formation of islands of soil fertility that may in turn facilitate the growth of other plants and positively influence the long term recovery of heath tundra ecosystems following disturbance.     

Assessing nitrogen fluxes from roots to soil associated to rhizodeposition by apple (<Emphasis Type="Italic">Malus domestica</Emphasis>) trees

The mass transfer from root to soil by means of rhizodeposition has been studied in grasses and forest trees, but its role in fruit trees is still unknown. In this study, N fluxes from roots to soil were estimated by applying a ¹⁵N mass balance technique to the soil–tree system. Apple (Malus domestica) trees were pre-labelled with ¹⁵N and then grown outdoors in 40 L pots for one vegetative season in (1) a coarse-textured, low organic matter soil, (2) a coarse-textured, high organic matter soil, and (3) a fine-textured, high organic matter soil. At tree harvest the ¹⁵N abundance of the soils was higher than at transplanting, but the total amount of ¹⁵N present in the tree–soil system was similar at transplanting and tree harvest. The soils had a strong effect on N fluxes from and to the soil. In the fine-textured soil, 11% of the total plant-derived nitrogen was transferred to the soil, compared with 2–5% in the two coarse-textured soils. Rhizodeposition was higher in the fine soil (18% of the primary production) than in the coarse-textured soils, whereas higher soil organic matter depressed rhizodeposition. Nitrogen uptake was almost double in the coarse-textured, high organic matter soil versus the other soils. Our results indicate that belowground primary productivity is significantly underestimated if based on root production data only. Rhizodeposition represents a major process, whose role should not be underestimated in carbon and nitrogen cycles in orchard ecosystems.