Nitrate removal, denitrification and nitrous oxide production in the riparian zone of an ephemeral stream

Riparian zones are important features of the landscape that can buffer waterways from non-point sources of nitrogen pollution. Studies of perennial streams have identified denitrification as one of the dominant mechanisms by which this can occur. This study aimed to assess nitrate removal within the riparian zone of an ephemeral stream and characterise the processes responsible, particularly denitrification, using both in-situ and laboratory techniques. To quantify rates of groundwater nitrate removal and denitrification in-situ, nitrate was added to two separate injection-capture well networks in a perched riparian aquifer of a low order ephemeral stream in South East Queensland, Australia. Both networks also received bromide as a conservative tracer and one received acetylene to inhibit the last step of denitrification. An average of 77 ± 2% and 98 ± 1% of the added nitrate was removed within a distance of 40 cm from the injection wells (networks with acetylene and without, respectively). Based on rates of N₂O production in the network with added acetylene, denitrification was not a major mechanism of nitrate loss, accounting for only 3% of removal. Reduction of nitrate to ammonium was also not a major pathway in either network, contributing <4%. Relatively high concentrations of oxygen in the aquifer following recent filling by stream water may have reduced the importance of these two anaerobic pathways. Alternatively, denitrification may have been underestimated using the in-situ acetylene block technique. In the laboratory, soils taken from two depths at each well network were incubated with four nitrate-N treatments (ranging from ambient concentration to an addition of 15 mg N l⁻¹), with and without added acetylene. Potential rates of denitrification, N₂O production and N₂O:N₂ ratios increased with nitrate additions, particularly in shallow soils. Potential rates of denitrification observed in the laboratory were equivalent in magnitude to nitrate removal measured in the field (mean 0.26 ± 0.12 mg N kg of dry soil⁻¹ d⁻¹), but were two orders of magnitude greater than denitrification measured in the field with added acetylene. The relative importance of assimilatory vs. dissimilatory processes of nitrate removal depends on environmental conditions in the aquifer, particularly hydrology and its effects on dissolved oxygen concentrations. Depending on seasonal conditions, aquifers of ephemeral streams like the study site are likely to fluctuate between oxic and anoxic conditions; nevertheless they may still function as effective buffers. While denitrification to N₂ is a desirable outcome from a management perspective, assimilation into biomass can provide a rapid sink for nitrate, thus helping to reduce short-term delivery of nitrate downstream. Longer-term studies are needed to determine the overall effectiveness of riparian buffers associated with ephemeral streams in mitigating nitrate loads reaching downstream ecosystems.     

Denitrification under different cultivated plants: effects of soil moisture tension,nitrate concentration,and photosynthetic activity

Summary Plant effects on the denitrification rate were investigated in pot experiments at different soil moisture tensions and nitrate concentrations. Nitrate concentrations and the soil moisture tension were regulated immediately before each measurement. The effects of the plants on denitrification rates were dependent on the soil moisture tension. At a low soil moisture tension (–7 cm H₂O), there was a 10-fold increase in the denitrification rate (planted versus unplanted soil). At a medium moisture tension (–30 cm H₂O) the plants had practically no effect, and at the highest tension (–60 cm H₂O) the effect was slightly negative. Large differences in denitrification rates under different plant species were observed. At a low soil moisture tension, the average denitrification rate (g N kg^–1 soil h^–1) was 39–42 under small grains (barley, wheat, and oats), 47–82 under the grasses (cocksfoot, meadow grass, meadow fescue, and timothy) and 18 under red clover. The differences between the monocots were attributable to differences in plant growth rates, rather than to any specific difference in stimulation or inhibition of denitrification, since the variations in photosynthetic activity fairly well predicted the differences in denitrification rates under different monocots. Clover, however, gave much lower denitrification rates than those predicted by the photosynthetic activity.     

Effects of Riparian Denitrification on Stream Nitrate-Evidence from Isotope Analysis and Extreme Nitrate Leaching during Rainfall-

The effects of riparian denitrification on stream nitrate were investigated by detailed soil water observations and isotope analysis at a small headwater catchment in an urban area near Tokyo, central Japan. In the base flow period, stream nitrate concentration (<100 µM) was comparable with that of riparian ground water which had less nitrate than unsaturated soil water. Nitrogen isotope analysis showed that the consumption of nitrate by denitrification took place in riparian ground water, suggesting that denitrification is an important process to control nitrate leaching to streams. During rainfall, the concentration of stream nitrate increased up to 400 µM, which was comparable with that of pre-event soil water. The fact that soil water nitrate directly leached to streams indicated that the riparian denitrification process did not work during rainfall because of the rapid discharge of water. A decrease of denitrification effects is a possible reason for high stream nitrate concentration during rainfall.     

Temporal and spatial patterns of denitrification enzyme activity and nitrous oxide fluxes in three adjacent vegetated riparian buffer zones

The aim of this study was to investigate temporal and spatial patterns of denitrification enzyme activity (DEA) and nitrous oxide (N₂O) fluxes in three adjacent riparian sites (mixed vegetation, forest and grass). The highest DEA was found in the surface (0–30 cm) soil and varied between 0.7±0.1 mg N kg^–1 day^–1 at 5°C and 5.9±0.4 mg N kg^–1 day^–1 at 15°C. There was no significant difference (P >0.05) between the DEA in the uppermost (0–30 cm and 60–90 cm) soil depths under different vegetation covers. In the two deepest (120–150 cm and 180–210 cm) soil depths the DEA varied between 0.0±0.0 mg N kg^–1 day^–1 at 5°C and 4.4±0.9 mg N kg^–1 day^–1 at 15°C and was clearly associated with the accumulation of buried organic carbon (OC). Two threshold values of OC were observed before DEA started to increase significantly, namely 5 and 25 g OC kg^–1 soil at 10–15°C and 5°C, respectively. In the three riparian sites N₂O fluxes varied between a net N₂O uptake of –0.6±0.4 mg N₂O-N m^–2 day^–1 and a net N₂O emission of 2.5±0.3 mg N₂O-N m^–2 day^–1. The observed N₂O emission did not lead to an important pollution swapping (from water pollution to greenhouse gas emission). Especially in the mixed vegetation and forest riparian site highest N₂O fluxes were observed upslope of the riparian site. The N₂O fluxes showed no clear temporal trend.     

Denitrification activity in the root zone of a sludge-amended desert soil

Summary We evaluated potential NO _inf3 ^sup- losses from organic and inorganic N sources applied to improve the growth of cotton (Gossypium hirsutum) on a Pima clay loam soil (Typic Torrifluvent). An initial set of soil cores (April 1989) was collected to a depth of 270 cm from sites in a cotton field previously amended with anaerobically digested sewage sludge or an inorganic N fertilizer. The denitrification potential was estimated in all soil samples by measuring N₂O with gas chromatography. Soils amended with a low or high rate of sludge showed increased denitrification activity over soil samples amended with a low rate or inorganic N fertilizer. All amended samples showed greater denitrification activity than control soils. The denitrification decreased with soil depth in all treatments, and was only evident as deep as 90 cm in the soils treated with the high sludge rate. However, when soils collected from depths greater than 90 cm were amended with a C substrate, significant denitrification activity occurred. These date imply that organisms capable of denitrification were present in all soil samples, even those at depths far beneath the root zone. Hence, denitrification was C-substrate limited. A second series of soil cores taken later in the growing season (July 1989) confirmed these data. Denitrification losses (under laboratory conditions) to a soil depth of 270 cm represented 1–4% of total soil N depending on treatment, when the activity was C-substrate limited. With additional C substrate, the denitrification losses increased to 15–22% of the total soil N.     

Denitrification enzyme activity and potential of subsoils under grazed grasslands assayed by membrane inlet mass spectrometer

The importance of subsoil denitrification on the fate of agriculturally derived nitrate (NO₃) leached to groundwater is crucial for budgeting N in an ecosystem and for identifying areas where the risk of excess NO₃ is reduced. However, the high atmospheric background of di-nitrogen (N₂) causes difficulties in assessing denitrification enzyme activity (DEA) and denitrification potential (DP) in soils directly. Here, we apply Membrane Inlet Mass Spectrometry (MIMS) technique to investigate indirectly DEA and DP in soils by measuring N₂/Ar ratio changes in headspace water over soil. Soils were collected from 0-10, 15-25 and 60-70 cm depths of a grazed ryegrass and grass-clover. The samples were amended with helium-flushed deionized water containing ranges of NO₃ and carbon (glucose-C) and were incubated for six hours in the dark at 21 °C. The peaks for N₂/Ar ratio, declined with increasing soil depth, indicating a reduced substrate requirements to initiate DEA en-masse (15-30 mg NO₃-N alone or with 60-120 mg glucose-C, kg⁻¹ soil). The dissolved N₂O concentrations were very small (0.004-0.269 μg N kg⁻¹ soil) but responded well to the added N and C, showing a reduction in DEA with soil depth. In three separate studies, only subsoils were incubated for 3 days at 12 °C with 20-30 mg NO₃-N ± 40-60 mg glucose-C, kg⁻¹ soil. Denitrification capacity (DC, NO₃ only treatment) was not statistically different to the control (no amendment) within a land use (0.03-0.05 vs. 0.07-0.22 mg N kg⁻¹ soil d⁻¹), the highest being in ryegrass subsoils receiving groundwater. The DP was significantly (P < 0.0001) higher in subsoils under ryegrass than under grass-clover (0.50-0.71 vs. 1.15 mg N kg⁻¹ soil d⁻¹). The rates of DP (NO₃ + glucose-C) increased significantly (P < 0.0001) in unsaturated and saturated subsoils (0.92 and 2.19 mg N kg⁻¹ soil d⁻¹, respectively) of grass-clover, due to the higher reductive state resulting from the 10 day pre-incubation. Available C accelerated denitrification in soils and superseded the temporary elevation in oxidative state due to NO₃ addition. The substrates load differences between the land uses regulated the degree of denitrification rates. Results suggest that both dissolved N₂O measured by gas chromatography and N₂/Ar ratio measured by MIMS to indirectly determine DEA, and the latter to quantify total DC/DP in soils can be used. However, interference of oxygen in the MIMS system should be considered if available C is added or is naturally elevated in soil or groundwater.     

Interannual and interseasonal soil CO2 efflux and VOC exchange rates in a Mediterranean holm oak forest in response to experimental drought

Climate models predict drier conditions in the next decades in the Mediterranean basin. Given the importance of soil CO₂ efflux in the global carbon balance and the important role of soil monoterpene and volatile organic compounds (VOCs) in soil ecology, we aimed to study the effects of the predicted drought on soil CO₂, monoterpenes and other VOC exchange rates and their seasonal and interannual variations. We decreased soil water availability in a Mediterranean holm oak forest soil by means of an experimental drought system performed since 1999 to the present. Measurements of soil gas exchange were carried out with IRGA, GC and PTR-MS techniques during two annual campaigns of contrasting precipitation. Soil respiration was twice higher the wet year than the dry year (2.27±0.26 and 1.05±0.15, respectively), and varied seasonally from 3.76±0.85 μmol m⁻² s⁻¹ in spring, to 0.13±0.01 μmol m⁻² s⁻¹ in summer. These results highlight the strong interannual and interseasonal variation in CO₂ efflux in Mediterranean ecosystems. The drought treatment produced a significant soil respiration reduction in drought plots in the wet sampling period. This reduction was even higher in wet springs (43% average reduction). These results show (1) that soil moisture is the main factor driving seasonal and interannual variations in soil respiration and (2) that the response of soil respiration to increased temperature is constrained by soil moisture. The results also show an additional control of soil CO₂ efflux by physiology and phenology of trees and animals. Soil monoterpene exchange rates ranged from −0.01 to 0.004 nmol m⁻² s⁻¹, thus the contribution of this Mediterranean holm oak forest soil to the total monoterpenes atmospheric budget seems to be very low. Responses of individual monoterpenes and VOCs to the drought treatment were different depending on the compound. This suggests that the effect of soil moisture reduction in the monoterpenes and VOC exchange rates seems to be dependent on monoterpene and VOC type. In general, soil monoterpene and other VOC exchange rates were not correlated with soil CO₂ efflux. In all cases, only a low proportion of variance was explained by the soil moisture changes, since almost all VOCs increased their emission rates in summer 2005, probably due to the effect of high soil temperature. Results indicate thus that physical and biological processes in soil are controlling soil VOC exchange but further research is needed on how these factors interact to produce the observed VOCs exchange responses.     

Seasonal variability of runoff and soil loss on forest road backslopes under simulated rainfall

Runoff and soil loss from forest road backslopes is a serious problem in Mediterranean areas. Surface runoff and sediment production on backslopes of forest roads in Los Alcornocales Natural Park (southern Spain) has been studied in this paper using a simple portable rainfall simulator at an intensity of 90 mm h^− 1. One hundred rainfall simulations were performed on bare and vegetated road backslopes during summer and winter in order to study seasonal differences. Runoff coefficients and soil loss rates were lower on the vegetated plots than on the bare ones. Runoff coefficients increased 1.7 (bare backslopes) and 3.1 times (vegetated backslopes) from summer to winter. Preserving the vegetation cover over 20% is recommended for keeping soil loss rates under low levels, especially during winter.     

Biological and physical effects of non-native earthworms on nitrogen cycling in riparian soils

Global nitrogen cycling is being altered by anthropogenic disturbances including invasion by non-native species. European and Asian earthworms have invaded northern temperate forests in North America with dramatic consequences for litter thickness, forest floor plant diversity, and soil nitrogen cycling. Invasive earthworms present at the boundary of terrestrial and aquatic ecosystems (i.e., riparian zones) may alter the flux of nitrogen into adjacent aquatic ecosystems. We examined how nitrogen cycling in riparian soil responds to amendments of invasive earthworms or artificial earthworm burrows. In earthworm-free riparian plots (0.25 m²), we established treatments of invasive earthworms (60 g fresh mass·m⁻²), artificial burrows (120 m⁻²), or control plots and sampled the plots after 30 days. Before and after treatment application we measured major soil characteristics (water-filled pore space, organic matter, and pH), nitrogen pools (exchangeable NH₄⁺ and NO₃⁻), and nitrogen transformation rates (net N-mineralization, net nitrification, and denitrification). Exchangeable NH₄⁺ and NO₃⁻ changed through time but did not differ among treatments. Net N-mineralization and net nitrification rates did not change through time and were similar across all treatments. However, denitrification rates in plots with added earthworms were 4 times greater than rates in control and burrow-only plots, which represents a large rapid increase in gaseous nitrogen flux out of these riparian soils. For all response variables, artificial burrows responded similarly to control plots, suggesting that earthworm biological activity (i.e., feeding, excretion, and mucus production) rather than physical effects (i.e., burrowing and soil aeration) drove the changes in nitrogen cycling. Examination of soil nitrogen pool and flux measurements suggest that this increase in denitrification was coupled with NH₄⁺ consumption by nitrifying bacteria, but future studies are needed to confirm this hypothesis. We conclude that the activity of invasive earthworms in riparian zones can increase the flux of N out of riparian zones, but the hydrologic context of the riparian zone (e.g., pore-water residence time) ultimately controls whether denitrification or nitrate leaching is the dominant flux of N.     

太湖流域平原区土壤水分动态分析

以太湖流域平原区两种土地利用方式(林地和菜地)为例,在长期定位观测土壤水分数据的基础上,采用时间序列法定量分析了土壤水分与降水之间的相关关系。结果表明:(1)降水量在时间上不相关,而各层土壤水分均为自相关序列;(2)降水与土壤水分之间存在一定的相关性,相关时长受季节、土壤深度、土地利用方式等的影响较为明显(1~8d不等);(3)林地相关时长的季节变化表现为:夏秋季春季冬季。随着土壤深度的增加,菜地土壤含水量与降水相关关系在夏秋季节呈减小趋势,而在冬春季节则相反。整体上林地降水对土壤含水量的有效影响时间长于菜地,且林地降水量与土壤含水量相关关系的规律性优于菜地。     

Litter decomposition and nitrogen mineralization in newly formed gaps in a Danish beech (Fagus sylvatica) forest

Gap formation is suggested as an alternative forest management approach to avoid extreme changes in the N cycle of forest ecosystems caused by traditional management practises. The present study aimed to investigate the effect of gap formation on N availability in beech litter and mineral soil on sites, which experienced only little soil disturbance during tree harvest. N pools, litter decomposition, and N mineralization rates in mineral soil were studied in two gaps (17 and 30 m in diameter) in a 75-year-old managed European beech (Fagus sylvatica L.) forest in Denmark and related to soil temperature (5 cm depth) and soil moisture (15 cm depth). Investigations were carried out during the first 2 years after gap formation in measurement plots located along the north-south transect running through the centre of each gap and into the surrounding forest.An effect of gap size was found only for soil temperatures and litter mass loss: soil temperatures were significantly increased in the northern part of the large gap during the first year after gap formation, and litter mass loss was significantly higher in the smaller gap. All other parameters investigated revealed no effect of gap size. Nitrification, net mineralization, and soil N concentrations tended to be increased in the gaps. Cumulative rates of net mineralization were two fold higher in the gaps during the growing season (June-October), but a statistically significant increase was found only for soil NH₄-N concentrations during this period. Forest floor parameters (C:N ratios, mass loss, N release) were not significantly modified during the first year after gap formation, neither were the total C content nor the C:N ratio in mineral soil at 0-10 cm depth.     

The Effect of Chronic High Groundwater Nitrate Loading on Riparian Forest Growth and Plant–Soil Processes

The effect of chronic high groundwater nitrate loading on riparian forests is poorly understood. The growth patterns of northern white cedar (Thuja occidentalis) and related plant–soil processes were examined at four riparian sites in southern Ontario, Canada which have similar vegetation, soils, and hydrology but have differed in adjacent land use for >60 years. Fertilized cropland at two riparian sites produced groundwater-fed surface flows with high mean NO₃–N concentrations of 9 and 31 mg l⁻¹, whereas mean concentrations were <0.5 mg l⁻¹ at two control sites down slope from forest. Tree-ring analysis at the two nitrate-rich sites indicated a positive growth trend in 1980–2004 and an absence of a positive growth trend in the 1945–1970 period that preceded high rates of synthetic nitrogen fertilizer use on cropland. However, a significant increase in growth also occurred in 1980–2004 at the two control riparian sites suggesting that high groundwater nitrate inputs did not influence tree growth. Cedar foliar and litter N content did not differ significantly between the high nitrate and control sites. Litter decomposition rates measured by the litterbag technique at a nitrate-enriched and control site were similar. Litter from a high nitrate and a control site produced a similar rate of potential denitrification in lab incubations of riparian surface peat. This study indicates that prolonged nitrate inputs in groundwater did not increase nitrogen uptake and growth of white cedar or stimulate decomposition and denitrification as a result of changes in the quality of plant material. In the absence of anthropogenic nitrate inputs, riparian wetland soils are typically high in ammonium and low in nitrate, and as a consequence, white cedar may have a limited ability to utilize nitrate.     

黄土性土壤剖面中N2O排放的研究初报

用田间原位土壤探头测定法和乙炔抑制未扰动土柱法 ,对黄土性土壤N2 O的排放进行了研究。试验结果表明 ,黄土性土壤N2 O的排放量存在着明显的季节和空间变异。季节变化与田间水分因子密切相关 ,N2 O高峰常出现在灌水 (降水 ) 3天后 ,施肥处理和对照的趋势完全一致。全年中八月份各土层N2 O浓度最高 ;在最高峰值时 ,施肥处理的N2 O浓度几乎是对照处理的 2 .5倍。土壤剖面中N2 O的浓度的顺序是 1 0cm <30cm <1 50cm <90cm <60cm ,以 60～90cm土层最高。影响黄土性土壤反硝化的主要因子是作为微生物能源和碳源的有机物质 ,在碳源充足时 ,土壤的硝态氮含量和水分因子是限制因子。     

N2O and NO emissions from an Andisol field as influenced by pelleted poultry manure

Animal manures from intensive livestock operations can be pelleted to improve handlings and recyclings of embodied nutrients. The aim of this study was to evaluate the influence of pelleted poultry manure on N₂O and NO fluxes from an Andisol field. In autumn 2006 and summer 2007, poultry manure (PM), pelleted poultry manure (PP), and chemical fertilizer (CF) were applied at a rate of 120 kg N ha⁻¹ in each cultivation period to Komatsuna (Brassica rapa var. peruviridis). Nitrous oxide and NO fluxes were measured using an automated monitoring system. A soil incubation experiment was also conducted to determine the influence of intact and ground pelleted manure on N₂O, NO, and CO₂ production with a water-filled pore space (WFPS) of 30 or 50%. In the field measurements, N₂O emission rates from the organic fertilizer treatments were larger than that from the CF treatment, possibly because organic C stimulated denitrification. The highest N₂O flux was observed from the PP treatment after a rainfall following fertilization, and the cumulative emission rate (2.72 ± 0.22 kg N ha⁻¹ y⁻¹) was 3.9 and 7.1 times that from the PM and CF treatments, respectively. In contrast, NO emission rates were highest from the CF treatment. The NO/N₂O flux ratio indicated that nitrification was the dominant process for NO and N₂O production from the CF treatment. Cumulative N₂O emission rates from all treatments were generally higher during the wetter cultivation period (autumn 2006) than during the drier cultivation period (summer 2007). In contrast, NO emission rates were higher in the drier than in the wetter cultivation period. The incubation experiment results showed a synergistic effect of soil moisture and the pelleted manure form on N₂O emission rates. The intact pelleted manure with the 50% WFPS treatment produced the highest N₂O and CO₂ fluxes and resulted in the lowest soil NO₃⁻ content after the incubation. These results indicate that anaerobic conditions inside the pellets, caused by rainfall and heterotrophic microbial activities, led to denitrification, resulting in high N₂O fluxes. Controlling the timing of N application by avoiding wet conditions might be one mitigation option to reduce N₂O emission rates from the PP treatment in this study field.     

Influence of tillage system on denitrification in maize-cropped soils

Denitrification losses show an irregular pattern through the year, often being caused by climatic conditions and management practices. The objectives of the present work were to quantify denitrification losses and to determine the influence of tillage system on the factors that control denitrification in fertilized soils. The modal profile of the soil was an Vertic Argiudoll, clay loam texture, located in Buenos Aires province, Argentina. The treatments were: (a) fertilized, (b) incorporated fertilization and (c) without fertilization for both no tillage and conventional tillage systems. Chambers were placed in the field to measure denitrification. In this clayish soil the estimated mean values of accumulated denitrification during the crop cycle (90 days) were 0.190kgNha^–1 for conventional tillage and 0.350kgNha^–1 for no tillage. In treatments with no tillage, losses by denitrification were approximately twice those of conventional tillage. These differences were also evidenced by the number of microorganisms, which were significantly higher (P<>;5%) for no tillage on all dates, except for at flowering. The increase at flowering coincided with the period of highest rainfall and consequently the highest water contents in the soil. The highest denitrification losses, except for sowing, were measured when soil moisture content was more than 30% (v/v). Denitrification increased in conjunction with an increase in the availability of carbon that is consumed by the heterotrophic microorganisms (including the denitrifiers). Received: 30 July 1996     

The variation of soil water regime, oxygen status and rooting pattern with soil type under Sitka spruce

Soil water regime, oxygen status and rooting pattern under Sitka spruce were characterized in five soil types: a waterlogged peaty gley, a peaty gley, a flushed peaty gley, a surface water gley and a brown forest soil during 1982. The waterlogged peaty gley had a high water-table which only dropped below 30 cm depth for 46 d during the summer. Below the water-table the profile was anoxic and rooting was restricted to a depth of 12cm. The peaty gley experienced seasonal waterlogging with a winter water-table around 15cm depth. Roots that grew further down the profile during the summer as the topsoil dried to –100 kPa matric potential were later killed when the water-table rose. The top 30 cm of the flushed peaty gley remained oxygenated even below the water-table. Roots survived below the winter water-table. The top 30 cm of the surface water gley and brown forest soil were freely draining and remained aerated throughout the year. The platinum electrode polarographic method demonstrated that there was a sharp transition to anoxic conditions just above the water-table at the first two sites.     

Changes in Soil Properties and Microbial Indices across Various Management Sites in the Mountain Environments of Azad Jammu and Kashmir

The mountainous region of the Himalayas is covered with forest, grassland, and arable land, but the variation in ecosystem functions has not been fully explored because of the lack of available data. This study appraises the changes in soil properties over the course of a year (spring, summer, autumn, winter) for forest, grassland, and arable soils in a typical hilly and mountainous region of Azad Jammu and Kashmir, Pakistan. Soil samples were collected from major land-cover types in the mountain region: natural forest, grassland, and cultivated land (arable). The natural forest served as a control against which changes in soil properties resulting from removal of natural vegetation and cultivation of soil were assessed. Soil samples were collected from depths of 0–15 and 15–30 cm six times during the year and examined for changes in temperature, moisture, electrical conductivity (EC), micronutrients [iron, manganese, copper, and zinc (Fe, Mn, Cu, Zn, respectively)], and microbial population. Significant differences were found in soil temperature, soil moisture, Fe, Mn, Cu, Zn, and number of bacteria, actinomycetes, and fungi among the three land-cover types. Soil under cultivation had 4–5 °C higher temperature and 3–6% lower moisture than the adjacent soils under grassland and forest. Electrical conductivity (EC) values of forest, grassland, and arable soil were 0.36, 0.30, and 0.31 dS m^?1, indicating that soil collected from the forest had 18–20% more EC than the adjacent arable and grassland soils. On average, amounts of Fe, Mn, Cu, and Zn in the soil collected from the arable site were 6.6, 5.7, 1.7, and 0.8 mg kg^?1, compared with 24.0, 12.1, 3.5, and 1.2 mg kg^?1 soil in the forest soil, showing that arable had two to four times less micronutrients than grassland and forest. Populations of bacteria, actinomycetes, and fungi in the forest were 22.3 (10⁵), 8.2 (10⁵), and 2.5 (10³), respectively, while arable land exhibited 8.2 (10⁵), 3.2 (10⁵), and 0.87 (10³). Season (temperature) and depth showed significant effects on microbial activity and nutrient concentration, and both decreased significantly in winter and in the subsurface layer of 15?30 cm. Different contents of the parameters among arable, grassland, and forest soils indicated an extractive effect of cultivation and agricultural practices on soil. Natural vegetation appeared to be a main contributor to soil quality as it maintained the moisture content and increased the nutrient status and microbial growth of soil. Therefore, it is important to sustain high-altitude ecosystems and reinstate the degraded lands in the mountain region.     

Hydraulic lift may buffer rhizosphere hyphae against the negative effects of severe soil drying in a California Oak savanna

Many studies have shown that the total abundance of hyphae in the soil covaries seasonally with soil moisture. We investigated the extent to which soil hyphal abundance varies as a function of depth and moisture availability within the soil profile during the dry season, and determined whether soil moisture compensation via hydraulic lift (HL) buffers rhizosphere fungi from the effects of severe soil drying. We measured soil water potential, isotopic composition of soil water and total hyphal length in a California coast live oak stand and adjacent grassland at the beginning and end of the 5-month summer drought period. Throughout the summer, oaks maintained predawn water potential values (−0.4±0.1 MPa) that were significantly above those recorded in the 0-200 cm soil depth interval, strongly suggesting root access to groundwater. Direct evaporation of soil water was much more intense and affected deeper layers of the profile in the grassland compared to the oak stand, as indicated by extremely negative water potential values and very enriched isotopic composition of soil water near the surface. Significantly higher soil water potential and less isotopically enriched soil water at 15-40 cm depth in the oak stand were consistent with oak root exudation of isotopically depleted groundwater or deep soil water not exposed to evaporation. Hyphal length in the soil profile declined markedly during the summer drought period in the grassland, particularly in upper layers (41-75% decrease at 0-40 cm depth), indicating rapid turnover of the arbuscular mycorrhizae (AMF) dominated hyphal carbon pool after grass senescence. By contrast, soil hyphal length in the ectomycorrhizal (EM)/AM oak stand remained remarkably constant throughout the summer drought period, with the only exception of the topsoil layer exposed to direct evaporation (49% decrease at 5 cm depth). The sustained exudation of water from roots to soil through HL may have buffered rhizosphere hyphae against the negative effects of extreme soil desiccation in the oak stand. These data suggest that HL by deep-rooted trees may influence the biogeochemical cycling of carbon and nutrients in seasonally dry ecosystems through effects on rhizosphere fungi.     

宁南山区保护性农业措施对冬小麦农田休闲期土壤水分的影响

[目的]研究宁南山区冬小麦农田休闲期保护性农业措施对土壤水分的影响,为该区降雨资源的高效利用以及保护性农业的可持续发展提供理论依据。[方法]基于不同时期多个保护性农业试验土壤水分数据比较分析。[结果](1)在干旱的情况下,免耕留茬处理能够显著增加土壤表层(0—20cm)含水量。(2)在免耕的情况下,随着秸秆留茬高度的增加,土壤水分呈现增加趋势。(3)在降雨量较大且降雨量具有连续性的情况下,常规耕作处理在土壤表层保蓄了较多的土壤水分(0—20cm),免耕秸秆覆盖处理能够增加土壤20—80cm土壤水分含量。(4)冬小麦休闲期种植豆科作物,降低了土壤水分,其降低幅度与降雨量以及种植密度有关,种植密度越大,对土壤水分的影响越大。[结论]免耕+留茬耕作措施能够提高冬小麦农田休闲期土壤水分含量,覆盖作物降低了休闲期土壤水分含量。     

Nitrifier dominance of Arctic soil nitrous oxide emissions arises due to fungal competition with denitrifiers for nitrate

Arctic soils emit nitrous oxide, which is a potent greenhouse gas and also represents an important loss of nitrogen to oligotrophic Arctic ecosystems. However, little is known about the temperature sensitivity of nitrous oxide release in Arctic soils or the organisms mainly responsible for it. We investigated controls on nitrous oxide emissions in an Arctic soil across a typical temperature range (between 4 and 13 °C) on Truelove Lowland, Devon Island, Canada (75°40′N 84°35′W) at two different moisture contents. When fertilized with ammonia or nitrate, nitrous oxide emissions and temperature dependence of nitrous oxide emissions were insensitive to soil moisture content but linked to nitrification rates. Stable isotope analysis revealed that nitrous oxide was predominantly released by nitrifiers. However, nitrous oxide emissions were not linked to nitrifier prevalence with an insignificant (P < 0.219) increase in amoA genes and a (P < 0.01) decrease in archaeal nitrifiers. In contrast, denitrifier nosZ prevalence was 10,000 times greater than that of nitrifiers and was related to nitrous oxide emission potential when soils were fertilized with nitrate. Manipulating water-filled pore space should have changed the pattern of N₂O emissions. We used selective inhibitors to further explore why denitrification did not occur under field conditions when we manipulated water-filled pore space or when we used ¹⁵N analysis. When fungi were inhibited in the soil, nitrous oxide emissions from denitrifiers increased with no change in nitrous oxide released by nitrifiers. When fungi were active in the soil, there was little available nitrate but when fungi were inhibited, available soil nitrate increased over the incubation period. The dominance of nitrifiers in nitrous oxide emissions from Arctic soils under field conditions is linked to the competition for nitrate between fungi and denitrifiers.