Comparison of soil nitrogen dynamics under beech,Norway spruce and Scots pine in central Germany

To investigate the effect of tree species on soil N dynamics in temperate forest ecosystems, total N (Nt), microbial N (Nmic), net N mineralization, net nitrification, and other soil chemical properties were comparatively examined in beech (64–68 years old) and Norway spruce (53–55 years old) on sites 1 and 2, and beech and Scots pine (45 years old) on site 3. The initial soil conditions of the two corresponding stands at each site were similar; soil types were dystric Planosol (site 1), stagnic Gleysols (site 2), and Podzols (site 3). In organic layers (LOf₁, Of₂, Oh), Nmic and Nmic/Nt, averaged over three sampling times (Aug., Nov., Apr.), were higher under the beech stands than under the corresponding coniferous ones. However, the Nmic in the organic layers under beech had a greater temporal variation. Incubation (10 weeks, 22 °C, samples from November) results showed that the net N mineralization rates in organic layers were relatively high with values of 8.1 to 24.8 mg N kg^–1 d^–1. Between the two corresponding stands, the differences in net N mineralization rates in most of the organic layers were very small. In contrast, initial net nitrification rates (0.2–17.1 mg N kg^–1 day^–1) were considerably lower in most of the organic layers under the conifer than under the beech. In the mineral soil (0–10 cm), Nmic values ranged from 4.1–72.7 mg kg^–1, following a clear sequence: August>November>April. Nmic values under the beech stands were significantly higher than those under the corresponding coniferous stands for samples from August and April, but not from November. The net N mineralization rates were very low in all the mineral soils studied (0.05–0.33 mg N kg^–1 day^–1), and no significant difference appeared between the two contrasting tree species.     

Effects of trees on paddy bund on soil fertility and rice growth in Northeast Thailand

A study was conducted in Northeast Thailand on six rice paddy fields on a farm with similar soil (Aquic Quartzipsamments) and with a single tree on the paddy bund. There were 4 tree species: Parinarium anamense, Dipterocarpus obtusifolius, D. intricatus, and Samanea saman. Samples of soil (0–10 cm depth) and rice were collected at 3 positions (1, 5–7, and 9–11 m) away from the tree base in 3 replicated tree-soil trasects in each paddy field. Significantly higher pH, organic matter, and nutrients (N, P, K, Ca, and Mg) were found in paddy fields with S. saman (a legume tree) but grain yield and biomass of rice were significantly lower. Higher soil fertility was found in the positions closer to tree base while grain yields, biomass, and number of tillers were lower, and rice was taller and had more unfilled grain. Shading was deduced to be the key factor responsible for the depressed rice yields and growth at positions closer to tree base especially in the highly shading S. saman.     

Soil respiration response to precipitation reduction in a grassland and a Mongolian pine plantation in semi-arid northeast China

Climate change is predicted to alter global precipitation regimes.However,the response of soil carbon and nitrogen cycles and soil microorganisms to precipitation reduction is poorly understood but is dependent on ecosystem type.To evaluate the impacts of reduced precipitation on soil respiration,soil inorganic nitrogen(i.e.,NH4^+–N and NO3^-–N),nitrogen mineralization,and soil microbial community composition,a precipitation manipulation experiment was initiated in a Mongolian pine plantation and a naturally restored grassland in semi-arid northeast China.Precipitation reduction led to decreases of soil respiration rates by 14 and 8%in 2014 and 2015 in the Mongolian pine plantation but no changes in the grassland.Soil inorganic nitrogen,ammonification and nitrification rate,and soil phospholipids fatty acids were not significantly changed by reduced precipitation but significantly differed between the two ecosystems and among growing seasons.Our results suggest that the impacts of precipitation reduction on soil respiration were different between the Mongolian pine plantation and the grassland,and that ecosystem type and growing season had more pronounced impacts on soil carbon and nitrogen cycles.     

Soil–water content and air-filled porosity affect height growth of Scots pine in afforested arable land in Finland

We examined the soil–water content (SWC) and air-filled porosity (AFP) of afforested arable land in situ and related them with tree growth, which was expressed as the total length of 5-year-height growth above 2.5 m stem height. A total of 34 randomly selected sites in western Finland afforested with Scots pine (Pinus sylvestris L.) were sampled and SWC was measured using time domain reflectometry (TDR). Increasing AFP up to 30% and correspondingly decreasing SWC significantly increased tree growth while concentrations of foliar nutrients and contents of soil nutrients had no effect. Increasing organic matter content (OMC) and decreasing bulk density (BD) were accompanied by increasing SWC and decreasing AFP in the 0–10 cm soil layer. SWC values above 70% indicated critical AFP below 10%. It was found that 44% of the studied sites had mean AFP lower than 10%. The results obtained indicate that the afforested Finnish arable land with high soil OMC is commonly characterized by high SWC and low soil aeration, which can limit tree growth.     

20%氟硅唑可湿性粉剂在梨和土壤中的残留分析方法及残留动态研究

研究了氟硅唑在梨和土壤中的残留分析方法及其残留动态。样品用乙腈提取,PSA分散吸附剂吸附净化,液相色谱-紫外测定。氟硅唑的最小检出量为9.8ng,方法的最低检出浓度为9.8×10-3 mg/kg。在0.02～0.2mg/L添加水平下,梨中的回收率85.0%～105.0%,相对标准偏差2.6%～7.4%;土壤中的回收率80.0%～108.0%;相对标准偏差1.5%～11.4%,符合农残分析要求。试验结果表明,氟硅唑在梨中消解较快,土壤中相对缓慢,半衰期分别为5.6～13.1d和5.7～31.4d。末次施药距收获间隔21d,梨中氟硅唑残留量均低于0.2mg/kg,该药按推荐剂量使用是安全的。     

Comparing indicators of N status of 50 beech stands (Fagus sylvatica L.) in northeastern France

We compared different potential indicators of nitrogen (N) availability across 50 beech forests growing on a wide range of soils in northeastern France. Among the 50 sites measured, high elevation acidic soils had the highest potential net N mineralization in the A horizon (PNM_0–5 cm), while low elevation neutral and calcareous soils had the lowest (PNM_0–5 cm). We found that (PNM_0–5 cm) was negatively correlated with soil pH (R² = 0.47***) and positively correlated with microbial C/N (R² = 0.34***). However, when high elevation sites were excluded from analyses, the relationship between PNM_0–5 cm and soil pH as well as microbial C/N became weaker (R² = 0.23*** for both variables). We found no relationship between PNM_0–5 cm and organic N concentration, soil C/N, or vegetation-based indices for N availability (Ellenberg N and Ecoplant C/N). Bivariate linear regression analyses showed that 69% of the variability in percent nitrification (%Nitrif) was explained by both soil pH (0–5 cm) and soil C/N. Percent nitrification was strongly correlated with vegetation-based indices for N availability. The Ellenberg N and R (pH index) values together explained 74% of the variation in %Nitrif. No relationship was found between %Nitrif and soil δ¹⁵N (natural abundance in ¹⁵N). Of the 76 plant species evaluated, the probability of presence of 61 plant species was significantly correlated with %Nitrif while the probability of presence of 27 plant species only was correlated with PNM_0–5 cm. From these results, we believe that the use of plant community composition or the combination of soil pH and C/N are robust indicators of N availability.     

Does soil organic matter variation affect the retention of NH4 and NO3 in forest ecosystems?

Human activities have fundamentally changed global nitrogen (N) cycling, leading to elevated N deposition in most parts of the world. The fate of deposited N, whether being retained to sustain plant growth or causing ecosystem N saturation, is critical to the global carbon (C) cycling and local environment. In a short-term laboratory experiment, we used ¹⁵N-labeled NH₄⁺ and NO₃⁻ to study the fate of N inputs in forest soils and what regulates N retention. Soils with a wide range of organic matter content and other attributes were collected from a 70-year-old plantation containing monotypic stands of Norway spruce (Picea abies), red pine (Pinus resinosa), sugar maple (Acer saccharum), and red oak (Quercus rubra), and separated into 0-5 cm and 5-15 cm layers. Nitrogen added to the soil was either immediately extracted (Time 0: T0) with K₂SO₄ solution, or incubated for 7 d (T7) and then extracted. Retention of ¹⁵N into the non-extractable soil pool at T0 was limited; but after the 7-d incubation, between 20 and 70% of the ¹⁵NH₄⁺ was retained. Nitrification transformed on average 50% of the ¹⁵NH₄⁺ into ¹⁵NO₃⁻ during the incubation while retention of ¹⁵NO₃⁻ at T7 remained low (7.40 ± 1.08%). Retention of ¹⁵NH₄⁺ into non-extractable soil at T7 was positively correlated to the percentage of soil organic matter (SOM) (r² = 0.323, P < 0.001), and was significantly higher (P < 0.001) in the high-SOM 0-5 cm layer than in the low-SOM 5-15 cm layer. Conversion of ¹⁵NH₄⁺ to ¹⁵NO₃⁻ during incubation significantly reduced the ¹⁵NH₄⁺ retention (P < 0.001). Our results suggest that the variations of SOM and other soil attributes play strong roles in the retention of newly deposited inorganic N and could affect forest ecosystem responses to chronic N deposition.     

Soil nutrients and microbial activity after early and late season prescribed burns in a Sierra Nevada mixed conifer forest

Restoring the natural fire regime to forested systems that have experienced fire exclusion throughout the past century can be a challenge due to the heavy fuel loading conditions. Fire is being re-introduced to mixed conifer forests in the Sierra Nevada through both early season and late season prescribed burns, even though most fires historically occurred in the late season. We assessed the impact of early and late season prescribed fires on soil biogeochemical and microbiological parameters that are important for ecosystem recovery. We found that the late season burns had more dramatic short-term effects on soil abiotic conditions (temperature, moisture and pH), mineral soil carbon levels, total inorganic nitrogen, and microbial activity than the early season burns, relative to unburned sites, suggesting a higher severity burn. However, the total soil nitrogen pools and fluxes and soil respiration rates were not differentially impacted by burn season. These burn season effects suggest that soil variables may be regulated more strongly by fire severity than by the season in which the prescribed fire is conducted.     

Competition for nitrogen between adult European beech and its offspring is reduced by avoidance strategy

Plant growth, reproduction, and biomass allocation may be affected differently by nitrogen availability depending on tree size and age. In this context, competition for limited N may be avoided by different strategies of N acquisition between different vegetation components (i.e., seedlings, mature trees, other woody and herbaceous understorey). This study investigated in a field experiment whether the competition for N between different vegetation components in beech forests was prevented via seasonal timing of N uptake and affected by microbial N use. For this purpose, a removal approach was used to study the seasonal effects on N uptake and N metabolites in adult beech trees and beech natural regeneration, as well as soil microbial processes of inorganic N production and utilisation. We found that the competition for N between beech natural regeneration and mature beech trees was reduced by seasonal avoidance strategies (“good parenting”) of N uptake regardless of the N sources used. In spring, organic and inorganic N uptake capacity was significantly higher in beech seedlings compared to adult beech trees, whereas in autumn mature beech trees showed the highest N uptake rates. Removal of vegetation components did not result in changes in soil microbial N processes in the course of the growing season. Thus, N resources released by the removal of vegetation components were marginal. This consistency in soil microbial N processes indicates that competition between plants and soil microorganisms for N was not avoided by timing of acquisition during the vegetation period, but existed during the entire growing season. In conclusion, N nutrition in the studied forest ecosystem seems to be optimally attuned to European beech.     

Early-stage impacts of sika deer on structure and function of the soil microbial food webs in a temperate forest: A large-scale experiment

Modification of forest vegetation caused by an overabundance of mammalian herbivores has been reported in temperate and subarctic regions all over the world. However, the indirect effects of these herbivores on the structure and functions of soil decomposer systems are not fully understood, especially in temperate forests. We investigated the early effects of sika deer invasion on soil decomposer systems in a Japanese temperate forest using two large-scale experimental enclosures with low and high densities of deer (LD: 25 ha, 4 deer km⁻²; HD: 6.25 ha, 16 deer km⁻²) including control plots without deer (WD). Three years after deer introduction the understory cover of dwarf bamboo (Sasa nipponica) declined due to deer browsing in both enclosures. At the same time, measurements were made of the soil microbial community, soil nematode community, soil nitrogen (N) mineralization rate, and carbon (C) and N content in dwarf bamboo leaves. In LD, soil microbial biomass was lower from WD, probably due to the decrease of fresh aboveground litter from dwarf bamboo. Surprisingly, there were no cascading effects on total abundance of soil nematodes and soil total N mineralization potential which were unaffected by deer in the LD treatment, while soil NH₄⁺-N content was lower and soil nematode community structure was different (abundance of 4 families was higher and that of 3 families was lower, but the functional structure was not different) from WD. Specifically, the responses to deer introduction varied between microbes and nematodes, and the change of balance in the microbial food webs may have altered N mineralization processes. In contrast, in the HD treatment, all the variables measured were not significantly different from those of WD treatment. Intensive browsing by deer may have cancelled out the effects of the decrease in aboveground litter input on the soil decomposer systems through other pathways, such as a transitory increase in belowground litter input caused by induced changes in allocation patterns of bamboo. No changes in total N mineralization potential, leaf N, and composition of understory vegetation in both enclosures indicated that deer introduction did not facilitate nor retard N cycling regardless of deer density. This study showed that sika deer browsing can affect soil decomposer systems at an early stage of invasion even at low density, which contrasts with previous studies on the subject. Linking our findings of early-stage effects of deer on soil decomposer systems to longer-term dynamics of understory vegetation and tree regeneration will be needed to evaluate the adequacy of deer management practices with respect to the sustainability of soil nutrient supplies.     

Soil properties under Acacia nilotica trees in a traditional agroforestry system in central India

Acacia nilotica (L.) Willd. ex Del is an important multipurpose tree of traditional agroforestry system in the central belt of the Indian sub-continent. The tree is reported to reduce crop yields under its canopy. However, information is lacking on the spatial variation in soil physical characters, nutrient pool sizes and their availability to crops under its canopy. The present study reports influence of three tree canopy positions, viz. mid canopy, canopy edge and canopy gap, of Acacia nilotica (≥ 12 years) on texture, organic C, total and mineral N and P, and soil pH, in 0 to 10, 10 to 20 and 20 to 30 cm depth of the soil at ten sites in a traditional agroforestry system. Sand particles declined by 10% and 9% whereas clay particles increased by 14% and 10% under mid canopy and canopy edge, respectively, compared to that under canopy gap. Clay particles did not decline significantly with soil depth under all canopy positions. Proportion of silt particles was not influenced by the canopy position. Soil organic C, total N, total P, mineral N (NO₃ ⁻-N and NH₄ ⁺-N) and P were greater under mid canopy and canopy edge positions compared to canopy gap. Soil organic C and N pool sizes were maximum in 0 to 10 cm and declined with the depth of soil. Total and mineral P contents were nearly uniform across the depths. C/N ratio tended to increase with the soil depth whereas C/P ratio declined. This revised version was published online in June 2006 with corrections to the Cover Date.     

黄土区人工林的土壤持水力与有效水状况

以晋西北黄土区河北杨林,小叶杨林和柠条灌木林为对象,就各人工林地的土壤持水力和在４个不同降水的土壤有效水动态特点进行了研究。结果表明,林地间土壤持水力和比水容量为：河北杨林〉小叶杨林〉柠条灌木林。各林地的土壤持水力和供水力普遍偏低。     

A geospatial and temporal framework for modeling gaseous N and other N losses from forest soils and basins,with application to the Turkey Lakes Watershed Project,in Ontario,Canada

This article quantifies pre- to post-harvest gaseous N emissions and other N losses from forest soils and basins geospatially and temporally via digital elevation and hydrological modeling, using daily rain, snow and air temperature records, annual atmospheric N deposition rates, and basin-specific soil and forest specifications as input. The approach relates gaseous N losses from soils to soil temperature and water-filled pore space (WFPS) as affected by the depth-to-water (DTW) below the soil surface. The approach is applied to the Turkey Lakes Watershed Project (TLW) in Ontario, 60 km north of Sault St. Marie, where basin-wide N losses due to denitrification would mostly be restricted to the wetland portions of the basin. Basin-wide N losses via denitrification and stream export (mineral N and dissolved organic N) were empirically related to upland N mineralization and soil leaching as controlling processes. The calibrated model calculations, set to conform to the field-monitored N concentrations in TLW streams, suggest that the harvest-induced nitrification and denitrification pulses would be strongest near the end of the first post-harvest year, dropping to background levels within about 4–5 years later. The article concludes with assessing basin-specific denitrification efficiencies in relation to atmospheric N deposition and basin-to-basin wetland coverage.     

Nitrate dynamics after clear felling monitored by in vivo nitrate reductase activity (NRA) and natural N abundance of Deschampsia flexuosa (L.) Trin.

The N dynamics following clear felling, focusing on NO₃⁻ turnover, were studied at four forested sites in southern Sweden. Two different methods were used to study N availability: (i) an in vivo nitrate reductase activity (NRA) bioassay and (ii) measurements of natural abundance of stable N isotopes in leaves of the grass species Deschampsia flexuosa, and in organic soil horizons. At each of the four sites, six plots were established and each year, for 5 consecutive years (1989–1993), one plot per site was felled. Thus, in 1993 there were five plots with different ages since clear felling and one control (closed forest) plot at each site. NRA was analyzed three times annually during the years 1989–1993. Samples for grass and soil analysis of δ¹⁵N, total N and soil pH were taken in 1993 only. NRA rapidly increased after the felling and remained high throughout the studied period. This suggests that there was an increased pool of plant-available soil NO₃⁻ more than 5 years after clear felling. Despite differences in site productivity and N deposition between the four sites, no significant differences in NRA were found between the sites. There were also rapid changes in δ¹⁵N in leaves of D. flexuosa, coinciding with the increases in NRA, during the first 3 years after felling. In contrast to NRA, shoot δ¹⁵N decreased 3–4 years after the felling at three out of four sites. Variations in the δ¹⁵N figures between sites may have been largely due to between-site differences in field-layer retention of N. At two of the sites, where NO₃⁻ leaching was also measured, a correlation was found between the NO₃⁻ concentration in the water and the difference in δ¹⁵N between D. flexuosa leaves from felled and closed forest plots. The data presented here suggest that NO₃⁻ leakage after clear felling is a rapid process, which is influenced by the development of field-layer biomass after the felling. Furthermore, losses of NO₃⁻ through leaching rapidly change the natural abundance of the plant available N pools in the soil.