Distribution of fertiliser N among fixed ammonium fractions as affected by moisture and fertiliser source and rate

The relevance of fixed in certain soils and its categorisation has made it necessary to re-examine N behaviour. A replicate factorial experiment was designed to investigate the influence of soil type, soil moisture and fertiliser source and rate on fixation dynamics with particular attention to the distribution between weakly and strongly fixed pools. Fixation of was <20% of added N for all soils except River Estate. The percentage of added N present as fixed was greater for the low application rate. Soil moisture did not significantly influence weakly fixed . However, the dry soil treatment showed greater fertiliser ¹⁵N present as strongly fixed . Fertiliser ¹⁵N present as weakly and strongly fixed decreased and increased, respectively, at the second sampling, indicating movement between the pools. The importance of the weakly fixed fraction as a transitory pool between strongly fixed and available was observed.     

Reducing NH3, N2O and {\text{NO}}_3^ - –N losses from a pasture soil with urease or nitrification inhibitors and elemental S-amended nitrogenous fertilizers

A 3-month field experiment comparing nitrogen (N) losses from and the agronomic efficiency of various N fertilizers was conducted on a sandy loam (Typic Hapludand) soil at Ruakura AgResearch farm, Hamilton, New Zealand during October to December 2003. Three replicates of seven treatments: urea, urea + the urease inhibitor N-(n-butyl) thiophosphoric triamide (trade name Agrotain), urea + Agrotain + elemental sulphur (S), urea + double inhibitor [DI; i.e., Agrotain + dicyandiamide (DCD)], diammonium phosphate (DAP), DAP + S, each applied at 150 kg N ha⁻¹, and control (no N). After fertilizer application, soil ammonium () and nitrate () concentrations (7.5-cm soil depth), ammonia (NH₃) volatilization, nitrate () leaching, nitrous oxide (N₂O) emission, pasture dry matter, and N uptake were monitored at different timings. Urea applied with Agrotain or Agrotain + S delayed urea hydrolysis and released soil at a slower rate than urea alone or urea + DI. Urea applied with DI increased NH₃ volatilization by 29% over urea alone, while urea + Agrotain and urea + Agrotain + S reduced NH₃ volatilization by 45 and 48%, respectively. Ammonia volatilization losses from DAP were lower than those from urea with or without inhibitors. Total reduction in leaching losses for urea + DI and urea + Agrotain compared to urea alone were 89% and 47%, respectively. Application of S with urea + Agrotain reduced leaching losses by an additional 6%. Nitrous oxide emissions were higher from the DAP and urea alone treatments. Urea applied with DI and urea + Agrotain reduced N₂O emissions by 37 and 5%, respectively, over urea alone. Compared to urea alone, total pasture production increased by 20, 17, and 15% for urea + Agrotain + S, urea + Agrotain, and urea + DI treatments, respectively, representing 86, 71, and 64% increases in N response efficiency. Total N uptake in urea + Agrotain, urea + Agrotain + S, and urea + DI increased by 29, 22, and 20%, respectively, compared to urea alone. These results suggest that the combination of both urease and nitrification inhibitors may have the most potential to reduce N losses and improve pasture production in intensively grazed systems.     

Gross nitrogen mineralisation and fungi-to-bacteria ratios are negatively correlated in boreal forests

In terrestrial ecosystems, gross nitrogen mineralisation is positively correlated to microbial biomass but negatively to soil organic matter C-to-N ratios; the influence of the microbial community structure is less well known. Here, we relate rates of gross N mineralisation to fungi-to-bacteria ratios in three natural forest types of contrasting N availability and in a long-term N-loading experiment in a boreal forest. We report, for the first time, a strong negative correlation between gross N mineralisation and the fungi-to-bacteria ratio ( = 0.91, P = 0.0005, N = 7). There was also a negative correlation between gross N mineralisation and the C-to-N ratio ( = 0.89, P = 0.001, N = 7), but a weaker positive correlation between gross N mineralisation and soil pH ( = 0.64, P = 0.019, N = 7). Our analysis suggests that soil fungi-to-bacteria and C-to-N ratios are interrelated and that they exert strong influences on soil N cycling in boreal forests.     

Contribution of nitrification happened in rhizospheric soil growing with different rice cultivars to N nutrition

A rhizobox with three compartments and soil slicing followed by quick freezing were used to study the spatiotemporal variations of nitrification of rhizospheric soil of Yangdao 6 (Indica) and Nongken 57 (Japonica). The results obtained revealed that ammonium () was the main N form in flooded paddy soil. A concentration gradient for was observed with the lowest concentration nearer to the root zone and the concentrations increased with increasing distance from the root zone. No concentration gradient was observed for nitrate (). The nitrification activities of both rice cultivars increased with the development of the incubation time. The nitrification activities were maximal in rhizospheric soil, followed by those in bulk soil and in the root zone. In the rhizosphere, nitrification activities decreased with increasing distance from the root zone. The maximal nitrification activity measured at 44, 51, and 58 days after sowing of Yangdao 6 and Nongken 57 rice cultivars was at a distance of 6 and 2 mm away from the root zone, respectively, and they were 0.88 and 0.73 mg kg⁻¹ h⁻¹, respectively. In this experiment, the nitrification activities were significantly and positively correlated with the ammonia-oxidizing bacteria (AOB) abundance (r=0.86, p<0.01). The nitrification activity, concentration, AOB abundance, dry matter and N accumulation and leaf reductase activity associated with Indica were always higher than those with Japonica. Therefore, nitrification in rhizosphere had more important significance for N nutrition, especially for the Indica rice cultivars.     

Influence of DCD and DMPP on soil N dynamics after incorporation of vegetable crop residues

We have investigated the effect of two nitrification inhibitors, 3,4-dimethylpyrazole phosphate (DMPP) and dicyandiamide (DCD), on the accumulation of and after incorporation of cauliflower residues in incubation experiments. Cauliflower leaves were incubated with soil and DCD or DMPP at two application rates [8.93 and 17.9 mg active component (ac) kg⁻¹ for DCD; 0.89 and 1.79 mg ac kg⁻¹ for DMPP]. Both doses of DCD and DMPP increased on average by 18.9 and 26.0 mg N kg⁻¹ for DCD1 (during 30 days) and DCD2 (during 45 days), respectively, and on average by 14.4 mg N kg⁻¹ for DMPP1 and DMPP2 during a period of at least 95 days. In DCD-treated soils, data followed an S-shaped curve, indicating that nitrification restarted during the experiment: inhibition was on average 24% during 35 days for DCD1 and on average 45% during 49 days for DCD2. Thereafter, amount in DCD-treated soils exceeded that of the cauliflower-only treatment by 31% for DCD1 and 78% for DCD2, probably due to a nitrogen release from DCD itself and a priming effect induced by DCD. In DMPP-treated soils, data followed a linear pattern since nitrification was inhibited during the complete incubation (95 days): inhibition was on average 56 and 64% for DMPP1 and DMPP2, respectively. DMPP did not affect the N mineralization of the crop residues. Under favourable conditions, DCD is able to inhibit the nitrification from crop residues for 50 days and DMPP for at least 95 days. Hence, especially DMPP shows a potential to reduce leaching after incorporation of crop residues.     

Nitrous oxide emissions from cattle-impacted pasture soil amended with nitrate and glucose

There is little information concerning N₂O fluxes in the pasture soil that has received large amounts of nutrients, such as urine and dung, for several years. The aims of this study were to (1) experimentally quantify the relationship between mineral N input and N₂O emissions from denitrification, (2) describe the time course of N₂O fluxes resulting in N inputs, and (3) find whether there exists an upper limit of the amount of nitrogen escaping the soil in the form of N₂O. The study site was a grassland used as a cattle overwintering area. It was amended with KNO₃ and glucose corresponding to 10–1,500 kg N and C per hectare, covering the range of nutrient inputs occurring in real field conditions. Using manual permanent chambers, N₂O fluxes from the soil were monitored for several days after the amendments. The peak N₂O emissions were up to 94 mg N₂O–N m⁻² h⁻¹, 5–8 h after amendment. No upper limit of N₂O emissions was detected as the emissions were directly related to the dose of nutrients in the whole range of amendments used, but the fluxes reflected the soil and environmental conditions, too. Thus, in three different experiments performed during the season, the total cumulative losses of N₂O–N ranged from 0.2 to 5.6% of the applied 500kg ha⁻¹. Splitting of high nutrient doses lowered the rate of N₂O fluxes after the first amendment, but the effect of splitting on the total amount of N₂O–N released from the soil was insignificant, as the initial lower values of emissions in the split variants were compensated for by a longer duration of gas fluxes. The results suggest that the cattle-impacted soil has the potential to metabolize large inputs of mineral nitrogen over short periods (∼days). Also, the emission factors for did not exceed values reported in literature.     

Nematode diversity,food web condition,and chemical and physical properties in different soil habitats of an organic farm

The aim of this paper was to assess biodiversity among different habitats of an organic farm and the relationships between some soil properties, nematode taxonomic diversity, and soil food web condition. Eight habitats were studied in the farm: ponds, ditches, a riparian corridor, hedgerows, and four agricultural fields (mustard, oats, fallow, and legumes). The undisturbed riparian corridor had higher soil and concentrations, and potentially mineralizable N and higher abundances of bacterivore nematodes and longer food webs. Canonical correlation analysis showed associations between habitats and nematode trophic groups: predatory and bacterial-feeding nematodes in the riparian corridor and hedgerows, omnivore nematodes in the ponds and ditches, and fungal-feeding nematodes in the legume field. Soil chemical and physical properties mirrored the aboveground farm patterns and were more similar among habitats that were or had been cultivated, compared to the riparian corridor. Soil food web indices, based on functional analysis of nematode faunal composition, reflected the aboveground landscape heterogeneity. Discriminant analysis indicated that soil food web indices separated the two most disturbed habitats (ponds and tailwater ditches) from the two least disturbed habitats (the riparian corridor and hedgerows). The indices correlated with soil functioning as inferred by soil properties. Abundance of nematode taxa was not associated with aboveground landscape patterns. The complexity of the soil food web may have been influenced by (1) environmental factors that differed between years, (2) different time periods since disturbance in the various habitats, and (3) movement of nutrients and organisms by water flow between habitats in the farmscale.     

Properties of anaerobically digested and composted municipal solid waste assessed by linking soil mesofauna dynamics and nitrogen modelling

We have studied the effect of anaerobically digested (ADMSW) and composted municipal solid waste (CMSW) on mineralization and foodweb dynamics to verify the hypothesis that ADMSW would immobilize N right after addition to soil in contrast to addition with CMSW. Another hypothesis was that the mesofauna (enchytraeids and microarthropods) would stimulate N release from the decomposer community. We measured excretion of -N and urea-N from the mesofauna and hypothesized that enchytraeids would release urea. ADMSW and CMSW were amended to pots with sandy loam and barley. The pots were divided into treatments with or without mesofauna. Mesofauna, plant N and biomass, soil N and ergosterol (fungal biomass) were measured over a 113-day period of four equidistant samplings. Soil respiration, N mineralization and N release by the mesofauna were modelled from concurrent studies. ADMSW- and CMSW-treated soils initially (<20 days) immobilized N. The amendments did not increase plant growth substantially, and this was probably due to N-limitation in the early stages of plant growth. Enchytraeid abundance was about three times higher in ADMSW- than CMSW-treated soils, indicating that ADMSW contained more labile compounds, bacteria, and microfauna. The mesofauna did not affect N-content, but the cumulated -N excreted by the mesofauna was estimated to be substantial and about one-fifth of total plant N in ADMSW. An explanation for this discrepancy might be that in the absence of the mesofauna, other members of the detrivore and microbivore community performed the mesofauna’s function. Neither enchytraeids nor microarthropods excreted urea.     

Gross nitrogen mineralisation rates in pastural soils and their relationships with organic nitrogen fractions, microbial biomass and protease activity under glasshouse conditions

In this study, gross nitrogen (N) mineralisation rates were determined in six pasture soils (Fleming, Kairanga, Karapoti, Lismore, Templeton and Waikoikoi) from three different regions of New Zealand. The soils were kept under controlled soil water potential (–10 to –30 kPa) and temperature (12–20°C) conditions in a glasshouse. The gross N mineralisation rates ranged from 0.76 to 5.87 g N g^–1 soil day^–1 in the six soils and were positively correlated with the amount of amino acid-N (AA-N), ammonia-N (NH₃-N), total hydrolysable-N (TH-N), microbial biomass-carbon (MB-C), microbial biomass-N (MB-N), protease activity and organic C and N. A stepwise regression was used to generate equations that could best describe gross N mineralisation rates. Microbial biomass-carbon and AA-N were included in the equation that best described the gross N mineralisation rate:

Effect of iron oxides and carboxylic acids on photochemical degradation of bisphenol A

γ-FeOOH was initially prepared by hydrothermal process and then sintered at 280°C, 310°C, and 420°C, and four kinds of iron oxides were obtained and named as lepidocrocite, IO-280, IO-310, and IO-420. They were characterized by XRD, SEM, and BET techniques to disclose the crystal composition, morphology, and surface area. The XRD results show that IO-280 and IO-310 should consist of maghemite and hematite, while IO-420 should be pure hematite. With the increase of temperature, the specific surface area significantly decreased. To test the photocatalytic activity of iron oxides, bisphenol A (BPA) was selected as a model chemical. The results show that BPA photocatalytic degradation should depend strongly on pH value, light source, and the crystal structure of iron oxides and that IO-310 had the highest activity in the absence of oxalic acid under UV or visible light illumination. The dependence of BPA photodegradation on carboxylic acids in lepidocrocite-carboxylate systems was investigated. BPA degradation was promoted greatly by the addition of oxalic and citric acid, and slightly by tartaric, malonic, and malic acid. The first-order kinetic constant (k) of BPA degradation follows the order oxaliccitrictartaric>malonic>malic>without acid≈succinic acid. In iron oxide–oxalic acid systems, the reaction rate and efficiency of BPA degradation under UV light was much more than under visible light illumination, and the k value follows the order: lepidocrocite>IO-280>IO-310>IO-420 under both UV and visible illumination. It is concluded that BPA photodegrdation should be dependent on the kind of carboxylic acid, iron oxides, and light sources in iron oxide–carboxylate system. The photochemical investigation of iron oxide–carboxylate complex systems is essential to the full understanding of the interactive mechanism of iron oxides and organic pollutants on the surface of soil in subtropical and tropical regions.     

Dissolved organic matter enhances the sorption of atrazine by soil

The influence of dissolved organic matter (DOM) on the sorption of atrazine (2-chloro-4-ethylamino-6-isopylamino-1,3,5-triazine) by ten soils was investigated. Batch sorption isotherm techniques were used to evaluate the important physiochemical properties of soil determining the sorption of atrazine in the presence of DOM. The sorption of atrazine as a representative of nonionic organic contaminants (NOCs) by soil with and without DOM could be well described by the Linear and Freundlich models. The n values of the Freundlich model were generally near to 1, indicating that linear partitioning was the major mechanism of atrazine sorption by soil samples. The apparent distribution coefficient, value, for atrazine sorption in the presence of DOM initially increased and decreased thereafter as the DOM concentration increased in the equilibrium solution. DOM at relatively lower concentrations significantly enhanced the sorption of atrazine by soil, while it inhibited the atrazine sorption at higher concentrations. For all the soil samples, the maximum of was 1.1~3.1 times higher than its corresponding K _d value for the control (without DOM). The maximum enhancement of the distribution coefficient () in the presence of DOM was negatively correlated with the content of soil organic carbon (SOC) and positively correlated with the clay content. The critical concentration of DOM, below which DOM would enhance atrazine sorption, was negatively correlated with SOC. The influence of DOM on atrazine sorption could be approximately considered as the net effect of the cumulative sorption and association of atrazine with DOM in solution. Results of this study provide an insight into the retention and mobility of a NOC in the soil environment.     

Dairy manure N mineralization estimates from incubations and litterbags

A laboratory incubation trial and a field litterbag study were conducted to determine the rate and magnitude of mineralization of dairy manure N components in a south central Wisconsin silt loam. Dairy manure components (urine, feces, or bedding, each ¹⁵N-labeled and the other components left unlabeled) were incubated in soil at 11, 18, or 25°C. Samples were taken at 14, 21, 42, 84, and 168 days and analyzed for mineralized N ( and ) and ¹⁵N abundance in the inorganic and organic fraction (at day 168 only). In the field study, nylon mesh (38 μm) litterbags filled with ¹⁵N-labeled manure (2000) or unlabeled manure (2000 and 2002) were placed 7.5 cm below the surface and excavated at 7, 14, 21, 28, 35 (2000 only), 42, 56, 84, 98, and 126 days after burial and at corn (Zea mays L.) harvest, after 142 days in 2002 and 154 days in 2000. In the incubation study, 50−60% of applied urine N was mineralized showing the importance of this manure N component as a source of plant available N. About 14−19% of applied N was mineralized from the fecal and bedding components. In the litterbag experiment, approximately 70% of the dry mass and 67% of the N was mineralized from the litterbags with similar amounts measured using either labeled or unlabeled N. Rates of manure organic matter decomposition and N mineralization were best predicted using single exponential models for both years with most of the release occurring during the first 21 days.     

Distribution and activity of hypolithic soil crusts in a hyperarid desert (Baja California, Mexico)

Widespread and ecologically important, biological soil crusts include those microbial communities living on the surface of the soil and those that live beneath semitranslucent rocks (a.k.a. hypolithic crusts). We examined the distribution, abundance, physiology, and potential soil N contributions of hypolithic, biological crusts in hyperarid ecosystems of the Baja California peninsula and islands in the midriff region of the Gulf of California, Mexico (Sonoran desert). Crusts were limited in distribution to areas with translucent quartz rocks less than 3 cm thick, were not found on areas of islands with seabird guano deposition, but covered as much as 1% (12,750 m²) of the surface area of one island. The percent of available rocks colonized by crusts was similar between the mainland (38%) and islands without seabird guano (26%). Carbon fixation rates in the field, which have not been previously reported, ranged between 0 and 1.23 μmol m⁻² s⁻¹, and in the lab ranged between 0.66 and 0.94 μmol m⁻² s⁻¹. Evidence of low rates of N fixation was inferred from δ ¹⁵N values of crust and soil. Hypolithic crusts were found to have minimal, if any, influence on soil salinity, pH, and , but may represent up to 14% of the biomass of primary producers on these islands and provide C and N to the belowground and possibly aboveground heterotrophic communities where crusts exist. The results of this study suggest a limited but potentially important contribution of hypolithic soil crusts to hyperarid ecosystems.     

Spraying of oxytetracycline and gentamicin onto field-grown coriander did not affect the abundance of resistant bacteria, resistance genes, and broad host range plasmids detected in tropical soil bacteria

Horticultural supplements containing oxytetracycline and gentamicin, two clinically relevant biocides, are widely marketed to prevent or control infections by bacterial phytopathogens. Despite their regular consumption in the world’s less developed countries, it is unknown whether exposure of tropical farmlands to these drugs results in an enrichment of resistant bacteria, resistance genes, and/or mobile genetic elements in the soil. These concerns were investigated under field conditions by repeatedly spraying recommended amounts of a commercial product containing oxytetracycline-HCl, and gentamicin- onto two coriander plots. Subsequent to five applications within 16 months, composite soil samples from control and treated sections were compared with respect to the abundance of resistant bacteria and the prevalence of conserved nucleotide sequences from tetracycline efflux proteins, tetracycline ribosomal protection proteins, four different families of gentamicin-modifying enzymes, and broad host range plasmids of the IncP-1 and IncQ incompatibility groups. The isolation frequency of oxytetracycline- and gentamicin-resistant bacteria and the detection rate of the aforementioned genes and elements were unrelated to application of the supplement. Despite the omnipresence of sequences from IncP-1 plasmids, conjugative plasmids conferring resistance to oxytetracycline or gentamicin were not captured in biparental matings. The widespread occurrence of resistant bacteria and resistance genes at the beginning of the trial emerges as a reasonable explanation for the lack of anticipated responses. Moreover, we assume that the biocides applied were inactivated by biotic and abiotic factors under tropical conditions.     

Interactions in microbial use of soluble plant components in soil

Glucose, glutamic acid, alanine, valine, and albumin were used as model substances to represent readily available soluble plant components. Their mineralisation in soil was monitored by making hourly measurements of total CO₂ evolution and periodic measurements of ¹⁴CO₂ during a 5-day incubation period. When glucose and the amino acids were combined in dual substrates the two components were mineralised simultaneously. In each combination the component with the shortest lag phase or highest specific growth rate () was mineralised preferentially. Glucose accelerated the mineralisation of the amino acids that had longer lag phases than glucose (i.e., alamine and valine). During the first 8 h, glucose mineralisation was reduced in the dual substrates compared with its mineralisation as a single substrate. The smaller the difference between the lag phase of glucose and the amino acid, the greater was the reduction in the glucose mineralisation rate. During the exponential phase, the effects on glucose mineralisation were dependent on the nature and concentration of the amino acid. The interactions observed between glucose and amino acid decomposition indicated that these substances were mineralised, at least partly, by the same microorganisms. In contrast, glucose and the soluble protein albumin were decomposed independently of each other.     

Chemical Composition of Precipitation in Beijing Area, Northern China

Variations of anions (SO₄ ^2-, NO₃ ^-,NO₂ ^-, Cl^- and F^-),cations (K⁺,Na⁺, Ca²⁺, Mg²⁺ and ⁺) and pH values in precipitation, througfall and stemflow samples collected overa four-year period (1995–1998) in Beijing (two sitesZhongguancun and Mangshan) are presented. The annualvolume-weighted range of pH values were 6.57–7.11 inprecipitation, 5.46–6.86 in thoughfall and 5.32–6.41 instemflow. The fominant anion was , while Ca⁺and NH₄ ⁺ were the main cations in precipitation,throughfall and stemflow. Most of ion concentrations with precipitation, throughfall and stemflow volume showed negative correlation, except for some ones. Significant correlationvalues were also found between ions (SO₄ ^2-,NO₃ ^-, Cl^-, F^-, Ca²⁺,Mg²⁺ andNa⁺) in precipitation, throughfall and stemflow indicatedthe common sources of these ions such as coal combustion,automobile emission and fertilizers application. Compared toprecipitation, there was an increased ion concentration inthroughfall or in stemflow. Changes of ion concentrations werein Quercus liatungensis Koiz. and Pinus tabulaefornisCarr. throughfall (or stemflow) because of different crown andbark qualities of tree species.     

Relationships among dust outbreaks, vegetation cover, and surface soil water content on the Loess Plateau of China, 1999–2000

We analyzed relationships among dust outbreaks, Normalized Difference Vegetation Indices (NDVI), and surface soil water content (0 to 2 cm depth) on the Loess Plateau, a significant dust source area of East Asia. World Surface Data for wind speed and current weather, coarse-resolution data for NDVI, and a three-layer soil model for surface soil water content were used. The threshold NDVI for preventing dust outbreaks was about 0.2 when the wind speed ranged from 7 to 8 m s^− 1. This threshold NDVI corresponds to a vegetation cover of 18%. The threshold ratio of surface soil water content to the field capacity (θ_r) was about 0.2. Conditions facilitating dust outbreaks on the Loess Plateau are when NDVI is less than 0.2 with wind speed  7 m s^− 1 and θ_r < 0.2, and when NDVI is greater than 0.2 with wind speed  9 m s^− 1 and θ_r < 0.2.     

Phenotypic diversity for qualitative and phenologic characters in germplasm collections of tef (Eragrostis tef)

A total of 3600 entries of tef [Eragrostis tef (Zucc.) Trotter] germplasm collections representing 36 populations from six regions of Ethiopia were evaluated for phenotypic diversity in six qualitative and phenologic characters. High values of Shannon–Weaver diversity index ( ) were recorded for most characters in each population. Monomorphism was high for anther color (purple) and panicle form (loose). Larger variances for were obtained among regions than among populations within a region from hierarchical analysis of variance. Significant (P<0.05) to highly significant (P<0.01) t-values were observed among values of some regions for seed color, panicle form and days to maturity. A substantial level of variance (P<0.05) was obtained for populations within altitudinal zones only for days to maturity. Altitudinal zones exhibited similar levels of diversity across characters for these collections. Generally, considerable variations important for tef improvement work have been observed and regions with highest diversity for some traits have been suggested for future in situ or ex situ germplasm conservation works. Although similar mean diversity indices were obtained among regions, they were non overlapping.     

Species‐specific differences in nitrogen uptake and utilization by six European tree species

Species‐specific uptake and allocation mechanisms for N are scarce, in particular when trees are cultivated in potted soil under more natural conditions than in hydroponic culture. The objective of this study was to compare specific N‐uptake rates for economically and ecologically important tree species in Central European forests: pine (Pinus sylvestris), spruce (Picea abies), oak (Quercus petraea), beech (Fagus sylvatica), lime (Tilia cordata), and ash (Fraxinus excelsior) when they grow in mineral soil from an old fallow site with a pH of 6. We used an ¹⁵N‐labeling method to measure tree seedling ¹⁵N uptake in potted soils (Humic Cambisol) when both N forms NH$ _4^+ $ and NO$ _3^- $ were simultaneously present in the soil solution for interspecies comparison and assessment of relationships between specific ¹⁵N‐uptake rates and amino acid–accumulation rates or relative growth rates (RGR). The results demonstrate that tree species varied significantly in their capacity to take up NH$ _4^+ $ or NO$ _3^- $ into roots, stems, or leaves, but indicate only marginal differences in their preference for NH$ _4^+ $ or NO$ _3^- $ when they grow in mineral soil. The ranking of specific ¹⁵N‐uptake rates for NH$ _4^+ $ and NO$ _3^- $ was oak < beech < spruce < pine < lime < ash. Fine roots of all species had the highest specific ¹⁵N‐uptake rates for both N forms, followed by total roots, leaves/needles, and stems. As regards tree seedling species, we found negative relationships between glutamine (Gln)‐accumulation rates in leaves/needles and total ¹⁵N‐uptake rates in fine roots. Noteworthy was the fact that, at high Gln‐accumulation rates, the N‐uptake system in fine roots of ash was probably lower under feedback inhibition by the amino acid.     

Direction-dependent behaviour of hydraulic and mechanical properties in structured soils under conventional and conservation tillage

The development of soil structure units with defined forms and dimensions (e.g. platy by soil compaction or prismatic up to subangular-blocky by swelling–shrinkage processes) can lead to direction-dependent behaviour of mechanical and hydraulic properties. However, little research has investigated direction-dependent behaviour directly. Undisturbed samples were collected at different horizons and orientations (vertical and horizontal) of Stagnic Luvisols derived from glacial till (Weichselian moraine region in Northern Germany). A direct shear test determined the cohesion (c) and the angle of internal friction (φ). The water retention curve (WRC), the saturated hydraulic conductivity (k_s) and the air permeability (k_a) were also measured. The air-filled porosity (_a) was determined and pore continuity indices (N) and blocked porosities (_b) were derived from the relationship between k_a and _a.Although the pore volume as a scalar is isotrop, the saturated hydraulic conductivity and air permeability can be anisotropic. In the seedbed (SB) and plough pan (PP) of conventionally managed soils the effective porosity is non-direction-dependent, however, differences in k_s as a function of sampling direction can reach one order of magnitude in PP (k_sh > k_sv). The shear strength parameters do not present a significant anisotropy, although, a pronounced spatial orientation of soil aggregates (e.g. induced by soil compaction in a plough pan) lead to direction-dependent shear strength (by σ_n: 10 kPa, σ_tv: 12 kPa and σ_th: 19 kPa). This behaviour was especially observed in pore continuity indices (e.g. vertical and horizontal oriented aggregates observed in Bvg and PP presented _bv < _bh and _bv > _bh, respectively) showing that the identification of soil structure can be used as the first parameter to estimate if hydraulic properties present a direction-dependent behaviour at the scale of the soil horizon, which is relevant in modelling transport processes.