Microalgal species variation at different successional stages in biological soil crusts of the Gurbantunggut Desert,Northwestern China

Biological soil crusts (BSC), most notably lichen crusts, develop and diversify in the Gurbantunggut Desert, the largest fixed and semi-fixed desert in China. Four different successional stages of BSC, including bare sand, microalgal crusts, lichen crusts, and moss crusts, were selected to determine successional changes in microalgal species composition and biomass and formation of BSC. A 10 × 10-m observation plot was established in an interdune region of the Gurbantunggut Desert and data were collected over an 8-year study period. The main results were: (1) different successional stages of BSC significantly affected the content of soil organic C and total and available N but not the total and available P and K content of soil; (2) composition of microalgal communities differed among the four successional stages; (3) significant differences in microalgal biomass were observed among the four successional stages; (4) bare sand was mainly uncompacted sand gains; (5) filamentous cyanobacteria, particularly Microcoleus vaginatus, were the dominant species in the early phase of crust succession. The presence of fungal mycelium and moss rhizoids prevented water and wind erosion.     

Feasibility of cyanobacterial inoculation for biological soil crusts formation in desert area

Practical testing of the feasibility of cyanobacterial inoculation to speed up the recovery of biological soil crusts in the field was conducted in this experiment. Results showed that cyanobacterial and algal cover climbed up to 48.5% and a total of 14 cyanobacterial and algal species were identified at the termination of inoculation experiment; biological crusts' thickness, compressive and chlorophyll a content increased with inoculation time among 3 years; moss species appeared in the second year; cyanobacterial inoculation increased organic carbon and total nitrogen of the soil; total salt, calcium carbonate and electrical conductivity in the soil also increased after inoculation. Diverse vascular plant communities composed of 10 and 9 species are established by cyanobacterial inoculation on the windward and leeward surface of the dunes, respectively, after 3 years. The Simpson index for the above two communities are 0.842 and 0.852, while the Shannon-Weiner index are 2.097 and 2.053, respectively. In conclusion, we suggest that cyanobacterial inoculation would be a suitable and effective technique to recover biological soil crusts, and may further restore the ecological system.     

Archaeal populations in biological soil crusts from arid lands in North America

Archaea are common and abundant members of biological soil crust communities across large-scale biogeographic provinces of arid North America. Regardless of microbial community development, archaeal populations averaged 2 × 10⁷ 16S rRNA gene copies per gram of soil, representing around 5% of the prokaryotic (total calculated bacterial and archaeal) numbers assessed by quantitative-PCR. In contrast, archaeal diversity, determined by denaturing gradient gel electrophoresis fingerprinting and clone libraries of 16S rRNA genes, was very restricted. Only six different phylotypes (all Crenarchaea) were detected, three of which were very dominant. Some phylotypes were widespread, while others were typical of Southern desert areas.     

Revisiting classic water erosion models in drylands: The strong impact of biological soil crusts

Soil erosion and subsequent degradation has been a contributor to societal collapse in the past and is one of the major expressions of desertification in arid regions. The revised universal soil loss equation (RUSLE) models soil lost to water erosion as a function of climate erosivity (the degree to which rainfall can result in erosion), topography, soil erodibility, and land use/management. The soil erodibility factor (K) is primarily based upon inherent soil properties (those which change slowly or not at all) such as soil texture and organic matter content, while the cover/management factor (C) is based on several parameters including biological soil crust (BSC) cover. We examined the effect of two more precise indicators of BSC development, chlorophyll a and exopolysaccharides (EPS), upon soil stability, which is closely inversely related to soil loss in an erosion event. To examine the relative influence of these elements of the C factor to the K factor, we conducted our investigation across eight strongly differing soils in the 0.8 million ha Grand Staircase-Escalante National Monument. We found that within every soil group, chlorophyll a was a moderate to excellent predictor of soil stability (R² = 0.21–0.75), and consistently better than EPS. Using a simple structural equation model, we explained over half of the variance in soil stability and determined that the direct effect of chlorophyll a was 3× more important than soil group in determining soil stability. Our results suggest that, holding the intensity of erosive forces constant, the acceleration or reduction of soil erosion in arid landscapes will primarily be an outcome of management practices. This is because the factor which is most influential to soil erosion, BSC development, is also among the most manageable, implying that water erosion in drylands has a solution.     

Carbon-to-nitrogen ratio is a poor predictor of low molecular weight organic nitrogen mineralization in soil

Carbon-to-nitrogen ratio (C:N) has frequently been shown to be a good predictor of the speed of organic residue decomposition and N mineralization in soil. While this relationship appears to work well for complex organic materials (e.g. plant litter), its applicability to smaller organic substrates containing N remains unknown. Here we evaluated whether the intrinsic properties of amino acids and peptides could be used to predict their rate of microbial uptake and subsequent N mineralization. In an agricultural grassland soil we found that C:N, molecular weight, aromaticity and sulphur content provided poor indicators of amino acid bioavailabilityand subsequent NH₄⁺ release into soil. We therefore hypothesize that the position of amino acids along microbial biosynthetic pathways together with internal demand for individual amino acids rather than their C or N content is the primary determinant of N mineralization.     

Protein breakdown represents a major bottleneck in nitrogen cycling in grassland soils

Proteins represent the dominant input of organic N into most ecosystems and they also constitute the largest store of N in soil organic matter. The extracellular protease mediated breakdown of proteins to amino acids therefore represents a key step regulating N cycling in soil. In this study we investigated the influence of a range of environmental factors on the rate of protein mineralization in a grazed grassland and fallow agricultural soil. The protein turnover rates were directly compared to the rates of amino acid mineralization under the same conditions. Uniformly ¹⁴C-labelled soluble protein and amino acids were added to soil and the rate of ¹⁴CO₂ evolution determined over 30 d. Our results indicate that the primary phase of protein mineralization was approximately 20 ± 3 fold slower that the rate of amino acid mineralization. The addition of large amounts of inorganic NO₃⁻ and NH₄⁺ to the soil did not repress the rate of protein mineralization suggesting that available N does not directly affect protease activity in the short term. Whilst protein mineralization was strongly temperature sensitive, the presence of plants and the addition of humic and tannic acids had relatively little influence on the rate of soluble protein degradation in this fertile grassland soil. Our results suggests that the extracellular protease mediated cleavage of proteins to amino acids rather than breakdown of amino acids to NH₄⁺ represents the limiting step in soil N cycling.     

Nitrogen fixation by biological soil crusts and heterotrophic bacteria in an intact Mojave Desert ecosystem with elevated CO2 and added soil carbon

Fixation of N by biological soil crusts and free-living heterotrophic soil microbes provides a significant proportion of ecosystem N in arid lands. To gain a better understanding of how elevated CO₂ may affect N₂-fixation in aridland ecosystems, we measured C₂H₂ reduction as a proxy for nitrogenase activity in biological soil crusts for 2 yr, and in soils either with or without dextrose-C additions for 1 yr, in an intact Mojave Desert ecosystem exposed to elevated CO₂. We also measured crust and soil δ¹⁵N and total N to assess changes in N sources, and δ¹³C of crusts to determine a functional shift in crust species, with elevated CO₂. The mean rate of C₂H₂ reduction by biological soil crusts was 76.9±5.6 μmol C₂H₄ m⁻² h⁻¹. There was no significant CO₂ effect, but crusts from plant interspaces showed high variability in nitrogenase activity with elevated CO₂. Additions of dextrose-C had a positive effect on rates of C₂H₂ reduction in soil. There was no elevated CO₂ effect on soil nitrogenase activity. Plant cover affected soil response to C addition, with the largest response in plant interspaces. The mean rate of C₂H₂ reduction in soils either with or without C additions were 8.5±3.6 μmol C₂H₄ m⁻² h⁻¹ and 4.8±2.1 μmol m⁻² h⁻¹, respectively. Crust and soil δ¹⁵N and δ¹³C values were not affected by CO₂ treatment, but did show an effect of cover type. Crust and soil samples in plant interspaces had the lowest values for both measurements. Analysis of soil and crust [N] and δ¹⁵N data with the Rayleigh distillation model suggests that any plant community changes with elevated CO₂ and concomitant changes in litter composition likely will overwhelm any physiological changes in N₂-fixation.     

Impact of grazed grassland management on total N accumulation in soil receiving different levels of N inputs

Nitrogen balances and total N and C accumulation in soil were studied in reseeded grazed grassland swards receiving different fertilizer N inputs (100–500 kg N ha^?1 year^?1) from March 1989 to February 1999, at an experimental site in Northern Ireland. Soil N and C accumulated linearly at rates of 102–152 kg N ha^?1 year^?1 and 1125–1454 kg C ha^?1 year^?1, respectively, in the top 15 cm soil during the 10 year period. Fertilizer N had a highly significant effect on the rate of N and C accumulation. In the sward receiving 500 kg fertilizer N ha^?1 year^?1 the input (wet deposition + fertilizer N applied) minus output (drainflow + animal product) averaged 417 kg N ha^?1 year^?1. Total N accumulation in the top 15 cm of soil was 152 kg N ha^?1 year^?1. The predicted range in NH₃ emission from this sward was 36–95 kg N ha^?1 year^?1. Evidence suggested that the remaining large imbalance was either caused by denitrification and/or other unknown loss processes. In the sward receiving 100 kg fertilizer N ha^?1 year^?1, it was apparent that N accumulation in the top 15 cm soil was greater than the input minus output balance, even before allowing for gaseous emissions. This suggested that there was an additional input source, possibly resulting from a redistribution of N from lower down the soil profile. This is an important factor to take into account in constructing N balances, as not all the N accumulating in the top 15 cm soil may be directly caused by N input. N redistribution within the soil profile would exacerbate the N deficit in budget studies.     

Microbial immobilization and mineralization of dissolved organic nitrogen from forest floors

Dissolved organic nitrogen (DON) plays a key role in the N cycle of many ecosystems, as DON availability and biodegradation are important for plant growth, microbial metabolism and N transport in soils. However, biodegradation of DON (defined as the sum of mineralization and microbial immobilization) is only poorly understood. In laboratory incubations, biodegradation of DON and dissolved organic carbon (DOC) from Oi and Oa horizons of spruce, beech and cypress forests ranged from 6 to 72%. Biodegradation of DON and DOC was similar in most samples, and mineralization of DON was more important than microbial immobilization. Nitrate additions (0-10 mg N L⁻¹) never influenced either DON immobilization by microorganisms or mineralization. We conclude that soil microorganisms do not necessarily prefer mineral N over DON for meeting their N demand, and that biodegradation of DON seems to be driven by the microbial demand for C rather than N. Quantifying the dynamics of DON in soils should include consideration of both C and N demands by microbes.     

Differential hydrological response of biological topsoil crusts along a rainfall gradient in a sandy arid area: Northern Negev desert,Israel

Drylands are regarded as highly sensitive to climatic change. The putative positive relationship between average annual rainfall and runoff, assumed for areas between 100 and 300 mm ignores the fact that climatic change in drylands is not limited to climatic factors alone, but is often accompanied by a parallel change in surface properties. Data on rainfall, runoff and soil moisture regime were collected at five monitoring sites in a sandy area, along a rainfall gradient from 86 to 160 mm. Despite the uniform sandy substratum the frequency and magnitude of runoff declined with increasing annual rainfall. Under wetter conditions a thick topsoil biological crust develops. This crust is able to absorb and retain large rain amounts, limiting the depth to which water can penetrate, and therefore water availability for the perennial vegetation. In the drier area, the thin crust can absorb only limited rain amounts, resulting in surface runoff and deeper water infiltration at run-on areas. Our findings demonstrate the important role played by different types of biological soil crusts along the rainfall gradient considered, and question the generally held belief that higher rainfall necessarily leads to deeper water infiltration in sandy arid areas; and higher water availability for the perennial vegetation.     

Direct effects of litter decomposition on soil dissolved organic carbon and nitrogen in a tropical rainforest

To clarify how litter decomposition processes affect soil dissolved organic carbon (DOC) and soil dissolved nitrogen (DN) dynamics, we conducted a field experiment on leaf litter and collected DOC and DN from the underlying soil in a tropical rainforest in Xishuangbanna, southwest China. Principal components analysis (PCA) showed the first PCA axis (corresponding to degraded litter quantity and quality) explained 61.3% and 71.2% of variation in DOC and DN concentrations, respectively. Stepwise linear regression analysis indicated that litter carbon mass controlled DOC and hemicellulose mass controlled DN concentrations. Litter decomposition was the predominant factor controlling surface-soil DOC and DN dynamics in this tropical rainforest.     

Effects of fallow or planting wheat (Triticum aestivum L.) and fertilizing P or fertilizing P and N practices on soil carbon and nitrogen in a low-organic-matter soil

Soil carbon (C) and nitrogen (N) are important for maintaining soil fertility, and they are considerably affected by soil use and management. In the present study, we conducted an 8-year ?eld experiment on loessial dryland soil (Eum-Orthic Anthrosol, Food and Agriculture Organization of the United Nations (FAO)) in the southern Loess Plateau, China. We tested four soil management regimes—i.e., winter wheat (Triticum aestivum L.) cultivation with phosphorus (P) fertilization (WP), winter wheat cultivation with N and P fertilization (WNP), natural fallow (NF) and bare fallow (BF)—to evaluate their effects on soil C and N fractions. After 8 years, compared with the WNP treatment, the total soil organic nitrogen (SON) in the WP treatment decreased by 14.6% and 36.8%, and microbial biomass nitrogen (MBN) by 35.6% and 61.1%, at 0–20 and 20–40 cm soil depths, respectively. The soil heavy fraction nitrogen (HFN) and light fraction nitrogen (LFN) in the WP treatment also decreased by 36.6% and 39.4%, respectively. Furthermore, BF treatment decreased total soil organic carbon (SOC), heavy fraction carbon (HFC), LFN and MBN at both soil depths with average reductions of 43.4%. The NF treatment decreased light fraction carbon (LFC) by 17.0% at 0–20 cm soil depth, as well as MBN by 24.8% and 71.2%, and inorganic C by 29.1% and 23.8%, at 0–20 and 20–40 cm soil depths, respectively. There was no significant difference of microbial biomass C concentration among the WP, NF and BF treatments. These results confirmed that a lack of N fertilization decreased SON, BF reduced both SOC and SON, and NF decreased soil inorganic C. Therefore, the managements of a recommended rate of N fertilizer application and shortened time of bare fallow are critical for maintaining or increasing SON fraction sequestration, and natural fallow management is not a useful method for maintaining soil fertility in dryland in the Loess Plateau in China.

Abbreviations: HFC: heavy fraction carbon; HFN: heavy fraction nitrogen; LFC: light fraction carbon; LFN: light fraction nitrogen; MBC: microbial biomass carbon; MBN: microbial biomass nitrogen; SOC: soil organic carbon; SON: soil organic nitrogen     

Dissolved organic carbon in soil solutions: a comparison of collection methods

A field study was undertaken to compare dissolved organic carbon (DOC) concentrations in soil solutions obtained with three different sampling methods over a range of soil types. The sampling devices used were a tension‐free collector, a tension Prenart collector and a tension Rhizon collector. Samples were collected fortnightly for a year at seven sites in northern England, each collection being replicated three times. The soil solution DOC ranged from 1.3 g m^?3 in an acid ranker to 34.7 g m^?3 in a peat. The DOC concentrations obtained with the three methods correlated reasonably well (r² = 0.6–0.8) but with an indication of bias, as the best fit line differed from the 1:1 line. The tension‐free collector gave generally higher DOC concentrations except at very low concentrations (in the acid ranker soil). The DOC concentrations measured with the tension‐free collectors were significantly (P < 0.05) higher than those obtained with Prenart and Rhizon collectors at four and six sites, out of seven, respectively. Subsequent laboratory tests on tension‐free collected samples showed no DOC loss on filtration through 0.1 and 0.22‐μm membranes, whereas a significant loss of DOC occurred when tension‐free collected samples were subsequently passed through Prenart and Rhizon collectors, indicating a probable sampling artefact with the tension devices. The difficulties of acquiring representative soil solution samples are discussed, together with the advantages and disadvantages of tension and tension‐free methods.     

Temporal variation in pools of amino acids, inorganic and microbial N in a temperate grassland soil

Plants can take up intact amino acids, even in competition with soil microbes, yet we lack detailed information on which amino acids dominate the soil and whether amino acid composition varies seasonally. This study tested the hypotheses that 1) the pool of amino acid N is generally larger than inorganic N; 2) temporal changes in the concentration of amino acid N is related to changes in the size of the microbial N pool; and 3) amino acid N is dominated by simple, neutral amino acids during warm months, whereas during cold months the amino acid N is dominated by more complex aromatic and basic amino acids. Approximately every month for two years we collected soil from a temperate, sub-alpine grassland in the Snowy Mountains of Australia. We quantified exchangeable pools of amino acids, nitrate and ammonium in 1 M KCl extracts. Microbial N was quantified by chloroform fumigation. Averaged across the 21 monthly samples, nitrate was 13% of the quantified pool of soluble non-protein N, ammonium was 34% and amino acid N was 53%. These data are consistent with our hypothesis that the pool of amino acid N is larger than inorganic N. There was substantial variation between months in concentrations of amino acids and inorganic N, but no clear temporal pattern. Microbial N did not vary between months, and thus changes in amino acid N were unrelated to microbial N. Principal components analysis indicated multivariate groupings of the different pools of N that were broadly indicative of function and/or biosynthetic relationships. Thus PCA identified a grouping of aromatic amino acids (Phe and Try) with amino acids derived from oxaloacetate (Asp, Ala, Val, Leu, Ile), and a second group comprising microbial N, nitrate and glycine. The pool of exchangeable amino acid N was dominated by Arg (26% of amino N) Val (20%) Gln (18%), Try (8%) and Asn (8%). Contrary to our hypothesis, the composition of the amino acid pool did not vary in a consistent way between months, and there was no evidence simple amino acids were relatively more abundant in warm months and complex amino acids in cool months.     

Depth distribution of soil organic C and N after long-term soybean cropping in Texas

Crop management practices have potential to enhance subsoil C and N sequestration in the southern U.S., but effects may vary with tillage regime and cropping sequence. The objective of this study was to determine the impacts of tillage and soybean cropping sequence on the depth distribution of soil organic C (SOC), dissolved organic C (DOC), and total N after 20 years of treatment imposition for a silty clay loam soil in central Texas. A continuous soybean monoculture, a wheat–soybean doublecrop, and a sorghum–wheat–soybean rotation were established under both conventional (CT) and no tillage (NT). Soil was sampled after soybean harvest and sectioned into 0–5, 5–15, 15–30, 30–55, 55–80, and 80–105 cm depth intervals. Both tillage and cropping intensity influenced C and N dynamics in surface and subsurface soils. No tillage increased SOC, DOC, and total N compared to CT to a 30 cm depth for continuous soybean, but to 55 cm depths for the more intensive sorghum–wheat–soybean rotation and wheat–soybean doublecrop. Averaged from 0 to 105 cm, NT increased SOC, DOC, and total N by 32, 22, and 34%, respectively, compared to CT. Intensive cropping increased SOC and total N at depths to 55 cm compared to continuous soybean, regardless of tillage regime. Continuous soybean had significantly lower SOC (5.3 g kg⁻¹) than sorghum–wheat–soybean (6.4 g kg⁻¹) and wheat–soybean (6.1 g kg⁻¹), and 19% lower total N than other cropping sequences. Dissolved organic C was also significantly higher for sorghum–wheat–soybean (139 mg C kg⁻¹) than wheat–soybean (92 mg C kg⁻¹) and continuous soybean (100 mg C kg⁻¹). The depth distribution of SOC, DOC, and total N indicated treatment effects below the maximum tillage depth (25 cm), suggesting that roots, or translocation of dissolved organic matter from surface soils, contributed to higher soil organic matter levels under NT than CT in subsurface soils. High-intensity cropping sequences, coupled with NT, resulted in the highest soil organic matter levels, demonstrating potential for C and N sequestration for subsurface soils in the southern U.S.     

Spatial distribution and physiology of biological soil crusts from semi-arid central Spain are related to soil chemistry and shrub cover

Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen (Cladonia foliacea) and one moss (Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5 m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5 m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems.     

Elevated CO2, but not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland

Elevated CO₂ and defoliation effects on nitrogen (N) cycling in rangeland soils remain poorly understood. Here we tested whether effects of elevated CO₂ (720 μl L⁻¹) and defoliation (clipping to 2.5 cm height) on N cycling depended on soil N availability (addition of 1 vs. 11 g N m⁻²) in intact mesocosms extracted from a semiarid grassland. Mesocosms were kept inside growth chambers for one growing season, and the experiment was repeated the next year. We added ¹⁵N (1 g m⁻²) to all mesocosms at the start of the growing season. We measured total N and ¹⁵N in plant, soil inorganic, microbial and soil organic pools at different times of the growing season. We combined the plant, soil inorganic, and microbial N pools into one pool (PIM-N pool) to separate biotic + inorganic from abiotic N residing in soil organic matter (SOM). With the ¹⁵N measurements we were then able to calculate transfer rates of N from the active PIM-N pool into SOM (soil N immobilization) and vice versa (soil N mobilization) throughout the growing season. We observed significant interactive effects of elevated CO₂ with N addition and defoliation with N addition on soil N mobilization and immobilization. However, no interactive effects were observed for net transfer rates. Net N transfer from the PIM-N pool into SOM increased under elevated CO₂, but was unaffected by defoliation. Elevated CO₂ and defoliation effects on the net transfer of N into SOM may not depend on soil N availability in semiarid grasslands, but may depend on the balance of root litter production affecting soil N immobilization and root exudation affecting soil N mobilization. We observed no interactive effects of elevated CO₂ with defoliation. We conclude that elevated CO₂, but not defoliation, may limit plant productivity in the long-term through increased soil N immobilization.     

Response of ecosystem respiration to soil water and plant biomass in a semiarid grassland

Abstract

The Mongolian steppe zone constitutes a major part of East Asian grasslands. The objective of this study was to evaluate the quantitative dependence of ecosystem respiration (R_eco) on the environmental variables of soil water and plant biomass in a semiarid grassland ecosystem. We determined R_eco using opaque, closed chambers in a Mongolian grassland dominated by graminaceous perennial grasses during six periods: July 2004, May 2005, July 2005, September 2005, June 2006, and August 2009. Using the data collected when soil water content and aboveground biomass were relatively constant, values of R_eco were fitted to an exponential temperature function, and the standardized rate of R_eco at 20°C (R₂₀) and temperature sensitivity (Q₁₀) of R_eco were calculated for each measurement plot and period. The results indicate that aboveground biomass significantly affected the variation in R₂₀, and the relationship was expressed with a linear model. The R₂₀ residuals of the linear biomass model were highly correlated with soil water content by a quadratic function. The Q₁₀ values showed a weak positive relationship with soil water content. Temporal and spatial variations in R_eco were well predicted by the exponential temperature model with R₂₀, which relates to aboveground biomass and soil water content, and with Q₁₀, which relates to soil water content.     

Predicting soil N mineralization: Relevance of organic matter fractions and soil properties

Distinct extractable organic matter (EOM) fractions have been used to assess the capacity of soils to supply nitrogen (N). However, substantial uncertainty exists on their role in the N cycle and their functional dependency on soil properties. We therefore examined the variation in mineralizable N and its relationship with EOM fractions, soil physical and chemical properties across 98 agricultural soils with contrasting inherent properties and management histories. Mineralizable N was determined by aerobic incubation at 20 °C and optimum moisture content for 20 weeks. We used multivariate statistical modelling to account for multi-collinearity, an issue generally overlooked in studies evaluating the predictive value of EOM fractions. Mineralization of N was primarily related to the size of OM pools and fractions present; they explained 78% of the variation in mineralizable N whereas other soil variables could explain maximally 8%. Both total and extractable OM expressed the same soil characteristic from a mineralization perspective; they were positively related to mineralizable N and explained a similar percentage of the variation in mineralizable N. Inclusion of mineralizable N in fertilizer recommendation systems should be based on at least one OM variable. The most appropriate EOM fraction can only be identified when the underlying mechanisms are known; regression techniques are not suitable for this purpose. Combination of single EOM fractions is not likely to improve the prediction of mineralizable N due to high multi-collinearity. Inclusion of texture-related soil variables or variables reflecting soil organic matter quality may be neglected due to their limited power to improve the prediction of mineralizable N.     

Comparison of uptake of different N forms by soil microorganisms and two wet-grassland plants: A pot study

Two common plant species of temperate wet grasslands, Carex acuta and Glyceria maxima, were tested for their preferences in the uptake of different nitrogen (N) sources (amino acid, ammonium, nitrate) and their ability to compete for these sources with soil microorganisms. The experiment was a one-day incubation study with plants growing in soil obtained from the field, which was supplied with a solution containing the three N sources, one at a time labeled with ¹⁵N. The bulk of the N demand of both species was covered by nitrate-N, which was the dominant N form in the soil at the time of the experiment. Ammonium-N was taken up less strongly, and organic N formed only a negligible part of their nutrition. The assimilated inorganic N was preferentially transported to apical meristem of the youngest leaf, while organic N remained mostly in the roots. The fast-growing Glyceria took up more N and was a better competitor vis-à-vis soil microbes for rarer N forms than Carex. However, both plants were poor competitors for N vis-à-vis soil microbes, irrespective of the N form. Microbes took up nitrate ca. five times faster and organic N more than a hundred times faster than plants. Correspondingly, the calculated turnover time of microbial N was 17 days, compared to 40 days for N in plant roots. A significant amount of added ¹⁵N was found at non-exchangeable sites in the soil, which points to the importance of microbial N transformation and abiotic N fixation for N retention in soil. In summary, the preferential assimilation of inorganic N by the wetland plants studied here and their poor ability to compete for N with soil microbes over the short term agree with the results of studies carried out with other species from temperate grasslands.