Inorganic Pool Research Articles

Plant Growth, Mycorrhizal Association, Nutrient Uptake and Phosphorus Dynamics in a Volcanic-Ash Soil in Colombia as Affected by the Establishment of Tithonia diversifolia

ABSTRACT Tithonia diversifolia has the ability to sequester nutrients from soil in its tissues, including P, and has been shown to be useful for cycling nutrients via biomass transfer and improved fallow. We investigated the effects of its establishment from bare root seedlings (plantlets) and vegetative stem cuttings (stakes) on shoot and root growth characteristics, arbuscular-mycorrhizae (AM) associations, nutrient acquisition and utilization, and P dynamics in a fine-textured volcanic-ash soil (Oxic Dystropept) of a mid-altitude hillside in southwestern Colombia. One year after establishment, the following determinations were made: leaf area index; shoot and root N, P, K, Ca, and Mg acquisition; AM root infection; AM fungal spores per 100 g soil; soil chemical characteristics; and P fractionation into inorganic (Pi) and organic (Po) pools. AM root infection in both coarse and fine roots was significantly greater in plants established from plantlets than those established from stakes with differences of 21 and 31%, respectively. Nutrient uptake efficiency (μg of shoot nutrient uptake per m of root length) and use efficiency (g of shoot biomass produced per g of shoot nutrient uptake) for N, P, K, Ca, and Mg were also greater with plants established from plantlets than those established from stakes. Improved nutrient acquisition could be attributed to relief from P stress and possibly uptake of some essential micronutrients resulting from AM association. High soil variability masked the effect of the establishment method on phosphorus pools, and neither the biologically available P (H2O-Po, resin-Pi, and NaHCO3-Pi and -Po) nor the moderately resistant P (NaOH-extractable P) was significantly affected, although plantlets had higher values. This study has shown that on this soil when Tithonia is to be used as a fallow species, the use of plantlets as compared to the stake method of establishment is better for nutrient acquisition and recycling.

Carbon dioxide fluxes in a semiarid environment with high carbonate soils

Carbon dioxide is increasing in the atmosphere due to human activities. Over long time periods semiarid soils have sequestered inorganic carbon to accumulate the third largest global carbon pool. The hypothesis for this study was that these soils are maintaining this carbon pool under present climatic conditions and are a sink for some of the increasing atmospheric carbon. Bowen ratio systems were used to measure CO 2 fluxes from a brush and a grass community with different soil types over 4 years in southeastern Arizona. Aboveground biomass and soil samples taken in spring and fall were analyzed to determine seasonal changes in carbon content. Contrary to the hypothesis, both sites were found to be losing carbon annually. Absence and presence of rainfall were important carbon flux driving forces. The brush site, with higher inorganic carbon in the soil, had an average annual loss of 144 g C m −2 and the grass site a loss of 128 g C m −2 from organic and inorganic sources. Average annual daytime CO 2 flux from the brush site was a loss of 26 g C m −2, while the grass site had a gain of 86 g C m −2. Based on measured average annual aboveground biomass data and estimates of belowground biomass, the brush site sequestered 80 g C m −2 and the grass site 135 g C m −2 into biomass during the growing season. Analysis of combined inorganic soil carbon data from both sites showed a significant seasonal difference with more in the fall season than in spring. The average annual fall season soil inorganic carbon was 22.5 g kg −1 and the spring season was 19.4 g kg −1 to a depth of 30 cm. This significant seasonal difference indicated that some of the measured CO 2 fluxes were into and out of the inorganic carbon pool. The source of carbon for the measured annual losses from these sites was concluded to be from the large inorganic carbon pool with carbon cycling through both the organic and inorganic pools at the sites.

Loss of mass and nitrogen from biosolids applied to a pine plantation

Application of biosolids (sewage sludge suitable for beneficial use) to tree plantations has the potential to increase productivity, but also to leach nitrogen (N) if application rates are excessive. This project aimed to determine rates and mechanisms of loss in mass and N from 3 dewatered biosolids (1 from aerobic and 2 from anaerobic digestion of waste water) applied to the surface of a radiata pine plantation. Biosolids were applied at operational rates (nominally 30�dry�t/ha), supplying between 965 and 1425 kg N/ha, of which 210–550 kg/ha was NH4+-N. Results showed that, for mass, biosolids were characterised by 2 discrete pools, an active pool comprising 52–67% of total mass, and a pool that can be considered inert within the time frame of 5 years. Most of the active organic matter and carbon was lost within 2 years; much of the loss of total mass was derived from the inorganic pool. There was a large and rapid loss of total N from biosolids during the first 3 weeks after application, most of which was accounted for by depletion of NH4+-N, presumably by volatilisation. Much of the subsequent release of N was apparently through mineralisation, with 300 kg/ha (anaerobically digested) to 500 kg/ha (aerobically digested) being released in the first year. Applications at these rates have the potential to contaminate waterways with N in the first year after application, the risk being greatest from aerobically digested biosolids. In subsequent years there appears to be little risk. However, applications at lower rates appear well suited to forests, with a small proportion of the inorganic N providing an initial input to soil and mineralisation of organic N providing a steady supply to the plantation over a number of years.

Seasonal Variations in Sediment Sulfur Cycling in the Ballastplaat Mudflat, Belgium

Sulfate reduction rate (SRR) and pools of reduced inorganic sulfur, acid volatile sulfide (AVS), chromium reducible sulfur (CRS), and elemental sulfur (So), were studied from June 1990 till March 1992 at two locations on the Ballastplaat mudflat in the Scheldt estuary. The sediment composition at station A was mainly sand with low organic content whereas sediments at station B were dominated by silt and clay with high organic content. SRR was positively related to temperature; more pronounced at station B (Ea=190 kJ mol−1) than at station A (Ea=110 kJ mol−1). The maximum SRR values observed equalled 14 μmol cm−3 d−1 at station B and 1 μmol cm−3 d−1 at station A. AVS was the dominant radiolabelled end product of the sulfate reduction reaction, except in surface sediments where pyrite and So were more dominant. However, CRS was the predominant reduced inorganic sulfur pool in the sediments. Both AVS and CRS pools showed temporal variations out of phase with SRR. SRR peaked in summer, while the concentrations of AVS and CRS were highest in fall. The accumulation of AVS and CRS started late summer after depletion of oxidants, which had accumulated during winter and spring. The estimated annual SRR and thus sulfide production in the upper 15 cm of station B was of the order of 100 mol m−2 yr−1, and at station A of the order of 12 mol m−2 yr−1. The sulfur mass balance shows that only a very small fraction, if any, of the produced sulfide is retained as reduced inorganic sulfur in the sediment.

Changes in organic and inorganic sulfur fractions and S mineralisation in a Typic Haplustert after long-term cropping with different fertiliser and organic manure inputs

Changes in the status of organic and inorganic pools of soil sulfur (S), total nitrogen (N), and organic carbon, and their interrelationships and S mineralisation were examined on a Typic Haplustert soil after a 27-year long-term cropping. The results indicated that NPK(+S) applied for 27 years at 50%, 100%, and 150% of optimum recommended rates and 100% NPK(+S) with farmyard manure (FYM) increased the organic C and total N status of soil compared with the control and the initial status of the soil. Intensive cropping with continuous use of 100% NPK without S resulted in depletion of total, organic, and inorganic S concentrations by 18.13%, 17.80%, and 21.72%, respectively, over the control, while the status of total, organic and inorganic S improved in plots that received graded rates of S with NPK and NPK plus FYM. Intensive cropping with continuous use of S-free NPK fertiliser (100% NPK-S) resulted in the widest C: N: S ratio in the soil. NaHCO 3 -extractable total, inorganic, and organic S fractions and NaOH-extractable total and inorganic S fractions were found to be better indices of soil S mineralisation than CaCl 2 -extractable inorganic S. Cumulative amounts of S mineralised during a 14-week incubation period varied between 2.3 and 21.3 mg S/kg soil and increased with an increase in the rates of S applied, along with NPK fertilisers. Incorporation of FYM with 100% NPK(+S) resulted in greater cumulative mineralised S over 100% NPK(+S) alone. The cumulative mineralised S showed a quadratic relationship with the duration of incubation in all the treatments.

Release rates of Al from inorganic and organic compounds in a sandy podzol, during laboratory experiments

Research with respect to release rates of Al and Al concentrations in the soil solution has led to two contrasting hypotheses, stressing either the importance of kinetically constrained dissolution reactions or the role of complexation of Al to soil organic matter. The existence of two contrasting hypotheses is partly due to the fact that the interpretation of existing results is hampered by the lack of specificity of the extractants used to quantify the organic and inorganic pools of Al and by the fact that simultaneous measurements on the release of Al and the change in Al pools are missing. This study was conducted to obtain more information on the release rates of Al from various amorphous pools and to evaluate the validity of the above hypotheses. To overcome the above-mentioned shortcomings both release rates of Al and changes in the various Al pools were measured during the experiment. Experiments were conducted for Ap, Bs and C horizons from a sandy podzol. Results indicated that Al was released from both the inorganic Al pool (oxalate minus pyrophosphate extractable Al) and the strongly bound organic Al pool (pyrophosphate minus CuCl 2 extractable Al) according to a first-order reaction. The decline in the weakly bound organic Al pool (CuCl 2 extractable minus adsorbed Al) could be described by a rapid dissolution of part of the pool, combined with an equilibrium reaction describing the complexation of Al to organic sites. Rate constants were highest for the non-complexed part of the weakly bound organic Al pool, intermediate for the inorganic Al pool and lowest for the strongly bound organic Al pool. Large differences exist in rate constants of the organic pools between the three horizons. Differences between the rate constants of the inorganic Al pool for the three horizons were not significant.

Phosphorus distribution and availability in response to dairy manure applications1

As livestock operations become larger and concerns about water quality become greater, attention must be paid to the composition of animal manure and its potential impact on the environment. One current concern involves the amount and forms of phosphorus (P) being added to land with manure. The objective of this experiment was to determine the forms and availability of P in soils receiving 4 years of continuous dairy manure applications. Soil samples were collected from lysimeter plots established in 1991 to study the impact of dairy manure applications on surface water and groundwater. Soil P was fractionated into available (NaHCO3), iron (Fe)‐ and aluminum (Al)‐bound (NaOH), and calcium (Ca)‐bound (HC1) forms. These data were related to manure application rates, soluble P concentrations, and anion exchange membrane (AEM) bound P. Results indicate that the potential to move P by leaching through these loessial soils is very low even at high manure application rates. Large manure additions resulted in increases in all P forms; however, the inorganic pools increased more than the organic pools. The AEM values were a good tool for predicting potential P movement by soil erosion or runoff with membrane bound P being strongly correlated with manure application rate (r2=0.82) and available P (NaHCO3). Best management practices for manure disposal need to consider the potential for P movement through erosion and runoff, and the AEM technique provides a means for evaluating this potential.

Fate and recovery of 15N-labelled fertilizer urea applied to winter wheat in spring in the Canterbury region of New Zealand

15N-labelled fertilizer urea was applied at increasing rates (0–200 kg N/ha), in spring, to winter wheat crops in the Canterbury region of New Zealand in three successive seasons (1993/94, 1994/95 and 1995/96). Recovery of fertilizer N by the crop (grain, chaff, straw and roots) ranged from 43–58% (mean 48%). The quantity of fertilizer N retained in the soil (0–40 cm), at harvest, ranged from 26–42%. Of the labelled N present in the soil, over 95% was present in organic form and 60–80% was retained in the surface 0–10 cm layer. Since soil organic matter represents a substantial sink for fertilizer N there is a need to characterize the nature of this organic pool of N more fully. The quantity of inorganic N present in the soil profile at harvest ranged from 20–46 kg N/ha and labelled fertilizer-derived N contributed less than 16% (mean 9·2%) to this inorganic pool. Loss of fertilizer N from the crop/soil system (i.e., labelled N not recovered in the crop or soil at harvest) varied from 12–26% (mean 18%). Losses were attributed mainly to denitrification since conditions were not conducive for ammonia volatilization or leaching of nitrate. In agreement with European research, it was concluded that almost all of the N at risk of leaching over the winter originates from mineralization of soil organic N and not from unused fertilizer-N applied in spring.

Photosynthesis and carbon storage between tides in a brown alga, Fucus vesiculosus

Intertidal macroalgae may spend a significant part of their lives in air. During photosynthesis in air, they encounter much lower concentrations of inorganic carbon than in seawater. Because they accumulate inorganic carbon from seawater, we investigated whether they similarly accumulate it from air. We measured photosynthesis in the intertidal species Fucus vesiculosus L. during 1990 and 1991 with a gas-phase O2 electrode or CO2-exchange apparatus in air and with a liquid-phase O2 electrode in seawater. Maximum rates were rapid and similar in air and seawater regardless of the method. Tissue from seawater could carry on photosynthesis in CO2-free air, indicating that carbon was stored in the tissue. After 2 h, this store was depleted and photosynthesis ceased. Supplying CO2 in air replenished the store. Under identical conditions, terrestrial C3 and C4 species showed no evidence of this store, but a CAM (crassulacean acid metabolism) species did. However, in contrast to the CAM behavior, F. vesiculosus did not store CO2 significantly in the dark. We found a small acid-releasable pool of carbon in the tissue that disappeared as photosynthesis depleted the carbon store. However, the pool was too small to account for the total carbon stored. While CO2 was being acquired or released from the store in the light, photosynthesis was not inhibited by 21% O2. These results indicate that there are two parallel paths for the supply of CO2 to photosynthesis. The first depends on inorganic carbon in seawater or in air and supports rapid photosynthesis. The second involves CO2 slowly released from an organic intermediate. The release protects CO2 fixation from the inhibitory effects of 21% O2. Photosynthesis in F. vesiculosus thus appears to be C3-like in its rapid fixation of CO2 from a small inorganic pool into phosphoglycerate. However, it is C4-like in its pre-fixation of carbon in an organic pool in the light, and is CAM-like in its ability to slowly use this pool as a sole source of CO2. The organic pool may serve to protect photosynthetic CO2 fixation against the inhibitory effects of O2 in air and in the boundary layer in seawater.

Stable isotope response to lake eutrophication: Calibration of a high‐resolution lacustrine sequence from Baldeggersee, Switzerland

Stable isotope analyses of discrete seasonal layers from a 108‐yr annually laminated freeze‐core from Baldeg‐gersee, a small, eutrophic lake in central Switzerland, provide information on the climatological and environmental factors, including lake eutrophication, that control oxygen and carbon isotopic composition of epilimnic biologically induced calcite precipitate.During the last 100 yr, Baldeggersee has undergone major increases in productivity and eutrophication in response to nutrient loading from agriculture and industrialization in the lake's watershed. Calibration of the isotopic signal in Baldeggersee to historical limnological data quantitatively links evidence of isotopic depletion in the sedimented calcite to trophic state of the lake. δ18O values from the spring/summer “light” sediment layers steadily diverged to more depleted values in response to historical eutrophication: measured δ18O values were up to 21.5‰ more negative than calculated equilibrium δ18O values. Evidence for 13C depletion in the calcite, relative to equilibrium values, is more difficult to ascertain because of an overall dominance of isotopic enrichment in the dissolved inorganic pool as productivity in Baldeggersee increases. A positive association exists between the degree of oxygen‐18 depletion and the calcite crystal size. Thus, large amorphous calcite grains can be used as a proxy for recognizing apparent isotopic nonequilibrium in sediment sequences from highly productive lacustrine environments from all geologic time scales.In contrast to the light layers, the oxygen isotopic composition of the calcite in the late summer/fall “dark” sediment layers is unaffected by the apparent isotope nonequilibrium. Oxygen and carbon isotope values from the dark laminae in the Baldeggersee sediment therefore provide environmental and climatological proxies that can be calibrated with known environmental and regional climate data for the last century.

Mineralization and microbial immobilization of N and P in arctic soils in relation to season, temperature and nutrient amendment

In situ summer, winter and annual net N and P mineralization and microbial immobilization of mineralized nutrients were measured in a low-altitude subarctic/alpine dwarf shrub heath and in a high-altitude fellfield. Net mineralization was determined by using the buried-bag technique, which was combined with fumigation–extraction to recover microbial nutrients and estimate microbial nutrient immobilization. The measurements were carried out in unperturbed plots and in plots, which had been subjected to elevated temperature and fertilizer addition for 5 years before the experiment started. During the growing season, the microbes in the unperturbed plots immobilized the major part of the mineralized nutrients, which resulted in low net mineralization. This occurred also during winter, when we assumed that nutrients should be released during microbial die-back. However, we found no evidence for a large microbial winter die-back and the surviving microbial biomass even immobilized extra nutrients. Soil temperature enhancement by ca. 2°C in general increased, or tended to increase, net mineralization. However, there was not necessarily any strong correlation between net mineralization and temperature because temperature-induced increase in gross mineralized nutrients could either lead to nutrient immobilization in the microbes or the nutrients could be released to the soil inorganic pool. Fertilizer additions had no major effect on net nutrient mineralization or nutrient immobilization. However, the ligno-cellulose index, which has been used as a predictor of substrate quality and usually correlates negatively with decomposition rate, also appeared to be a good predictor of gross mineralization, but a poor predictor of net mineralization.

Methane fluxes and nitrogen dynamics from a drained fenland peat

Rates of methane uptake were measured in incubation studies with intact cores from adjacent fenland peats that have been under arable management and woodland management for at least the past 30 years. On two separate occasions the woodland peat showed greater rates of uptake than the arable peat. These rates ranged from 23.1 to 223.3 μg CH4 m–2 day–1 for the woodland peat and from 29.6 to 157.6 μg CH4 m–2 day–1 for the arable peat. When the peats were artificially flooded there was a decrease in the rate of methane oxidation, but neither site showed any net efflux of methane. 15N isotopic dilution was used to characterise nitrogen cycling within the two peats. Both showed similar rates of gross nitrogen mineralisation (3.58 mg N kg–1 day–1, arable peat; 3.54 N kg–1 day–1, woodland peat) and ammonium consumption (4.19 arable peat and 4.70 mg N kg–1 day–1 woodland peat). There were significant differences in their inorganic ammonium and nitrate pool sizes, and the rate of gross nitrification was significantly higher in the woodland peat (4.90 mg N kg–1 day–1) compared to the arable peat (1.90 mg N kg–1 day–1). These results are discussed in the light of high atmospheric nitrogen deposition.

Forms of Soil Phosphorus in Selected Hydrologic Units of the Florida Everglades

AbstractThe Florida Everglades wetland ecosystem is subject to changes in hydroperiod and nutrient loading, resulting in soil P enrichment and changes in vegetation communities. The objectives of this study were to: (i) quantify the forms of inorganic and organic P in soils from four hydrologic units of the Everglades, and (ii) develop empirical relationships among various soil P forms. Soil samples from selected hydrologic units, including the Water Conservation Areas (WCAs) and the Holey Land Wildlife Management Area (HWMA), were obtained at various locations along transects perpendicular to each nutrient input source, while selected field sites were sampled in the Everglades Agricultural Area (EAA). Spatial distribution of total P in the surface 0‐ to 10‐cm soil depth showed distinct gradients in the WCAs and HWMA soils, with high total P in soils closer to sources (canals and inflow structures) than in interior, unimpacted areas. Soil ash content and bulk density were also altered as a result of soil subsidence (for EAA soils), hydrology, and nutrient loading (for the WCAs and the HWMA soils). Influence of P loading was primarily confined to the top 30‐cm soil depth, with about one‐third of the P stored in the inorganic pool (primarily as Ca‐ and Mg‐bound P), and the remainder present as organic P. Inorganic P content was higher in surface soils and decreased with depth. Soil P enrichment indicated that for approximately 5 km from the inflow structures or canals, soils have been impacted by nutrient loading. Empirical relationships developed in this study should be useful for estimating soil P forms at the landscape level, using total P data available for a large number of sites throughout the Everglades region.

Soil microbiological parameters as indicators of soil quality under improved fallow management systems in south-western Nigeria

Soil microbiological and soil biochemical parameters (pH, exchangeable basic cations, inorganic and organic phosphorus pools, total organic carbon and total nitrogen, microbial biomass carbon, acid and alkaline phosphatase, β-glucosidase and protease activity) were identified as indicators of soil quality under improved fallow management systems with senna, leucaena and pueraria on severely degraded and non-degraded soil. Principal component analysis demonstrated that soil organic matter related nutrient dynamics was the major contributor to explain the total variance (>80%) of the sites under the prevailing experimental conditions. Highest loadings with the major principal component were provided by microbial biomass, alkaline phosphatase, total N, β-glucosidase and organic C. Contrasting fallow management systems (alley cropping, live mulch, planted fallow, controls in long-term experiments) at three sites differing in degree of soil degradation could be evaluated adequately by these indicators. β-Glucosidase indicated soil quality changes better than total organic carbon. Alkaline phosphatase was more sensitive than microbial biomass in characterizing the effects of improved fallow management on site degradation. Acid phosphatase and protease were not considered sensitive indicators for soil quality evaluations of these long-term management trials. Pueraria sustained soil quality on the non-degraded site but did not improve the severely degraded site, suggesting that pueraria is a soil fertility maintenance crop. In contrast, senna improved the degraded sites and more so on the most severely degraded site. Apparently, senna can be considered a suitable candidate for soil restoration purposes.

Short-term immobilisation and crop uptake of fertiliser nitrogen applied to winter wheat: effect of date of application in spring

Previous studies on the fate of fertiliser nitrogen applied to winter wheat in temperate climates have shown that nitrogen (N) applied early, at tillering for wheat, was less efficiently taken up than N applied later in the growth cycle. We examined the extent to which the soil microbial N immobilisation varied during the wheat spring growth cycle and how microbial immobilisation and plant uptake competed for nitrogen. We set up a pulse-15N labelled field experiment in which N was applied at eight development stages from tillering (beginning of March) to anthesis (mid-June). Each application was 50 kg N ha-1 as 15N labelled urea except for the first application which was 25 kg N ha-1. The distribution of fertiliser 15N in shoots, roots, mineral and organic soil N was examined by destructive sampling 7 and 14 days after each 15N pulse. The inorganic 15N pool was almost depleted by day 14. The N uptake efficiency increased with later applications from 45% at tillering to 65% at flowering. N immobilisation was rather constant at 13–16% of N applied, whatever the date of application. The increase in plant 15N uptake resulted in an increase in the total 15N recovery in the plant-soil system (15N in soil +15N in plant), suggesting that gaseous losses were lower at the later application dates.

Modelling of soil nitrogen forms after organic amendments under controlled conditions

This N model is derived from the C model (MOMOS) published by Sallih and Pansu (1993). Both models were fitted to experimental data obtained from an incubation experiment of 14C- and 15N-labelled plant material in two soils with contrasting characteristics over 2 y under controlled laboratory conditions. The N model uses the same structure as the C model with five organic compartments: labile plant material (V L); stable plant material (V R); microbial biomass (B); labile humified material (A); and stable humified material (H). Two additional compartments are included: exchangeable NH 4N and NO 3N. The transfers of N between the organic compartments are described according to first-order kinetics. Nitrogen transferred to the NH 4 compartment results from the balance between the output and input of all the organic compartments. Ammonium N output is split between nitrification and immobilisation into B (77%) and H (23%). Nitrification is controlled by a microbial growth law. The N model uses the parameters defined in the C model, with a constant multiplying factor for the N kinetic constants ( fn = 1.4 and 1.9 in soil 1 and 2, respectively). The additional parameters defined for inorganic pools are comparable in both soils. The predicted 14C-to- 15N ratios of each organic compartment agree with the experimental data, showing a rapid incorporation of 15N into microbial biomass and a gradual build-up into stable humified compounds. The model was adjusted to 15N experimental data from five time series (each series containing from five to eight sampling occasions), and was validated using five series of corresponding total N data. MOMOS-C and -N formulation is relatively simple, combining mechanistic first-order kinetic models and growth models. The predictions are in agreement with 16 data sets including different forms of organic 14C (three series), total C (three series), organic and inorganic 15N and total N.

Organic and inorganic nitrogen pools in talus fields and subtalus water, Green Lakes Valley, Colorado Front Range

Organic and inorganic pools of nitrogen (N) were measured in talus fines or ‘soils’ and subtalus water during the summer of 1995 in the alpine Green Lakes Valley catchment of the Colorado Front Range. Nineteen talus soil samples were divided into four classes: subtalus dry, subtalus wet, surface vegetated and surface bare. The size of the individual talus soil patches ranged from 0·5 to 12·0 m2 in area, with bulk density ranging from 0·98 to 1·71 kg m−3 and soil texture ranging from sandy gravel in the subsurface talus to a loam in the vegetated surface. All samples contained KCl-extractable NH+4 and NO−3, organic N and carbon (C), and 17 of 19 samples contained microbial biomass. The mean subtalus values for KCl-extractable NH−4, of 3·2 mg N kg−1, and NO−3, of 1·0 mg N kg−1, were comparable with developed alpine soils on Niwot Ridge. Average microbial biomass in subtalus soils of 5·4 mg N kg−1 and total N of 1000 mg N kg−1 were about an order of magnitude lower than alpine tundra soils, reflecting the reduced amount of vegetation in talus areas. However, these measurements in surface-vegetated patches of talus were comparable with the well-developed soils on Niwot Ridge. These measurements in talus of microbial biomass, total N and KCl-extractable NH+4 and NO−3, show that there is sufficient biotically conditioned ‘soil’ within talus fields to influence the solute content of interstitial waters. Mean NO−3 concentrations of 20 μeq l−1 from 29 samples of subtalus water were significantly higher than the 6·7 μeq l−1 in snow, while NH+4 concentrations in subtalus water of 0·7 μeq l−1 was significantly lower than in snow at 5·2 μeq l−1 (p = 0·001). Nitrate concentrations in subtalus water were significantly (p < 0·0001) correlated with concentrations of geochemical weathering products such as Ca2+ (r2 = 0·84) and silica (r2 = 0·49). The correlation of NO−3 in subtalus water with geochemical weathering products suggests that NO−3 concentrations in subtalus water increased with increased residence time, consistent with a biological source for this subtalus water NO−3. The high NO−3 concentrations in subtalus water compared with atmospheric deposition of NO−3 suggests that NO−3 in talus fields may contribute to NO−3 in stream waters of high-elevation catchments. © 1997 John Wiley & Sons, Ltd.

Organic and inorganic nitrogen pools in talus fields and subtalus water, Green Lakes Valley, Colorado Front Range

Organic and inorganic pools of nitrogen (N) were measured in talus fines or 'soils' and subtalus water during the summer of 1995 in the alpine Green Lakes Valley catchment of the Colorado Front Range. Nineteen talus soil samples were divided into four classes: subtalus dry, subtalus wet, surface vegetated and surface bare. The size of the individual talus soil patches ranged from 0.5 to 12.0 m 2 in area, with bulk density ranging from 0.98 to 1.71 kg m -3 and soil texture ranging from sandy gravel in the subsurface talus to a loam in the vegetated surface. All samples contained KCI-extractable NH 4 + and NO 3 - , organic N and carbon (C), and 17 of 19 samples contained microbial biomass. The mean subtalus values for KCI-extractable NH 4 - , of 3.2 mg N kg -1 , and NO 3 - of 1.0 mg N kg -1 , were comparable with developed alpine soils on Niwot Ridge. Average microbial biomass in subtalus soils of 5.4 mg N kg -1 and total N of 1000 mg N kg -1 were about an order of magnitude lower than alpine tundra soils, reflecting the reduced amount of vegetation in talus areas. However, these measurements in surface-vegetated patches of talus were comparable with the well-developed soils on Niwot Ridge. These measurements in talus of microbial biomass, total N and KCl-extractable NH 4 + and NO 3 - , show that there is sufficient biotically conditioned 'soil' within talus fields to influence the solute content of interstitial waters. Mean NO 3 - concentrations of 20 μeq l -1 from 29 samples of subtalus water were significantly higher than the 6.7 μeq l -1 in snow, while NH 4 + concentrations in subtalus water of 0.7 μeq l -1 was significantly lower than in snow at 5.2 μeq l -1 (p = 0.001). Nitrate concentrations in subtalus water were significantly (p < 0.0001) correlated with concentrations of geochemical weathering products such as Ca 2+ (r 2 = 0.84) and silica (r 2 = 0.49). The correlation of NO 3 - in subtalus water with geochemical weathering products suggests that NO 3 - concentrations in subtalus water increased with increased residence time, consistent with a biological source for this subtalus water NO 3 - . The high NO 3 - concentrations in subtalus water compared with atmospheric deposition of NO 3 - suggests that NO 3 - in talus fields may contribute to NO 3 - in stream waters of high-elevation catchments.

Nitrogen cycling in brigalow clay soils under pasture and cropping

Clay soils previously under native brigalow (Acacia harpophylla) forest are highly productive under annual cropping in central and southern Queensland. Grass pastures sown on these soils are initially productive, but deteriorate after several years because of N-stress (rundown). The aim of this work was to compare the patterns of N cycling in these pasture and cropping systems, in order to understand the rundown of the pastures. A small pulse of 15N-labelled ammonium sulfate was applied in the field to sites cropped with sorghum (Sorghum bicolor) and under green panic (Panicum maximum var. trichoglume) pasture, and its movement through the soil and plant pools was followed over 2 growing seasons. There were large differences in the cycling of 15N in the cropping and pasture systems. Under sorghum, 60% of the applied 15N was immobilised by microorganisms after 4 days, after which it was re-mineralised. Plant uptake and stabilisation in soil organic matter and clay were relatively slow. The first sorghum crop assimilated 14% of the applied 15N. During the second season, most of the 15N was stabilised in soil organic matter and clay (maximum 42%). A significant proportion of the 15N remained in the soil inorganic pool over the 2 seasons. Under green panic, 82% of the 15N left the soil inorganic pool within 4 days and entered the microbial biomass, soil organic matter, and the plant. Uptake and re-release of 15N were most rapid in the microbial biomass (maximum uptake 34% of applied after 4 days). Microbial immobilisation and re-mineralisation were, however, slower under green panic than under sorghum. The pasture plant accumulated 32% of the applied 15N, two-thirds of which was re-released in the second season. Stabilised N represented up to 62% of the applied 15N, and was consistently greater under green panic than under sorghum. After 2 seasons, 15N was released from the stabilised N pool in both systems, at approximately the same rate as it had been stabilised. At the end of the experiment, 40% of the applied 15N was unaccounted for in the pasture system, and 66% in the crop system. The reduced N availability in the pasture system was attributed to immobilisation of N in soil organic matter and clay, plant material, and, to a lesser extent, soil microbial biomass. This immobilisation resulted from the large accumulation of carbonaceous plant residues.

Nitrogen limitation of the microbial biomass in an old-growth forest soil

:We assessed whether the microbial biomass of an old-growth mixed-conifer forest soil was N limited by applying the equivalent of 450 kg N/ha as (NH4)2SO4 at a 2-cm depth to mineral soil contained in 10-cm deep microplots. An increase in the size of the soil microbial biomass following N fertilization was used as the test of N limitation in the soil microflora.We estimated the impact of this fertilization treatment on microbial activity and net N transformations by conducting long-term aerobic laboratory incubations of soil, and measuring changes in microbial respiration and inorganic pool sizes. The impact of NH4+ fertilization on autotrophic nitrifiers also was assessed using a nitrification potential assay. Microbial C, N and respiration rate were significantly higher in the 4–10 cm soil layer 6 and 12 months following fertilization. However, in the 0–4 cm layer, these microbial parameters were similar in fertilized and unfertilized soils. Laboratory estimates of net N mineralization rates were higher at both depths and on both dates in fertilized soils. Mean net nitrification rates and nitrification potentials were consistently higher in fertilized soils, but many of these differences were not statistically significant. Our results suggest the biomass and activity of the soil microbial community can be altered by N addition to organically rich, old-growth mineral soils.