Vergleich von Elektro-Ultrafiltration (EUF) und Extraktion mit 0,01 molarer CaCl2-Lösung zur Bestimmung des pflanzenverfügbaren Stickstoffs im Boden

Comparison of electro-ultrafiltration (EUF) and extraction by 0.01 M CaCl₂ for the determination of “available” soil nitrogen The NO₃-N, NH₄-N and N_org content of 25 different farm soils were analysed using the EUF-technique and CaCl₂ extraction in order to compare the reproducibility and reliability of the two methods. Nitrogen available to plants was measured in a micro pot experiment with Lolium multiflorum. - CaCl₂ values were better reproducibel (cv = 2.6%) than those of the EUF method (cv = 6.2%). - Close correlations were found between the soil content of CaCl₂-and EUF-extractable nitrogen (r² = 0.917). - The EUF-extractable N_org-fraction was positivly correlated with the total N-uptake by the test plants (r = 0.921). Most importently these data did not significantly deviate from the correlation of the CaCl₂ extractable N_org-fraction with total N-uptake (r = 0.932).     

Available Nitrogen for Corn and Winter Cereal in Spanish Soils Measured by Electro‐ultrafiltration,Calcium Chloride,and Incubation Methods

Abstract

Comparison of methods is necessary to develop a quick and reliable test that can be used to determine soil‐available nitrogen (N) in an attempt to increase the efficiency of N fertilizers and reduce losses. The objectives of this research were to compare the fractions extracted by the calcium chloride (CaCl₂) and the electro‐ultrafiltration (EUF) methods and to correlate them to the mineralization rate (k) obtained from a 112‐d incubation of 61 soil samples. Thirty‐five soil samples were collected from cornfields and 26 from winter cereal fields. Subsamples were either aerobically incubated to calculate k or extracted by the EUF and CaCl₂ methods to identify three fractions: nitrate (NO₃ ^?)‐N, ammonium (NH₄ ⁺)‐N, and Norg‐N. The Norg‐N extracted by both methods was larger in soils from cornfields than in soils from winter cereal fields. In samples from cornfields, the Norg‐N fraction obtained by the EUF method was correlated to the Norg‐N measured by the CaCl₂ method (r=0.46). Soil N content was related to k in samples from cornfields (r=0.40) but not in samples from winter cereal fields. Also, k was correlated to inorganic N content extracted by both chemical methods. The CaCl₂ method was a reliable alternative for laboratories to determine soil‐available N for corn but not for winter cereal.     

An evaluation of plant-available soil nitrogen in selected sandy soils by electro-ultrafiltration, KCl, and CaCl2 extraction methods

 Improving the precision in estimating the nitrogen (N) requirement for citrus trees on sandy soils is important for increasing N efficiency by the trees and minimizing potential losses of N in commercial citrus production areas. In this study, representative Florida soils were sampled from major citrus production areas and the electro-ultrafiltration (EUF) technique was used to measure the concentrations of total EUF-extractable nitrogen (EUF-N_t), ammonium-N (EUF-NH₄ ⁺–N) and nitrate-N (EUF-NO₃ ^––N). Available organic N (N_org) was calculated as: EUF-N_t–(NH₄ ⁺–N+NO₃ ^––N). The N concentrations in the EUF extraction were greater than those by the KCl or CaCl₂ method. The N_org fraction, estimated by the EUF method, varied from 4.4 to 40.8 mg kg^–1 soil, equivalent to 10 to 91 kg N ha^–1 (for the top 15 cm depth soil) and was positively correlated with the total soil N determined by the Kjeldahl method. The presence of appreciable amounts of N_org in these soils indicates that these soils contain high proportions of the total soil N in easily mineralizable N_org forms. This study demonstrates that the EUF-extractable organic bound N must be considered in developing N fertilizer recommendations for citrus. Received: 13 January 1999     

Bestimmung von Mikronährstoffen und Schwermetallen in Böden mit dem Verfahren der Elektro‐Ultrafiltration (EUF) durch Zugabe von DTPA

With the electro–ultrafiltration (EUF) technique, the plant availability of several plant nutrients in soils can be characterized. The basic principle of EUF is that an electric field is induced using platinum electrodes. Ions in the soil suspension move either to the cathode or to the anode and are filtrated through ultra‐membrane filters. In the standard EUF procedure, two extractions steps are used: 30 min at 20°C and 5 min at 80°C. However, the determination of micronutrients and heavy metals with the standard EUF procedure is not possible, because the solubility of these elements in water is low and most of the watersoluble elements are precipitated when passing the platinum electrodes. The addition of DTPA, a well known complexing agent, during a third EUF fraction (5 min at 80°C) enables extraction of micronutrients and heavy metals. Highest concentrations in the 33 soils of the study were found for iron, followed by zinc, manganese, lead, copper, and nickel. Lower concentrations were obtained for cobalt, chromium, cadmium, and molybdenum. For two soils, the EUF/DTPA procedure was compared to CaCl₂/DTPA and EDTA soil extraction methods, showing that higher or comparable amounts were found with CaCl₂/DTPA and much higher amounts with the EDTA method. These results reveal that the EUF/DTPA technique in principle can be used for the determination of plant‐available micronutrients and heavy metals. However, in a next step the relationship between EUF/DTPA‐extractable elements and their availability for plants needs to be quantified.     

Extractability of labelled microbial biomass N by electro-ultrafiltration and CaCl2 extraction

A laboratory soil incubation and a pot experiment with ryegrass were carried out in order to examine the extractability of microbial biomass N by using either 10-mM CaCl₂ extraction or the electro-ultrafiltration (EUF) method. The aim of the experiment was to test the hypothesis whether the organic N (Norg) extracted by EUF or CaCl₂ from dried soil samples represents a part of the microbial biomass. For the laboratory incubation a ¹⁵N-labelled Escherichia coli suspension was mixed with the soil. For the pot experiment a suspension of ¹⁵N-labelled bacteria was applied which had previously been isolated from the soil used. Soil samples of both treatments, with and without applied bacterial suspension, were extracted by EUF and CaCl₂. The extractability of applied microbial biomass was estimated from the difference in extractable Norg between the two treatments. In addition, the N isotopic composition in the upper plant matter, in the soil, and in organic and inorganic N fractions of EUF and CaCl₂ extracts was analysed. Both experiments showed that the applied microbial biomass was highly accessible to mineralization and thus represented potentially mineralizable N. However, this mineralizable N was not extractable by CaCl₂ or by the EUF method. It was, therefore, concluded that the organic N released on soil drying and which was thus extractable was derived from the non-biomass soil organic matter. The result suggests that both extraction methods may provide a suitable index for mineralizable N only in cases where the decomposable organic substrates are derived mainly from sources other than the living soil biota.Dedicated to Professor J. C. G. Ottow on the occasion of his 60th birthday     

Dynamic of different C and N fractions in a Cambisol under five year succession fallow in Saxony (Germany)

The dynamic of different soil C and N fractions in a Cambisol under succession fallow was investigated from June 1996 until May 2001. Mineral soil samples (0 – 10 and 10 – 30 cm) were analyzed for their concentrations of organic C (C_org), total N (N_t), hot water extractable C and N (HWC and HWN), and KCl extractable C and N (C_org(KCl), N_org(KCl), NH₄⁺‐N, NO₃^–‐N). The values of all C and N fractions revealed a distinct depth gradient. While the concentrations of C_org increased after set aside significantly from 7.7 to 8.9 g kg^–1 at 0 – 10 cm, those at 10 – 30 cm depth decreased from 7.2 to 6.1 g kg^–1. N_t remained rather constant throughout the whole observation period. The HWC concentrations increased from 0.33 to 0.49 g kg^–1, while HWN decreased slightly at 0 – 10 cm with time. In contrast, both HWC and HWN increased at 10 – 30 cm soil depth. HWC showed close significant correlations to C_org, and HWN to N_t as well as to NH₄⁺‐N and NO₃^–‐N, respectively. In comparison to hot water‐extractable C and N, C_org(KCl) and N_org(KCl) accounted only about one tenth of those and showed a decreasing trend with time of succession. C : N ratio of the KCl fraction was in the same order of magnitude as the HWC : HWN ratio, except the last phase of the experiment where hot water extract values increased above 10.     

Nitrogen compounds extracted by electroultrafiltration (EUF) or CaCl2 solution and their relationships to nitrogen mineralization in soils

The objective of the investigation was to identify the most important organic N-containing fractions extracted from soils by electroultrafiltration (EUF) or a CaCl₂ solution, respectively, and their importance for nitrogen mineralization. The investigation comprised 19 agricultural and one forest top soil. Net N mineralization was tested in Mitscherlich pot experiments with three treatments: (1) fallow soil without N fertilizer, (2) soil cultivated with rye grass without N fertilizer, (3) soil cultivated with rye grass with N fertilizer. The highest proportion of N in the extracts was the amino N fraction (amino acids + peptides) amounting to approximately 60% of the total N extracted by CaCl₂ and to about 40% of the total N extracted by EUF. The proportion of amino sugars from total N extracted was in average 10% for the CaCl₂ and 5.2% for the EUF extracts. The proportion of heterocyclic N bases derived from nucleic acids amounted in average to 4.8% and 3.6% for the CaCl₂ and EUF extract, respectively. Amino N (amino acids + peptides) were correlated best with net N mineralization (EUF, r = 0.81***, CaCl₂, r = 0.86***). The correlation between amino sugars and net N mineralization was r = 0.55* for the EUF extract and r = 0.49* for the CaCl₂ extract. The heterocyclic N bases did not correlate with net N mineralization. Correlations between Norg extracted by CaCl₂ versus net N mineralization were higher than those obtained by the EUF extract. Net N mineralization was about four times higher in the fallow soils than in the treatment with grass and no N fertilizer. In the treatment with grass + N fertilizer on average no net N mineralization occurred, moreover there was a tendency of N immobilization. It is assumend that in the treatments with grass cultivation, organic C released by roots stimulated the assimilation of mineral N and amino acids by soil microorganisms resulting in a low net N mineralization. Net N mineralization led to a highly significant depletion in the Norg pools and particularly in the amino N and amino sugar pools in the treatment with grass and without N fertilizer. This depletion was particularly evident in the CaCl₂ extracts. The results justify the conclusion that the Norg obtained with both extraction methods originates from a dynamic N pool into which N flows in and out. The amino N extractable with EUF or CaCl₂ is a reliable indicator for the net N mineralization potential of soils.     

Evaluation of euf‐potassium quotient as a measure of potassium buffering capacity of illite‐dominant soils under intensive cropping

Abstract

Suitability of the electro‐ultrafiltration (EUF)‐potassium (K) quotient, a ratio between strongly held and easily desorbable EUF fractions of K to measure K‐buffering capacity of illite‐dominant soils under intensive cropping, was studied. EUF‐K quotients which ranged from 0.333 to 0.580 before cropping increased to the range of 0.405 to 0.710 after the cropping. As a result of cropping, the mean K‐ replenishment rate decreased from 6.98 kg/ha/day to 0.2 kg/ha/day, whereas the mean EUF‐K quotient increased from 0.43 to 0.53. EUF‐K quotient showed significant positive correlation with dry matter yield (r = 0.78*) and K uptake (r = 0.79*) in the first harvest of crop and in the subsequent harvests it maintained correlations of lower order. While other EUK‐K fractions, i.e. EUF 20°C, EUF 80°C, and EUF 35 min increased or maintained the same extent of correlation with conventional soil test methods after cropping, EUF‐K quotient failed to have significant correlation with those soil test methods after cropping. The current study brought out that exhaustive cropping differentially depleted the EUF fractions leading to an increase in the estimated buffering capacity (EUF‐Q) which failed to show satisfactory relationship with K removal by the crop.     

0.01molL-1CaCl2作为土壤不同N素形态浸提剂的研究

本文用荷兰8种表土测试不同温度下0.01molL^-1CaCl₂提取液和淋滤液中N素各形态。试验结果表明温度对NO₃^--N提取量和淋滤量无影响,但对NH₄⁺-N、可溶性总N和还原态N影响显著;可溶性有机态N的释放服从一级动力学方程:N_t=N₀(1-e^-kt),非线性最小二乘法能满意地拟合动力学实验结果。测定0.01molL^-1CaCl₂提取液中的可溶性有机态N对预测土壤N素矿化、合理推荐施肥及防止N素污染可能是一个很有前途的指标。     

Nitrogen turnover in bare soil planted subsequently with grass as investigated by electro‐ultrafiltration (EUF)

Changes of EUF‐extractable nitrogen (N) (nitrate, ammonium, organic N) in 20 arable bare soils, subsequently planted with ryegrass (Lolium multiflorum L.) and cutting three times were investigated in pot experiments. All 20 soils responded qualitatively in the same way. During the period of bare soil, there was a significant increase of EUF‐extractable nitrate (EUF NO ), while extractable ammonium (EUF NH ) remained on the same level and organic N (EUF N_org) decreased. This decrease, however, was not significant. From sowing until the first cutting of the grass, EUF‐NO concentration decreased to almost zero. This low EUF‐NO level was maintained throughout the subsequent experimental period (three cuttings of grass). During the growth of the first cutting, EUF N_org decreased while EUF NH remained constant, however, on a low level. EUF NH fell during the growth of the second and third cutting. In this period, however, the N supply of the grass was insufficient. EUF N_org decreased during the growth of the second cutting, but increased during the growth of the third cutting. This shows that the EUF‐N_org fraction represents a transient pool, which gains and loses N. EUF NO , EUF NH , and EUF N_org correlated with the N uptake of the grass. Strongest correlation for EUF NO was found for the first cutting (p < 0.001), and for EUF NH and EUF N_org for the second and third cutting (p < 0.001). Total soil N was not correlated with the N uptake of the grass. EUF N_org was only about 2% of the total N. This relatively small EUF‐N_org fraction, however, is relevant for the mineralization of organic soil N, and the N quantity indicated by EUF N_org is in the range of the N amount mineralized in arable soils within a growing season.     

Amino acids and amino sugars extracted by EUF from a sandy soil incubated with green manure,bacterial biomass or cellulose

The extraction of soils by the electro-ultrafiltration (EUF) method yields organic N which has been used as an index for mineralisable N in soils. This EUF extractable organic fraction contains a mixture of various N compounds not yet completely identified. It has been proposed that the amino N compounds are more indicative for the potentially mineralisable N in soils than the total organic N extracted (Mengel et al., 1999). An amendment of soils with easily mineralisable organic matter may, therefore, alter the amino N concentrations of the organic N extracted. Our determination of the amino N compounds aimed to prove this hypothesis. The principle of our experiment was to mix soil with green manure, bacterial biomass and cellulose, respectively, and to incubate the treated soil aerobically for 80 days at 20°C in the laboratory. Control treatments without organic amendment were also incubated. Soil samples were taken several times during the incubation period and analysed for the inorganic N (NO₃⁻-N and NH₄⁺-N) and for the EUF extractable organic N. Amino acids and amino sugars were determined in the hydrolysed EUF extracts. The concentrations of amino acids and amino sugars in the organic N extracted varied with time and differed between the treatments. Glutamic acid has been found to be the most relevant amino acid in the EUF extracts and was particularly indicative for the existence of mineralisable green manure in the soil. Glucosamine was the most relevant amino sugar in the EUF extracts and this amino sugar appears to be indicative for the easily mineralisable relics of microbial cells in the soil.     

Organic C levels of Ap horizons in North German Pleistocene sands as influenced by climate,texture, and history of land‐use

We evaluated the contents of organic carbon (C_org) of Ap horizons from 11 North German study areas along a Southeast to Northwest precipitation gradient with respect to their general levels and as related to C : N ratio, soil texture (clay content), bulk soil density, climate, and historical land‐use since 1780. The focus was on sandy soils, with the largest group of samples originating from 308 km² of the Fuhrberg catchment north of Hannover/Lower Saxony. Data from loess areas were used for comparisons. Major aims were (1) to quantify current C_org stocks, (2) to provide data on site‐specific, steady‐state C_org levels in old arable soils, and (3) to identify the main controls of C_org levels in the studied sands. The mean C_org content in sandy, well‐drained, old Ap horizons (uplands, > 200 years under cultivation, near steady‐state) increased with precipitation from < 8 g kg^—1 in the dry eastern parts of the study area (530 mm year^—1, 8.3°C) to 25 g kg^—1 in the moist Northwest (825 mm year^—1, 8.4°C). The C_org levels in lowlands which have been drained for more than 40 years were approximately 3 g kg^—1 higher than those of uplands under a similar climate. The factor clay content had no predictive value because low contents were associated with high C_org levels. Large proportions of refractory organic matter in sands resulting from specific features of historical land‐use and soil development (calluna heathland, heath plaggen fertilization, podzolization) appeared to be the most probable reason for such high C_org levels. However, the high C_org levels of these old arable sites were still exceeded by those of younger arable areas formerly under continuos grassland. A chrono‐sequence suggested that a period of about 100 years is necessary until a new steady‐state C_org level is established after conversion of grassland into arable land. Elevated C_org levels in current Ap horizons were also found for former woodland and heathland soils. The main conclusion is that sands can contain a lot of stable organic matter, sometimes more than finer textured soils.     

Comparing tests for soil fertility. III. Evaluation of phosphate availability in Alfisols by Olsen,Mehlich 3, electro‐ultrafiltration,and the paper‐strip methodology procedures

Abstract

The suitability of the “iron‐impregnated paper‐strip”; (Pi) methodology for the assessment of phosphate (P) availability in soils has been tested on the basis of a comparison with the classical Olsen method as well as with the new‐generation electro‐ultrafiltration (EUF) and Mehlich No. 3 (M3) extraction procedure. The EUF extractions were performed by a first 30 min run at low (L) energy conditions (20°C, max 15 mA, and 200V), followed by a 10 min run at high (H) energy conditions (80°C, max 150 mA, and 400V). The total EUF‐P extracted (T‐EUF‐P), given by the sum L‐EUF‐P + H‐EUF‐P, was also considered. The investigation was carried out on twenty samples of representative Alfisols ("Terra Rossa") from the pedoclimatic environments of the Mediterranean area. The soil samples were characterized by a large variability of their available‐P values, whose ranges were, as mg‐kg^‐1 soil, 0.1–34.0 for L‐EUF‐P, 0.2–39.3 for HEUF‐P, 0.3–73.3 for T‐EUF‐P, 0.8–113.6 for Olsen‐P, 1.0–122.4 for Pi‐P, 1.1–224.6 for M3‐P. The respective mean values were 4.6, 5.1, 9.6, 17.2, 20.1, and 29.7 mg P/kg soil, with ratios L‐EUF‐P:H‐EUF‐P:T‐EUF‐P:Olsen‐P:Pi‐P:M3‐P increasing as 0.27:0.29:0.56: 1.00:1.17:1.73. A good compliance among all methods was verified; the most significant correlations were determined for Pi‐P versus M3‐P (R²=0.993***) and for Pi‐P versus Olsen‐P (R²=0.988***), clearly indicating the liability of the Pi procedure for the assessment of the P fertility in Alfisols. A multiple linear regression analysis revealed that all the available P values, afar from the individual procedure, were significantly (P<0.01) dependent on M3 exchangeable iron (Fe) (positively) and aluminum (Al) (negatively). This could lead one to argue that the prevailing available soil P source in the investigated Alfisol samples was that connected with the labile Fe‐P pool. Withal, one could infer that the A1‐M3 and Fe‐M3 parameters should also be considered for the making of P fertilizer recommendations. From this standpoint, the M3 is a well‐proved multielement extractant suitable for the simultaneous determination of available P and exchangeable Fe and Al as well.     

A procedure for the determination of soluble boron in soils ranging widely in boron concentrations,sodicity, and pH.

Abstract

Soils in southern Australia within the Mediterranean‐type climate zone vary widely in boron concentrations, from potentially deficient to toxic for plant growth. A general method is needed for the determination of boron in soils ranging from acidic to alkaline, with wide ranges of clay content and sodicity.

The amounts of boron extracted were compared following boiling in 0.01M CaCl₂ in test tubes in temperature‐controlled programmable digestion blocks, or in Erienmeyer flasks on hot plates under different analytical conditions. Comparisons were also made between CaCl₂, hot water, and mannitol extractable boron. All analyses of boron were made by inductively coupled plasma spectrometry (ICPS).

The extraction of boron was dependent on extraction time and temperature of the heating block. Addition of 20 ml of 0.01M CaCl₂ to 10g of soil in 250 ml test tubes fitted with air condensers and placed in a pre‐heated temperature‐controlled digestion block set at 140°C and boiled for 30 minutes. This method was found to be a suitable extractant of boron in soils with a wide range of sodicity, pH and extractable boron (from concentrations potentially deficient to toxic for plant growth).     

An inverse abundance approach to separate soil nitrogen pools and gaseous nitrogen fluxes into fractions related to ammonium,nitrate and soil organic nitrogen

The soil nitrogen (N) cycle exhibits a variety of complex biochemical reactions in which N species such as NO₂⁻, NO and N₂O are produced and consumed by co‐existing processes that respond differently to the local environmental conditions. Key to understanding the soil N cycle in its full complexity is the development and application of methods that allow a quantification of individual pathways and processes that are responsible for the build‐up and/or emission of N compounds. Triplet ¹⁵N tracer experiments (TTE) have been developed and applied to allow a source‐related quantification of N species such as NO₂⁻, and N₂O by different biochemical pathways (e.g. ammonia oxidation and nitrate reduction) that are related to multiple N sources (NH₄⁺, NO₃⁻ and N_org). An analysis of a TTE requires the application of either a numerical or analytical model. Because of the ease of application it is desirable to use analytical models. However, available analytical solutions suffer from serious drawbacks concerning the quantification of N fluxes related to soil organic N. In this paper we describe the development and application of a new inverse abundance approach (IAA) to analyse a TTE. Theoretical and experimental data sets of soil N₂O release were analysed by the new method. The IAA was also applied to an already existing data set to identify fractions of the soil nitrite pool related to NH₄⁺, NO₃⁻ and N_org. We show that the IAA provides a reliable and comprehensive data evaluation of a TTE.     

Applicability of 0.01 M CaCl2 as a single extraction solution for the assessment of the nutrient status of soils and other diagnostic purposes

Abstract

The applicability of 0.01 M CaCl₂ solution as a single extraction agent for soils as a basis for fertilizer recommendation was tested on a variety of soils both from the Netherlands and from some tropical countries. Air‐dry soil samples were subjected to extraction with 0.01 M CaCl₂ and to several conventional extraction procedures, and the results were compared. In the soil suspensions pH was measured, whereas in the extracts Na, K, Mg, P, different extractable N‐forms and Zn were measured. The values found in CaCl₂ extracts are discussed in relation to results of other extraction procedures and as to their potential value in soil quality assessment. It is concluded that a single extraction procedure with 0.01 M CaCl₂ can be applied for fertilizer recommendation purposes. The possibility of determining different extractable N‐forms (NH₄, NO₃, soluble organic N) significantly enhances the value of the method in predicting the N‐fertilizer needs. Furthermore it was found that the concentration of Zn in 0.01 M CaCl₂ extracts was a good indicator of phytotoxicity in a polluted area. Additional advantages of this extraction are low costs, simplicity and repro‐ducibility.     

Microbial C,N, and P relationships in moisture‐stressed soils of Potohar,Pakistan

In 11 rain‐fed arable soils of the Potohar plateau, Pakistan, the amounts of microbial‐biomass C (C_mic), biomass N (N_mic), and biomass P (P_mic) were analyzed in relation to the element‐specific total storage compartment, i.e., soil C_org, N_t, and P_t. The effects of climatic conditions and soil physico‐chemical properties on these relationships were highlighted with special respect to crop yield levels. Average contents of soil C_org, N_t, and P_t were 3.9, 0.32, and 0.61 mg (g soil)^–1, respectively. Less than 1% of P_t was extractable with 0.5 M NaHCO₃. Mean contents of C_mic, N_mic, and P_mic were 118.4, 12.0, and 3.9 µg (g soil)^–1. Values of C_mic, N_mic, P_mic, soil C_org, and N_t were all highly significantly interrelated. The mean crop yield level was closely connected with all soil organic matter– and microbial biomass–related properties, but showed also some influence by the amount of precipitation from September to June. Also the fraction of NaHCO₃‐extractable P was closely related to soil organic matter, soil microbial biomass, and crop yield level. This reveals the overwhelming importance of biological processes for P turnover in alkaline soils.     

Effect of temperature on the mineralization of C and N of fresh and mature compost in sandy material

Information about the mineralization rate of compost at various temperatures is a precondition to optimize mineral N fertilization and to minimize N losses in compost‐amended soils. Objectives were to quantify the influence of the temperature on the mineralization rate and leaching of dissolved organic carbon (DOC) and nitrogen (DON), NO₃^—, and NH₄⁺ from a fresh (C : N = 15.4) and a mature (C : N = 9.2) organic household waste compost. Compost samples were mixed with quartz sand to ensure aerobic conditions, incubated at 5, 10, 15, 20, and 25°C and irrigated weekly for 112 days. For the fresh compost, cumulative CO₂ evolution after 112 days ranged from 36% of the initial C content at 5°C to 54% at 25°C. The CO₂ evolution was only small in the experiments with mature compost (1 to 6% of the initial C content). The data were described satisfactorily by a combined first‐order (fresh compost) or a first‐order kinetic model (mature compost). For the fresh compost, cumulative DOC production was negatively related to the temperature, probably due to leaching of some of the partly metabolized easily degradable fractions at lower temperatures. The production ratios of DOC : CO₂‐C decreased with increasing temperature from 0.094 at 5°C to 0.038 at 25°C for the fresh and from 1.55 at 5°C to 0.26 at 25°C for the mature compost. In the experiments with fresh compost, net release of NO₃^— occurred after a time lag which depended on the temperature. Cumulative net release of NO₃^— after 112 days ranged from 1.8% of the initial N content at 5°C to 14.3% at 25°C. Approximately 10% of the initial N content of the mature compost was released as NO₃^— after 14 days at all temperatures. The DOC : DON ratios in the experiments using fresh compost ranged from 11.5 to 15.7 and no temperature dependency was observed. For the mature compost, DOC : DON ratios were slightly smaller (7.4 to 8.9). The DON : (NH₄⁺ + NO₃^—) ratio decreased with increasing temperature from 0.91 at 5°C to 0.19 at 25°C for the fresh compost and from 0.21 at 5°C to 0.12 at 25°C for the mature compost. The results of the dynamics of C and N mineralization of fresh and mature compost can be used to assess the appropriate application (timing and amount) of compost to soils.     

Limitations when quantifying microbial carbon and nitrogen by fumigation‐extraction in rooted soils

Soil microbial C and N (C_mic, N_mic) estimation by the chloroform fumigation‐extraction method is erroneous in densely rooted soils due to CHCl₃‐labile C and N compounds. The effect of a pre‐extraction with 50 mM K₂SO₄ and a pre‐incubation (conditioning at 25 °C for 7 days) on the flush in extractable, CHCl₃‐labile C (C‐flush) and N (N‐flush) was tested with reference to rooting density (0.3—75 mg root dry matter g^—1) in one arable and 3 grassland soils. In the arable soil and in the second horizon (10—20 cm) of a grassland soil, C‐flush values were not affected by the pre‐extraction. However, the pre‐extraction considerably reduced C‐flush values in the top soils of the grassland (above 10 cm). Only about 42 % was found in the pre‐extracted roots and the rest was lost during the pre‐extraction. The estimated concentrations of N_mic decreased due to pre‐extraction of soil samples with low root biomass. Clearly, the concentrations of N_mic were underestimated by introducing the pre‐extraction. Soil pre‐incubation reduced C‐flush values only slightly, whereas N‐flush values were not affected. It can be concluded that (1) CHCl₃‐labile root C and N is partly extracted with K₂SO₄ after pre‐incubation and (2) CHCl₃‐labile C and N removed with the roots during pre‐extraction is partly derived from microbial biomass. Soils with low rooting density (arable soils, grassland soils below approximately 10 cm depth) should therefore be fumigated and extracted without pre‐extraction. In densely rooted soils, fumigation extraction with and without pre‐extraction probably gives estimates for the minimum and maximum of C_mic and N_mic.     

Soil microbial biomass C and N along a climatic transect in the Mongolian steppe

The relationships of soil microbial biomass C (C_mic) or N (N_mic) with mean annual precipitation and temperature were studied along a climatic transect in the Mongolian steppe. Soil organic C (C_org) and total N (N_t), respiration rate, C_mic and N_mic at depths of 0–5 and 5–10 cm decreased with increasing aridity. The contents of C_org and N_t in the 0- to 5-cm soil layers decreased linearly with precipitation reduction along the transect. C_mic and N_mic changes with precipitation were not linear, with higher changes between 330 and 128 mm mean annual precipitation. C_mic/C_org and N_mic/N_t increased with increasing aridity. The metabolic quotient qCO₂ of 0- to 5-cm soil layers was low between 330 and 273 mm precipitation. The relationship between the qCO₂ of the 0- to 5-cm soil layers and the mean annual precipitation was well fitted with a quadratic function y =0.0006x² –0.40x +86.0, where y is the qCO₂ (µmol CO₂-C mmol^–1 C_mic) and x is the mean annual precipitation (mm). C_org, N_t, C_mic, N_mic and respiration rate decreased exponentially with increasing mean annual temperature in both the 0- to 5- and 5- to 10-cm soil layers, and change rate was lower when the mean annual temperature was higher than 2.6°C. The close relationships of the mean annual precipitation or temperature with soil C_org, N_t, C_mic, N_mic, C_mic/C_org and qCO₂ indicate that each parameter can be calculated by determining the other parameters in this specific climatic range.