Origin and properties of humus in the subsoil of irrigated rice paddies

Stability of humus in the plow layer soil is considered to affect the quantity and quality of dissolved organic matter leached from the plow layer soil. Therefore, a model experiment was conducted to analyze the effect of soil reduction under submerged conditions on the stability of humus in the plow layer soil. The changes in the stability of humus in the plow layer soil during submerged incubations with and without rice straw application were evaluated based on the changes in the binding type of humus. Binding type of humus in the plow layer soil was analyzed by successive extractions of organic matter with water, 0.25 M Na₂SO₄, 0.1 M Na₄P₂O₇ (pH 7.0), 0.1 M Na₄P₂O₇ (pH 10.5), and 0.1 M Na₄P₂O₇ (pH 10.5) with NaBH₄. Amounts of Fe, Mn, and Mg in each fraction were also determined to estimate the relationships between humus and metals.

The successive extraction of humus indicated that the amount of organic carbon which was extractable with the (NaBH₄ +0.1 M Na₄P₂O₇) solution decreased while that of the 0.1 M Na₄P₂O₇ (pH 7.0}-extractable organic carbon increased during submerged incubation with rice straw application. The origin of the increase in the amount of organic carbon in the Na₄P₂O₇ (pH 7.0)-extractable fraction during submerged incubation was investigated further by another incubation experiment using ¹³C-glucose as a reducing agent. Atom- 13C% analysis showed that the contribution of organic carbon derived from compounds other than glucose to the increase in the contents of humic acids and fulvic acids in the Na₄P₂O₇ (pH 7.0)-extractable fractions was ca. 80%. Therefore, it was concluded that the binding type of humus changed from (NaBH₄ + Na₄P₂O₇)-extractable to Na₄P₂O₇ (pH 7.0)-extractable humus under reducing conditions. Since the amounts of organic carbon and Fe increased in the Na₄P₂O₇ (pH 7.0)-extractable fraction and decreased in the (NaBH₄ +0.1 M Na₄P₂O₇)-extractable fraction simultaneously, iron reduction was presumably associated with the change in the binding type of humus in submerged paddy soil.     

Sulfur transformations in relation to carbon and nitrogen in incubated soils

Changes in biomass-S in relationship to biomass-C and N were evaluated, and the transformation of ³⁵S-labelled SO₄^2? among organic matter fractions were followed during incubation of a Black Chernozemic (Udic Haploboroll) and Orthic Gray Luvisol (Typic Cryoboralf) soils. There was a net immobilization of S with and without the addition of cellulose or sulfate after 64 days. In contrast, a net mineralization of N occurred. Cellulose decomposition rates responded to supplies of S available for new microbial cell synthesis.Fluctuations in the amounts of biomass-S during incubation of both soils followed biomass-C and biomass-N changes and C/S and C/N ratios of the biomass ranged between 47–121 and 4.9–7.7, respectively. Microbially-incorporated S was found concentrated within the biomass or partially transformed into soil organic matter.Fractionation of soils after incubation, by a 0.1 m NaOH-0.1 m Na₄P₂o₇ extraction-separation technique showed significant increases in the C and N contents of the conventional humic acid (HA-A) and fulvic acid (FA-A), and humin (<2 μm) fractions. Biomass C accounted for 20–64% of the observed increases in these fractions suggesting that the differences were due partly to transformed microbial products and partly to microbial cell organic constituents released on lysis of cells during incubation. In contrast to C and N, the contents of total S and HI-reducible S increased in the FA-A fraction only and accounted for 45–76% of the immobilized labelled S.     

Turnover of microbial tissue in soil under field conditions

The microbial population of a Brown Chernozemic soil was labelled in situ by adding ¹⁴C-glucose and ¹⁵NH₄¹⁵NO₃ to the plow layer. The loss of ¹⁴C, nitrogen immobilization-mineralization reactions, bacterial numbers (plate count, direct count) and fungal hyphal lengths were determined periodically throughout the growing period in amended and unamended microplots and in the surrounding field soil. After 5 days, 90 per cent of the labelled N occurred in the organic form with little subsequent mineralization. Of the labelled C added, 63, 56 and 39 per cent, remained in the soil after 3, 14 and 104 days, respectively.The ratio of fungal C to bacterial C increased as soil moisture decreased. Viable (plate count) and total numbers of bacteria in samples from unamended plots and field soil were significantly correlated with each other and with soil moisture. Fungal hyphal lengths from amended soil were also significantly related to moisture but the rate of loss of ¹⁴C and mineralization of ¹⁵N were not. The synthesized microbial material (tissue and metabolites) exhibited a high degree of stability throughout the study. The half-life of labelled C remaining in the soil after 30 days was calculated to be 6 months compared to only 4 days for the added glucose C. The amount of energy used for maintenance by the soil population under field conditions was calculated from measurements of biomass C, respired labelled C and respired soil C.     

Rhizodeposition of C and N in peas and oats after C-N double labelling under field conditions

Compounds released by plant roots during growth can make up a high proportion of below-ground plant (BGP) carbon and nitrogen, and therefore influence soil organic matter turnover and plant nutrient availability by stimulating the soil microorganisms. The present study was conducted to examine the amount and fate of C (CdfR) and N rhizodeposits (NdfR), in this study defined as root-derived C or N present in the soil after removal of roots and root fragments, released during reproductive growth. BGP biomass of peas (Pisum sativum L.) and oats (Avena sativa L.) was successfully labelled in situ with a ¹³C-glucose-¹⁵N-urea mixture under field conditions using a stem feeding method. Pea plants were labelled at the beginning of flowering and harvested 36 and 52 days after labelling at pod filling (P_P) and maturity (P_M), respectively. Oat plants were labelled at grain filling and harvested 42 days after labelling at maturity (O_M). CdfR was 24.2% (P_P), 29.6% (P_M) and 30.8% (O_M) of total recovered plant C. NdfR was 32.1% (P_P), 36.4% (P_M) and 30.0% (O_M) of total plant N. Due to higher N assimilation, amounts of NdfR were four times higher in peas in comparison with oats. The results for NdfR in peas were higher than results from other studies. The C-to-N ratio of rhizodeposits was lower under peas (17.3) than under oats (41.9) at maturity. At maturity, microbial CdfR at 0-30 cm soil depth was 37% of the microbial biomass C in peas and 59% in oats. Microbial NdfR was 15% of microbial N in peas and 5% in oats. Furthermore, inorganic NdfR was 34% in peas and 9% in oats at 0-30 cm at maturity. These results show that rhizodeposits of peas provide a more easily available substrate to soil microorganisms, which are incorporated to a greater extent and turned over faster in comparison with oats. Beside the higher amounts of N released from pea roots, this process contributes to the higher N-availability for subsequent crops.     

Mineralisation dans le sol de materiaux microbiens marques au carbone 14 et a l'azote 15: Quantification de l'azote de la biomasse microbienne

The mineralization of microbial material of different C-to-N ratios (5.2, 7.9, 10.2, 12.7) was followed in fumigated soil. The microbial materials used were from Aspergillus flavus cultures, grown in liquid media and labelled with [¹⁴C]glucose and (¹⁵NN4)3804. Three contrasting soils were used and the microbial materials incubated with the fumigated soils for 28 days at 28°C.The evolution of the added organic microbial C was fast: 80% of the [¹⁴C]CO₂ produced during the whole 28 days incubation was evolved in the first week. Microbial C mineralization was mainly related to soil type; the C-to-N ratio had small effect on the ratio (mineralized microbial carbon-to-added microbial carbon). Calculation of the K_c- coefficient (the fraction of the added microbial C mineralized in 7 days) shows that K_c values lie between 0.38 and 0.43 in the 3 soils.Organic N in the added microbial material also breaks down quickly: between 60 and 100% of the organic nitrogen mineralized was evolved during the first week of incubation. Mineralization kinetics are related to soil type and to the C-to-N ratio of the microbial material.The proportion of N mineralized in 7 days was lower in an acid soil than in near neutral soils and lower with high C-to-N ratio material than with low C-to-N ratio material. The ratio (mineralized microbial N-to-added microbial N) depends on soil type and is negatively correlated with the C-to-N ratio of the microbial material. The K_N value (the fraction of the added microbial N mineralized in 7 days) lies between 0.22 and 0.47 for the three soils and four materials investigated. The added microbial material induced a priming effect on soil native N: materials with C-to-N ratios of 10.2 and 12.7 produced negative priming effects whereas materials with C-to-N ratios of 5.2 and 7.9 sometimes produced a positive priming action.From the relationship between the C-to-N ratio of the added material and the (mineralized microbial C-to-mineralized microbial N) ratio, the soil native microbial biomass was estimated using the fiush-C-to-flush-N ratio. Biomass nitrogen was then calculated from the formula biomass-N = biomassC/(biomass C-to-N ratio). Calculated in this way, 2–4% of the total nitrogen in the three soils was in microbial biomass.     

Interference by organic complexation of Fe and A1 on the SO2-4 adsorption in Spodic B horizons in Sweden

The relations between pH, different fractions of Fe and A1 and Na₄P₂O₇-soluble C and the amount of adsorbed SO^2-₄ were assessed by analysing 63 soil samples from 14 podsolized soils in Sweden. The amount of adsorbed SO^2-₄ was significantly better correlated with the calculated amount of the inorganic fraction of Fe and A1 oxides obtained by subtracting Na₄P₂O₇-soluble Fe and A1 from oxalate-soluble Fe and Al than with the oxalate extraction alone. There was a close correlation between C and organically-bound S in the Na₄P₂O₇ extract which shows that the C:S ratio of the extracted fulvic acids is about constant in the soils studied. It was found that, as the proportion of organically-complexed Fe and Al increases, the ability of the soil to adsorb SO^2-₄ decreases. The amount of adsorbed SO^2-₄ expressed on the basis of the amounts of oxalate-soluble Fe and Al was generally smaller in areas with low S deposition (< 60 mmol m^-2 a^-1). The ratio between pyrophosphate-soluble C and oxalate-extractable Fe and Al was negatively correlated with pH in water. It was concluded that Fe and Al associated with organic matter cannot adsorb SO^2-₄ and that the degree of this association is pH dependent. These observations have important implications regarding the effects of anthropogenic acidification.     

Changes in enzymic activity and distribution of acid-soluble,amino acid-nitrogen in soil during nitrogen immobilization and mineralization

During a period of immobilization of nitrate-¹⁵N and mineralization of organic N in a sandy-loam, changes were recorded in: (a) the concentration of an added carbon source, glucose-¹⁴C: (b) evolution of ¹⁴CO₂: (c) bacterial populations; (d) distribution and concentration of newly-synthesized, acid-soluble, amino acid-¹⁵N; and (e) distribution and activities of several oxidative and hydrolytic enzyme systems.Added glucose-¹⁴C was rapidly metabolized by the soil microflora. After 1.5 day's incubation, when only 3.6 per cent of the added glucose was present, 68 per cent of the ¹⁴C remained in the soil-microbial system. During this period there was a marked increase in viable bacterial numbers and an almost complete immobilization of nitrate-¹⁵N. On continued incubation, microbial metabolites were oxidized at decreasing rates, the more rapid phase corresponding to a period of net decline in the viable bacterial population.Soil was fractionated by a relatively mild procedure into components containing: (a) extractable proteins; (b) extractable amino acids and peptides; (c) particulate material containing microbial cells, cell debris and material bound to larger soil particles; and (d) microbial metabolites mainly bound to soil colloids. Although the total, acid-soluble, amino acid-¹⁵N remained relatively constant for about 50 days, there were marked changes in their concentration in different fractions, especially in the extracts and in the fraction containing fine colloidal material. However, the relatively large decline in labelled, acid-soluble, amino acid-¹⁵N occurred during the active phase of oxidation of microbial metabolites when little net mineralization of labelled compounds occurred.Increases in enzymic activities generally coincided with increased viable bacterial populations although there were some exceptions, notably casein and benzoyl arginine amide-hydrolysing enzymes. The stabilities of the newly-formed enzymes varied markedly. The greatest relative changes in activity occurred with the casein-hydrolysing enzymes. Their activity reached a maximal value after the main flush of bacterial growth, was short-lived and was to a large extent extractable. The formation and disappearance of this extracellular proteolytic activity coincided approximately with that of a secondary peak of extractable, newly-synthesized, protein-¹⁵N. In general however, changes in enzymic activity could not be identified with changes of protein-¹⁵N concentrations of the different fractions.     

A Hybrid Approach for PAHs and Metals Removal from Field-Contaminated Sediment Using Activated Persulfate Oxidation Coupled with Chemical-Enhanced Washing

The aim of this study was to investigate the removal of both polycyclic aromatic hydrocarbons (PAHs) and heavy metals from field-contaminated sediments by activated persulfate oxidation. Various chemicals, including hydroxypropyl-??-cyclodextrin (HPCD), S,S-ethylenediaminedisuccinic acid (EDDS), tetrasodium pyrophosphate (Na₄P₂O₇), and hydrochloric acid (HCl), were applied individually before or after activated persulfate oxidation to enhance the co-removal of both types of pollutants. It was found that the organic removal efficiency was not significantly enhanced by increasing the concentration of HPCD from 2.5 to 5.0?mM. The removal efficiency of heavy metals was not improved even at an excess amount of EDDS after activated persulfate oxidation. However, the addition of EDDS acted as the Fe²⁺ carrier for activated persulfate oxidation. In addition, no significant enhancement of heavy metal removal was observed by increasing the concentrations of Na₄P₂O₇ and HCl from 0.01 to 0.1?M after activated persulfate oxidation. However, comparing 0.1?M HCl with 0.1?M Na₄P₂O₇, HCl was shown to be more effective in promoting the removal of organic pollutants. With further adjustments on the experimental conditions, the highest removal amount of metals and PAHs was achieved by adding 2?M of HCl with 3?days mixing, followed by Fe²⁺-activated persulfate oxidation (PS/Fe²⁺ molar ratio at 4:1) for further 6?h mixing. The removal efficiency of low and high molecular weight PAHs was about 70 and 20?%, respectively, while the removal efficiency of metals was 70, 100, 40, 65, 65, 80, and 100?% for Cr, Cu, Hg, Mn, Ni, Pb, and Zn, respectively.     

Chloroform fumigation and the release of soil nitrogen: The effects of fumigation time and temperature

Fumigation with CHC1₃ (24 h, 25°C) increased the amount of NH₄-N and total N extracted by 0.5 M K₂SO₄ from two soils (one arable, one grassland). The amount of N released by CHC1₃ increased with the duration of fumigation up to 5 days, when it levelled off. Between about 10–34% of the total N released by CHC1₃ was in the form of NH₄-N, the proportion increasing with duration of exposure.When a grassland soil that had received a field application of ¹⁵N-labelled fertilizer 1 yr previously was fumigated, the N released by CHC1₃ was 4 times more heavily labelled than the soil N as a whole. Prolonging the exposure of this soil to CHC1₃ increased the amount of total N released, but hardly altered the proportion of labelled N in the CHC1₃-released N, suggesting that N is being released from a single soil fraction. The most likely soil fraction is the soil microbial biomass. It is suggested that CHC1₃ does not alter the K₂SO₄-extractability of soil-N fractions other than microbial N and that the extra N released by CHC1₃ and extracted by K₂SO₄ gives a direct measure of soil microbial biomass N.In contrast to fumigation done at lower temperatures, less total N was released by soil fumigated at 60°C, or above, than was released from unfumigated soil held at the same temperature. The greater release of N in the non-fumigated soils above 60°C could have been due to soil enzymic processes which were inhibited by CHC1₃ in the fumigated soil.     

Differential response of mineral-associated organic matter in tropical soils formed in volcanic ashes and marine Tertiary sediment to treatment with HCl,NaOCl, and Na4P2O7

Quantitative knowledge of the amount and stability of soil organic matter (SOM) is necessary to understand and predict the role of soils in the global carbon cycle. At present little is known about the influence of soil type on the storage and stability of SOM, especially in the tropics. We compared the amount of mineral-associated SOM resistant to different chemical treatments in soils of different parent material and mineralogical composition (volcanic ashes – dominated by short-range-order aluminosilicates and marine Tertiary sediments – dominated by smectite) in the humid tropics of Northwest Ecuador. Using ¹³C isotope analyses we traced the origin of soil organic carbon (SOC) in mineral-associated soil fractions resistant to treatment with HCl, NaOCl, and Na₄P₂O₇ under pasture (C₄) and secondary forest (C₃). Prior to chemical treatments, particulate organic matter was removed by density fractionation (cut-off: 1.6 g cm^?3). Our results show that: (1) independent of soil mineralogical composition, about 45% of mineral-associated SOC was resistant to acid hydrolysis, suggesting a comparable SOM composition for the investigated soils; (2) oxidation by NaOCl isolated a SOM fraction with enhanced stability of mineral-bound SOM in soils developed from volcanic ashes; while Na₄P₂O₇ extracted more SOC, indicating the importance of Al-humus complexes in these soils; and (3) recently incorporated SOM was not stabilized after land use change in soils developed from volcanic ashes but was partly stabilized in soils rich in smectites. Together these results show that the employed methods were not able to isolate a SOM fraction which is protected against microbial decay under field conditions and that the outcome of these methods is sensitive to soil type which makes interpretation challenging and generalisations to other soils types or climates impossible.     

Assessing soil quality under intensive cultivation and tree orchards in Southern Italy

Concerns about groundwater contamination as well as pesticide residues in food and soil have fuelled vigorous debates about the sustainability of chemical-intensive agriculture. Search has been prompted for agronomic strategies with lower environmental hazards. In this multidisciplinary study we compared the characteristics of soils from 20 agricultural farms selected in five geographical areas of Southern Italy with different soil types. In each farm, fields with management regime classified as high-input (HIMR, intensive cultivation under plastic tunnels) or low-input (LIMR, tree orchards) were selected. Soil samples were analyzed for 31 parameters including physical and chemical properties (bulk density, water holding capacity, texture, pH, limestone, electrical conductivity, organic C to a depth of 0–20 and 20–40 cm, total N, P₂O₅, Ca²⁺, Mg²⁺, K⁺, Na⁺, cation exchange capacity), enzymatic activities (dehydrogenase, arylsulphatase, β-glucosidase, phosphatase and urease) and microbiological features (potential respiration, functional diversity of microbial populations by BIOLOG EcoPlates™, microbial biomass, fungal mycelium, culturable actinomycetes, bacteria and fungi, pseudomonads and bacterial species richness by 16S rDNA-DGGE). Finally, a soil bioassay was performed in order to evaluate the plant growth of a biotest plant (Lactuca sativa) and soil suppressiveness of the Rhizoctonia solani–L. sativa pathosystem.Results showed that many soil properties were influenced by management regime more than by the sampling area. Compared to LIMR, HIMR soils consistently had reduced soil organic C (−24%), enzymatic activities, microbial biomass and fungal mycelium (−40% and −18%, respectively), functional diversity (−18%) and bacterial species richness (−14%). On the contrary, the same soils showed a remarkable increase in the values of the parameters related to the mineral soil fraction (electrical conductivity +370%; P₂O₅ +72%; Na⁺ +86%). Management regime did not affect cation exchange capacity, pH, limestone and soil texture. The lettuce bioassay showed a higher plant growth (+17%) in the LIMR compared to HIMR soils, despite the lower content of mineral nutrients. Suppression of R. solani was not influenced by management regime, but significant differences were recorded among farms. Differences among the assessed soil parameters indicate a trend of soil quality deterioration under the high-input management regime.     

FTIR‐spectroscopic characterization of humic acids and humin fractions obtained by advanced NaOH,Na4P2O7, and Na2CO3 extraction procedures

Aim of our study was the development of the methodological basis for the characterization of humic fractions of a long‐term field experiment. Humic acids (HAs) were extracted from three layers of a nontilled soil using three different extractants (1 M NaOH, 0.1 M Na₄P₂O₇, 1 M Na₂CO₃), and the humin fraction was enriched. NaOH as extractant for FTIR analysis of humic substances yields higher resolved IR spectra, especially in the important regions of stretching vibrations including aromatic and aliphatic groups and in the fingerprint area including amides, aliphats, and aromats than the other extractants. The NaOH extraction has lower extraction yields as compared to Na₄P₂O₇ and Na₂CO₃ and represents a different part of the soil organic matter (SOM). This is reflected by lower C : N ratios and higher E₄ : E₆ and fulvic acid–to–humic acid ratios as compared to the other extractants. The FTIR band areas of HA fraction obtained by NaOH showed an increase of the aromatic and carbonyl groups and a decrease of amide groups with increasing soil depth. Aliphatic groups showed contradicting results: The bands of the stretching vibrations increased, and the band of the bending vibrations decreased. We assume that band interactions in the bending vibrations were responsible for that phenomenon under the assumption of an increase of aliphatic groups with increasing soil depth. The IR bands of the enriched humin fraction showed a decreasing trend in case of both aliphatic bands deriving from stretching vibrations and an increase of aromatic characteristics with depth. Our study led to the conclusion that HA fractions obtained by 1 M NaOH represent a small and dynamic fraction indicated by the measured yields in combination with values of N_t, C : N, E₄ : E₆ ratios, and ratios of fulvic acids (FA) to HA. The humin fraction has a high contribution to the total organic C and represents a more stabilized fraction of SOM which still shows changes in its aromatic and aliphatic characteristics with soil depth.     

Determination of kC and kNin situ for calibration of the chloroform fumigation-incubation method

¹⁴C-labelled glucose and ¹⁵N-labelled KNO₃ were added to soil and the microbial biomass during 42 days' incubation was estimated using the chloroform fumigation-incubation method (CFIM). By day 1, most of the glucose (1577 μgCg^?1 soil) was metabolized and 110 μg NO^?₃-Ng^?1 soil were immobilized. In situ values for the proportions of biomass C (k_C) and biomass N (k_N) mineralized during the 10 days after CHCl₃ fumigation were determined on the basis that the immobilized labelled C and N remaining in the soil at this time were present as living microbial cells and their associated metabolites. The tracer data indicated that biomass C could be calculated by applying a k_c value of 0.41 to the CO₂-C evolved from the fumigated sample without subtraction of an unfumigated “control”. Biomass N was estimated from the net NH₄^?-N accumulation during the fumigation-incubation. The problem of reimmobilization of NH⁺₄-N where organisms of wide C:N ratio occur was overcome by adjusting the value of k_N according to the ratio of CO₂-C evolved: net NH₄⁺-N accumulated during the fumigation-incubation (C_F:N_F).A C_F:N_F ratio of 6:1 resulted in a k_N of 0.30 whereas a ratio of 13:1 indicated a k_N of 0.20.     

Decomposition de corps microbiens dans des sols fumiges au chloroforme: Effets du type de sol et de microorganisme

Five microbial species (Aspergillus flavus, Trichoderma viride, Streptomyces sp., Arthrobacter sp., Achromobacter liquefaciens) were cultivated in liquid media containing ¹⁴C-labelled glucose. The decomposition of these microorganisms was recorded in four different soils after chloroform fumigation by a technique related to that proposed by Jenkinson and Powlson, to determine the mineralization rate of microbial organic matter (K_c coefficient). Three treatments were used: untreated soil, fumigated soil alone and fumigated soil supplied with ¹⁴C-labelled cells. Total evolved CO₂ and ¹⁴CO₂ were measured after 7 and 14 days at 28°C.The labelled microorganisms enabled the calculation of mineralization rate K_c (K_c = mineralized microbial carbon/supplied microbial carbon). The extent of mineralization of labelled microbial carbon depended on the type of soil and on the microbial species. Statistical analysis of results at 7 days showed that 58% of the variance is taken in account by the soil effect and 32% by the microorganism effect. Between 35 and 49% of the supplied microbial C was mineralized in 7 days according to the soil type and the species of microorganism. Our results confirmed that the average value for K_c = 0.41 is acceptable, but K_c variability according to soil type must be considered.The priming effect on organic C and native microbial biomass mineralization, due to microbial carbon addition was obtained by comparison between the amount of non-labelled CO₂-C produced by fumigated soils with or without added labelled microorganisms: this priming effect was generally negligible.These results indicate that the major portion of the error of microbial biomass measurement comes from the K_c estimation.     

Discrete functional pools of soil organic matter in a UK grassland soil are differentially affected by temperature and priming

We show that both temperature and priming act differently on distinct C pools in a temperate grassland soil. We used SOM which was ¹⁴C-labelled in four different ways: by labelling soil with ¹⁴C-glucose, by adding leaf litter from plants pre-labelled with ¹⁴CO₂, and by labelling in situ with ¹⁴CO₂ applied to the ryegrass canopy either 6 or 18 months earlier. Samples of each type of ¹⁴C labelled soil were incubated at either 4, 10, 15, or 20 °C and the exponential loss of ¹⁴CO₂ used to characterise treatment effects. ¹⁴C allocation to microbial fractions was greater, and so overall mineralization by microbes was greater, as temperature rose, but turnover of the microbial labile pool was temperature-insensitive, and the turnover of microbial structural material was reduced as temperature rose. The ability of the microbial population to degrade just one fraction of plant litter was increased greatly by temperature. A pool of SOM with a half-life of about 70 d was degraded faster at higher temperatures. Less tractable but abundant pools of SOM were not accessed more readily at higher temperatures by the microbial population. Priming with glucose or amino-acids only speeded the mineralization of recent SOM (probably from the living microbial biomass), and was not altered by temperature. These results have implications for the impacts of climate change on soil C cycling.     

Extractability of microbial N as influenced by C : N ratio in the flush after drying or fumigation

 Extractability of microbial N was estimated using in situ labelling of the microbial population with ¹⁵N. Four arable soils (one grey forest soil and three chernozems with different long-term fertilization) were amended with (NH₄)₂SO₄ (unlabelled or labelled with ¹⁵N) and d-glucose with a C : N ratio of 10 : 1 or 20 : 1 for the grey forest soil and 50 : 1 for the chernozems. d-glucose and labelled N with a C : N ratio of 20 : 1 did not cause microbial immobilization of unlabelled N. The use of substrates with a C : N ratio of 50 : 1 led to a pronounced priming action on soil N and decreased the extractability of immobilized ¹⁵N. Values of the extractable biomass N fraction (k _EN ) assessed for the fumigation-extraction and rehydration procedures were similar and varied in inverse proportion to the C : N ratio of the flush. The k _EN factor was calculated using values of the C : N ratio in flushes and the fixed C : N ratio of structural cell components, with the assumption that the C : N ratio of the extractable cytoplasmic cell fraction is variable. The ratio between the extractable and non-extractable biomass N fraction (k _EC ) and the C : N ratio of non-extractable cell components were assessed as equation parameters optimized for the measured k _EN and C : N ratio of flush data. Received: 31 October 1997     

Chloroform fumigation and the release of soil nitrogen: A rapid direct extraction method to measure microbial biomass nitrogen in soil

A new “direct extraction” method for measuring soil microbial biomass nitrogen (biomass N) is described. The new method (fumigation-extraction) is based on CHC1₃ fumigation, followed by immediate extraction with 0.5 M K₂SO₄ and measurement of total N released by CHC1₃ in the soil extracts. The amounts of NH₄-N and total N extracted by K₂SO₄ immediately after fumigation increased with fumigation time up to 5 days. Total N released by CHC1₃ after 1 day fumigation (1 day CHC1₃-N) and after 5 days fumigation (5 day CHC1₃-N) were positively correlated with the flush of mineral N (F_N) in 37 soils that had been fumigated, the fumigant removed and the soils incubated for 10 days (fumigation-incubation). The regression equations were 1 day CHC1₃-N = (0.79 ± 0.022) F_N and 5 day CHC1₃-N = (1.01 ± 0.027) F_N, both regressions accounting for 92% of the variance in the data.In field soils previously treated with ¹⁵N-labelled fertilizer, the amounts of labelled N, measured after fumigation-extraction, were very similar to the amounts of labelled N mineralized during fumigation-incubation; both were about 4 times as heavily labelled as the soil N as a whole. These results suggest that fumigation-extraction and fumigation-incubation both measure the same fraction of the soil organic N (probably the cytoplasmic component of the soil microbial biomass) and that measurement of the total N released by CHC1₃ fumigation for 24 h provides a rapid method for measuring biomass N.     

Simulating the dynamics of glucose and NH4+ in alkaline saline soils of the former Lake Texcoco with the Detran model

The turnover of organic matter determines the availability of plant nutrients in unfertilized soils, and this applies particularly to the alkaline saline soil of the former Lake Texcoco in Mexico. We investigated the effects of alkalinity and salinity on dynamics of organic material and inorganic N added to the soil. Glucose labelled with ¹⁴C was added to soil of the former Lake Texcoco drained for different lengths of time, and dynamics of ¹⁴C, C and N were investigated with the Detran model. Soil was sampled from an undrained plot and from three drained for 1, 5 and 8 years, amended with 1000 mg ¹⁴C‐labelled glucose kg^?1 and 200 mg NH₄⁺‐N kg^?1, and incubated aerobically. Production of ¹⁴CO₂ and CO₂, dynamics of NH₄⁺, NO₂^– and NO₃^–, and microbial biomass ¹⁴C, C and N were monitored and simulated with the Detran model. A third stable microbial biomass fraction had to be introduced in the model to simulate the dynamics of glucose, because > 90 mg ¹⁴C kg^?1 soil persisted in the soil microbial biomass after 97 days. The observed priming effect was mostly due to an increased decay of soil organic matter, but an increased turnover of the microbial biomass C contributed somewhat to the phenomenon. The dynamics of NH₄⁺ and NO₃^– in the NH₄⁺‐amended soil could not be simulated unless an immobilization of NH₄⁺ into the microbial biomass occurred in the first day of the incubation without an immediate incorporation of it into microbial organic material. The dynamics of C and a priming effect could be simulated satisfactorily, but the model had to be adjusted to simulate the dynamics of N when NH₄⁺ was added to alkaline saline soils.     

Studies of nitrogen immobilization and mineralization in calcareous soils—V. Formation and distribution of isotope-labelled biomass during decomposition of 14C- and 15N-labelled plant material

Medicago littoralis leaf material, labelled with ¹⁴C and ¹⁵N, and of C:N ratio 8.7:1, decomposed rapidly in a calcareous soil. One half of the plant-C and two thirds of the plant-N remained in the soil as organic residues after 34 days. The rates of decomposition and the changes in the distribution of organic-¹⁴C and -¹⁵N residues followed similar patterns.Incorporation of ¹⁴C and ¹⁵N into microbial cells, formed during plant breakdown, reached a maximum after 62 days. At this time the microbial biomass accounted for 21.9 and 23.3%, respectively, of residual organic-¹⁴C and -¹⁵N. Thereafter, the amounts of isotope-labelled biomass decreased with the percentage decrease slightly exceeding that of the total labelled soil residues.During plant decomposition, changes occurred in the concentrations of organic-¹⁴C and -¹⁵N in some of the soil components, these having been fractionated according to density and particle size. Especially evident was the rapid and extensive decrease of labelled material from the fine clay-size components. This was partly due to the decrease in the biomass-¹⁴C of this fraction. Changes in biomass-¹⁴C of some physical fractions were approximately reflected by changes in their numbers of viable microorganisms.     

Organic manure input and straw cover improved the community structure of nitrogen cycle function microorganism driven by water erosion

Water erosion process induces differences to the nitrogen (N) functional microbial community structure, which is the driving force to key N processes at soil-water interface. However, how the soil N transformations associated with water erosion is affected by microorganisms, and how the microbial respond, are still unclear. The objective of this study is to investigate the changes of microbial diversity and community structure of the N-cycle function microorganisms as affected by water erosion under application of organic manure and straw cover. On the basis of iso-nitrogen substitution, four treatments were set up: 1) only chemical fertilizer with N 150 kg ha^?1, P₂O₅ 60 kg ha^?1 and K₂O 90 kg ha^?1 (CK); the N was substituted 20% by 2) organic manure (OM); 3) straw (SW); and 4) organic manure + straw (1:1) (OMSW). The results showed that applying organic manure and straw to sloping farmland can increase soil N contents, but reduce runoff depth, K_w, sediment yield and N loss, especially in the OMSW. Straw cover and straw + organic manure increased the diversity (Chao1) of nitrifier (AOB), and both diversity and uniformity (Shannon) of denitrifier (nirK/S) were increased in the OMSW. All erosion control measures reduced N-fixing bacteria diversity and increased their uniformity, and the combined application of organic manure and straw cover was a better erosion control measure than the single application of them. Improved soil chemistry and erodibility were the main drives for the changes of N-functional microbial community structure and the appearance of dominant bacteria with different organic materials.