Leaching of dissolved organic and inorganic nitrogen from legume-based grasslands

Leaching of dissolved inorganic N (DIN) and dissolved organic N (DON) is a considerable loss pathway in grassland soils. We investigated the white clover (Trifolium repens) contribution to N transport and temporal N dynamics in soil solution under a pure stand of white clover and white clover-ryegrass (Lolium perenne) mixed stand. The temporal white clover contribution to N leaching was analysed by ¹⁵N incorporation into DIN and DON in percolating soil solution collected at 25-cm depth following white clover ¹⁵N leaf labelling that was applied at different times during the growing season. The white clover contribution to N transport in the soil profile was investigated over 2 years by analysing ¹⁵N in DIN and DON in percolating soil solution collected at 25-, 45- and 80-cm depth following ¹⁵N leaf labelling of white clover. The results showed that clover was a source of both DIN and DON. White clover autumn deposition contributed the most to N leaching. The leaching of DIN from the white clover in pure stand exceeded that of the mixed stand and confirmed that leaching of DIN is a function of N loadings and N demand. The DON leaching was unaffected by the presence of a companion grass, suggesting that the DON leaching from our grassland derived from the lysis of soil microbial biomass living on recent white clover deposits. White clover contributed to the leaching of DIN and DON at all depths, and the fact that the contents of DI ¹⁵N and DO ¹⁵N did not change with depth indicated that surplus of DIN and DON, formed in the uppermost soil layer, was transported in the soil profile.     

Contribution of dissolved organic nitrogen deposition to nitrogen saturation in a forested mountainous watershed in Tsukui, Central Japan

Nitrogen (N) budget was estimated with dissolved inorganic N (DIN) and dissolved organic N (DON) in a forested mountainous watershed in Tsukui, Kanagawa Prefecture, about 50 km west of Tokyo in Central Japan. The forest vegetation in the watershed was dominant by Konara oak (Quercus serrata) and Korean hornbeam (Carpinus tschonoskii), and Japanese cedar (Cryptomeria japonica). Nitrate (NO₃ ^?) concentration in the watershed streamwater was averagely high (98.0 ±± 19 (±± SD, n = 36) μmol L^?1) during 2001–2003. There was no seasonal and annual changes in the stream NO^? ₃ concentration even though the highest N uptake rate presumably occurred during the spring of plant growing season, a fact indicating that N availability was in excess of biotic demands. The DON deposition rates (DON input rates) in open area and forest area were estimated as one of the main N sources, accounting for about 32% of total dissolved N (TDN). It was estimated that a part of the DON input rate contributed to the excessive stream NO^? ₃ output rate under the condition of the rapid mineralization and nitrification rates, which annual DON deposition rates were positively correlated with the stream NO₃ ^? output rates. The DON retention rate in the DON budget had a potential capacity, which contributed to the excessive stream NO^? ₃ output rate without other N contributions (e.g. forest floor N or soil N).     

Role of dissolved organic nitrogen (DON) in soil N cycling in grassland soils

Dissolved organic nitrogen (DON) represents a significant pool of soluble N in many soils and freshwaters. Further, the low molecular weight (LMW) component of DON represents an important source of N for microorganisms and can also be utilized directly by some plants. Our purpose was to determine which of the pathways in the decomposition and subsequent ammonification and nitrification of organic N represented a significant block in soil N supply in three agricultural grassland soils. The results indicate that the conversion of insoluble organic N to LMW-DON and not LMW-DON to NH₄⁺ or NH₄⁺ to NO₃⁻ represents a major constraint to N supply. We hypothesize that there are two distinct DON pools in soil. The first pool comprises mainly free amino acids and proteins and is turned over very rapidly by the microbial community, so it does not accumulate in soil. The second pool is a high molecular weight pool rich in humic substances, which turns over slowly and represents the major DON loss to freshwaters. The results also suggest that in NO₃⁻ rich soils the uptake of LMW-DON by soil microorganisms may primarily provide them with C to fuel respiration, rather than to satisfy their internal N demand.     

Microbial immobilization and mineralization of dissolved organic nitrogen from forest floors

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

Potential fluxes of N2O and CH4 from soils of three forest types in Eastern Canada

We conducted laboratory incubation experiments to elucidate the influence of forest type and topographic position on emission and/or consumption potentials of nitrous oxide (N₂O) and methane (CH₄) from soils of three forest types in Eastern Canada. Soil samples collected from deciduous, black spruce and white pine forests were incubated under a control, an NH₄NO₃ amendment and an elevated headspace CH₄ concentration at 70% water-filled pore space (WFPS), except the poorly drained wetland soils which were incubated at 100% WFPS. Deciduous and boreal forest soils exhibited greater potential of N₂O and CH₄ fluxes than did white pine forest soils. Mineral N addition resulted in significant increases in N₂O emissions from wetland forest soils compared to the unamended soils, whereas well-drained soils exhibited no significant increase in N₂O emissions in-response to mineral N additions. Soils in deciduous, boreal and white pine forests consumed CH₄ when incubated under an elevated headspace CH₄ concentration, except the poorly drained soils in the deciduous forest, which emitted CH₄. CH₄ consumption rates in deciduous and boreal forest soils were twice the amount consumed by the white pine forest soils. The results suggest that an episodic increase in reactive N input in these forests is not likely to increase N₂O emissions, except from the poorly drained wetland soils; however, long-term in situ N fertilization studies are required to validate the observed results. Moreover, wetland soils in the deciduous forest are net sources of CH₄ unlike the well-drained soils, which are net sinks of atmospheric CH₄. Because wetland soils can produce a substantial amount of CH₄ and N₂O, the contribution of these wetlands to the total trace gas fluxes need to be accounted for when modeling fluxes from forest soils in Eastern Canada.     

林分类型和氮添加对亚热带森林土壤氧化亚氮排放的影响

为揭示亚热带森林土壤N₂O排放对林分类型和氮添加的响应特征,选取位于福建省三明市的中亚热带米槠次生林、杉木人工林和马尾松人工林土壤为研究对象,分别设置无氮添加(N0 mg/kg)、低氮添加(N10 mg/kg)、中氮添加(N25 mg/kg)和高氮添加(N50 mg/kg)4个氮添加水平,进行微宇宙培养试验,测定土壤N₂O排放。结果表明：与无氮添加处理相比,氮添加整体上降低3种林分土壤pH,增加土壤NH₄⁺-N和NO₃^--N含量。无氮添加处理中杉木人工林和马尾松人工林土壤N₂O累积排放量分别为9.67和9.62 mg/kg,显著高于米槠次生林土壤N₂O累积排放量6.81 mg/kg。低氮添加处理中杉木人工林和马尾松人工林土壤N₂O累积排放量显著高于米槠次生林。但在中氮和高氮添加处理中,3种林分土壤N₂O累积排放量均无显著性差异。不同氮添加处理均促进3种林分土壤N     

Dissolved organic nitrogen dynamics in a Mediterranean vineyard soil

Dissolved organic carbon (DOC) and nitrogen (DON) have been hypothesized to play a central role in nutrient cycling in agricultural soils. The aim of this study was to investigate the annual dynamics of DOC and DON in a Greek vineyard soil and to assess the potential role of DON in supplying N to the vines. Our results indicated that significant quantities of DOC and DON existed in soil throughout the year and that peaks in concentration appeared to correlate with discrete agronomic events (e.g. onset of irrigation and plowing). Both field and laboratory experiments showed that free amino acids were rapidly mineralized in soil and that consequently free amino acids represented only a small proportion of the soil's total soluble N. Due to rapid nitrification the soil solution N was dominated by NO₃⁻. Based upon the calculation of a plant-soil N budget and previous studies on N uptake in Vitis vinifera L., it is likely that DON uptake does not directly supply significant amounts of N to the plant. As the soil was not N limited we hypothesize that amino acids are used by the microbial community more as a source of C rather than a source of N. While we conclude that DON constitutes a significant N pool in vineyard soils further work is required to chemically characterize its constituent units and their relative bioavailability so that their overall role in N cycling can be determined.     

Effects of nickel addition on nitrogen mineralization,nitrification, and nitrogen leaching in some boreal forest soils

The effects of Ni additions on nitrification, N mineralization, and N leaching were examined in soils from boreal jack pine (Pinus banksiana Lamb.) forests. The results of a series of incubation experiments suggested that under certain conditions, Ni at 100 μg g^?1 soil can stimulate nitrification, and at 500 μg g^?1 can stimulate N mineralization. Nitrification rates were very low overall, but were higher in soils from the vicinity of the Sudbury, Ontario Ni-Cu smelters than in uncontaminated soils. The nitrifier populations, estimated by the most probable number method, were extremely low in uncontaminated soils, but also increased following some Ni additions. Increased leaching of NO_f3 ^p was observed in soil columns treated with Ni. Since N tends to be in low supply in boreal forests, and therefore tightly cycled, the observed disruptions caused by Ni inputs could have an effect on forest productivity.     

Contribution of dissolved organic nitrogen to N leaching from four German agricultural soils

Dissolved organic nitrogen (DON) substantially contributes to N leaching from forest ecosystems. However, little is known about the role of DON for N leaching from agricultural soils. Therefore, the aim of our study was to quantify the contribution of DON to total N leaching from four agricultural soils. Concentrations and fluxes of DON and mineral N were monitored at two cropped sites (Plaggic Anthrosols) and two fallow plots (Plaggic Anthrosol and Gleyic Podzol) from November 1999 till May 2001 by means of glass suction plates. The experimental sites were located near the city of Münster, NW Germany. Median DON concentrations in 90 cm depth were 2.3 mg l^—1 and 2.0 mg l^—1 at the cropped sites and 1.6 mg l^—1 and 1.3 mg l^—1 at the fallow sites. There was only a slight (Anthrosols) or no (Gleyic Podzol) decrease in median DON concentrations with increasing depth. Total N seepage was between 19 kg N ha^—1 yr^—1 and 46 kg N ha^—1 yr^—1 at the fallow sites and 16—159 kg N ha^—1 yr^—1 at the cropped sites. For the fallow plots, DON seepage contributed 10—21 % to the total N flux (4—5 kg DON ha^—1 yr^—1), at the cropped sites DON seepage was 6—21 % of the total N flux (6—10 kg DON ha^—1 yr^—1). Thus, even in highly fertilized agricultural soils, DON is a considerable N carrier in seepage that should be considered in detailed soil N budgets.     

Dissolved organic carbon and nitrogen dynamics in temperate coniferous forest plantations

Dissolved organic matter (DOM) plays a central role in driving many chemical and biological processes in soil; however, our understanding of the fluxes and composition of the DOM pool still remains unclear. In this study we investigated the composition and dynamics of dissolved organic carbon (DOC) and nitrogen (DON) in five temperate coniferous forests. We subsequently related our findings to the inputs (litterfall, throughfall, atmospheric deposition) and outputs (leaching, respiration) of C and N from the forest and to plant available sources of N. With the exception of NO₃^?, most of the measured soil solution components (e.g. DOC, DON, NH₄⁺, free amino acids, total phenolics and proteins) progressively declined in concentration with soil depth, particularly in the organic horizons. This decline correlated well with total microbial activity within the soil profile. We calculated that the amount of C lost by soil respiration each day was equivalent to 70% of the DOC pool and 0.06% of the total soil C. The rapid rate of amino acid mineralization and the domination of the low molecular weight soluble N pool by inorganic N suggest that the microbial community is C‐ rather than N‐limited and that C‐limitation increases with soil depth. Further, our results suggest that the forest stands were not N‐limited and were probably more reliant on inorganic N as a primary N source rather than DON. In conclusion, our results show that the size of the DON and DOC pools are small relative to both the amount of C and N passing through the soil each year and the total C and N present in the soil. In addition, high rates of atmospheric N deposition in these forests may have removed competition for N resources between the plant and microbial communities.     

Conversion of grassland to coniferous woodland has limited effects on soil nitrogen cycle processes

In the last century, conversion of native North American grasslands to Juniperus virginiana forests or woodlands has dramatically altered ecosystem structure and significantly increased ecosystem carbon (C) stocks. We compared soils under recently established J. virginiana forests and adjacent native C₄-dominated grassland to assess changes in potential soil nitrogen (N) transformations and plant available N. Over a 2-year period, concentrations of extractable inorganic N were measured in soils from forest and grassland sites. Potential gross N ammonification, nitrification, and consumption rates were determined using ¹⁵N isotope-dilution under laboratory conditions, controlling for soil temperature and moisture content. Potential nitrification rates (V_max) and microbial biomass, as well as soil physical and chemical properties were also assessed. Extractable NH₄⁺ concentrations were significantly greater in grassland soils across the study period (P ≤ 0.01), but analysis by date indicated that differences in extractable inorganic N occurred more frequently in fall and winter, when grasses were senescent but J. virginiana was still active. Laboratory-based rates of gross N mineralization (ammonification) and nitrification were greater in grassland soils (P ≤ 0.05), but only on one of four dates. Potential nitrification rates (V_max) were an order of magnitude greater than gross nitrification rates in both ecosystems, suggesting that nitrification is highly constrained by NH₄⁺ availability. Differences in plant uptake of N, C inputs, and soil microclimate as forests replace grasslands may influence plant available N in the field, as evidenced by seasonal differences in soil extractable NH₄⁺, and total soil C and N accumulation. However, we found few differences in potential soil N transformations under laboratory conditions, suggesting that this grassland-to-forest conversion caused little change in mineralizable organic N pools or potential microbial activity.     

Interactive effects of tree species and soil moisture on methane consumption

Methane consumption by temperate forest soils is a major sink for this important greenhouse gas, but little is known about how tree species influence CH₄ uptake by soils. Here, we show that six common tree species in Siberian boreal and temperate forests significantly affect potential CH₄ consumption in laboratory microcosms. Overall, soils under hardwood species (aspen and birch) consumed CH₄ at higher rates than soils under coniferous species and grassland. While NH₄⁺ addition often reduces CH₄ uptake, we found no effect of NH₄⁺ addition, possibly because of the relatively high ratio of CH₄-to-NH₄⁺ in our incubations. The effects of soil moisture strongly depended on plant species. An increase in soil moisture enhanced CH₄ consumption in soils under spruce but had the opposite effect under Scots pine and larch. Under other species, soil moisture did not affect CH₄ consumption. These results could be explained by specific responses of different groups of CH₄-oxidizing bacteria to elevated moisture.     

Soil biochemical and chemical changes in relation to mature spruce (Picea abies) forest conversion and regeneration

To investigate soil changes from forest conversion and regeneration, soil net N mineralization, potential nitrification, microbial biomass N, L‐asparaginase, L‐glutaminase, and other chemical and biological properties were examined in three adjacent stands: mature pure and dense Norway spruce (Picea abies (L.) Karst) (110 yr) (stand I), mature Norway spruce mixed with young beech (Fagus sylvatica) (5 yr) (stand II), and young Norway spruce (16 yr) (stand III). The latter two stands were converted or regenerated from the mature Norway spruce stand as former. The studied soils were characterized as having a very low pH value (2.9 – 3.5 in 0.01 M CaCl₂), a high total N content (1.06 – 1.94 %), a high metabolic quotient (qCO₂) (6.7 – 16.9 g CO₂ kg^–1 h^–1), a low microbial biomass N (1.1 – 3.3 % of total N, except LOf₁ at stand III), and a relatively high net N mineralization (175 – 1213 mg N kg^–1 in LOf₁ and Of₂, 4 weeks incubation). In the converted forest (stand II), C : N ratio and qCO₂ values in the LOf₁ layer decreased significantly, and base saturation and exchangeable Ca showed a somewhat increment in mineral soil. In the regenerated forest (stand III), the total N storage in the surface layers decreased by 30 %. The surface organic layers (LOf₁, Of₂) possessed a very high net N mineralization (1.5 – 3 times higher than those in other two stands), high microbial biomass (C, N), and high basal respiration and qCO₂ values. Meanwhile, in the Oh layer, the base saturation and the exchangeable Ca decreased. All studied substrates showed little net nitrification after the first period of incubation (2 weeks). In the later period of incubation (7 – 11 weeks), a considerable amount of NO₃‐N accumulated (20 – 100 % of total cumulative mineral N) in the soils from the two pure spruce stands (I, III). In contrast, there was almost no net NO₃‐N accumulation in the soils from the converted mixed stand (II) indicating that there was a difference in microorganisms in the two types of forest ecosystems. Soil microbial biomass N, mineral N, net N mineralization, L‐asparaginase, and L‐glutaminase were correlated and associated with forest management.     

Soluble organic nitrogen in agricultural soils

 The existence of soluble organic forms of N in rain and drainage waters has been known for many years, but these have not been generally regarded as significant pools of N in agricultural soils. We review the size and function of both soluble organic N extracted from soils (SON) and dissolved organic N present in soil solution and drainage waters (DON) in arable agricultural soils. SON is of the same order of magnitude as mineral N and of equal size in many cases; 20–30 kg SON-N ha^–1 is present in a wide range of arable agricultural soils from England. Its dynamics are affected by mineralisation, immobilisation, leaching and plant uptake in the same way as those of mineral N, but its pool size is more constant than that of mineral N. DON can be sampled from soil solution using suction cups and collected in drainage waters. Significant amounts of DON are leached, but this comprises only about one-tenth of the SON extracted from the same soil. Leached DON may take with it nutrients, chelated or complexed metals and pesticides. SON/DON is clearly an important pool in N transformations and plant uptake, but there are still many gaps in our understanding. Received: 10 June 1999     

地中海生态系统中可溶性有机N研究

Dissolved organic nitrogen (DON) in soils has recently gained increasing interest because it may be both a direct N source for plants and the dominant available N form in nutrient-poor soils, however, its prevalence in Mediterranean ecosystems remains unclear. The aims of this study were to i) estimate soil DON in a wide set of Mediterranean ecosystems and compare this levels with those for other ecosystems; ii) describe temporal changes in DON and dissolved inorganic nitrogen (DIN) forms (NH4+ and NO3? ), and characterize spatial heterogeneity within plant communities; and iii) study the relative proportion of soil DON and DIN forms as a test of Schimel and Bennett’s hypothesis that the prevalence of different N forms follows a gradient of nutrient availability. The study was carried out in eleven plant communities chosen to represent a wide spectrum of Mediterranean vegetation types, ranging from early to late successional status. DON concentrations in the studied Mediterranean plant communities (0-18.2 mg N kg-1) were consistently lower than those found in the literature for other ecosystems. We found high temporal and spatial variability in soil DON for all plant communities. As predicted by the Schimel and Bennett model for nutrient-poor ecosystems, DON dominance over ammonium and nitrate was observed for most plant communities in winter and spring soil samples. However, mineral-N dominated over DON in summer and autumn. Thus, soil water content may have an important effect on DON versus mineral N dominance in Mediterranean ecosystems.     

Fate of N and C from labelled plant material: Recovery in perennial ryegrass–clover mixtures and in pore water of the sward

The belowground C and N dynamics leading to organic and inorganic N leaching from perennial ryegrass–clover mixtures are not well understood. Based on the hypothesis that four different plant materials would degrade differently, a 16 months field experiment was conducted to determine (i) the source strength of labelled plant residues in dissolved inorganic N (DIN) and dissolved organic N (DON) in pore water from the plough layer, and (ii) the plant uptake of organically bound N. Litterbags containing ¹⁴C- and ¹⁵N-labelled ryegrass or clover roots or leaves were inserted into the sward of a ryegrass–clover mixture in early spring. The fate of the released ¹⁴C and ¹⁵N was monitored in harvested biomass, roots, soil, and pore water percolating from the plough layer. No evidence of plant uptake of dual-labelled organic compounds from the dual-labelled residues could be observed. N in pore water from the plough layer during autumn and winter had a constant content of dissolved organic N (DON) and an increasing content of dissolved inorganic N (DIN). A positive correlation between aboveground clover biomass harvested in the growth season and total-N in pore water indicated that decaying roots from the living clover could be a major source of the 10 kg N ha⁻¹ being lost with pore water during autumn and winter. The presence of ¹⁵N in pore water shifted from the DON fraction in autumn to the DIN fraction in late winter, with strong indications that ¹⁵N originated from the living ryegrass. However, ¹⁵N in pore water originating from plant residues only constituted 1.5% of the total dissolved N from the plough layer.     

Importance of organic nitrogen fractions in sandy soils,obtained by electro-ultrafiltration or CaCl2 extraction,for nitrogen mineralization and nitrogen uptake of rape

Summary Sandy soils have low reserves of mineral N in spring. Therefore organic-bound N is the most important pool available for crops. The objective of the present investigation was to study the importance of the organic-bound N extracted by electro-ultrafiltration and by a CaCl₂ solution for the supply of N to rape and for N mineralization. Mitscherlich-pot experiments carried out with 12 different sandy soils (Germany) showed a highly significant correlation between the organic N extracted (two fractions) and the N uptake by the rape (electroultrafiltration extract: r=0.76^***; CaCl₂ extract: r=0.76^***). Organic N extracted by both methods before the application of N fertilizer was also significantly correlated with N mineralization (electro-ultrafiltration extract: r=0.75^***; CaCl₂ extract: r=0.79^***). N uptake by the rape and the mineralization of organic N increased with soil pH and decreased with an increasing C:N ratio and an increasing proportion of sand in the soils. Ninety-eight percent of the variation in N uptake by the rape was determined by the differences in net mineralization of organic N. This show that in sandy soils with low mineral N reserves (NO _inf3 ^sup- -N, NH ₄ ⁺ -N) the organic soil N extracted by electro-ultrafiltration or CaCl₂ solutions indicates the variance in plant-available N. Total soil N was not related to the N uptake by plants nor to N mineralization.     

Soil nitrogen pools and turnover in native woodland and managed pasture soils

Dissolved organic nitrogen (DON) is a significant nitrogen (N) pool in most soils and is considered to be important for N cycling. The present study focused on paired sites of native remnant woodland and managed pasture at three locations in south-eastern Australia. Improved understanding of N cycling is important for assessing the impact of agriculture on soil processes and can guide conservation and restoration soil management strategies to maintain remnant native woodland systems, which currently exist as small pockets of woodland within extensive managed pasture landscapes. Organic and inorganic N pools were quantified, as well as the rates of amino acid and peptide mineralisation in the paired native woodland and managed pasture systems. Soil DON dominated the soil N pool in both land uses, and the proportion of DON to other N pools was greatest at the most N-limited site (up to ∼70% of extractable N). In both land uses soil ammonium and free amino acid concentrations were similar (∼20% of extractable N), and soil nitrate formed the smallest N pool (<∼5% of extractable N). Mineralisation of ¹⁴C-labelled amino acid and peptide substrates was rapid (<3 h), and more amino acid was respired than peptide in both the native woodland and managed pasture soils. Soil C:N ratio was important in separating site and land use differences, and contrasting relationships between soil physico-chemical properties and organic N uptake rates were identified across sites and land uses.     

Amino acid,peptide and protein mineralization dynamics in a taiga forest soil

The availability of inorganic N has been shown to be one of the major factors limiting primary productivity in high latitude ecosystems. The factors regulating the rate of transformation of organic N to nitrate and ammonium, however, remain poorly understood. The aim of this study was to investigate the nature of the soluble N pool in forest soils and to determine the relative rate of inorganic N production from high and low molecular weight (MW) dissolved organic nitrogen (DON) compounds in black spruce forest soils. DON was found to be the dominant N form in soil solution, however, most of this DON was of high MW of which >75% remained unidentified. Free amino acids constituted less than 5% of the total DON pool. The concentration of NO₃⁻ and NH₄⁺ was low in all soils but significantly greater than the concentration of free amino acids. Incubations of low MW DON with soil indicated a rapid processing of amino acids, di- and tri-peptides to NH₄⁺ followed by a slower transformation of the NH₄⁺ pool to NO₃⁻. The rate of protein transformation to NH₄⁺ was slower than for amino acids and peptides suggesting that the block in N mineralization in taiga forest soils is the transformation of high MW DON to low MW DON and not low MW DON to NH₄⁺ or NH₄⁺ to NO₃⁻. Calculated turnover rates of amino acid-derived C and N immobilized in the soil microbial biomass were similar with a half-life of approximately 30 d indicating congruent C and N mineralization.     

Assessment of Suitable Extractants for Predicting Plant-available Potassium in Indian Coastal Soils

The suitability of seven chemical extractants was evaluated on 24 Indian coastal soils for prediction of plant-available potassium (K) to rice (Oryza sativa L. var. NC 492) grown in modified Neubauer technique. Average amounts of soil K extracted were in descending order: 0.5 M NaHCO₃ > neutral 1 N NH₄OAc > 0.02 M CaCl₂ > Bray and Kurtz No.1 > 1 N HNO₃ > 0.1 N HNO₃ > distilled water. The highest simple correlation with plant K uptake was obtained with 0.1 N HNO₃-K (r = 0.848) and lowest with CaCl₂-K (r = 0.805). Predictive models were developed using plant K uptake as the dependent variable and extractable soil K, sand, silt, soil pH, and electrical conductivity as the independent variables. Based on the final R² and ease of measurement, distilled water, 1 N NH₄OAc, and 0.1 N HNO₃ models were the best predictors of plant-available K in coastal soils when used along with sand or soil pH.