Arylamidase Activity as an Index of Nitrogen Mineralization in Soils

Abstract

The enzyme arylamidase [EC 3.4.11.2] catalyzes the hydrolysis of N‐terminal amino acids from arylamides. Because it has been proposed that this enzyme may play a major role in nitrogen (N) mineralization in soils, studies were carried out using short‐term laboratory incubations under aerobic and anaerobic conditions and chemical hydrolysis of soil organic N to assess the N mineralization in a range of 51 soils from six agroecological zones of the North Central region of the United States. The enzyme activity was assayed at its optimal pH value. With the exception of the values obtained for field‐moist soils incubated under anaerobic conditions, the amounts of N mineralized by all the biological and chemical methods studied were significantly correlated with arylamidase activity, with r values of 0.54*** for the amounts of inorganic N produced under aerobic incubation, of 0.44** for anaerobic incubation of air‐dried soils, of 0.53*** and 0.55*** for the amounts of ammonium (NH₄ ⁺)‐N released by steam distillation with PO₄‐B₄O₇ for 4 and 8 min, respectively; and of 0.49*** and 0.53*** for the amounts of NH₄ ⁺‐N released by steam distillation with disodium tetraborate (Na₂B₄O₇) for 4 min or 8 min, respectively. The amounts of N extractable with hot potassium chloride (KCl) were most significantly correlated with arylamidase activity (r=0.56***). Arylamidase activity was significantly correlated with organic carbon (C) (r=0.49***), organic N (r=0.55***), and fixed ammonium (NH₄ ⁺)‐N (r =0.42**).     

Tillage and residue management effects on β-glucosaminidase activity in soils

There is an increasing interest in assessing the effects of tillage systems and residue management on biochemical processes, especially enzyme activities, of soils. This study was carried out to investigate the effects of three tillage systems (no-till, chisel plow and moldboard plow) and four residue placements (bare, normal, mulch and double mulch) on the activity of N-acetyl-β-glucosaminidase (NAGase, EC 3.2.1.30) involved in C and N cycling in soils. The activity values were significantly affected by tillage and residue management practices, being greatest in soils with no-till/double mulch and least with no-till/bare and moldboard/normal. Also, they were the highest under no-till/ double mulch-treated soils. Linear regression analyses showed that the activity of NAGase was significantly correlated with organic C in the surface soils (r=0.89***) and with organic C content at different depths (r=0.97***). The NAGase activity values were significantly correlated with the arylamidase activity values of the soils (r=0.63**), suggesting that tillage and residue management practices have similar impacts on the activities of these enzymes. The activity of this enzyme decreased markedly with increasing depth of the surface soil (0-15 cm) of the no-till/ double mulch-treated plots.     

Phosphate buffer–extractable organic nitrogen as an index of soil‐N availability for sorghum and pearl millet

The availability of soil nitrogen (N) is usually quantified by the amount of mineralized N as determined after several weeks of soil incubation. Various alternative methods using chemical solvents have been developed to extract the available organic N, which is easily mineralized. We compared one such solution, neutral phosphate buffer (NPB), with conventional incubation and 0.01 M–CaCl₂ extraction, as measures of soil N available to two major cereal crops of the semiarid tropics, based on the total N uptake by plants in a pot experiment. Mineralized N had the highest correlation with N uptake by pearl millet (Pennisetum glaucum L., r = 0.979***) and sorghum (Sorghum bicolor [L.] Moench, r = 0.978***). NPB‐extractable N was also highly correlated with N uptake (pearl millet, r = 0.876***; sorghum, r = 0.872***). Only one major peak was detected when NPB extracts were analyzed using size‐exclusion high‐performance liquid chromatography, regardless of soil properties. In addition, the organic N extracted with NPB was characterized by determining the content of peptidoglycan, the main component of bacterial cell walls. Although the characteristics of NPB‐extractable organic N are still unclear, it offers a promising quick assay of available N.     

Organic fertilizers derived from plant materials Part I: Turnover in soil at low and moderate temperatures

The aim was to investigate different organic fertilizers derived from plant materials with respect to their nitrogen and carbon turnover in soil in comparison with organic fertilizers derived from animal‐waste products. In a 64‐day incubation study at 5°C and 15°C, the following fertilizers were used: coarse faba bean–seed meal (Vicia faba L.), coarse meals of yellow and white lupin seeds (Lupinus albus L. and Lupinus luteus L.), Phytoperls^® (waste products of maize [Zea mays L.] processing), coarse meal of castor cake (Ricinus communis L.) as a widely used organic fertilizer, and horn meal as a reference fertilizer‐derived from animal waste products. At 15°C, horn meal showed the highest apparent net N mineralization of fertilizer‐derived N, followed by castor cake and the two lupin meals. At 5°C, apparent net N mineralization of fertilizer‐derived N from horn meal and coarse meal of yellow lupin seeds was nearly identical, followed by castor‐cake meal. Net N mineralization from legume‐seed meals showed no or even a negative temperature response, at least temporarily. In contrast, the other fertilizers showed a positive temperature response of net N mineralization. The content in recalcitrant structural components and the decoupling of decomposition of N‐rich and C‐rich tissue components in time are discussed as controlling factors of fertilizer‐N turnover at low temperature. Microbial residues seem to be an important temporary sink of fertilizer‐derived C and N. Legume‐seed meals induced considerable N‐priming effects. Temperature induced differences in the decomposition of total fertilizer C, indicated by changes in the sum of cumulative CO₂‐C evolution, total K₂SO₄‐soluble organic C and microbial‐biomass C were much smaller than indicated by cumulative CO₂‐C evolution alone. Our results indicate that legume‐seed meals have the potential to replace horn meal and castor‐cake meal in organic vegetable production, especially when soil temperatures in early spring are still low.     

Nitrogen mineralization of leguminous crops in soils

Organic‐N production by legumes is a key benefit of growing cover crops and green manures. A soil sample was mixed with legume residue commonly used as green manure in Kenya at a rate of 500 mg N (kg soil)^–1. Silica sand equal to the weight of the soils was added and mixed thoroughly. The mixture was packed in a leaching tube and leached with 100 mL of 5 mM of CaCl₂ ^. 2H₂O and incubated at 30°C. The leaching was repeated every 2 weeks for a total of 16 weeks and analyzed for N as NH , NO , and NO . Five legume residues and five different soils were used in this study. Nitrogen mineralization of the legume residues conformed to an exponential model. Application of a two‐components exponential model showed two phases of N mineralization. The relationship between the organic N remaining after each incubation period and time of incubation was controlled by two first‐order reactions. The initial fast rate (k₁) changed to a slow rate (k₂) at incubation times ranging from 2 to 8 weeks, depending on the legume residue and the soil used. The percentage of N in each phase varied among the legume residue and soils. Linear regression analyses showed that net cumulative amounts of N mineralized from individual legume residues was significantly correlated with the total polyphenols and polyphenol‐to‐N ratios for two soils. Nitrogen mineralization of dolichos and field bean was significantly and negatively correlated with clay and sand, respectively; of field bean and alfalfa was significantly correlated with C_mic; and of dolichos significantly but negatively correlated with the total N and organic N in soils. Linear regression analysis of the pooled data showed that net cumulative amounts of N mineralized and percentage N mineralized were significantly correlated with C : N ratios of the residues (r = 0.44 and 0.48 at p < 0.05, respectively), and that net cumulative N mineralized was significantly correlated with (lignin + polyphenols)‐to‐N ratios (r = 0.41 at p < 0.05) and with lignin contents (r = 0.61 at p < 0.001).     

Mineralization of Nitrogen in Soils Amended with Dairy Manure as Affected by Wetting/Drying Cycles

Abstract

Interest in manure management and its effects on nitrogen (N) mineralization has increased in recent years. The focus of this research was to investigate the N‐mineralization rates of different soil types in Coastal Plain soils and compare them to a soil from Illinois. Soils with and without dairy composted manure addition were subjected to different wetting/drying cycles [constant moisture at 60% water‐filled pore space (WFPS) and cycling moisture from 60 to 30% WFPS] under laboratory conditions at three different temperatures (11°C, 18°C, and 25°C). Samples were collected from three different soil types: Catlin (Mollisols), Bama (Ultisols), and Goldsboro (Utilsols). Soil chemical and physical properties were determined to help assess variations in N-mineralization rates. Addition of composted manure greatly impacted the amount of N mineralized. The amount of manure‐derived organic N mineralized to inorganic forms was mainly attributed to the soil series, with the Catlin (silt loam) producing the most inorganic N followed by the Goldsboro (loam) and then Bama (sandy loam). This was probably due to soil texture and the native climatic conditions of the soil. No significant differences were observed between the constant and cycling moisture regimens, suggesting that the imposed drying cycle may not have been sufficient to desiccate microbial cells and cause a flush in N mineralization upon rewetting. Nitrogen mineralization responded greatly to the influence of temperature, with the greatest N mineralization occurring at 25°C. The information acquired from this study may aid in predicting the impact of manure application to help increase N‐use efficiency when applied under different conditions (e.g., climate season) and soil types.     

Pedotransfer functions for the pool size of slowly mineralizable organic N in sandy arable soils

The major aim of this study was to evaluate how the pool size of slowly mineralizable, ‘old’ soil organic N can be derived from more easily accessible soil and site information via pedotransfer functions (PTF). Besides modeling, this pool size might be of great importance for the identification of soils with high mineralization potential in drinking‐water catchments. From long‐term laboratory incubations (ca. 200 days) at 35 °C, the pool sizes of easily mineralizable organic N (N_fast), mainly in fresh residues, and slowly mineralizable, ‘old’ soil organic N (N_slow) as well as their first‐order rate coefficients were obtained. 90 sandy arable soils from NW Germany served to derive PTFs for N_slow that were evaluated using another 20 soils from the same region. Information on former land‐use and soil type was obtained from topographical, historical, and soil maps (partly from 1780). Pool size N_slow very strongly depends on soil type and former land‐use. Mean pool sizes of N_slow were much lower in old arable lowland (105 mg N kg^–1) than upland soils (175 mg N kg^–1) possibly due to lower clay contents. Within lowlands, mean pool sizes in former grassland soils (245 mg N kg^–1) were 2 to 3 times larger than in old arable soils due to accumulation of mineralizable N. In contrast, mean pool sizes of N_slow were lowest in recently cleared, former heath‐ and woodland (31 mg N kg^–1) as a result of the input of hardly decomposable organic matter. Neither N nor C in the light fraction (density < 1.8 g cm^–3) was adequate to derive pool size N_slow in the studied soils (r² < 0.03). Instead, N_slow can be accurately (r² = 0.55 – 0.83) derived from one or two basic soil characteristics (e.g. organic C, total N, C : N, mineral fraction < 20 μm), provided that sites were grouped by former land‐use. Field mineralization from N_slow during winter (independent data set) can be predicted as well on the basis of N_slow‐values calculated from PTFs that were derived after grouping the soils by former land‐use (r² = 0.51***). In contrast, using the PTF without soil grouping strongly reduced the reliability (r² = 0.16).     

Nitrogen mineralization in relation to site history and soil properties for a range of Australian forest soils

Rates of N mineralization were measured in 27 forest soils encompassing a wide range of forest types and management treatments in south-east Australia. Undisturbed soil columns were incubated at 20°C for 68 days at near field-capacity water content, and N mineralization was measured in 5-cm depth increments to 30 cm. The soils represented three primary profile forms: gradational, uniform and duplex. They were sampled beneath mature native Eucalyptus sp. forest and from plantations of Pinus radiata of varying age (<1 to 37 years). Several sites had been fertilized, irrigated, or intercropped with lupins. The soils ranged greatly in total soil N concentrations, C:N ratios, total P, and sand, silt, and clay contents. Net N mineralization for individual soil profiles (0–30 cm depth) varied from 2.0 to 66.6 kg ha^-1 over 68 days, with soils from individual depths mineralizing from <0 (immobilization) to 19.3 kg ha^-1 per 5 cm soil depth. Only 0.1–3.1% of the total N present at 0–30 cm in depth was mineralized during the incubation, and both the amount and the percentage of total N mineralized decreased with increasing soil depth. N fertilization, addition of slash residues, or intercropping with lupins in the years prior to sampling increased N mineralization. Several years of irrigation of a sandy soil reduced levels of total N and C, and lowered rates of N mineralization. Considuring all soil depths, the simple linear correlations between soil parameters (C, N, P, C:N, C:P, N:P, coarse sand, fine sand, silt, clay) and N mineralization rates were generally low (r<0.53), but these improved for total N (r=0.82) and organic C (r=0.79) when the soils were grouped into primary profile forms. Prediction of field N-mineralization rates was complicated by the poor correlations between soil properties and N mineralization, and temporal changes in the pools of labile organic-N substrates in the field.     

Contributions of Organic Nitrogen Forms to Mineralized Nitrogen during Incubation Experiments of the Soils on the Loess Plateau

Interval leaching long‐term water‐logged incubation and interval leaching long‐term aerobic incubation were carried out to study the changes of different soil organic nitrogen (N) forms and their contributions to mineralized N during N‐mineralization process on 10 kinds of farmland soils with markedly different physical and chemical properties on the Loess Plateau. The results showed that the N‐mineralization capability and capacity using the two incubation methods were evidently different. After 217 days, cumulative mineralized N, the decreased amounts of total acid‐hydrolyzable N, acid‐hydrolyzable ammonia N, and acid‐hydrolyzable amino acid N in the water‐logged incubation were about twice those in the aerobic incubation. Soluble organic N leached in the aerobic incubation was four times that in the water‐logged incubation, which implied that organic N in the aerobic incubation mineralized more thoroughly than that in the water‐logged incubation. The correlation analysis of the changes of soil organic N forms with crop N uptake showed that the reduced amount of total acid‐hydrolyzable N in the water‐logged incubation was closely associated with total N uptake by two successive‐season crops (winter wheat and summer maize), and the decreased amounts of total acid‐hydrolyzable N and amino acid N in the aerobic incubation were highly significantly related to the N uptake of both the first season crop (winter wheat) and successive‐season maize. Multiple regression analysis, path analysis, and partial correlation analysis of the changes of soil organic N forms with mineralized N indicated that ammonia N was a main contribution to mineralized N in the water‐logged incubation, whereas both acid‐hydrolyzable amino acid and ammonia N were main contributions to mineralized N in the aerobic incubation. These results suggested that acid‐hydrolyzable ammonia was the primary contribution to mineralized N during N mineralization process with the two incubation methods.     

Effect of cropping systems on nitrogen mineralization in soils

 Understanding the effect of cropping systems on N mineralization in soils is crucial for a better assessment of N fertilizer requirements of crops in order to minimize nitrate contamination of surface and groundwater resources. The effects of crop rotations and N fertilization on N mineralization were studied in soils from two long-term field experiments at the Northeast Research Center and the Clarion-Webster Research Center in Iowa that were initiated in 1979 and 1954, respectively. Surface soil samples were taken in 1996 from plots of corn (Zea mays L.), soybean (Glycine max (L.) Merr.), oats (Avena sativa L.), or meadow (alfalfa) (Medicago sativa L.) that had received 0 or 180 kg N ha^–1 before corn and an annual application of 20 kg P and 56 kg K ha^–1. N mineralization was studied in leaching columns under aerobic conditions at 30  °C for 24 weeks. The results showed that N mineralization was affected by cover crop at the time of sampling. Continuous soybean decreased, whereas inclusion of meadow increased, the amount of cumulative N mineralized. The mineralizable N pool (N _o) varied considerably among the soil samples studied, ranging from 137 mg N kg^–1 soil under continuous soybean to >500 mg N kg^–1 soil under meadow-based rotations, sampled in meadow. The results suggest that the N _o and/or organic N in soils under meadow-based cropping systems contained a higher proportion of active N fractions. Received: 10 February 1999     

Sulfur mineralization rates and potentials of soils

Summary Field-moist soil and glass beads mixtures were packed in glass tubes and leached with 100 ml of 5 mM CaCl₂ and incubated at 20 or 30°C. The leaching procedure was repeated every 2 weeks for 14 weeks. The leachates were analysed for SO _inf4 ^sup2– and NO₃ ^–. The S uptake by three successive croppings of corn (Zea mays L.) or soybean [Glycine max (L.) Merr.] at 40- or 60-day intervals, respectively, or three cuttings of ryegrass (Lolium multiflorum L.) at 30-day intervals were studied under greenhouse conditions. Results showed that significantly greater amounts of S were mineralized at 30°C than at 20°C in each of 13 Iowa and 7 Chilean surface soils. Expressed as percentages of organic S in soils, the amounts of S mineralized in the Iowa surface soils in 14 weeks at 20 and 30°C ranged from 1.2% to 9.8% and from 2.4% to 17.5%, respectively. The corresponding values for the Chilean soils ranged from 0.9% to 7.2%6 and from 1.4% to 12.1%. The Q₁₀ values of S mineralization ranged from 1.7 to 4.4 (average 2.5) for the Iowa soils and from 1.7 to 3.1 (average 2.1) for the Chilean soils. The cumulative S mineralized at 20°C in 14 weeks was significantly correlated with the cumulative N mineralized (linear model, r=0.72^**; quadratic model, r=0.84^***). Similarly, the cumulative S mineralized at 30°C was significantly correlated with the cumulative N mineralized at this temperature (linear model, r=0.81^***; quadratic model, r = 0.82^***). The potentially mineralizable S pool (S₀), calculated by using an exponential equation for the S mineralized at 20°C, ranged from 5 to 44 mg kg^–1 for the Iowa soils and from 10 to 25 mg kg^–1 for the Chilean soils. The corresponding values obtained by using a reciprocal-plot technique ranged from 6 to 48 mg kg^–1 and from 12 to 26 mg kg^–1, respectively. The S₀ values calculated for S mineralized at 30°C, in general, were higher than those obtained at 20°C. The S mineralization rate constant (k) and the time required to mineralize 50% of S₀ (K _t), calculated by using the cumulative SO _inf4 ^sup2– released during 14 weeks of incubation, varied considerably among the soils. Up take of S by corn and soybean (tops+roots) were, in general, lower than the total SO _inf4 ^sup2– mineralized in 14 weeks at 20°C.     

Alkaline Hydrolyzable Organic Nitrogen as an Index of Nitrogen Mineralization in Soils: Relationship with Activities of Arylamidase and Amidohydrolases

ABSTRACT

This study investigated the relationship between a recently proposed alkaline hydrolysis method for estimating the chemical index of nitrogen (N) mineralization potential of soils and the activities of arylamidase and four amidohydrolases involved in hydrolysis of organic N (ON) in soils. Nitrogen mineralization was studied in 13 soils from uncultivated fields in Iowa, USA, by direct steam distillation of 1 g field-most soil treated with 1 M KOH or 1 M NaOH. The distillate was collected in boric acids, which was changed every 5 min for a total of 40 min. The NH₄ ⁺-N in the distillate was determined by titration with 0.005 M H₂SO₄. The cumulative amounts of N hydrolyzed were fitted to the first-order exponential equation to determine the “potentially hydrolyzable N (N_max )” for the soils. The activities of arylamidase, L-asparaginase, L-glutaminase, amidase, and L-aspartase were assayed at their optimal pH values. Results showed that estimated N_max values were strongly correlated with the activities of arylamidase and amidohydrolases. The activities of arylamidase and the amidohydrolases were significantly correlated, indicating that the activities of the two groups of enzymes are coupled in mineralization of ON in soils. Based on the specificity of enzyme reactions and the strong relationship between estimated N_max values and the activities of arylamidase and amidohydrolases, we concluded that similar amide-N bonds were susceptible to enzymatic and alkaline hydrolysis, and that alkaline hydrolyzable ON can be used as an index of N mineralization in soils.     

Sulfur mineralization in two soils amended with organic manures,crop residues,and green manures

The mineralization of sulfur (S) was investigated in a Vertisol and an Inceptisol amended with organic manures, green manures, and crop residues. Field‐moist soils amended with 10 g kg^—1 of organic materials were mixed with glass beads, placed in pyrex leaching tubes, leached with 0.01 M CaCl₂ to remove the mineral S and incubated at 30 °C. The leachates were collected every fortnight for 16 weeks and analyzed for SO₄‐S. The amount of S mineralized in control and in manure‐amended soils was highest in the first week and decreased steadily thereafter. The total S mineralized in amended soils varied considerably depending on the type of organic materials incorporated and soil used. The cumulative amounts of S mineralized in amended soils ranged from 6.98 mg S (kg soil)^—1 in Inceptisol amended with wheat straw to 34.38 mg S (kg soil)^—1 in Vertisol amended with farmyard manure (FYM). Expressed as a percentage of the S added to soils, the S mineralized was higher in FYM treated soils (63.5 to 67.3 %) as compared to poultry manure amended soils (60.5 to 62.3 %). Similarly the percentage of S mineralization from subabul (Leucaena leucocephala) loppings was higher (53.6 to 55.5 %) than that from gliricidia (Gliricidia sepium) loppings (50.3 to 51.1 %). Regression analysis clearly indicated the dependence of S mineralization on the C : S ratio of the organic materials added to soil. The addition of organic amendments resulted in net immobilization of S when the C : S ratio was above 290:1 in Vertisol and 349:1 in Inceptisol. The mineralizable S pool (S_o) and first‐order rate constant (k) varied considerably among the different types of organic materials added and soil. The S_o values of FYM treated soils were higher than in subabul, gliricidia, and poultry manure treated soils.     

Mineralization and microbial biomass formation in upland soil amended with some tropical plant residues at different temperatures

A model experiment was carried out at 15, 25, and 35°C to investigate the changes in microbial biomass and the pattern of mineralization in upland soil during 8 weeks following the addition of 8 organic materials including 6 tropical plant residues, ipil ipil (Leucaena leucocephala), azolla (Azolla pinnata), water hyacinth (Eichhornia crassipes), dhaincha (Sesbania rostrata), cowpea (Vigna unguiculata), and sunhemp (Crotalaria juncea). The amounts of CO₂-C evolved and inorganic N produced at 35°C were about 2 times larger than those at 15°C. At any temperature, the flush decomposition of C was observed within the first week and thereafter the rate of mineralization became relatively slow. A negative correlation was observed between inorganic N and C/N ratios of the added organic materials. The relationships between the amounts of cellulose or cellulose plus hemicellulose and the amount of mineralized N of the added organic materials were also negative.

The changes in the microbial biomass were affected by temperatures. The amount of biomass C and N was maximum after 42 d of incubation at 15°C, and after 7 d at 25 and 35°C, and thereafter decreased. The rate of biomass decline was slower at 15°C and faster at 35°C than at 25°C. Regardless of the temperatures, the addition of organic materials enhanced microbial biomass formation throughout the incubation periods.     

N mineralization parameters of sandy arable soils

The objective of this study was to experimentally investigate net N mineralization in sandy arable soils and to derive adequate N mineralization parameters for simulation purposes. Long‐term incubations at 35 °C were done for at least 200 days with 147 sandy arable soils from Northwest Germany. To cumulative net N mineralization curves the simultaneous two‐pool first‐order kinetic equation was fitted in order to differentiate between N mineralization from an easily decomposable, fresh organic matter pool (N_fast) and from a slowly decomposable pool (N_slow) of more humified OM. North German loess soils served as a reference, since available model parameters were mainly derived from those soils. Although curve patterns in sandy soils often somewhat deviated from typical double‐exponential patterns, the mineralization equation generally could be fitted. Two pools were clearly revealed, but a transfer of the standard parameters was found to be not appropriate — except maybe for the pool size of the fast decomposable N pool. The mean k_fast at 35 °C (0.1263 d^—1) is about 46% higher than the known ’︁standard’ loess value, indicating better conditions for decomposition of fresh residues at this temperature. The mean k_slow at 35 °C (0.0023 d^—1), which is 60% lower than reported earlier from loess soils, and much lower mineralization rates of the slowly decomposable N pool give reason to the presence of generally more resistant organic material in these sandy soils. The relation between N_slow and total N was found to be not close enough to derive the pool size of slowly decomposable N just from total N as done for loess soils. Reducing the variability is necessary, promising approaches exist. The eight reference loess soils revealed — on an average — the known N mineralization parameters.     

Estimates of Gross Transformation Rates of Dairy Manure N Using 15N Pool Dilution

Abstract

Most measurements of dairy manure nitrogen (N) availability depend on net changes in soil inorganic N concentration over time, which overlooks the cycling of manure N in the soil. Gross transformations of manure N, including mineralization (m), immobilization (i), and nitrification (n), can be quantified using ¹⁵N pool dilution methods. This research measures gross m, n, and i resulting from application of four freeze‐dried dairy manures that had distinctly different patterns of N availability. A sandy loam soil (coarse‐loamy, mixed, frigid Typic Haplorthod) was amended with four different freeze‐dried dairy manures and incubated at 25°C with optimal soil water content. The dilution of ¹⁵ammonium (NH4⁺) during a 48‐h interval (7–9 d and 56–58 d after manure application) was used to estimate m, whereas the dilution of ¹⁵nitrate (NO₃ ^?) was used to estimate n. Gross immobilization was calculated as gross minus net mineralization. Gross mineralization in the unamended soil was similar at 7‐ to 9‐d and 56‐ to 58‐d intervals and was significantly increased by the application of manures. For both amended and unamended soil, m was much greater (i.e., three‐ to nine‐fold) than estimated net mineralization, illustrating the degree to which manure N can be cycled in soil. At the early interval, both m and i were directly related to the manure C input, demonstrating the linkage between substrate C availability and N utilization by soil microbes. This research clearly shows that the application of dairy manures stimulates gross N transformation rates in the soil, improving our understanding of the impact of manure application on soil N cycling.     

Mobile and readily available C and N fractions and their relationship to microbial biomass and selected enzyme activities in a sandy soil under different management systems

Within different land‐use systems such as agriculture, forestry, and fallow, the different morphology and physiology of the plants, together with their specific management, lead to a system‐typical set of ecological conditions in the soil. The response of total, mobile, and easily available C and N fractions, microbial biomass, and enzyme activities involved in C and N cycling to different soil management was investigated in a sandy soil at a field study at Riesa, Northeastern Germany. The management systems included agricultural management (AM), succession fallow (SF), and forest management (FM). Samples of the mineral soil (0—5, 5—10, and 10—30 cm) were taken in spring 1999 and analyzed for their contents on organic C, total N, NH₄⁺‐N and NO₃^—‐N, KCl‐extractable organic C and N fractions (Corg_(KCl) and Norg_(KCl)), microbial biomass C and N, and activities of β‐glucosidase and L‐asparaginase. With the exception of Norg_(KCl), all investigated C and N pools showed a clear relationship to the land‐use system that was most pronounced in the 0—5 cm profile increment. SF resulted in greater contents of readily available C (Corg_(KCl)), NH₄⁺‐N, microbial biomass C and N, and enzyme activities in the uppermost 5 cm of the soil compared to all other systems studied. These differences were significant at P ≤ 0.05 to P ≤ 0.001. Comparably high C_mic:C_org ratios of 2.4 to 3.9 % in the SF plot imply a faster C and N turnover than in AM and FM plots. Forest management led to 1.5‐ to 2‐fold larger organic C contents compared to SF and AM plots, respectively. High organic C contents were coupled with low microbial biomass C (78 μg g^—1) and N contents (10.7 μg g^—1), extremely low C_mic : C_org ratios (0.2—0.6 %) and low β‐glucosidase (81 μg PN g^—1 h^—1) and L‐asparaginase (7.3 μg NH₄‐N g^—1 2 h^—1) activities. These results indicate a severe inhibition of mineralization processes in soils under locust stands. Under agricultural management, chemical and biological parameters expressed medium values with exception for NO₃^—‐N contents which were significantly higher than in SF and FM plots (P ≤ 0.005) and increased with increasing soil depth. Nevertheless, the depth gradient found for all studied parameters was most pronounced in soils under SF. Microbial biomass C and N were correlated to β‐glucosidase and L‐asparaginase activity (r ≥ 0.63; P ≤ 0.001). Furthermore, microbial biomass and enzyme activities were related to the amounts of readily mineralizable organic C (i.e. Corg_(KCl)) with r ≥ 0.41 (P ≤ 0.01), suggesting that (1) KCl‐extractable organic C compounds from field‐fresh prepared soils represent an important C source for soil microbial populations, and (2) that microbial biomass is an important source for enzymes in soil. The Norg_(KCl) pool is not necessarily related to the size of microbial biomass C and N and enzyme activities in soil.<?show $6#>     

Nitrogen mineralization from mature bio‐waste compost in vineyard soils. I. Long‐term laboratory incubation experiments

The steadily increasing utilization of bio‐waste compost in German viticulture requires a more detailed investigation of nitrogen (N) mineralization parameters for mature bio‐waste compost applied to vineyard soils. N mineralization kinetics were described with two superposing exponential equations. Long‐term aerobic laboratory incubation experiments of 12 soil‐compost substrates revealed that 5±2.8% of its total N content could be released from a rapidly decomposable fraction (half‐life period t₅₀ = 41 d at 15°C) and another 60±2.9% from a slower decomposable fraction (t₅₀ = 490 d). The remaining proportion (35%) is considered not to be released in the medium term. The obtained potentially mineralizable nitrogen of 65% of total compost N significantly differs from current fertilizer recommendations, which were adopted from calculations for agricultural conditions. For fertilizer recommendations in viticulture, we recommend the consideration of a higher N‐mineralization potential for organic fertilizers.     

Boiling Potassium Chloride–Extractable Nitrogen as an Index of Potentially Mineralizable and Plant‐Available Nitrogen in Soil

Abstract

Mineralization of soil organic nitrogen (N) and its contribution toward crop N uptake is central to developing efficient N‐management practices. Because biological incubation methods are time consuming and do not fit into the batch‐analysis techniques of soil‐testing laboratories, an analytical procedure that can provide an estimate of the mineralizable N would be useful as a soil‐test method for predicting plant‐available N in soil. In the present studies, the ability of boiling potassium chloride (KCl) to extract potentially mineralizable and plant‐available N in arable soils of semi‐arid India was tested against results from biological incubations and uptake of N by wheat in a pot experiment. Mineralization of organic N in soils was studied in the laboratory by conducting aerobic incubations for 112 days at 32°C and 33 KPa of moisture. Cumulative N mineralization in different soils ranged from 8.2 to 75.6 mg N kg^?1 soil that constituted 2.7 to 8.8% of organic N. The amount of mineral N extracted by KCl increased with increase in length of boiling from 0.5 to 2 h. Boiling for 0.5, 1, 1.5, and 2 h resulted in an increase in mineral‐N extraction by 9.3, 12.7, 19.6, and 26.1%, respectively, as compared to mineral N extracted at room temperature. The boiling‐KCl‐hydrolyzable N (ΔN_i) was directly dependent upon soil organic N content, but the presence of clay retarded hydrolysis for boiling lengths of 0.5 and 1 h. However, for boiling lengths of 1.5, and 2 h, the negative effect of clay was not apparent. The ΔN _i was significantly (P=0.05) correlated to cumulative N mineralized and N‐mineralization potential (N₀). The relationship between N₀ and ΔN _i was curvilinear and was best described by a power function. Boiling length of 2 h accounted for 78% of the variability in N₀. Results of the pot experiment showed that at 21‐ and 63‐day growth stages, dry‐matter yield and N uptake by wheat were significantly correlated to boiling‐KCl‐extractable mineral N. Thus, boiling KCl could be used to predict potentially mineralizable and plant‐available N in these soils, and a boiling time of 2 h was most suitable to avoid the negatively affected estimates of boiling‐KCl‐hydrolyzable N in the presence of clay. The results have implications for selecting length of boiling in soils varying widely in clay content, and this may explain why, in earlier studies, longer boiling times (viz. 2 or 4 h) were better predictors of N availability as compared to 0.5 and 1 h.     

Temperature responses of nitrogen processes cannot be inferred from carbon turnover in a boreal Norway spruce forest

Cold season processes contribute substantially to annual carbon (C) and nitrogen (N) budgets in boreal forest ecosystems, but little is known about how decomposition processes are affected at temperatures prevalent during wintertime. The aim of this study was to evaluate temperature responses of soil C and N processes and to test the hypothesis that there is a switch towards decomposing N‐rich material when soil temperatures drop to near 0°C. In the laboratory, soils from a boreal forest long‐term nutrient fertilization experiment were exposed to different temperatures varying from +2 to +15°C, and C mineralization, gross as well as net N mineralization/immobilization were estimated. Carbon mineralization declined exponentially as temperature decreased, whereas the response of N processes to temperature varied, with some indication that soil C and N processes are decoupled at low temperatures. We could only partially confirm that the decoupling between C and N processes at low temperature was due to a switch to N‐rich material, i.e., a change in the material undergoing decomposition. Overall, our results clearly showed that temperature responses of N processes cannot be inferred from C processes in boreal forest ecosystems, and that there is a need to improve our understanding of the relationship between the two across the range of temperatures experienced throughout the year. In particular, further research is required to establish and evaluate appropriate proxies for modelling the relationship of C and N processes at temperatures close to the freezing point.