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

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

Nitrate Transformation and N2O Emission in a Typical Intensively Managed Calcareous Fluvaquent Soil: A 15-Nitrogen Tracer Incubation Study

A 56-day aerobic incubation experiment was performed with 15-nitrogen (N) tracer techniques after application of wheat straw to investigate nitrate-N (NO₃-N) immobilization in a typical intensively managed calcareous Fluvaquent soil. The dynamics of concentration and isotopic abundance of soil N pools and nitrous oxide (N₂O) emission were determined. As the amount of straw increased, the concentration and isotopic abundance of total soil organic N and newly formed labeled particulate organic matter (POM-N) increased while NO₃-N decreased. When ¹⁵NO₃-N was applied combined with a large amount of straw at 5000 mg carbon (C) kg^?1 only 1.1 ± 0.4 mg kg^?1 NO₃-N remained on day 56. The soil microbial biomass N (SMBN) concentration and newly formed labeled SMBN increased significantly (P < 0.05) with increasing amount of straw. Total N₂O-N emissions were at levels of only micrograms kg^?1 soil. The results indicate that application of straw can promote the immobilization of excessive nitrate with little emission of N₂O.     

Soil N Losses by Denitrification Evaluated Using the 15N Tracer Method

Molecular nitrogen (N₂) and nitrous oxide (N₂O) generated by denitrification increase N losses in the soil–plant system. This study aimed to quantify N₂ and N₂O from potassium nitrate (K¹⁵NO₃) applied to soils with different textures and moisture contents in the absence and presence of a source of carbon (C) using the ¹⁵N tracer method. In the three soils used (sandy texture (ST), sandy clay loam texture (SCLT), and clayey texture (CT)), three moisture contents were evaluated (40%, 60%, and 80% of the water holding capacity (WHC)) with (D⁺) and without (D^?) dextrose added. The treatments received 100 mg N kg^?1 (KNO₃ with 23.24 atom% ¹⁵N). N₂ emissions occurred in all of the treatments, but N₂O emissions only occurred in the D⁺ treatment, showing increases with increasing moisture content. SCLT with 80% WHC in the D⁺ treatment exhibited the highest accumulated N emission (48.26 mg kg^?1). The ¹⁵N balance suggested trapping of the gases in the soil.     

The effect of inorganic nitrogen on the added nitrogen interaction of soils in incubation experiments

A laboratory incubation experiment was conducted to study the effect of indigenous inorganic N on the immobilization of applied N and on the occurrence of an added N interaction (ANI). Samples of six Mollisols from Illinois were incubated with ¹⁵N-labelled (NH₄)₂SO₄ (100 or 200 mg N kg^-1 soil), with or without the use of 0.01 M CaCl₂ to extract inorganic N (mainly NO _inf3 ^sup- ) before incubation. From 6 to 49% of the N applied was immobilized, higher percentages being obtained with unextracted soils than with the extracted soils and with the higher rate of N addition. Net mineralization of native N occurred in both the unextracted and extracted soils, but was more extensive in the unextracted soil and increased with the addition of N. The increases were accompanied by a positive ANI, which usually exceeded the amount of applied N immobilized and increased with the rate of addition. The ANI values observed with extracted soils were attributed to increased mineralization of native organic N.     

Mineralization of soil organic nitrogen solubilized by aqua ammonia fertilizer

Organic N solubilized by NH_3(aq) was extracted from ¹⁵N-labelled or unlabelled soil, concentrated and added to non-extracted soil, which was incubated under aerobic conditions at 27±1°C. Gross N mineralization, gross N immobilization, and nitrification in soils with or without addition of unlabelled soluble organic N were estimated by models based on the dilution of the NH ₄ ⁺ or NO _inf3 ^sup- pools, which were labelled with ¹⁵N at the beginning of incubation. Mineralization of labelled organic N was measured by the appearance of label in the mineral N pool. Although gross N mineralization and gross N immobilization were increased in two soils between day 0 and day 7 following addition of unlabelled organic N solubilized by NH_3(aq), there was no increase in net N mineralization. Solubilization of ¹⁵N-labelled organic N increased and the ¹⁵N enrichment of the soluble organic N decereased as the concentration of NH_3(aq) added increased. A constant proportion of approximately one-quarter of the labelled organic N added at different rates to non-extracted soil was recovered in the mineral N pool after an incubation period of 14 days, and the availability ratios calculated from net N mineralization data were 1.1:1 and 2.1:1 for 111 and 186 mg added organic-N kg^-1 soil, respectively, indicating that the mineralization of organic N was increased by solubilization.     

C and N turnover in structurally intact soils of different texture

The turnover of native and applied C and N in undisturbed soil samples of different texture but similar mineralogical composition, origin and cropping history was evaluated at −10 kPa water potential. Cores of structurally intact soil with 108, 224 and 337 g clay kg⁻¹ were horizontially sliced and ¹⁵N-labelled sheep faeces was placed between the two halves of the intact core. The cores together with unamended treatments were incubated in the dark at 20 °C and the evolution of CO₂-C determined continuously for 177 d. Inorganic and microbial biomass N and ¹⁵N were determined periodically. Net nitrification was less in soil amended with faeces compared with unamended soil. When adjusted for the NO₃-N present in soil before faeces was applied, net nitrification became negative indicating that NO₃-N had been immobilized or denitrified. The soil most rich in clay nitrified least N and ¹⁵N. The amounts of N retained in the microbial biomass in unamended soils increased with clay content. A maximum of 13% of the faeces ¹⁵N was recovered in the microbial biomass in the amended soils. CO₂-C evolution increased with clay content in amended and unamended soils. CO₂-C evolution from the most sandy soil was reduced due to a low content of potentially mineralizable native soil C whereas the rate constant of C mineralization rate peaked in this soil. When the pool of potentially mineralizable native soil C was assumed proportional to volumetric water content, the three soils contained similar proportions of potentially mineralizable native soil C but the rate constant of C mineralization remained highest in the soil with least clay. Thus although a similar availability of water in the three soils was ensured by their identical matric potential, the actual volume of water seemed to determine the proportion of total C that was potentially mineralizable. The proportion of mineralizable C in the faeces was similar in the three soils (70% of total C), again with a higher rate constant of C mineralization in the soil with least clay. It is hypothesized that the pool of potentially mineralizable C and C rate constants fluctuate with the soil water content.     

Transformation of 15N-labelled leguminous plant material in three contrasting soils

Summary Two soils from Pakistan (Hafizabad silt loam and Khurrarianwala silt loam) and one from Illinois, USA (Drummer silty clay loam) were incubated with ¹⁵N-labelled soybean tops for up to 20 weeks at 30°C. Mineralization of soybean ¹⁵N was slightly more rapid in the Pakistani soils, and after 20 weeks of incubation, 50%, 53%, and 56% of the applied ¹⁵N was accounted for as (NH₄ ⁺+NO₃ ^–)-N in Drummer, Hafizabad, and Khurrarianwala soils, respectively. Potentially mineralizable N (determined by anaerobic incubation) varied between 1.5% and 10% of the applied ¹⁵N in the three soils at different stages of incubation; somewhat higher percentages were mineralizable in the Pakistani soils than in the Drummer soil. From 3.7% to 9% of the applied ¹⁵N was accounted for in the microbial biomass. From 10% to 32% of the applied N was recovered in the humic acid and fulvic acid fractions of the organic matter by sequential extraction with Na₄P₂O₇ and NaOH; from 12% to 49% was recovered in the humin fraction. Of the three soils, Drummer soil contained more ¹⁵N as humic and fulvic acids. In all cases, the ¹⁵N was approximately equally distributed between the humic and fulvic acid fractions. A significant percentage of the humin ¹⁵N (52%–78%, equivalent to 8%–34% of the applied ¹⁵N) occurred in non-hydrolyzable (6 N HCl) forms. Of the hydrolyzable ¹⁵N, 42%–51% was accounted for as amino acid-N followed in order by NH₃ (17%–30%), hydrolyzable unknown forms (20%–22%), and amino sugars (6%–2%). The recovery of applied ¹⁵N for the different incubation stages was 87±22%. Recovery was lowest with the Khurrarianwala soil, presumably because of NH₃ volatilization losses caused by the high pH of this soil.     

Mineralization of carbon and nitrogen from fresh and anaerobically stored sheep manure in soils of different texture

A sandy loam soil was mixed with three different amounts of quartz sand and incubated with (¹⁵NH₄)₂SO₄ (60 g N g^-1 soil) and fresh or anaerobically stored sheep manure (60 g g^-1 soil). The mineralization-immobilization of N and the mineralization of C were studied during 84 days of incubation at 20°C. After 7 days, the amount of unlabelled inorganic N in the manure-treated soils was 6–10 g N g^-1 soil higher than in soils amended with only (¹⁵NH₄)₂SO₄. However, due to immobilization of labelled inorganic N, the resulting net mineralization of N from manure was insignificant or slightly negative in the three soil-sand mixtures (100% soil+0% quartz sand; 50% soil+50% quartz sand; 25% soil+75% quartz sand). After 84 days, the cumulative CO₂ evolution and the net mineralization of N from the fresh manure were highest in the soil-sand mixutre with the lowest clay content (4% clay); 28% fo the manure C and 18% of the manure N were net mineralized. There was no significant difference between the soil-sand mixtures containing 8% and 16% clay, in which 24% of the manure C and -1% to 4% of the manure N were net mineralized. The higher net mineralization of N in the soil-sand mixture with the lowest clay content was probably caused by a higher remineralization of immobilized N in this soil-sand mixture. Anaerobic storage of the manure reduced the CO₂ evolution rates from the manure C in the three soil-sand mixtures during the initial weeks of decomposition. However, there was no effect of storage on net mineralization of N at the end of the incubation period. Hence, there was no apparent relationship between net mineralization of manure N and C.     

NH3 Volatilization,N2O Emission and Microbial Biomass Turnover from 15N-Labeled Manure Under Laboratory Conditions

On irrigated agricultural soils from semi-arid and arid regions, ammonia (NH₃) volatilization and nitrous oxide (N₂O) emission can be a considerable source of N losses. This study was designed to test the capture of ¹⁵N loss as NH₃ and N₂O from previous and recent manure application using a sandy, calcareous soil from Oman amended one or two times with ¹⁵N labeled manure to elucidate microbial turnover processes under laboratory conditions. The system allowed to detect ¹⁵N enrichments in evolved N₂O-N and NH₃-N of up to 17% and 9%, respectively, and total N, K₂SO₄ extractable N and microbial N pools from previous and recent ¹⁵N labeled manure applications of up to 7%, 8%, and 15%. One time manured soil had higher cumulative N₂O-N emissions (141 µg kg^?1) than repeatedly manured soil with 43 µg kg^?1 of which only 22% derived from recent manure application indicating a priming effect.     

A comparison of different indices for nitrogen mineralization

Indices of N mineralization in soils of contrasting texture, pH, and organic matter contents were compared at different dates during the growing season. The indices were derived from a 12-week aerobic incubation, determination of the amount of microbial biomass at the start of the incubation, determination of the increase in NH ₄ ⁺ after boiling with 2 M KCl for 2h, and extraction of total soluble N with 0.01 M CaCl₂. Cumulative mineral N increased linearly with time in the course of the incubations. Rates of mineralization in soil samples taken in March 1989 and 1990 were significantly correlated with soluble organic N, while correlations between the mineralization rate and the increase in NH ₄ ⁺ after boiling with 2 M KCl for 2 h were poor for sandy soils and absent for loamy soils. Correlations between NH ₄ ⁺ after boiling with 2 M KCl for 2h and the soil N concentration were highly significant, but no general relationship was found between the mineralization rate and the soil N concentration. Neither biomass N nor biomass C was significantly correlated with the mineralization rate or with one of the chemical indices. Among the methods tested, soluble organic N extracted with 0.01 M CaCl₂ was the only method with any promise for routine measurement of the mineralization capacity of the individual sites.     

Effect of extracellular-enzyme activities on solubilization rate of soil organic nitrogen

In a sandy soil containing ¹⁵N-labeled active (soluble and easily degradable) and non-labelled passive (recalcitrant) fractions of soil organic matter, the rate of net N mineralization (solubilization) was determined during a 55-day incubation at 25°C, 63% water-holding capacity and different levels of soil extracellular-enzyme activities. The active fraction of soil N was labelled by preincubation (at 5°C and 74% water-holding capacity for 6 months) of soil amended with ¹⁵N-labeled plant material. Increases in the activity of extracellular-enzymes in soil were induced by the addition of glucose and KH₂PO₄ at the beginning of the incubation. The results show that the contents of total soluble N (NO ₃ ^– –N+NH ₄ ⁺ –N + soluble organic N) were significantly higher in glucose-amended soil compared to the unamended soil. The increases in soluble N in soil amended with 1 and 2 mg glucose g^-1 dry soil corresponded to a mean rate of net solubilization of 7.9±1.4 and 18.8±0.7 nmol N g^-1 dry soil day^-1, respectively. The mean rate of net N solubilization (3.6±1.0 nmol N g^-1 dry soil day^-1) in unamended soil was significantly lower than those of glucose amended soils. The content of ¹⁵N in total soluble N in soil amended with 2 mg glucose, for example, was diluted from 3.11±0.08 atom% before the incubation to 2.77±0.03 atom% after 55 days. This indicates that 89% of soluble-N accumulated in soil by the end of the incubation originated from the active fraction of soil N and the rest, estimated at 11%, originated from the passive fraction. The activities of soluble and total proteases as well as the rate of N solubilization in the soil increased with the application of glucose. The activity of these extracellular enzymes was highly correlated with the rates of net N solubilization. Thus, increases in extracellular-enzyme activities in glucose-amended soils had a priming effect on the solubilization of ¹⁵N-labeled active and non-labeled passive fractions of soil organic N. It seems that the activity of extracellular-enzymes expressed in terms of total and soluble protease activities could be a rate-limiting factor in the processes of soil organic N solubilization.     

好氧条件下两种水稻土的氮素总转化速率比较

Although to date individual gross N transformations could be quantified by ~(15)N tracing method and models,studies are still limited in paddy soil.An incubation experiment was conducted using topsoil(0-20 cm) and subsoil(20-60 cm) of two paddy soils,alkaline and clay(AC) soil and neutral and silt loam(NSL) soil,to investigate gross N transformation rates.Soil samples were labeled with either ~(15)NH4_NO_3 or NH_4~(15)NO_3,and then incubated at 25 °C for 168 h at 60%water-holding capacity.The gross N mineralization(recalcitrant and labile organic N mineralization) rates in AC soil were 1.6 to 3.3 times higher than that in NSL soil,and the gross N nitrification(autotrophic and heterotrophic nitrification) rates in AC soil were 2.4 to 4.4 times higher than those in NSL soil.Although gross NO_3~- consumption(i.e.,NO_3~- immobilization and dissimilatory NO_3~- reduction to NH_4~+ rates increased with increasing gross nitrification rates,the measured net nitrification rate in AC soil was approximately 2.0 to 5.1 times higher than that in NSL soil.These showed that high NO_3~- production capacity of alkaline paddy soil should be a cause for concern because an accumulation of NO_3~- can increase the risk of NO_3~- loss through leaching and denitrification.     

GREENHOUSE GAS EMISSIONS FROM AN ALKALINE SALINE SOIL CULTIVATED WITH MAIZE (ZEA MAYS L.) AND AMENDED WITH ANAEROBICALLY DIGESTED COW MANURE: A GREENHOUSE EXPERIMENT

Sludge derived from cow manure anaerobically digested to produce biogas (methane; CH₄) was applied to maize (Zea mays L.) cultivated in a nutrient-low, alkaline, saline soil with electrolytic conductivity 9.4 dS m^?1 and pH 9.3. Carbon dioxide (CO₂) emission increased 3.1 times when sludge was applied to soil, 1.6 times when cultivated with maize and 3.5 times in sludge-amended maize cultivated soil compared to the unamended uncultivated soil (1.51 mg C kg^?1 soil day^?1). Nitrous oxide (N₂O) emission from unamended soil was -0.0004 μg nitrogen (N) kg^?1 soil day^?1 and similar from soil cultivated with maize (0.27 μg N kg^?1 soil day^?1). Application of sludge increased the N₂O emission to 4.59 μg N kg^?1 soil day^?1, but cultivating this soil reduced it to 2.42 μg N kg^?1 soil day^?1. It was found that application of anaerobic digested cow manure stimulated maize development in an alkaline saline soil and increased emissions of CO₂ and N₂O.     

Studies of nitrogen immobilization and mineralization in calcareous soils—II: Mineralization of immobilized nitrogen from soil fractions of different particle size and density

¹⁵NO^?₃ was immobilized in a calcareous sandy soil and a calcareous clay soil each incubated with glucose and wheat straw. Net mineralization of organic-¹⁵N was more rapid in the sandy soil, irrespective of C amendment, and in soils amended with glucose. Intermittent drying and wetting of soils during incubation stimulated mineralization of ¹⁵N-labelled and native soil organic-N in all treatments. The availability (percentage mineralization) of recently-immobilized ¹⁵N consistently exceeded that of the native soil N. Ratios of the availability of labelled and unlabelled N were similar in the sandy and clay soils but varied according to C amendment, drying and wetting cycle and incubation period.Changes in the distribution of immobilized N amongst soil extracts and soil fractions of different particle size and density were determined during periods of net N mineralization. In straw-amended soils, the organic-¹⁵N of a light fraction, sp.gr. < 1.59, decomposed relatively rapidly during the late mineralization period. Decreases of organic ¹⁵N of the fine clay fraction were also recorded. In glucose-amended soils, net N mineralization was accompanied by significant decreases in the concentrations of organic-¹⁵N of the silt and fine clay fractions.Drying and rewetting of soils hastened or magnified changes occurring in the organic-¹⁵N of soil fractions, but qualitatively, the pattern of change was similar to that observed with soils incubated under uniformly-moist conditions.The percentage distribution of labelled and unlabelled N suggested that in the long term, the silt fraction will accumulate an increasing proportion of the more stable nitrogenous residues.     

Transformations of chemically fixed liquid anhydrous ammonia by soil microorganisms

Summary The application of liquid anhydrous NH₃ to soil leads to chemical fixation of NH₃ by organic matter and of NH _inf4 ^sup+ by clay minerals. A laboratory study was conducted to ascertain the biological transformations of newly fixed liquid anhydrous ¹⁵NH₃ in a Drummer silty clay loam by incubation of the ¹⁵N-labelled soil with glucose for 0, 7, 30, and 90 days and by sequential extraction of organic-matter-fixed ¹⁵NH₃ with 0.15 M Na₄P₂O₇, 0.15 M KOH, 0.1 M NaOH, and acidified dimethyl sulfoxide. About 16% of the ¹⁵NH₃ injected was fixed, of which 52% was accounted for by clay fixation. The various humic fractions (fulvic acid, humic acid, and humin) were obtained, and the distribution patterns of the fixed ¹⁵NH₃-N in these fractions were compared. The potential availability of the fixed ¹⁵NH₃-N was also estimated. The percentage of the ¹⁵NH₃ recovered as organic-matter-fixed ¹⁵NH₃ decreased as the length of incubation increased (to 28% after 90 days); the decrease was attributed in part to an increase in the amount recovered as clay-fixed NH _inf4 ^sup+ (from 52 to 64%). Changes in the distribution of the organic-matter-fixed ¹⁵NH₃-N in the humic fractions included: (1) an increase in the relative amount of the fixed ¹⁵NH₃ as humic acid in both the Na₄P₂O₇ and KOH extracts, (2) an increase in the percentage of organic-matter-fixed ¹⁵NH₃-N in the fulvic acid fractions as high-molecular-weight components (determined by dialysis) or as generic fulvic acid (determined by sorption-desorption from XAD-8 resin), and (3) an increase in the percentage of the organic-matter-fixed ¹⁵NH₃ as humin. The potential availability of the organic-matter-fixed ¹⁵NH₃-N decreased as the length of the incubation increased, from 22 to 4% over the 90-day incubation period, and was correlated significantly (0.05 level) with Na₄P₂O₇-extractable N. These results suggest that organic-matter-fixed liquid anhydrous NH₃ is initially more labile than the native soil N but becomes less labile with time.     

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.     

Mineral-fixed ammonium in clay- and silt-size fractions of soils incubated with 15N-ammonium sulphate for five years

Summary Four soils with 6, 12, 23, and 47% of clay were incubated for 5 years with ¹⁵N-labeled (NH_{4 2}SO₄ and hemicellulose. The incubations took place at 20°C and 55% water-holding capacity. Samples of whole soils, and clay- (<2 m) and silt-(2–20 m) size fractions (isolated by ultrasonic dispersion and gravity sedimentation) were analysed for labeled and native mineral-fixed ammonium. Mineral-fixed ammonium in non-incubated soil samples accounted for 3.4%–8.3% of the total N and showed a close positive correlation with the soil clay content (r ² = 0.997). After 5 years of incubation, the content of mineral-fixed ammonium in the clay fraction was 255–430 g N g^–1, corresponding to 71%–82% of the mineral-fixed ammonium in whole soils. Values for silt were 72–166 g N g^–1 (14%–33% of whole soil content). In the soils with 6% and 12% clay, less than 1 % of the labeled clay N was present as mineral-fixed ammonium. In the soil with 23% clay, 3% of the labeled N in the clay was mineral-fixed ammonium. Labeled mineral-fixed ammonium was not detected in the silt fractions. For whole soils, and clay and silt fractions, the proportion of native N present as mineral-fixed ammonium varied between 3% and 6%. In contrast, the proportion of labeled N found as mineral-fixed ammonium in the soil with 4701o clay was 23%, 38% and 31% for clay, silt, and whole-soil samples, respectively. Corresponding values for native mineral-fixed ammonium were 12%, 16%, and 10%. Consequently, studies based on soil particle-size fractions and addressing the N turnover in clay-rich soils should consider the pool of mineral-fixed ammonium, especially when comparing results from different size fractions with those from fractions isolated from soils of a widely different textural composition.     

Interactive effects from combining fertilizer and organic residue inputs on nitrogen transformations

Concerns about sustainability of agroecosystems management options in developed and developing countries warrant improved understanding of N cycling. The Integrated Soil Fertility Management paradigm recognizes the possible interactive benefits of combining organic residues with mineral fertilizer inputs on agroecosystem functioning. However, these beneficial effects may be controlled by residue quality. This study examines the controls of inputs on N cycling across a gradient of (1) input, (2) residue quality, and (3) texture. We hypothesized that combining organic residue and mineral fertilizers would enhance potential N availability relative to either input alone. Residue and fertilizer inputs labeled with ¹⁵N (40–60 atom% ¹⁵N) were incubated with 200 g soil for 545 d in a microcosm experiment. Input treatments consisted of a no-input control, organic residues (3.65 g C kg⁻¹ soil, equivalent to 4 Mg C ha⁻¹), mineral N fertilizer (100 mg N kg⁻¹ soil, equivalent to 120 kg N ha⁻¹), and a combination of both with either the residue or fertilizer ¹⁵N-labeled. Zea mays stover inputs were added to four differently textured soils (sand, sandy loam, clay loam, and clay). Additionally, inputs of three residue quality classes (class I: Tithonia diversifolia, class II: Calliandra calothyrsus, class III: Z. mays stover) were applied to the clay soil. Available N and N₂O emissions were measured as indicators for potential plant N uptake and N losses. Combining residue and fertilizer inputs resulted in a significant (P < 0.05) negative interactive effect on total extractable mineral N in all soils. This interactive effect decreased the mineral N pool, due to an immobilization of fertilizer-derived N and was observed up to 181 d, but generally became non-significant after 545 d. The initial reduction in mineral N might lead to less N₂O losses. However, a texture effect on N₂O fluxes was observed, with a significant interactive effect of combining residue and fertilizer inputs decreasing N₂O losses in the coarse textured soils, but increasing N₂O losses in the fine textured soils. The interactive effect on mineral N of combining fertilizer with residue changed from negative to positive with increasing residue quality. Our results indicate that combining fertilizer with medium quality residue has the potential to change N transformations through a negative interactive effect on mineral N. We conclude that capitalizing on interactions between fertilizer and organic residues allows for the development of sustainable nutrient management practices.     

Dynamics of 15N-labeled ammonium sulfate in various inorganic and organic soil fractions of wetland rice soils

Summary The dynamics of basally applied ¹⁵N-labeled ammonium sulfate in inorganic and organic soil fractions of five wetland rice soils of the Philippines was studied in a greenhouse experiment. Soil and plant samples were collected and analyzed for ¹⁵N at various growth stages. Exchangeable NH₄ ⁺ depletion continued after 40 days after transplanting (DAT) and corresponded with increased nitrogen uptake by rice plants. Part of the applied fertilizer was fixed by 2:1 clay minerals, especially in Maligaya silty clay loam, which contained beidellite as the dominant clay mineral. After the initial fixation, nonexchangeable ¹⁵N was released from 20 DAT in Maligaya silty clay loam, but fixation delayed fertilizer N uptake from the soil. Part of the applied N was immobilized into the organic fraction. In Guadalupe clay and Maligaya silty clay loam, immobilization increased with time while the three other soils showed significant release of fertilizer N from the organic fraction during crop growth. Most of the immobilized fertilizer N was recovered in the nondistillable acid soluble (alpha-amino acid + hydrolyzable unknown-N) fraction at crop maturity. Between 61% and 66% of applied N was recovered from the plant in four soils while 52% of fertilizer N was recovered from the plant in Maligaya silty loam. Only 20% – 30% of the total N uptake at maturity was derived from fertilizer N. Nmin (mineral N) content of the soil before transplanting significantly correlated with N uptake. Twenty-two to 34% of applied N was unaccounted for possibly due to denitrification and ammonia volatilization.     

Priming effect of biuret addition on native soil N mineralisation under laboratory conditions

This study was carried out to quantify the priming effect of biuret on native soil nitrogen (N) mineralisation during a 112-day incubation. Addition of biuret (100 mg ¹⁵N-labelled biuret kg⁻¹ soil) increased the turnover rate constant of soil organic matter and had a positive priming effect on native soil N mineralisation in two soils. The additional mineralisation was 0.65% of the total soil N (equivalent to 47.1 kg N ha⁻¹) in a sandy loam soil and 0.62% of the soil N (equivalent to 46.5 kg N ha⁻¹) in a silt loam soil.