Mineralisation of <Superscript>15</Superscript>N-labelled sheep manure in soils of different texture and water contents

In order to investigate the effect of soil water and texture on C and N mineralisation of applied organic matter, sheep manure was sandwiched between two halves of intact soil cores and incubated at 20°C. The soils contained 10.8% (L1), 22.4% (L3) and 33.7% (L5) clay, respectively, and were drained to seven different matric potentials in the range -15 to -1,500 hPa. Evolution of CO₂-C was determined during 4 weeks of incubation. Contents of NO₃^--N, ¹⁵N and microbial biomass N were determined at the end of the incubation. The net release of CO₂-C from the manure (estimated as the difference between soils with and without manure) and the total CO₂-C evolution from soils with manure was not related to soil water content. Most CO₂-C evolved from manure-amended soils in the least clayey L1 soil. The manure caused immobilisation of soil NO₃^--N but the soil matric potential had no major effects on the net NO₃^--N production. Less than 1% of the manure ¹⁵N was found as NO₃^--N at the end of the incubation. When unamended, the sandy L1 soil held the least N in microbial biomass but the largest increases in biomass N caused by manure application were found in this soil. Despite the higher increases in microbial biomass N in the L1 soil, the total content of microbial biomass N in soils with manure application peaked in the most clayey soil (L5). The recovery of manure ¹⁵N at the end of the incubation ranged from 89% to 102%. The variation in ¹⁵N recovery was not related to soil clay content nor to soil matric potential. The experimental set-up was designed to mimic field conditions where manure is left as a discrete layer surrounded by structurally intact soil. In this situation the soil clay content and the soil water level appeared to have little influence on the C and N turnover in the manure layer.     

Sequential extraction of low concentrations of pyrene and formation of non-extractable residues in sterile and non-sterile soils

In this study, temporal changes in the extractability of ¹⁴C-pyrene, at native concentrations, were followed in two soils with differing organic matter contents, under sterile and non-sterile conditions over 24 weeks by a sequential solvent extraction scheme. No significant loss of the added ¹⁴C-pyrene was observed during the incubation. Significant decreases in methanol:water and n-butanol extractability were observed with increasing soil-pyrene contact time. Significant non-extractable residues were formed in all soils, with the largest increases found in the non-sterile soils. After 8 weeks soil-pyrene contact time, there was a significant increase in the rate and extent of sequestration of pyrene in the biologically active soils. This indicated that the aging of pyrene was initially a physical process, with active microbial communities increasing the rate and extent of residue formation after 8 weeks soil-pyrene contact time. These findings suggest that there is a need for longer term ageing experiments following the role of microbial communities on the formation of solvent non-extractable residues. The humin fraction of the soil organic matter contained the majority of the ¹⁴C-pyrene associated activity which was not extractable using the scheme of sequential solvents. Saponification of the soil humin resulted in the release of similar amounts of ¹⁴C-pyrene associated activity from sterile and non-sterile soils. Solvent extraction with methanol:water was found to significantly underestimate the bioavailable fraction, whereas n-butanol overestimated the bioavailability of the ¹⁴C-pyrene-associated activity when assessed by bacterial mineralization after 24 weeks soil-pyrene contact time.     

The nature and origins of diester phosphates in soils: a 31P-NMR study

Soils of two climosequences in Russia were investigated by ³¹P-NMR spectroscopy. They comprised Dystric Podzoluvisols, Haplic Greyzems, Calcic Chernozems, and Gypsic Kastanozems, which are located along temperature and precipitation gradients of the Russian Plain. Another sequence of soils included forest Humic Cambisols and Umbric Leptosols of subalpine and alpine meadows, which are formed in different climatic conditions along a climosequence of the Mt. Malaya Khatipara (northern Caucasus). The results showed that accumulation of DNA was high in the cold, wet, and acid soils (Dystric Podzoluvisol, alpine Umbric Leptosol), while phospholipids and teichoic acids mainly accumulated in the more microbially active soils. We performed a laboratory incubation experiment to test the relationship between microbial biomass P and P species identified in soil extracts. The proportions of P compounds resonating at 0.5-3.0 ppm in the NaHCO₃ and H₂SO₄ extracts from the incubated Humic Cambisol increased. The amounts of phosphate diesters resonating at 0 ppm in the same extracts and in the subsequent NaOH extracts decreased after incubation. Based on the results of ³¹P-NMR spectroscopy of native soils and of the laboratory incubation experiment we concluded that signals at 0 ppm in spectra of soil alkaline extracts belong to DNA P which is mainly stabilised in soil organic matter outside microbial cells (at least in soils with relatively low microbial activity). Phospholipids-teichoic acids P extracted with 0.5 M NaHCO₃ seems to be derived from soil microbial biomass, and its proportion can reflect the microbial activity in the soil.     

Bioavailability of triazine herbicides in a sandy soil profile

The bioavailability of atrazine was evaluated in a Danish soil profile (Drengsted) using a combination of soil sorption, transport and mineralisation methods as well as inoculation using Pseudomonas ADP. Sorption of atrazine decreased markedly with depth as indicated by K_d values of 5.2 l kg^-1 for the upper soil and 0.1 l kg^-1 for the subsoils. The transport of atrazine was evaluated using soil TLC plates and the resulting R_f values were 0.1 for the upper soil and 0.9 for the subsoil. Only a relatively small amount of atrazine leached through undisturbed soil columns taken from the upper 60 cm. Inoculating with Pseudomonas strain ADP (1᎒⁶ CFU g^-1 dry weight soil) revealed that the degradation of 0.01 ppm atrazine was fully completed (80% mineralisation) within 10 days in the subsoil, while it reached less than 15% in the upper soil. Over a period of 500 days, a total mineralisation of 37% of added atrazine in the upper soil was found (2 mg kg^-1 incubated at 20° C). However, in the subsurface soil where 0.02 mg kg^-1 of atrazine was incubated at 10°C, the degradation was slower, only reaching about 12%. Terbuthylazine mineralisation was found to be temperature-dependent and low (less than 5%) in the upper soil and very much lower in the subsoil. Desethylterbuthylazine was the most frequently found metabolite. Finally, Pseudomonas strain ADP inoculated into soils from different depths increased the mineralisation of terbuthylazine dramatically. Modelling using a "two-compartment model" indicated that desorption of terbuthylazine is the limiting step for its mineralisation.     

Decomposition of carbon-14-labelled wheat straw in repeatedly fumigated and non-fumigated soils with different levels of heavy metal contamination

We analysed the decomposition of ¹⁴C-labelled straw at five different levels of heavy metal contamination (100-20,000 µg total Zn g^-1 soil) in non-fumigated and repeatedly fumigated soils. The soils were not spiked with Zn, but were taken from sites containing different heavy metal concentrations. Zn was only used as a reference and the effects observed are most likely due to this metal. Microbial biomass decreased with increasing heavy metal content of soils, paralleled generally by the decreasing amount of wheat straw ¹⁴C incorporated into the microbial biomass. In addition, the newly synthesised microbial biomass declined more rapidly as the incubation proceeded. In the repeatedly fumigated soils, microbial biomass ¹⁴C corresponded to roughly 50% of the maximum ¹⁴C incorporation of the non-fumigated soil. The relative decline during incubation was similar to that of the non-fumigated soil at the respective contamination level. These results reveal clearly that heavy metal effects on straw decomposition do not depend on the ratio of substrate C to microbial biomass C. In contrast to microbial biomass C, the mineralisation of the wheat straw was not seriously affected by heavy metal contamination. The same was true for all of the repeatedly fumigated treatments, where a much smaller microbial biomass mineralised nearly the same amount of straw as in the non-fumigated soils. However, repeated fumigation caused a strong reduction in the decomposition of soil organic matter. The ratio of CO₂-¹⁴C to microbial biomass ¹⁴C after 60 days was linearly related to the Zn concentration in both non-fumigated and repeatedly fumigated samples, clearly indicating that an additional energy cost is required by soil microorganisms with increasing heavy metal concentrations.     

Effects of temperature, soil water status, and soil type on swine slurry nitrogen transformations

Manure N dynamics are affected by manure characteristics, soil factors, and environmental conditions. An incubation experiment was conducted to assess the relationship of these factors. The effects of temperature (11, 18, and 25°C), soil texture (three soils, silt loam to sandy loam), and soil water status (constant at 60% water filled pore space, WFPS, and fluctuating between 30% and 60% WFPS) on net mineralization and nitrification of swine manure N were assessed. Swine manure was applied at an equivalent rate of 350 kg total N ha^-1 to 250 g air-dry soil in 2-l canning jars. Subsamples were taken from each jar for NO₃^- and NH₄⁺ determination when fluctuating moisture treatment dried to 30% WFPS, with sampling continuing through four wet-dry cycles at each temperature. Manure NH₄⁺ was rapidly nitrified to NO₃^-. The relationship between NO₃^- accumulation and degree days after application (DDAA, 0°C base) could be described across temperatures using a single pool exponential model for each soil. More NO₃^- accumulated in coarser-textured soils (150-200 mg N kg^-1 soil), compared to 130 mg N kg^-1 soil in the silt loam soil. Fluctuating soil water status did not alter estimates of rate and extent of NO₃^- accumulation, but slowed NH₄⁺ disappearance somewhat.     

Short-term patterns of carbon and nitrogen mineralisation in a fallow field amended with green manures from agroforestry trees

The mineralisation of green manure from agroforestry trees was monitored with the objective to compare the temporal dynamics of mineralisation of litter from different species. Green manures from five agroforestry tree species were used on a fallow field during the long rainy season of 1997 (March-August) and from two species in the following short rainy season (September-January) in western Kenya. Different methods, i.e. measurements of isotopic ratios of C in respired CO₂ and of soil organic matter (SOM) fractions, soil inorganic N and mass loss from litterbags, were used in the field to study decomposition and C and N mineralisation. Soil respiration, with the separation of added C from old soil C by using the isotopic ratio of ¹³C/¹²C in the respired CO₂, correlated well with extractable NH₄⁺ in the soil. Mineralisation was high and very rapid from residues of Sesbania sesban of high quality [e.g. low ratio of (polyphenol+lignin)/N] and low and slow from low quality residues of Grevillea robusta. Ten days after application, 37% and 8% of the added C had been respired from Sesbania and Grevillea, respectively. Apparently, as much as 70-90% of the added C was respired in 40 days from high quality green manure. Weight losses of around 80%, from high quality residues in litterbags, also indicate substantial C losses and that a build-up of SOM is unlikely. For immediate effects on soil fertility, application of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application.     

Mitigation of N2O emission from an alluvial soil by application of karanjin

An incubation experiment was conducted to study N₂O emissions from a Typic Ustochrept, alluvial soil, fertilized with urea and urea combined with different levels of two nitrification inhibitors, viz karanjin and dicyandiamide (DCD). Karanjin [a furano-flavonoid, obtained from karanja (Pongamia glabra Vent.) seeds] and DCD were incorporated at rates of 5, 10, 15, 20 and 25% of applied urea-N (100 mg kg^-1 soil), to the soil adjusted to field capacity moisture content. The highest N₂O flux (366 µg N₂O-N kg^-1 soil day^-1) was obtained on day 1 after incubation from soil fertilized with urea without any inhibitor. The presence of the inhibitors appreciably reduced the mean N₂O flux from the urea-treated soils. The application of karanjin resulted in a higher mitigation of total N₂O-N emission (92-96%) compared to DCD (60-71%). Rates of N₂O flux ranged from 0.9 to 140 µg N₂O-N kg^-1 soil day^-1 from urea combined with different levels of the two inhibitors (coefficient of variation=24-272%). Karanjin (62-75%) was also more effective than DCD (9-42%) in inhibiting nitrification during the 30-day incubation period.     

Influence of soil parameters on the effect of 3,4-dimethylpyrazole-phosphate as a nitrification inhibitor

Nitrification inhibitors specifically retard the oxidation of NH₄⁺ to NO₂^- during the nitrification process in soil. In this study, the influence of soil properties on the nitrification-inhibiting effect of 3,4-dimethylpyrazole-phosphate (DMPP), a newly developed nitrification inhibitor, has been investigated. Based on short-term incubation experiments, where the degradation of DMPP could be largely disregarded, the oxidation of the applied NH₄⁺ was more inhibited in sandy soils compared with loamy soils. The influence of soil parameters on the relative NO₂^- formation could be described by a multiple regression model including the sand fraction, soil H⁺ concentration and soil catalase activity (R²=0.62). Adsorption studies showed that the binding behaviour of DMPP was influenced markedly by soil textural properties, viz. the clay fraction (r²=0.61). The adsorption of DMPP was found to be an important factor for the inhibitory effect on NH₄⁺ oxidation in a short-term incubation (r²=0.57). It is concluded that the evaluated soil properties can be used to predict the short-term inhibitory effect of DMPP in different soils. The significance of these results for long-term experiments under laboratory and field conditions needs further investigation.     

Soil organic matter distribution as influenced by enchytraeid and earthworm activity

Loam and sandy soils, and the earthworm casts produced with ¹⁴C-labelled plant material in both soils, were incubated in airtight glass vessels with and without enchytraeids to evaluate the effects of soil fauna on the distribution and fragmentation of organic matter. After 1, 3, and 6 weeks, the amount of C mineralised was determined in soils and earthworm casts, and the soil was fractionated into particulate organic matter (POM), the most active pool of soil organic matter, after complete physical dispersion in water. The percentage weight of fine fractions (0-50 µm) was 67.4% in the loam soil. Sand (coarse, i.e. 150-2,000 µm and fine 50-150 µm) represented 87.2% of total weight in sandy soil, while the percentages of C (PC) were 23.2% in coarse POM (2,000-150 µm) and 11.9% in fine POM (150-50 µm). These percentages were higher than those in loam soil, i.e. 3.4% (coarse POM) and 5.4% (fine POM). The PC in coarse POM (9.50%) and fine POM (16.4%) remained higher in casts from sandy soil than in casts from loam soil (4.7% in coarse and 14.3% in fine POM). The highest percentages of ¹⁴C-labelled leaves were found in fine fractions, 55.9% in casts from loam soil and 48.8% in casts from sandy soil. The C mineralisation of the added plant material was higher in casts from the sandy soil (20.3%) than from the loam soil (13.5%). Enchytraeids enhanced C mineralisation in the bulk sandy soil, but did not affect the mineralisation of added plant material in either soil. The main enchytraeid effect was enhancement of the humification process in the bulk sandy soil, the casts from this soil, and the bulk loam soil.     

Sulphur speciation and turnover in soils: evidence from sulphur K-edge XANES spectroscopy and isotope dilution studies

Sulphur K-edge X-ray absorption near edge structure (XANES) spectroscopy was used to quantify S species in humic substance extracts from ten soils from the UK, China and New Zealand, which differ in land use and agricultural management. XANES spectroscopy showed the presence of most reduced (sulphides, disulphides, thiols and thiophenes), intermediate (sulphoxides and sulphonates) and highly oxidised S (ester sulphates) forms, with the three groups representing 14-32%, 33-50% and 22-53% of the organic S in the humic substance extracts, respectively. Land use had a profound influence on the relative proportions of S species. Well-drained arable soils generally had a higher proportion of organic S present in the most oxidised form than the grassland soils collected nearby, whereas paddy soils showed a more reduced profile due to episodic flooding. In the Broadbalk Classical Experiment at Rothamsted, reversion of an arable system to grassland or woodland in the 1880s resulted in an increase of the most reduced and intermediate S species at the expense of the most oxidised S species. Long-term applications of farmyard manure to an arable plot also shifted S species from the most oxidised to the intermediate and the most reduced species. Sulphur immobilisation and gross mineralisation were determined in seven soils using the ³⁵S isotope dilution method. Gross mineralisation during a 53-day incubation correlated more closely with the amounts of the most reduced and intermediate S species than with the most oxidised S species, suggesting that the former (C-bonded S) were the main source of organic S for mineralisation in the short-term.     

Kinetics of microbial phosphorus uptake in cultivated soils

Knowledge about the role of microorganisms in P cycling at conditions of constant soil respiration rates and constant size of microbially bound P is lacking. To study the kinetics of microbial P uptake and cycling under such conditions, soils differing in biological activity were ³³PO₄ labelled by introducing a carrier-free tracer solution and incubated for 56 days. The ³³PO₄ incorporation into the fraction of microbial P releasable by chloroform treatment (P_chl) was assessed and the isotopic composition [=specific activity (SA); SA=³³PO₄/³¹PO₄] of P_chl and soil solution P compared. Soils were taken from a 20-year-old field experiment including a non-fertilised control (NON), a minerally fertilised conventional (MIN) and two organic farming systems [bio-organic (ORG); bio-dynamic (DYN)]. Tracer P incorporation continuously increased during incubation in DYN, ORG and MIN soils. It decreased in the order DYN>ORG>MIN, with differences in ³³PO₄ uptake between the farming systems being higher than suggested by the differences in the amount of P_chl. In the P-deficient NON soil, the highest initial incorporation of tracer P was found, but no additional uptake could be detected thereafter. In all soils, the SA of P_chl converged to the SA of the soil solution with increasing time. Since P_chl remained almost constant during the experiment, the findings suggest an intensive uptake of P from the soil solution into P_chl and concomitant release of P back to the soil solution and, thus, a rapid cycling through P_chl. Intensive P cycling between P_chl and the soil solution was confirmed in an additional experiment where microbial activity was stimulated by glucose and N additions.     

Soil metabolism of a new herbicide, [14C]Pyribenzoxim, under flooded conditions

To elucidate the fate of a new pyrimidinyloxybenzoic herbicide, pyribenzoxim, a soil metabolism study was carried out with [14C]pyribenzoxim applied to a sandy loam soil under flooded conditions. The material balance of applied radioactivity ranged from 96.4 to 104.4% and from 96.1 to 101.9% for nonsterile and sterile soils, respectively. The half-life of [14C]pyribenzoxim was calculated to be approximately 1.3 and 9.4 days for nonsterile and sterile soils, respectively. The metabolites identified during the study were 2,6-bis(4,6-dimethoxypyrimidin-2-yloxy)benzoic acid (M1) and 2-hydroxy-6-(4,6-dimethoxypyrimidin-2-yloxy)benzoic acid (M2), resulting from the cleavage of the ester bond and subsequent hydrolysis. The nonextractable radioactivity levels increased to 37.8% for nonsterile conditions at 50 days after treatment and to 38.2% for sterile conditions at 60 days after treatment. Fractionation of the nonextractable soil residues indicated that bound radioactivity was associated mainly with humin fraction. No significant volatile products or [14C]carbon dioxide was observed during the study. On the basis of these results, pyribenzoxim is considered to undergo rapid degradation in soil by microbial and chemical reactions, mainly hydrolysis, which limits its transfer to and accumulation in lower soil layers and groundwater. Therefore, the possibility of environmental contamination from the use of pyribenzoxim is expected to be very low.     

Mineralization-immobilization of soil organic S and oxidation of elemental S in subtropical soils under flooded and nonflooded conditions

Information on the influence of soil moisture on elemental sulphur (S⁰) oxidation and transformation into organic S in semi-arid subtropical soils is scarce. We studied the impact of three moisture regimes on the mineralization of soil organic S, and the oxidation and immobilization of S⁰ in acidic (pH 4.9), neutral (pH 7.1) and alkaline (pH 10.2) subtropical soils. Repacked soil cores were incubated under aerobic (40% and 60% water-filled pore space, WFPS) and flooded soil conditions (120% WFPS) for 0, 14, 28 and 42 days with and without incorporated S⁰ (500 µg g^-1 soil). Soil moisture had profound effects on these processes and the mineralization of native soil organic S, oxidation of applied S⁰ and transformation of S⁰ into soil organic S proceeded most rapidly at 60% WFPS, irrespective of soil pH. Mineralization of native soil organic S resulted in the accumulation of 34, 49 and 44 g SO₄^2--S g^-1 soil in acidic, neutral and alkaline soil in a 42-day period at 60% WFPS. The oxidation rate of added S⁰ during the initial 14-day period at 60% WFPS was highest in alkaline soil (428 µg S cm^-2 day^-1), followed by neutral soil (326 µg S cm^-2 day^-1), and lowest in acidic soil (235 µg S cm^-2 day^-1). These rates are several folds higher than those reported in earlier studies because now we computed the oxidation rates by including the amount of S⁰ that was immobilized to organic S. Of the applied S⁰ at 40% and 60% WFPS, 2.6% and 6.0%, 3.4% and 10.0%, and 9.4% and 14.4% oxidized to SO₄^2-, and 15.0% and 17.6%, 17.6% and 19.6%, and 17.6% and 23.6% transformed into organic S in the 42-day period in acidic, neutral and alkaline soil, respectively. These results suggest that in order to synchronize the availability of S with plant need, S⁰ may be applied well before the seeding of crops especially in acidic soils and in rainfed regions where soil moisture remains at less than 60% WFPS. Apparently no oxidation of S⁰ and significant reduction of SO₄^2--S (7, 53 and 78 µg SO₄^2--S g^-1 in acidic, neutral and alkaline soil, respectively) under flooded conditions suggest that S⁰ is least effective for correcting S deficiency in flooded soil systems such as rice fields.     

Degradation of isoproturon in biobeds

The fate of isoproturon {N,N-dimethyl-N'-[4-(1-methylethyl)phenyl]urea} in biobeds with and without inoculation with the white rot fungus Phanerochaete chrysosporium was studied. Total extractable isoproturon, its metabolites and formation of non-extractable residues were evaluated. Studies with ¹⁴C-isoproturon were also included. A strong decrease in isoproturon was observed in non-inoculated biobeds. Total extractable isoproturon decreased by 76% after 100 days. The decrease was even larger in biobeds inoculated with the white rot fungus P. chrysosporium. After 28 days, total extractable isoproturon decreased by 78%, and after 100 days >99% had disappeared in the inoculated biobeds. However, the studies with ¹⁴C-isoproturon showed that 30% of the initially recovered ¹⁴C-isoproturon remained in the non-inoculated biobeds as non-extractable residues. As no studies with ¹⁴C-isoproturon were performed in inoculated biobeds, it is unclear if the higher rate of disappearance was due to higher biodegradation or higher formation of bound residues.     

Effects of earthworms on Zn fractionation in soils

Laboratory incubation experiments were conducted to examine the effect of earthworm (Pheretima sp.) activity on soil pH, zinc (Zn) fractionation and N mineralization in three soils. No Zn uptake by earthworms was observed. Zinc addition decreased pH of red soil (soil 1) and hydragric paddy soil (soil 3) by 0.5 and 0.2 unit, respectively, but had no effect on alluvial soil (soil 2). The effect of Zn on soil pH was possibly due to a specific adsorption mechanism between Zn and oxides. Earthworm activity significantly decreased the pH of the red soil, a key factor affecting Zn solubility, but not of the other two soils. Earthworm activity significantly increased DTPA-Zn (DTPA-extractable) and OxFe-Zn (NH₂OH-HCl-extractable) in the red soil, but had little effect on other fractions. In the alluvial soil, earthworm activity significantly increased OxFe-Zn but decreased organic-Zn (organic-associated Zn). In the hydragric paddy soil, earthworm activity significantly increased MgCl₂-Zn (MgCl₂-extractable) and organic-Zn. The level of CaCl₂-extractable Zn in all three soils was not affected by earthworm activity. Nitrogen mineralized as a result of earthworm activity was equivalent to 110, 120 and 30 kg N ha^-1 in soils 1, 2 and 3, respectively. Zinc added at rates less than 400 mg Zn kg^-1 did not seem to affect the activity of N-mineralizing microorganisms. The present results indicated the possibility of increasing the metal bioavailability of relatively low level metal-contaminated soils, with a higher organic matter content, by earthworm inoculation.     

Denitrification, N2O and CO2 fluxes in rice-wheat cropping system as affected by crop residues, fertilizer N and legume green manure

Use of renewable N and C sources such as green manure (GM) and crop residues in rice-wheat cropping systems of South Asia may lead to higher crop productivity and C sequestration. However, information on measurements of gaseous N losses (N₂O+N₂) via denitrification and environmental problems such as N₂O and CO₂ production in rice-wheat cropping systems is not available. An acetylene inhibition-intact soil core technique was employed for direct measurement of denitrification losses, N₂O and CO₂ production, in an irrigated field planted to rice (Oryza sativa L.) and wheat (Triticum aestivum L.) in an annual rotation. The soil was a coarse-textured Tolewal sandy loam soil (Typic Ustochrept) and the site a semi-arid subtropical Punjab region of India. Wheat residue (WR, C:N=94) was incorporated at 6 t ha^-1 and sesbania (Sesbania aculeata L.) was grown as GM crop for 60 days during the pre-rice fallow period. Fresh biomass of GM (C:N.=18) at 20 or 40 t ha^-1 was incorporated into the soil 2 days before transplanting rice. Results of this study reveal that (1) denitrification is a significant N loss process under wetland rice amounting to 33% of the prescribed dose of 120 kg N ha^-1 applied as fertilizer urea-N (FN); (2) integrated management of 6 t WR ha^-1 and 20 t GM ha^-1 supplying 88 kg N ha^-1 and 32 kg FN ha^-1 significantly reduced cumulative gaseous N losses to 51.6 kg N ha^-1 as compared with 58.2 kg N ha^-1 for 120 kg FN ha^-1 alone; (3) application of excessive N and C through applying 40 t GM ha^-1 (176 kg N ha^-1) resulted in the highest gaseous losses of 70 kg N ha^-1; (4) the gaseous N losses under wheat were 0.6% to 2% of the applied 120 kg FN ha^-1 and were eight- to tenfold lower (5-8 kg N ha^-1) than those preceding rice; (5) an interplay between the availability of NO₃^- and organic C largely controlled denitrification and N₂O flux during summer-grown flooded rice whereas temperature and soil aeration status were the primary regulators of the nitrification-denitrification processes and gaseous N losses during winter-grown upland wheat; (6) the irrigated rice-wheat system is a significant source of N₂O as it emits around 15 kg N₂O-N ha^-1 year^-1; (7) incorporation of WR in rice and rice residue (C:N=63) in wheat increased soil respiration, and increased CO₂ production in WR- and GM-amended soils under anaerobic wetland rice coincided with enhanced rates of denitrification; and (8) with adequate soil moisture, most of the decomposable C fraction of added residues was mineralized within one crop-growing season and application of FN and GM further accelerated this process.     

Carbon and net nitrogen mineralisation in two forest soils amended with different concentrations of biuret

Biuret is a known contaminant of urea fertilisers that might be useful as a slow release N fertiliser for forestry. We studied carbon (C), net nitrogen (N) mineralisation and soil microbial biomass C and N dynamics in two forest soils (a sandy loam and a silt loam) during a 16-week long incubation following application of biuret (C 23.3%, N 40.8%, O 30.0% and H 4.9%) at concentrations of 0, 2, 10, 100 and 1000 mg kg⁻¹ (oven-dried) soil to assess the potential of biuret as a slow-release N fertiliser. Lower concentrations of biuret specifically increased C mineralisation and soil microbial biomass C in the sandy loam soil, but not in the silt loam soil. A significant decrease of microbial biomass C was found in both soils at week 16 after biuret was applied at higher concentrations. C mineralisation declined with duration of incubation in both soils due to decreased C availability. Biuret at concentrations from 10 to 100 mg kg⁻¹ soil had a significantly positive priming effect on soil organic N mineralisation in both soils. The causes for the priming effects were related to the stimulation of microbial growth and activity at an early stage of the incubation and/or the death of microbes at a later stage, which was biuret-concentration-dependent. The patterns in NH₄⁺-N accumulation differed markedly between the two soils. Net N mineralisation and nitrification were much greater in the sandy loam soil than in the silt loam soil. However, the onset of net nitrification was earlier in the silt loam soil. Biuret might be a potential slow-release N source in the silt loam soil.     

Nitrogen and phosphorus mineralization potentials of soils receiving repeated annual cattle manure applications

Manure application rates are generally calculated to balance nutrient inputs with crop requirements, based on a projected crop yield and estimates of nutrient release from recently applied manure during a growing season. Often, the contribution to plant nutrition of manure applied in the past is not considered explicitly. We obtained archived soil samples collected every 5 years during a 25-year period (1973-1998) from a long-term study in Lethbridge, Alberta, Canada to evaluate the effects of long-term manure applications on soil N and P mineralization potentials (N_max and P_max, respectively). Soils from experimental plots receiving 0, 30, 60, 90, 120 and 180 Mg manure (wet weight) ha^-1 year^-1 were incubated aerobically for 20 weeks under four different combinations of soil temperature (10°C and 20°C) and moisture [50% and 75% of field capacity (FC)] conditions. N_max and P_max were fit using a first-order rate equation. N_max and P_max were related linearly to the cumulative amount of N and P applied in manure, suggesting long-term manure applications increased the proportion of potentially mineralizable N and P in soils. Soil storage and handling in the laboratory (e.g., weekly rewetting during incubations) affected the slopes of the regression equations describing N_max and P_max. The slopes of regression lines relating N_max and P_max to cumulative manure applications were highest when soils were incubated at 20°C and 75% of FC. Adjusting manure application rates on agricultural land with a history of manure amendments, based on the increase in potentially mineralizable N and P from past manure applications, could help minimize nutrient export and environmental pollution from manure-amended soils.     

Adsorption and Desorption of Cry1Ab Proteins on Differently Textured Paddy Soils

In recent years, selected cry genes from Bacillus thuringiensis (Bt) encoding the production of Cry proteins (Bt toxins) have been engineered into crop plants (Bt-crops). Through the cultivation of Bt crops and the application of Bt pesticides, Cry proteins could be introduced into arable soils. The interaction between the proteins and soils was analyzed in this study to investigate the affinity of Cry proteins in paddy soil ecosystems. Four Paddy soils were selected to represent different soil textures. Cry proteins were spiked in soils, and the amount of protein adsorbed was measured over 24 h. Desorption of Cry1Ab proteins from paddy soils was performed by washing with sterile Milli-Q water (H₂O_MQ), and subsequently extracted with an extraction buffer. The paddy soils had a strong affinity for Cry1Ab proteins. Most of the Cry1Ab proteins added (> 98%) were rapidly adsorbed on the paddy soils tested. More Cry1Ab proteins were adsorbed on non-sterile soils than on sterile soils. Less than 2% of the adsorbed Cry1Ab proteins were desorbed using H₂O_MQ, while a considerable proportion of the adsorbed proteins could be desorbed with the buffer, ranging from 20% to 40%. The amount of proteins desorbed increased with the increases in the initial amount of Cry1Ab proteins added to the paddy soils. The concentration of Cry1Ab proteins desorbed from the paddy soils was higher for sterile soils than non-sterile ones. Our results indicate that Bt toxins released via the cultivation of Bt crops, the application of Bt pesticides can be adsorbed on paddy soils, and soil texture could impose an impact on the adsorption capability.