Biol Fertil Soils (1999) 29 : 386–393
Q Springer-Verlag 1999
ORIGINAL PAPER
P. Martín-Olmedo 7 R. M. Rees
Short-term N availability in response to dissolved-organic-carbon from poultry manure, alone or in combination with cellulose
Received: 29 May 1998
Abstract Short-term changes in N availability in a sandy soil in response to the dissolved organic carbon (DOC) from a poultry manure (application rate equivalent to approximately 250 kg N ha P1) were evaluated in a 44-day aerobic incubation experiment. The treatments included poultry manure alone and two treatments in which an extra source of C, of low water solubility, was added with the poultry manure in the form of a low (1.05 g kg P1) and a high (4.22 g kg P1) amount of cellulose. All treatments were fertilised with the equivalent of 60 kg N ha P1 of ( 15NH4)2SO4 in solution. A control treatment consisted of sieved field-moist soil plus 60 kg N ha P1 of ( 15NH4)2SO4 in solution. Measurements were made of N2O and CO2 emissions, inorganic N, DOC, biomass N, biomass C and labelled N contained in the inorganic N and biomass N pools. The dynamics of N turnover in this study were driven mainly by processes of mineralisation–immobilisation with little significant loss of N by volatilisation or denitrification. The DOC supplied with the poultry manure played a more important role in N2O emissions than differences in C/N ratio. Changes in DOC and cumulative CO2-C production during the first 11 days were also highly correlated (R 2p0.88–0.66, P~0.01). An initial net immobilisation of N, with significant increases in biomass C and biomass N (P~0.05) for all treatments over the control at day 11, indicated a high availability of C from the DOC fraction. The presence of additional C from the applied cellulose did not enable a massive N immobilisation. Total inorganic N and unlabelled inorganic N concentrations were highest in soils treated with poultry manure alone (P~0.05), indicating that an active gross mineralisation of the added poultry manure and a possible positive priming effect were taking place during the incubation. P. Martín-Olmedo (Y) 7 R. M. Rees Scottish Agricultural College, West Mains Road, Edinburgh EH9 3JG, Scotland e-mail:
[email protected] Fax: c44-131-6672601
Key words Dissolved organic carbon 7 Poultry manure 7 N availability 7 Cellulose 7 N turnover
Introduction Several studies have implied that the amount of dissolved organic carbon (DOC) is a measure of the readily available resource for microbial growth and biological decomposition, often being considered as a good index of C availability (Burford and Bremner 1975; Reinertsen et al. 1984; Paul and Beauchamp 1989; Liang et al. 1995, 1996; Jensen et al. 1997). However, few studies have satisfactorily considered the dynamic nature of the DOC fraction and its role in N turnover. The immediate effect of the application of manures to agricultural land is an increase in mineral nutrients and DOC, which generally increases both soil organic C and microbial biomass (McGill et al. 1986; Liang et al. 1995, 1996). Very high concentrations of mineral N have been recorded following the application of organic manures to agricultural land (Vaidyanathan et al. 1991), the fate of which is likely to be influenced by interactions with DOC. Poultry manures contain essential plant nutrients and are well known to be excellent fertilisers (Simpson 1990; Sims and Wolf 1994). The DOC composition of poultry manures varies widely depending on poultry type, diet and dietary supplements, litter type, handling and storage operations, but it can account for up to 25% of total organic C (Stevenson 1986). Considerable research has been carried out in order to identify application rates, timing and incorporation practices that contribute to improved agricultural uses for poultry manure in terms of maximising crop yields and minimising negative environmental problems (Bitzer and Sims 1988; Edwards and Daniel 1992). However, very little information is available on the fate of the DOC pool supplied by the poultry manure, and the influence that this pool has on the N mineralisation–immobilisation turnover in poultry manureamended soils. A better understanding of these processes will help to make more efficient use of the inor-
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ganic and organic N pools that poultry manures contain as well as providing a better understanding of N turnover in manure-amended soils. A laboratory incubation was conducted to study short-term changes in N availability in response to DOC supplied with a poultry manure, alone or in combination with different amounts of cellulose. The addition of cellulose was made in order to introduce an additional source of C of low solubility in water, and investigate changes in N mineralisation–immobilisation in response to alterations of C : N ratios.
Materials and methods Soil and products used The soil used in this experiment was a sandy loam of the Biel Series (Soil Survey of Scotland 1982), collected in May 1996 from the Ap horizon of an arable field at the Bush Estate in southeastern Scotland. Selected properties of the soil (layer of 0–20 cm depth) were as follows: water content, 0.23 g g P1 soil; pH, 6.1; organic C, 2.6%; total N, 0.22%. Large pieces of green residue were removed by hand and field-moist soil was sieved to ~4 mm and thoroughly mixed but not allowed to dry. Experimental treatments included additions of an aerobically stabilised and pelleted poultry manure and a finely ground spruce cellulose (Fluka, Biochemica). The poultry manure was characterised by a water content of 0.12 g g P1, total N content of 4.2%, P1 total C content of 40%, NHc , NOP 4 -N concentration of 7.5 g kg 3P1 N concentration of 0.05 g kg and DOC concentration of 94 g kg P1. The cellulose presented a water content of 0.07 g g P1, total C of 41% and DOC concentration of 5.3 g kg P1. Treatments and incubation design Replicate treatments were established in 1 l sealable kilner jars provided with a short-tube (3 mm) inserted through a hole in the lids. A three-way stopcock was attached to the top of the tube to sample gases in the jars, and 600 g sieved field-moist soil was placed in each jar. Poultry manure was added at an N content equivalent to approximately 250 kg N ha P1; this required 2.40 g poultry manure (equivalent dry weight) per kg dry soil (PM1 treatment). In order to supply an additional C source with low solubility in water, the poultry manure was combined with cellulose at a rate of either 1.05 g kg P1 dry soil or 4.22 g kg P1 dry soil (PM2 and PM3 treatments, respectively). All treatments were fertilised with the equivalent of 60 kg N ha P1 of ( 15NH4)2SO4 (10.4 atom % 15N enrichment) in solution. The C/N ratio of the different combinations added to the soil was: 7.5, 10.6 and 19.9 for PM1, PM2 and PM3 treatments, respectively. A control treatment consisted of 600 g sieved field-moist soil to which 60 kg N ha P1 as ( 15NH4)2SO4 (10.4 atom % 15N enrichment) in solution had been added. Each treatment was replicated four times. The jars containing the treated soil were loosely stoppered to allow aeration and incubated in the dark at 12 7C (mean annual temperature in the arable depth in soils of Scotland) for 44 days. The 44-day incubation period was chosen in view of the fact that microbial mineralisation–immobilisation activities are reported to be more intense during the first few days of incubation (Azam et al. 1994). At intervals of 3 days, the jars were weighed, and deionised water added to compensate for moisture loss. Sampling and analyses Subsamples from every jar were removed 3, 5, 11, 17, 34 and 44 days after the start of the incubation for the determination of P inorganic N (NHc 4 -N and NO3 -N) and DOC. Inorganic N was ex-
tracted by shaking with 1 M KCl (soil : solution, 1 : 5) for 1 h followed by filtration through Whatman No. 42 filter paper (Keeney and Nelson 1982). The concentration of NHc 4 -N was determined by the salicylate-hypochlorite method (Crooke and Simpson 1971) and NOP 3 -N by the Griess-Ilosvay technique (Best 1976), both using a flow injection autoanalyser (Chemlab Instruments). To extract DOC, soil was shaken with distilled water (soil : solution, 1 : 2 ratio) for 15 min followed by centrifugation (5000 g for 20 min) and filtration through a 0.45 mm Millipore filter. The filtrate was analysed for DOC by UV digestion in a persulphate reagent using a Dohrmann DC 80 carbon analyser. The concentrations of N2O and CO2 in the headspace of the incubation jars were measured at 0, 1, 2, 4, 14 days and 0, 1, 2, 3, 11 days, respectively. Gas sampling was carried out prior to any alteration in soil water or soil sampling. One hour prior to sampling, the jars were sealed and gas samples taken using 5 ml glass hypodermic syringes. These gas samples were analysed within 1 h by gas chromatography (Hewlett Packard 5890 Series 2, with an ECD detector for N2O and a TCD detector for CO2). Cumulative emissions of N2O and CO2 over the period of the first 14 and 11 days, respectively, were calculated following McTaggart et al. (1997) by linear interpolation between successive flux measurements and integration over the whole period. Jars were also sampled after 11 and 44 days for the concentration of C and N in the microbial biomass and the percentage of 15 N present in the inorganic N and biomass N pools. C and N in the microbial biomass were determined by the chloroform fumigation-incubation method (Voroney and Paul 1984) on a 10-g (fresh weight) sample. Moist soil samples were placed in a sealable glass tube and exposed to ethanol-free CHCl3 vapour for 24 h at room temperature in a vacuum oven. After removal of the CHCl3, the tubes were sealed with gas-tight suba-seals and transferred to an incubator at 20–25 7C for 10 days in the dark. The CO2 concentration in the headspace was then analysed by gas chromatograph (as described above). After CO2 sampling, the soil was extracted with 50 ml of 1 M KCl and analysed for NHc 4 -N (as described above). The biomass C and N were calculated as follows: Biomass C (mg C kg P1 soil)pCf/0.41 and Biomass N (mg N kg P1 soil)pNf/[(P0.014pCf/Nf)c0.39] (Voroney and Paul 1984), where CfpCO2-C (mg C kg P1) emitted P1 c and Nfp(NHc ). 4 -N)fumigatedP(NH4 -N)unfumigated (mg N kg The method of Brooks et al. (1989) was used to determine the 15 N enrichment of the inorganic N and biomass N pools. Soil samples were analysed for available N (as described above). A volume of 1 M KCl extract containing between 100 and 350 mg N was placed in a kilner jar which had a disc of Whatman GF/D filter paper inserted on a syringe needle attached to the inside of the lid, on to which 10 ml of KHSO4 had been pipetted. Exposure of the filter paper and KHSO4 to the atmosphere was minimised. 15 For 15NHc NHc 4 -N analysis 0.3 g MgO, or for both 4 -N and 15 NOP -N analysis 0.4 g Devarda’s alloy followed by 0. 3 g MgO, 3 were added to the solution in the kilner jar which was then sealed and left in the dark at room temperature for 6 days. After 6 days the needle and filter paper were dried in a desiccator for 24 h, after which the filter paper was placed in a tin foil cup, ready for analysis by mass spectrometry. Results are expressed as the absolute amount of N derived from the labelled inorganic fertiliser (NdfF) using the expression: NdfFpTotal amount of N p [atom% 15N excess treated soilPbackground enrichment] [fertiliser enrichmentPbackground enrichment] The total amount of N derived from the soil plus the added manure (NdfScPM) was calculated as follows: (NdfScPM) p[total amount of N]P[NdfF] Statistical analyses All results were expressed on an oven-dry soil weight basis (105 7C, 24 h). The data were analysed by standard analysis of
388 variance techniques. Significant main effects were separated with a paired Student’s t-test (P~0.05). All analyses were carried out using the MINITAB statistics package. Results are expressed as meanB1 SE.
Results Mineral N pool At the start of the experiment, there was a high concentration of inorganic N in the soil (136B7 mg N kg P1) as a result of the addition of (NH4)2SO4 and the NHc 4 -N initially present in the poultry manure (7.5 g N kg P1). The concentration of NHc 4 -N (Fig. 1a) declined steadily during the incubation period in all treatments, reaching values of around 1 mg N kg P1 after 44 days. From day 11 to day 44, the highest concentrations of NHc 4 -N were
always measured in PM1 and the lowest in PM3 and control (P~0.05). The concentration of NOP 3 -N (Fig. 1b) generally increased in all treatments during the period in which the concentration of NHc 4 -N declined. The largest increase was observed in the PM1 treatment (P~0.05) after day 11. Soils supplemented with the high amount of cellulose (PM3 treatment) showed very little increase, with lower NOP 3 -N concentrations (P~0.05) than those in PM1, PM2 and control treatments after day 17. The reduction in available NHc 4 -N and the increase in NOP -N concentrations with time were highly corre3 lated for PM,1 and control treatments (R 2p0.91 and 0.89, respectively P~0.01), but not so well correlated in the PM2 and PM3 treatments (R 2p0.63 and 0.34, respectively P~0.01). P The change in total inorganic N (NHc 4 -NcNO3 -N) (Fig. 1c) measured from PM1 and control treatments was relatively constant throughout the incubation period, with an initial apparent immobilization from day 3 to day 5. The addition of increasing amounts of cellulose with the poultry manure resulted in an increase of the initial apparent immobilisation in PM2 and PM3 treatments by comparison with the soil receiving manure only (Fig. 1c). Dissolved organic carbon The concentrations of DOC were found to be higher for PM1, PM2 and PM3 treatments than in the control treatment (Fig. 2). This fact was mainly due to the high amount of DOC supplied with the poultry manure. However, a large part of this C was rapidly used in the first 5 days, decreasing steadily until the end of the experiment (44 days) when no differences between treatments were observed.
P Fig. 1 Concentrations of NHc 4 -N (a), NO3 -N (b) and total inorganic N (c) measured for soils receiving inorganic N plus poultry manure ([), inorganic N plus poultry manure and the low rate of cellulose (N), inorganic N plus poultry manure and the high rate of cellulose (n) and inorganic N only (}). Bars show SE
Fig. 2 Dissolved organic carbon (DOC) concentrations measured for soils receiving inorganic N plus poultry manure ([), inorganic N plus poultry manure and the low rate of cellulose (N), inorganic N plus poultry manure and the high rate of cellulose (n) and inorganic N only (}). Bars show SE
389 Fig. 3 Emission rates and cumulative emissions of N2O (a, b) and CO2 (c, d) measured for soils receiving inorganic N plus poultry manure ([, PM1), inorganic N plus poultry manure and the low rate of cellulose (N, PM2), inorganic N plus poultry manure and the high rate of cellulose (n, PM3) and inorganic N only (}, C). Bars show SE
N2O and CO2 emissions On day 1, combined additions of both organic and inorganic N in soils receiving poultry manure raised emissions of N2O (Fig. 3a) in PM1, PM2 and PM3 treatments compared with that measured from the control (P~0.05). After a decline between days 2 and 4, emissions of N2O from PM1 and PM2 treatments increased slightly by day 14, with fluxes significantly greater (P~0.05) than that from the control. Emission rates of N2O from soils supplemented with the high amount of cellulose (PM3 treatment) followed a diminishing trend from day 1 to 14. This decline is connected with lack of increase in the NOP 3 -N concentrations (Fig. 1b). Over the first 14 days of incubation, the cumulative N2O emissions (Fig. 3b) measured from PM1, PM2 and PM3 treatments were very similar but were almost twice as high as that measured from the control (P~0.05). The different treatments used in this experiment also influenced the release of CO2 over the first 11 days. The highest CO2 emission rate (Fig. 3c) and cumulative CO2 emission (Fig. 3d) were measured in the treatment with the largest organic C input (PM3 treatment) although differences with PM1 and PM2 treatments were not always significant (P~0.05). Proportionally, however, 35% of the total organic C added with the poultry manure in PM1 treatment was evolved as CO2 after 11 days, a percentage that was more than double that corresponding to PM2 and PM3 treatments.
The cumulative CO2 released from the PM1 treatment was highly correlated (R 2p0.88, P~0.001) with the DOC concentrations measured for this treatment in the first 11 days of the incubation. There was also a significant correlation between cumulative CO2 and DOC in PM2 and PM3 treatments (R 2p0.78 and 0.66, respectively, P~0.01).
Biomass C and biomass N At day 11, the concentrations of biomass C (data not presented) in the three treatments with added organic substrates were very similar (357–490 mg C kg P1) but were greater (P~0.05) than that for the control (236B34 mg C kg P1). The determination of biomass C in the PM3 treatment at this time was associated with high variability (covariation 40%) indicating the limitations of fumigation-incubation technique in soils which contain large quantities of microbially available C (Martens 1995). Average variation of biomass C determination at day 11 in the remaining treatments was 13%. By the end of the experiment (44 days), a general decline in the biomass C concentrations was observed in PM1, PM2 and PM3 treatments (208–249 mg C kg P1), reaching values similar to that for the control (194B5 mg C kg P1). The determination of biomass C at this stage was more reliable, with an average of 7% variation for all treatments.
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After 11 days, soils receiving poultry manure, whether alone (PM1 treatment) or in combination with the low (PM2 treatment) and the high amount of cellulose (PM3 treatment), also showed an increase (P~0.05) in the concentrations of biomass N (Fig. 4a) over the control (136%, 136% and 78%, respectively). By day 44, the concentrations of biomass N decreased significantly in PM1 and PM2 treatments (P~0.05) but not in the control or in the PM3 treatment. The determination of biomass N concentrations for all treatments was associated with an average of 2–9% variation.
1.6 and 2.7 times, respectively, those measured for PM3 treatment. In general, the ratio of NdfF to NdfScPM measured from PM1 treatment (Fig. 4) in the two N pools (biomass-N and inorganic-N) was smaller than the control. The concentrations of NdfF measured in the biomass N pool had declined substantially for all treatments by day 44. At this time, the greatest NdfF concentration in the biomass N was measured for PM3 treatment (P~0.05).
Discussion Biomass- 15N and mineral- 15N pools: NdfF and NdfScPM concentrations The concentrations of NdfF in the biomass N and inorganic N pools at day 11 for PM1 treatment were about
Fig. 4 Concentrations of N derived from the labelled fertilizer (NdfF) and concentrations of N derived from the soil and the poultry manure (NdfScPM) in the biomass-N (a) and the inorganic-N (b) pools measured for the soils receiving inorganic N plus poultry manure (PM1), inorganic N plus poultry manure and the low rate of cellulose (PM2), inorganic N plus poultry manure and the high rate of cellulose (PM3) and inorganic N only (C). Treatments with the same letter for NdfF and NdfscPM do not differ statistically (P~0.05)
The total N content and C : N ratio of organic substrates have often been considered to be useful indices of potential N availability (Reinertsen et al. 1984; Douglas and Magdoff 1991; Cook and Allan 1992). However, several studies have proposed that other chemical components of organic substrates such as volatile fatty acids, proteins, cellulose, lignin or polyphenols need to be considered for better prediction of N mineralisationimmobilisation potential and decomposition rate of substrates (Lerch et al. 1992; Tian et al. 1992; Kirchmann and Lundvall 1993). In the present study the importance of DOC on influencing the dynamic of N in soils amended with poultry manure has been examined. Changes in mineral N concentration in incubation studies with organic wastes can result from mineralisation–immobilisation transformations in addition to volatilisation and denitrification processes. The highest concentrations of inorganic N found in soils amended with poultry manure but not cellulose (PM1 treatment) showed that active gross mineralisation of the added poultry manure took place during the experiment. Similar results have been summarised by Sims and Wolf (1994). It is important to note that the addition of the manure might have also stimulated the decomposition of the native soil organic matter due to what is called priming effect (Azam et al. 1993). This argument is supported by the higher unlabelled inorganic-N (NdfScPM) in the PM1 treatment than in the control at days 11 and 44. The concentrations of biomass C and biomass N in soils receiving amendment with organic substrates, which were higher than those in the control after day 11, provide evidence for a parallel process of microbial immobilisation of N. Increased fluxes of C and N into the microbial biomass due to the addition of exogenous organic matter to soils have been extensively reported (Ghoshal and Singh 1995; Jensen et al. 1997). In the present study, the larger concentrations of biomass-labelled N in PM1 than in PM3 at day 11 suggests: (1) that increasing amounts of cellulose did not increase microbial immobilisation of N; and (2) that during the first 11 days, most N immobilisation resulted from the presence of poultry manure. These observations could be explained by the high concentration of
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DOC in the poultry manure (24% of total organic C) compared to the low concentration of DOC in the added cellulose (1.3% of total organic C). This hypothesis is supported by the simultaneous decreases in DOC and total inorganic N concentrations from days 1 to 5 in soils treated with poultry manure (PM1, PM2 and PM3). The high correlation found between cumulative CO2 production and DOC in the PM1 treatment also substantiates the assumption that the dissolved organic substances were in general quite labile and particularly susceptible to microbial decomposition. Burford and Bremner (1975), Paul and Beauchamp (1989) and Liang et al. (1995, 1996) found strong relationships between DOC concentrations and CO2-C mineralisation in a variety of soil types, and in response to increasing amounts of added DOC. Similarly, Zack et al. (1990) reported that microbial biomass C was highly correlated with DOC. However, the quantity of the DOC supplied with the poultry manure in our study (241 mg C kg P1 dry soil) was insufficient to account for the total CO2-C evolved (364B5.3 mg CO2-C kg P1 dry soil) plus the increment over the control in biomass C concentrations (121B28 mg C kg P1 dry soil) 11 days after the application of PM1 treatment. Therefore, part of the available C in soils treated with poultry manure must have been derived either from the decomposition of insoluble organic fractions of the poultry manure or from the native soil organic matter as the result of a priming effect. This finding is in agreement with that reported by Liang et al. (1996) who observed that most of the C mineralised after the addition to soils of DOC extracted from composted dairy manures came from the indigenous soil organic C pool. Cellulose decomposition in soils has been reported to be accelerated by the presence of easily decomposable substances, which serve as an energy supply for the cellulose decomposers (Dalenbergh and Jager 1989). The DOC added to soil with the poultry manure in our study could have been used as an energy source in the formation of cellulose decomposers. This argument would explain the largest cumulative CO2-C production recorded in PM3 treatment after 11 days. Entry et al. (1997) observed that the combined addition to soils of different organic by-products (newsprint, woodchips, cotton gin waste) and poultry manure did not consistently affect microbial biomass, but resulted in higher cellulose degradation rates than soils amended with the same organic by-products and NH4NO3. The low concentration of labelled (NdfF) contained in the biomass and inorganic N pools of soils treated with the high amount of cellulose (PM3 treatment) at day 11 would appear to be surprising if it is assumed from the decrease in the concentration of inorganic N that high rates of immobilisation were associated with this treatment. However, this effect could result from an earlier N immobilisation, with inorganic N being transferred to a non-biomass labile pool (N incorporated into the soil organic pool through the death of microbial tissues), which could not be recorded in the
labelled pools measured at day 11. Another explanation could be an abiotic N immobilisation by binding the inorganic N to the organic matter. Ruterford and Juma (1992) observed that amendment of soil with glucose leads to the conversion of large amounts of N into non-microbial organic N fractions which are slowly mineralised. Once the DOC fraction has been completely utilised by the microbes, the microbial availability of C would depend on the degradation of cellulose and the more recalcitrant organic fractions of poultry manure and soil. In this sense, cellulose appeared to be an intermediate labile C source between DOC and the insoluble fractions of poultry manure and soil, being responsible for the highest biomass N concentrations in PM3 treatment by day 44. The largest NdfF concentration of biomass N in PM3 treatment at this stage could be due to the re-immobilisation of the non-biomass N pool or to the release of part of the labelled N bound to the organic matter. The general re-mineralisation of labelled N (NdfF) from the microbial biomass recorded by day 44 responded to the intricate cycle of organic matter degradation and microbial population turnover. The existence of two compartments in the microbial biomass, an easily mineralisable compartment with zero-order kinetics, and a more stable compartment with first order kinetics, has been described in different models (Van Veen et al. 1984; Nicolardot et al. 1994). Significant losses of NH3 by volatilisation were unlikely to have occurred in this study due to the homogeneous incorporation of the manure and (NH4)2SO4 made at the beginning of the experiment, and the aerobic pre-treatment of the poultry manure. Sims and Wolf (1994) noted in their review that immediate incorporation of poultry wastes reduces average NH3 volatilisation losses to 3% of total N, relative to 20% for surface application. Bernal and Kirchmann (1992) reported losses of NH3 through volatilisation of only 4% N from aerobically treated pig manures, by comparison with losses of around 14% from anaerobically treated pig manures. N2O emissions measured from soil receiving both organic and inorganic N, which were greater than that measured for the control on day 1, confirm the results of other authors such as Granli and Bøckman (1994), who reported that the rate of N2O production is partially controlled by C susceptible to mineralisation. Biogenic production of N2O in soils occurs primarily by microbially mediated nitrification and denitrification (Sahrawat and Keeney 1986). These processes are govP erned by N substrate availability (NHc 4 -N and NO3 -N) and the course of organic matter decomposition, as well as other factors (Granli and Bøckman 1994; Mosier et al. 1998). For NHc 4 -N containing fertilisers and under aerobic conditions, as the ones used in the present study, mainly microbiological nitrification reactions could be expected to be involved in the N2O emissions recorded (Granli and Bøckman 1994). However, the
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organic C input made with the poultry manure and the cellulose might have created anaerobic microsites and so reduced autotrophic nitrification and enhanced N2O production via denitrification (Sahrawat and Keeney 1986). The lack of significant differences between PM1, PM2 and PM3 treatments in cumulative N2O emissions observed after 14 days suggests that the DOC fraction supplied with the poultry manure might play an important role in the N2O emissions in our experiment. The importance of readily decomposable organic compounds in contributing to denitrification processes in soil is well documented in previous studies (Beauchamp et al. 1989; Dendooven et al. 1996). However, measurements over the first 14 days indicated that the loss of N2O-N from all treatments was always less than 2 mg N kg P1 h P1, which is equivalent to a loss of no more than 1 mg N kg P1 over the first 14 days of the experiment. Losses of N2 were not measured in this study, but given the aerobic nature of the incubation, it seems unlikely that these would be significantly greater than N2O losses (Arah et al. 1991; McTaggart et al. 1997). Therefore, this study demonstrates that in a relatively aerobic environment such as reported here, the most significant effects in soils receiving poultry manure alone or in combination with cellulose were on mineralisation–immobilisation turnover, with no significant losses of N by volatilisation or denitrification. The DOC supplied with the poultry manure acted as a highly biodegradable fraction, being efficiently used by the soil microbial biomass. This suggests that the manure DOC content is an important factor to be considered when predicting short-term availability of N in soils treated with poultry manure. Acknowledgements The authors wish to thank F. Wright and R. Howard for technical support. Financial support was provided by Scottish Office of Agriculture, Environment and Fisheries Department and the Ministerio de Educación y Ciencias de Espan˜a.
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