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Nitrogen transformations after the spreading of pig slurry on bare soil and ryegrass using 15N-labelled ammonium
European Journal of Agronomy 7 (1997) 181–188
Nitrogen transformations after the spreading of pig slurry on bare soil and ryegrass using 15N-labelled ammonium T. Morvan a,*, Ph. Leterme a, G.G. Arsene b, B. Mary c a
Unite´ INRA d’Agronomie, ENSAR, 65 rue de Saint-Brieuc, F35042 Rennes, France b USAMVB Timisoara, Colea Arodulini, 19, Timisoara 1900, Romania c Unite´ INRA d’Agronomie, rue Fernand Christ, BP 101, F02004 Laon Cedex, France Accepted 13 April 1997
Abstract A short field experiment (27 days) was carried out in summer 1995, to study the effect of an actively growing grass sward on nitrogen transformations of a pig slurry. The ammonium fraction of the slurry was labelled with (15NH4)2SO4. The slurry was spread manually on microplots in mid-June, at the rate of 3 l/m2, on a cut ryegrass sward, and compared with bare soil. Absorption of 15N-labelled NH4 by the grass occurred very rapidly, attaining 41% after 13 days and showing no further change at 27 days. The gaseous losses, mainly through volatilization of ammonia, were considerable. 15N recovery in soil and plant material on day 27 was 42.5% (±1.2) on the bare soil, versus 57.4% (±3.1) on the ryegrass. The grass sward significantly reduced: (i) volatilization, as shown by the difference of 14.9% in 15N recovery, on the 6th day; (ii) immobilization, which was 25% (±2.2) on day 27 on bare soil and 16.4% (±2.9) in the presence of ryegrass. 15N-labelled inorganic nitrogen was completely depleted beneath the ryegrass, 27 days after application, whereas ammonium was depleted and the nitrate was equal to 16.4% (±1.6) of the applied NH4 on the bare soil. It is clearly apparent that the ammonia from the slurry is more efficiently used when applied to an actively growing sward, rather than to bare soil, even though a significant portion of the plant is involved in internal recycling. 1997 Elsevier Science B.V. Keywords: Slurry; 15N; Grassland; Volatilization; Immobilization
1. Introduction Slurries, because of their high ammonium content, provide nitrogen which is quickly available for crops. The availability of the ammonium fraction is determined by gaseous loss and microbial immobilization. The first occurs mainly through the volatilization of
* Corresponding author. Tel.: +33 99 287231; fax: +33 99 287230; e-mail: [email protected]
ammonia, and is highly variable, (20–70% of total ammonium nitrogen (TAN) applied) (Lauer et al., 1976; Beauchamp et al., 1982; Pain et al., 1989; Ge´nermont, 1996), whereas microbial immobilization represents 15–35% of the TAN (Morvan et al., 1996). Slurry incorporation slightly reduces ammonia volatilization, but is not always possible, especially as it may lead to plant injury, for example after late winter applications on wheat or rapeseed. Furthermore, high nitrogen utilization efficiencies have been obtained for slurry ammonium
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after surface spreading of diluted pig slurries in actively growing wheat and were correlated with low levels of volatilization. A reduction in volatilization brought about by living plants has been reported and might be expected from: (i) absorption by plant leaves of ammonia volatilized from the underlying soil, as reported by Denmead et al., 1976; (ii) absorption of ammonium through the roots and (iii) microclimatic effects due to the canopy (Faurie and Bardin, 1979). Contradictory results have however been reported by other authors (Thompson et al., 1990), which could be explained by the greater surface area resulting from slurry retention on the leaves. What effect do plants have upon nitrogen immobilization? We might expect high rates of microbial immobilization under a grass sward, because of the large amounts of root exudate. Short-duration experiments, involving the measurement of actual rates of NH4 uptake by plants and microbes, have shown that microbial immobilization may be five times higher than plant uptake (Jackson et al., 1989). Ledgard et al. (1989), studying the partitioning of 15N-labelled ammonium applied to grass-clover pasture confirmed that microbial immobilization was a significant component of 15N balance, but also observed that more fertilizer was immobilized when plant growth was slow, due to lower temperatures. Immobilization is known to depend on the amount and persistence of ammonia in the soil. Thus, we may suppose that an actively growing plant able to rapidly absorb significant amounts of inorganic nitrogen, will reduce microbial immobilization. The few studies describing the effect of plants on nitrogen transformations after slurry addition sometimes give contradictory conclusions about volatilization, and rarely provide a complete description of soil-plant behaviour and competition, either because only one process was studied, or because the time scale of the experiment was too short, or too long. The aim of the present work was to obtain a better understanding of the effect of an actively growing plant on nitrogen transformations of a pig slurry ammonia pool, using labelled ammonium. A short field experiment was therefore carried out from midJune to mid-July, in order to ensure that climatic conditions were favourable to plant growth.
Table 1 Physical and chemical properties of the soil, and slurry composition Soil properties Particle size distribution (%) Clay Silt Sand Total N (%) pH KCl Bulk density of the soil layers 0–10 cm 10–20 cm Slurry composition N-NH4 (g/l) (after addition of ammonium sulfate) Total N (Kjeldhal) (g/l) pH Dry matter (%)
14.4 72.5 13.1 0.13 6.2 1.53 1.50 4.02 6.12 7.36 1.4
2. Materials and methods 2.1. Site and design The experiment was conducted at Le Rheu Experimental Station (INRA), in western France, on a loamy soil. Some of the chemical and physical properties of this soil are summarized in Table 1. The fate of the ammonia fraction of a pig slurry was studied using 15 N-labelled NH4. Two treatments were compared: surface spreading on a ryegrass sward, and on bare soil. Daily temperatures were relatively high, varying from 15°C to 25°C (the mean air temperature was 20.3°C over the 27 days), and were favourable both to plant growth and to ammonia volatilization and nitrogen biotransformations, such as nitrification, mineralization and immobilization. The amount of rainfall and change in soil moisture in the soil surface layer are shown in Fig. 1; 2 × 10 mm were applied by irrigation, the day before the start of the experiment, and on day 3, to prevent the soil from drying out. The soil moisture therefore varied from 60 to 100% of the field capacity over the first 13 days and was favourable for microbial activity and plant growth. No significant differences were noticed between the two treatments during this period. The plots were 4.70 m × 2.30 m and laid out in a
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nium at the rate of 188 g/ha), followed by removal of the aerial residues, just before spreading. The characteristics of the soil surface (structure, bulk density) and soil moisture, controlling initial infiltration of the slurry, and consequently the intensity of ammonia volatilization, were therefore similar in each treatment. Chemical weed control probably had no effect on the biological activity of the soil as the kinetics and level of immobilization of 15N-labelled nitrogen were similar to those observed after a comparable slurry application to a soil that was not subjected to chemical weed control (unpublished data). Fig. 1. Rainfall plus irrigation, and change in soil moisture on the bare soil (BS) and under ryegrass (R).
2.2. Soil and plant sampling
randomized block design with three replications. Each plot was divided into five microplots of 0.60 m2, corresponding to the sampling area for a given date of measurement. Microplots were separated from each other by a 30-cm discard area and from the boundaries by 50-cm guard strips. The ammonium fraction of a pig slurry, (composition in Table 1), was enriched with 15N using a solution of (15NH4)2SO4 10% atom excess, which was added and thoroughly mixed to the slurry. The initial atom excess of the ammonium pool (1.089%) was determined just after spreading, by sampling the soil surface. The enriched slurry was applied at the rate of 3 l/m2 on June 20th 1995, between 1500 h and 1645 h, in warm, sunny conditions. The amounts of ammonium and organic nitrogen applied were equal to 121 and 63 kg N/ha respectively. A watering can with a distribution bar was used to obtain as even a distribution as possible, and the plots were divided into microplots each receiving the same quantity of slurry. No run-off was observed after spreading, despite the high soil compaction (bulk density of the 0–10 cm soil layer: 1.53 g/cm3). The slurry was applied: (i) to a 2-year-old cut ryegrass (Lolium perenne) sward, which had not been fertilized with nitrogen since the date of sowing. The nitrogen content in plant material harvested at a height of 0.5 cm was therefore very low (1,12%) at the beginning of the trial. The grass sward was cut one week before the experiment, and was 10–12 cm high on the day of application; (ii) to bare soil, obtained by chemical destruction of the grass (glifosate ammo-
The microplots were sampled 1, 3, 6, 13 and 27 days after slurry spreading. The aerial parts were cut just above the soil, on a square area of 0.25 m2. The plant material was washed free of soil and slurry, the volume of washing water was measured, and the plants and water sampled for inorganic nitrogen and 15 N analysis. Soil samples were taken from the 0–10 cm and 10– 20 cm depths, to determine root biomass, organic and inorganic soil nitrogen. A 60 mm diameter probe was used for root sampling, and a 20 mm diameter probe for soil sampling. For soil nitrogen analysis, samples were obtained from each microplot by mixing 27 cores from each soil layer, and passing the whole sample through an 8-mm mesh sieve. Soil samples for roots were obtained by mixing six cores taken with the 60-mm diameter probe. The roots were separated from the soil by washing on a 2-mm mesh sieve; herbage residues were removed by hand. Above-ground parts and roots were dried at 60°C, finely ground to powder and put into tin containers for total nitrogen content determinations and 15N atom excess analysis. 2.3. Analytical procedures Inorganic nitrogen in the soil was determined in a KCl extract (600 ml 1 M KCl/300 g fresh soil, shaken for 30 min, then filtered through a Whatmann 42 filter), using the fractionated steam distillation with MgO for ammonium and Dewarda’s alloy for nitrate analysis (Drouineau and Gouny, 1947). Organic plus clay-fixed nitrogen and 15N was measured on a sample
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of moist soil, after removal of the inorganic nitrogen, as described by Recous et al., 1988; the sample was dried at 60°C, and finely ground. Atom excess was determined on subsamples of plant, soil, and dried solutions resulting from the steam distillations, using a total combustion technique linked to a VG SIRA 9 mass spectrometer (Recous et al., 1988).
3. Results 3.1. Inorganic nitrogen dynamics Inorganic nitrogen occurred mainly in the surface soil layer (0–10 cm). The amounts of 15NH4-labelled ammonium observed in the 10–20 cm soil layer did not exceed 0.5% of the applied ammonium. Small amounts of 15N-labelled nitrate, not exceeding 1.3% of the applied nitrogen, were measured in the lower layer of the bare soil, on days 13 and 27. The fate of the ammonium nitrogen is shown in Fig. 2: the curves represent the change in the amount of ammonium in the 0–20 cm soil layer, and ammonia deposition on the leaves in the ryegrass treatment. The amount of ammonium deposited on the plant leaves was relatively large on day 1: this suggests that the initial deposition was probably considerable, if comparable with the great decrease of the ammonium pool in the soil observed during the first day. Ammonium amounts decreased sharply during the
Fig. 3. The proportions of the 15N-labelled NH4 present as 15Nlabelled NO3 in the 0–20 cm soil layer. (Vertical bars indicate the standard deviation of the three replicates; SD not shown are smaller than symbol size.)
first 6 days. The kinetics of 15N-labelled NH4 were similar in each treatment; the plant tended to stimulate the depletion of ammonium, particularly between days 6 and 13. The change in 15N-labelled NO3 in the soil is shown in Fig. 3. Some nitrification of the slurry ammonium had occurred by the end of the experiment, because the ammonium pool had been depleted by that time, in both treatments. 15N-labelled NO3 content steadily increased until the end of the experiment in the bare soil, attaining only 16% of the nitrogen applied, but it was completely depleted beneath the sward. We did not observe a true latent period at the beginning of the trial as is often reported (Le Pham et al., 1984). 3.2. Dry matter and nitrogen absorption dynamics
Fig. 2. The proportions of 15N-labelled NH4 in the 0–20 cm soil layer, and deposition on ryegrass leaves, after application of 121 kg N-NH4/ha from a pig slurry. (Vertical bars indicate the standard deviation of the three replicates; SD not shown are smaller than symbol size.)
The pattern of 15N absorption by the whole plant is shown in Fig. 4; nitrogen absorption occurred rapidly: the nitrogen utilization efficiency had already reached 11.5% the day after spreading and had attained 41% by day 13 and remained stable till day 27. Because the soil moisture, temperature and soil ammonia content were similar in both treatments (Figs. 1 and 2) during the first 6 days, it can be assumed that nitrification occurred at the same rate in both treatments, during this period. It then follows from the comparison of 15N-labelled nitrate dynamics and 15N inorganic nitrogen absorption that absorption was mainly due to the uptake of ammonium or ammonia. The two routes of assimilation, through the roots and leaves, were probably efficient (see Section 4),
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iod corresponding to the amount of nitrogen taken up by the grass. 3.4. 15N balance
Fig. 4. The proportions of the 15N-labelled NH4 present in the above-ground parts and roots following the pig slurry application. (Vertical bars indicate the standard deviation of the three replicates; SD not shown are smaller than symbol size.)
and equal quantities can be found in aerial plant parts and roots on days 1 and 3. Plant dry matter and nitrogen content increased simultaneously during the experiment. The aboveground parts of the ryegrass rose from 4.1 to 6.8 t dry matter/ha, representing a mean growth rate of 0.1 t dry matter/ha per day. The nitrogen content of the aerial parts rose from 1.12 to 1.74%. The proportion of 15N-labelled nitrogen measured in the whole plant was 15.8% of the amount of 15N-labelled NH4 applied, on day 1, and reached a maximum value of 33.4% on day 13. 3.3. Immobilization The pattern of immobilization is shown in Fig. 5. In both treatments, 15N immobilization rose sharply during the first 3 days; on bare soil, it exceeded the value in the ryegrass with 6% as early as the first day, and continued to increase, whereas it remained steady in the ryegrass sward. The difference observed on day 1 might be related to the significant deposition of ammonia and organic carbon from the slurry on the leaves (ammonia deposition representing 7% of the total applied ammonium, on day 1) (Fig. 2), which thus reduced the amount of ammonium and organic carbon available in the soil. On the other hand, the increased margin between days 6 and 13 can be explained by the competitive effect of the plants which absorbed the inorganic 15N-labelled nitrogen, the depletion of the ammonium pool during this per-
Fig. 6 shows the change in non-recovery of the 15N, corresponding to gaseous losses. Denitrification was probably low during this experiment, in view of the climatic conditions and soil moisture. Ammonia volatilization probably accounts for most of the gaseous losses, all the more so as the kinetics of the 15N nonrecovery exhibit a typical pattern of volatilization, as described by Sommer and Olesen (1991) and by Jarvis and Pain (1990). In fact, cumulative volatilization can generally be described by an asymptotic exponential curve, and is well fitted to the following equation: v = vmax (1-e-kt) (Moal, 1995). The optimal values of the parameters of this equation were calculated for each treatment, using the Nlin procedure in the SAS package (SAS, 1988). The losses were the same on day 1, in both treatments, attaining 40% of the applied nitrogen. This suggests that there was no significant microclimatic effect of the canopy, or that differences compensated. Volatilization was significant on the bare soil between days 1 and 3 (+19% according to the adjusted curve), but almost stopped after day 1 on the ryegrass treatment (+2% according to the adjusted curve). This halting of volatilization in the grass sward was unexpected, as (i) the amounts of ammoniacal nitrogen measured in the surface layer were still high on days 1, 3 and 6, and were only a few kg N/ha less than the amounts measured on bare soil, (ii) volatilization after
Fig. 5. Proportions of 15N-labelled NH4 immobilized in the two treatments. (Vertical bars indicate the standard deviation of the three replicates.)
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Fig. 6. Proportions of 15N-labelled NH4 not recovered, corresponding to gaseous losses, during the days following pig slurry application. Values were adjusted using the equation: y = a (1- e-kt) (Vertical bars indicate the standard deviation of the three replicates.)
slurry spreading generally takes place over a period of 6 days (Jarvis and Pain, 1990; Moal, 1995) or even longer (Ge´nermont, 1996). The difference in 15N balance between the two treatments attained a maximum from the 6th day onwards, and remained steady until the end of the experiment. The high rate of volatilization was the result of the climatic conditions, high ammonium content of the slurry, and surface spreading.
4. Discussion The difference of 14.5% in nitrogen recovery shows that the grass sward significantly reduced volatilization. Similar results were obtained by Whitehead and Raistrick (1992) who used a direct method to quantify ammonia volatilization after spreading livestock urine; they observed a difference of 16% between cumulative volatilization over 8 days, on ryegrass, compared with bare soil. We observed, in agreement with these authors, that the effect of the canopy on volatilization was negligible during the first day, but substantial during the next 2–3 days. This effect could also be due to the rapid growth and great increase in nitrogen content during the 13 days following spreading. The low dry matter content of the slurry might also have reduced retention of the slurry on the canopy, despite the significant deposition of ammonium observed 24 h after the application. An
application of cattle slurry would probably have led to much higher gaseous losses from the canopy (Thompson et al., 1990), because of the higher dry matter content of cattle slurry. We also observed that volatilization halted after day 1 on the grass sward, despite the presence of significant amounts of ammoniacal nitrogen in the surface soil layer, between days 1 and 6. It is therefore almost certain that significant amounts of ammonia were given off at the soil surface in this treatment, the flux being slightly lower than that produced on bare soil, due to the lower temperatures of the soil surface (shade of sward) and slightly lower amounts of ammonium under grassland, on days 1 and 3. The total halting of net volatilization in the ryegrass treatment can only therefore be explained by the plant’s absorption of the ammonia given off at the soil surface. It is consistent with the findings of Denmead et al. (1976) that ammonium concentration measured under a grass-clover pasture was greatest near the ground surface and exponentially decreased with height above the ground. The sink effect of the grass sward towards ammonium would in our experimental conditions thus be due both to absorption by the roots and direct assimilation of ammonia by the leaves (Faurie and Bardin, 1979, Jarvis and Pain, 1990). This is also in good agreement with the results of Porter et al. (1972) and Hutchinson et al. (1972) who showed that plant leaves from different species can absorb significant amounts of ammonia from the air. Lockyer and Whitehead (1986) and Whitehead and Lockyer (1987), measuring the uptake of gaseous ammonia by the leaves of Italian ryegrass exposed in chambers to different contents of ammonia in the air, observed that the amount of ammonia absorbed increased linearly with ammonia air concentration. In our experiment, the very low initial inorganic content of the soil probably contributed to the stimulation of ammonia absorption by the canopy, but contents comparable to those that we observed are frequently measured under grassland, outside the periods of fertilizer application; the situation under study was therefore not exceptional for this criterion. We also observed that the rate of immobilization was lower beneath the grass sward (Fig. 5). The driving force of immobilization is the amount of decomposable carbon (Recous et al., 1990). In our case, the carbon that could be assimilated by microorganisms
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came (i) from the slurry which was supplied in equal quantities in both treatments, (ii) from dying leaves and roots and from root exudates in the case of ryegrass, and dead roots in the bare soil treatment, and (iii) from native organic matter. Mineral nitrogen is often a limiting factor of decomposition, which explains the very high stimulation of immobilization in the days immediately following an application, even without the addition of organic carbon; in this case, however, the measured levels of immobilization remain moderate, in the order of several mg N/kg soil, and are much greater following the addition of a carbon substrate (Recous et al., 1990). In this experiment, several factors could explain the differences between the two treatments: (i) the amounts of ammoniacal nitrogen measured at the soil surface during the first 6 days were not limiting for immobilization; the lower level of immobilization under ryegrass was therefore due to a lower availability of C-substrate, attributable in part to the deposition of organic matter from the slurry on the sward and the presence in bare soil of dead roots that had not yet been decomposed at the time of slurry application; (ii) the halting of immobilization of 15N labelled nitrogen after the 6th day in the ryegrass treatment (whereas this continued on the bare soil) coincided in contrast with the total disappearance of mineral nitrogen under this treatment, due to plant absorption. Immobilization over 27 days accounted for 25% of the 15N labelled nitrogen applied to the bare soil, and 16% in the ryegrass treatment. These rates of immobilization are relatively low, given that: (i) high levels of immobilization are expected under grassland (attaining 40–60% of 15N recovery, according to Jackson et al. (1989)) and that; (ii) soluble C was provided by the slurry. According to earlier findings (Ledgard et al., 1989; Guiraud et al., 1992), this low rate of immobilization might be due to low persistence of the ammonium pool, in our experimental conditions. Thus, this experiment shows that on a short time scale an actively growing grass sward can significantly modify partitioning of the ammonium pool following the application of a pig slurry. The amounts of 15 N-labelled NH4 absorbed by the plants, attaining at the end of the experiment 41% of nitrogen applied for the whole plant, and 29% for the aerial parts, should be compared with the 16.4% of inorganic nitrogen available on the bare soil.
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A study of the factors determining the infiltration and deposition of the slurry and the foliar assimilation of gaseous ammonia is required, as these factors govern the amounts of nitrogen volatilized, and thus have a considerable effect on nitrogen efficiency.
Acknowledgements We thank B. Blaise, Y. Fauvel for valuable assistance in the field and laboratory, R. Aubre´e for assistance in the field sampling, and O. Delfosse for 15N analysis. References Beauchamp, E.G., Kidd, G.E. and Thurtell, G., 1982. Ammonia volatilization from liquid dairy cattle manure in the field. Can. J. Soil Sci., 62: 11–19. Denmead, O.T., Freney, J.R. and Simpson, J.R., 1976. A closed ammonia cycle within a plant canopy. Soil Biol. Biochem., 8: 161–164. Drouineau, G. and Gouny, P., 1947. Contribution a` l’e´tude du dosage de l’azote nitrique par la me´thode Devarda. Ann. Agron., 17: 154–164. Faurie, G. and Bardin, R., 1979. La volatilisation de l’ammoniac. II. Influence des facteurs climatiques et du couvert v :e´ge´tal. Ann. Agron., 30: 401–414. Ge´nermont, S., 1996. Mode´lisation de la Volatilisation d’Ammoniac apre`s E´pandage de Lisier sur Parcelle Agricole. Thesis, University Paul Sabatier, Toulouse. Guiraud, G., Marol, C. and Fardeau, J.C., 1992. Balance and immobilization of (15NH4)2SO4 in a soil after the addition of Didin as a nitrification inhibitor. Biol. Fert. Soils, 14: 23–29. Hutchinson, G.L., Millington, R.J. and Peters, D.B., 1972. Atmospheric ammonia: absorption by plant leaves. Science, 175: 771– 772. Jackson, L.E., Schimel, J.P. and Firestone, M.K., 1989. Short-term partitioning of ammonium and nitrate between plants and microbes in an annual grassland. Soil Biol. Biochem., 21: 409–415. Jarvis, S.C. and Pain, B.F., 1990. Ammonia volatilisation from agricultural land. Proc. Fert. Soc., 298: 3–35. Lauer, D.A., Bouldin, D.R. and Klausner, S.D., 1976. Ammonia volatilization from dairy manure spread on the soil surface. J. Environ. Qual., 5: 134–141. Ledgard, S.F., Brier, G.J. and Sarathchandra, S.U., 1989. Plant uptake and microbial immobilization of 15N-labelled ammonium applied to grass-clover pasture – Influence of simulated winter temperature and time of application. Soil Biol. Biochem., 21: 667–670. Le Pham, M., Lambert, R. and Laudelout, H., 1984. Estimation de la valeur fertilisante azot :e´e du lisier par simulation nume´rique. Agronomie, 4: 63–74.
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Lockyer, D.R. and Whitehead, D.C., 1986. The uptake of gaseous ammonia by the leaves of Italian ryegrass. J. Exp. Bot., 37: 919– 927. Moal, J.F., 1995. Volatilisation de l’Azote Ammoniacal des Lisiers apre`s E´pandage: Quantification et E´tude des Facteurs d’Influence. Cemagref Dicova 229 pp. Morvan, T., Leterme, P. and Mary, B., 1996. Quantification par le marquage isotopique 15N des flux d’azote conse´cutifs a` un e´pandage d’automne de lisier de porc sur triticale. Agronomie, 16: 541–552. Pain, B.F., Phillips, V.R., Clarkson, C.R. and Klarenbeek, J.V., 1989. Loss of nitrogen through ammonia volatilization during and following the application of pig or cattle slurry to grassland. J. Sci. Food Agric., 47: 1–12. Porter, L.K., Viets, F.G. and Hutchinson, G.L., 1972. Air containing nitrogen-15 ammonia: foliar absorption by corn seedlings. Science, 175: 759–761. Recous, S., Fresneau, C., Faurie, G. and Mary, B., 1988. The fate of labelled 15N urea and ammonium nitrate applied to a winter wheat crop. Plant Soil, 112: 205–214. Recous, S., Mary, B. and Faurie, G., 1990. Microbial immobiliza-
tion of ammonium and nitrate in cultivated soils. Soil Biol. Biochem., 7: 913–922. SAS, 1988. SAS/STATy User’s Guide, Release 6.03 Edition. SAS Institute Inc. Cary, NC, 1028 pp. Sommer, S.G. and Olesen, J.E., 1991. Effects of dry matter content and temperature on ammonia loss from surface-applied cattle slurry. J Environ. Qual. 20: 679–683. Thompson, R.B., Pain, B.F. and Lockyer, D.R., 1990. Ammonia volatilization from cattle slurry following surface application to grassland. I. Influence of mechanical separation, changes in chemical composition during volatilization and the presence of the grass sward. Plant Soil, 125: 109–117. Whitehead, D.C. and Lockyer, D.R., 1987. The influence of the concentration of gaseous ammonia on its uptake by the leaves of Italian ryegrass, with and without an adequate supply of nitrogen to the roots. J. Exp. Bot., 38: 818–827. Whitehead, D.C. and Raistrick, N., 1992. Effects of plant material on ammonia volatilization from simulated livestock urine applied to soil. Biol. Fert. Soils, 13: 92–95.
Nitrogen transformations after the spreading of pig slurry on bare soil and ryegrass using 15N-labelled ammonium T. Morvan a,*, Ph. Leterme a, G.G. Arsene b, B. Mary c a
Unite´ INRA d’Agronomie, ENSAR, 65 rue de Saint-Brieuc, F35042 Rennes, France b USAMVB Timisoara, Colea Arodulini, 19, Timisoara 1900, Romania c Unite´ INRA d’Agronomie, rue Fernand Christ, BP 101, F02004 Laon Cedex, France Accepted 13 April 1997
Abstract A short field experiment (27 days) was carried out in summer 1995, to study the effect of an actively growing grass sward on nitrogen transformations of a pig slurry. The ammonium fraction of the slurry was labelled with (15NH4)2SO4. The slurry was spread manually on microplots in mid-June, at the rate of 3 l/m2, on a cut ryegrass sward, and compared with bare soil. Absorption of 15N-labelled NH4 by the grass occurred very rapidly, attaining 41% after 13 days and showing no further change at 27 days. The gaseous losses, mainly through volatilization of ammonia, were considerable. 15N recovery in soil and plant material on day 27 was 42.5% (±1.2) on the bare soil, versus 57.4% (±3.1) on the ryegrass. The grass sward significantly reduced: (i) volatilization, as shown by the difference of 14.9% in 15N recovery, on the 6th day; (ii) immobilization, which was 25% (±2.2) on day 27 on bare soil and 16.4% (±2.9) in the presence of ryegrass. 15N-labelled inorganic nitrogen was completely depleted beneath the ryegrass, 27 days after application, whereas ammonium was depleted and the nitrate was equal to 16.4% (±1.6) of the applied NH4 on the bare soil. It is clearly apparent that the ammonia from the slurry is more efficiently used when applied to an actively growing sward, rather than to bare soil, even though a significant portion of the plant is involved in internal recycling. 1997 Elsevier Science B.V. Keywords: Slurry; 15N; Grassland; Volatilization; Immobilization
1. Introduction Slurries, because of their high ammonium content, provide nitrogen which is quickly available for crops. The availability of the ammonium fraction is determined by gaseous loss and microbial immobilization. The first occurs mainly through the volatilization of
* Corresponding author. Tel.: +33 99 287231; fax: +33 99 287230; e-mail: [email protected]
ammonia, and is highly variable, (20–70% of total ammonium nitrogen (TAN) applied) (Lauer et al., 1976; Beauchamp et al., 1982; Pain et al., 1989; Ge´nermont, 1996), whereas microbial immobilization represents 15–35% of the TAN (Morvan et al., 1996). Slurry incorporation slightly reduces ammonia volatilization, but is not always possible, especially as it may lead to plant injury, for example after late winter applications on wheat or rapeseed. Furthermore, high nitrogen utilization efficiencies have been obtained for slurry ammonium
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after surface spreading of diluted pig slurries in actively growing wheat and were correlated with low levels of volatilization. A reduction in volatilization brought about by living plants has been reported and might be expected from: (i) absorption by plant leaves of ammonia volatilized from the underlying soil, as reported by Denmead et al., 1976; (ii) absorption of ammonium through the roots and (iii) microclimatic effects due to the canopy (Faurie and Bardin, 1979). Contradictory results have however been reported by other authors (Thompson et al., 1990), which could be explained by the greater surface area resulting from slurry retention on the leaves. What effect do plants have upon nitrogen immobilization? We might expect high rates of microbial immobilization under a grass sward, because of the large amounts of root exudate. Short-duration experiments, involving the measurement of actual rates of NH4 uptake by plants and microbes, have shown that microbial immobilization may be five times higher than plant uptake (Jackson et al., 1989). Ledgard et al. (1989), studying the partitioning of 15N-labelled ammonium applied to grass-clover pasture confirmed that microbial immobilization was a significant component of 15N balance, but also observed that more fertilizer was immobilized when plant growth was slow, due to lower temperatures. Immobilization is known to depend on the amount and persistence of ammonia in the soil. Thus, we may suppose that an actively growing plant able to rapidly absorb significant amounts of inorganic nitrogen, will reduce microbial immobilization. The few studies describing the effect of plants on nitrogen transformations after slurry addition sometimes give contradictory conclusions about volatilization, and rarely provide a complete description of soil-plant behaviour and competition, either because only one process was studied, or because the time scale of the experiment was too short, or too long. The aim of the present work was to obtain a better understanding of the effect of an actively growing plant on nitrogen transformations of a pig slurry ammonia pool, using labelled ammonium. A short field experiment was therefore carried out from midJune to mid-July, in order to ensure that climatic conditions were favourable to plant growth.
Table 1 Physical and chemical properties of the soil, and slurry composition Soil properties Particle size distribution (%) Clay Silt Sand Total N (%) pH KCl Bulk density of the soil layers 0–10 cm 10–20 cm Slurry composition N-NH4 (g/l) (after addition of ammonium sulfate) Total N (Kjeldhal) (g/l) pH Dry matter (%)
14.4 72.5 13.1 0.13 6.2 1.53 1.50 4.02 6.12 7.36 1.4
2. Materials and methods 2.1. Site and design The experiment was conducted at Le Rheu Experimental Station (INRA), in western France, on a loamy soil. Some of the chemical and physical properties of this soil are summarized in Table 1. The fate of the ammonia fraction of a pig slurry was studied using 15 N-labelled NH4. Two treatments were compared: surface spreading on a ryegrass sward, and on bare soil. Daily temperatures were relatively high, varying from 15°C to 25°C (the mean air temperature was 20.3°C over the 27 days), and were favourable both to plant growth and to ammonia volatilization and nitrogen biotransformations, such as nitrification, mineralization and immobilization. The amount of rainfall and change in soil moisture in the soil surface layer are shown in Fig. 1; 2 × 10 mm were applied by irrigation, the day before the start of the experiment, and on day 3, to prevent the soil from drying out. The soil moisture therefore varied from 60 to 100% of the field capacity over the first 13 days and was favourable for microbial activity and plant growth. No significant differences were noticed between the two treatments during this period. The plots were 4.70 m × 2.30 m and laid out in a
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nium at the rate of 188 g/ha), followed by removal of the aerial residues, just before spreading. The characteristics of the soil surface (structure, bulk density) and soil moisture, controlling initial infiltration of the slurry, and consequently the intensity of ammonia volatilization, were therefore similar in each treatment. Chemical weed control probably had no effect on the biological activity of the soil as the kinetics and level of immobilization of 15N-labelled nitrogen were similar to those observed after a comparable slurry application to a soil that was not subjected to chemical weed control (unpublished data). Fig. 1. Rainfall plus irrigation, and change in soil moisture on the bare soil (BS) and under ryegrass (R).
2.2. Soil and plant sampling
randomized block design with three replications. Each plot was divided into five microplots of 0.60 m2, corresponding to the sampling area for a given date of measurement. Microplots were separated from each other by a 30-cm discard area and from the boundaries by 50-cm guard strips. The ammonium fraction of a pig slurry, (composition in Table 1), was enriched with 15N using a solution of (15NH4)2SO4 10% atom excess, which was added and thoroughly mixed to the slurry. The initial atom excess of the ammonium pool (1.089%) was determined just after spreading, by sampling the soil surface. The enriched slurry was applied at the rate of 3 l/m2 on June 20th 1995, between 1500 h and 1645 h, in warm, sunny conditions. The amounts of ammonium and organic nitrogen applied were equal to 121 and 63 kg N/ha respectively. A watering can with a distribution bar was used to obtain as even a distribution as possible, and the plots were divided into microplots each receiving the same quantity of slurry. No run-off was observed after spreading, despite the high soil compaction (bulk density of the 0–10 cm soil layer: 1.53 g/cm3). The slurry was applied: (i) to a 2-year-old cut ryegrass (Lolium perenne) sward, which had not been fertilized with nitrogen since the date of sowing. The nitrogen content in plant material harvested at a height of 0.5 cm was therefore very low (1,12%) at the beginning of the trial. The grass sward was cut one week before the experiment, and was 10–12 cm high on the day of application; (ii) to bare soil, obtained by chemical destruction of the grass (glifosate ammo-
The microplots were sampled 1, 3, 6, 13 and 27 days after slurry spreading. The aerial parts were cut just above the soil, on a square area of 0.25 m2. The plant material was washed free of soil and slurry, the volume of washing water was measured, and the plants and water sampled for inorganic nitrogen and 15 N analysis. Soil samples were taken from the 0–10 cm and 10– 20 cm depths, to determine root biomass, organic and inorganic soil nitrogen. A 60 mm diameter probe was used for root sampling, and a 20 mm diameter probe for soil sampling. For soil nitrogen analysis, samples were obtained from each microplot by mixing 27 cores from each soil layer, and passing the whole sample through an 8-mm mesh sieve. Soil samples for roots were obtained by mixing six cores taken with the 60-mm diameter probe. The roots were separated from the soil by washing on a 2-mm mesh sieve; herbage residues were removed by hand. Above-ground parts and roots were dried at 60°C, finely ground to powder and put into tin containers for total nitrogen content determinations and 15N atom excess analysis. 2.3. Analytical procedures Inorganic nitrogen in the soil was determined in a KCl extract (600 ml 1 M KCl/300 g fresh soil, shaken for 30 min, then filtered through a Whatmann 42 filter), using the fractionated steam distillation with MgO for ammonium and Dewarda’s alloy for nitrate analysis (Drouineau and Gouny, 1947). Organic plus clay-fixed nitrogen and 15N was measured on a sample
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of moist soil, after removal of the inorganic nitrogen, as described by Recous et al., 1988; the sample was dried at 60°C, and finely ground. Atom excess was determined on subsamples of plant, soil, and dried solutions resulting from the steam distillations, using a total combustion technique linked to a VG SIRA 9 mass spectrometer (Recous et al., 1988).
3. Results 3.1. Inorganic nitrogen dynamics Inorganic nitrogen occurred mainly in the surface soil layer (0–10 cm). The amounts of 15NH4-labelled ammonium observed in the 10–20 cm soil layer did not exceed 0.5% of the applied ammonium. Small amounts of 15N-labelled nitrate, not exceeding 1.3% of the applied nitrogen, were measured in the lower layer of the bare soil, on days 13 and 27. The fate of the ammonium nitrogen is shown in Fig. 2: the curves represent the change in the amount of ammonium in the 0–20 cm soil layer, and ammonia deposition on the leaves in the ryegrass treatment. The amount of ammonium deposited on the plant leaves was relatively large on day 1: this suggests that the initial deposition was probably considerable, if comparable with the great decrease of the ammonium pool in the soil observed during the first day. Ammonium amounts decreased sharply during the
Fig. 3. The proportions of the 15N-labelled NH4 present as 15Nlabelled NO3 in the 0–20 cm soil layer. (Vertical bars indicate the standard deviation of the three replicates; SD not shown are smaller than symbol size.)
first 6 days. The kinetics of 15N-labelled NH4 were similar in each treatment; the plant tended to stimulate the depletion of ammonium, particularly between days 6 and 13. The change in 15N-labelled NO3 in the soil is shown in Fig. 3. Some nitrification of the slurry ammonium had occurred by the end of the experiment, because the ammonium pool had been depleted by that time, in both treatments. 15N-labelled NO3 content steadily increased until the end of the experiment in the bare soil, attaining only 16% of the nitrogen applied, but it was completely depleted beneath the sward. We did not observe a true latent period at the beginning of the trial as is often reported (Le Pham et al., 1984). 3.2. Dry matter and nitrogen absorption dynamics
Fig. 2. The proportions of 15N-labelled NH4 in the 0–20 cm soil layer, and deposition on ryegrass leaves, after application of 121 kg N-NH4/ha from a pig slurry. (Vertical bars indicate the standard deviation of the three replicates; SD not shown are smaller than symbol size.)
The pattern of 15N absorption by the whole plant is shown in Fig. 4; nitrogen absorption occurred rapidly: the nitrogen utilization efficiency had already reached 11.5% the day after spreading and had attained 41% by day 13 and remained stable till day 27. Because the soil moisture, temperature and soil ammonia content were similar in both treatments (Figs. 1 and 2) during the first 6 days, it can be assumed that nitrification occurred at the same rate in both treatments, during this period. It then follows from the comparison of 15N-labelled nitrate dynamics and 15N inorganic nitrogen absorption that absorption was mainly due to the uptake of ammonium or ammonia. The two routes of assimilation, through the roots and leaves, were probably efficient (see Section 4),
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iod corresponding to the amount of nitrogen taken up by the grass. 3.4. 15N balance
Fig. 4. The proportions of the 15N-labelled NH4 present in the above-ground parts and roots following the pig slurry application. (Vertical bars indicate the standard deviation of the three replicates; SD not shown are smaller than symbol size.)
and equal quantities can be found in aerial plant parts and roots on days 1 and 3. Plant dry matter and nitrogen content increased simultaneously during the experiment. The aboveground parts of the ryegrass rose from 4.1 to 6.8 t dry matter/ha, representing a mean growth rate of 0.1 t dry matter/ha per day. The nitrogen content of the aerial parts rose from 1.12 to 1.74%. The proportion of 15N-labelled nitrogen measured in the whole plant was 15.8% of the amount of 15N-labelled NH4 applied, on day 1, and reached a maximum value of 33.4% on day 13. 3.3. Immobilization The pattern of immobilization is shown in Fig. 5. In both treatments, 15N immobilization rose sharply during the first 3 days; on bare soil, it exceeded the value in the ryegrass with 6% as early as the first day, and continued to increase, whereas it remained steady in the ryegrass sward. The difference observed on day 1 might be related to the significant deposition of ammonia and organic carbon from the slurry on the leaves (ammonia deposition representing 7% of the total applied ammonium, on day 1) (Fig. 2), which thus reduced the amount of ammonium and organic carbon available in the soil. On the other hand, the increased margin between days 6 and 13 can be explained by the competitive effect of the plants which absorbed the inorganic 15N-labelled nitrogen, the depletion of the ammonium pool during this per-
Fig. 6 shows the change in non-recovery of the 15N, corresponding to gaseous losses. Denitrification was probably low during this experiment, in view of the climatic conditions and soil moisture. Ammonia volatilization probably accounts for most of the gaseous losses, all the more so as the kinetics of the 15N nonrecovery exhibit a typical pattern of volatilization, as described by Sommer and Olesen (1991) and by Jarvis and Pain (1990). In fact, cumulative volatilization can generally be described by an asymptotic exponential curve, and is well fitted to the following equation: v = vmax (1-e-kt) (Moal, 1995). The optimal values of the parameters of this equation were calculated for each treatment, using the Nlin procedure in the SAS package (SAS, 1988). The losses were the same on day 1, in both treatments, attaining 40% of the applied nitrogen. This suggests that there was no significant microclimatic effect of the canopy, or that differences compensated. Volatilization was significant on the bare soil between days 1 and 3 (+19% according to the adjusted curve), but almost stopped after day 1 on the ryegrass treatment (+2% according to the adjusted curve). This halting of volatilization in the grass sward was unexpected, as (i) the amounts of ammoniacal nitrogen measured in the surface layer were still high on days 1, 3 and 6, and were only a few kg N/ha less than the amounts measured on bare soil, (ii) volatilization after
Fig. 5. Proportions of 15N-labelled NH4 immobilized in the two treatments. (Vertical bars indicate the standard deviation of the three replicates.)
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Fig. 6. Proportions of 15N-labelled NH4 not recovered, corresponding to gaseous losses, during the days following pig slurry application. Values were adjusted using the equation: y = a (1- e-kt) (Vertical bars indicate the standard deviation of the three replicates.)
slurry spreading generally takes place over a period of 6 days (Jarvis and Pain, 1990; Moal, 1995) or even longer (Ge´nermont, 1996). The difference in 15N balance between the two treatments attained a maximum from the 6th day onwards, and remained steady until the end of the experiment. The high rate of volatilization was the result of the climatic conditions, high ammonium content of the slurry, and surface spreading.
4. Discussion The difference of 14.5% in nitrogen recovery shows that the grass sward significantly reduced volatilization. Similar results were obtained by Whitehead and Raistrick (1992) who used a direct method to quantify ammonia volatilization after spreading livestock urine; they observed a difference of 16% between cumulative volatilization over 8 days, on ryegrass, compared with bare soil. We observed, in agreement with these authors, that the effect of the canopy on volatilization was negligible during the first day, but substantial during the next 2–3 days. This effect could also be due to the rapid growth and great increase in nitrogen content during the 13 days following spreading. The low dry matter content of the slurry might also have reduced retention of the slurry on the canopy, despite the significant deposition of ammonium observed 24 h after the application. An
application of cattle slurry would probably have led to much higher gaseous losses from the canopy (Thompson et al., 1990), because of the higher dry matter content of cattle slurry. We also observed that volatilization halted after day 1 on the grass sward, despite the presence of significant amounts of ammoniacal nitrogen in the surface soil layer, between days 1 and 6. It is therefore almost certain that significant amounts of ammonia were given off at the soil surface in this treatment, the flux being slightly lower than that produced on bare soil, due to the lower temperatures of the soil surface (shade of sward) and slightly lower amounts of ammonium under grassland, on days 1 and 3. The total halting of net volatilization in the ryegrass treatment can only therefore be explained by the plant’s absorption of the ammonia given off at the soil surface. It is consistent with the findings of Denmead et al. (1976) that ammonium concentration measured under a grass-clover pasture was greatest near the ground surface and exponentially decreased with height above the ground. The sink effect of the grass sward towards ammonium would in our experimental conditions thus be due both to absorption by the roots and direct assimilation of ammonia by the leaves (Faurie and Bardin, 1979, Jarvis and Pain, 1990). This is also in good agreement with the results of Porter et al. (1972) and Hutchinson et al. (1972) who showed that plant leaves from different species can absorb significant amounts of ammonia from the air. Lockyer and Whitehead (1986) and Whitehead and Lockyer (1987), measuring the uptake of gaseous ammonia by the leaves of Italian ryegrass exposed in chambers to different contents of ammonia in the air, observed that the amount of ammonia absorbed increased linearly with ammonia air concentration. In our experiment, the very low initial inorganic content of the soil probably contributed to the stimulation of ammonia absorption by the canopy, but contents comparable to those that we observed are frequently measured under grassland, outside the periods of fertilizer application; the situation under study was therefore not exceptional for this criterion. We also observed that the rate of immobilization was lower beneath the grass sward (Fig. 5). The driving force of immobilization is the amount of decomposable carbon (Recous et al., 1990). In our case, the carbon that could be assimilated by microorganisms
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came (i) from the slurry which was supplied in equal quantities in both treatments, (ii) from dying leaves and roots and from root exudates in the case of ryegrass, and dead roots in the bare soil treatment, and (iii) from native organic matter. Mineral nitrogen is often a limiting factor of decomposition, which explains the very high stimulation of immobilization in the days immediately following an application, even without the addition of organic carbon; in this case, however, the measured levels of immobilization remain moderate, in the order of several mg N/kg soil, and are much greater following the addition of a carbon substrate (Recous et al., 1990). In this experiment, several factors could explain the differences between the two treatments: (i) the amounts of ammoniacal nitrogen measured at the soil surface during the first 6 days were not limiting for immobilization; the lower level of immobilization under ryegrass was therefore due to a lower availability of C-substrate, attributable in part to the deposition of organic matter from the slurry on the sward and the presence in bare soil of dead roots that had not yet been decomposed at the time of slurry application; (ii) the halting of immobilization of 15N labelled nitrogen after the 6th day in the ryegrass treatment (whereas this continued on the bare soil) coincided in contrast with the total disappearance of mineral nitrogen under this treatment, due to plant absorption. Immobilization over 27 days accounted for 25% of the 15N labelled nitrogen applied to the bare soil, and 16% in the ryegrass treatment. These rates of immobilization are relatively low, given that: (i) high levels of immobilization are expected under grassland (attaining 40–60% of 15N recovery, according to Jackson et al. (1989)) and that; (ii) soluble C was provided by the slurry. According to earlier findings (Ledgard et al., 1989; Guiraud et al., 1992), this low rate of immobilization might be due to low persistence of the ammonium pool, in our experimental conditions. Thus, this experiment shows that on a short time scale an actively growing grass sward can significantly modify partitioning of the ammonium pool following the application of a pig slurry. The amounts of 15 N-labelled NH4 absorbed by the plants, attaining at the end of the experiment 41% of nitrogen applied for the whole plant, and 29% for the aerial parts, should be compared with the 16.4% of inorganic nitrogen available on the bare soil.
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A study of the factors determining the infiltration and deposition of the slurry and the foliar assimilation of gaseous ammonia is required, as these factors govern the amounts of nitrogen volatilized, and thus have a considerable effect on nitrogen efficiency.
Acknowledgements We thank B. Blaise, Y. Fauvel for valuable assistance in the field and laboratory, R. Aubre´e for assistance in the field sampling, and O. Delfosse for 15N analysis. References Beauchamp, E.G., Kidd, G.E. and Thurtell, G., 1982. Ammonia volatilization from liquid dairy cattle manure in the field. Can. J. Soil Sci., 62: 11–19. Denmead, O.T., Freney, J.R. and Simpson, J.R., 1976. A closed ammonia cycle within a plant canopy. Soil Biol. Biochem., 8: 161–164. Drouineau, G. and Gouny, P., 1947. Contribution a` l’e´tude du dosage de l’azote nitrique par la me´thode Devarda. Ann. Agron., 17: 154–164. Faurie, G. and Bardin, R., 1979. La volatilisation de l’ammoniac. II. Influence des facteurs climatiques et du couvert v :e´ge´tal. Ann. Agron., 30: 401–414. Ge´nermont, S., 1996. Mode´lisation de la Volatilisation d’Ammoniac apre`s E´pandage de Lisier sur Parcelle Agricole. Thesis, University Paul Sabatier, Toulouse. Guiraud, G., Marol, C. and Fardeau, J.C., 1992. Balance and immobilization of (15NH4)2SO4 in a soil after the addition of Didin as a nitrification inhibitor. Biol. Fert. Soils, 14: 23–29. Hutchinson, G.L., Millington, R.J. and Peters, D.B., 1972. Atmospheric ammonia: absorption by plant leaves. Science, 175: 771– 772. Jackson, L.E., Schimel, J.P. and Firestone, M.K., 1989. Short-term partitioning of ammonium and nitrate between plants and microbes in an annual grassland. Soil Biol. Biochem., 21: 409–415. Jarvis, S.C. and Pain, B.F., 1990. Ammonia volatilisation from agricultural land. Proc. Fert. Soc., 298: 3–35. Lauer, D.A., Bouldin, D.R. and Klausner, S.D., 1976. Ammonia volatilization from dairy manure spread on the soil surface. J. Environ. Qual., 5: 134–141. Ledgard, S.F., Brier, G.J. and Sarathchandra, S.U., 1989. Plant uptake and microbial immobilization of 15N-labelled ammonium applied to grass-clover pasture – Influence of simulated winter temperature and time of application. Soil Biol. Biochem., 21: 667–670. Le Pham, M., Lambert, R. and Laudelout, H., 1984. Estimation de la valeur fertilisante azot :e´e du lisier par simulation nume´rique. Agronomie, 4: 63–74.
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