Relationship between P and N concentrations in timothy

Transcription

1 Relationship between P and N concentrations in timothy G. Bélanger 1 and J. E. Richards 2 1 Soils and Crops Research and Development Research Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Boulevard, Sainte-Foy, Québec, Canada G1V 2J3; 2 Atlantic Cool Climate Crops Research Centre, Agriculture and Agri-Food Canada, 308 Brookfield Road, Box 39088, St. John s, Newfoundland, Canada A1E 5Y7. Received 22 October 1997, accepted 20 August Bélanger, G. and Richards, J. E Relationship between P and N concentrations in timothy. Can. J. Plant Sci. 79: Tools quantifying the status of N and P in plants may help to achieve efficient management of these nutrients and to optimize crop growth and yield. The objective of this study was to establish the relationship between P and N concentrations during the regrowth of timothy (Phleum pratense L.) and, in particular, to estimate the critical P concentration required to diagnose P deficiency. The relationship between P and N concentrations was determined for timothy grown in two experiments conducted with early- and latematuring cultivars under non-limiting N conditions in spring of 1991 and 1992, and in two experiments with four rates of N fertilization conducted in the spring of 1993 and the summer of Shoot biomass and P and N concentrations were determined weekly during each regrowth cycle. The P and N concentrations decreased with time in all four experiments. The decrease in P concentration with increasing shoot biomass was generally similar to the decrease in N concentration. The relationship between P concentration and shoot biomass was not different for early- and late-maturing timothy cultivars. This relationship, however, was affected by N fertilization. For a given shoot biomass, increasing N fertilization rates increased P concentration. The relationship between P and N concentrations under non-limiting N conditions is described by a linear relationship (P = N, R 2 = 0.79, P < 0.001, n = 48) in which P concentration (P) and N concentration (N) are expressed in g kg 1 DM. The relationship between P and N concentrations was different under N limiting conditions. For a given N concentration, the P concentration was greater under limiting N conditions than under non-limiting N conditions. Our results show that the critical P concentration for shoot growth is a function of the N concentration in the shoot biomass and the level of N deficiency. The present study provides the relationship required to estimate the critical P concentration which is essential for quantifying levels of P deficiency in timothy, and in developing models to predict the quantity of fertilizer P needed to correct that deficiency. Key words: Phleum pratense L., timothy, nitrogen, phosphorus, grasses Bélanger, G. et Richards, J. E Relation entre les teneurs en P et N de la fléole des prés. Can. J. Plant Sci. 79: Le développement d outils permettant de quantifier le statut de nutrition en N et P dans les plantes est une étape essentielle pour atteindre une gestion efficace de ces éléments nutritifs, et pour optimiser la croissance et le rendement. L objectif de cette étude était d établir la relation entre les teneurs en P et N au cours de repousses de fléole des prés (Phleum pratense L.), et plus particulièrement, de déterminer la teneur critique en P laquelle pourrait être utilisée pour diagnostiquer les déficiences en P. Cette relation entre les teneurs en P et N a été développée à partir de deux expériences avec des cultivars hâtifs et tardifs de fléole des prés réalisées sous des conditions non-limitantes en N aux printemps de 1991 et 1992, et de deux expériences avec quatre niveaux de fertilisation azotée réalisées au printemps 1993 et à l été La biomasse des parties aériennes, et les teneurs en P et N ont été déterminées par des échantillonnages hebdomadaires au cours de chaque repousse. Les teneurs en P et N ont diminué avec le temps dans les quatre expériences. La diminution de la teneur en P avec une augmentation de la biomasse des parties aériennes était généralement similaire à la diminution de la teneur en N. La relation entre la teneur en P et la biomasse des parties aériennes n était pas différente pour les cultivars hâtifs et tardifs. Cette relation, cependant, était affectée par la fertilisation azotée. Pour un niveau donné de biomasse aérienne, l augmentation des niveaux de fertilisation azotée a augmenté la concentration en P. La relation entre les teneurs en P et N sous des conditions non-limitantes en N est décrite par une relation linéaire (P = N, R 2 = 0.79, P < 0.001, n = 48) dans laquelle les teneurs en P (P) et N (N) sont exprimées en g kg 1 DM. La relation entre les teneurs en P et N était différente sous des conditions limitantes en azote. Pour une teneur donnée en N, la teneur en P était plus grande sous des conditions limitantes en azote que sous des conditions non-limitantes. Nos résultats montrent que la teneur critique en P pour la croissance des parties aériennes est fonction de la teneur en N des parties aériennes et du niveau de déficience azotée. Cette étude fournit la relation requise pour estimer la teneur critique en P laquelle est essentielle pour quantifier les niveaux de déficience en P de la fléole des prés, et pour développer des modèles d estimation de la quantité de phosphore nécessaire pour corriger cette déficience. Mots clés: Phleum pratense L., fléole des prés, azote, phophore, graminées The development of tools to diagnose N and P crop status is an essential step in achieving efficient management of these nutrients. Recent studies have shown that the diagnosis of N and P deficiencies in grasses can be based on the relationship between the concentration of N and P in plant tissue, and shoot biomass (Salette and Huché 1991; Duru et al ). Salette and Lemaire (1981) demonstrated that for grasses growing under a non-limiting N supply, the N concentration of the sward was negatively related to DM accumulation. The critical N concentration (N c ), the minimum N concentration required to achieve maximum shoot growth, was defined as a function of shoot biomass by Lemaire and

2 66 CANADIAN JOURNAL OF PLANT SCIENCE Salette (1984) for tall fescue (N c = 48 DM 0.33 ). In our paper on the growth analysis of timothy (Phleum pratense L.) grown with varying N nutrition (Bélanger and Richards 1997), we successfully validated the approach first proposed by Salette and Lemaire (1981) on tall fescue (Festuca arundinacea Schreb.). The possibility of using this relationship for diagnostic purposes of N deficiency is thoroughly discussed by Lemaire and Meynard (1997). Salette and Huché (1991) extended this approach to other nutrients such as P and K. Because of the similarities in the decrease in N and P concentrations with increasing shoot biomass, it was proposed to use the P:N ratio for diagnostic purposes (Salette and Huché 1991; Duru 1992; Duru and Ducrocq 1997). The use of P:N ratio to detect the nature of nutrient limitations was also proposed by Koerselman and Meuleman (1996) on natural ecosystems in the Netherlands and by Sinclair et al. (1997) on cut white clover/ryegrass swards in New Zealand. Our objective was to establish the relationship between P and N concentrations of timothy using data from experiments with various N rates, and with timothy cultivars differing in maturity grown under non-limiting N conditions. More specifically, we wanted to determine the critical P concentration for shoot growth which could be used to diagnose P deficiency in timothy. MATERIALS AND METHODS The relationship between P and N concentrations in timothy (Phleum pratense L.) was studied using data obtained from four experiments conducted on a Fredericton soil (Orthic Dystric Brunisol) (Rees and Fahmy 1984) at the Fredericton Research Centre of Agriculture and Agri-Food Canada (Lat N; long ). Two of the experiments were conducted in 1991 and 1992 on the primary growth of timothy cultivars differing in maturity and established in 1988 and 1990, respectively. Four cultivars were grown under nonlimiting N conditions in a randomized complete block design with four replications and a plot size of 8 m 1.25 m (Bélanger and Richards 1995). The early-maturing cultivars were Champ, Nike, and G in 1991 and Clair and Axel in The late-maturing cultivars were Farol in 1991 and Farol and Hokushu in The delay in heading between the early and late-maturing cultivars was 14 d (Bélanger and Richards 1995). In the other two experiments, four rates of fertilizer N, as ammonium nitrate, were surface-broadcast on 1-yr-old stands of timothy (cv. Champ) in the spring of 1993 (primary growth) and in the summer of 1994 (secondary growth) in a randomized complete block design with five replications (Bélanger and Richards 1997). The plot size was 9 m 1.5 m. At the time of seeding, soil P extraction with 0.03 M NH 4 F in M HCl (Bray 2) indicated that the soil (0 15 cm) contained 188 kg P ha 1 (spring 1991), 224 kg P ha 1 (spring 1992), and 428 kg P ha 1 (spring 1993; summer 1994). Furthermore, 50 kg P ha 1, as superphosphate, was surface-broadcast before growth started in all three experiments conducted in spring and after defoliation for the experiment conducted in summer In the two experiments with cultivars differing in maturity, 168 and 200 kg N ha 1, as ammonium nitrate, were surface-broadcast in 1991 and 1992, respectively. The experiments and the sampling methodology are described in detail by Bélanger and Richards (1995, 1997). Cultivars, N rates and sampling dates for each experiment are presented in Tables 1 and 2. Briefly, shoot biomass was sampled weekly for 5 wk in spring and 4 wk in summer during each regrowth on a 1.25 m 0.9 m area using a plot harvester at a cutting height of 5 cm. A subsample of approximately 500 g was dried at 55 C in a forced-draft oven for 3 d, ground to pass a 1-mm screen in a Wiley mill, and stored at room temperature in translucent plastic jars. The P and N concentrations in plant tissue were determined by the H 2 SO 4 -H 2 O 2 digestion method described by Richards (1993). The N and P concentrations were subjected to analyses of variance and standard errors of the means (SEM) were calculated (Genstat 5 Committee 1993). A linear parallel curve analysis with grouped data was used to determine if the response curves of P concentration to shoot biomass and N concentration differed among cultivars in spring of 1991 and For the analysis of the relationship between P concentration and shoot biomass, a logarithmic transformation was used. Using the FIT routine of Genstat 5 (Genstat 5 Committee 1993), the response curves were described by the following model: Y = k + lx where Y is the response variable, X is the explanatory variable, and l and k are estimated parameters. The procedure initially calculated one equation (for the four cultivars) to describe the average response to the explanatory variable. On the next step, separate k parameters were estimated for each cultivar to determine the vertical separation between parallel lines (i.e. response curves). The next step estimated separate linear parameters (l) for the slope (i.e. the linear portion of the cultivars by the explanatory variable interaction). At each step, the statistical significance was calculated for the change in the sum of squares explained by the addition of another parameter to the model. RESULTS AND DISCUSSION Decrease in P and N Concentrations with Time Phosphorus and N concentrations in the shoot biomass decreased with time in all four experiments (Tables 1 and 2). The decrease in P and N concentrations with time or advancing maturity is well documented for many grass species including timothy (Gervais and St-Pierre 1979; Jeffrey 1988; Duru 1992; Whitehead 1995). Increasing N fertilization rates had a positive effect on P concentration, but this effect decreased with time (Table 2). On the last sampling dates, which correspond to the usual harvest dates of timothy, the difference in P concentration caused by the N fertilization rates was relatively small (Table 2). MacLeod (1965) and Grant and MacLean (1966) also reported a limited effect of N fertilization on P concentration in timothy.

3 BÉLANGER AND RICHARDS P AND N CONCENTRATIONS IN TIMOTHY 67 Table 1. Plant P and N concentrations of early- and late-maturing timothy cultivars on different sampling dates in spring of 1991 and P (g kg 1 DM) N (g kg 1 DM) Sampling dates Champ (E) z Nike (E) G (E) Farol (L) SEM Champ (E) Nike (E) G (E) Farol (L) SEM 21 May May June June June P (g kg 1 DM) N (g kg 1 DM) Sampling dates Claire (E) Axel (E) Hokushu (L) Farol (L) SEM Clair (E) Axel (E) Hokushu (L) Farol (L) SEM 26 May June June June June z E: early; L: late. Table 2. Plant P and N concentrations of timothy cv. Champ fertilized with various rates of N in spring (1993) and summer (1994) Spring 1993 Applied N (kg N ha 1 ) 0 z Sampling dates P (g kg 1 DM) SEM N (g kg 1 DM) SEM 25 May June June June June Summer 1994 Applied N (kg N ha 1 ) Sampling dates P (g kg 1 DM) SEM N (g kg 1 DM) SEM 26 July August August August z N rates in kg N ha 1. Decrease in P Concentration with Shoot Biomass The decrease in P concentration with increasing shoot biomass (Figs. 1 and 2) was generally similar to the decrease in N concentration reported previously (Bélanger and Richards 1997). There were no significant differences among cultivars in the relationship between P concentration and shoot biomass (Fig. 1, Table 3). For a given shoot biomass, P concentration increased with increasing N fertilization rates (Fig. 2). Similar results were reported by Duru (1992) on permanent pastures and Salette (1990) on pure swards of perennial ryegrass (Lolium perenne L.) and tall fescue. Our results show that, as for N, there is dilution of P in increasing shoot biomass. Since shoot biomass is much greater under non-limiting N conditions than with limiting N conditions, the greater extent of P dilution under nonlimiting N conditions masks the direct positive effect of N fertilization on P concentration. This may explain why previous studies reported a limited effect of N fertilization on P concentration of timothy (MacLeod 1965; Grant and MacLean 1966). Relationship between P and N Concentrations NON-LIMITING N CONDITIONS. In our study, the N concentration in 1991 and 1992 of all cultivars at all sampling dates was near the critical N concentration predicted by the model of Lemaire and Salette (1984). Hence, we assume N was not limiting shoot growth in the spring of 1991 and In a previous paper, we concluded that maximum shoot growth was reached with fertilization rates of 140 kg N ha 1 in the spring of 1993 and 120 kg N ha 1 in the summer of 1994 (Bélanger and Richards 1997). There were no significant differences in the relationship between P and N concentrations of early and late-maturing timothy cultivars (Table 3). Hence, the data from the exper-

4 68 CANADIAN JOURNAL OF PLANT SCIENCE Fig. 1. Phosphorus concentration as a function of shoot biomass of timothy cultivars differing in maturity (E, early; L, late) in the spring of 1991 and iments conducted in spring of 1991 and 1992 under nonlimiting N conditions were pooled with data obtained under non-limiting N conditions in the spring of 1993 (140 kg N ha 1 ) and in the summer of 1994 (120 kg N ha 1 ) (Fig. 3). The relationship between P and N concentrations can be described by the following linear relationship: P = N (R 2 = 0.79; P < 0.001; n = 48) (1) in which P concentration (P) and N concentration (N) are expressed in g kg 1 DM. This relationship represents an approximation of the critical P concentration under non-limiting N conditions i.e. the minimal P concentration required to achieve maximum shoot growth. It is based on the assumption that soil P availability was not limiting shoot growth. Duru and Ducrocq (1997) in France reported a similar linear relationship between P and N concentrations on permanent grasslands (P = N with P and N in g kg 1 DM). It is perhaps Fig. 2. Phosphorus concentration as a function of shoot biomass of timothy cv. Champ fertilized with various N rates in the spring of 1993 and the summer of Solid lines, data from one rate; dashed lines, data from one sampling date. significant that the intercept and the slope of the relationship between P and N concentrations observed in our study on timothy are very similar to those reported by Duru and Ducrocq (1997) on permanent grasslands (Fig. 3). Further research is required to validate these models under a wider range of environmental conditions and species. N LIMITING CONDITIONS. The relationship between P and N concentrations was different under non-limiting than under limiting N conditions. For a given N concentration, the P concentration was greater under limiting N conditions than under non-limiting N conditions (Fig. 4). Duru (1992) made a similar observation. Salette (1990), however, reported that the relationship between P and N concentrations took into account the effect of both the N deficiency and the decrease in N concentration by dilution in shoot biomass. In our data with timothy, the relationship between P and N concentration did not account for all of the effect of the N deficiency as suggested by Salette (1990). The relationship between P

5 BÉLANGER AND RICHARDS P AND N CONCENTRATIONS IN TIMOTHY 69 Table 3. Linear parallel curve analysis of cultivars (C) for P concentrations as a function of shoot biomass (SB) and N concentration in spring of 1991 and 1992 Mean square values Estimated parameters d.f Log P vs. log SB z SB *** y *** C NS NS SB C linear interaction NS NS Residual R P vs. N N *** 7.977*** C NS 0.027NS N C linear interaction NS 0.037NS Residual R z A logarithmic transformation of shoot biomass and P concentration was used. y Statistical significance of the additional mean square contributed to the model y = k + lx as individual parameters k and l for each cultivar estimated in steps. ***Significant at P < 0.001; NS, not significant. Fig. 3. Phosphorus concentration (P) as a function of N concentration (N) of timothy in four experiments under non-limiting N conditions. The solid line represents the linear regression [P = N, R 2 = 0.79, P < 0.001, n = 48]; the dashed line represents the model of Duru and Ducrocq (1977). and N concentrations obtained under non-limiting N conditions should therefore be modified to take into account the N deficiency. However, we have insufficient data to relate adequately the parameters of the relationship and the level of N deficiency. The indices of N nutrition defined as the ratio between measured N concentration in the shoot biomass and the critical N concentration were 0.36 in the spring of 1993 and 0.39 in the summer of 1994 with no applied N (Bélanger and Richards 1997). With 70 and 60 kg N ha 1, the indices of N nutrition were 0.74 in the spring of 1993 and 0.69 in the summer of Even though the levels of N deficiency Fig. 4. Phosphorus concentration as a function of N concentration of timothy cv. Champ fertilized with various N rates in the spring of 1993 and the summer of The solid line represents the linear regression (P = N) for non-limiting N conditions as presented in Fig. 3. were different with no N applied and the second N rate (70 and 60 kg N ha 1 ), the relationships between P and N concentrations were similar (Fig. 4). Salette and Huché (1991) concluded that the absorption and translocation of P is adjusted to the growth rate corresponding to the absorption and utilization of N by the crop. Kamprath (1987) also reported that N supply was the main factor affecting the P content of corn on soils with adequate levels of P. He also reported that leaf P concentration at silking was increased by N fertilization and was highly correlated with leaf N concentration. Our results are to a large extent in agreement with the conclusions of Salette and Huché (1991) and Kamprath (1987). Implications for P and N Diagnostic in Timothy In diagnostic methods based on plant analyses, N and P concentrations in shoot biomass are used to determine the pres-

6 70 CANADIAN JOURNAL OF PLANT SCIENCE ence of a deficiency. Our results show that P and N concentrations decrease during growth, and that critical P concentration is a function of the N concentration and the level of N deficiency. The level of N deficiency can be estimated by the index of N nutrition i.e. the observed N concentration divided by the critical N concentration. Since the critical N concentration is a function of shoot biomass, we conclude that the critical P concentration should be determined as a function of shoot biomass and the N concentration of that shoot biomass. In practice, however, the determination of shoot biomass is time- and labour-consuming. Duru (1992) and Koerselman and Meulman (1996) suggested using the P:N ratio for diagnostic purposes because it eliminates the need to determine shoot biomass. It might be feasible to obtain a valid diagnostic of P and N deficiencies by only measuring P and N concentrations if we assume a negligible effect of N deficiency on the relationship between P and N concentrations. The implications of this assumption, however, should be further investigated. For the local conditions and the analytical method used in New Brunswick, soil P availability for forage grasses in our study was considered high to very high. Furthermore, we applied a relatively high rate of P before growth started. We therefore assumed that P was not limiting shoot growth. Further research is required to study the relationship between P and N concentrations in timothy grown with varying rates of P and N fertilization on soils with low P availability. The relationship between the index of P nutrition estimated using the concept of a critical P concentration and the measures of soil P availability should also be investigated. Bélanger et al. (1989) showed that balanced applications of N, P, and K were required to achieve long-term productivity of timothy. This implies that imbalances in plant P and N concentrations would negatively affect timothy productivity. To our knowledge, there are no tools available to quantify simultaneously P and N deficiencies, and to diagnose P and N imbalances in timothy. The present study provides the relationship required to estimate the critical P concentration, which is essential for quantifying levels of P deficiencies. Combined with the index of N nutrition developed in an earlier paper (Bélanger and Richards 1997), it provides the tools required to diagnose N and P deficiencies and imbalances in timothy. As suggested by Salette and Huché (1991) and based on the results of Duru and Ducrocq (1997), this approach should be applicable to many grass species. ACKNOWLEDGEMENTS We thank J. O. Monteith and G. P. Demarchant for excellent technical assistance, and R. Michaud and N. 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