Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer

Transcription

1 Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer Y. K. Soon 1 and S. S. Malhi 2 1 Agriculture and Agri-Food Canada, Beaverlodge Research Farm, P. O. Box 29, Beaverlodge, Alberta, Canada T0H 0C0 ( soony@agr.gc.ca); and 2 Agriculture and Agri-Food Canada, Melfort Research Farm, P. O. Box 1240, Melfort, Saskatchewan, Canada S0E 1A0. Received 3 December 2004, accepted 18 June Soon, Y. K. and Malhi, S. S Soil nitrogen dynamics as affected by landscape position and nitrogen fertilizer. Can. J. Soil Sci. 85: The influence of landscape position on the dynamics of N in the soil-plant system has not been adequately studied. Our aim with this study on a predominantly Black Chernozem soil was to evaluate the effect of slope position (upper vs. lower) and N fertilizer application (none vs. 60 kg N ha 1 ) on soil and wheat (Triticum aestivum L.) N through the growing season. Landscape position had a dominant effect on soil NO 3 and soluble organic N (SON) concentrations, especially in the surface 15 cm. These pools of soil N and net N mineralization were greater at the lower than at the upper slope position. The landscape effect is attributed to higher organic matter content (as measured by organic C) and water availability in lower compared with upper slope positions. Nitrogen application had no measurable effect on soil NO 3 and SON concentrations. Exchangeable and non-exchangeable NH 4 + were little affected by slope position or N fertilization. Nitrogen application increased wheat N uptake; however, its influence was less than that of slope position, especially on N accumulation in wheat heads during grain-filling. Although N application increased wheat yields, landscape position exerted the greater influence: grain yield was less on upper than lower slope positions due to earlier onset of crop maturity. During grain filling, net N mineralization was suppressed at the upper slope position and by N application. The increase in crop yield and N uptake due to N application was not significantly different between slope positions. This study demonstrated that landscape position had a greater influence on N dynamics and availability than the application of typical amounts of fertilizer N and that the two effects were mostly independent of each other. Key words: Available N, landscape position, N uptake, net N mineralization, soluble organic N Soon, Y. K. et Malhi, S. S Incidence de l emplacement dans le relief et des engrais azotés sur la dynamique de l azote dans le sol. Can. J. Soil Sci. 85: On ne s est pas assez intéressé à l incidence de l emplacement dans le relief sur la dynamique de l azote (N) dans le système sol-plante. En se penchant sur un sol principalement constitué de tchernoziom noir, les auteurs voulaient établir comment l emplacement de la pente (haut ou bas) et l application d un engrais azoté (aucune ou 60 kg de N par hectare) agissent sur l azote du sol et celui présent dans le blé (Triticum aestivum L.) durant la période végétative. L emplacement dans le relief a un effet dominant sur la concentration de NO 3 - dans le sol et sur le N organique soluble (NOS), surtout dans la couche supérieure de 15 cm. Les réservoirs de N du sol et de N net minéralisé sont plus importants au bas des pentes que dans le haut. On attribue cette incidence du relief à la plus grande teneur en matière organique (sous forme de C organique) et disponibilité d eau observée au bas des pentes. L application d un engrais azoté n a aucun effet quantifiable sur la concentration de NO 3 et de NOS dans le sol. La concentration d ions NH 4 + échangeables et non échangeables n est guère affectée par l emplacement de la pente ni par l amendement azoté. L application d engrais N augmente l absorption du N par le blé, mais cette influence est plus faible que celle de l emplacement de la pente, surtout pour ce qui est de l accumulation de N dans l épi lors du remplissage du grain. Bien que l application d engrais N accroisse le rendement du blé, l emplacement de la pente a plus d influence : le rendement grainier est plus élevé au bas des pentes que dans le haut, sans doute à cause d une maturité plus hâtive. L application de N freine la minéralisation de cet élément sur le haut des pentes, phénomène qui survient aussi à cet endroit pendant le remplissage du grain. L accroissement du rendement et la plus grande absorption de N attribuable à l application d engrais N ne varient pas significativement avec l emplacement de la pente. L étude montre que l emplacement dans le relief exerce plus d influence sur la dynamique et la disponibilité du N que l application d une quantité typique d engrais N et que les deux effets sont essentiellement indépendants. Mots clés: N disponible, emplacement dans le relief, absorption de N, minéralisation nette du N, N organique soluble Crop productivity and N uptake can vary with positions within a landscape, particularly on the rolling plains and hummocky landforms of prairie landscapes (Fiez et al. 1994, 1995; Walley et al. 2001). Manning et al. (2001b) found that, for a growing season with precipitation 62% higher than average, wheat yield and N uptake were significantly less at lower slope than at upper slope positions when N fertilizer was applied at 45 and 90 kg N ha 1. However, the opposite trend in wheat yield was observed in another year with growing season precipitation 37% below average. Malhi et al. (2004) also reported that plant N 579 uptake and the recovery of applied fertilizer N were influenced by landscape position, and the effect varied with the year, presumably reflecting soil water availability. Jowkin and Schoenau (1998) found that landscape position had a greater effect than tillage practices (no-till vs. conventional tillage) on N availability and crop yield: yield and N uptake by spring wheat were lower at shoulder positions than at footslope positions. The soil moisture and fertility status of landscape units depend on the slope position they occupy Abbreviations: DM, dry matter; SON, soluble organic N

2 580 CANADIAN JOURNAL OF SOIL SCIENCE (Pennock et al. 1987). The variations in crop productivity and nutrient availability due to landscape position have been attributed, usually, to greater soil water availability in level or lower slope position (Walley et al. 2001; Malhi et al. 2004). Soil organic matter also tends to be higher in footslope compared with shoulder positions (Fiez et al. 1995; Malhi et al. 2004). The footslope has been identified as the slope segment where soil eroded from upper slopes is deposited, and generally has deep and productive soils (Pennock and de Jong 1990). Several ecosystem processes affecting N cycling, such as denitrification, have been shown to proceed at a higher rate in footslope than in shoulder or upper slope positions (Pennock et al. 1992). Manning et al. (2001a) reported that, during a wet growing season, apparent net N mineralization decreased from upper to lower slope positions. They suggested that this resulted from greater N loss at lower than at upper slope positions. Reduced crop yields (and corresponding N uptake) and N loss by denitrification can occur in footslope or shallow depressions under anaerobic conditions associated with wet soil. Recently, SON has attracted considerable research for its role in N cycling in ecosystems (McDowell 2003). Several studies found that as much SON as extractable inorganic N, if not more, can be present in agricultural soils (Smith 1987; Murphy et al. 2000). Murphy et al. (2000) reported that, in the United Kingdom, inorganic N in the soil tended to vary more seasonally than SON. Several field studies have shown that the non-exchangeable NH 4 + pool decreased significantly as the growing season progressed (Kudeyarov 1981; Mengel and Scherer 1981; Soon 1998). This suggests that at least a fraction of non-exchangeable NH 4 + was mobilized from the interlayers of clay minerals and became bio-available during crop growth. The influence of landscape position on soil N pools, such as SON, non-exchangeable NH 4 +, and plant N, and their dynamics during the growing season have not been adequately investigated. Knowledge of N fluxes and their dynamics, as affected by slope position and N application, is required to understand N cycling in soil-plant systems better and to manage more efficiently N fertilizer added to agricultural soils across variable landscapes. Our aim in this study was to compare the influence of landscape position vs. N fertility level on soil N pools, plant N uptake and net N mineralization through the growing season. This study was part of a larger study reported by Malhi et al. (2004). MATERIALS AND METHODS A field experiment was conducted 1997 to 1999 on a Black Chernozem soil near Prince Albert (53.12 N W) in Saskatchewan, Canada. This portion of the study was conducted during 1999 only. Crop yields and recovery of 15 N- labelled urea fertilizer from 1997 to 1999 were reported by Malhi et al. (2004). The experiment was a factorial design with two N fertilizer rates (0 and 60 kg N ha 1 as urea) and two landscape positions upper (shoulder) or lower (level to depressional) slope position (Pennock et al. 1987) with six replications. At each landscape position (constituting the main plot), N treatments were randomly assigned to each of two subplots. Subplots were 2.1 m wide and at least 60 m long (plot length varied somewhat between replicates due to landscape or topographic variation). The experiment was managed under no-till. Weeds were suppressed with glyphosphate (N-[phosphonomethyl]glycine) 10 d before seeding. Spring wheat (Triticum aestivum L. AC Barrie ) was seeded at a row spacing of 203 mm using a hoe-drill. In 1999, plots were seeded on May 31, and harvested on Sep. 16. All plots received 10 kg ha 1 of P as ammonium phosphate banded in the seed furrow. Soil from fertilized and non-fertilized plots was sampled to 60-cm depth four times during the year using a truckmounted hydraulically driven 38-mm corer: prior to seeding (on May 28), at % heading (Feekes on Jul. 26) and mid-dough growth stages (Feekes on Aug. 20) of wheat, and after crop harvest (Sep. 24). The first sampling after N fertilizer application was at % heading by which time most of the applied N was either immobilized by microbial biomass or absorbed by wheat plants. A minimum of four cores were taken per plot and composited after sectioning into depth increments of: 0 15, 15 30, and cm. The soils were dried at room temperature and passed through a 2-mm sieve. Subsamples of soils were ground to pass a 60-mesh sieve. Wheat plants were sampled from six 1-m rows at Feekes and growth stages, split into heads and vegetative (i.e., stems and leaves) portions, dried at 65 C to a constant weight and subsamples were ground in a Wiley mill to pass a 40-mesh sieve. Five grams of soil (< 2 mm) were extracted with 25 ml of 1 M KCl (1 h shaking followed by filtration) for determination of NO 3 and exchangeable NH 4 +. Total soluble N was extracted with 30 ml of 0.5 M K 2 SO 4 using 6 g of soil (< 2 mm ). After 1 h of shaking, 3 ml of the filtered extract was oxidized with 3 ml of an alkaline persulphate oxidant, prepared according to Cabrera and Beare (1993), in an autoclave for 30 min at 120 C. Reduced forms of soluble N were oxidized to NO 3, which was determined by automated colorimetry, as described below. 1 M KCl-extractable inorganic N was subtracted from total soluble N to derive SON. Non-exchangeable (or fixed) NH 4 + was extracted from 0.5 g of ground sample (60 mesh) using HF-HCl (Silva and Bremner 1966). Extracted N from all the above procedures was determined by automated colorimetry using an Alpkem RFA 300 auto-analyser: NH 4 + determination was based on indophenol formation with sodium salicylate, and NO 3 was determined following its reduction by Cd to NO 2 and subsequent diazotization. Soil ph was determined in 0.01 M CaCl 2, using a 1:2.5 soil:solution (m/v) ratio. Organic C was determined using a modified Mebius procedure (Soon and Abboud 1991). Organic C and ph were analyzed only on the top layer of soil but for all sampling dates. Total N in plant tissue samples was determined by a dry combustion N analyzer (LECO FP428). Nitrogen uptake was calculated by multiplying dry matter by the N concentration. Changes in soil N pools and plant N uptake between preseeding and % heading (nominally, the vegetative period), and between heading and mid-dough of wheat

3 SOON AND MALHI LANDSCAPE POSITION AND FERTILIZER N EFFECTS ON N DYNAMICS 581 (nominally, the grain-filling period) were determined. Net N mineralization was calculated as plant N uptake plus extractable soil inorganic N at sampling minus extractable soil inorganic N at sowing minus plant N derived from fertilizer. Uptake of fertilizer N was determined using 15 N data reported by Malhi et al. (2004). Data were subjected to analysis of variance (ANOVA) separately for each sampling occasion, and repeated measures ANOVA with profile transformation for the combined data (four times of sampling). The analysis was that of a split plot design with slope position as main plot and N fertilizer rate as sub-plot. A probability (P) level of 0.05 was selected for significance. Least significant difference (LSD 0.05 ) values were calculated for means separation. SAS programs were used for all statistical analyses (SAS Institute, Inc. 1990). RESULTS In 1999 the growing season precipitation was: May, 95 mm; June, 108 mm; July, 170 mm; and August, 58 mm. Total precipitation received between seeding and harvesting was 341 mm compared with a 30-yr mean of 226 mm, and the growing-degree-days (base 5 C) was 1194 vs. a 30-yr mean of Soil organic C in the top 15 cm of soil was significantly higher in lower than in upper slope positions (Table 1). Data for the heading of wheat sampling only are shown since variation between samplings was slight. Soil ph was slightly but significantly higher in upper than in lower slope positions (Table 1). At sowing, soil water content to 90-cm depth was 76 and 130 mm on upper and lower slope positions, respectively (Malhi et al. 2004). Soil Nitrogen Pools The overall trend, common to all three layers of soil, was that of a relatively large decline in mean NO 3 -N concentrations between seeding and heading of wheat, and a smaller (but statistically significant) decrease in NO 3 -N between heading and mid-dough of wheat (Table 2). Mean soil NO 3 - N concentrations tended to increase from mid-dough to post-harvest of wheat. There was also a trend for more NO 3 - N to be present in lower than in upper slope positions, although few of the differences (mainly in the surface 15 cm) were significant because of data variability. Mean NO 3 - N concentration in the lower slope position tended to be twice as high as in the upper slope position (P values varied from 0.02 for the top layer to 0.09 for the deepest layer). Landscape position accounted for 23 38% of the variation in NO 3 -N content early in the season, with more variation in the surface than in subsurface layers. By the mid-dough growth stage of wheat, however, landscape position accounted for 15% of the NO 3 -N variation in the surface soil, and less than 5% in the cm depth. Nitrogen fertilizer application had no significant effect on NO 3 -N or NH 4 -N (data not shown). Average NH 4 -N concentration was higher in lower than in upper slope positions only in the top 15 cm of soil (Table 3). The differences in NH 4 -N concentration in the surface soil layer due to slope position were also significant on three of the four samplings. With one exception, differences between landscape positions were mostly not significant at Table 1. Soil organic C and ph in top 15 cm of soil as influenced by landscape position z Slope position Soil organic C (g C kg 1 ) ph y Upper Lower LSD z At % heading of wheat. y Soil ph in 0.01 M CaCl 2 solution, 1:2.5 soil:solution ratio. soil depths greater than 15 cm. Mean NH 4 -N concentrations also tended to be more uniform among samplings than NO 3 - N concentrations, indicating little net change in exchangeable soil NH 4 + over the growing season. At all times, there was more SON than extractable inorganic N in each of the three soil layers. Soluble organic N concentration in the 0 60 cm depth was at least twice that of inorganic N. Fertilizer N application had no effect on SON (data not shown). Soluble organic N in the 0 60 cm depth of soil at the upper slope position was 102 kg ha 1 at the beginning of the growing season, and decreased to about 80 kg N ha 1 for subsequent samplings. For the lower slope position, SON initially was also 102 kg ha 1 ; it increased to 121 kg ha 1 at heading and subsequently levelled off to about 102 kg ha 1. Soil SON content was higher in lower than in upper slope positions at the heading and mid-dough growth stages. Soluble organic N concentrations in the soil layers displayed dynamics somewhat different from those of inorganic N (Fig. 1). In the top 15 cm SON was higher in lower than in upper slope positions, although the differences were significant for only two of the four samplings and the means of the samplings. There was a slight but significant increase in mean SON after harvest compared to the prior sampling (indicated by repeated measures ANOVA). The changes in SON in the cm depth showed a strong (P < 0.01) slope position time of sampling interaction. Except at prior to seeding, SON concentrations in the lower slope position were higher than those in the upper slope position. Soluble organic N in the upper slope position decreased from about 46 kg N ha 1 before seeding to about 18 kg N ha 1 at mid-dough and after harvest of wheat. The SON in the lower slope position was about 41 kg N ha 1 at heading of wheat and kg N ha 1 at other times. There was an overall trend for mean SON in the cm depth to decrease between heading and mid-dough of wheat samplings (indicated by repeated measures ANOVA). In the cm depth, SON in the upper slope position was maintained in the kg N ha 1 range through the growing season. At the lower slope position, SON increased slightly between seeding and heading of wheat, and showed a significant decrease between heading and mid-dough of wheat. Landscape position accounted 24 27% of SON variation in the surface 15 cm of soil and 16 33% of the variation in the cm layer. Landscape position was a major factor in SON variation (16 20%) in the cm depth early in the season only. Non-exchangeable NH 4 + was usually not affected by N fertilizer or landscape position (data not shown). Mean nonexchangeable NH 4 -N was 232 kg ha 1 [standard error (SEM) = 4.1] in the 0 15 cm depth, 274 kg ha 1 (SEM =

4 582 CANADIAN JOURNAL OF SOIL SCIENCE Table 2. Slope position and N fertilizer effects on soil nitrate at various depths and times of sampling Time of sampling Slope position Pre-seeding Wheat % headed Mid-dough of wheat Post-harvest Mean (SEM) z 0 15 cm Nitrate-N (kg ha 1 ) Upper Lower 12.0* y 3.4* * Mean ** x 1.5** 2.4** 3.9 (0.54) cm Upper Lower Mean ** 0.6** (0.55) cm Upper Lower * Mean ** 0.8** 1.2** 3.7 (0.58) z SEM, pooled standard error over time of sampling (5 DF, n = 48). y *Following a value in a row designated lower denotes a significance difference between slope positions. x * or ** following a mean for each time of sampling indicates that the mean was significantly different from the preceding mean at P = 0.05 or P = 0.01, respectively, based on repeated measures ANOVA using a profile transformation. Table 3. Slope position and N fertilizer effects on exchangeable ammonium at various depths and times of sampling Time of sampling Slope position Pre-seeding Wheat % headed Mid-dough of wheat Post-harvest Mean (SEM) z 0 15 cm Exchangeable NH 4 -N (kg ha 1 ) Upper Lower ** y 6.6* 6.7* 7.1** Mean (0.40) cm Upper Lower * Mean ** x 4.8** (0.47) cm Upper Lower Mean (0.88) z SEM, pooled standard error over time of sampling (5 DF, n = 48). y * and ** following a value in a row designated lower denotes a significance difference between slope positions at P = 0.05 or P = 0.01, respectively. x * or ** following a mean for each time of sampling indicates that the mean was significantly different from the preceding mean at P = 0.05 or P = 0.01, respectively, based on repeated measures ANOVA using a profile transformation. 6.0) in the cm depth, and 514 kg ha 1 (SEM = 13.9) in the cm depth. Between heading and mid-dough of wheat, non-exchangeable NH 4 -N in the surface 15 cm increased from 210 to 251 kg ha 1 ; between seeding and heading of wheat it decreased in the cm depth from 566 to 510 kg ha 1. Non-exchangeable NH 4 + at the intermediate depth showed little change with time. Crop Production, N Uptake and Other Fluxes of N At heading of wheat, vegetative (i.e., stems and leaves) dry matter (DM) production was increased by N fertilizer application (which accounted for 59% of the variation) and at the lower slope compared with the upper slope position (8% of variation). The DM increase in response to N at the upper slope position was twice that at the lower slope position (Table 4). However, there was no N rate slope position interaction for other plant part DM or other samplings. Slope position resulted in a major portion of the variation (38%) in head DM production at heading of wheat. Wheat head DM was less in the lower slope position than in the upper slope position and exhibited no significant response to N application at this early stage of head development. At mid-dough growth stage, vegetative as well as head DM production was increased by N application, but slope position accounted for a considerably greater portion (48 and 56%, respectively) of the total variation (Table 4). During grain filling, head DM increased an average of 10- fold on the upper slope position vs. 29-fold on the lower slope position, i.e., the slower start to heading on the lower slope position did not affect the final yield advantage associated with that slope position. Landscape position exerted a strong influence on N concentration in wheat tissues, with wheat plants grown on the lower slope position having consistently higher concentra-

5 SOON AND MALHI LANDSCAPE POSITION AND FERTILIZER N EFFECTS ON N DYNAMICS 583 tions than plants grown on the upper slope position (Table 4). Landscape position accounted for 36 49% of the variation in N concentration of stems and leaves, and 26 31% of the variation in head N concentration. With the exception of head N concentration at the mid-dough growth stage, N fertilizer application had no effect on wheat tissue N concentration although the difference for the N concentration of vegetative tissues at heading was just barely not significant (P = 0.06). At the mid-dough stage of wheat, however, the N concentration of stems and leaves increased in response to N application at the lower slope position only (i.e., there was a slope position N rate interaction). Nitrogen uptake by stems and leaves at heading showed a similar pattern to DM accumulation, except that there was no N rate slope position interaction (Fig. 2). Forty-four percent of the variation in N uptake of stems and leaves was due to N application and 23% was due to slope position. Nitrogen uptake by wheat heads at heading showed a similar pattern to DM accumulation, with slope position causing a major portion (25%) of the variation. Nitrogen fertilizer application had no effect on head N uptake at that stage of wheat development. At mid-dough growth stage, landscape position resulted in 54% of the variation in N accumulation in wheat vegetative tissues, although the effect of N application rate was also significant (but accounting for only 6% of the variation)(fig. 2). Both N fertilizer and slope position significantly affected head N uptake at the mid-dough growth stage, with 54% of the variation associated with slope position compared to 26% with N application. During grain-filling, an average of 70% of N that had previously accumulated in the vegetative tissue of wheat grown on the upper slope position was apparently translocated to developing heads compared with an average of 41% for wheat grown on the lower slope position. Between the heading and mid-dough growth stages, wheat head N uptake on upper slope position increased from 6 to 44 kg ha 1 and on lower slope position from 4 to 76 kg ha 1. Since the average amounts of N translocated in that time period were 25 and 21 kg N ha 1, respectively, for the upper and lower slope positions, the bulk of N uptake by wheat grown on the lower slope position during that time period must have come from soil N. Nitrogen uptake by wheat grown on the upper slope position was less during grain-filling than during vegetative growth whereas it was similar for the two growth phases for wheat grown at the lower slope position. Changes in soil and plant N, and net N mineralization during vegetative growth and grain-filling are shown in Table 5. The depletion of soil inorganic N during the vegetative growth and grain-filling periods tended to be greater for the lower than for the upper slope position. Most of the decrease in available N occurred in the NO 3 -N pool (Table 2). Nitrogen application had no effect on depletions of the inorganic N pool. During the vegetative growth phase, SON was decreased in the upper slope position and increased in the lower slope position. There was an overall loss of SON from the upper slope position during the growing season, but no measurable change in SON at the lower slope position. The relationship between changes in SON and plant N Fig. 1. Influence of landscape position and time on soluble organic N in (a) 0 15 cm, (b) cm and (c) cm depth of a Black Chernozem soil. For (a) there was no slope position time of sampling interaction: at any sampling time, position means showing a common letter are not significantly different (P = 0.05). For (b) and (c), the slope position time of sampling interaction was significant, and means for different sampling times at a given slope position are not significantly different when identified with a common letter. Pooled standard errors of mean (SEM) are shown for each depth of soil.

6 584 CANADIAN JOURNAL OF SOIL SCIENCE Table 4. Dry matter and N concentration of stems and leaves, and heads of wheat plants at heading and at mid-dough stages of development as influenced by landscape position and N fertilizer application Slope Stems and leaves Heads position No N 60 kg N ha 1 Mean No N 60 kg N ha 1 Mean Dry matter at % heading (kg ha 1 ) Upper Lower * z ** Mean ** SEM (10 DF) 129* y 26 Dry matter at mid-dough growth stage (kg ha 1 ) Upper Lower ** ** Mean ** ** SEM (10 DF) N concentration at % heading (g kg 1 ) Upper Lower ** ** Mean SEM (10 DF) 1.31 N concentration at mid-dough (g kg 1 ) Upper Lower ** ** Mean ** SEM (10 DF) 0.305** 0.65 z * and ** following a mean for slope position or N treatment indicate that the treatment effect is significant at P = 0.05 and P = 0.01 levels, respectively. y * and ** following a SEM indicates that the N rate slope position interaction is significant at P = 0.05 and P = 0.01, respectively. uptake was difficult to discern, although a trend of decreasing SON during grain-filling of wheat was apparent. There was a net decrease in non-exchangeable NH 4 + during the vegetative period and a small net gain during grain filling, but the variability was high and there was no apparent relation with plant N uptake. Slope position or N application had little influence on net N mineralization rates during the vegetative period (Table 5). However, during the grain-filling period, net N mineralization was greater at the lower than at the upper slope position, and greater in the no-n treatment than in the N-fertilized treatment. Net N mineralization during grainfilling of wheat was greater at the lower than at the upper slope position, but it was similar between slope positions during the vegetative growth phase. Over the entire growth period, net N mineralization was about 10 kg N ha 1 at the upper slope position compared with about 44 kg N ha 1 at the lower slope position. Net N mineralization over the entire growth period was about 40 kg N ha 1 in soil that received no N fertilizer compared with 14 kg N ha 1 in soil that received N fertilizer, indicating that net soil N turnover was suppressed by N application. DISCUSSION AND CONCLUSION This study was done during a year with 90% more precipitation from May through August than the 30-yr average, so the results may apply more to wet years than dry ones. The SON pool was considerably larger than the inorganic N pool, probably reflecting the rich organic matter status of the Black Chernozem soil. However, the contribution of SON to the N nutrition of crops was difficult to assess. Our data on SON dynamics suggest that it has little direct relationship to plant N uptake (Table 5). Appel and Mengel (1990, 1992) also found that SON did not decline during plant growth, and Murphy et al. (2000) reported only a weak correlation between the size of the SON pool and potentially mineralizable N (as measured by anaerobic incubation). However, a weak correlation with plant N uptake should not be taken to infer that SON was not turning over and supplying N to the bio-available pool. The characterization and quantification of SON as a source of bio-available N may need to be made at a more detailed level. Soluble organic N in soil is now thought to be composed of a relatively stable fraction and a more dynamic fraction (Murphy et al. 2000). Jones et al. (2004) found two distinct SON pools in grassland soils: a pool comprised mainly of free amino acids and proteins that turned over rapidly so that it did not accumulate in the soil, and a pool of high molecular weight humic substances, which turned over slowly. We need to characterize and measure the labile fraction of SON if we are to gain insights into its role in soil N cycling and dynamics. In contrast to some previous studies (Mengel and Scherer 1981; Soon 1998), we found no indication that nonexchangeable NH 4 + made a measurable contribution to available N. The pool was large, however, containing about 1 Mg N ha 1 within the surface 60 cm, so that any contribution to available N would have been hard to detect. Addition of 60 kg N ha 1 had little persistent effect on soil inorganic N and SON levels. Its main influence was on DM and N accumulation in wheat shoot at the head formation, and even this diminished by the mid-dough growth stage. Net N mineralization during grain-filling of wheat was suppressed by N application. Landscape position had a predominant effect on soil N levels (particularly on NO 3 to 15 cm and SON to 30 cm), and plant DM and N accumulation at most sampling times. Malhi et al. (2004) found that fertilizer N recovery by wheat plant was similar, about 48 49%, for both landscape positions. Therefore, the greater uptake of N at the lower slope position must have been derived from native soil N. This was provided by net

7 SOON AND MALHI LANDSCAPE POSITION AND FERTILIZER N EFFECTS ON N DYNAMICS 585 Fig. 2. Influence of landscape position and fertilizer N application on N uptake in wheat heads, and stems and leaves, at % heading and at mid-dough growth stage of wheat. N denotes no N, and +N denotes 60 kg N ha 1. Standard errors (with 10 degrees of freedom) are: 18.7 for stem and leaves and 1.78 for wheat heads at % heading; 5.9 for stem and leaves and 12.3 for wheat heads at mid-dough of wheat. N mineralization, which was significantly greater at the lower slope position after heading (when it apparently ceased at the upper slope position) and, presumably, in the spring before sowing. The proportion of N derived from fertilizer (%Ndff) was lower at the lower slope than at the upper slope position (19 vs. 27%; Malhi et al. 2004), indicating that the larger available N pool at the lower slope position resulted in a greater dilution of the labelled added N. The predominant influence of landscape position can be attributed partly to the higher organic C (i.e., organic matter) at the lower slope position (Table 1). Available water in the spring has been reported to be higher in the lower slope position compared with the upper slope position at this site (Malhi et al. 2004) as well as others (Walley et al. 2001). Since the water-holding capacity of soils typically increases with organic matter content, the lower slope position has an additional advantage over the upper slope position in this regard. Soil ph was slightly higher at the upper than at the lower slope position, but these ph differences likely had little effect on plant growth and N availability. The earlier cessation of net N mineralization and reduced N uptake at the upper slope position during head filling may be related to lower water availability there compared with the lower slope position in late summer. A major aim of farmers is the efficient use of applied N. The few significant interactions between N application and

8 586 CANADIAN JOURNAL OF SOIL SCIENCE Table 5. Changes in N pools and N fluxes during the growing season as influenced by slope position and N fertilizer Time interval between: Seeding and % heading of wheat % heading and mid-dough of wheat N pool or flux z Position N added Position N added (kg N ha 1 ) Upper Lower No N 60 kg ha 1 Upper Lower No N 60 kg ha 1 Exch. NH 4 - and NO 3 -N SEM y Soluble organic N SEM 6.3* Non-exch. NH 4 -N SEM * Plant N uptake SEM 1.8** 2.5** 4.8* 2.3 Net N mineralization x SEM * 2.1** z All soil data are based on 0 60 cm sampling depth. Negative numbers denote a decrease in the pool size and vice versa. y * and ** following a standard error of the mean (SEM) indicate that the pair of treatments was significantly different at P = 0.05 and P = 0.01, respectively. x Net N mineralization = plant N uptake plus soil inorganic N at sampling minus plant N derived from fertilizer minus soil inorganic N at sowing. landscape position indicate that those factors are mostly independent of each other, i.e., slope position did not significantly influence soil and crop response to N fertilization. Our observations, along with those over three years by Malhi et al. (2004), suggest that a variable rate fertilizer strategy may have little advantage in crop production and N uptake. Crop response at the lower slope position may have been limited by factors other than N (e.g., environmental variables), and at the upper slope position, yields may have been limited by water availability as well as N supply. Walley et al. (2001), another site in Saskatchewan, also found that a variable N fertilizer rate strategy had no apparent advantage. In conclusion, on rolling plains and hummocky landscapes, positions within the landscape can exert a greater influence on N dynamics and availability than addition of normal amounts of fertilizer N, at least under conditions of above-average precipitation. Wheat DM and N accumulation, and net N mineralization were less at the upper than at the lower slope position mainly because of lower soil moisture and organic matter content. Crop response to N addition tended to be similar at both landscape positions in spite of observable differences in N dynamics. ACKNOWLEDGEMENTS We thank Anwar Haq and Darwin Leach for technical assistance. Appel, T. and Mengel, K Importance of organic fractions in sandy soils, obtained by electro-ultrafltration or CaCl 2 extraction, for nitrogen mineralization and nitrogen uptake of rape. Biol. Fertil. Soil 10: Appel, T. and Mengel, K Nitrogen uptake of cereals grown on sandy soils as related to nitrogen fertilizer application and soil nitrogen fractions obtained by electroultrafiltration (EUF) and CaCl 2 extraction. Eur. J. Agron. 1: 1 9. Cabrera, M. C. and Beare, M. H Alkaline persulfate oxidation for determination of total nitrogen in microbial biomass extracts. Soil Sci. Soc. Am. J. 57: Fiez, T. E., Miller, B. C. and Pan, W. L Winter wheat and grain protein across varied landscape positions. Agron. J. 86: Fiez, T. E., Pan, W. L. and Miller, B. C Nitrogen use efficiency of winter wheat among landscape position. Soil Sci. Soc. Am. J. 59: Jones, D. L., Shannon, D., Murphy, D. V. and Farrar, J Role of dissolved organic nitrogen (DON) in soil N cycling in grasslands soils. Soil Biol. Biochem. 36: Jowkin, V. and Schoenau, J. J Impact of tillage and landscape position on nitrogen availability and yield of spring wheat in a brown soil zone in southwestern Saskatchewan. Can. J. Soil Sci. 78: Kudeyarov, V. N Mobility of fixed ammonium in soil. Pages in F. E. Clarke and T. Rosswall, eds. Terrestrial nitrogen cycles. Ecol. Bull. 33. Stockholm, Sweden. Malhi, S. S., Johnston, A. M., Gill, K. S. and Pennock, D. J Landscape position effects on the recovery of 15 N-labelled urea applied to wheat on two soils in Saskatchewan, Canada. Nutr. Cycl. Agroecosyst. 68: Manning, G., Fuller, L. G., Eilers, R. G. and Florinsky, I. 2001a. Soil moisture and nutrient variation within an undulating Manitoba landscape. Can. J. Soil Sci. 81: Manning, G., Fuller, L. G., Flaten, D. N. and Eilers, R. G. 2001b. Wheat yield and grain protein variation within an undulating soil landscape. Can. J. Soil Sci. 81: McDowell, W. H Dissolved organic matter in soils future directions and unanswered questions. Geoderma 113: Mengel, K. and Scherer, H. W Release of non-exchangeable (fixed) ammonium under field conditions during the growing season. Soil Sci. 131: Murphy, D. V., Macdonald, A. J., Stockdale, E. A., Goulding, K. W. T., Fortune, S., Gaunt, J. L., Poulton, P. R., Wakefield, J. A., Webster, C. P. and Wilmer, W. S Soluble organic nitrogen in agricultural soils. Biol. Fertil. Soils 30: Pennock, D. J. and de Jong, E Spatial pattern of soil redistribution in boroll landscapes, southern Saskatchewan, Canada. Soil Sci. 150: Pennock, D. J., Zebarth, B. J. and de Jong, E Landscape classification and soil distribution in hummocky terrain, Saskatchewan, Canada. Geoderma 40: Pennock, D. J., van Kessel, C., Farrell, R. E. and Sutherland, R. A Landscape-scale variations in denitrification. Soil Sci. Soc. Am. J. 56:

9 SOON AND MALHI LANDSCAPE POSITION AND FERTILIZER N EFFECTS ON N DYNAMICS 587 SAS Institute, Inc SAS/STAT user s guide. Version 6. 4th ed. SAS Institute, Inc., Cary, NC. Silva, J. A. and Bremner, J. M Determination and isotope ratio analysis of different forms of nitrogen in soils. 5. Fixed ammonium. Soil Sci. Soc. Am. Proc. 30: Smith, S. J Soluble organic nitrogen losses associated with recovery of mineralized nitrogen. Soil Sci. Soc. Am. J. 51: Soon, Y. K. and Abboud, S A comparison of some methods for soil organic carbon determination. Commun. Soil Sci. Plant Anal. 22: Soon, Y. K Nitrogen cycling involving non-exchangeable ammonium in a gray luvisol. Biol. Fertil. Soils 27: Walley, F., Pennock, D., Solohub, M. and Hnatowich, G Spring wheat (Triticum aestivum) yield and grain protein responses to N fertilizer in topographically defined landscape positions. Can. J. Soil Sci. 81:

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