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Mineral nitrogen in an oxisol from the Brazilian cerrados in the presence of Brachiaria spp.
Eur. J. Agron., 1994, 3(4), 333-337
Mineral nitrogen in an oxisol from the Brazilian cerrados in the presence of Brachiaria spp. C. H. B. Miranda
1
,
G. Cadisch 3 , S. Urquiaga 2 C. H. B. Miranda 3 , R. M. Boddey 2 and K. E. Giller3 1
EMBRAPA-CNPGC. CP 154. 79100, Campo Grande, MS, Brazil. EMBRAPA-CNPAB. Km 47, 23851-970, Seropedica, ltaguai, RJ, Brazil. H'ye College, University of London. H'ye, Ashford, Kent TN25 5AH, England. 2
3
Received 3 May 1993; accepted 19 April 1994
Abstract
The effects of commercial cultivars of Brachiaria decumbens, B. humidicola, and B. brizantha on nitrogen mineralization and nitrification were examined in a Dark Red Latosol from the Cerrados region of Central Brazil. B. decumbens responded most rapidly to the addition of fertilizer N. B. brizantha also absorbed the fertilizer N rapidly but grew more slowly. By contrast, B. humidicola showed a slower response to the fertilizer addition, both in N uptake and growth. The concentrations of mineral N in the soil under both B. decumbens and B. brizantha indicated that most of the fertilizer N remaining in the soil was quickly immobilized, as also indicated by the results of incubated soil samples. On the other hand, the soil under B. humidicola showed strong mineralization of native organic N between three and six days after the addition of N in the presence of the plants, but not during the incubation. In this species there was also a clear indication of early nitrification, whereas in the soil under the other two species the build up of the nitrifier population seemed to be delayed. It is concluded that the species studied had different rates of N uptake and assimilation and stimulated microorganisms differently in their rhizosphere, which lead to contrasting transformations on the soil mineral N pool. The observed patterns of mineralization/immobilization and nitrification changed so quickly that analysis of the effects of plants on soil N transformations based on a few harvests or over long time intervals could be misleading. Key-words : tropical grasses, tropical soil, pasture, nitrogen mineralization, nitrification, soil incubation, plant N uptake, 15 N, Brachiaria spp.
INTRODUCTION Nitrogen fertilizer is rarely added to pastures in the Brazilian cerrados area and the use of pasture legumes is not common. Consequently, the grasses are dependent on the soil available N, relying on an efficient natural recycling of N. There are some studies suggesting that plants could stimulate net mineralization of soil organic N, through the exudation of carbon compounds from their roots (Wheatley et al., 1990; Whipps and Lynch, 1985). A species with such a potential would thus have an advantage in obtaining otherwise limiting soil N, as in the cerrados soils. The significance of such a process is still controversial and, as stated by Griffiths and Robinson (1992), it is unlikely a plant would obtain any benefit from such a process, since the theoretical gains would be small. ISSN l/61-0301/941041$ 4.001© Gauthier-Villars - ESAg
In this paper we present the results of an experiment conducted with three commercial cultivars of Brachiaria spp., the most common grasses used in the Brazilian cerrados area. When these species are introduced together with P fertilization, pastures are initially productive, but productivity gradually declines a process known as pasture degradation. Rates of pasture degradation appear to be greater under other B. humidicola than under other Brachiaria species. The objectives of the experiment were to examine the response of the plants to N fertilizer (added as ammonium-N) and the transformations which occur in the soil mineral N pool following the addition of N to pots in which the three species were grown. MATERIAL AND METHODS Sixty pots containing 300 g of a clay cerrados soil (dark-red Latosol with 70 per cent clay, 15 per cent
C. H. B. Miranda et al.
334
sand, 0.12 per cent N, pH 5 .1) collected from an area under native pasture near Brasilia, Brazil, were prepared for this experiment. Each pot received a basic fertilization of P, K, Ca, Mg, B, Zn and Mo. The soil was moistened to 30 per cent moisture content (weight basis) and incubated for two weeks in a growthchamber, at 25 °C and 12 h of light per day. Settings were kept constant throughout the experimental period, the moisture content being restored daily. After two weeks, three seedlings of the Brazilian commercial cultivars of either B. decumbens, B. humidicola, or B. brizantha, pre-germinated in perlite, were transplanted to each pot, with twenty replications of each species. Sixty days after planting, four pots of each species were harvested and a solution containing 45 mg of N as (NH4 hS0 4 enriched with 4.524 atom per cent 15 N excess was added to the remaining pots. Thereafter, four replications of each species were harvested 3, 6, 12, and 24 days after the N addition. At the time of the harvest, a soil core was carefully extracted (using a PVC cylinder of 25 mm diameter and 150 mm height), and incubated aerobically for 7 days before mineral N extraction. At the same time, soil samples were collected around the hole made by the cylinder for a direct extraction of the mineral N, and measurements of soil moisture, soil pH, soil total N, and soil 15 N enrichment. The soil mineral N was extracted by transferring 30 g of soil to a flask, adding 150 ml of a 1M KCl solution, and shaking the mixture for two hours. The solution was then filtered using glass-fibre filter paper (previously washed with KCl). Ammonium-N was determined by the salicylate method, and nitrate-N was determined by an automatic colorimetric method, as described by Keeney and Nelson (1982). The total soil mineral N was considered as the sum of both ammonium-N and nitrate-N fractions. Two subsamples of the air-dried soil were directly analyzed for total N content and 15N enrichment using a CN auto-analyzer (Roboprep CN, Europa Scientific Instruments) linked to a Micromass 622 mass spectrometer (VG Isogas). After soil sampling, the shoots and roots were separately collected, dried for 3 days at 65 °C, weighed and ground in a roller mill and directly analyzed for N content and 15N enrichment, as described above.
RESULTS AND DISCUSSION The shoot dry weight of B. decumbens increased linearly from the day of the N addition, whilst there was a measurable increase in that of B. humidicola only after six days (Figure 1). B. brizantha increased dry weight at a similar rate to B. decumbens over the first four harvests, but growth rate was reduced
between the last two harvests. Root growth response was delayed in relation to shoot growth in all species on the first two harvests, but thereafter showed a tendency to increase.
4.5
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Figure 1. Dry 11111tter (g per pot) and total N (mg per pot) of shoots and roots of B. decumbens. B. humidicola, and B. brizantha (n = 4 ± SEM).
B. decumbens and B. brizantha were initially much more aggressive than B. humidicola with respect to fertilizer N uptake and their N contents increased sharply after the fertilizer addition. The N content of B. decumbens then remained constant over the last three harvests, whilst that of B. brizantha decreased. Root N contents were similar between species but again B. humidicola showed the smallest increase in N content. The species also showed a different behaviour regarding the uptake of native soil N. B. humidicola took up N mainly from the fertilizer during the experimental period, maintaining a constant N content derived from the soil (Table 1). B. brizantha also used mainly fertilizer N, reducing its content of N derived from the soil by the last harvest. B. decumbens, constantly increased the uptake of N from the soil during the experiment, suggesting a stimulatory effect of the fertilizer, probably due to a priming effect. Ew: J. Agron.
Mineral nitrogen in a Brazilian oxisol in the presence of Brachiaria spp.
335
Table 1. Plant N derived from the applied fertilizer (45 mg per pot applied at day 0) or from the native soil N (mg per pot) and total recovery of the applied N on the plant-soil system (n = 4 ± SEM).
Days after N addition
0
3
6
12
24
15.8 ± 2.5 5.6 ± 1.1 11.8 ± 0.6
21.0 ± 0.6 9.3 ± 1.3 18.6 ± 0.9
21.2 ± 0.8 16.8 ± 0.9 21.1 ± 0.4
20.6 ± 0.5 19.0 ± 1.8 17.7 ± 1.6
13.9 ± 1.3 17.0 ± 1.3 20.4 ± 0.9
15.8 ± 0.2 17.4 ± 1.4 19.8 ± 0.6
18.7 ± 0.4 17.9 ± 1.8 18.0 ± 1.1
20.6 ± 0.5 18.9 ± 0.9 16.1 ± 0.8
B. decumbens
45.2 ± 2.5
B. humidicola B. brizantha
41.6 ± 0.8 44.3 ± 0.6
44.2 ± 0.6 41.4 ± 0.8 43.1 ± 0.9
41.4 ± 1.2 41.5 ± 1.5 42.6 ± 0.6
41.2 ± 0.6 40.5 ± 0.9 40.1 ± 1.7
Plant N derived from the fertilizer N (mg per pot)
B. decumbens B. humidicola B. brizantha Plant N derived from the soil (mg per pot)
B. decumbens B. humidicola B. brizantha
12.4 ± 0.8 15.7 ± 1.1 19.8 ± 0.7
Total plant and soil N derived from the fertilizer N (mg per pot)
The balance between the N inputs to the system (applied N plus initial mineral N) and theN outputs in the following harvest (total N accumulated in the plants plus soil mineral N extracted) provides a reliable indication of the transformations occurring in the soil mineral N pool. This balance can be made for each period between two consecutive harvests, considering as inputs the concentration of soil mineral N extracted in the previous harvest, and as outputs the variation in plant N content during the same period. If the balance is positive it is due to de novo N production, as a result of native organic N mineralization ; if the balance is negative it would be the result of mineral N immobilization. To obtain comparable values the results were divided by the length of the period between harvests. Making this calculation based on the total increase in plant N uptake (N from the soil plus N from the fertilizer - Table 1) and total soil mineral N (sum of the ammonium-N and nitrate-N concentrations at the harvests - Table 2), a negative balance was recorded for B. decumbens in the first two harvests (- 5.5 and - 0.5 mg N poC 1 day- 1). In the last two harvests a small positive balance was observed (0.3 and 0.1 mg N pot- 1 day- 1 ). A similar result was observed for B. brizantha, indicating that immobilization was the dominant N transformation in the soil under these two species, which was confirmed by the results of incubated soils samples. For B. humidicola a different picture was obtained. The balance was negative for the first period (- 1.9 mg N pot- 1 day- 1), but highly positive in the following interval (9.5 mg N por- 1 day- 1), indicating a strong mineralization of the soil organic N from day 3 to 6. The occurrence of mineralization was confirmed Vol. 3, U 0 4- 1994
by the increase in the ammonium-N fraction. During the following periods the balance was negative (-5.2 and -0.8 mg N por- 1 day- 1), indicating dominance of immobilization, following the pattern of the other two species. There was a small increase in the nitrate-N concentrations at the two final harvests of B. brizantha and more significantly in the two intermediate harvests of B. humidicola, suggesting that nitrification was active in the presence of the plants. No variations were observed under B. decumbens. Since this species was the only one actively taking N from the soil as well as the fertilizer N, it could indeed include any nitrate-N formed, thus not accumulating enough nitrate-N to detect nitrification. Considering the ammonium-N and nitrate-N fractions in the soil after seven days of incubation it can be seen that in the soil taken from B. humidicola, nitrification was stimulated (Table 2). This was indicated by the increase of nitrate-N during the incubation period at all harvests except the last one, where the amount of ammonium-N in the soil at the harvest was perhaps not enough to stimulate nitrification. The high rate of nitrification under B. humidicola at the harvest on day 3 seemed to be due, at least in part, to the slow uptake of N by the plant, thus leaving large concentrations of ammonium in the soil. It is known that nitrification is inducible by substrate availability (Prosser, 1986). At the same time the nitrifiers seemed to be particularly active in that period, since a proportional concentration of the ammonium-N that disappeared was recovered as nitrate-N. At the next harvest the nitrifiers seemed to have lost the competition for the available ammonium-N, since the nitrate produc-
C. H. B. Miranda et al.
336
Table 2. Concentrations of ammonium-N and nitrate-N (mg por 1 ) at harvest or after seven days of incubation of the soil under B. decumbens, B. humidicola, and B. brizantha (n = 4 ± SEM).
Days after N addition
B. decumbens NH4 at harvest NH4 after incubation Variation N0 3 at harvest N0 3 after incubation Variation
0
3
6
12
24
0.4 ± 0.0 0.7 ± 0.1 0.3 0.7 ± 0.1 0.1 ± 0.0 -0.6
11.5 ± 2.8 13.0 ± 3.1 1.5 ± 0.1 0.8 ± 0.1 -0.7
3.2 ± 0.8 2.5 ± 0.4 -0.7 1.0 ± 0.2 2.8 ± 0.1 1.8
0.8 ± 0.1 2.7 ± 0.2 0.9 1.5 ± 0.1 1.7 ± 0.1 0.2
1.4 ± 0.1 1.0 ± 0.1 -0.4 1.2 ± 0.1 1.0 ± 0.1 -0.2
0.5 ± 0.1 0.7 ± 0.1 0.2 0.8 ± 0.1 0.9 ± 0.1 0.1
32.7 ± 0.2 15.5 ± 0.6 -17.1 1.0 ± 0.2 16.7 ± 0.6 15.7
51.1 ± 0.1 11.0 ± 0.1 -40.1 6.7 ± 1.4 8.3 ± 1.4 1.6
14.9 13.4 3.7 6.5
± 0.3
2.8
1.7 ± 0.3 0.6 ± 0.1 -1.1 1.3 ± 0.1 1.2 ± 0.1 -0.1
0.5 ± 0.1 0.7 ± 0.1 0.2 1.0 ± 0.1 0.1 ± 0.0 -0.9
17.3 ± 3.1 16.2 ± 2.4 -1.1 0.4 ± 0.1 0.8 ± 0.1 0.4
3.0 ± 0.6 3.2 ± 0.6 0.2 0.7 ± 0.2 2.9 ± 0.3 2.2
2.0 ± 0.1 3.6 ± 0.2 1.6 1.6 ± 0.1 2.4 ± 0.2 0.8
1.7 ± 0.3 0.8 ± 0.1 -0.9 1.2 ± 0.1 1.1 ± 0.1 -0.1
1.5
B. humidico1a NH4 at harvest NH4 after incubation
Variation N0 3 at harvest N0 3 after incubation
Variation B. brizantha NH4 at harvest NH4 after incubation Variation N0 3 at harvest N0 3 after incubation Variation
± 0.7
1.5 ± 0.3 ± 0.4
Variation in NH4 (or N0 3 ) = concentration after incubation - concentration at harvest.
tion was now less than 5 per cent of the ammonium available at the beginning of the incubation. In the soil under the other two species, which were initially very aggressive in N uptake, less ammonium-N was available, but the amounts present should still have been enough to induce nitrification. However, there were no clear signs of early nitrification (day 3) under these species. Some nitrate was formed at later harvests (days 6 and 12) perhaps suggesting that the build-up of the nitrifier population was delayed under these species. In the harvest at day 3 there was complete recovery of the applied N in the B. decumbens and B. brizantha plant and soil systems (Table 1), with some losses recorded under B. humidicola. In the following harvests around 10 per cent of the applied N was missing from the plant-soil system. Since the moisture was strictly controlled, losses from leaching could be disregarded ; ammonia volatilization probably did not occur either, since the soil became acidic as a result of the N uptake (data not shown). Some of the lost N may have been present in the very small roots, which were impossible to collect, and which were excluded from the soil analysis. However, this may not fully account for all the missing N, and it is probable that
the main losses were the result of denitrification. Denitrification as the main pathway of N losses could explain the early lower total fertilizer recovery of B. humidicola (Table 1). This species had a much higher initial nitrification than the other ones, hence more nitrate-N available for denitrification. This could also explain the low recovery of nitrate-N observed in soil under this species in the field when compared with other Brachiaria spp., as reported by SylvesterBradley et al. ( 1985). Based on the overall results, it can be concluded that the species studied had different patterns of N uptake and assimilation, mainly related to the speed of such processes. These features indirectly influenced the soil N dynamics, as differences in the available soil mineral N stimulated the growth of soil microorganisms differently. It is possible that the species studied excrete different amounts of substrate, or support a distinct microftora in their rhizosphere, since a different pattern of mineralization/immobilization of the organic soil N was observed among the species tested. The variations observed in the ammonium-N and nitrate-N both in the presence of active plants or in soil incubated in the absence of the plants are complex and analysis of the effects of plants on mineral N Eur. J. Agron.
Mineral nitrogen in a Brazilian oxisol in the presence of Brachiaria spp.
337
transformations based on a few harvests or over long time intervals can be misleading.
Keeney D. R. and Nelson D. W. (1982). Nitrogen - inorganic forms. In : Methods of soil analysis. Madison : American Society of Agronomy, 2, pp. 643-698.
ACKNOWLEDGEMENTS
Prosser J. I. (1986). Experimental and theoretical models of nitrification. In : Nitrification. Special Publications of the General Society for General Microbiology, 20, pp. 63-78
C. Miranda and S. Urquiaga gratefully acknowledge a scholarship assistance from the Brazilian National Council of Research (CNPq).
Sylvester-Bradley R., Mosquera D. and Mendez J. E. (1988). Inhibition of nitrate accumulation in tropical grassland soil : effect of nitrogen fertilization and soil disturbance. J. Soil Sci., 39, 407-416.
REFERENCES
Wheatley R., Ritz K. and Griffiths B. (1990). Microbial biomass and mineral N transformations in soil planted with barley, ryegrass, pea or turnip. Plant Soil, 127, 157-167.
Griffiths B. and Robinson D. (1992). Root-induced nitrogen mineralization : a nitrogen balance model. Plant Soil, 139, 253-263.
Whipps J. M. and Lynch J. M. (1985). Energy losses by the plant in rhizodeposition. Ann. Proc. Phytochem. Soc. Eur., 26, 59-71.
Vol. 3, n" 4- 1994
Mineral nitrogen in an oxisol from the Brazilian cerrados in the presence of Brachiaria spp. C. H. B. Miranda
1
,
G. Cadisch 3 , S. Urquiaga 2 C. H. B. Miranda 3 , R. M. Boddey 2 and K. E. Giller3 1
EMBRAPA-CNPGC. CP 154. 79100, Campo Grande, MS, Brazil. EMBRAPA-CNPAB. Km 47, 23851-970, Seropedica, ltaguai, RJ, Brazil. H'ye College, University of London. H'ye, Ashford, Kent TN25 5AH, England. 2
3
Received 3 May 1993; accepted 19 April 1994
Abstract
The effects of commercial cultivars of Brachiaria decumbens, B. humidicola, and B. brizantha on nitrogen mineralization and nitrification were examined in a Dark Red Latosol from the Cerrados region of Central Brazil. B. decumbens responded most rapidly to the addition of fertilizer N. B. brizantha also absorbed the fertilizer N rapidly but grew more slowly. By contrast, B. humidicola showed a slower response to the fertilizer addition, both in N uptake and growth. The concentrations of mineral N in the soil under both B. decumbens and B. brizantha indicated that most of the fertilizer N remaining in the soil was quickly immobilized, as also indicated by the results of incubated soil samples. On the other hand, the soil under B. humidicola showed strong mineralization of native organic N between three and six days after the addition of N in the presence of the plants, but not during the incubation. In this species there was also a clear indication of early nitrification, whereas in the soil under the other two species the build up of the nitrifier population seemed to be delayed. It is concluded that the species studied had different rates of N uptake and assimilation and stimulated microorganisms differently in their rhizosphere, which lead to contrasting transformations on the soil mineral N pool. The observed patterns of mineralization/immobilization and nitrification changed so quickly that analysis of the effects of plants on soil N transformations based on a few harvests or over long time intervals could be misleading. Key-words : tropical grasses, tropical soil, pasture, nitrogen mineralization, nitrification, soil incubation, plant N uptake, 15 N, Brachiaria spp.
INTRODUCTION Nitrogen fertilizer is rarely added to pastures in the Brazilian cerrados area and the use of pasture legumes is not common. Consequently, the grasses are dependent on the soil available N, relying on an efficient natural recycling of N. There are some studies suggesting that plants could stimulate net mineralization of soil organic N, through the exudation of carbon compounds from their roots (Wheatley et al., 1990; Whipps and Lynch, 1985). A species with such a potential would thus have an advantage in obtaining otherwise limiting soil N, as in the cerrados soils. The significance of such a process is still controversial and, as stated by Griffiths and Robinson (1992), it is unlikely a plant would obtain any benefit from such a process, since the theoretical gains would be small. ISSN l/61-0301/941041$ 4.001© Gauthier-Villars - ESAg
In this paper we present the results of an experiment conducted with three commercial cultivars of Brachiaria spp., the most common grasses used in the Brazilian cerrados area. When these species are introduced together with P fertilization, pastures are initially productive, but productivity gradually declines a process known as pasture degradation. Rates of pasture degradation appear to be greater under other B. humidicola than under other Brachiaria species. The objectives of the experiment were to examine the response of the plants to N fertilizer (added as ammonium-N) and the transformations which occur in the soil mineral N pool following the addition of N to pots in which the three species were grown. MATERIAL AND METHODS Sixty pots containing 300 g of a clay cerrados soil (dark-red Latosol with 70 per cent clay, 15 per cent
C. H. B. Miranda et al.
334
sand, 0.12 per cent N, pH 5 .1) collected from an area under native pasture near Brasilia, Brazil, were prepared for this experiment. Each pot received a basic fertilization of P, K, Ca, Mg, B, Zn and Mo. The soil was moistened to 30 per cent moisture content (weight basis) and incubated for two weeks in a growthchamber, at 25 °C and 12 h of light per day. Settings were kept constant throughout the experimental period, the moisture content being restored daily. After two weeks, three seedlings of the Brazilian commercial cultivars of either B. decumbens, B. humidicola, or B. brizantha, pre-germinated in perlite, were transplanted to each pot, with twenty replications of each species. Sixty days after planting, four pots of each species were harvested and a solution containing 45 mg of N as (NH4 hS0 4 enriched with 4.524 atom per cent 15 N excess was added to the remaining pots. Thereafter, four replications of each species were harvested 3, 6, 12, and 24 days after the N addition. At the time of the harvest, a soil core was carefully extracted (using a PVC cylinder of 25 mm diameter and 150 mm height), and incubated aerobically for 7 days before mineral N extraction. At the same time, soil samples were collected around the hole made by the cylinder for a direct extraction of the mineral N, and measurements of soil moisture, soil pH, soil total N, and soil 15 N enrichment. The soil mineral N was extracted by transferring 30 g of soil to a flask, adding 150 ml of a 1M KCl solution, and shaking the mixture for two hours. The solution was then filtered using glass-fibre filter paper (previously washed with KCl). Ammonium-N was determined by the salicylate method, and nitrate-N was determined by an automatic colorimetric method, as described by Keeney and Nelson (1982). The total soil mineral N was considered as the sum of both ammonium-N and nitrate-N fractions. Two subsamples of the air-dried soil were directly analyzed for total N content and 15N enrichment using a CN auto-analyzer (Roboprep CN, Europa Scientific Instruments) linked to a Micromass 622 mass spectrometer (VG Isogas). After soil sampling, the shoots and roots were separately collected, dried for 3 days at 65 °C, weighed and ground in a roller mill and directly analyzed for N content and 15N enrichment, as described above.
RESULTS AND DISCUSSION The shoot dry weight of B. decumbens increased linearly from the day of the N addition, whilst there was a measurable increase in that of B. humidicola only after six days (Figure 1). B. brizantha increased dry weight at a similar rate to B. decumbens over the first four harvests, but growth rate was reduced
between the last two harvests. Root growth response was delayed in relation to shoot growth in all species on the first two harvests, but thereafter showed a tendency to increase.
4.5
4.5 ohoot
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15
11!1
21
24
after N addition
Figure 1. Dry 11111tter (g per pot) and total N (mg per pot) of shoots and roots of B. decumbens. B. humidicola, and B. brizantha (n = 4 ± SEM).
B. decumbens and B. brizantha were initially much more aggressive than B. humidicola with respect to fertilizer N uptake and their N contents increased sharply after the fertilizer addition. The N content of B. decumbens then remained constant over the last three harvests, whilst that of B. brizantha decreased. Root N contents were similar between species but again B. humidicola showed the smallest increase in N content. The species also showed a different behaviour regarding the uptake of native soil N. B. humidicola took up N mainly from the fertilizer during the experimental period, maintaining a constant N content derived from the soil (Table 1). B. brizantha also used mainly fertilizer N, reducing its content of N derived from the soil by the last harvest. B. decumbens, constantly increased the uptake of N from the soil during the experiment, suggesting a stimulatory effect of the fertilizer, probably due to a priming effect. Ew: J. Agron.
Mineral nitrogen in a Brazilian oxisol in the presence of Brachiaria spp.
335
Table 1. Plant N derived from the applied fertilizer (45 mg per pot applied at day 0) or from the native soil N (mg per pot) and total recovery of the applied N on the plant-soil system (n = 4 ± SEM).
Days after N addition
0
3
6
12
24
15.8 ± 2.5 5.6 ± 1.1 11.8 ± 0.6
21.0 ± 0.6 9.3 ± 1.3 18.6 ± 0.9
21.2 ± 0.8 16.8 ± 0.9 21.1 ± 0.4
20.6 ± 0.5 19.0 ± 1.8 17.7 ± 1.6
13.9 ± 1.3 17.0 ± 1.3 20.4 ± 0.9
15.8 ± 0.2 17.4 ± 1.4 19.8 ± 0.6
18.7 ± 0.4 17.9 ± 1.8 18.0 ± 1.1
20.6 ± 0.5 18.9 ± 0.9 16.1 ± 0.8
B. decumbens
45.2 ± 2.5
B. humidicola B. brizantha
41.6 ± 0.8 44.3 ± 0.6
44.2 ± 0.6 41.4 ± 0.8 43.1 ± 0.9
41.4 ± 1.2 41.5 ± 1.5 42.6 ± 0.6
41.2 ± 0.6 40.5 ± 0.9 40.1 ± 1.7
Plant N derived from the fertilizer N (mg per pot)
B. decumbens B. humidicola B. brizantha Plant N derived from the soil (mg per pot)
B. decumbens B. humidicola B. brizantha
12.4 ± 0.8 15.7 ± 1.1 19.8 ± 0.7
Total plant and soil N derived from the fertilizer N (mg per pot)
The balance between the N inputs to the system (applied N plus initial mineral N) and theN outputs in the following harvest (total N accumulated in the plants plus soil mineral N extracted) provides a reliable indication of the transformations occurring in the soil mineral N pool. This balance can be made for each period between two consecutive harvests, considering as inputs the concentration of soil mineral N extracted in the previous harvest, and as outputs the variation in plant N content during the same period. If the balance is positive it is due to de novo N production, as a result of native organic N mineralization ; if the balance is negative it would be the result of mineral N immobilization. To obtain comparable values the results were divided by the length of the period between harvests. Making this calculation based on the total increase in plant N uptake (N from the soil plus N from the fertilizer - Table 1) and total soil mineral N (sum of the ammonium-N and nitrate-N concentrations at the harvests - Table 2), a negative balance was recorded for B. decumbens in the first two harvests (- 5.5 and - 0.5 mg N poC 1 day- 1). In the last two harvests a small positive balance was observed (0.3 and 0.1 mg N pot- 1 day- 1 ). A similar result was observed for B. brizantha, indicating that immobilization was the dominant N transformation in the soil under these two species, which was confirmed by the results of incubated soils samples. For B. humidicola a different picture was obtained. The balance was negative for the first period (- 1.9 mg N pot- 1 day- 1), but highly positive in the following interval (9.5 mg N por- 1 day- 1), indicating a strong mineralization of the soil organic N from day 3 to 6. The occurrence of mineralization was confirmed Vol. 3, U 0 4- 1994
by the increase in the ammonium-N fraction. During the following periods the balance was negative (-5.2 and -0.8 mg N por- 1 day- 1), indicating dominance of immobilization, following the pattern of the other two species. There was a small increase in the nitrate-N concentrations at the two final harvests of B. brizantha and more significantly in the two intermediate harvests of B. humidicola, suggesting that nitrification was active in the presence of the plants. No variations were observed under B. decumbens. Since this species was the only one actively taking N from the soil as well as the fertilizer N, it could indeed include any nitrate-N formed, thus not accumulating enough nitrate-N to detect nitrification. Considering the ammonium-N and nitrate-N fractions in the soil after seven days of incubation it can be seen that in the soil taken from B. humidicola, nitrification was stimulated (Table 2). This was indicated by the increase of nitrate-N during the incubation period at all harvests except the last one, where the amount of ammonium-N in the soil at the harvest was perhaps not enough to stimulate nitrification. The high rate of nitrification under B. humidicola at the harvest on day 3 seemed to be due, at least in part, to the slow uptake of N by the plant, thus leaving large concentrations of ammonium in the soil. It is known that nitrification is inducible by substrate availability (Prosser, 1986). At the same time the nitrifiers seemed to be particularly active in that period, since a proportional concentration of the ammonium-N that disappeared was recovered as nitrate-N. At the next harvest the nitrifiers seemed to have lost the competition for the available ammonium-N, since the nitrate produc-
C. H. B. Miranda et al.
336
Table 2. Concentrations of ammonium-N and nitrate-N (mg por 1 ) at harvest or after seven days of incubation of the soil under B. decumbens, B. humidicola, and B. brizantha (n = 4 ± SEM).
Days after N addition
B. decumbens NH4 at harvest NH4 after incubation Variation N0 3 at harvest N0 3 after incubation Variation
0
3
6
12
24
0.4 ± 0.0 0.7 ± 0.1 0.3 0.7 ± 0.1 0.1 ± 0.0 -0.6
11.5 ± 2.8 13.0 ± 3.1 1.5 ± 0.1 0.8 ± 0.1 -0.7
3.2 ± 0.8 2.5 ± 0.4 -0.7 1.0 ± 0.2 2.8 ± 0.1 1.8
0.8 ± 0.1 2.7 ± 0.2 0.9 1.5 ± 0.1 1.7 ± 0.1 0.2
1.4 ± 0.1 1.0 ± 0.1 -0.4 1.2 ± 0.1 1.0 ± 0.1 -0.2
0.5 ± 0.1 0.7 ± 0.1 0.2 0.8 ± 0.1 0.9 ± 0.1 0.1
32.7 ± 0.2 15.5 ± 0.6 -17.1 1.0 ± 0.2 16.7 ± 0.6 15.7
51.1 ± 0.1 11.0 ± 0.1 -40.1 6.7 ± 1.4 8.3 ± 1.4 1.6
14.9 13.4 3.7 6.5
± 0.3
2.8
1.7 ± 0.3 0.6 ± 0.1 -1.1 1.3 ± 0.1 1.2 ± 0.1 -0.1
0.5 ± 0.1 0.7 ± 0.1 0.2 1.0 ± 0.1 0.1 ± 0.0 -0.9
17.3 ± 3.1 16.2 ± 2.4 -1.1 0.4 ± 0.1 0.8 ± 0.1 0.4
3.0 ± 0.6 3.2 ± 0.6 0.2 0.7 ± 0.2 2.9 ± 0.3 2.2
2.0 ± 0.1 3.6 ± 0.2 1.6 1.6 ± 0.1 2.4 ± 0.2 0.8
1.7 ± 0.3 0.8 ± 0.1 -0.9 1.2 ± 0.1 1.1 ± 0.1 -0.1
1.5
B. humidico1a NH4 at harvest NH4 after incubation
Variation N0 3 at harvest N0 3 after incubation
Variation B. brizantha NH4 at harvest NH4 after incubation Variation N0 3 at harvest N0 3 after incubation Variation
± 0.7
1.5 ± 0.3 ± 0.4
Variation in NH4 (or N0 3 ) = concentration after incubation - concentration at harvest.
tion was now less than 5 per cent of the ammonium available at the beginning of the incubation. In the soil under the other two species, which were initially very aggressive in N uptake, less ammonium-N was available, but the amounts present should still have been enough to induce nitrification. However, there were no clear signs of early nitrification (day 3) under these species. Some nitrate was formed at later harvests (days 6 and 12) perhaps suggesting that the build-up of the nitrifier population was delayed under these species. In the harvest at day 3 there was complete recovery of the applied N in the B. decumbens and B. brizantha plant and soil systems (Table 1), with some losses recorded under B. humidicola. In the following harvests around 10 per cent of the applied N was missing from the plant-soil system. Since the moisture was strictly controlled, losses from leaching could be disregarded ; ammonia volatilization probably did not occur either, since the soil became acidic as a result of the N uptake (data not shown). Some of the lost N may have been present in the very small roots, which were impossible to collect, and which were excluded from the soil analysis. However, this may not fully account for all the missing N, and it is probable that
the main losses were the result of denitrification. Denitrification as the main pathway of N losses could explain the early lower total fertilizer recovery of B. humidicola (Table 1). This species had a much higher initial nitrification than the other ones, hence more nitrate-N available for denitrification. This could also explain the low recovery of nitrate-N observed in soil under this species in the field when compared with other Brachiaria spp., as reported by SylvesterBradley et al. ( 1985). Based on the overall results, it can be concluded that the species studied had different patterns of N uptake and assimilation, mainly related to the speed of such processes. These features indirectly influenced the soil N dynamics, as differences in the available soil mineral N stimulated the growth of soil microorganisms differently. It is possible that the species studied excrete different amounts of substrate, or support a distinct microftora in their rhizosphere, since a different pattern of mineralization/immobilization of the organic soil N was observed among the species tested. The variations observed in the ammonium-N and nitrate-N both in the presence of active plants or in soil incubated in the absence of the plants are complex and analysis of the effects of plants on mineral N Eur. J. Agron.
Mineral nitrogen in a Brazilian oxisol in the presence of Brachiaria spp.
337
transformations based on a few harvests or over long time intervals can be misleading.
Keeney D. R. and Nelson D. W. (1982). Nitrogen - inorganic forms. In : Methods of soil analysis. Madison : American Society of Agronomy, 2, pp. 643-698.
ACKNOWLEDGEMENTS
Prosser J. I. (1986). Experimental and theoretical models of nitrification. In : Nitrification. Special Publications of the General Society for General Microbiology, 20, pp. 63-78
C. Miranda and S. Urquiaga gratefully acknowledge a scholarship assistance from the Brazilian National Council of Research (CNPq).
Sylvester-Bradley R., Mosquera D. and Mendez J. E. (1988). Inhibition of nitrate accumulation in tropical grassland soil : effect of nitrogen fertilization and soil disturbance. J. Soil Sci., 39, 407-416.
REFERENCES
Wheatley R., Ritz K. and Griffiths B. (1990). Microbial biomass and mineral N transformations in soil planted with barley, ryegrass, pea or turnip. Plant Soil, 127, 157-167.
Griffiths B. and Robinson D. (1992). Root-induced nitrogen mineralization : a nitrogen balance model. Plant Soil, 139, 253-263.
Whipps J. M. and Lynch J. M. (1985). Energy losses by the plant in rhizodeposition. Ann. Proc. Phytochem. Soc. Eur., 26, 59-71.
Vol. 3, n" 4- 1994