African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 8, Num. 3, 2000, pp. 327-336

African Crop Science Journal, Vol. 8. No. 3, pp. 327-336

SHORT COMMUNICATION

EFFECT OF LIME, urea and triple super phosphate on NITROGEN AND PHOSPHORUS MINERALISATION IN AN ACID SOIL DURING INCUBATION

J.J. LELEI, B.O. MOCHOGE and R.N. ONWONGA
Egerton University, Soil Science Department, P.O. Box 536, Njoro, Kenya

(Received 29 July, 1999; accepted 4 February, 2000)

Code Number: CS00035

INTRODUCTION

A significant portion of soil N and P occurs in organic forms that are not available for crop uptake unless mineralised ( Stevenson, 1986; Tisdale et al., 1990). Slowed nitrification rates (Weber and Gainey, 1962) and P fixation (Stevenson, 1986) greatly hinder the conversion of organic N and P to their mineral forms in acid soils. Other factors include soil temperature, moisture, pH, fertiliser additions and the C/N ratio of the organic material (Dalal, 1977; Jansson and Persson, 1982; Hendrickson, 1985).

Organic N and P mineralisation in acid soils are stimulated mainly through liming (Dalal, 1977; Kamprath and Foy, 1985; Hue, 1989) and/or P fertiliser application (Dalal, 1977; Evans, 1985). Liming raises soil pH, thereby creating favourable conditions for microbial growth, especially nitrifyers and actinomycetes (Anderson and Domesch, 1980), and decreases the solubility of Al- and Fe-hydroxides but increases the solubility of Al- and Fe-phosphates. After P application, there is competition between inorganic and organic P compounds for soil sorption sites resulting in a substantial increase in dissolved organic P (Evans, 1985).

Laboratory incubation experiments are a convenient way of quantifying and studying the N mineralisation processes (Bremner, 1965a; Keeney, 1982). Bremner (1965b) and Keeney (1982) found that incubation of soil under favourable conditions provides a rational measure of N availability. This is because the agents responsible for release of mineral N during incubation are the same ones which avail N from the organic soil pool for crop growth during the growing season. Lathwell et al. (1972) also found that N produced during incubation was highly correlated with N released to crops in the field. This method (Laboratory incubation) is however, unsatisfactory because either several simultaneous occuring processes are measured (in situ net mineralisation rate) or they establish potential nitrification rates rather than actual rates (Woldendorp and Laanbroek, 1989). There is scanty literature on the behaviour of P mineralisation under laboratory incubation. Nevertheless, the factors controlling P mineralisation are more or less the same as those of N (Vaughan and Malcolm, 1985).

The purpose of this study, therefore, was to investigate the effect of various soil amendments on N and P mineralisation in an acid soil through laboratory incubation.

MATERIALS AND METHODS

Soil sampling. The soil for the study (Table 1) was obtained from Kenya Agricultural Research Institute (KARI), located 5 km from Molo Town, Nakuru District. The research station (0°12¹S, 35°41¹E) is at an elevation of 2500 m above sea level (Jaetzold and Schmidt, 1982). The soils are well-drained, deep, dark reddish brown in colour with a mollic A horizon and are classified as mollic Andosols (FAO/UNESCO, 1990). The field had been under grass (Cynodon dactylon) and maize stubble for a year from the previous crop. The soil was sampled from six profile pits at three depths; 0-15, 15-30 and 30-60 cm. To avoid mixing of soil from the three depths, sampling was started from the 30-60 cm depth upwards. The samples, from the six profile pits, were mixed according to their respective depths and one composite sample for each depth was obtained.

Incubation procedure. Two kg of soil from each composite was weighed in triplicate and treated with 2.5 t lime ha^-1, 75 kg P ha^-1 as tripple superphosphate (TSP) and 50 kg N ha^-1 as urea. Each treatment was replicated thrice. A control was also included. The lime and fertilisers were first dissolved in distilled water and then sprayed on the thin soil layers. The rates chosen were based on a study by Kamprath (1970), which showed that 1.65 t ha^-1 of lime is needed to neutralise 1.0 cmol kg^-1 of aluminium in the exchange complex and extension recomme-ndations for the area, respectively. Soil moisture of the treated samples was adjusted to field capacity (approximately 60% of the water holding capacity). The samples were then transferred to transparent polythene bags (30 x 40 cm), sealed and incubated in the laboratory at room temperature (19 - 22°C) for 120 days.

Chemical analysis. Soil for available-N (NH₄⁺-N and NO₃^--N) and P analysis was sampled from polythene bags of each replicate at 0, 15, 30, 60, 90, and 120 days of incubation. Available N and P in the soil samples were extracted with neutral 2 M KCl and double acid extractant (0.025M H₂SO₄ and 0.1M HCl), respectively. Their concentrations were determined according to Keeney and Nelson (1982) and Olsen and Dean (1965) procedures, respectively. N and P mineralised for urea and TSP treatments were obtained by subtracting the amount supplied by respective fertilisers from total mineralised N and P. The moisture content, bulk density (from undisturbed soil samples) and pH (H₂O) were also analysed according to Gardener (1965), Blake (1965) and in 1:2.5 soil to water suspension using a glass electrode pH meter, respectively.

Statistical analysis. For each treatment, mean values and standard deviations were calculated. The net mineralised N and P were calculated as the difference between available N and P in soil after and prior to the 120 days of incubation. The mineralisation rate constants were calculated by simple regression analysis. To determine differences among treatments with respect to net mineralised N and P during incubation, the student t-test (MSTAT-C, 1990) was used.

RESULTS

Soil N mineralisation. Cummulative N production in the control, lime, TSP and urea treatments during the incubation period is shown in Table 2. In all treatments, NH₄⁺-N was higher than NO₃^- -N in the first two to three weeks of incubation except for lime in the 0-15 and 15-30 cm depths. Thereafter, NO₃^-N concentration was high in all treatments and depths except for the 30-60 cm depth of the control treatment. The control and TSP treatments exhibited lag periods in the first two weeks of incubation (Table 2).

Lime application increased net mineral N production substantially compared to control, TSP and urea treatments (Table 3). Rates of N mineralisation were highest in lime and lowest in urea treatment. There was a gradient decrease of cummulative net mineralised N with depth for all treatments. Significant differences (P<0.05) in net mineral-N released (0-60 cm depth) were observed among the treatments (Table 3).

Soil P mineralisation. Table 4 shows cumulative mineralised P in the 0-15, 15-30 and 30-60 cm soil depths of the control, urea, TSP and lime treatments during incubation. P release was continuous with incubation time, but inconsistent in all depths and treatments.

TSP-treated samples had the highest cumulative net mineralised P followed by lime, control and urea in that order (Table 5). Rates of P mineralised per day were highest in TSP and least in urea treatment. Mineralisation rates were nearly constant in all depths of urea but differed from one depth to another without following any specific pattern in other treatments (Table 5).

DISCUSSION

Effect of lime, urea and TSP on soil N mineralisation. Higher levels of NH₄⁺-N than NO₃-N at the onset of incubation is attributable to soil acidity and high soil moisture regimes before sampling. The latter affects nitrification but not mineralisation (Tietema et al., 1992). The micro-organisms involved in mineralisation are many and can thrive in extreme soil conditions which is not the case with nitrifiers (Tisdale et al., 1990). The dominance of NH₄⁺-N throughout incubation in the 30-60 cm depth of control treatment could be attributed to low nitrifier activities. Nitrifiers are usually low at lower soil depths relative to the upper depths due to reduced oxygen availability (Tisdale et al., 1990; Guto, 1997).

Sudden change of environment before adaptation was the likely cause of the lag periods observed in control and TSP treatments during the first two weeks of incubation. This especially applies to chemoautotrophs which are quite sensitive to abrupt environmental changes. The increases, thereafter, were due to microbial adaptation. Stanford and Smith (1972) found relatively low rates of N mineralisation during the first four weeks of incubation. N mineralisation flushes as well as lag phases have also been observed during incubation experiments (Stanford et al., 1974). Limed soils did not exhibit lag phases during mineralisation probably due to favourable conditions for nitrifiers especially the rise of pH in some soil microsites that had been established. Lyngstad (1992) reported that liming increased nitrification rate and nearly all NH₄⁺-N was nitrified during the incubation period. This was the case in this study where decline in NH₄⁺-N corresponded to a rise in NO₃^--N (Table 2). Absence of lag periods in urea treatment could be attributable to the urea N which was readily available to the soil microoganisms for their energy and nutritional requirements at the initial stages of incubation (Tisdale et al., 1990).

The gradient decrease in mineral N with depth in all treatments could be due to decreasing organic matter with depth increase (Table 1). Other researchers also made similar observations (Odhiambo, 1989; Guto, 1997). Low concentration of NH₄⁺ - N in soil resulting from reduction of easily mineralisable organic matter with time could have led to gradual decrease in nitrification towards the end of incubation in all treatments and depths. Tietema et al. (1992) indicated that nitrification decreases with incubation time, and Dendooven et al. (1992) attributed this to the decrease in readily decomposable organic materials. The 0-15 cm depth of control treatment contributed only 42.7% of total net mineral - N in the soil profile. Thus, the soil’s acidic nature retarded N mineralisation due to depressed biological activity normally associated with this depth (Hendricks, 1992; Kirchner et al., 1993).

Application of urea fertiliser depressed the soil nitrification process (Tables 2 and 3). This could be due to production of H+ ions by urea fertiliser during nitrification and, consequently, conversion of soil organic N to its mineral forms was inhibited (Tisdale et al., 1990). Alternatively, this could be due to nitrite formation in acidic conditions which can escape during digestion and, therefore, not detectable by the Kjeldahl method (Peterson and Smith, 1982). Martikainen (1985) also attributed inhibition of nitrification upon salt application to decrease in soil pH. This concurred with our results for the pH values were lowest in urea treatment (Table 6). The results of this study, however, show that TSP application resulted to higher N mineralisation rates than urea application, indicating that P fertilisation favoured microbial activities in this soil. Other researchers reported similar observations (Virginia and Jarell, 1983; Simard et al., 1988).

Effect of lime, urea and TSP on soil P mineralisation. Higher levels of mineralised P in TSP than in other treatments could be attributed to saturation of P sorption sites upon addition of TSP which consequently availed excess P in soil solution. Evans (1985) reported that competition between inorganic and organic P for soil sorption sites could take place resulting in increased dissolved organic phosphorus directly after fertiliser application. The increase in cumulative mineralised P with incubation time for all treatments suggests that P was continually released into soil solution from the organic pool. Seeling and Zasoski (1993) found that solution organic P was continuously replenished after crop removal and concluded that, if P was not derived from microbial synthesis, at least solubilisation of soil organic P by micro-organisms must have occurred. Urea had a depressing effect on P mineralisation (Table 5). This is because it leaves an acidic residue in soil which lowers soil pH leading to reduced net P mineralisation (Tisdale et al., 1990). Lack of significant differences in mineralised P between control and lime (Table 5) may be attributed to reimmobilisation of P released in the latter by soil micro-organisms. Vaughan and Malcon (1985) reported that mineralised P may not be immediately available due to reimmo-bilisation by micro-organisms.

Rates of P mineralisation were higher in lower profile depths of the control and lime than in urea and TSP treatments (Table 5). This scenario was different from that of N, where mineralisation rates declined with increasing depth (Table 3). This could be due to increasing amounts of P with decreasing depth resulting from inorganic combinations compared to N which decreases with depth as a result of declining organic matter. Tisdale et al. (1990) reported that the inorganic fraction of soil phosphorus occurs in numerous combinations with Fe²⁺, Al³⁺, Ca²⁺, and other elements which facilitate its downward mobility. The lag phases observed in N mineralisation (control and TSP treatments) were not very visible under P implying that organisms involved in P mineralisation could thrive normally even upon changes due to experiment.

In conclusion, application of lime and TSP fertiliser to the acid soil enhanced N and P mineralisation, respectively but urea fertiliser depressed both. Field studies to support the current study are recommended.

ACKNOWLEDGEMENTS

The authors acknowledge the Board of Post Graduate Studies, Egerton University, for funding the research. The Department of Soil Science, Egerton University and Kenya Agricultural Research Institute, Molo are also acknowledged for providing facilities and soil used in the study.

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