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Published in J. Environ. Qual. 33:1855-1865 (2004).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Waste Management

Nitrogen, Phosphorus, and Potassium Uptake by Wheat and Their Distribution in Soil following Successive, Annual Compost Applications

A. Bar-Tala,*, U. Yermiyahub, J. Berauda,d, M. Keinana, R. Rosenberga, D. Zoharc, V. Rosena,e and P. Finea

a Department of Soil Chemistry and Plant Nutrition, Institute of Soil, Water and Environmental Sciences, the Volcani Center, Agricultural Research Organization, P.O.B. 6, Bet Dagan 50250, Israel
b Agricultural Research Organization, Gilat Research Center, D.N. Negev 85280, Israel
c Shaham, Ministry of Agriculture and Rural Development, Bet Dagan 50250, P.O.B. 6, Israel
d Present address: Réseau des Missions Déchets, APCA–Chambres d'Agriculture, 9, Avenue George V, F-75008 Paris, France
e Present address: Interdepartmental Equipment Unit, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot, Israel

* Corresponding author (abartal{at}agri.gov.il).

Received for publication August 7, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The overall objective of the present study was to determine the loading limits of composts that should be applied annually to irrigated wheat. We conducted a container experiment in a greenhouse during four years. It included eight treatments: sewage sludge compost (SSC) and cattle manure compost (CMC), each applied annually to a sandy soil, at rates equivalent to 3, 6, and 12 kg m–2, and two controls, one fertilized and one unfertilized. Total dry matter (DM), grain production, and the amount of N, P, and K taken up by plants increased with increasing compost rate. Nitrogen uptake by the plants of the fertilized control was much higher than by the plants of the highest compost rate. Phosphorus and K uptake by the plants amended with the highest compost rate was much higher than by the fertilized control plants. Inorganic N quantity in the soil increased with increasing compost rate and with successive applications. The net N mineralization during the first year of wheat growth was very low, less than 3.5% of the applied organic N under all compost application rates. The contribution of the organic N mineralization increased during the second and third years. Most of the N increase in the compost treatment was found in the upper layer of 0 to 15 cm, whereas in the fertilized treatment N accumulated from the surface to the bottom of the container, 0 to 55 cm. The successive application of high rates of composts resulted in P and K accumulation in the soil profile.

Abbreviations: CMC, cattle manure compost • DM, dry matter • SSC, sewage sludge compost


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
APPLICATION OF ORGANIC WASTES to arable soils is probably the most economical and environmentally sound solution to the problem of their disposal. Domestic sludge and compost disposal rates in agriculture depend on agronomic and environmental concerns. The USEPA published the 40 CFR Part 503 regulations (USEPA, 1993), which allow the beneficial use of sewage biosolids (sewage sludge) produced by municipal wastewater facilities as long as they are applied at the "agronomic rate" for a given crop. The agronomic rate is defined as the biosolids application rate that ensures that the amount of nitrogen required by the crops is supplied over a defined growth period, so that amount of nitrogen that passes below the root zone and into ground water is minimal. Cogger et al. (2001) showed that nutrients rather than toxicity due to metals control sustainable site management for repeated domestic biosolids applications. The same guidelines can be applied to other organic wastes, such as manure and composts. To follow these regulations it is necessary to know the rates of N supply required by the crop, and of N release from the soil and from the applied organic wastes.

Application of organic wastes in the form of composts is preferable to direct use of the raw material, since the former is a more hygienic and uniform product. However, the composting process stabilizes the available N and reduces the value of the biosolids (manure or sewage sludge) as N fertilizers (Castellanos and Pratt, 1981; Witter and Lopez-Real, 1987). The short- and long-term effects on N availability from the application of composts to soils have been studied for various crops in several different soils and climatic regions (Eriksen et al., 1999; Hyatt, 1995; Mamo et al., 1999; Sims, 1990). Soon after application, compost was found to have a positive effect on crops only when applied with additional N fertilizer; however, organic matter content and net N mineralization increased in compost-treated soils with time. Thus, compost application increased crop yields compared with those from untreated control plots (Hyatt, 1995). Net immobilization of N was found in the first year after application of various composts followed by net mineralization of N a year later (Eriksen et al., 1999; Mamo et al., 1999). Sims (1990) found that supplemental fertilizer N, in excess of that necessary to meet crop demand, might be required to prevent N deficiency that resulted from immobilization of soil mineral N by sewage sludge compost (SSC). Bernal et al. (1998) found a period of net immobilization of N during the first stage of SSC decomposition in soil; it lasted 3 and 70 d for mature and immature compost, respectively.

Preusch et al. (2002) showed that the composting process had no significant effect on P availability from broiler litter while N availability decreased. Sharpley et al. (1994) reported that soils amended with organic wastes and composts may be a source of P contamination of surface water. Accumulation and increased mobility of phosphorus in soils, following application of organic matter, were found by Eghball et al. (1996) and Whalen and Chang (2001). Therefore, the accumulation and leaching of P have to be considered in testing the load limits of composts on soils.

Potassium in organic wastes and composts is in mineral form and the total amount of K is equal to the water extracted (Hadas, personal communication, 2003). Mineral K is available for uptake by plants and for exchange on soil surface, fixation, and leaching (Mengel and Kirkby, 1978). Therefore, the concentration of K in the compost and the governing reactions in soil determine the quantity of compost required to meet K demand by the crop.

The overall objective of the present study was to estimate the loading limits of composts that should be applied annually to wheat under specific conditions. The specific objectives were to investigate the effects of annual applications of composts on (i) plant growth and grain production; (ii) the uptake of N, P, and K by the plants; and (iii) the distribution of N, P, and K in the soil profile.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experiment was conducted at Bet Dagan, Israel (35° E, 31° N; 50 m altitude), in a greenhouse. The soil was a Typic Rhodoxeralf, very sandy, 4% clay, 2% silt, and 94% sand, with 1.7% calcium carbonate, and pH 7.5 in saturated paste. The total organic matter content was 5.6 g kg–1, as measured by the dichromate oxidation–titration method (Nelson and Sommers, 1982). The total N content was 96 mg kg–1, as measured after semimicro-Kjeldahl digestion and colorimetric analysis in an autoanalyzer (Lachat Instruments, Milwaukee, WI).

The two types of compost were a commercial CMC and SSC from the Netanya sewage treatment plant. The composts were prepared in heaps of the organic residues mixed with unknown quantities of pruned branches to improve the aeration and to supply extra carbon. The heaps were mixed frequently to ensure that the process was aerobic and to avoid excessive heating and they were irrigated to maintain moisture content. The composting process was halted by air-drying the materials and the dried composts were stored in sealed containers. The SSC was a less mature compost than the CMC as can be deduced from its high NH4 content (Table 1). The pH and EC were measured in a 1:5 (weight basis) compost-to-water extract. Organic matter content in the compost was estimated by the same method as that used for the soil. Total N, P, and K contents were determined after digestion with H2SO4 and H2O2; N and P were measured colorimetrically with a Lachat autoanalyzer, and K was measured by flame spectrometry. Soluble NH4 and NO3 in the composts were measured with the same autoanalyzer after extraction with 1 M KCl, in a 1:5 (weight basis) compost to solution ratio. Calcium carbonates were estimated with a calcimeter in which carbonates were dissolved in 10% HCl and the volume of the evolved CO2 was measured.


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  		Table 1. Chemical analyses of the sewage sludge compost (SSC) and the cattle manure compost (CMC).

 
Greenhouse Container Experiment
The average weekly minimum and maximum air temperature ranged from 20 to 30°C and 12 to 18°C, respectively. The soil and composts were air-dried and ground to pass through a 2-mm sieve. At the beginning of the first year, December 1997, thirty-two 100-L plastic containers (diameter = 40 cm, height = 60 cm) were each filled with 80 kg of soil. The composts were applied to two successive years of wheat crops, then a third year of wheat crops after one year of break without planting, irrigation, or compost application. Six treatments included three application rates of each of the two composts of 325, 650, and 1300 g (dry weight) of compost per container, or an equivalent of 3, 6, and 12 kg m–2, respectively. Thus, a wide range of compost loads was obtained according to the main goal of the research. Two additional treatments were controls without composts: one was fertilized and one was unfertilized. A randomized design of the eight treatments in four blocks was applied: one container for each treatment in each block. The upper 15-cm soil layer of each container was taken out and compost was added to it and thoroughly mixed, according to one of the eight treatments (including the controls without composts), and then replaced on top of the remaining soil. The containers were irrigated until leaching began, which indicated the attainment of water-holding capacity. Wheat (Triticum aestivum L. cv. Ayalon) was then sown at 100 seeds per container (926 seeds m–2). Soil moisture was controlled according to weight loss and adjusted by daily manual irrigation with deionized water in the first two years, and by drip irrigation with tap water of electrical conductivity of 0.9 to 1.0 dS m–1 in the last year. The quantity of obtained drainage was negligible, less than 1% of the applied water. The fertilized control was irrigated with nutrient solutions (Table 2). The wheat was harvested 135 to 148 d after seeding, and the soil was left to dry.


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  		Table 2. Mineral compositions of the solutions of the fertilized control in the container experiment.

 
At the beginning of the second and third years, the top 15-cm soil layer was removed and mixed with an aliquot of compost identical to the one it had received. The soil mixed with compost was returned to the container and irrigated until the water-holding capacity was attained. Seeds were sown at a density of 60 seeds in each container (556 seeds m–2).

Soil and plants were sampled during the course of the growing period. Plants were sampled in the first year on Days 32, 57, 85, and 135 (harvest) after seeding; in the second year on Days 32, 57, and 140 (harvest) after seeding; and in the last year on Days 36, 69, 97, and 148 (harvest) after seeding. The aboveground biomass of 10 plants was removed from each container, weighed, dried at 65°C, and analyzed for dry weight and for N, P, and K concentrations by wet digestion, as described above for the compost. Four replicate soil cores were collected from each container during the first year on Days 32 and 135 (harvest) after seeding; in the second year on Days 48, 64, and 140 (harvest) after seeding; and in the last year on Days 36, 69, 97, and 148 (harvest) after seeding. The initial soil and the soil samples from the cores, divided into layers of 0 to 15, 15 to 35, and 35 to 55 cm depth, were analyzed for ammonium and nitrate, following extraction in 1 M KCl at a 1:5 soil to solution ratio. In the last samples of the second year and all samples of the third year, Olsen P and K were analyzed in extracts in NaHCO3 at a 1:20 soil to solution ratio (Bar-Yosef and Akiri, 1978).

Statistical Analyses and Calculations
Plant and soil data were analyzed by the Fit Model procedure of the JMP 5.1 software (SAS Institute, 2002). Analysis of variance was used to obtain an F value for the significant effect of the model, where each combination of compost type and rate or a control is a treatment, and the Tukey HSD (honestly significant difference) test was used to find significant differences among treatments.

The net amount of N mineralized per season (Nmin, g m–2) was calculated according to Eq. [1]:

[1]
where Nup (g m–2) is the amount of N taken up by the plants, Nf (g m–2) is the final amount of mineral N in the soil, Ninput and Nir (g m–2) are the amounts of inorganic N applied via the compost and via the irrigation water, respectively, and Ni (g m–2) is the initial amount of mineral N in the soil. Net N mineralized assumes negligible gaseous N losses and losses through ammonium fixation by clay minerals and no losses through drainage.

A first-order kinetic was assumed for organic N mineralization (Eq. [2]):

[2]
where Nt (g kg–1) is the quantity of N mineralized at time t, N0 (g kg–1) is the quantity of mineralizable N in the compost, k is the rate constant (d–1), and t is elapsed time (d).

Nonlinear regression was applied by means of the JMP 5.0.1 software (SAS Institute, 2002) to estimate k. The term N0 in Eq. [2] was assumed to be equal to the total organic N in the composts. No attempt was made to estimate k and N0 simultaneously, because the two were highly correlated.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Aboveground Dry Matter Production
Dry matter (DM) produced from both compost treatments was significantly greater than the unfertilized control in each year of the trial (Table 3). The SSC was slightly more effective than the CMC but the difference between treatments was significant only in the first year. The lowest rate of CMC or SSC application increased the yields of shoots by more than twice that of the unfertilized control, which yielded almost no grain at all. As the rates of compost application increased from 3 to 12 kg m–2 yr–1 the production of shoot and grain DM increased two- to threefold (Table 3). Shoot and grain yields from mineral fertilization were significantly larger than those with the highest rate of compost in the first two years, but no significant difference was observed in the last year. The reason for the large differences in DM production in the first two years is not clear but could be due to more N available from mineral-fertilizer treatments than for compost treatments. The larger number of shoots in the mineral-fertilized treatments supports this idea (Table 3). In addition, fertilization in the last year started about three weeks after germination and might explain why no significant difference in the numbers of shoots and total DM production was observed between treatments.


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  		Table 3. Results of three years of compost application at three rates on aboveground dry matter (DM) production of shoots and numbers of shoots, and grain of wheat plants at harvest.

 
Cumulative aboveground DM production increased with the two compost treatments and the mineral fertilizer treatment in each year (Fig. 1). Cattle manure compost and SSC had similar effects on plant growth, the latter being only slightly more effective. The differences between the effects of the different rates of compost application increased with time until harvest. In the second year no significant differences were observed in DM at the early sampling times although final yield from the mineral fertilizer treatment was greater than other treatments. At the first sampling date in the first year, none of the compost treatments showed any major differences in DM from the fertilized control, whereas the highest rates of compost application resulted in greater DM production at the second and third sampling dates.



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  		Fig. 1. Dry matter production by aboveground wheat plants with time in the first, second, and third years of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) plants and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates (10–20 plants per replicate) and the bars represent the SE.

 
Nitrogen, Phosphorus, and Potassium Contents in Aboveground Wheat Organs and Total Uptake
Large differences were observed among the plants receiving the various compost application rates in the early stages of plant development (data not shown). However, the difference became insignificant as plant aged (Table 4), because the N concentration in the shoot (stem + leaves) DM declined due to remobilization of N to the grain (Palta and Fillery, 1993, 1995; Palta et al., 1994). The main effect of the compost application treatments on N concentration was observed in the grain in the last year, when the concentration increased with increasing application rate. Significantly higher N concentrations were obtained in the DM of both the shoots and the grain in the fertilized control than in the compost treatments and the unfertilized control. This result is in agreement with the measured and simulated results of Meinke (1996) and Asseng et al. (2002) on the effects of a wide range of N application rates, from 0 to 36 kg ha–1, on the protein content in the grain of irrigated wheat. In the present study there was also a trend toward higher N concentrations in the DM of the shoots and grain in the control without fertilization than in the compost treatments.


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  		Table 4. Nitrogen, P, and K concentrations in the shoots and grain of wheat at harvest in each of three years of treatment.

 
The difference in accumulated N uptake by the aboveground organs from the fertilized control and the other treatments increased throughout the growing season (Fig. 2). The largest difference (Year 2, Fig. 2) also correlated with the largest DM production in the mineral-fertilizer treatment (Year 2, Fig. 1). In the first 36 d of the last year, N uptake by the fertilized control was less than that in the highest compost-rate treatment, but for the remainder of the growing season N uptake in the former treatment was significantly greater in the mineral-fertilizer treatment than in composts. The total amount of N taken up by plants in the fertilized control was nearly equal to the amount of mineral N applied in the first two years, but in the last year only about 55% of that applied was taken up (Table 5). The rate of nitrogen uptake was significantly different for the mineral-fertilized treatment than for compost treatments or the control treatment (Fig. 2). In the first two years there were no significant differences in N uptake in plants from the different compost rates and the unfertilized control; in the third year N uptake in the compost treatments was greater than the unfertilized control and it increased with increasing compost rate (Fig. 2). The pattern of postanthesis continuous uptake of N is typical for irrigated wheat when there is a sufficient N supply (Meinke, 1996). In each compost treatment, N uptake was nearly equal to the amount of mineral N applied in the first year but increased two to three times in the second and third years (Table 5).



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  		Fig. 2. Nitrogen uptake by wheat (aboveground organs) with time in the first, second, and third years of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) plants and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates (10–20 plants per replicate) and the bars represent the SE.

 

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  		Table 5. The amounts of N, P, and K applied in compost treatments and mineral fertilizer and the amounts taken up by the aboveground organs of wheat plants.

 
Phosphorus concentrations in shoot DM increased by 3 to 13 times compared with the control, and grain P was also greater than controls (Table 4). Increasing the compost rate from 3 to 12 kg m–2 yr–1 resulted in increases in P concentration in the DM of the shoots and grain at the end of the first and third years, but no significant effect was found in the second year. Phosphorus concentrations in shoot DM from the fertilized control were less than that in the highest compost-rate treatment, but more P was found in the grain from the fertilized control. Generally, the concentrations of P in the shoot and the grain in all compost rates and the fertilized control were in the range of sufficient P supply, 1.0 to 1.5 and 3.5 to 5.0 mg g–1, respectively (Mengel and Kirkby, 1978).

The pattern of P uptake by the aboveground organs depended on the treatment and the year (Fig. 3). In the first year the P accumulation in most treatments reached its peak between 57 and 85 d after sowing. Phosphorus uptake continued after Day 85 only at the highest SSC application rate and in the fertilized control treatments. In the second and the last year the P uptake in all treatments except the unfertilized control continued until harvest, and the uptake rates increased with increasing compost application rates. Phosphorus uptake by the fertilized control was highest in the second year, when the total DM production in this treatment was much greater than those in the compost treatments, but in the last year P from fertilized control treatments was less than that in the highest compost-rate treatment.



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  		Fig. 3. Phosphorus uptake by wheat (aboveground organs) with time in the first, second, and third years of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) plants and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates (10–20 plants per replicate) and the bars represent the SE.

 
The total amounts of P taken up by the plants in all the compost-rate treatments were less than 10% of the amount applied throughout the three years; whereas 30 to 50% of the P applied by mineral fertigation was taken up (Table 5). Thus, it is clear that P was not a limiting factor in the compost treatments because excess P was applied even at the lowest compost rate. The increased availability of P as compost application increased is consistent with previously published results (Eghball et al., 1996; Preusch et al., 2002; Sharpley et al., 1994; Whalen and Chang, 2001).

The potassium concentration in the DM of the shoots from the control treatment was significantly less than the potassium concentration in the DM from the low-rate SSC treatment in each of the three years. In contrast, potassium concentrations in shoots from the CMC treatment were generally greater than those from the control treatment, but the differences were not always significant (Table 4). Further increase in the CMC and SSC rates from 3 to 12 kg m–2 yr–1 significantly increased the K concentration in the shoot DM. There was a trend toward higher K concentration in the DM of the shoots in the SSC treatment than in those in the CMC treatment (Table 4), although the K content of CMC was greater than that of SSC (Table 1). In the second year there was no significant difference in grain K concentration between treatments, whereas in the last year a significant difference was observed between the control and the fertilized control but not between other treatments. The typically K concentration in the tops of plants at maturity was between 10 and 15 mg kg–1 (Beaton and Sekhon, 1985), whereas the average shoot and grain concentrations of wheat with sufficient K supply were 8.6 and 5.0 mg g–1 (Mengel and Kirkby, 1978). The data from the present study indicate that all compost rates maintained sufficient K. Potassium concentrations in shoots and grain of various wheat cultivars under optimal K fertilization range from 9.2 to 19.5 and 3.5 to 6.5 mg g–1 (Guoping et al., 1999).

Potassium uptake by aboveground organs peaked between 60 to 89 d after sowing in most treatments, but uptake continued until harvest in the fertilized control in each year and in the highest compost-rate treatment in the last year (Fig. 4). In addition the difference between the highest and lowest compost applications increased until harvest. The K uptake in the fertilized control was less than that in the medium and high compost-rate treatments during much of the growing season in the first and third years, but only until 48 d from sowing in the second year. The K uptakes in the SSC treatments were greater than those in the corresponding CMC treatments, in spite of the greater K content in CMC than in SSC.



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  		Fig. 4. Potassium uptake by wheat (aboveground organs) with time in the first, second, and third years of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) plants and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates (10–20 plants per replicate) and the bars represent the SE.

 
The amounts of K taken up annually by the plants were less than the K applied annually in the composts. However, the maximum amount of K taken up by the plants in the third year of the highest compost-rate treatment was greater than the amount applied in the two lower rates of compost application or by fertilization (Table 5). These results suggest that excess K was supplied in the highest rates of compost application and may have been a limiting factor in the lowest compost-rate treatment in all three years and in the fertilized control in the last year. Even though wheat is considered a low-K-consuming crop with average value of 3.3 g m–2 (Mengel and Kirkby, 1978), K tends to accumulate in the tissue in excess of critical level required for its specific roles in plant physiology, around 20 mg g–1 (Leigh and Wyn Jones, 1984; Walker et al., 1996). The relatively high K concentrations in the shoots in the medium and low compost-rate treatments during all three years of the experiment (Table 4) indicate that K was not the limiting factor for wheat yield under these treatments.

Nitrogen, Phosphorus, and Potassium Dynamics in the Soil Profile
The initial concentrations of mineral N in soil in the upper soil layer increased with compost rate and were greater in the SSC than in the CMC treatments (Fig. 5). The initial inorganic N concentration in the fertilized control in the first year was equal to that in the unfertilized control, but increased concentrations were observed in the second and third years of treatments because of the accumulation of residual fertilizer N from the previous years. During the second and third years of treatments, inorganic N concentration also increased in the 15- to 35- and 35- to 55-cm layers as a result of downward movement of the applied mineral N. The initial inorganic N concentration in the top layer of the highest compost-rate treatment was greater than that in the fertilized control, but smaller or similar values were obtained throughout the season until harvest every year. As the compost rate increased the inorganic N concentration increased; the largest values were observed in the top 0- to 15-cm layer immediately following application, then decreased during the next 45 to 60 d, followed by slight increase in inorganic N concentration until harvest. Unlike the fertilization treatment, the main change in inorganic N concentration following compost application occurred in the top layer, whereas there were small changes in the deeper layers.



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  		Fig. 5. Dynamics of mineral N concentration in the soil layers of the containers during three growing seasons of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) treatments and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates and the bars represent the SE.

 
During the third year of treatments the concentration of the Olsen P in the top soil layer was greater than that in the deeper layers because of its low mobility in soil (Fig. 6). However, application of the two composts at all rates resulted in considerable increases in available P concentration in the middle and the deepest layers compared with those in the unfertilized and the fertilized controls. Accumulation of P in the deeper layer at the end of the third year under compost application was probably a result of increased mobility of P following application of organic matter, as shown by Eghball et al. (1996) and Whalen and Chang (2001). The available P concentration in the soil profile increased with increasing compost application rate. Variation in available P concentration throughout the growing season indicates that the application of sufficient compost to meet N demands may result in the accumulation of excess P in the soil.



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  		Fig. 6. Dynamics of available P in soil layers in the third growing seasons of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) treatments and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates and the bars represent the SE.

 
The concentration of potassium in the soil during the third year of treatments increased as the compost application rate increased and was greater in the SSC treatments than in the CMC treatments (Fig. 7). The K concentration in the top soil layer following compost application was greater than in the deeper layers, but during the season higher values were measured in the middle 15- to 35-cm layer as a result of downward movement that was faster than that of P, but lower than that of N. The increase in K concentration in compost treatments indicates that excess K accumulated when compost was applied to meet N demand. The values of K concentration in the unfertilized control indicate that K uptake becomes restricted as K concentration decreases below 5 mg kg–1.



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  		Fig. 7. Dynamics of available K in soil layers in the third growing seasons of fertilized (solid lines and filled symbols) and control (dashed lines and empty symbols) treatments and application of sewage sludge compost (SSC, solid lines and filled symbols) and cattle manure compost (CMC, dashed lines and empty symbols) at three rates (3, 6, and 12 kg m–2 yr–1). Each point represents the mean of four replicates and the bars represent the SE.

 
Net Nitrogen Mineralization
The net amounts of N mineralized per year following SSC and CMC applications were not affected by compost treatments in the first year of application (Table 6). The net amount of N mineralized increased considerably from the first to the second year with both composts, and increased further in the third year of the SSC treatment but not with the CMC treatment. Application of compost at 12 kg m–2 increased net amount of N mineralized by a factor of 2.3 to 3.8 compared with the low compost-rate treatment (Table 6). During Years 2 and 3 of the treatments the mean values of mineralized N in the SSC and CMC treatments were 1.7 to 2.4 and 1.15 to 2.00 g kg–1, respectively. The smaller mineralized N following the application of composts in the first year compared with the same values in the two following years might have been a result of immobilization. Nitrogen dynamics in compost-amended soils have been found to range from drastic immobilization to net N mineralization (Hue and Sobieszczyk, 1999), depending on the compost maturity and the C to N ratio (Bernal et al., 1998). Net N immobilization has been reported in the first year after compost applications (Eriksen et al., 1999; Mamo et al., 1999), and Sims (1990) found that application of mineral N was required to prevent N deficiency after compost application.


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  		Table 6. Estimated net N mineralization, inorganic N applied, the initial amount of inorganic N in soil before seeding and at harvest, and estimated N taken up by wheat plants throughout growth until harvest.

 
On the assumptions that all the organic N from the composts is mineralizable N in the compost and that it decomposes according to first-order kinetics, the values of k for SSC and CMC (calculated from Eq. [2]) would be 2.39 x 10–4 and 2.58 x 10–4 d–1, respectively. The difference between the coefficients was less than the asymptotic SE of the estimated values (1.6–1.9 x 10–5). This difference does not account for the larger rate of net N mineralization from SSC treatment compared with CMC application. Thus, the difference in mineralized N rate of the two compost treatments stemmed from the difference in total organic N content. Using the estimated k and assuming 150 d of mineralization during the growing season, about 3.46% of the applied organic N should be released in the first season, and the release should continue at slowly decreasing rates of 3.35 and 3.23%, respectively, during the next two years. This nearly constant rate of mineralization of the applied organic N conflicts with previously published results on the decay series of mineralization rate with time (Klausner et al., 1994) and the observation of rapid and slow mineralized components in various composts (Hadas and Portnoy, 1994, 1997). Bernal et al. (1998) identified a labile component in a mature sludge compost, which had a decomposition constant of 3.3 x 10–2 d–1, and which contained 19.8% of its total C and 9.1% of its total N. Hadas and Portnoy (1994) also identified in three different composts a very labile component (the soluble fraction) which had a decomposition constant of 1 d–1; but these composts had different decomposition rate constants of the resistant fraction, which varied from 3.9 x 10–4 to 1.1 x 10–8 d–1. They concluded that each of the composts was composed of a labile component and two resistant components. Thuries et al. (2001) compared different models for predicting the kinetics of organic matter decomposition and concluded that a two-compartment model with a very labile and a stable fraction gave good predictions with the minimum number of parameters.

In the present study, the results of the greenhouse experiment and the analysis of the mineralization of the organic N of the two composts clearly showed that the major difference between the two composts in N supply was related to their inorganic and organic N contents. In the first season the main supply of N to the wheat plants was from inorganic N, with only a small fraction obtained from the organic N. The contribution of organic N by net N mineralization was found to increase in the second and third years of application. Furthermore, CMC and SSC are found to have similar mineralization rate constants if it is assumed that the total organic N is available for mineralization. The two composts were derived from different fresh materials and showed clear differences between their total analyses (Table 1), but the composting process seemed to produce materials with similar mineralization behavior.

Most N uptake from the composts occurred during the first 50 to 60 d, in contrast to N uptake in the fertilized treatment. However, the amount of organic N applied in the composts in the first year was about 10 times greater than the amount of N taken up, whereas in the third year N application was three to six times greater than the amount taken up. Thus, the contribution from organic N to plant uptake increased progressively from the first to the last year.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Large amounts of compost are required in the short term to supply the inorganic N demand of wheat because of the small net mineralization rate of organic N. In the long term, however, the compost residues contribute to mineral N supply. It seems that the main constraint to compost application should be the contents of P, soluble salts, and other hazardous elements. Our results indicate that dynamic N transformations and N, P, and K uptake models would be an important tool to develop to manage current and future compost applications.


   ACKNOWLEDGMENTS
 
The authors are very grateful to the associate editor, Dr. Mike Ebinger, and two anonymous reviewers for their critical reading of the manuscript. This research received financial support from the Chief Scientist of the Israeli Ministry of Agriculture and Rural Development, and the Service de Cooperation et d'Action Culturelle de l'Ambassade de France en Israel supported the stay of J. Beraud in Israel as a cooperating participant.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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