The pot culture experiment with Italian ryegrass (Lolium multiflorum L.) was conducted from September to October, 2011 in Okayama, Japan under room condition where air temperature was maintained at 35˚C/25˚C (day/night). The sandy soil was air-dried and passed through a 2 mm sieve. The soil had pH 7.1 (in 1:2.5 soil: water suspension), total carbon of 0.2 g∙kg⁻¹, total nitrogen of 0.051 g∙kg⁻¹, a C:N ratio 3.9. Biosolids compost (Bcomp) was co-processed for 50 days using a blower system with aerated windrows method mixing sewage sludge (raw material), brewery waste (sub-raw material) and wood chips as bulking agent. Bcomp was collected from Chugoku Yuki composting facility in Japan. Municipal solids waste compost (Scomp) was previously obtained from Thuy Phuong composting plant in Vietnam, and then carried to Japan for investigation. The chemical properties of two composts are summarized in Table 1.

The preliminary experiment was conducted from April to August, 2011 to evaluate above biosolids compost quality through maturity index and biomass yield of Italian ryegrass [11] . The result showed that the total biomass yield significantly increased with increasing biosolids compost-N rates up to 150 kg∙N∙ha⁻¹. These rates of compost were used as reference data for the current study. For treatments with compost, a sample of 650 g soil (dry weight basis) was hand mixed well with either Scomp or Bcomp equivalent to 130 mg∙N∙pot⁻¹, and transferred into plastic Neubauer’ pots 100 cm² (11.28 cm in diameter × 6.6 cm in depth). For treatments with CF, the same amount of soil was added with two rates (130 mg∙N∙pot⁻¹, 1N and 260 mg∙N∙pot⁻¹, 2N) as ¹⁵N-labeled ammonium sulfate (13.172 atom%) in solution. All treatments except control received 100 mg∙P∙pot⁻¹ as Ca₃(PO₄)₂. The details of different treatments are given in Table 2.

Ryegrass was sown at density of 20 seeds∙pot⁻¹, which was equivalent to 2000 seeds∙m⁻², over a 0.5 cm soil surface layer to facilitate seed germination. After sowing 45 days, all plant parts (including shoot and root) were harvested separately, dried at 100˚C for 26 hours. Plant parts N contents were determined by dry combustion method using CN corder (TM-700). Shoot and root dry weight were multiplied by the respective N content, and then summarized in total plant N uptake. All treatments were arranged in a randomized complete block design with three replications.

The incubation study was conducted to determine gross total N mineralization and gross total N immobilization rates after combining Scomp with CF. Soil, compost and ¹⁵N-labeled (NH₄)₂SO₄ (13.172 atom%) were prepared

^†Numbers in parentheses are available heavy metals.

as same method with above pot culture experiment without plants. The pots were additional covered with alu- minum foil perforated five small needle holes to ensure gas exchange and minimize water loss. Then the amended soil samples were incubated in dark at 25˚C and 35˚C for 10 days. During the incubation time, soil was weighed daily and adjusted with distilled water to 60% holding capacity. The sampling was destructed at 2, 5, and 10 days by removing pots in triplicate for each treatment. To ensure the homogeneity without changing inorganic nitrogen contents, soil was dried at low temperature in vacuum chambers before sampling.

Inorganic N was completely extracted from the soil samples by 2 M KCl (w: v = 1:5) after shaking for 1 hour. The filtered solution was immediately analyzed for NH₄-N and NO₃-N using indophenols-blue and Vanadium (III) chloride reduction method [12] , respectively. After extraction, the residual soil was washed in alcohol 60%, dried and ground to a very fine powder before analyzing ¹⁵N immobilized in organic soil by the CN-corder- coupled with isotope Quadrupole mass spectrometry (Ulvac).

Total plant N uptake derived from chemical N fertilizer (N_dfCF) was calculated directly using the formula of Hauck and Bremner [13] :

where, TNU is total N uptake in plant part; a, b, and c are the atom % ¹⁵N-labeled (NH₄)₂SO₄ (13.172%), plant part or soil receiving no ¹⁵N, and plant part receiving ¹⁵N, respectively. The denominator b value in the formula (1) was assumed to be equal to natural abundance (0.366%). Total plant N uptake derived from compost (D_dfComp), from soil (N_dfS) and nitrogen uptake efficiency (NUE) were calculated using difference method of Westerman and Kurtz [14] .

The gross N mineralization rate was simply calculated using the following derivation of isotope dilution:

where, ¹⁵N_Org(0) and ¹⁵N_Inorg(0) are ¹⁵N atom % in organic and inorganic pools, respectively, on day 0. ∆¹⁵N_Org is difference of ¹⁵N content (mg∙N∙kg⁻¹) in organic pools between day t and day 0. ∆N_Inorg is the difference (mg∙N∙kg⁻¹) of total inorganic nitrogen (NH₄ + NO₃) between day t and day 0. For isotopic signal data from mass spectrometry, ¹⁵N atom % in organic soil was calculated from following formula:

To obtain G_mi of each interval, ¹⁵N_Inorg(t) was calculated indirectly by subtracting ¹⁵N_Org(t) on day t from total initial known ¹⁵N_{[Org+Inorg](0)} on day 0. Therefore, in this model the loss of N as unaccounted proportions were assumed to be negligible.

Gross N immobilization rate (G_im) was calculated following Murphy et al. [10] :

where, N_mi is net mineralization rate calculated by difference of total inorganic nitrogen between day t and 0.

Means of values were compared with those of others by Fisher’s protected least significant difference (LSD) after ANOVA test, using the EXCEL^® macro add-ins DSAASTAT Version 1.512 [15] .

(1) Growth

Total dry weight (sum of shoot and root) significantly increased (P < 0.01) with CF and/or compost additions compared to control treatment, with the exception of Scomp alone (Figure 1). From 63% to 95% of the total dry weight was contributed by shoot biomass (data not shown), indicating that plant growth was greatly affected by variability of nitrogen sources from different treatments. The results show that either Scomp or Bcomp combined with two rates of CF (1N and 2N) had no effect on yield other than that produced by CF alone. Incremen- tal yield was observed by Scomp + 1N compared to Scomp alone. Total dry weight by CF (1N) was not different from CF (2N), while total dry weight by Bcomp was significantly higher (P < 0.01) than Scomp (Figure 1).

(2) Total Plant N Uptake and Partitioning of Applied Nitrogen

Table 3 shows the partitioning of N_dfComp, N_dfCF, and N_dfS as influenced by different fertilizer treatments. Thus, N_dfComp indicates more clearly difference between compost alone and blend treatments. N_dfComp was all signifi- cantly decreased (P < 0.01) with two additional N rates from CF compared to compost alone, with the exception

Means in a column followed by the same lower case are not least significantly different at the 0.01^** level.

of Scomp + 1N treatment. When combining Bcomp with 1N and 2N, the percentage of added compost-N was both decreased from 62% to 19%. For Scomp, those were 29% and 4%, respectively. The percentages of added compost-N between Scomp and Scomp + 1N treatments were similar, which were equal to a half of Bcomp alone (62%) (Table 3). The N_dfCF values were useful to compare difference between CF alone and blend treatments. Similarly, combining CF with either Scomp or Bcomp caused to significant decrease (P < 0.01) of N_dfCF compared to CF alone, with the exception of Scomp + 1N. Consequently, the percentages of added chemical fertilizer-N were also lowered from 43% - 58% to 23% - 56% and 22% - 30% for Scomp and Bcomp blended treatments, respectively.

N_dfS of CF fertilized treatments were observed to be higher than CF unfertilized treatments. These values were not used for statistical analysis within because data were simulated from control and CF alone treatments.

Total nitrogen uptake (TNU) was generally similar to total dry weight data (Table 3 and Figure 1). The only differences were that N uptake from Scomp + 1N was significantly higher (P < 0.01) than Scomp + 2N, Scomp + 2N was higher than Scomp, and Scomp was higher than control treatment. The results also show that combining composts with CF did not increase the TNU and NUE compare to CF alone. With the exception of Scomp + 1N treatment, TNU was remarkably enhanced by 137.69 mg∙N∙pot⁻¹. Interestingly, TNU and NUE of Bcomp (98.83 mg∙pot⁻¹ and 62.0%) were similar to CF (1N) (103.31 mg∙pot⁻¹ and 65.4%), but significantly higher (P < 0.01) than Scomp alone (55.24 mg∙pot⁻¹ and 28.2%, respectively).

The relationships between nitrogen uptake and total dry weight are illustrated in Figures 2-5. The results of the correlation analysis indicate that total biomass was closely associated with TNU (R² = 0.900), while less closely associated with N_dfCF (R² = 0.530), poorly associated with N_dfS and N_dfComp (R² = 0.396 and R² = 0.111, respectively).

(1) Soil Inorganic Nitrogen Contents and Recovery of Applied Nitrogen

Changes in the ammonium (NH₄-N) and nitrate nitrogen (NO₃-N) contents during incubation are shown in Table 4 and Figure 6. There was a considerable decrease of NH₄-N from 200 - 400 mg∙kg⁻¹ on day 0 to 15.1 - 95.1 mg∙kg⁻¹ on day 10. At the same time, there is a remarkable increase of NO₃-N from day 5. After 10 days of incubation, total inorganic nitrogen, NH₄-N, and NO₃-N contents were significantly higher at 2N than 1N may due to different initial contents. The increase of temperature from 25˚C to 35˚C does not seem to affect NH₄-N content, but significantly increased (P < 0.01) NO₃-N as well as total inorganic N content (Table 4).

^†Means in a column and same N fraction followed by the same lower are not least significantly different at the 0.01^** and 0.05^* levels. Ns = not sig- nificantly different.

Recovery of applied nitrogen in organic-N, NH₄-N, and NO₃-N are given in Figure 6. The results indicate that the recovery of applied nitrogen in organic form significantly increased within 10 days of incubation, while those in NH₄-N sharply decreased from day 2 especially at 25˚C. At the same rate of added N, effects of temperature on recovery of organic N or NH₄-N behave in a similar manner. At the same temperature, the recovery of applied nitrogen in the organic N was higher at 1N than 2N, whereas those of NH₄-N in 2N were higher. After 10 days of incubation, about 6% - 9% of total applied N were recovered in NO₃-N form (Figure 6).

(2) Net Mineralization and Gross Total N Transformation

Table 5 summarizes soil nitrogen transformation rates between organic and inorganic pools. Some ¹⁵N values of Scomp + 2N treatment on day 5 could not be measured because of failure of isotopic analysis system. Net mineralization rates (N_mi) were previously calculated by the difference of total inorganic nitrogen. The results show that temperature and added N rates had significant effects (P < 0.05 and P < 0.01, respectively) on net mineralization rates during the incubation. The results show that N_mi rates in all treatments had large negative values, suggesting a net immobilization in soil (since immobilization > mineralization). With the exception of Scomp + 1N at 35˚C during 5 - 10 days of incubation, N_mi shows a small positive value (+2.68 mg∙kg⁻¹∙soil∙day⁻¹).

Although N_mi rates are useful to evaluate the size of inorganic nitrogen in soil, they do not provide further in-

^†Means in a column followed by the same capital letters are not least significantly different at the 0.01^** and 0.05 level. Ns = not significantly different.

formation regarding flow of nitrogen into or out of each N pool. Therefore, the gross total N mineralization (G_mi) and gross total N immobilization (G_im) are more suitable to understand the interaction between organic and inorganic sources. Interestingly, the data show that G_im rates were observed to be dominant and much higher than G_mi at almost intervals of incubation. G_im was also affected by the change of temperature, which was significantly higher (P < 0.05) at 25˚C than 35˚C. At the same temperature, Scomp + 2N gave larger G_im than did Scomp + 1N. In contrast, G_mi rates were not affected by temperature, while those of Scomp + 2N were significantly higher (P < 0.01) than Scomp + 1N.

After 10 days of incubation, the results show that 16.08 - 29.62 mg N∙kg⁻¹∙day⁻¹ soil from inorganic-N were immobilized into organic pools, while only 0.40 - 0.66 mg N∙kg⁻¹∙day⁻¹ soil from organic-N were mineralized to inorganic pools (Table 5).