Ammonium and nitrate concentrations, and ammonium/nitrate ratio
The concentrations of ammonium in soil varied significantly due to fertilizer amendments (short rainy season: χ2 = 123.7, df = 2, p < 0.001, long rainy season: χ2 = 129.2, df = 2, p < 0.001), incubation time (short rainy season: χ2 = 69.6, df = 5, p < 0.001, long rainy season: χ2 = 34.3, df = 5, p < 0.001). The interaction effect of fertilizer amendments and incubation time was also significant (short rainy season: χ2 = 52.4, df = 10, p < 0.001, long rainy season: χ2 = 30.3, df = 5, p < 0.001). Initial ammonium concentrations ranged between 1.1 and 402 mg kg−1, whereby the control soil and BSFFF treated soil had the lowest and highest ammonium concentrations, respectively (Table 1). The ammonium concentrations of BSFFF and commercial organic fertilizer (SAFI) treated soils were 2–287 and 4–122 times higher than those of the unamended soil, respectively. The ammonium concentration of soil amended with BSFFF and SAFI followed a decreasing trend throughout experiments. On the other hand, the ammonium concentration of unamended soil was observed to increase considerably to reach its peak values at the 60th day of incubation.
There were significant decreases in the ammonium concentration of BSFFF treated soil from initial values up to the 60th day of incubation during the short rainy season (p < 0.001) and from the 60th to 90th day after incubation during the long rainy season (p < 0.001). Soil amended with BSFFF had significantly higher ammonium concentration than the control treatment throughout experiments, except at 60 days after incubation during both seasons. Likewise, soil treated with BSFFF achieved significantly higher ammonium concentration than SAFI treatments, except at 15 and 60 days after incubation during the short and long rainy season experiments, respectively. At the end of experiments, soil amended with BSFFF had the highest ammonium concentration, which was 34 and 40 times higher than those of the control and SAFI treatments, respectively (Table 1).
Significant differences in soil nitrate concentration due to fertilizer treatments (short rainy season: χ2 = 242.0 df = 2, p < 0.001, long rainy season: χ2 = 299.0, df = 2, p < 0.001) and the interaction of treatment and incubation time (short rainy season: χ2 = 23.2, df = 10, p = 0.01, long rainy season: χ2 = 31.3, df = 10, p < 0.001) were observed. The effect of incubation time was significant during the long rainy season only (short rainy season: χ2 = 11.0, df = 5, p = 0.05, long rainy season: χ2 = 13.0, df = 5, p = 0.02). Initial nitrate concentrations ranged between 0.2 and 400 mg kg−1, while the soil in the control treatment and that treated with SAFI had the lowest and highest values, respectively (Table 1). The nitrate concentration of the control treatment increased to peak values (14–18 mg kg−1) at the 90th day after incubation during both seasons and decreased afterwards.
The nitrate concentrations of soil amended with BSFFF and SAFI did not follow a uniform trend. Soil treated with BSFFF reached peak nitrate concentration 125 days after incubation, while SAFI amended soil achieved the highest nitrate values after 90 and 15 days of incubation during the short and long rainy seasons, respectively. The nitrate concentration of soil amended with SAFI fertilizer significantly (p = 0.02) decreased from the 30th to 60th day after incubation during the long rainy season. The peak nitrate concentration of SAFI treated soil was 17–27 and 26–28 times higher than those achieved using the control and BSFFF treatments, respectively. Soil amended with SAFI fertilizer maintained significantly higher nitrate concentrations than the control and BSFFF treatments, except at 60 days after incubation during both seasons. At 125 days after incubation, the nitrate concentration soil amended with SAFI was 20 and 19 times higher than those of the control and BSFFF treatments, respectively (Table 1).
The ammonium/nitrate ratio was significantly influenced by fertilizer treatments (short rainy season: χ2 = 366.9, df = 2, p < 0.001, long rainy season: χ2 = 316.4, df = 2, p < 0.001), incubation time (short rainy season: χ2 = 167.6, df = 5, p < 0.001, long rainy season: χ2 = 80.2, df = 5, p < 0.001) and their interactions (short rainy season: χ2 = 329.3, df = 10, p < 0.001, long rainy season: χ2 = 160.4, df = 10, p < 0.001). The initial ammonium/nitrate ratio ranged between 0.5 and 90, while SAFI and BSFFF amended soils had the lowest and highest values, respectively (Table 1). There was a spike in ammonium/nitrate ratios of the control soil to peak values at 60 days after incubation during both seasons. The same ammonium/nitrate ratio was observed in soil amended with SAFI during the long rainy season. The ammonium/nitrate ratio of soil amended with BSFFF significantly (p < 0.001) increased to peak values at 15 after incubation and decreased significantly up to 60 days of incubation. Thereafter, the ammonium/nitrate ratio decreased until the end of experiments.
Soils amended with SAFI maintained a significantly lower ammonium/nitrate ratio than BSFFF treated soils (Table 1). The BSFFF amendment caused significantly higher ratios of ammonium/nitrate than the control treatment at 30 days (F = 247.3, df = 2, 6, p < 0.001), 90 days (F = 6.4, df = 2, 6, p = 0.03) and 125 days (F = 12.8, df = 2, 6, p < 0.001) after incubation during the short rainy season. Furthermore, BSFFF amended soil had significantly higher ammonium/nitrate ratio than the control during long rainy season, except at 60 days after incubation (F = 7.3, df = 2, 6, p = 0.03) (Table 1). At the end of experiments, the ammonium ratios to nitrate ranged between 0.01 and 13.8, while SAFI and BSFFF amended soils had the lowest and highest values, respectively.
Mineralization and nitrification rates
The soil nitrogen mineralization rate was significantly influenced by incubation time (short rainy season: χ2 = 25.7, df = 4, p < 0.001, long rainy season: χ2 = 11.0, df = 4, p = 0.027) and the interaction of incubation time and fertilizer amendment (short rainy season: χ2 = 34.1, df = 8, p < 0.001, long rainy season: χ2 = 45.0, df = 8, p < 0.001). The effect of fertilizer amendment was significant during the short rainy season only (χ2 = 6.1, df = 2, p = 0.046). The fluxes of nitrogen mineralization of soil amended using different treatments are presented in Table 2.
Amendment with BSFFF triggered net mineralization rates at 60 and 125 days after incubation during the short rainy season and most of the long rainy season, except at 90 days after incubation. Soil amended with SAFI achieved net mineralization in 90 days after incubation during both seasons and at 15 days after incubation during the long rainy season. For the unamended soil, net mineralization rates were recorded at 15 and 60 days after incubation during the short rainy season, and most of the long rainy season, except at 90 days after incubation (Table 2).
Soil amended with SAFI experienced longer periods (60–95 days) of N immobilization during both seasons than BSFFF treatments (30–60 days) (Table 2). During the short rainy season, the highest immobilization rate (− 11 mg N kg−1 day−1) was recorded at 30 days after incubation in soil amended with SAFI, and this was significantly (p < 0.038) higher than that of BSFFF and control treatments. Soil amended with BSFFF experienced the highest immobilization rate (− 12 mg N kg−1 day−1) during the long rainy season, which was not significantly different (p < 0.01) from that of the untreated soil.
All treatments achieved peak N mineralization rates (0.5–5.2 mg N kg−1 day−1) between 60 and 90 days after incubation (Table 2). Soil amended using SAFI achieved the highest net mineralization rate during the short rainy season at 90 days after incubation. During the long rainy season, the unamended soil had the highest mineralization rate (7.4 mg N kg−1 day−1) 60 days after incubation, which was not significantly different from that of BSFFF treatment. Generally, all treatments experienced rises in the rates of N mineralization during the long rainy seasons, with higher increases (three–ten folds) observed in BSFFF treated soils.
The soil nitrification rate was significantly influenced by incubation time (short rainy season: χ2 = 21.6, df = 4, p < 0.001, long rainy season: χ2 = 10.9, df = 4, p = 0.03) and the interaction of incubation time and fertilizer treatments (short rainy season: χ2 = 43.2, df = 8, p < 0.001, long rainy season: χ2 = 24, df = 8, p = 0.002). Table 2 presents the changes in nitrification rates during experiments. All treatments reached peak net nitrification rates 60–90 days after incubation. Soil amended with SAFI had the highest net nitrification rates (1.5–5.6 mg N kg−1 day−1), which were 22–25 and 17–75 times higher than the values achieved using BSFFF and control treatments, respectively. However, amendment with SAFI depressed the nitrification rate (− 0.18 to − 8.1 mg N kg−1 day−1) more than BSFFF treatments (− 0.04 to − 0.35 mg N kg−1 day−1).
Soil amended with BSFFF achieved a longer period of net nitrification (80 days) than SAFI amended soil (45 days) (Table 2). The net nitrification rates of soils amended with BSFFF were significantly higher than those achieved using SAFI at 30 and 60 days after incubation during the short (p < 0.001) and long rainy seasons (p = 0.019), respectively. The net nitrification rates of BSFFF amended soils increased by two–four folds during the long rainy season.
Soil pH, and populations of bacteria and fungi
The soil pH was significantly influenced by fertilizer treatments (short rainy season: χ2 = 733.7, df = 2, p < 0.001, long rainy season: χ2 = 192.8, df = 2, p < 0.001), incubation time (short rains season: χ2 = 119.2, df = 5, p < 0.001, long rainy season: χ2 = 387.2, df = 5, p < 0.001) and their interaction (short rainy season: χ2 = 127.1, df = 10, p < 0.001, long rainy season: χ2 = 315.4, df = 10, p < 0.001). The initial pH of soils amended with different fertilizer treatments ranged between 5.2 and 8.7. Thereafter, the soil pH decreased until the end of the experiments (Fig. 1a,d). Soil amended with BSFFF maintained significantly (p < 0.001) higher pH values than control soil up to 90 days of incubation and throughout experiments for SAFI treated soil. The unamended soil had significantly higher pH than that of soil treated with SAFI, except at 90 days after incubation during the short rainy season (Fig. 1a). At the end of experiments, soil pH ranged between 4.8 and 6.2, whereby the control soil had the highest pH while soil amended with SAFI had the lowest pH.
Changes in pH (a,d), and populations of bacteria (b,e) and fungi (c,f) of soil treated with different fertilizers during the short (a–c) and long rainy (d–f) seasons incubation experiments.
The soil bacteria population was significantly influenced by incubation time during both seasons (short rainy season: χ2 = 24.7, df = 5, p < 0.001, long rainy season: χ2 = 73.9, df = 5, p < 0.001) and fertilizer amendment during the short rainy season only (χ2 = 19.9, df = 2, p < 0.001). However, the interaction of incubation time and fertilizer amendments was not significant (short rainy season: χ2 = 8.9, df = 10, p = 0.541, long rainy season: χ2 = 8.1, df = 10, p = 0.618). The bacterial population significantly increased from initial values to peak levels (9.5–10.3 log10 CFU g−1) between 30 and 60 days after incubation and decreased afterwards (Fig. 1b,e). Soil treated with BSFFF achieved the highest bacterial populations during both seasons. Its peak bacterial population during the long rainy season was equivalent to the highest value obtained using SAFI (Fig. 1e). Amendment with BSFFF caused significantly higher bacterial populations than other treatments at 30 and 90 days during the short rainy season and at 125 days after incubation during both seasons. At the end of experiments, the bacterial populations ranged between 7.3 and 9.3 log10 CFU g−1, whereby BSFFF and control treatments had the highest and lowest values, respectively.
The different fertilizer amendments caused significant differences in soil fungi populations during the short rainy season only (χ2 = 15.5, df = 2, p < 0.001). The effect of incubation time was significant during both seasons (short rainy season: χ2 = 11.9, df = 5, p = 0.04, long rainy season: χ2 = 53.7, df = 5, p < 0.001) while the interaction of fertilizer treatments and incubation time was significant during the long rainy season only (χ2 = 18.7, df = 10, p = 0.044). Minimal changes in fungi populations were observed in the first 30 days of incubation (Fig. 1c,f). The soil fungi populations significantly increased to peak values (7.9–9.0 log10 CFU g−1) between 60 and 90 days after incubation and decreased afterwards. However, SAFI treated soil achieved peak fungi populations after 125 days during the long rainy season. Soil amended with BSFFF maintained significantly (p < 0.001) higher fungi populations than other treatments during the entire short rainy season and 125 days after incubation during the long rainy season. At the end of the experiments, the control treatment had the lowest fungi population (7.1 log10 CFU g−1) while BSFFF treated soil had the highest (9.0 log10 CFU g−1). Also, soils treated with BSFFF and SAFI had a significantly (p < 0.001) higher fungi population than the unamended soil at the end of the experiments.
Soil nutrients concentration
The total N concentration was significantly influenced by fertilizer amendments (short rainy season: χ2 = 1011.7, df = 2, p < 0.001, long rainy season: χ2 = 1509.5, df = 2, p < 0.001), incubation time (short rainy season: χ2 = 143.1, df = 4, p < 0.001, long rainy season: χ2 = 30.2, df = 4, p < 0.001) and their interaction (short rainy season: χ2 = 261.9, df = 8, p < 0.001, long rainy season: χ2 = 231.6, df = 8, p < 0.001). The total N concentration of soil treated with BSFFF significantly increased to peak values (0.74–0.83%) at 30 and 60 days after incubation during the short and long rainy season, respectively, and decreased afterwards (Fig. 2a,d). On the other hand, there was minimal changes in the total N concentration in soil amended with SAFI during experiments, while total N concentration in unamended soil was observed to increase from day 60 after incubation and peaked on the 125th day.
Effects of different fertilizers on concentrations of total nitrogen (a,d), available phosphorus (b,e) and exchangeable potassium (c,f) of soils during the short (a–c) and long rainy (d–f) seasons incubation experiments.
Soils amended with BSFFF maintained significantly (p < 0.001) higher total N concentration than other treatments throughout experiments. The total N concentration of soil treated with BSFFF was 2.5–5.4 and 2.5–4.4 times higher than soil amended with SAFI during the short and long rainy seasons, respectively. Likewise, soil amended with SAFI achieved significantly (p < 0.001) higher total N concentration than the unamended soil, except at 30 days during the long rainy season and on the 125th day during both seasons. At the end of the experiments, soils treated with BSFFF had the highest N concentration (0.14–0.34%), which was significantly (p < 0.001) higher than that of the SAFI and control treatments by 149–152% and 32–44%, respectively.
The concentration of available phosphorus (P) was found to vary significantly due to fertilizer amendments (short rainy season: χ2 = 5920.2, df = 2, p < 0.001, long rainy season: χ2 = 6128.9, df = 2, p < 0.001), incubation time (short rainy season: χ2 = 55.1, df = 4, p < 0.001, long rainy season: short rainy season: χ2 = 68.0, df = 4, p < 0.001) and the interaction effect of fertilizers and incubation time (short rainy season: short rainy season: χ2 = 45.4, df = 8, p < 0.001, long rainy season: short rainy season: χ2 = 43.3, df = 8, p < 0.001). Initial P concentration ranged between 8 and 694 ppm and significantly (p < 0.001) increased to peak values (279–804 ppm) between 90 and 125 days after incubation (Fig. 2b,e).
All fertilizer treatments maintained significantly (p < 0.001) higher P levels than the control treatment throughout experiments. The P concentration of the control soil did not vary significantly during experiments. The P concentration of soil treated with BSFFF was significantly (p < 0.001) higher than that of SAFI treatment by 1.8–2.1 times. At the end of experiments, the P concentration of soil treated with BSFFF was about 83% higher than that of SAFI treatment (p < 0.001).
The concentrations of exchangeable potassium (K) also varied significantly due to fertilizer amendments (short rainy season: χ2 = 384.8, df = 2, p < 0.001, long rainy season: χ2 = 386.7, df = 2, p < 0.001), incubation time (short rainy season: χ2 = 154.7, df = 4, p < 0.001, long rainy season: χ2 = 140.9, df = 4, p < 0.001) and their interaction (short rainy season: χ2 = 127.6, df = 8, p < 0.001, long rainy season: χ2 = 118.7, df = 8, p < 0.001). The initial K concentration ranged between 1.1 and 9.1 cmol kg−1 from where it decreased throughout experiments, with the highest decreases observed in SAFI treated soils (Fig. 2c,f). Soil amended with SAFI maintained significantly (p < 0.001) higher K levels than other treatments, except at 125 days during the long rainy season. Soil treated with BSFFF had significantly (p < 0.001) higher K concentration than the control soil up to 90 days of incubation. The final K concentration of SAFI treated soil (1.6–2.0 cmol kg−1) was 87–89% and 42–63% higher than those achieved using BSFFF and the control treatment, respectively.
The exchangeable calcium (Ca) ions varied significantly due to fertilizer amendments during experiments (short rainy season: χ2 = 59.9, df = 2, p < 0.001, long rainy season: χ2 = 48.9, df = 2, p < 0.001) and incubation time (short rainy season: χ2 = 19.9, df = 4, p < 0.001, long rainy season: χ2 = 92.2, df = 4, p < 0.001) but the interaction effects were not significant (short rainy season: χ2 = 6.3, df = 8, p = 0.61, long rainy season: χ2 = 14.0, df = 8, p = 0.08). The Ca concentration decreased to minimum values at 90 and 60 days of incubation during the short and long rainy seasons, respectively and increased thereafter (Table 3). Soil amended with SAFI achieved significantly (p < 0.001) higher Ca concentration than BSFFF treatments by 24–96 and 26–91% during short and long rainy seasons, respectively.
The control treatment attained significantly (p < 0.001) higher Ca concentration than BSFFF amended soil during the first 30 days of incubation. At the end of experiments, the Ca concentration did not vary significantly (long rainy season: p = 0.096, long rains season: p = 0.098), but SAFI amended soils had the highest. At the same time, BSFFF treated soil had the lowest Ca concentration.
The exchangeable magnesium (Mg) ions in the soil also varied significantly due to fertilizer amendments (short rainy season: χ2 = 1341.0 df = 2, p < 0.001, long rainy season: χ2 = 1339.4, df = 2, p < 0.001) and incubation time (short rainy season: χ2 = 12.0, df = 4, p = 0.018, long rains season: χ2 = 14.7, df = 4, p = 0.005) during both seasons, but their interactions were not significant (short rainy season: χ2 = 10.8, df = 8, p = 0.213, long rainy season: χ2 = 10.9, df = 4, p = 0.209). Initial Mg concentrations ranged between 0.78 and 2.86 cmol kg−1, while the control and BSFFF treatments had the lowest and highest Mg concentrations, respectively (Table 3).
The Mg concentration increased to peak values of 0.68 to 3.10 cmol kg−1 between 30 and 60 days of incubation, after which the concentrations kept decreasing up to the end of the experiments. The Mg concentration of soil amended with BSFFF was 2.6–3.7 and 3.4–4.3 times higher than those of SAFI treated and unamended soil, respectively. Also, soils treated with BSFFF maintained significantly (p < 0.001) higher Mg concentration than other treatments, except the control at 125 days of incubation during the long rainy season. At the end of the experiments, soil treated with BSFFF had the highest Mg concentration (2.4–2.9 cmol kg−1), which was 3.7 and 4.3 times higher than those of SAFI and control treatments.
Nutrients released by unamended soil and soils amended with organic fertilizers
The amount of N (short rainy season: F = 5.5, df = 2, 6, p = 0.044, long rainy season: F = 6.2, df = 2, 6, p = 0.035), that of available P (short rainy season: F = 740.9, df = 2, 6, p < 0.001, long rainy season: F = 740.2, df = 2, 6, p < 0.0001) and exchangeable Mg (short rainy season: F = 135.7, df = 2, 6, p < 0.001, long rainy season: F = 28.6, df = 2, 6, p < 0.001) released by the unamended soil and soils amended with different organic fertilizers varied significantly during the experiments (Table 4). However, the quantities of K (short rainy season: F = 3.4, df = 2, 6, p = 0.102, long rainy season: F = 4.0, df = 2, 6, p = 0.078) and Ca (short rainy season: F = 4.2, df = 2, 6, p = 0.073, long rainy season: F = 4.1, df = 2, 6, p = 0.074) released were not significantly influenced by fertilizer amendments.
Soil amended with SAFI and BSFFF released significantly (p < 0.001) higher N than unamended soil during the short and long rainy seasons, respectively (Table 4). The N released by soil treated with SAFI and BSFFF was 7–16 times and 8–11 times higher than N released by the control soil, respectively. Soils treated with SAFI released 107% higher N than BSFFF treated soil during the short rainy season. On the other hand, soil treated with BSFFF released the highest N during the long rainy season, 58 and 998% higher than SAFI amended soil and the control soil, respectively.
The quantity of P released by BSFFF amended soils was significantly (p < 0.001) higher than those of SAFI and control treatments by about two- and forty-folds, respectively. In contrast, the P released by soil treated with SAFI was 22 times higher than that released by the control soil (Table 4). Soil treated with SAFI had the highest K, which was 1.9 times and 1.4–1.6 times higher than those of BSFFF and control treatments, respectively. The Ca released by soil amended with SAFI was 44 and 18% higher than that released by BSFFF and the control treatments, respectively.
The highest quantity of Mg was released by soil amended with BSFFF, and this was significantly (p < 0.001) higher than those of SAFI and control treatments by 82–268% and 319–326%, respectively (Table 4). However, soil amendment with SAFI released significantly higher Mg than the control treatment.
Source: Ecology - nature.com
