Long-term application of fertilizer and manures affect P fractions in Mollisol


Total P and available P

Fertilizer application significantly (P < 0.05) increased total P concentration within 0–20 cm soil depth and available P concentration within 0–40 cm soil depth (Fig. 2). Compared to CK, total P concentration in the 0–20 cm soil depth was increased by 26.6%, 4.1%, and 68.2% in NPK, NPK + S, and OM treatments, respectively, while available P concentration was increased by 315.9%, 108.3% and 667.5% in NPK, NPKS and OM treatments, respectively. Phosphorous was accumulated in the 0–40 cm soil depth in both NPK and OM treatments, majority of which was in the cultivated 0–20 cm soil depth (Fig. 2). The majority of P was accumulated in the 60–80 cm soil depth under NPK + S treatment, but no significant difference was observed among treatments (P > 0.05). The highest increase in available P concentration in NPK + S treatment observed in the 60–100 cm soil depth, with the increase of 111% and 115% in 60–80 cm and 80–100 cm soil depths, respectively, over CK treatment.

Figure 2

Effect of long-term application of chemical fertilizer, organic manure, and straw on total phosphorus (P) and available P concentrations. * indicates significant difference at P < 0.05 level at same soil depth, ns indicates no significant difference between the total phosphorus (P) and available P concentrations at the same soil depth. CK, no fertilizer application; NPK, chemical fertilizer application; NPK + S, chemical fertilizer plus maize straw incorporation; OM, organic manure application.

Phosphorous activity constant

The phosphorus activity coefficient (PAC, the ratio of available P to total P) is an important indicator of soil P availability and the transformation of P fractions37. When the PAC is less than 2.0%, the total P is not easily converted to available P38. The PAC ranged from 1.5% to 13.8% within 0–100 cm soil depth and the largest value (> 3.6%) was associated with OM treatment, especially at the 0–20 and 20–40 cm soil depths (Fig. 3). The PAC values under NPK, NPK + S and OM treatments increased by 7.6%, 4.5% and 11.5% in the 0–20 cm soil depth and 4.2%, 1.3%, and 5.8% in 20–40 cm soil depth, respectively as compared to the CK treatment. However, PAC value for soil depth below 40 cm showed the trend, NPK < CK < OM < NPK + S treatments (P < 0.05).

Figure 3
figure3

Effect of long-term fertilization on phosphorous activation constant in Mollisol profiles. Different letters in the bars of same depth indicates significant difference at P < 0.05 level among different treatments. CK, no fertilizer application; NPK, chemical fertilizer application; NPK + S, chemical fertilizer plus maize straw incorporation; OM, organic manure application.

The accumulation of total P and available P in soil profile

Total P accumulation under NPK, NPK + S and OM treatments within the 0–100 cm soil profile was 511, 1,068 and 1,353 kg P ha−1 more than that under CK treatment (Table 2). Increase in total P in NPK and OM treatments within the top 0–20 cm soil depth accounted for 95.9% and 79.5% of total P increase within 0–100 cm soil profile, respectively. However, total P in NPK + S treatment was mainly accumulated between 20–100 cm soil depths, with increased values of 215, 201, 556 and 90 kg P ha−1 for 20–40, 40–60, 60–80 and 80–100 cm soil depths, respectively over the CK treatment (Table 2). The NPK and OM treatments showed increase in available P concentration at the 0–40 cm soil depths (P < 0.05). The available P concentrations under NPK and OM treatments increased by 181.7 and 356.1 kg P ha−1 in the 0–20 cm soil depths, and 69.7 and 127.0 kg P ha−1 for 20–40 cm soil depth compared to that under CK treatment. Available P under NPK + S treatment increased throughout the entire 0–100 cm soil profile compared to that associated with the CK treatment (P < 0.05) (Table 2).

Table 2 The effect of different treatments on total phosphorus (P) and available P accumulation in soil profiles (kg ha−1).

P fractions

Organic P (Po) concentration decreased with the increase of soil depth regardless of treatment, while inorganic P (Pi) concentration increased under CK and NPK + S treatments and decreased under NPK and OM treatments (Fig. 4). The P fertilizer application significantly increased Pi concentration in the 0–100 cm soil depth (P < 0.05) and Po concentration at 40–80 cm soil depth (P < 0.05) compared with that of the CK treatment. The Pi under NPK and OM treatments accumulated mainly in the 0–20 cm soil depth resulting in higher Pi concentration of 12.4% and 62.1%, respectively, than that at deeper soil depths (Fig. 4). Applied P fertilizer mainly presented as Pi in 0–40 cm soil depth for the NPK and OM treatments, and in 0–100 cm soil depth for NPK + S treatment over CK treatment.

Figure 4
figure4

Effect of long-term application of chemical fertilizer, organic manure and straw on the distribution of organic-P and inorganic-P contents. * indicates significant difference at P < 0.05 level at the same soil depth, ns indicates no significant difference regarding the total inorganic and organic P concentrations at the same soil depth. CK, no fertilizer application; NPK, chemical fertilizer application; NPK + S, chemical fertilizer plus maize straw incorporation; OM, organic manure application.

In general, concentration of all Pi fraction under NPK + S, NPK and OM treatments increased depending on soil depth except for Ca10-P compared to that associated with the CK treatment (Fig. 5). Different fertilizer treatments have significantly impacted Ca2-P and Ca8-P concentrations as deep as 0–40 cm depth (P < 0.05), showing a decreasing trend as OM > NPK > NPK + S. Compared with CK treatment, the Ca2-P concentrations associated with OM, NPK and NPK + S treatments showed significant increase of 5.2 times, 7.5 times and 1.4 times, respectively in the 0–20 cm soil depth and 2.4 times, 2.8 times and 0.5 times, respectively in 20–40 cm soil depth (P < 0.05). The Ca8-P concentration associated with NPK, OM and NPK + S treatments showed significant increase of 4.5 times, 10.4 times and 1.1 times, respectively in the 0–20 cm soil depth and 1.3 times, 2.9 times and 0.5 times, respectively in the 20–40 cm soil depth (P < 0.05). The NPK and NPK + S treatments showed significant increase in Al-P concentrations of 1.30 times and 7.61 times, respectively within 0–100 cm soil depth (P < 0.05), while OM treatment showed significant increase of Fe-P concentration of 79.0–235.8% within 0–60 cm soil depths (P < 0.05), compared with the CK treatment. Fertilizer application increased O-P concentration with significant differences observed between different treatments (P < 0.05).

Figure 5
figure5

Effect of long-term application of chemical fertilizer, organic manure and straw on the distribution of inorganic P fractions. * indicates significant difference at P < 0.05 level at the same soil depth, ns indicates no significant difference regarding the Ca2-P, Ca8-P, Al-P, Fe-P, O-P and Ca10-P concentrations at the same soil depth. CK, no fertilizer application; NPK, chemical fertilizer application; NPK + S, chemical fertilizer plus maize straw incorporation; OM, organic manure application.

The concentrations of different Po fractions decreased with the increase of soil depth (Fig. 6) except for HSOP. The added P fertilizer mainly transformed to MSOP and HSOP (Fig. 5). The LOP and MLOP forms under OM treatment increased by 77.4% and 14.45%, respectively in the 0–20 cm soil depth as compared to that associated with the CK treatment. The LOP under NPK + S increased by 17.7% and 3.5% for 0–20 cm and 20–40 cm soil depth, respectively, compared with CK treatment. MSOP concentrations under NPK and NPK + S treatments decreased by 0.6% and 2.4% within 0–20 cm soil depth, while increased by 6.0–38.3% and 4.0–12.4% within 20–80 cm soil depths, respectively. OM treatment significantly increased MSOP concentration by 11.6–30.3% within 0–80 cm soil depth with the peak at 60–80 cm soil depth. Generally, P fertilizer application increased HSOP concentration at 0–100 cm soil profile by 8.5–45.0%, 0.7–17.0% and 1.6–23.8% for NPK, NPK + S and OM treatments, respectively, with significantly different at 20–60 cm soil depth.

Figure 6
figure6

Effect of long-term application of chemical fertilizer, organic manure and straw on the distribution of organic P fractions. * indicates significant difference at P < 0.05 level at the same soil depth, ns indicates no significant difference regarding labile organic P, moderately labile organic P, moderately stable organic P and highly stable organic P concentrations at the same soil depth. CK, no fertilizer application; NPK, chemical fertilizer application; NPK + S, chemical fertilizer plus maize straw incorporation; OM, organic manure application.

Accumulation of different P fractions

Total P accumulation increased mainly in the 0–20 cm soil depth as impacted by long-term fertilization in this study (Table 2). Compared with CK treatment, the accumulation of Fe-P, Ca8-P, Ca2-P, O-P and Al-P in NPK treatment increased by 144.9, 93.5, 68.0, 67.8, and 36.8 kg P ha−1, respectively; while the accumulation of Fe-P, Ca8-P, Ca2-P, MSOP, and Al-P associated with OM treatment increased by 347.3, 203.8, 91.8, 129.5 and 39.2 kg P ha−1, respectively. The accumulation of Fe-P and Al-P associated with NPK + S treatment were increased as well (Table 3).

Table 3 The effect of treatments on the accumulation of different P fractions in the 0–20 cm soil depth (kg ha−1).

Proportions of P fractions

The concentrations of different P fractions increased significantly in the 0–40 cm soil depth (P < 0.05) (Fig. 7). Compared with CK, the proportions of Po fractions of the total P associated with NPK and NPK + S treatments decreased by 0.3–3.5%, while the proportions of Ca2-P, Ca8-P and Fe-P of total P associated under NPK and NPK + S treatments increased by 1.0–4.0 percentage. The greatest variations among these fractions were observed at the 0–20 cm soil depth. However, OM treatment did not impact the proportion of different Po fractions of the total P compared with CK treatment. Field treatments did not impact the proportion of different P fractions of the total P below 40 cm soil depths.

Figure 7
figure7

Effect of long-term application of chemical fertilizer, organic manure and straw on the percentage of different P fractions. LOP, labile organic P; MLOP: moderately labile organic P; MSOP: moderately stable organic P; HSOP: highly stable organic P. CK: no fertilizer application, NPK: chemical fertilizer application, NPK + S: chemical fertilizer plus maize straw incorporation, OM: organic manure application.



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