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The Olsen phosphorus (Olsen-P) concentration of soil is generally a good indicator for estimating the bioavailability of P and environmental risk in soils. To maintain soil Olsen-P at adequate levels for crop growth and environmental sustainability, the relationship between soil Olsen-P and the P budget (the P input minus the output) as well as the variations of soil Olsen-P and P budget were investigated from three long-term (22 years) experiments in China. Five treatments were selected: (1) unfertilized control (CK); (2) nitrogen and potassium (NK); (3) nitrogen, phosphorous, and potassium (NPK); (4) nitrogen, phosphorous, potassium and straw; (5) nitrogen, phosphorous, potassium and manure. The results showed that without P fertilizers (CK, NK), there was a soil P deficit of 75-640 kg ha-1, and the lowest P deficit (mean of CK and NK) was in Eutric Cambisol. Soil Olsen-P decreased by 0.11-0.39 mg kg-1 year-1 in the order of Luvic Phaeozems > Eutric Cambisol > Calcaric Cambisol. Soil Olsen-P and the P deficit had a significantly (P<0.01) positive linear relationship. For every 100 kg of P ha-1 of deficit, soil Olsen-P decreased by 0.44-9.19 mg kg-1 in the order of Eutric Cambisol > Luvic Phaeozems > Calcaric Cambisol. Under the P fertilizer treatments (NPK, NPKS, and NPKM), soil Olsen-P showed an obvious surplus (except the NPK and NPKS in Luvic Phaeozems) of 122-2190 kg ha-1, and the largest P surplus was found under the NPKM treatment at each site. The relation between soil Olsen-P and the experimental years could be simulated using quadratic equation of one unknown in Calcaric Cambisol for the lower P input after 14 years of fertilization. And soil Olsen-P increased by 1.30-7.69 mg kg-1 year-1 in the order of Luvic Phaeozems > Eutric Cambisol. The relation between soil Olsen-P and the P surplus could be simulated by a simple linear equation except under NPK and NPKS in Luvic Phaeozems. With 100 kg ha-1 P surplus, soil Olsen-P increased by 3.24-7.27 mg kg-1 in the order of Calcaric Cambisol (6.42 mg kg-1) > Eutric Cambisol (3.24 mg kg-1). In addition, the change in soil Olsen-P with a 100 kg P ha-1 surplus (soil Olsen-P efficiency) was affected by the soil organic matter (SOM), pH, and CaCO3 content, etc. In the practice of fertilization, it's not necessary to increase the amount of P fertilizers, farmers should take measure to solve the local problem, for adjust the soil pH of Eutric Cambisol and Calcaric Cambisol, and apply more nitrogen in Luvic Phaeozems. In the area of serious soil P surplus, it is encouraged to stop applying P fertilizers for a few years to take advantage of soil accumulated P and make the high Olsen-P content decrease to a reasonable level., Competing Interests: The authors have declared that no competing interests exist.

The phosphorus activation coefficient (PAC, the ratio of available P to total P) is an important indicator of soil P availability and the transformation of P fractions. Understanding the details of the PAC is useful to estimate soil available P status and to provide P management guidance. In this research, soils from five long-term (23 years) fertilization treatments in three croplands were selected to examine the relationships between the PAC and P fractions and to analyse the influencing factors. PAC was affected by both soil types and fertilization treatments. Compared to the unfertilized control (CK) treatment, long-term P application significantly increased the PAC, all of the inorganic P (Pi) fractions and most of the organic P (Po) fractions in all the three soils, particularly in chemical fertilizer combined with manure treatment (NPKM). The PAC was significantly correlated to all of the Pi fractions proportions (P<0.05) except for Dil. HCl-Pi and Conc. HCl-Pi. Compared with CK, the chemical P and chemical P combined with manure treatments increased the ratio of total Pi fractions to total Po fractions (Pit/Pot); furthermore, NPKM significantly increased the organic C (Co) content and decreased the Co/Pot ratio. Stepwise multiple regressions showed that PAC = 0.93 Co+0.69 Pit/Pot-0.07 Co/Pot-0.27CaCO3-3.79 (R2 = 0.924, P<0.001). In addition, the variance partitioning analysis showed that more variance of PAC is explained by soil factors (29.53%) than by P input (0.19%) and climate (0.25%) factors. Our findings demonstrate that P application increased the PAC by changing the Co content and the proportion of P fractions. Moreover, soil factors were the most important drivers of P transformations, and NPKM was optimal for improving soil fertility in Chinese croplands.

Objective Soil contamination by cadmium (Cd) and lead (Pb) is a great environmental concern for food safety, and breeding cultivars that can reduce their uptake is one way to grow crops in Cd-and Pb-contaminated soils. In this paper, we experimentally studied how wheat lines with different phosphorus (P) uptake affect their uptake, translocation and accumulation of P, Cd and Pb in the crops. Method The experiment consisted of a lab part and a field part. The lab part considered one factor -- wheat varieties, and the field part was arranged randomly in soil that had been moderately contaminated by Cd and Pb. In both parts, we measured Cd and Pb uptake and their subsequent translocation. Result The average P content in the roots of lines with high P (HP) use efficiency was 7.69 mg/g, significantly lower than that in the lines with low P (LP) use efficiency, which was 8.36 mg/g (P<0.05). The average P content in the shoots of HP varieties was 9.73 mg/g, significantly higher than the 8.19 mg/g in the LP cultivars (P<0.05). The transport coefficient of P in HP varieties was 1.45, significantly higher than the 1.19 of the LP varieties (P<0.05). The kinetic uptake of Cd and Pb by roots and their translocation to shoots followed Michaelis- Menten equation (R2>0.94), with the maximum absorption rates (Vmax) and the Michaelis constants (Km) differing significantly between varieties (P<0.05). The Vmax of the roots of HP varieties was 23.9 and 411 μg/(g·d), respectively, for Cd and Pb, in comparison with 80.0 and 835 μg/(g·d) (P<0.01) for the LP lines. In contrast, Vmax for Cd and Pb in the shoots of the HP varieties was much higher than that for the LP varieties. The difference in Km between the HP and LP varieties was consistent with their difference in Vmax. Correlation analysis showed that the transport coefficients of both Cd and Pb were negatively correlated with root P content (r=-0.638 and -0.714, respectively) at significant level. The transport coefficient of Cd was positively correlated with P transport coefficient (r=0.671, P<0.01). The Pb transport coefficient was also positively correlated with P transport coefficient though not at significant level (r=0.391, P>0.05). The increase in root P content helps reduce the translocation of Cd and Pb; an increase in translocation coefficient of P did not show a noticeable impact on Pb translocation, but significantly boosted Cd transport from roots to the shoots. Conclusion The roots of LP varieties showed high uptake but low translocation of P, Cd and Pb, as opposed the HP varieties which showed a low uptake but high translocation. This suggests that LP varieties are more suitable for growing in soils contaminated by Cd and/or Pb. [ABSTRACT FROM AUTHOR]

With the increasing pressure on phosphorus (P) resources and global water eutrophication, the development of sustainable land management is pushed forward by detailed knowledge on the nature and fate of phosphorus (P) in agricultural soils under various long-term fertilizer regimes. Using bulk- (solution 31P nuclear magnetic resonance; P K-edge X-ray absorption near-edge, XANES) and micro- (X-ray fluorescence and XANES) spectroscopic techniques, we investigated the molecular speciation of P in an alkaline soil from plots under wheat-maize rotation with or without chemical P fertilizers for 26 years, since 1990. The long-term experimental results show that P buildup and drawdown were mainly in the inorganic forms in response to P addition and depletion. The added P increased P accumulation in the forms of brushite and deoxyribonucleic acid that preferentially depleted without P addition, indicating that these species were phytoavailable. Although P forms as hydroxyapatite and orthophosphate monoesters were increased under P fertilization, no significant depletions of these P forms were observed when P fertilization was withheld. In contrast, iron-associated P increased with and without P fertilization. These results from this long-term experimental site facilitate understanding of dynamics and availability of fertilizer P and the existing legacy P in soils, with important implications for sustainable P management in these soils and similar soils in other regions. • P Speciation in alkaline soils characterized by NMR, XANES and microprobe • Soil P transformation after long-term P fertilization and unfertilization • Brushite and DNA enriched and depleted with and without fertilization, respectively • HAP and Mono-P enriched and unchanged with and without fertilization, respectively • Fe-P increased insignificantly regardless of P fertilization. [ABSTRACT FROM AUTHOR]

Abstract: Soil degradation and water pollution could have resulted from inappropriate phosphorus (P) supply. Soil Olsen P is generally a good indicator to estimate bio-availability of P and environmental risk in alkaline soil. The change in Olsen P is always strongly affected by soil organic carbon (SOC) when different forms of inorganic and/or organic P are applied to farmlands with chemical fertilizer and/or manure. We related soil Olsen P to apparent P balance (APB) and SOC in alkaline soils at three 15-year (1991–2005) fertilization sites of northern China. Six treatments were examined: unfertilized control, chemical nitrogen (N), chemical NP, chemical N plus potassium (NK), chemical NPK, and chemical NPK plus animal manure (NPKM, same total N but 20–80% more P). Compared to the initial Olsen P in 1990, after 15years Olsen P was increased under P fertilization but decreased under no-P fertilization. At the three 15-year fertilization sites, annual mean Olsen P was 4.9–12.3 times higher under NPKM, 1.9–2.8 times higher under NP and NPK, but only 28.3–84.8% under Control, N and NK. Annual mean percentage of Olsen P to total P was higher under NPKM (4.6–8.1%) than under other five fertilizations (0.4–2.9%). Change in Olsen P significantly positively correlated with accumulated APB under all fertilizations (r 2 =0.10–0.31, P <0.05). At these three sites, Olsen P could be increased by 5.2, 5.8 and 12.1% respectively under NP, NPK and NPKM when 1kgP/ha was in surplus, meanwhile Olsen P was decreased by 2.1, 4.0 and 6.4% under Control, N and NK when 1kgP/ha was in deficit. Significantly higher SOC accumulation was under NPKM than under other five fertilizations when calculated by the same unit of P input. The combination of chemical P and manure P with organic C input is a better strategy to increase soil Olsen P and SOC accumulation on farmland with maize–wheat rotation in northern China. [Copyright &y& Elsevier]

Highlights: [•] Long-term field research proved that manure application with chemical fertilizer improved the nitrogen use efficiency (NUE) of wheat–corn rotation system the most especially in acid soil. [•] Phosphorus is found to be another important factor to improve NUE up to 40–60% if soil is P deficient. [•] Between the rotation crops, both grain yield and NUE of wheat were determined more responsive to P fertilization, and that for corn were more responsive to manure application. [Copyright &y& Elsevier]

Abstract: Long-term (over 15 years) winter wheat (Triticum aestivum L.)–maize (Zea mays L.) crop rotation experiments were conducted to investigate the accumulation of phosphorus (P) at five sites differing geographically and climatically in China. The results showed that, in soils without P added, the concentration of soil P extracted by 0.5molL−1 NaHCO3 at pH 8.5 (Olsen-P) decreased with cultivation time until about 3mgkg−1, afterwards it remained constant. The trend of decrease in Olsen-P in soils without P added could be described by an exponential function of time. The concentration of Olsen-P in soils with P fertilizers increased with cultivation time and the model of accumulation of Olsen-P in soils could be described using P application rate, crop yield and soil pH. The accumulation rate of Olsen-P in the long-term wheat–maize crop rotation experiments was 1.21mgkg−1 year−1 on average. If the target yield of wheat and maize is 10tonha−1 in the soil with pH 8, the increasing rates of Olsen-P in soils as estimated by the model will be 0.06, 0.36, 0.66, 0.95, 1.25 and 1.55mgkg−1 year−1 when P application rates are 30, 40, 50, 60, 70 and 80kgPha−1 year−1, respectively. The models of accumulation of Olsen-P in soils were validated independently and could be used for the accurate prediction of accumulation rate of Olsen-P in soils with wheat–maize rotation systems. Also the application of the model was discussed for best management of soil P in agricultural production and environment protection. [Copyright &y& Elsevier]