【Objective】 Fertilizer is generally added to agricultural soil to meet the needs of crop production, but long-term over fertilization changes soil phosphorus (P) pool and soil properties. This study evaluated the characteristics change of P adsorption and desorption and its response to soil properties under long-term fertilization, to do a favor to provide theoretical basis of rational fertilizer application and improve the P availability of black soil.【Method】 Four treatments, including no fertilizer (CK), urea and potash sulphate (NK), urea, super-calcium phosphate and potash sulphate (NPK), and NPK plus pig manure (NPKM), were investigated in a 21-year (1989-2010) long-term fertilization experiment at Gongzhuling (Jilin Province) of China. The crop of cropping system was maize. Soil samples were collected in 1990, 2000 and 2010 at 0-20 cm depth to analyze soil properties and to measure soil P adsorption and desorption characteristics. Langmuir equation was used to fit the P adsorption curve, and then the maximum adsorption capacity (Qmax), adsorption constant (K), buffering capacity of soil P (MBC), and P sorption saturation (DPS) were calculated according to Langmuir equation.【Result】 There was a good fitness between the P adsorption curve and Langmuir equation (R 2=0.93-0.99, P＜0.01). There existed difference for P adsorption and desorption characteristic under the four treatments. Over time, compared with initial year, for CK and NK treatments, the Qmax value increased by 1.83 and 1.61 times, MBC value increased by 0.80% and 49.40%, DPS value decreased by 92.04% and 87.50%, Readily Desorbable Phosphorus (RDP) value decreased by 20.00% and 82.83%, respectively; for NPK treatment, Qmax and DPS value increased by 81.87% and 79.56%, MBC and RDP value decreased by 79.37% and 48.57%, respectively, while under NPKM treatment, the Qmax and MBC value decreased by 33.35% and 78.52%, DPS and RDP values increased by 11.36 and 1.48 times, respectively. After 21 years experiments, compared with CK and NPK treatments, the Qmax and MBC value of NPKM treatment decreased by 64.66% and 49.52%, 81.87% and 79.56%, respectively; the DPS and RDP value of NPKM treatment increased by 110 and 3.81 times, 4.36 times and 78.57%, respectively. Compared with other treatments, the Total-P, Olsen-P, soil organic matter (SOM) and CaCO3 contents increased and SSA decreased significantly, but the pH, free Fe2O3 and Al2O3 value kept unchanged under NPKM treatment. RDA test showed that SOM and Total-P were the main factors that explained 49.5% and 18.7% of the total variation (P＜0.05) which caused the difference of P adsorption desorption characteristic parameters among four treatments.【Conclusion】 Long-term combination of NPK fertilizers with manures could significantly increase SOM and P accumulation contents, decrease the soil adsorption capacity and increase desorption capacity, and improve P availability in soil, but it significantly increased the DPS value, easily thereby caused the risk of phosphorus loss. Therefore, various management practices and inorganic and organic P fertilizer input amounts should be considered to reduce P losses from this area.
【Objective】 Change characteristic of phosphorus (P) in red soil were quantified under long term different fertilizations, and effects of P on soil productivity were studied, so as to provide a theoretical basis for P management in red soil regions in southern China. 【Method】 Change of soil Olsen-P and total P content was compared under long term no P fertilization (CK, N, NK), chemical P fertilizer (PK, NP, NPK), application of chemical fertilizer combined with half crop straw return (NPKS) and application of organic manure and chemical fertilizer combined with organic manure (1.5NPKM, NPKM, M) using long term trial platform in the upland red soil (from 1991-2016). Responses of soil P to P balance were analyzed. Different models were used to fit the response curve of crops yield to Olsen-P in the red soil. The critical P value of soil Olsen-P for crop yield (CV) was calculated. 【Result】 The Olsen-P and total P increased, and P activation coefficient (PAC) promoted under long term P fertilization in the red soil. PAC was higher under application of organic manure and chemical fertilizer combined with organic manure (1.5NPKM, NPKM, M) than that under application of chemical fertilizer combined with half crop straw return (NPKS) and chemical P fertilizer (PK, NP, NPK). Changes of Olsen-P and total P were significantly affected by P balance (P＜0.01). Soil Olsen-P increased by 3.00-5.22 mg·kg -1, and total P increased by 0.02-0.06 g·kg -1 with 100 kg P·hm -2 cumulative surplus in the red soil. The Olsen-P decreased with years under long term no P fertilization (CK, N, NK), and they decreased by 1.85 mg·kg -1, 0.40 mg·kg -1, and 1.76 mg·kg -1, respectively, with 100 kg P·hm -2 cumulative deficiency in the red soil. Grain yields of wheat and maize were significant higher under application of organic manure and chemical fertilizer combined with organic manure (1.5NPKM, NPKM, M) than that under application of chemical fertilizer combined with half crop straw return (NPKS) and chemical P fertilizer (PK, NP, NPK) than under no P fertilizations (CK, N, NK). Sustainability index of grain yield (SYI) were higher under application of organic manure and chemical fertilizer combined with organic manure (1.5NPKM, NPKM, M) than that under the other treatments. Three models (linear-linear model, linear-platform model and Michelice model) were better fit the response of crop yield to Olsen-P in red soil (P＜0.01). Linear-linear model was recommended for the higher R 2. The critical value of Olsen-P content in agronomic in red soil of wheat and maize were 13.5 mg·kg -1, and 23.4 mg·kg -1, respectively, calculated by linear-linear mode. 【Conclusion】 Application of chemical fertilizer combined with organic manure were recommended in red soil region of southern China. That had the beneficial for P accumulation and promoting P availability. Applications of chemical fertilizer combined with organic manure were also used to keep the high and stable production. Linear-linear model was recommended to calculate the critical value of Olsen-P content in agronomic. Application rates of P fertilizer should be adjusted timely according to the difference between actual Olsen-P content in the soil and critical value of Olsen-P content in agronomic in productivity.
【Objective】 It is of great significance to maintain and improve soil quality and land productivity, meanwhile enhance the phosphorus (P) use efficiency by studying the distribution characteristics and changes of soil phosphorus in winter wheat/summer maize cropping in Shaanxi Guanzhong Plain in recent 30 years. 【Method】 Based on 458 soil samples collected in total in 2011 from 10 typical counties in winter wheat/summer maize cropping in Guanzhong Plain analysis data, we compared its Olsen P content with the soil survey data in 1980s, and also explored the distribution characteristics of the current soil phosphorus pools at the regional level.【Result】 The results showed that the soil Olsen P content in the plough layer soils significantly increased during the past 30 years. The mean values of soil Olsen P in Baoji, Xianyang and Weinan were 26.09 mg·kg -1, 27.50 mg·kg -1 and 21.53 mg·kg -1, respectively, compared with soil survey data in 1980s, the corresponding increasing rate were 334.83%, 276.71% and 231.23%, respectively. The proportions of cultivated land with higher soil Olsen P levels were significantly increased, with the largest increase was observed in 15-30 mg·kg -1 and the largest decrease in Olsen P＜10 mg·kg -1. The total distribution of soil Olsen P and inorganic phosphorus was higher in the west (Baoji City) and the middle (Xianyang City) than in the east (Weinan City), and organic phosphorus content was opposite. At present, the soil Olsen P level in the large area of Guanzhong Plain under winter wheat/summer maize cropping was higher than the agronomic threshold, which could meet the supply of phosphorus in high yield crops. Meanwhile, the cumulative distribution probability of soil Olsen P content (greater than 37 mg·kg -1) was more than 20% in Baoji and Xianyang, the cumulative distribution probability of water-soluble P content (greater than 2 mg·kg -1) was more than 22%, so there would be a higher risk of water environment pollution. 【Conclusion】 Excessive application of phosphorus fertilizer under long-term intensive and high-intensity planting conditions significantly increased the Olsen P content in soil. Considering the protection of environment and rational utilization of limited phosphorus resources, the application of phosphorus fertilizer should be regulated reasonably, so as to apply fertilizer according to soil and supply fertilizer according to demand.
【Objective】 Investigating the contribution of inorganic phosphorus (P) fractions to soil available phosphorus under equivalent soil organic matter content might be helpful in development of phosphate fertilizer management strategy whereby to improve phosphorus use efficiency in a given soil.【Method】 We collected and screened the soils with a gradient of Olsen P level but the quite similar soil organic matter content (SOC ranges from 10.03 to 10.68 g·kg -1) in a Tier soil under winter wheat-summer maize cropping in Guanzhong Plain of Shaanxi Province. The Olsen P contents of the selected soil samples were 10.73 mg·kg -1, 18.06 mg·kg -1, 20.61 mg·kg -1, 24.01 mg·kg -1, 30.73 mg·kg -1, 43.69 mg·kg -1, and 58.58 mg·kg -1, respectively. We then analyzed the soil inorganic phosphorus forms with the phosphorus fractionation method developed by Chang & Jackson and modified by Jiang & Gu.【Result】 The results showed that the calcium bounded phosphorus fraction was the dominant form in the cultivated soil in the northwestern winter wheat-summer maize planting area, accounting for 66.67% of the total amount of inorganic phosphorus, of which dicalcium phosphate (Ca2-P), octa-calcium phosphate (Ca8-P) and apatite (Ca10-P) account for 2.80%, 16.80% and 47.09%, respectively, on average; and aluminum bounded phosphate (Al-P), iron bounded phosphate (Fe-P) and occluded phosphate (O-P) composed of 16.28%, 5.23% and 11.81%, respectively. Soil Olsen P was increased significantly and linearly with the increasing content of Ca2-P, Ca8-P, Ca10-P, Al-P, Fe-P and O-P; while the phosphorus activation coefficients, defined as the ratio of soil Olsen P to total P, were correlated significantly positively and linearly to the content of Ca2-P, Ca8-P, Al-P, Fe-P and O-P. The results of path analysis showed that the contribution of inorganic phosphorus to soil available phosphorus (Olsen P) in Guanzhong Plain area of Shaanxi Province was in the order of Ca2-P (0.974)＞Al-P (0.186)＞Ca8-P (0.182)＞Fe-P (0.150)＞Ca10-P (0.007)＞O-P (-0.074), the contribution of inorganic phosphorus to phosphorus activation coefficient (PAC) was Ca2-P (0.768)＞Al-P (0.082)＞Ca8-P (0.071)＞Fe-P (-0.018)＞Ca10-P (-0.055)＞O-P (-0.388), which was consistent with the contribution of soil phosphorus component to available phosphorus. The results of stepwise regression analysis showed that Ca2-P and Ca8-P were the major two contributors to Olsen P, but the Ca2-P contributed the most to PAC.【Conclusion】 Under the same or similar soil organic matter condition, Ca2-P was the most effective phosphorus source in Tier soil of wheat-maize growing area in Guanzhong Plain of Shaanxi Province. The increase of soil phosphorus availability was mainly achieved by increasing the proportion of phosphorus forms of readily available and relatively high availability such as Ca2-P, Ca8-P and Al-P, and reducing the proportion of very low availability forms, i.e. Ca10-P. Generally, under current cropping system, the application of phosphate fertilizers mainly kept the soil phosphorus in pools of available forms in the Guanzhong Plain.
【Objective】 In this study, the changes of phosphorus profit and loss as well as various forms of inorganic phosphorus in fluvo-aquic soil under long-term fertilizer application, and the effect of soil phosphorus profit and loss on inorganic phosphorus were discussed, which provided a theoretical basis for the rational application of phosphorus fertilizer in fluvo-aquic soil.【Method】 Based on the “National Long-term Monitoring Station for Soil Fertility and Fertilizer Benefit in Fluvo-aquic Soil”, the four fertilization modes in North China including NPK (single chemical fertilizer), SNPK (straw return), MNPK (organic-inorganic combined application), 1.5 MNPK (high amount of organic-inorganic combined application), were studied with the control of NK (non-phosphate fertilizer) to study the changes of apparent phosphorus profit and loss, cumulative phosphorus profit and loss, the content and relative content of each form of inorganic phosphorus, and the effect of soil phosphorus profit and loss on each form of inorganic phosphorus.【Result】 Under the condition of not applying phosphate fertilizer for 25 years, the phosphorus in the soil was always in a state of deficit, the cumulative deficit of phosphorus in soil was 431.8 kg·hm -2, after 25 years, the cumulative surplus of soil phosphorus on four phosphate fertilizer models (NPK, SNPK, MNPK, and1.5MNPK) was 291.2, 398.4, 1 742.4, and 2 676.9 kg·hm -2, respectively. When phosphorus fertilizer was not applied for a long time, Ca2-P decreased most in soil inorganic phosphorus by 49.0%. In the first 13 years, the soil Ca2-P of the above four fertilizer models increased by 1.2-5.4 times, with an average annual increase of 1.26-5.73 mg·kg -1. In the latter 12 years, the growth rate of soil Ca2-P decreased by 99.2%-112.6% by applying chemical fertilizer single, straw returning and organic-inorganic combined application model; and the content of Ca2-P in the soil decreased by 2.0 mg·kg -1 per year. The relative content of soil Ca2-P increased by 1.0%-3.5% in 25 years above four fertilizer models. After 25 years of application of phosphate fertilizer, the content of Ca8-P, Al-P, and Fe-P in soil increased 1.4-6.5, 1.8-3.3, and 1.1-2.2 times, respectively, with an average annual increase of 4.69-19.81, 1.67-3.10, and 1.23-2.37 mg·kg -1, respectively; the relative content increased 8.4%-3.0%, 3.3%-4.0%, 1.8%-3.3%, respectively. The contents of Ca10-P and O-P were oscillatory between 350-410 and 100-160 mg·kg -1 for a long time, but their relative contents decreased by 11.4%-29.7% and 3.1%-8.9%, respectively. Under the condition of not applying phosphate fertilizer for 25 years, for every 100 kg P·hm -2 cumulative deficit, Ca2-P, Ca8-P, Al-P, Fe-P, Ca10-P, and O-P in soil decreased by 1.2, 2.7, 1.1, 1.5, 0.8, and 7.5 mg·kg -1, respectively. Under the mode of single fertilizer and straw returning for 25 years, for every 100 kg P·hm -2 cumulative surplus, Ca2-P, Ca8-P, Al-P, Fe-P, Ca10-P, and O-P in soil increased by 3.9-5.0, 21.5-21.6, 6.5-7.4, 4.8-5.6, 4.0-7.5, and 2.4-7.2 mg·kg -1, respectively. Under the mode of organic and inorganic combined application for 25 years, for every 100 kg P·hm -2 cumulative surplus, Ca2-P, Ca8-P, Al-P, Fe-P, Ca10-P, and O-P in soil increased by 1.8-2.8, 14.2-16.4, 2.5-3.2, 1.9-2.6, -0.2-1.2, and 0.3-1.9 mg·kg -1, respectively. 【Conclusion】 The long-term application of phosphorus fertilizer could increase the surplus of phosphorus and the content and its relative content of Ca2-P, Ca8-P, Al-P, and Fe-P in fluvo-aquic soil, and the effect of organic and inorganic combined application was higher than that of single chemical fertilizer and straw returning. When the surplus of phosphorus in fluvo-aquic soil was the same, the increment of Ca8-P was the most in all fertilizer models, followed by Al-P and Fe-P, and the increment of Ca2-P, Ca8-P, Al-P, and Fe-P in soil with single chemical fertilizer was higher than that of straw returning and organic and inorganic combined application.
