Retention And Release Of Soil Phosphorus From Various Aggregates In Upland Red Soils With Long-term Fertilization

Red soil is the most important grain producing soils in Southern China.Naturally poor with respect to their physical conditions, red soils are characterized as low pH and high iron-aluminum oxides resulting in lower concentrations of phosphate in soil solution and frequent phosphorus (P) deficiencies to plants.Application of chemical fertilizer alone and combined with organic materials such as crop straw (e.g. rice straw, fresh residues etc.) and framyard manture (e.g. pig manure), is the basic approch to improve P fertility in red soils. However, soil accumulated P easily to loss into water body and pollute the water environment after long-term application of chemical and/or organic amendments. Soil aggregates in the soil are important media capable of storing, transforming and acitvating soil P, and distribution and stability of soil aggregates is influenced by long-term fertilization.However, little has been reported on release pattern and adsorption capacity of P across various soil aggregates and their relationships in red soils as affected by long-term fertilization, which are essential to sustainable management of P sources in agricultural fields. The objectives of this study are to: (ⅰ) investigate the distribution of various aggregates in a red soil under long-term fertilization; (ii) changes in TP,Bray P, CaClh-P, PSI and DPS (SPSC) in soil aggregates of different particle-size fractions; (ⅲ) to explore relationships between CaCl2-P, Bray P, DPS (SPSC); and (ⅳ)to study how DPS and Bray P affect P loss potential.A long-term fertilization experiment was established in 1988 and soil samples were collected from the 11 fertilization treatments, designed in the experiment as follows: NPK, NK, PK, NP, NPKCa, NPKCaS; and Control (CK), CK + peanut straw (PS), CK + rice straw (RS), CK + fresh radish (FR) and CK + pig manure(PM). The soil samples were fractionated, using the wet sieving method,into aggregates of different particle sizesb (> 2.0 mm, 2.0～1.0 mm,1.0～0.25 mm and 0.25～0.053 mm) transformation and mobility. Then the various fractions of aggregates were analyzed for total P (TP),Bray P,CaCl2-P,P sorption index (PSI)degree of P saturation (DPS), soil phosphate sorption capacity (SPSC) and relationships between the parameters. Results show that::(1) The proportion of aggregates in the upland red soil after long-term application of inorganic fertilizer was in the order of: 1～0.25 mm(45.8%～64.5%) > 0.25～0.053 mm(14.0%～21.5%) > (> 2 mm, 2.2%～18.9%) > (<0.053 mm,7.0%～12.7%) > 2～1 mm (5.7%～9.6%). The proportion of aggregates in the upland red soil after long-term combined application of organic fertilizer was in the order of: 1～0.25 mm (44.1%～62.2%) >(> 2 mm, 13.2%～24.9%) > 0.25～0.053 mm (9.0%～16.1%) > 2～1 mm (6.0%～10.4%) >(< 0.053 mm,5.0%～12.0%).Soil aggregate size in > 2 mm was significantly increased after long-term fertilization.The mean weight diameter (MWD) of aggregates in upland soils affected by the application of inorganic fertilizer and combined with organic fertilizer were 0.66～0.79 and 0.86～0.92,respectively.(2) Long-term application of phosphate fertilizer can effectively maintain soil P supplied level in upland red soil. Compared with Treatment NK,soil TP of various aggregates was significantly increased by the order of: (> 2 mm) (122.6%) > 0.250.053 mm (103.6%) > 2～1 mm(88.7%) > 1～0.25 mm (85.5%).Compared with Treatment CK, soil TP of various aggregates was increased by the order of: 1～0.25 mm (78.7%) >(> 2 mm) (76.6%) > 0.25～0.053 mm (61.6%) > 2～1 mm (50.7%),but only PM treatment significantly increased the soil TP content of various aggregates. All the fertilization treatments, irrespective of their pattern, significantly increased P content in the > 2.0 mm fraction of aggregates with PEC (P enrichment coefficient) >1, but in Treatment NPK + PM soil P was more easily enriched in macro-aggregates (> 0.25 mm).(3) Compared with Treatment NK, the PSI of various aggregates in Treatment NPK was decreased in order of: (> 2 mm) (141.2%) > 1～0.25 mm (127.5%) > 2～1 mm (112%) > 0.25～0.053 mm (70.5%). Compared with Treatment CK, the PSI of various aggregates was increased in order of: 1～0.25 mm (48%) > 2 ～1mm(45.6%) >(> 2 mm) (38.9%) > 0.25～0.053 mm (35.1%).(4) Compared with Treatment NK, soil Bray P, DPS and CaCl2-P of various aggregates in Treatment NPK were significantly increased by 465%～758%,281%～620% and 23.9% ～ 50%,respectively. Compared with Treatment CK,soil Bray P,DPS and CaCl2-P of various aggregates in Treatment PM were significantly increased by 856%～1283%,4270%～355%,1000%～849%,respectively.(5) In upland red soil, with the increased of proportion of aggregates size > 1 mm in red soil, soil TP, Bray P, CaCl2-P and DPS increased significantly, but with the increased of proportion of aggregates size < 1 mm , soil TP, Bray P, CaCl2-P and DPS decreased significanly. With the increased of soil TP, soil Bray P, CaCl2-P and DPS significantly increased, but PSI and SPSC significantly decreased.(6) Correlation analysis indicates that the P accumulated in the soil maybe subject to potential lossto the safety of water quality when Bray P was range in 168～260 mg kg-1 or DPS < 28% in the red soil; otherwise P loss would be significant.Therefore, new strategies of using both commercial fertilizers and animal manures must be established and implemented to prevent P build up in the soil and to minimize P loss to water bodies in the red soil region.