Phosphorus（P）is an essential element for all forms of life on earth.P is by far the most important mineral nutrient for crop production,after nitrogen（N）.The production of crops for food requires an adequate supply of P in the soil.However,surplus application of P is common in China for both cereal and other systems,how P transforms and accumulates in deep soil profile needs more investigation to further understand P performance.The Loess Plateau region is an important and large arable crop production area in Northwest China.Dryland farming is the dominant approach in this region,with single crops of winter wheat（Triticum aestivum L.）or spring maize（Zea mays L.）each year.The double cropping with irrigated winter wheat and summer maize plays the key role in supplying food for local people.In addition,in past 20 years land-use changes from arable crop to greenhouse vegetable or kiwi-orchard happen widely in this area,which led to great changes in nutrient input.However,limited information is available about how continuously applying P fertilizers from different sources affects P efficiency,P dynamics and P transport in the soil profiles.This study investigated 1)P efficiency,critical P levels,soil P availability and distribution of P fractions in soil profile based on 25-year long-term fertilization experiments;2)accumulation and distribution of P in soil profile after conversion cereal crops to greenhouse vegetable and kiwi-orchard.Long-term experiments involved both single and double cropping systems.The single cropping included four treatments,i.e.control（CK）,application of mineral P and potassium（PK）,combining application of NPK,organic manure combined with NPK（MNPK）.The double cropping system involved five treatments,in which four was same as those under single cropping,and the additional one was crop straw combined with NPK（SNPK）.The three land-uses were also involved:（1）wheat-maize double cropping system;（2）greenhouse vegetable;and（3）kiwi-orchard.The main results obtained were as follow:The P efficiency ranged from 20 to 54%under the double cropping system and from 13to 33%in the single cropping system over the 25 years studied,and the highest P efficiency was in the NPK treatment in both systems.Application of P significantly increased soil total-P,Olsen-P and CaCl₂-P over the years,especially in the MNPK treatment.Two segment regression analyses indicated that Olsen-P was about 14 mg kg^-1 or 16 mg kg^-1 for optimal crop yield under double cropping or single cropping.In addition,Olsen-P content above 37mg kg^-1 or 57 mg kg^-1 led to a significant increase in CaCl₂-P content under double cropping or single cropping,which might indicate risk of P leaching.After 24 years of P applications,total P accumulated at various depths depending on treatment under double cropping,but there was no accumulation under single cropping.While Olsen-P leached down to a depth of300 cm in the MNPK treatments under both cropping systems,and higher concentrations were recorded in the deeper layers in single cropping than in double cropping systems.The Olsen-P leached to a shallower depth in the PK and NPK treatments under single cropping compared to double cropping.It is concluded that P supply in excess of the crop’s requirement（e.g.MNPK）or an unbalanced nutrient supply（e.g.PK）resulted in not only low P efficiency and massive accumulation of P in the topsoil but also leaching out of root zone under both irrigated and rain-fed conditions in the loess soil.Thus,P fertilization recommendations need to be adjusted based on changes in soil P over time.After long-term P fertilization under double cropping system soil labile P,moderately labile P and less labile P pools in the surface soil（0-20 cm）was significantly higher under MNPK than under other treatments,and the differences between NPK and SNPK treatments was non-significant.All P pools in NPK and SNPK treatments were also significantly higher those in CK treatment.Furthermore,in subsoil layers（20-100 cm）,these P pools were consistently greater under MNPK treatment compared with CK.Moderately labile P and less labile P were also significantly higher in NPK and SNPK treatments than in CK.For labile P pool,long-term fertilization regimes（NPK,SNPK and MNPK）mainly increased the organic P extracted by NaHCO₃ throughout soil profile（0-100 cm）.But MNPK also significantly enhanced both water and NaHCO₃extracted inorganic labile P.While long-term fertilization increased moderately labile P pool through increasing NaOH extracted organic P.Results suggest that long-term fertilization regimes mainly enhanced moderately labile P pool throughout soil profile.Compared with arable land,vegetable greenhouse or kiwi orchard showed significantly greater soil P accumulation at surface layer,i.e.CaCl₂-P increased by 10-20 times,Olsen-P by3-5 times,Mehlich-3 extracted P by 3-6 times and organic P by maximum of 4 times.Degree of P saturation（DPS）increased from 4%under arable crop to 14%under kiwi-orchard and22%under greenhouse vegetable.Land conversion also markedly increased soil available P（Olsen-P and Mehlich-3 extracted P）distributing in deep soil layers down to 300 cm,especially for greenhouse vegetable,where Olsen-P or Mehlich-3 extracted P contents reached the critical P value for arable crop to achieve maximum yield.This indicates a huge amount of P had been further leached down to deep soils under the current management.Conversion from cereals to greenhouse vegetable significantly increased the easily-available-P and moderately available-P pools in the top 60 cm depth,and non-available-P pool at top 40 cm depth.The increase in labile inorganic fractions was the main contribution for increases of easily-available-P and moderately available-P pools in comparison to the labile organic fractions.However,conversion from cereals to kiwi-orchard significantly increased three P pools at top 20 cm depth.Easily-available P fractions（i.e.,H₂O-Pi,NaHCO₃-Pi and NaHCO₃-Po）were strongly and positively correlated with organic carbon content,suggesting that SOM plays a key role in maintaining P availability.Overall,long-term surplus P application has led to great amount of P accumulating and distributing in deep subsoil layers which were prone to economic losses and posing the high potential environmental risks.Hence,there is an urgent need to update the P management strategies for both cereal cropping system and greenhouse vegetable or kiwi orchard to prevent the situation from getting worsen in future.