Effect Of Phosphorus Fertilizer Rate On Crop Yield,Phosphorus Utilization And Soil Phosphorus Transformation In A Rice-Oilseed Rape Rotation

Rice-oilseed rape rotation is the main cropping system in the Yangtze River basin,which has made a significant contribution to ensuring the food and oil supply security in China.The problems of excessive phosphorus（P）fertilizer application and unreasonable annual distribution are prominent in production,limiting the improvement of crop yield and P fertilizer utilization,and aggravating the environmental problems caused by P loss.P application is an important measure to ensure high crop yield,and P fertilizer management affects the soil-crop system P cycle characteristics.The seasonal alternation of soil wet and dry in rice-oilseed rape rotation and the difference of crop response to P make the P cycling process complex.Therefore,it is important to clarify the effects of different P fertilizer rates on crop P uptake,soil P transformation and migration in rice-oilseed rape rotation to optimal P fertilizer application,improve crop P utilization efficiency and reduce P loss.In this study,245 typical soil P contents in 14 provinces of the Yangtze River basin were analyzed to clarify the current status of regional soil P;relying on a positioning experiment platform during 2016-2021,the effects of different P fertilizer rates（0,45,90,135 and 180 kg P₂O₅/hm²）on crop yield,P uptake,and soil P transformation and migration in the rice-oilseed rape rotation were elucidated.The main results are as follows:（1）Soil total P（TP）was abundant in the cultivated layer（0-20 cm）of the rice-oilseed rape rotation planting area in the Yangtze River basin,and the deficiency and excess of available P（Olsen-P）coexisted.The average contents of TP,Olsen-P and soluble P（Ca Cl₂-P）were 0.62 g/kg,23.2 mg/kg and 0.49 mg/kg,respectively.23.1%of the soils were deficient in Olsen-P,mainly in the middle reaches of the Yangtze River.The soil Olsen-P leaching threshold was 39.9 mg/kg,and soil Olsen-P content in 13.0%area was higher than the leaching threshold.The regional soil P pool was dominated by inorganic P（Pi）,with an average of 82.2%.The labile P（Na HCO₃-Pi）and moderately labile P（Na OH-Pi）contents were significantly and positively correlated with Olsen-P content,and the stable P pool（HCl-Pi and Residual-P）at different Olsen-P levels were not significantly different.（2）P fertilizer application significantly increased crop yield and shoot P uptake in the rice-oilseed rape rotation.P application mainly contributed to yield improvement by increasing the number of pods and panicles per plant of rice and oilseed rape,which increased 5.7%-17.1%and 164.1%-1254.3%for rice and oilseed rape,respectively,compared with the no P treatment（P₀）.The recommended P application rates for maximum yield in rice and oilseed rape were 35.4 kg P₂O₅/hm²and 57.2 kg P₂O₅/hm²,respectively.Excess P fertilizer inputs reduced P harvest index（PHI）and P recovery efficiency（PRE）.P fertilizer rates significantly affected soil P balance,with annual P deficit of 31.8 kg P/hm² in the P₀ treatment and soil apparent P balance（APB）of-4.7-97.1 kg P/hm² in the P application treatment.There was a significant positive correlation between the annual soil APB and the change in soil Olsen-P content,with an average increase of 5.0 mg/kg of Olsen-P content in the cultivated soil for every 100 kg P₂O₅/hm² surplus.（3）P application significantly promoted crop leaf growth and P uptake.Leaf P allocation was sensitive to the response of P supply,especially leaf Pi concentration,which increased by 77.4%and 216.8%on average in rice and oilseed rape under P application compared to P₀ treatment.P application promoted the allocation of leaf P to Pi to meet the requirement of maintaining higher photosynthetic capacity.Compared with oilseed rape,rice achieved significant photosynthetic capacity return with a small amount of residual P（P_R）as the cost and a corresponding increase in Pi with higher P uptake efficiency（PUp E）,utilization efficiency（PUt E）,and photosynthetic P utilization efficiency（PPUE）.（4）P fertilizer inputs significantly increased soil TP,Olsen-P and Hedley P fractions,with increases in labile Pi（H₂O-Pi,Na HCO₃-Pi）and moderately labile Pi（Na OH-Pi）dominating.Newly applied P fertilizer into the soil was mainly stored in the labile and moderately labile P pools,and gradually transformed to the stable P pool with the extension of the reproductive period.Lower soil Eh in the rice season induced an increase in Fe activation and promoted the transformation of Na OH-Pi to H₂O-Pi,thereby enhancing soil P bioavailability;lower soil p H in the oilseed rape season mobilized the transformation of HCl-Pi to Na HCO₃-Pi,Na OH-Pi and organic P（Po）.Paddy and upland showed different potential biologically available P supply pathways,with both Na OH-Pi and Na HCO₃-Pi having direct positive effects on H₂O-Pi in the rice season and only Na HCO₃-Pi having a direct positive effect on H₂O-Pi in the oilseed rape season,and soil P bioavailability（DGT-P）was better in rice season than in oilseed rape season.（5）P fertilizer application increased field ponding water and percolation water P concentrations in the rice-oilseed rape rotation,while increasing runoff and leaching P losses.P concentrations in both field ponding water and percolation water in the rice season peaked after 1 d of P application and then decreased rapidly.Field ponding water P concentrations decreased to a minimum and stabilized after 10 d of P application in the medium P treatment(P₉₀),while the high P treatment(P₁₈₀)extended this period to 20 d.Percolation water P concentrations in oilseed rape season reached the highest at the first sampling and then decreased continuously.Annual P losses of the P application treatments ranged from 0.62-1.49 kg P/hm² with loss coefficients of 0.20%-0.57%,which increased P losses by 55.0%-109.9%compared to the P₀ treatment.P fertilizer,soil P content and precipitation together led to different P losses within and between seasons,with runoff from heavy precipitation during the rice season being the main pathway for annual P losses.In addition,compared to no P treatment,the deep（40-60 cm）soil TP increased by 11.4%in the P application treatment,and excess P fertilizer inputs increased soil P surplus and vertical transport.In conclusion,reasonable P fertilizer inputs promote crop P uptake by increasing the intensity of cultivated soil labile P supply,and thereby improved crop yield by regulating leaf photosynthetic P allocation.The increased soil P bioavailability in the rice season and the strong uptake capacity of rice to the relatively more stable P in the soil and the high utilization capacity of leaf photosynthetic P allocation cause lower P demand in the rice season than in the oilseed rape season.Pay attention to upland season P fertilizer input based on annual balanced P application to maintain high crop yield and high P fertilizer efficiency in rice-oilseed rape rotation and reduce soil P surplus and loss.