| Rice(Oryza sativa L.)is the most important food crop globally.It also plays an important role in China's grain production.Application of nitrogen(N)fertilizer is an important measure to raise rice yield.However,the unreasonable application of N fertilizer not only leads to a large of N loss and low N fertilizer use efficiency(NUE),but also causes a series of negative environmental problems.Improving NUE is the key to reduce N application rate and increase rice yield.Many studies have paid attention to factors influencing NUE in rice production,but it mainly focuses on rice varieties,climatic and environmental factors,N fertilizer application,planting management and other agronomic measures.Although paddy soil N transformation characteristic has an important impact on rice N utilization and yield,the studies on the effect of paddy soil N transformation on rice NUE has not attracted extensive attention.To clarify the mechanism and key factors of paddy soil N transformation characteristic on NUE is the theoretical premise of targeted N regulation measures.In this study,the mechanism of soil N transformation characteristic affecting NUE was studied by field experiment,laboratory 15N labeling experiment combined with numerical analysis model and pot experiment,as well as effect of targeted N fertilizer regulation measures,taking typical paddy soils in major rice producing areas of China as the object.The main results are as follows.(1)Two sites with similar distance and same climatic conditions,but significant differences in soil N transformation characteristics(i.e.,acid soil with low nitrification rate and alkaline soil with high nitrification rate)were selected.Under the condition of the same management measures and rice varieties,a 2-year strictly comparable rice field experiment was conducted under different N application rates to determine the rice yield and apparent NUE.A 15N paired labelling experiment was used to determine the N transformation rates of the two soils.The results showed that the match of soil N transformation characteristic and rice N preference had an important impact on NUE and yield.The acid paddy soil with a low gross nitrification rate(1.98±0.09 mg N kg-1 d-1)and a high ammonium(NH4+-N)assimilation rate(2.46±0.82 mg N kg-1 d-1)was beneficial to the supply and retention of NH4+-N.Its N transformation characteristic was in good match with rice N preference,which was beneficial to improve yield and NUE.While in alkaline paddy soil,the gross nitrification rate was very high(7.91±0.15 mg N kg-1 d-1),and the NH4+-N assimilation rate was low(0.05±0.02 mg N kg-1 d-1),which were not conducive to the retention of NH4+-N,and even caused a large amount of N loss via denitrification.The N transformation characteristic of alkaline paddy soil was not consistent with rice N preference,resulting in the decline of yield and NUE.These results indicated that nitrification was the key factor affecting the retention time of NH4+-N,and may further affect the N loss via denitrification and NUE of rice.Soil pH was an important factor affecting the nitrification rate,which explained the phenomenon that the rice yield and apparent NUE of Guanxi(acid paddy soil)were higher than that of Shimeitang(alkaline paddy soil)under the same N application rate.(2)Based on the above experiment,a larger-sample study was carried out to further prove the effect of the match relationship on NUE and yield.In this study,50 typical paddy soils were collected from 18 provinces with large rice planting area in China.Soil gross N transformation rates and the retention time of NH4+-N fertilizer were quantified by 15N paired labelling experiment combined with numerical optimization model and injection method of membrane mass spectrometry.Pot experiment using 15N labelled urea was carried out to quantify the NUE and N loss of rice planted in different paddy soils under the same management measures,water and temperature.The results showed that NUE was closely related to the retention time of NH4+-N fertilizer.With the increase of average retention time of NH4+-N fertilizer,NUE of rice showed a logarithmic growth trend(p<0.05).The gross nitrification rate was positively correlated with soil pH,which was the key factor to regulate the retention time of NH4+N fertilizer.This was confirmed by the logarithmic decrease of NH4+-N retention time with the increase of the gross nitrification rate(p<0.05).Denitrification rate increased with the increase of gross nitrification rate(p<0.01),indicating that gross nitrification rate was also a key factor of N loss in nitrification-denitrification coupled process.Therefore,compared with alkaline soil,the N loss in acid soil was lower,while NUE was higher.These results explicated that soil N transformation characteristic and the factor controlling N transformation played an important role in rice yield and the improvement of NUE.(3)The effects of N transformation inhibitors on soil N transformation rate,rice yield,NUE and N loss were studied by 15N tracer experiment in two paddy soils with different pH(acid 5.18 and alkaline 7.83,respectively)in both laboratory and field.The results showed that the nitrification inhibitor and the double nitrification and urease inhibitors had no significant effects on the retention time of NH4+-N and the potential N loss risk(gross nitrification rate+ammonia volatilization rate,ONH4+VNH3)in acidic soils.However,in alkaline soil,the nitrification inhibitor had a significant effect on the gross nitrification rate,which reduced the total N loss of NH4+-N fertilizer(ONH4+VNH3)by 19.7%and prolonged the retention time of NH4+-N fertilizer.Similarly,the double nitrification and urease inhibitors also reduced the total N loss of NH4+-N fertilizer by 15.8%and increased the retention time of NH4+-N fertilizer in alkaline soil.The results of field experiment showed that the application of nitrification inhibitor and the double nitrification and urease inhibitors could prolong the retention time of NH4+-N fertilizer in alkaline soil,thus improving rice yield and NUE,but had no significant effect in acidic soil.These results elucidated the N process mechanism of the effect of soil pH on rice yield,NUE and N loss caused by nitrification inhibitor or the double nitrification and urease inhibitors in rice system.(4)Field experiment was conducted to determine the rice yield in alkaline paddy soil by setting new types of N fertilizer,N transformation inhibitors,N fertilizer postponing application and increasing topdressing times.As a control,field experiment was carried out in the acid soil with similar distance and the same climatic conditions at the same time.The results showed that the rice yield in acid soil was generally higher than that in alkaline soil under the same N application rate and planting seasons.N fertilizer regulation measures,such as new types of N fertilizer,N transformation inhibitors,N fertilizer postponing application and increasing topdressing times,could effectively increase rice yield in alkaline soil and reach the comparable yield as that in acid soil.These results indicated that the N fertilizer regulation measures by prolonging the retention time of NH4+-N fertilizer have achieved good practical effects on improving the yield of alkaline paddy soil.In conclusion,this study indicated that the match of soil N transformation characteristic with rice N preference had an important impact on NUE and yield.Soil pH is the key factor to control the gross nitrification rate of paddy soil.Gross nitrification rate is the key factor to regulate the retention time of NH4+-N fertilizer and N loss in nitrification-denitrification coupled process,which has an important impact on NUE and yield.Nitrification inhibitor and the double nitrification and urease inhibitors can significantly reduce the gross nitrification rate and ammonia volatilization rate of alkaline soil,respectively,so as to reduce the potential N loss risk,prolong the retention time of NH4+-N,and promote the NUE.However,they had no significant effect on the retention time of NH4+-N and the potential N loss risk in acidic soil.N fertilizer regulation measures,such as new types of N fertilizer,N transformation inhibitors,N fertilizer postponing application and increasing topdressing times,have achieved good practical effects on improving the yield of alkaline paddy soil by prolonging the retention time of NH4+-N fertilizer.This study provides a new theoretical support for clarifying the effect mechanism and key factors of N transformation characteristic on NUE in paddy soil,and practical example for implementing targeted N fertilizer regulation measures,so as to achieve N fertilizer reduction and efficiency increase improvement. |
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