Studies Of The Relationship Between Soil N Transformation Processes And Crops N Uptake And N Loss

Nitrogen(N)is one of the most important elements for crop growth,and is also a main factor regulating soil productivity.However,high N fertilizer inputs in ecosystems have resulted in decreased N use efficiency(NUE)and negative environment effects.Therefore,it is critical to increase NUE,while decrease N loss to maximize the economic benefit and minimize the environmental impacts of N fertilizers.To date,only a limited studies have addressed the direct connection among N forms,soil N transformations,crops N preferences and climate condition,and its effects on NUE,despite the fact that the fate of N and soil internal N transformations are known to be intimately linked.In this study,a series of 15N incubation experiments in laboratory plot experiments as well as 15N microplot experiments in the fileds in Fujian(FJ),Sichuan(SC)and Hunan(HN)province in China were conducted to investigate the effects of the coupling degree of crop N preference,soil N transformation characteristics and climate conditions on crop yield,NUE and N loss.On this basis,the effectiveness and applicable situations of several N management practices(such as N application at later stage,optimize nitrogen fertilizer rate and urea stabilizers,etc.)were also discussed in the thesis.In this study,A 15N-tracing experiment were carried out in labotratory to study the soil N transformation rates,and a series of field experiment combined with 15N tracing in micro-plot were conducted to investigate the growth and N fates for rice(preferring NH4+)and wheat(preferring NO3-)planted in FJ and SC.FJ is characterized by a typical acidic soil,and SC is characterized by a typical alkaline purple soil.The results showed that,the gross N mineralization rates were similar between FJ and SC sites,but the gross autotrophic nitrification rate of FJ was significantly higher than that of SC.Under field condition,the NH4+ remained in the soil solution for a longer time after urea application into FJ site,and rice planted in FJ have a significaltly higher yield and NUE,and a significantly lower N loss than that in SC.The NO3-concentrations in soil solutions remained higher for a longer time in SC during the wheat season,and wheat planted in SC have a significantly higher yield and NUE,and a significantly lower N loss than that in FJ.These results indicated that the matching of crop N preference with the soil N transformation characteristics(especially the nitrification rate)is of critical importance to enhancing crop’s yield and NUE,and reducing N loss in the field.Replacement of NH4+ based fertilizer with NO3-based fertilizer improved the yiele and NUE for wheat planted in SC;but not in FJ because of the higher NO3-loss driven by precipitation in humid condition.Thus,climatic condition,e.g.precipitation,should also be considered in the development of the N fertilizer management strategies by matching crop N preference,soil N transformation characteristics,and N source to increase NUE and reduce N losses.We further differentiated the results into base fertilizer and topdressing,and found that the differences in NUE and N loss between two study sites were mainly reflected in basal fertilizer period but not in topdressing period,irrespective of N fertilizer forms and crop species.The NUE of base fertilizer for wheat planted in SC soil were significantly higher than that in FJ soil by 80%-333%(Fig.1a);whereas the NUE of topdressing in SC soil were higher than that in FJ soil by only 5%-24%.The amount of base fertilizer-N uptake by rice in FJ was 22%was significantly higher than that in SC(12%);while the N loss of base fertilizer in FJ(48%)were significantly lower than that in SC(63%).However,there were no significant differences in NUE,N remaining in soils and N loss of topdressing for wheat planted in FJ and SC.The yield and biomass of crops were significantly and positively correlated with the NUE of base fertilizer rather than topdressing.Such results may imply that the difference in NUE of base fertilizer was primarily responsible for significantly different productivity of rice or wheat planted in FJ and SC with different soil N transformation characteristics and climate conditions.This is primarily attributed to the higher N uptake capacity for crops in the topdressing stage,which reduced the impact caused by the mismatch among crop’s N preference,soil N transformation and climate.Therefore,"N application at later stage"(i.e."increase the ratio of topdressing to total N application")is likely a suitable N management to improve NUE and reduce N loss,when crop’s N preference are mismatch with soil N transformation characteristics and climate conditions.Increasing the N application rate is an important approaches to improve crop yield.However,the effects of N application rate on rice yield in soil with different pH are still not clear.In this study,a series of field experiment were conducted in FJ,SC and HN to study the effects on N application rates on rice yield in soils with different pH.The results showed that the yield of rice was 9.9t ha-1 in FJ with N application rate at 100 kg N ha-1,which was significantly higher than that in SC by 30.3%.When N application rate was 180 kg N ha-1 in HN,the yields of early season rice and late rice were 6.8 t ha-1 and 7.9 t ha-1 in acidic soil,respectively which were significantly higher than those in alkaline soil by 27.7%and 17.7%respectively.An increase in the N application rate from 100 kg N ha-1 to 200 kg N ha-1 did not significantly improve the rice yield in FJ and SC.Similarly,N application also have no significant effects on the yield of early rice and late rice in acidic soil of HN.An increase in the N application rate from 180 kg N ha-1 to 240 kg N ha-1 had no significant influence on the yields of early season rice and late rice in alkaline soil in HN.Furthermore,increasing N application rate from 180 kg N ha-1 to 300 kg N ha-1,significantly improved the early rice yield of alkaline soil in Hunan by only 10%,but had no significant effect on the yield of late rice.Therefore,a higher rice yield could be achieved in acidic soil with lower N application rate;while the rice yield in alkaline soil is hard to be improved by increasing N application rate.This was likely due to the higher nitrification rate in alkaline soil,which is mismatch rice’s N preference.The addition of urease and nitrification inhibitors with N fertilizers is a common N fertilizer management practices in agroecosystems.In this study,a series of 15N tracing experiments were conducted in laboratory and field in FJ and SC to study the effects of urease and nitrification inhibitors on gross N transformation rates and its relationship with rice yield and N fate in paddy soils with different pH.The treatments was same in two study sites,which including urea(U),urea+nitrification inhibitor(UD)and urea+nitrification inhibitor+urease inhibitor(UDN).In FJ site,UD and UDN significantly reduced the autotrophic nitrification rate(ONH4),and significantly increased the ammonia volatilization rate(VNH3),but did not affect the sum of autotrophic nitrification rate and ammonia volatilization rate(ONH4+VNH3),compared with U treatment.This indicate that the UD and UDN treatments did not change the risk of NH4+ loss from acidic soils,and therefore had no significant effect on N uptake,N loss and yield of rice FJ.However,compared with U treatment in SC,UD significantly increased the rate of VNH3,significantly reduced the rates of ONH4 and significantly reduced ONH4+VNH3 by 19.7%;UDN significantly reduced VNH3,and ONH4 in the initial period,and thus significantly reduced ONH4+VNH3 reduced by 15.8%.It is in favour of the maintenance of NH4+ in soil and N uptake by rice,and therefore,UD and UDN treatment in SC significantly improved the NUE of rice by 46.1-66.2%,and significantly reduced N loss by 26.4-25.2%,compared with U treatment in SC.Therefore,soil pH and its effects on soil N transformation characteristics should be considered for the application of nitrification inhibitors and urease inhibitors in rice fields.