Fate And Mechanisms Of Residual Fertilizer Nitrogen In Soils With Different Fertility

Nitrogen fertilizer application plays an important role in increasing agriculturalproductivity. The application of inorganic nitrogen (N) fertilizer in China has increasedrapidly since the1960s. Excessiveapplication of inorganic N fertilizers has led to a largeamount of fertilizer N (20%～50%) remaining in soil at harvest, which caused a series ofenvironmental problems. So, it is of great significance to quantify the uptake and loss ofresidual fertilizer N in soil. Long-term different fertilization affected soil organic carbon, totalN, soil microbial biomass and soluble organic matter, and so on. Thus it may affect patterns ofuptake and loss of residual N. Therefore, it is important to understand the fate of the residualfertilizer N in soils with different fertility after crop harvest for both agronomic andenvironmental reasons.The study was carried out on three soils with different fertilization history (19years),1)no fertilizer (No-F), or2) inorganic NPK fertilizers, or3) cattle manure plus NPK fertilizershad been applied. The effects of long-term fertilization managements on the fate of residualfertilizer N in soils were studied combining field, pot with incubation experiments using15Nlabeled method. The main conclusions were showed as follows:（1）An incubation experiment was conducted to evaluate the effects oflong-termdifferent fertilization treatments on N mineralization in the0-20cm,20-40cm and40-60cmsoil layers in winter wheat-summer fallow rotation systems. Compared with the NPK soil, theMNPK soil significantly increase the soil organic carbon, total nitrogen, cumulativemineralized N and potentially mineralizable nitrogen (N0) in0-20cm and20-40cm soil layers.The cumulative N and N0in0-60cm soil layers were significantly higher in the MNPK soilthan in No-F soil. For all the three soils, the proportion of N0to total N was19%~23%,9%~12%and6%in0-20cm,20-40cm and40-60cm soil layers, respectively. The proportionof N0to total N was highest in the MNPK soil. The MNPK soil had the highest value ofpercentage of cumulative N in0-20cm to0-60cm soil layer. We concluded that long-termcombined application of cattle manure with inorganic fertilizer was an efficient way toenhance the nitrogen supplying capacity of soil in0-60cm depth.（2）A pot experiment was conducted to study the fate of15N labeled fertilizer appliedalone and/or together with crop straw in soils with long-term different fertilization managements. Compared with the treatment of addition of fertilizer N alone (+N), thecombined addition of straw and fertilizer N (+1/2N+1/2S) significantly decreased the uptakeof fertilizer N and soil N in the first growing season. The uptake of fertilizer N was higher inthe MNPK soil than in NPK and No-F soils under the two N fertilizer treatments. After thefirst cropping of wheat was harvest,82.6%～95.1%of the residual fertilizer N in all the threesoils under the+1/2N+1/2Streatment and NPK and MNPK soils under the+N treatment wasin the organic form, and47.7%of the residual fertilizer N in the No-F soil under the+Ntreatment was in the mineral form. For the+N treatment, the residual fertilizer N useefficiency (RNUE) was significantly higher in the No-F soil (20.0%) than that in NPK (9.1%)and MNPK (12.4%) soils. For the+1/2N+1/2S treatment, the RNUE in the MNPK soil(12.0%) was significantly higher than that in NPK (9.4%) and No-F (8.8%) soils. Totalfertilizer N use efficiency in two growing seasons was34%for the No-F soil,54%for theNPK soil and64%for the MNPK soil under the+N treatment, and17%,27%and36%under+1/2N+1/2S treatment. Under the+N treatment the loss rates of fertilizer N during twogrowing seasons were25%,15%and12%for No-F, NPK and MNPK soils, respectively, and21%,18%and16%under+1/2N+1/2S treatment.24%~41%of the fertilizer was remained insoil under+N treatment after the second wheat harvest, and48%~62%for the+1/2N+1/2Streatment.In conclusion, short-term application of straw (with high C/N ratio) plus fertilizer Ndecreases N uptake due to N immobilization, while long-term application of manure withorganic fertilizer has a high ability to adjust the N supplying of soil, and increases theutilization of fertilizer N.（3）A pot experiment coupled with short-term mineralization incubation wereconducted to explore characteristics of N mineralization and uptake of15N-labeled N fertilizerapplied alone and/or together with crop straw in the soils with long-term different fertilizertreatments. After28days of incubation, the net N mineralization in the MNPK soil increasedsignificantly, by39%~49%over that in the NPK soil. In NPK and MNPK soils,1.23to1.90mg kg-1of the residual fertilizer N was mineralized, accounting for2.78%~5.53%of the totalresidual fertilizer N in the soils. The net mineralization rates of residual fertilizer N in NPKand MNPK soils were significantly higher than that in No-F soil. Compared to the+Ntreatment, the+1/2N+1/2S treatment significantly increased the net N mineralization ratesinthe three soils. N uptake by the second crop of wheat during the vegetative growing periodwas found to be significantly and positively related to the net soil N mineralization rate duringthe short incubation period.To sum up, long-term combined application of organic manurewith chemical fertilizer could increase the mineralization rate of soil N, and residual fertilizerN could mineralized gradually during the following seasons. （4） Amicro-plot experiment in the field was conducted to evaluate the fate of residualfertilizer N in soils over summer fallow and the second winter wheat growing season. Duringthe summer fallow period, the loss rate of residual fertilizer N in No-F soil was as high as48%, and significantly higher than that in the NPK (22%) and MNPK soils (19%). Theresidual fertilizer N use efficiency by the second winter wheat was12.8%in the No-F soil,6%in the NPK soil, and8%in the MNPK soil. These were equivalent to9.0%,2.0%and2.2%of the applied fertilizer N. The total fertilizer N recovery (fertilizer N uptake by cropsand residual in0-100cm soil layers) in the MNPK and NPK soils (84.5%and86.6%,respectively) were both significantly higher than that in the No-F soil (59%) after twogrowing seasons. The residual fertilizer N in0-100cm soil layers was17.8%~30.7%in allsoils after the second growing season, mainly in the0-40cm soil layers. The conclusion wasthat high proportion of the residual fertilizer N is lost during the summer fallow in the drylandfarming, and long-term combined application of cattle manure with inorganic fertilizer NPKcould increase fertilizer recovery in plant and soil system and decrease its loss.（5） Using the15N labeled method, a micro-plot experiment in the field was conductedto study the fate of the residual fertilizer N after three growing seasons in soils with long-termdifferent fertilization managements in winter wheat-summer maize rotation systems. The totalfertilizer N use efficiency in the NPK and MNPK soils (57%and65%, respectively) duringthree growing seasons was significantly higher than that in the No-F soil (28%). The uptakerate of the residual fertilizer N by the crops growing in the subsequent two seasons were17%for the No-F soil,15%for the NPK soil and3%for the MNPK soil. And7%of the residualfertilizer N was taken up by the third crops in the No-F soil,2%in the NPK soil and3%in theMNPK soil. Total fertilizer N recovery rates (fertilizer N uptake by the three crops andfertilizer N residual in the0-100cm soil layers) were50%in the No-F soil,77%in the NPKsoil and84%in the MNPK soil. Furthermore, the total residual rate of fertilizer N in the0-100soil layers was about20%in all soils after the three growing seasons, mainly in the0-20cm soil layers. The conclusion is that the residual fertilizer N has high availability for thesubsequent crops, and the long-term combined application of cattle manure and inorganicNPK can increase fertilizer N use efficiency and decrease its loss.（6）Three short-term incubation experiments were used to evaluate the effects oflong-term different fertilization history on the biotic and abiotic immobilization, nitrification,NH3volatilization,and loss following the addition of15N labeled (NH4)2SO4. The resultsshowed that the immobilization of the added NH4+-15N by abiotic and biotic processesoccurred simultaneously in all soils. Total NH4+-15N immobilization was48.2,41.1and38.2mg kg-1for No-F, NPK and MNPK soils, accounting for32.1%,27.4%and25.5%of the added N in14-days N transformation experiment. Biotic15N immobilization amounted for53.6%,83.4%and96.2%of the total immobilization15N. The maximum fixed-15N in NPK(8.8mg kg-1) and MNPK (2.1mg kg-1) soils were accounted for35%and8%of that in No-Fsoil (24.7mg kg-1). The NPK and MNPK soils showed higher nitrification potential, about61.7%and67.1%of applied NH4+-15N were found in the NO3--N form after3days ofincubation. Compared with No-F soil, MNPK and NPK soils significantly decreased NH3volatilization. Under the19-days N leaching experiment, the15N loss rates were56%,76%and85%. We conclude that long-term combined application of manure with inorganicfertilizer increased N biotic immobilization and nitrification, decreased N abioticimmobilization and NH3volatilization.In general, residual fertilizer N has availability for the subsequent crops at a certainextent, and the residual fertilizer N use efficiency was6%~20%. Long-term combinedapplication of cattle manure plus inorganic fertilizer decreases residual fertilizer N loss, and alarge amount of the residual fertilizer N still remained in soil to replenish the consumption ofsoil N. Hence, long-term combined application of manure with inorganic NPK fertilizers is aneffective andenvironmentally friendly way for food production security.