The Effects Of Fertilization On Soil Fertility And Crop Yields In Newly Built Dryland Terrace

Building level terrace is an important measure to control soil erosion and develop dryland agriculture in the Loess Plateau of China.However,the soil of the newly built terrace is barren,and it is difficult to increase crop yield in the short term.Therefore,it is necessary to improve soil quality and crop yield through fertilization measures.Many studies and practices have shown that a combined application of organic fertilizer and inorganic fertilizer is a better method to increase yield than the application of organic or inorganic fertilizer alone.However,there are few long-term fertilization studies on the newly built terrace,and the process,mechanism,and effect of fertility improvement on yield are still unclear,so more long-term in situ experiments are needed for in-depth research.In this study,the effects of different fertilization measures on soil physicochemical properties and crop yield in a newly built terrace were studied through an 18-year（for years 2004-2021,the author worked from 2017 to 2022）long-term in situ experiment.In particular,we focused on the joint fertilization efficacy of organic and inorganic fer-tilizers on crop yield（JFE）and the effect of fertilization on soil organic carbon（SOC）sequestration of the terrace.The main research contents were as follows:（1）Based on the pea-wheat-potato rotation,four treatments with three repeats per treatment were imposed（CK:no fertilized control;NP:applied inorganic nitrogen and phosphorus fer-tilizers;M:applied sheep manure;MNP:combined manure plus the inorganic N and P fertilizer）to study JFE;（2）Based on this,we collected 79 literatures data in global and further studied JFE;（3）We investigated SOC and its components（free light SOC frac-tion:LFOC,particulate SOC fraction:POC,mineral associated SOC fraction:MAOC）of five land use types in the local area,and analyzed the sequestration potential of SOC and its components in the terrace;（4）Based on the results that the MAOC in the terrace tended to be stable in the last five years,but still significantly lower than that of local natural grasslands,an incubation experiment with different ratios of carbon to nitrogen（C:N）of plant samples was conducted to analyze the possibility to further increase in MAOC in the terrace;（5）Based on the significantly negative correlation between p H and MAOC during the 18-year periods of the fertilization experiments,an incubation experiment was conducted to analyze the impact of p H reduction on MAOC;（6）Using the Denitrification Decomposition Model（DNDC）,we analyzed the impacts of climate changes on crop yield and SOC under different fertilization treatments in terrace.The main results are as follows:1.Compared with CK,fertilization significantly increased crop yields,and MNP was an optimized fertilization method for rapid,stable,and sustainable obtaining high crop yields.The average pea yields in CK,NP,M,and MNP were 347,655,798,and1132 kg ha^-1,respectively.The average wheat yield was 427,871,1005,and 1528 kg ha^-1,respectively.The average potato yields were 950,2285,2244,and 3352 kg ha^-1,respectively.With the extension of years,the yield in NP and CK decreased,the yield in M increased first and tended to be stable since the tenth year,and the yield in MNP was the highest and relatively stable.During the 18 years,the water use efficiency of crop yield in CK,NP,M,and MNP was 2.8,6.2,7.4,and 10.8 kg ha^-1mm^-1,respectively;the yield variation coefficient was 21%,11%,17%,and 6%,respectively;the sustaina-bility yield index was 61%,77%,70%,and 87%,respectively.2.Compared with CK,fertilization increased soil microbial biomass carbon（MBC）.The increase of MBC in MNP treatment was the fastest,and for years 2-6,it was significantly higher than other treatments.In the long run,MBC in MNP and M treatments was equal and significantly higher than NP and CK;it was higher in NP than CK,but the difference was not significant.Manure application significantly increased the activities ofβ-glucosidase,urease,and alkaline phosphatase.Long-term fertilization significantly increased soil total phosphorus,available phosphorus（AP）,and inorganic nitrogen（IN）.AP and IN were the highest and increased the fastest in MNP.The in-crease of MBC,AP,and IN in MNP in years 2-8 were 20%,163%,and 106%higher than the sum of the increase of M and NP,respectively.Long-term crop production reduced 0-2 m soil water storage,MNP and M treatments could keep 0-1 m soil water storage relatively stable and significantly higher than NP and CK.3.The dynamics of the soil quality of the terrace during the 18-year were evaluated using the soil quality index（SQI）.The results showed that there was no significant difference in SQI in MNP and M and showed a trend of continuous increase,which was significantly higher than NP and CK,while NP and CK increased first and then stabi-lized since the fifth crop rotation cycle.Manure application was the key to rapidly and continuously increasing the soil quality of the terrace.4.For years 2-8,the yield increase in MNP treatment relative to CK(Y_MNP-Y_CK,ΔY_MNP)was 10-80%higher than the sum of M and NP[(Y_M-Y_CK)+(Y_NP-Y_CK),ΔY_M+NP],and joint fertilization had a synergistic effect on yield increase.For years 9-12 and 16-18,theΔY_MNP was 19%and 20%lower than that ofΔY_M+NP on average,and JFE showed an offset effect.For years 13-15,theΔY_MNP was similar to that ofΔY_M+NP,and JFE was 5%on average,indicating JFE had a summing effect.Structural equation model analysis showed that the main reason for the synergistic effect of joint fertiliza-tion was the rapid increase of IN and AP caused by the rapid increase of microbial activity in MNP treatment,which led to the rapid increase of crop yield.With the in-crease in soil fertility and the gradual rise in M yield,the synergistic effect turned into an offset effect.The summing effect of JFE was mainly related to the yield decrease in NP caused by soil water deficit.5.The meta-analysis showed that JEF was mainly related to soil fertility.On the whole,JFE decreased with the increase in initial SOC,soil total nitrogen（TN）,phos-phorus,potassium,and available nitrogen,phosphorus,and potassium concentration in the plow soil layer,but only SOC and TN could fully reflect the three effects of JFE.However,only the initial plow layer SOC and TN can fully reflect the three effects of JFE.There was a significant negative exponential function relationship between JFE and SOC and TN,with the lowest value being-35%.When the initial SOC<4.3 g kg^-1 or TN<0.52 g kg^-1,the yield increase of joint fertilization of organic and inorganic fertilizers(ΔY_OMF)was significantly higher than the sum of applying organic（OM）and inorganic fertilizers（F）alone(ΔY_OM+F),and JFE showed a synergistic effect.When SOC or TN increased to less than 7.3 g kg^-1 and 0.73 g kg^-1,respectively,there was no significant difference between theΔY_OMF andΔY_OM+F,and JFE had a summing effect.When SOC or TN continued to increase,theΔY_OMF was significantly lower than that ofΔY_OM+F,and JFE had an offset effect.In addition,JFE was higher in soil with neutral p H and coarse texture,under irrigation and multi-crop planting system,in higher aver-age annual air temperature,lower annual precipitation,lower altitude,and lower lati-tude regions,and vice versa.6.Manure application significantly increased TN,SOC,LEOC,POC,and MAOC.The apparent conversion rate of organic carbon in manure to SOC decreased with the year extension and gradually stabilized at about 36%after the tenth year.In the 18th year,the SOC of CK,NP,M,and MNP in the newly built terrace increased by 1.7,3.1,13.6,and 12.4 t ha^-1 over the initial value(5.8 t ha^-1),respectively;the sequestration rate was 0.09,0.17,0.75 and 0.69 t ha^-1yr^-1,respectively;the carbon sequestration efficiency was 76%,70%,40%,and 32%,respectively.With the cumulative organic carbon（OC）input rise,SOC increased first and then tended to be stable and increased again linearly when the cumulative OC input exceeded 22 t ha^-1.With the increase in cumulative OC input,MAOC in M and MNP increased rapidly for the first four years,then continued to decrease because of the reduction of manure application,and MAOC began to increase again when the cumulative OC input exceeded 22 t ha^-1 in the eleventh year but tended to stable in the last five years of the experiment.LFOC and POC con-tinuously improved with the cumulative OC input increase,which was the main com-ponent of SOC rising in the later stage of the experiment.7.Compared with local natural grassland,SOC and its components in the terrace still have great retention potential after 18 years of manure application.In the 18th year,the 0-20 cm SOC,LFOC,POC and MAOC of the terrace were 45%,23%,55%and 52%lower than that of the local natural grassland,respectively.The incubation experiment with different C:N of plant samples showed that except in natural grassland,adding plant litters with low C:N could significantly increase the MAOC of the terrace,aban-doned cropland,and alfalfa land.And the lower the C:N of the plant sample,the higher the MAOC increase.This result indicates that MAOC in the terrace still has the poten-tial for further improvement,and organic matter input with a low C:N contributes to further improving MAOC in the terrace.8.Fertilization significantly reduced soil p H in the newly built terrace.The incu-bation experiment showed that the p H of terrace-soil samples decreased from 8.34-8.56to 0.49-0.64 and 0.64-0.82 under p H1 and p H2 treatment,respectively,after adding acid（p H0:no acid,10 ml and 15 ml 0.01M HCL/100 g soil for p H1 and p H2,respec-tively）.There was no significant difference in the cumulative mineralization of organic carbon under p H1 and p H2,but it was significantly lower than that in p H0.MAOC increased with p H decreasing,but there was no significant difference among treatments.This result indicates that soil p H changes from alkaline to neutral in the terrace could be beneficial to the storage of MAOC.9.Under the condition of simulated climate change scenarios during 2022-2100,the order of yield of pea,wheat,and potato between treatments was MNP>M>NP>CK;pea yield was higher than the baseline period（2004-2021）;the wheat yield in CK was similar to the baseline period but it was higher in NP and M and lower in MNP than baseline period;the potato yield in M was higher than baseline period and it was lower in other treatments.Comparatively,MNP and M had higher yield stability and sustainability than NP.Fertilization decreased the response of crop yield to precipitation during the growing season in the baseline period,which contributed to improving yield stability.And the combined application of manure and inorganic fertilizers could reduce the response of crop yield to annual precipitation and average annual air temperature under climate changes condition.The SOC increased in all four treatments,but the SOC growth rate retarded with the time and climate scenarios increase.In conclusion,the combined application of manure and inorganic fertilizers could obtain high crop yield rapidly,and improve soil quality rapidly and continuously in the newly built terrace.The joint fertilization efficacy of organic and inorganic fertilizers on yield has synergistic,summing,and offset effects,and is mainly affected by soil fertility.Manure application could significantly increase SOC and its components of the terrace,and it still has a large potential for further sequestration after 18 years of manure application.After the long-term application of manure,the MAOC in the ter-race tended to be stable,and the input of organic matters with low C:N has the potential to improve the MAOC further.Compared with the application of manure and inorganic fertilizers alone,the combined application of manure and inorganic fertilizer is an op-timized measure to obtain stable and sustainable high crop yields while significantly increasing SOC both under the baseline period and in future climate change conditions.