Characteristics Of Soil Carbon And Nitrogen Turnover With Organic Fertilizer Application On The Loess Plateau

The Loess Plateau in China is characterized by an arid and semi-arid climate with low precipitation and infertility.Excessive chemical fertilizers?CF?have been applied in an attempt to improve crop yield in this region.However,the long-term application of CFs has caused a serious soil degradation problem.Therefore,the application of organic fertilizer is important to the local sustainable development of agriculture.The quantity of organic fertilizer can not meet the needs of agricultural cultivation,thus the integrated use of organic and inorganic fertilizer is a main recommended measure.Considering the current fertilization situation and enrichment of calcium carbonate?CaCO₃?in the soil on Loess Plateau,this research studied?1?the effects of the application of composted manure?CM?plus CF on soil C and N turnover;?2?the effects of CaCO₃ on soil C and N turnover after CM application;and?3?the effects of the application of CM plus CF on soil physical properties,the drought tolerance of apple trees,and apple yield and storability.Overall,we hoped to reveal the turnover characteristics of C and N in the soil and the effects on orchard soil properties and the growth of apple trees when CM amended with CF or CaCO₃.The main results of this study are as follows:?1?In this study,the characteristics of organic C turnover after CM with CF application were revealed.After 360 days incubation,the cumulative C mineralized of CM was50%–60%.Compared with CM application merely,the application of CM plus CF?CM-CF?stimulated soil microbial activity,and shifted the peak of soil microbial C?MBC?content forward about 80 days.However,the effects of CM-CF on soil organic carbon?SOC?fractions were closely related to CM application rate and incubation time.Specifically,compared with CM only,CM-CF decreased soil total organic carbon?TOC?content slightly but increased MBC content by 2.7%–26.5%.The 0.5%M–NP increased soil water soluble organic C?WSOC?content by 9.3%on day 30,but decreased it by 7.2%during 30–360 d.During the whole incubation period,the 2%M–NP and 4%M–NP treatments increased WSOC content by an average of 20.7%and 19.2%respectively.With the application of CM plus CF,soil hot water soluble organic C?HWC?content was increased by 13.1%–14.6%in comparison with CM only during 0–180 d,but the effect almost disappeared thereafter.The0.5%M-NP decreased soil particulate orgainic C?POC?content 20.1%,but 2%M-NP and4%M-NP increased soil POC content by 4.9%and 14.9%.The effects of CM-CF on soil readily oxidizable organic C?ROC?content was similar to that of POC.The 0.5%M-NP decreased soil ROC content by 15.3%,but 2%M-NP and 4%M-NP increased soil ROC content by 10.3%and 10.0%.Overall,low dosage of CM combined with CF?0.5%M–NP?had little effect on SOC fractions and even decreased them,but the application of medium and high dosage of CM plus CF?2%M–NP,4%M–NP?could significantly increase the content of SOC fractions?MBC,WSOC,HWC,ROC and POC?.?2?The characteristics of organic N turnover after CM with CF application were revealed.Compared with CM only,CM-CF promoted the mineralization of organic N.Soil total N?TN?content in the treatment of 2%M–NP had the least loss?14.4%?relative to 0.5%M–NP and4%M–NP after 360 days of incubation.Thus,the combination of 2%CM and CF was more conducive to the preservation of N in the soil.The application of CM plus CF increased soil N fractions content,and the effect was positively correlated with CM application rate but decreased with time.Compared with CM only,CM-CF increased soil microbial N?MBN?content by 7.0%–13.7%.During 0–180 d,the effect of CM-CF on soil alkali-hydrolyzable N?AH-N?content was obscured by CF applied,but CM-CF significantly increased soil AH-N content in comparison with CK from 180 to 360 d,but there was no significant difference relative to CM only.Compared with CF only,CM-CF increased soil MBN and AH-N content by 35.1%–115.6%and 3.5%–55.8%respectively from 0 to 180 day.Additionally,CM-CF also increased soil ammonium N?NH₄–N?and nitrate N?NO₃–N?content,but the effect disappeared with time.Overall,2%M–NP and 4%M–NP can increase soil N fractions content,resulting in an improvement of N supply capacity;however 45 t ha^-1 CM plus 450 kg ha^-1 N and 157.5 kg ha^-1 P?2%M–NP?has better effect on N retention.?3?The effects of CaCO₃ content on C turnover and SOC fractions after CM application were revealed.The effects of CaCO₃ on SOC fractions were related to soil types and CM application.After 105 days of incubation,medium and high CaCO₃ increased soil MBC content by 2.3%and 15.2%relative to low CaCO₃ when CM was not applied in soil Y,but decreased soil WSOC and ROC contents by 7.5%and 18.7%respectively,and 5.7%and 7.2%respectively.When soil Y was amended with CM,soil MBC,WSOC and ROC contents respectively increased by 4.2%–12.8%,1.6%–5.2%and 4.1%–6.7%with CaCO₃ content.Additionally,the increase of CaCO₃ content had no significant effect on calcium combined humus?Ca-Hs?C content whether soil Y was amended or not amended with CM.In the soil T,medium and high CaCO₃ increased soil MBC and Ca-Hs C contents by 3.7%and 7.3%respectively,and 3.4%and 9.2%respectively,but decreased soil WSOC and ROC contents by9.1%–16.1%and 14.8%–16.8%when CM was not applied.When soil T was amended with CM,soil MBC,WSOC and ROC content decreased with CaCO₃ content,but Ca-Hs C content still increased by 3.5%and 3.7%.On day 105,the application of CM significantly increased CO₂ emissions by 51.8%and 56.8%,respectively,in soil Y and soil T,but the effect of CaCO₃on soil CO₂ emissions was dependent on whether CM was applied.When soil Y and soil T were not amended with CM,soil CO₂ emissions decreased by 5.6%–12.8%and 1.6%–10.3%with CaCO₃ content.When soil Y was amended with CM,there was a positive interaction between high CaCO₃ and CM,resulting in an increase of 4.5%in CO₂ emissions than the application of CM only.When soil T was amended with CM,there was a positive interaction between CM and medium CaCO₃ and a negative interaction between CM and high CaCO₃,resulting in an increase of 1.1%and a decrease of 9.4%in CO₂ emissions respectively than the application of CM only.Overall,the increase of soil labile organic C fractions content will promote SOC mineralization,but Ca-Hs hinder it.The effect of CaCO₃ content and CM application on the turnover of SOC is determined by the combined effect of the protective effect of CaCO₃ on soil organic matter?SOM?and the priming effect of soil microbes on SOC mineralization.?4?The application of CM significantly increased soil TN,MBN,AH-N and mineral N contents in soil Y and soil T.However,the effect of CaCO₃ content on soil N was complicated.The apparent content of soil N fractions was manily affected by the propagation of soil microbes and by the decomposition of organic N and the utilization of inorganic N from microbes.In the soil Y,compared with low CaCO₃,medium and high CaCO₃ decreased soil TN and AH-N contents by 1.9%and 6.2%respectively,and 6.2%and 8.1%respectively when no CM was applied,but increased soil MBN content by 4.5%and 12.7%.When soil Y was amended with CM,medium and high CaCO₃ respectively increased soil TN and MBN contents by 2.8%–5.9%and 5.1%–12.2%,but respectively decreased soil AH-N and mineral N contents by 2.9%–5.8%and 7.2%–13.1%.In the soil T,when no CM was applied,the effects of CaCO₃ content on soil N fractions were consistent with soil Y.However,when soil T was amended with CM,soil TN,MBN,AH-N and mineral N contents decreased by4.3%–5.9%,7.8%–13.5%,6.0%–6.6%and 12.5%–20.7%respectively with CaCO₃ content.Overall,soil N pool decreased with CaCO₃ content without CM application,but the effects of CaCO₃ on soil N turnover after application of CM were dependent on the degree of protection of SOC provided by CaCO₃.The CM applied to soil Y had more chances to combine with CaCO₃,which was beneficial to the preservation of N,but the CM applied to soil T was less protected by CaCO₃ and more susceptible to decomposition by microorganisms,resulting in a reduction in soil N pool with CaCO₃ content.?5?The effects of the application of CM and CaCO₃ on soil structure,SOC distribution and stability were studied.The soil classified as Cumulic Haplustoll was mainly composed of<0.25 mm aggregates,accounting for 69.4%of the total soil mass.The application of CM was conducive to the formation of>0.25 mm size of soil aggregates(WR_0.25).The addition of 20and 40 g kg^-1 CM respectively increased soil WR_0.25 content by 15.7%and 19.6%,and improved the mean weight diameter?MWD?of soil aggregates by 16.4%and 18.8%.However,the application of CaCO₃ promoted the dispersion of soil particles after 105 days of incubation.Specifically,the addition of 30 and 50 g kg^-1 CaCO₃ decreased WR_0.25 content by7.8%and 9.8%,and decreased the MWD of soil aggregates by 4.6%and 11.3%respectively.The application of CM could increase the content of TOC in different sizes of soil aggregates,but the effect of CaCO₃ on the distribution of SOC in different sizes of soil aggregates was different.The addition of 50 g kg^-1 CaCO₃ decresed the TOC content in>2 mm and 1–2 mm soil aggregates by 37.9%and 12.6%respectively.However,the addition of 30 and 50 g kg^-1CaCO₃ respectively incressed the TOC content in 0.053–0.25 mm soil aggregates by 4.9%and 13.5%.In this study,soil TOC content in>2 mm,1–2 mm and 0.5–1 mm aggregates had the highest concentration and the greatest variation,while SOC in<0.053 mm aggregates had the lowest concentration and the highest oxidation stability,meanwhile it accounted for31%–50%of soil TOC due to the most mass.The path analysis showed that the effects of TOC and ROC on the stability of SOC were different in different sizes of aggregates.The stability of SOC was mainly dependent on the TOC content in 0.053–0.25 mm aggregates.Soil ROC content in the 0.5–1 mm and 1–2 mm sizes of aggregates determined the stability of SOC,but the instability of SOC was main dependent on the ROC content in the<0.053 mm size of aggregates due to the most organic C distribution in this size of aggregates.?6?In the apple orchard soil,the application of CM plus CF increased soil TOC content,decreased soil bulk density,and optimized soil porosity?increased aeration porosity and capillary porosity,decreased inactive porosity?.The application of low?5 kg per tree;CM_L?and high?10 kg per tree;CM_H?dosage of CM plus CF respectively increased soil water storage in the 20–60 cm layer by 1.4%and 4.8%,increased available soil water content in the0–100 cm soil layer by 4.1%–44.8%and 10.4%–60.2%,elevated water content in apple tree leave by 1.3%and 3.7%,and improved leaf potential of apple trees by 15.9%and 30.0%.It indicated that the application of CM plus CF could improve the drought resistance of apple trees,proved by that the proline content of apple tree leaves was decreased by 16.1%and28.5%.Meanwhile,the application of CM plus CF increased the stomatal conductance,intercellular CO₂ concentration and chlorophyll content,but reduced the transpiration rate of apple trees,resulting in an increase in photosynthesis and assimilation of CO₂.Eventually,the application of CM plus CF increased apple yield by 5.1%–13.3%.Furthermore,polygalacturonase?PG?and malondialdehyde?MDA?contents of apple fruits were reduced by 8.9%–16.3%and 19.4%–39.5%respectively,resulting in an increase in the storage performance of apple fruits.Overall,the application of CM plus CF was an effective fertilization strategy for mitigating aridity stress,promoting apple tree growth and improving apple yield and quality on the Loess plateau.The effect was related to CM types and dosage,and increased with the increase of the input of organic C.In sum,the turnover characteristics of organic fertilizer in the Loess Plateau soil are affected by inorganic fertilizers and CaCO₃.The effect of the application of organic fertilizer plus chemical fertilizer on soil C and N transformation is related to C/N ratio.The medium and high amount of organic fertilizer combined with chemical fertilizer is beneficial to the rapid decomposition of organic fertilizer under low temperature conditions,and obviously increases the content of soil organic C and N fractions,and meets the demand for nutrients of apple trees in autumn.The effects of CaCO₃ on soil C and N turnover are dependent on its stimulating effect on soil microbial activity and the protection of SOM.The increase of soil labile organic C fractions will promote soil organic C mineralization,but the formation of Ca-Hs will hinder the decomposition of SOM.The application of organic fertilizer plus chemical fertilizer can enhance the drought resistance of apple trees and increase the yield and quality of apples,and the effect increases with the increase of organic C input.