Acidification Characteristics And Regulation Mechanisms Of Red Soil Under Long-term Combined Application Of Organic Fertilizer And Chemical Fertilizer

Soil acidification intensified from overuse of chemical N fertilizer has been seriously limiting the sustainable development of agriculture in in the red soil region of South China.Soil acidification increased concentration of toxic elements such as aluminum,and decreased concentrations of beneficial elements such as calcium,magnesium and phosphorus,thus limiting crop growth.Fertilization is one of the main factors affecting soil acidification in farmland.It is of great guiding significance to explore the variation characteristics of acidity and buffering capacity of red soils under different fertilization treatments.Two-long-term filed experiments were conducted in 1990 and 2009,respectively,in Qiyang Red Soil Experiment Station of Chinese Academy of Agricultural Sciences.The red soil fertility and fertilizer efficiency started in 1990 included 9 treatments:no fertilizer（CK）,single fertilizer（N,NP,NK,PK,NPK）,chemical fertilizer and straw counters-field（NPKS）,and single or combined application of organic fertilizer（M,NPKM）.The organic and inorganic combined application experiments started in 2009 included 4 treatments:single application of chemical fertilizer（NPKM0,liming to improve acidity in 2018）,organic fertilizer replacing 20%,40%and 60%of chemical fertilizer nitrogen（NPKM1,NPKM2 and NPKM3）.An incubation experiment was conducted to investigate the resistance of two red soils differing in initial properties to external acid,and to elucidate the main factors and potential mechanisms.The changes in red soil acidity,acid-base buffer capacity,soil aluminum forms and other indicators in the two experiments were determined.The main results were showed below:（1）As compared with no fertilization treatment(16.89 mmol_（+）·kg^-1·p H^-1),the acid-base buffer capacity of red soil in fertilization treatments significantly increased to 19.08～28.01 mmol_（+）·kg^-1·p H^-1after 30-year fertilization.The highest increase was found in NPKM and M treatments.Soil available phosphorus,CEC,organic matter,available nitrogen,total nitrogen,and total phosphorus were significantly positively correlated with soil acid-base buffer capacity（P<0.01）,and soil calcium carbonate content was significantly positively correlated with soil acid-base buffer capacity（P<0.05）.Long-term application of organic fertilizer to increase CEC and SOM is one of the main reasons for increasing soil acid-base buffer capacity.（2）Under the long-term combined application of different organic and inorganic fertilizers,the soil p H value of NPKM0 treatment decreased by 0.48 units,and the exchangeable acid and aluminum increased by 2.74 and 1.06 cmol_（+）·kg^-1respectively.After liming in the end of 2018,the soil p H increased by 0.58 units,and the exchangeable acidity and aluminium decreased by 2.62 and 1.45 cmol_（+）·kg^-1respectively.NPKM2 and NPKM3 treatments can effectively prevent soil acidification.By 2020,the soil p H of NPKM3 treatment increased by 0.78 units as compared with the initial value in 2009,and the exchangeable acidity and aluminium decreased by 1.10 and 1.25 cmol_（+）·kg^-1respectively.The NPKM2 and NPKM3treatments significantly increased soil cation exchange capacity as compared with NPKM0,but there was no significant change after liming in NPKM0 treatment.Soil exchangeable acid had a very significant positive correlation with exchangeable aluminum,and a very significant negative correlation with p H value and exchangeable calcium content;There was a significant negative correlation between soil exchangeable aluminium content and cation exchange capacity（P<0.05）;There was a very significant positive correlation between soil cation exchange capacity and organic matter content.（3）The contents of exchangeable aluminum,adsorbed hydroxyl aluminum and organic complex aluminum in soil increased with the decrease of p H value.As compared with NPKM3 treatment,the contents of exchangeable aluminum,adsorbed hydroxy aluminium,and organically bound aluminium in MPKM1 treatment increased by 159.04,96.34 and 152.88 mg·kg^-1,respectively.As compared with NPKM1 treatment,NPKM0 treatment soil exchangeable aluminium and adsorbable hydroxy aluminum had no significant difference after liming in,but significantly reduced the content of soil organically bound aluminium by 71.43 mg·kg^-1.The correlation analysis showed that soil acidity and cation exchange capacity may be the main factors causing the transformation of active aluminium forms in the above three soils.（4）The soil acid buffer curve showed that as soil p H decrease,the increase rates of soil exchangeable aluminium in NPKM0,NPKM1,NPKM2 and NPKM3treatments were 2.71,2.42,1.93 and 0.16 cmol_（+）·kg^-1·p H^-1,respectively;Correlation analysis showed that the increase rate of exchangeable aluminium was significantly negatively correlated with soil p H value,exchangeable magnesium,cation exchange capacity,exchangeable potassium and organic matter content.（5）The results showed that exchangeable acidity increased by 1.32 cmol_（+）·kg^-1and 0.76 cmol_（+）·kg^-1in NPKM0 and NPKM3 treatments,respectively,as soil p H decreased by 1 unit.Exchangeable aluminium increased by 1.23 cmol_（+）·kg^-1and 0.40cmol_（+）·kg^-1,respectively,while exchangeable hydrogen increased by 0.35 cmol_（+）·kg^-1and 0.37cmol_（+）·kg^-1,respectively.NPKM0 treatment reduced the content of adsorbed hydroxy aluminium by 163.07 cmol_（+）·kg^-1,while NPKM3 treatment had no significant change.There was no significant change of organically bound aluminium as p H value decrease.The results of correlation analysis showed that soil p H value and cation exchange capacity may be the main factors affecting the release of soil active aluminium.In conclusion,compared with short-term application of lime to improve soil acidity,long-term application of organic fertilizer can effectively prevent red soil acidification,in which sourced 40%of total N from organic fertilizer not only effectively prevent the reduction of red soil p H value,but also inhibit the activation of soil aluminum and significantly reduce the content of soil active aluminum;In addition,long-term application of organic fertilizer was helpful to increase the content of soil organic matter,cation exchange capacity and other nutrient elements.Therefore,it can be seen that the combined application of organic fertilizer not only prevent red soil acidification,but also improve the fertility level.It is one of the measures to realize the sustainable utilization of red soil resources.