Response Of Soil Chemical Characteristics Under Short-term Nitrogen Addition In Cunninghamia Lanceolata Plantation

With the development of modern industry and agriculture,the nitrogen compounds released into the atmosphere by human activities increased sharply.Nitrogen deposition has gradually become one of the main factors affecting the global carbon and nitrogen cycle.Excessive nitrogen deposition would cause serious ecological problems,such as soil acidification,water eutrophication,and loss of biodiversity.In this context,it is of great theoretical and practical significance to study the response mechanism of forest ecosystem to nitrogen deposition in China.Cunninghamia lanceolate is one of the most important tree species for artificial afforestation in subtropical region of China.It occupies an important position in forestry production because of the advantages of rapid growth,wide cultivation range and high economic value.The study was conducted in C.lanceolata plantation at its fast-growing stage in state-owned forest farms of Hongya county,Sichuan Province,and the plantation soil was taken as the study object.From April to November 2019,we monthly sprayed ammonium nitrate beneath fir plantations to simulate nitrogen deposition.This field experiment included three N treatments:control(CK,0 g N·m^-2·a^-1),low-nitrogen(LN,5 g N·m^-2·a^-1)and high-nitrogen(HN,10 g N·m^-2·a^-1).The purpose was to study the initial response of C.lanceolata stand land to nitrogen addition,and provide data support and theoretical basic for nitrogen management of C.lanceolata plantations.The main results were as follows:（1）Nitrogen addition promoted soil acidification.The p H value of soil decreased first and then increased with the addition of nitrogen,with the largest difference among treatments in November.Compared with CK,soil p H decreased by 2.71%under LN condition and 3.31%under HN treatment.The p H of soil decreased with the deepening of soil layer.（2）Short-term nitrogen addition increased the NO₃^--N content in the two forest age soils,and the nitrate nitrogen content in the soil of 11a C.lanceolata plantations changed more obviously.Compared with CK,the nitrate nitrogen content in LN and HN increased by 33.3%and 32.0%respectively.The contents of NO₃^--N and NH₄⁺-N in the soil of the two plantations showed significant temporal changes,and the contents in July and August were the lowest.Nitrate nitrogen content was significantly affected by soil layer,and showed as0-20 cm>20-40 cm soil layer.（3）The content of total carbon（TC）,total nitrogen（TN）and total phosphorus（TP）in soil was significantly affected by time,and showed significant seasonal changes.TC content of soil was directly proportional to nitrogen concentration.Soil TC content of C.lanceolata plantations at different ages showed 11 a>18 a.The TN content in the soil of 11-year-old C.lanceolata plantations was directly proportional to nitrogen concentration.Compared with CK,TN content in HN significantly increased by 22.5%.TN content of soil in the 0-20 cm soil layer was more than that in the 20-40 cm soil layer in the two-age C.lanceolata plantations.Nitrogen addition increased TP content of soil.Compared with CK,TP content of soil in 11 a C.lanceolata plantations increased by 42.4%under the condition of high nitrogen.Soil P was more effective in the growing season in C.lanceolata plantations of both ages.The response of soil N and P contents in 11 a C.lanceolata plantation to nitrogen addition was more sensitive than that in 18 a.（4）The C:N of the two forest age soils was relatively stable.Both C:P and N:P were affected by seasons,and their variation trends were similar.These two parameters are mainly affected by element P.Compared with different forest ages,the soil N:P in the 18 a was more susceptible to nitrogen addition.It indicates that under the background conditions of yellow soil,the effectiveness of P is low in this area,and the ecosystem is mainly limited by P.（5）The content of Zn in soil showed obvious seasonal changes,showing as the growth season<Non-growing season.Nitrogen addition inhibited the increase of Zn and Mn content in soil of 11 a C.lanceolata plantations but promoted the accumulation of Zn and Mn in the soil of 18 a C.lanceolata plantations.Under the two stand conditions,the content of Zn and Mn in soil under the treatment of LN had a greater change range,indicating that LN may be more likely to cause Zn&Mn morphological changes and soil leaching.In summary,short-term nitrogen addition aggravated the acidification of C.lanceolata plantations of all ages,and the nitrate nitrogen content in C.lanceolata plantations of 11 a significantly increased,while the nitrogen and phosphorus content were in direct proportion to the nitrogen addition concentration.The changes of trace element Zn&Mn are more sensitive under the condition of low concentration of nitrogen.In addition,the response of soil chemical characteristics to nitrogen addition was regulated by forest age and soil layer.These results can provide a scientific basis for further understanding the response of soil nutrient cycling to nitrogen deposition in the plantation ecosystem of C.lanceolata plantations of Hongya.