Effect Of Nitrogen And Phosphorus Fertilization On Dynamic Change Of Nitrogen Mineralization Of Soil Aggregates In Pinus Elliotti Plantation

Atmospheric nitrogen deposition is one of the hot spots in global environmental research.Large amounts of nitrogen enter the atmosphere in the form of gases,which seep into terrestrial ecosystems.The status quo of phosphorus deficiency in subtropical soils is also subtly affecting the soil environment,and there is an interaction between nitrogen and phosphorus in soils.Therefore,it is extremely urgent to study the law of nitrogen and phosphorus action in soil.Soil aggregates are the basic unit of soil structure.The distribution of aggregates can reflect soil fertility.Nitrogen and phosphorus fertilization have direct or indirect effects on the formation and distribution of soil aggregates.Soil nitrogen mineralization is an important process of nitrogen cycle in forest ecosystem,and the agglomeration of soil has an important effect on the fixation of organic nitrogen.Pinus elliotti Engelm is one of the main tree species introduced in south China.To study the effect of nitrogen and phosphorus fertilization on the nitrogen mineralization of soil aggregates of Pinus elliotti is not only of great significance to solve the problem of atmospheric deposition,but also helpful to make full use of nitrogen and phosphorus fertilizer to improve soil fertility and provide theoretical basis for the sustainable management of Pinus elliotti plantation.In this experiment,four fertilization treatments were designed based on the Pinus elliotti plantation constructed in 2008 in the red soil hilly area of south China.CK(0 kg hm^-2 a^-1),N(50 kg N hm^-2 a^-1),P(100 kg P hm^-2 a^-1),and NP(50 kg N hm^-2 a^-1+100 kg P hm^-2 a^-1)were repeated for 4 times respectively.In the four sampling time of December 2018,March 2019,June 2019 and September 2019,five-point sampling method was adopted to take 0-10 cm(upper layer)and 10-20 cm(lower layer)of soil in the quadrat.Soil aggregates with diameters less than 0.25 mm,0.25-1 mm,1-2 mm and 2-5 mm were obtained through dry sieving.The annual dynamic changes of nitrogen mineralization rate of soil aggregates with different soil layers and different grain sizes under different fertilization treatments were studied by laboratory culture experiments.Main results:(1)Soil aggregates with a particle size of 0.25-1 mm and a particle size greater than 5 mm were the main forms of soil presence in Pinus elliotti plantation.The deeper the soil aggregate with a particle size larger than 5 mm is,the weaker the mechanical stability is,indicating that the soil aggregate with a particle size larger than 5 mm is richer in nutrients.In September,the mechanical stability of soil aggregates with particle size of0.25-1 mm was significantly lower than that of the other three sampling times.Nitrogen and phosphorus fertilizer had no significant effect on the proportion of aggregates with particle size less than 5 mm,but single phosphate fertilizer significantly increased the size of aggregates with particle size greater than 5 mm.(2)The ammonia-nitrogen and total nitrogen contents in Pinus elliotti soil in September were significantly higher than those in the other three sampling times.The deeper the soil is,the lower the nitrogen content is.All n and P fertilization treatments had a certain effect on soil N content,but n application alone was the best.Single application of n fertilizer had the most obvious effect on soil microbial n content,while single application of P fertilizer mainly increased soil nitrate n content.(3)In the soil nitrogen mineralization test of Pinus elliotti,the soil ammonia nitrogen and nitrate nitrogen content reached the maximum in September.With the increase of culture time,the soil ammonia nitrogen and nitrate nitrogen accumulation also increased,and the soil ammonia nitrogen and nitrate nitrogen content increased more obviously after nitrogen and phosphorus fertilization.From the perspective of soil layer,ammonia nitrogen and nitrate nitrogen content of the upper soil is more abundant,corresponding to the net nitrogen mineralization rate and nitrification rate of the upper soil is also faster.From the perspective of particle size,the content of ammonia nitrogen and nitrate nitrogen in soil decreases with the increase of particle size of aggregates,and the net nitrogen mineralization rate and nitrification rate of corresponding small particle size aggregates are larger.Single application of phosphorus fertilizer had the best effect on the increase of ammonia-nitrogen content in soil,so it had the most obvious effect on the increase of net nitrogen mineralization rate.However,single application of nitrogen fertilizer had the best effect on the increase of soil nitrate nitrogen content and the corresponding nitrification rate was faster.(4)Nitrogen and phosphorus fertilizers had no significant effect on TN,TP,TK,AP,AK and MBN contents in the soil of Pinus elliotti.Four fertilizer treatments,soil SOC content of single phosphorus,nitrogen and phosphate at the same time there was no significant difference with control group,but single nitrogen decreased soil SOC content,shows that fertilization of nitrogen and phosphorus fertilizer and phosphate fertilization interaction has no effect on soil nutrient or a degree inhibits the absorption of nutrients from soil,and the single nitrogen promoted the absorption of nutrients from soil.Continuous n and P fertilization can increase the content of ammonia nitrogen and nitrate nitrogen in the soil.Single application of P fertilizer significantly increased the p H value of the upper soil,while the application of N and P fertilizer increased the content of AP in the soil.SOC was significantly positively correlated with soil AP,and negatively correlated with soil AK.There was significant positive correlation between ammonia nitrogen and MBN,extremely significant positive correlation between nitrate nitrogen,and extremely significant positive correlation between soil MBN and p H value.