The Effect Of Land Use On Soil Nitrogen Transformation In Red Soil And Its Driving Factors

Nitrogen（N）plays an essential role in biogeochemical cycles and shows a fundamental part in controlling of ecosystem structure and function.Soil N mineralization,nitrification and denitrification are the most vital N cycling processes for agricultural and environmental concerns,influencing the quantity of available N for plant and microbial uptake.Land use change can affect soil organic C,total N,soil temperature and other soil properties,which in turn affect soil N cycling.It is important for sustainable agricultural development to clarify the effects of land use change on soil N transformation.Humid subtropical arable soils in China cover about 446 890 km²,which account for approximately 4%of the world’s subtropical arable land surface or 37%of China’s arable area.These soils support 22.5%of the population of China.To date,over the past 60 years,characteristics of subtropical region of China is significantly agricultural associated with rapid alterations land use patterns to meet an increasing demand for food,cash crops,and fiber.Land use and management practices,such as the use of inorganic N fertilizer,organic manure,and lime,could markedly influence the physical and chemical properties of the soil,which would in turn affect soil N cycling.However,the expanded knowledge and further work especially about the rates of N mineralization and nitrification associated with land use effect is needed on showing soil nutrients characteristics of specific site of the region,e.g.humid subtropical region in China.In this study,soil samples were collected from Jiangxi province,which covers an area of 166 900 km²（14.8%of the country’s subtropical area）and has a typical subtropical monsoon climate.The land is now covered mainly by vegetation of forest,tea,upland,and paddy,which account for 63.1%,0.3%,2.0%and 10.5%coverage,respectively.The area of bamboo is account for 10.1%of forest coverage.In this study,soil samples were collected from four land use types,i.e.forest（F）,bamboo（B）,tea plantation（TP）and upland（U）.The net N mineralization and nitrification rates,the denitrification potential and N₂O and N₂ emission were determined in the soils under four land use types.The abundances of AOA,AOB,nir S,nir K,and nos Z were also measured,to identify the effects of land use on soil N transformation in subtropical region of China.The result showed that the soil p H value was the highest and ratio of C to N（C/N）was the lowest under U among the 4 land use types.The numbers of AOA ranged from6.20?10⁶ to 6.58?10⁶ copies/g soil and those of AOB varied from 4.18?10⁶ to 7.41?10⁶copies g^-1 soil.The soil AOA and AOB were the highest under U among the 4 land use types.The net N mineralization rates(3.46 and 1.62 mg N kg^-1,respectively)in U soil at 0-7 days and 0-14 days were significantly higher than those in the soils under the other land use types mainly due to higher soil p H value and lower C/N ratio.The net nitrification rates were 5.33 and 3.06 mg N kg^-1 soil at 0-7 days and 0-14 days,respectively,which was significantly higher under U than those under the other land use types.Net nitrification rates were positively correlated with ammonium N（NH₄⁺-N）（P<0.01）,AOB（P<0.01）,and p H（P<0.05）.These results indicated that higher p H value and content of available NH₄⁺-N stimulated the abundance of AOB and were responsible for the higher net nitrification rate in U soil.However,there was no significant difference in the average N₂O emission rate among the different land use types,which indicated that the land use did not affect the N₂O emissions.This was contradict with the results of net nitrification rate under U land use.One reason was probably that there were some other production pathways of N₂O in soils besides nitrification process.In the future,the effects of land use types on the production pathways of N₂O should be studied,in order to clarify the mechanisms of N₂O emissions in Red Soil.The results showed that there were significant differences in the potential denitrification in soils under different land use types.The denitrification potential was in the following order:B?TP?F?U.The denitrification potential was significantly positively correlated with SOC（P<0.05）,CO₂ emission（P<0.01）,nir S（P<0.05）and nir K（P<0.05）.The stepwise regression analysis showed that the mineralized C was the key factor controlling the denitrification potential,which could explain 66%of variation of the denitrification potential in soils under different land use types.The differences in N₂O and N₂emission were also significant in soils under different land use types,the lowest N₂O and N₂emission were observed in soil of upland,indicating the enhancing p H did not increase the potential N loss via denitrification and N₂O emission.The mineralized C was also the key factor controlling N₂O and N₂emission in soils under different land use types.The significant relationship between nir S,nir K,nos Z and the mineralized C further supported our conclusions.Soil p H was the key factor regulating the ratio of N₂/N₂O,but the mechanisms of p H regulating the ratio of N₂/N₂O need further study.A p H manipulation experiment（p H=4.6,5.7,6.8,7.5,8.2）was performed,and net N mineralization rate,net nitrification rate,the abundance of AOB and AOA were measured,to testify the role of p H in soil N transformations.The results showed that net rate of nitrogen mineralization（P<0.05）and nitrification（P<0.01）increased significantly with the increase of soil p H.Net nitrification rate was positively correlated with net nitrogen mineralization rate（P<0.05）.The abundance of AOB increased significantly with the increase of soil p H（P<0.05）,but no significant change in AOA.The results of p H treatment showed that the mineralization of organic N in subtropical acid forest soil was sensitive to change of soil p H,and the N mineralization rate increased with the increase of p H.This response not only increased the substrate concentration of soil nitrification,but also increased the p H neutral microsites suitable for autotrophic nitrification.Application of lime directly increased soil p H and increased p H microsites due to organic N mineralization affecting the abundance of AOB and nitrification activity.The soil samples in a long-term pig manure and lime applications in upland red soils were collected,and gross rate of soil N trasnformations were determined using ¹⁵N tracing experiment,to testity the effects of fertilization and liming on soil N transformation.There were four treatments:1)No Manure（CK）;2)Low-rate Manure(LM,N 150 kg ha^-1 y^-1);3)Highrate manure(HM,N 600 kg ha^-1 y^-1);4)High-rate manure and lime(HML,N 600kg ha^-1 y^-1+lime 3 000 kg ha^-1（3a）^-1).The results showed that long-term application of manure could improve the gross N mineralization and NH₄⁺-N assimilation rate of red soil,but only reached the significant level when HM and HML were treated（P<0.05）.The application of pig manure+lime dosing（HML treatment）further increased the gross N mineralization and assimilation rate of NH₄⁺-N in red soil（P<0.05）.The NH₄⁺-N assimilation rate and the gross mineralization rate of all the soil N treated in red soil showed a significant positive correlation（R²=0.87,P<0.001）,indicating that NH₄⁺-N assimilation was related with the mineralization of N.The gross N mineralization and the assimilation rates of NH₄⁺-N in all the treated soils were significantly positively correlated with the total soil N,C content and C/N ratio（R²>0.70,p<0.001）.Long-term application of manure also significantly increased the O_NH4（gross autotrophic nitrification）rate of red soil（p<0.05）,and HM treatment was more significantly improved than LM treatment.Compared with HM treatment,O_NH4 rate of HML treatment in red soil was further improved（p<0.05）.O_NH4was positively correlated with MN（gross mineralization rate）（R²=0.90,p<0.001）and soil p H（R²=0.96,p<0.001）.There was a significant positive correlation between crop yield and soil gross N mineralization rate,and there was a significant positive correlation between N₂O emission and NO₃^--N leaching and gross nitrification rate.Therefore,the gross mineralization process and the gross nitrification process are the main control objects to achieve the agricultural production improvement and emission reduction.These results will be helpful to understand the mechanism of soil nitrogen conservation and its determinant factors,and provide scientific basis for nitrogen management according to the characteristics of soil nitrogen transformation process and crop nitrogen form preference.In line with the above findings,we would like to present the premise of this thesis document:i.e.,the fundamental things we learned from the transformation or complex system of the nitrogen cycle in the soil is how to keep the soil management via agriculture activities to be able to support requisites of present and future human generations.There has been a shift in environmental ecology where land（soil+atmosphere+vegetation）is being exploited to meet growing food needs due to the increasing number of people.The primary thing to resolve the effect of land-use change can be as straightforward as how people take care of themselves in order to be able to control their behavior wisely.Due to the account of nature’s limited carrying capacity,we need to stay in line with nature.This interpretation of concepts requires human continuity and consciousness.The ability to disregard the principles of proper and sustainable management must not be disrespectful to humans.In short,the ecosystem balance and its services point of view are becoming issues that need to be of paramount concern.