Soil Nitrogen Accumulation And Transformation Impact On Soil Carbon Pool Under Different Plantations In The Loess Hilly Region Of China

Clearing the characteristics of soil nitrogen(N)dynamic change from former farmland to forests and the relationship between soil carbon(C)and N pool is significant to soil N sequestration,soil quality and ecological environment effect improvement for the cognitive vegetation restoration.Under the background of returning farmland to forest project,the typical plantations and robinia pseudoacacia L.,caragana korshinskii and grassland with different restoration years in the Loess Hilly Region were selected as the research objects in this study.Using time and space substitution method,through combining field experiment and indoor analysis to study soil N composition accumulation and transformation of different plantation and the regular with restoration age,and combining with the indoor incubation test to analyse litter species,moisture and temperature on the influence of soil N transformation,and comparing soil carbon pool of different plantations and robinia pseudoacacia L.,caragana korshinskii and grassland with different restoration years,and clearing the relationship between soil N accumulation,transformation characteristics and soil carbon pool,we obtained the following main conclusions:(1)Vegetation reconstruction from farmland had obvious effects on soil N fraction contents,stocks and sequestration at soil depth of 0-200 cm.The change rule of soil total N(TN),nitrate N(NO3-N)and total inorganic N(TIN)contents of different plantations was similar with soil deepen,and showed that shallow soil(0-40 cm)change was greater than that of deep soil(40-200 cm).However,the ammonium N(NH4-N),dissolvable total N(DTN)and dissolvable organic N(DON)contents did not change significantly with soil deepen.Compared with farmland,the stocks of soil NO3-N,TIN,DTN and DON obviously declined under platycladus orientalis(L.)Franco and robinia pseudoacacia L.+ platycladus orientalis(L.)Franco,but the stocks of TN,NH4-N,NO3-N,TIN,DTN and DON obviously raised under caragana korshinskii,and were higher than farmland of 2.24 Mg/ha,8.49 kg/ha,15.00 kg/ha,23.54 kg/ha,429.63 kg/ha and 406.44 kg/ha at a depth of 0-200 cm,respectively,and the N fraction accumulation effect was remarkable.There results showed that vegetation restoration was the important factors influencing the N accumulation in the soil,and the soil N fraction accumulation effect is varied under different plantations.(2)Soil N fraction stocks and sequestration were different under robinia pseudoacacia L.,caragana korshinskii and grassland with different restoration years,and deep soil occupied the important role.The change rules of soil N fraction stocks with increasing restoration years were inconsistent,and shallow soil was different from the deep soil.The distribution percentages of soil N fraction stocks were significant difference under various soil layers,and deep soil occupied large proportion.The soil depths of 100-200 cm contained the highest percentages of soil N fraction with the range of 34.18%-57.47%,and the soil depths of 40-100 cm contained 20.11%-37.03%.In most cases,the sequestrations of soil TN and NO3-N were decreased with soil depth of 0-10 cm,10-40 cm,40-100 cm and 100-200 cm under robinia pseudoacacia L.,caragana korshinskii and grassland with different restoration years,but the NH4-N sequestrations showed increased.(3)Litter species and temperature were the main factors affecting soil N transformation and played an important role in soil N transformation.Soil NO3-N content,mineralization N(MN)content,nitrification rate and mineralization rate were significant difference under different litter species treatments,and showed that caragana korshinskii litter treatment hold the highest,followed by robinia pseudoacacia L.,9:1 robinia pseudoacacia L.+ Amygdalus davidiana(Carrière)de Vos ex Henry treatments and control,grass and 1:1 robinia pseudoacacia L.+ platycladus orientalis(L.)Franco were the lowest.Different soil moistures had no significant effects on soil N transformation.Temperature had significant effect on the contents of soil NH4-N,NO3-N and TIN.The average NH4-N content under 38 ? treatment was 5.87 mg/kg,and significant higher than that of 12? and 25? treatments.The NO3-N and MN contents under 38 ? treatment were higher than 12? and 25? treatments of 11.87 mg/kg and 2.67 mg/kg(NO3-N),11.35 mg/kg and 7.57 mg/kg(MN),respectively.Under same temperature and moisture condition,soil NO3-N and MN contents were significant difference under litter species treatments.In the process of cultivation,the highest soil NO3-N and MN contents were capacity korshinskii litter traetment under the condition of 25? and 60% field water capacity,but the highest soil NO3-N and MN contents of the other litter treatments were under the condition of 38? and 60% field water capacity.(4)Vegetation type,restoration year and soil depth were the main factors affecting soil C pool and common regulated soil C stocks.The pools of soil organic C(SOC),dissolvable oraganic C(DOC),dissolvable inorganic C(DIC)and dissolvable total C(DTC)were significant difference.The SOC pool of caragana korshinskii was the highest at soil depth of 0-10 cm,0-40 cm,0-100 cm and 0-200 cm,and higher than farmland of 2.41 Mg/ha,2.64 Mg/ha,3.12Mg/ha and 5.99 Mg/ha,respectively.The DOC,DIC and DTC pools of grassland were the highest,and higher than farmland of 10.20%,8.34% and 8.62% at soil depth of 0-200 cm,respectively.The C pool change trends were different with increasing restoration years under robinia pseudoacacia L.,caragana korshinskii and grassland.(5)Vegetation type,restoration year and soil depth were the main factors affecting the relationship between soil N accumulation,transformation and soil C pool,and common regulated the relationship of soil C and N.The effects of soil N fraction accumulation on soil C pool were similar in each soil layer under different plantations,namely 0-10 cm and 0-40 cm soil layer was relatively consistent,and 0-100 cm and 0-200 cm soil layer was relatively consistent,but soil N fraction transformation rates on the influence of soil C pool were different in various soil depth.TN accumulation and transformation were most sensitive to SOC under all platations in every layer.The effects of soil N fraction accumulation and transformation on soil C pool were varied in each soil layer under robinia pseudoacacia L.,caragana korshinskii and grassland with different restoration years.Under different restoration years of robinia pseudoacacia L.,soil DON and DTN accumulation were more sensitive to DOC and DTC pools,and soil DON and DTN transformation were more sensitive to soil DTC pool.Soil N accumulation at soil depth of 0-200 cm were more sensitive to soil C pool than the other soil layers under caragana korshinskii,and soil NH4-N,NO3-N and TIN transformation were more sensitive to soil DIC pool.Soil N fraction accumulation and transformation on soil C pool were varied with different soil layers under grassland with different restoration years.