Effects Of Following Afforestation On Nitrogen Retention Capacity In Red Soil And Calcareous Soil In Karst Area

Soil ammonium?NH₄⁺?and nitrate?NO₃^-?are the main bioavailable N forms for plant uptake.Understanding soil nitrogen?N?transformation processes,which can control N dynamic and availability,is critical to maintain forest growth and productivity during afforestation?the conversion of cropland to forest plantations?.However,only few studies have been conducted to investigate soil N transformation processes involved in N availability in afforested soils.In a 15N tracing study,red soils and calcareous soils were sampled to investigate the changes in gross N transformation rates and influencing factors in the karst area of southwest China.Red soils were collected from cropland,10-y and 50-y afforested woodlands,and calcareous soils were collected from cropland,shrub and forest soils following 50-y succession of abondened cropland.The results of the study are as follows:1.Afforestation can significantly increase organic carbon,total N,C/N,NH₄⁺,WHC and cation exchange capacity in red soils and calcareous soils,but pH,NO₃^-and calcium?Ca?concentrations showed different trends in both soils.Afforestation sifnificantly decreased pH,Ca and NO₃^-concentration in red soils.Oppositely,calcium concentration was increased significantly and pH was not significantly different in calcareous soils after afforestation.NO₃^-concentrations was significantly reduced in calcareous soil under shrub,but was kept at the relative level in the forest soils.2.N retention capacity recovered rapidly in red soils after afforestation.Afforestation stimulated the rates of mineralization,microbial NH₄⁺ immobilization and adsorption of NH₄⁺ leading to a faster turnover of NH₄⁺ pool in afforested soils.Moreover,afforestation inhibited autotrophic nitrification rate,but increased microbial NO₃^-immobilization rate,resulting in low net NO₃^-production.Soil NO₃^-retention capacity was correlated positively with organic carbon,total N,WHC,cation exchange capacity,aluminum oxide,free aluminum oxide and exchangeable aluminum,but negatively with pH,calcium oxide and exchangeable calcium,indicating that the absence of agricultural management?e.g.,N fertilizer and liming?and alteration in soil environment by tree establishment are responsible for the recovery of N retention capacity in acidic soils during afforestation.3.N retention capacity varies greatly due to weathering intensity and soil texture in calcareous soils after afforestation.Higher weathering intensity and clay concentrations increased the rates of microbial NH₄⁺ immobilization,but inhibited autotrophic nitrification rate,resulting in the negative rate of net NH₄⁺ and NO₃^-production in calcareous soils following afforestation.NH₄⁺ can be qucikly transformed into NO₃^-due to the relatively higher autotrophic nitrification rate in lower weathering intensity and loose texture calcareous soils.The higher potential for NO₃^-production reduces N retention capacity in hot,rainy karst area.4.In general,soil N retention capacity was significantly different in red soils and calcareous soils after afforestation.N retention capacity recovered rapidly in red soils,while varied greatly due to weathering intensity in calcareous soils after afforestation.N retention capacity was strong in higher weathering intensity calcareous soils,as well as red soils,but the mechanism is different.The higher potential for NO₃^-production reduces N retention capacity in lower weathering intensity and loose texture calcareous soils.