Rhizosphere Effect And Influencing Factors Of Four Coniferous Plantations At Different Stand Ages

Rhizosphere effects of soil nitrogen and phosphorus plays an important role in the cycle of nitrogen and phosphorus nutrients.The soil characteristics and root traits may change at different stand ages.Rhizosphere effects are often influenced by soil characteristics and root exudates,nevertheless,the variation rhizosphere effects of nitrogen and phosphorus and its influencing mechanism are not clear at present.In this study,young stand stage（8-15 years old）,middle stand stage（25-35 years old）and mature stand stage（45-65 years old）of Pinus sylvestris var.mongolica,Picea koraiensis,Pinus koraiensis and Larix gmelinii plantations in Mengjiagang Forest Farm in Jiamusi City,Heilongjiang Province were used as research materials.In August 2021,we measured fine root（the first three orders）traits（diameter,specific root length,tissue density,mycorrhizal colonization rate,respiration rate and exudation rate）and soil characteristics（total nitrogen,available nitrogen,total phosphorus,available phosphorus,total carbon and p H）,then the rhizosphere effects of nitrogen and phosphorus were calculated.The aims of this study were to reveal the variation rule and influence mechanism of rhizosphere effects of N and P at different stand ages.Our results show that:（1）Soil characteristics and rhizosphere effects of N and P were significantly different among stand ages and tree species.Soil total carbon and total nitrogen concentrations of young stand were lower than those of middle and mature stand in four tree species.The highest soil total carbon concentration was 86.79 g/kg at mature stand of P.koraiensis,and the lowest was12.35 g/kg at young stand of P.sylvestris var.mongolica.Soil total nitrogen concentration was the highest at mature stand of P.koraiensis（6.58 g/kg）and the lowest at young stand of P.sylvestris var.mongolica（0.71 g/kg）.The concentrations of soil ammonium,nitrate and available nitrogen were the lowest at young stand,but except nitrate of P.koraiensis and L.gmelinii.The concentrations of soil available P and total P of P.koraiensis,P.koraiensis and L.gmelinii were the lowest at young stand,and the highest at mature stand,but the concentrations of soil available P and total P of P.sylvestris var.mongolica were the lowest at middle-aged stand.With the increase of stand age,the rhizosphere effects of available P,total P and total N of all tree species increased gradually,while the rhizosphere effect of soil ammonium increased firstly and then decreased.In addition,the rhizosphere effects of soil available nitrogen,total nitrogen,available phosphorus and total phosphorus in P.sylvestris var.mongolica,soil available nitrogen,total nitrogen and total phosphorus P.koraiensis,soil total nitrogen and total phosphorus in P.koraiensis,and soil nitrate nitrogen in L.gmelinii showed no significant differences between middle-age and mature stands.（2）Root morphology,physiological traits and mycorrhizal colonization rate were significantly different among stand ages and tree species.With the increase of stand age,root diameter and mycorrhizal colonization rate increased gradually.The maximum diameter was0.47 mm at the mature stand of P.koraiensis,and the minimum diameter was 0.27 mm at the young stand of P.koraiensis.The maximum mycorrhizal colonization rate was 79.39%at mature stand of L.gmelinii,and the minimum was 51.90%in young stand of P.sylvestris var.mongolica.With the increase of stand age,both root respiration rate and root exudation rate showed a decreasing trend（root respiration rate of P.sylvestris var.mongolica first increased and then decreased）.The maximum root respiration rate was 21.65 n mol CO2 g^-1 s^-1 at the middle-aged stand of P.sylvestris var.mongolica,and the minimum was 7.52 n mol CO2 g^-1 s^-1at the mature stand of P.koraiensis.The maximum root exudation rate was 0.63 mg C g^-1 h^-1 at young stand of P.koraiensis and the minimum was 0.04 mg C g^-1 h^-1 at mature stand of P.koraiensis.The changes of specific root length and tissue density of the four tree species were complicated and did not show a consistent pattern.In addition,with the increase of stand age,the changes of root functional traits（especially diameter,mycorrhizal colonization rate,respiration rate and exudation rate）indicated that the resource acquisition strategy of roots became more conservative.（3）The relationship between rhizosphere effects of nitrogen,phosphorus and root characters,soil characteristics was different among stand ages and tree species.The results of Pearson correlation analysis between the first principal component of root traits,the first principal component of soil characteristics and the rhizosphere effects of soil nitrogen and phosphorus showed that roots and soil mainly affected the rhizosphere effects of soil ammonium,available nitrogen and total phosphorus.The results of Pearson correlation analysis between the first principal component of rhizosphere effect and root traits,soil characteristics showed that rhizosphere effects of nitrogen and phosphorus were mainly affected by root exudation rate,respiration rate,specific root length,tissue density and soil ammonium,available nitrogen and total nitrogen.The results of Pearson correlation analysis between rhizosphere effect of the first principal component and the first principal component of root traits,the first principal component of soil characteristics showed that the influence of root traits on rhizosphere effect was significant in all three ages of P.koraiensis and P.koraiensis,but only at young stand of P.sylvestris var.mongolica and L.gmelinii.The influence of soil characteristics on the rhizosphere effects were mainly significant at young stand of P.koraiensis and P.koraiensis,while the influence of soil characteristics on the rhizosphere effects was only significant only at middle-aged stand of P.sylvestris var.mongolica and L.gmelinii.In conclusion,this study confirmed that during the development of coniferous plantations in temperate regions,soil characteristics,rhizosphere effects of soil nitrogen and phosphorus and root function traits had obvious dynamic changes.The relationship between rhizosphere effects of nitrogen,phosphorus and root traits,soil characteristics is not only affected by tree development stage,but also has great relationship with tree species.The relationship between root traits and rhizosphere effects of N,P showed the coordination of root functional traits in nutrient acquisition.The results of this study will provide scientific basis for further understanding of plant-soil interaction and plantation cultivation.