Root Decomposition And Nutrient Release Characteristics Of Main Tree Species In Cold Temperate Zone Of China

Root system is the main component of underground litter and its death and decomposition are important sources of forest soil nutrients.Root decomposition plays an important role in root turnover and nutrient return.The study of root decomposition is helpful to understand the biogeochemical cycle of forest ecosystem.The northern forest of Daxing ’anling is the only cold temperate forest in China,which plays an important role in regional climate regulation,water conservation and soil and water conservation.In this study,four main forest types(Betula platyphylla,Populus davidiana,Larix gmelinii and Pinus sylvestris var.mongolica)were selected as the research objects.The root systems of Betula platyphylla,Populus davidiana,Larix gmelinii and Pinus sylvestris var.mongolica forest in three diameter sizes(fine root(≤2 mm),middle root(2 ～ 5 mm)and thick root(5 ～ 10 mm))were collected by field bag method.In August 2020,different soil depths(0 ～ 10 cm,10 ～ 20 cm and 20 ～ 30cm)were buried for decomposition experiments to investigate the decomposition and nutrient release characteristics of roots of different tree species with different diameters.The results show that:(1)With the increase of decomposition time,the root weight retention rate showed a decreasing trend.The results of variance analysis showed that tree species,diameter size,soil depth and time all had significant effects on the root decomposition rate(P<0.01).The root weight retention rate of the four tree species decreased significantly on the first 61 d of decomposition,the root decomposition rate of Betula platyphylla of three soil depths varied from 12.52% to 26.29%,and the root decomposition rate of Populus davidiana varied from16.95% to 26.43%.Larix gmelinii and Pinus sylvestris var.mongolica ranged from 4.86% to16.43% and 5.62% to 11.00% respectively.After 61 d of decomposition,the decomposition rate slowed down significantly.After 1a of decomposition,the decomposition rates of roots at all diameter sizes of Betula platyphylla and Populus davidiana ranged from 24.94% to 39.92%,while the decomposition rates of roots at all diameter sizes of Larix gmelinii and Pinus sylvestris var.mongolica ranged from 16.67% to 34.07%.The root decomposition rate of the same tree species with different diameter sizes in the same soil depth was fine root > medium root > coarse root,and the decomposition rate decreased with the increase of soil depth.The root decomposition rate of broad leaf was higher than that of conifer among different tree species.(2)According to Olson exponential decay model fitting calculation,the annual decomposition coefficients(K)of roots of four tree species in three soil depths all decreased with the increase of diameters.The average annual decomposition coefficients of the three soil depth of Betula platyphylla root varied from 0.3090 to 0.5253,Populus davidiana from 0.4316 to 0.5134,Larix gmelinii from 0.1821 to 0.3115,and Pinus sylvestris var.mongolica from0.1981 to 0.3340.The annual decomposition coefficients of Betula platyphylla and Populus davidiana were higher than those of Larix gmelinii and Pinus sylvestris var.mongolica.(3)The mass loss rates of fine,medium and coarse roots were positively correlated with initial N,P and K concentrations(P<0.01),and negatively correlated with C,C/N and C/P concentrations(P<0.01).There was a significant positive correlation between root decomposition rate and soil temperature(P<0.05),but no significant correlation between root decomposition rate and soil moisture(P>0.05).(4)The remaining rates of C,N,P and K in the roots of the four tree species showed consistent changes in the three soil depths.At the initial stage of decomposition(August ～October 2020),the remaining rate of C decreased rapidly,and then showed a slow trend of decline.At the early stage of decomposition,roots released N and P elements,and at the late stage of decomposition(July to August 2021),roots released small amounts of N and P elements.The K remaining rate decreased significantly at the initial stage of decomposition,and gradually decreased after 61 d of decomposition.After 365 d decomposition,the release rates of C,N,P and K from the roots of broad-leaf species in the three soil depths were higher than those of conifer species.The nutrient release rate of C was negatively correlated with the initial C concentration,the nutrient release rate of N,P and K was positively correlated with their initial concentration,and the nutrient release rate of N and P was negatively correlated with C/N and C/P,respectively.It can be seen from the root decomposition experiment of three diameter sizes of four tree species: The decomposition rate of fine roots was the highest,followed by medium roots and coarse roots.The decomposition rate of broad-leaved trees was higher than that of coniferous trees.The decomposition rate of roots in different soil depth was lower with deeper soil depth.Nutrient elements C,N,P and K in roots were released at the initial stage of decomposition,and were enriched with the increase of decomposition time.After one year of decomposition,the release rate of nutrient elements showed K > P > C > N.