Fine-Root Decomposition Of Manchurian Ash And Davurian Larch Plantations

Fine root decomposition is one of the major pathways of carbon and nutrient cycling in terrestrial ecosystem.The most of fine-root decomposition processes we known is derived from litterbag studies.However,the environmental conditions of roots decay within litterbag were changed in litterbag studies.In order to characterize the root decomposition in nature,we employed minirhizotron technique to study root decay.In contrast,this technique requires no a priori root processing,retains natural rhizosphere associations,and maintains in situ decay conditions.Fine-root decomposition were measured using minirhizotrons.In October 2003,six 20 m×30 m plots were established in each plantation(three fertilization and three control),and six minirhizotron tubes(90 cm in length,5.5 cm inside diameter)were installed randomly throughout the plot at an angle of 45°to the soil surface,providing an average viewable tube surface extending to 45 cm vertical depth.From April in 2004 to October in 2007,video images were collected at approximately two weeks intervals during the growing season.Data recorded for each root include of the date of appearance,date of death(apparently blackening or shriveling),date of disappear,depth,diameter and order.The interval from the date of birth to disappear represented decomposition time of individual root.Video images were digitized via Root Tracker software(NC,USA).Kaplan-Meier method in survival analysis was used to generate cumulative remaining rate and median root decomposition time(MRDT),differences in cumulative remaining rate between tree species,diameter classes,root order,soil depths, season,fertilization and control were compared by Log-rank test.Cumulative remaining rate of fine-roots in both species decreased gradually with time. The decreasing of cumulative remaining rate of larch roots was significantly faster than that of ash(p＜0.001).Median root decomposition time(MRDT)of larch was 101±18 d compared with 296±16 d for ash.The MRDT of both species differed in three diameter classes(≤0.3 mm, 0.3～0.6.mm,＞0.6 mm).The finest roots(≤0.3 mm)of both ash and larch decayed more slower than the other two diameter size,however,only the MRDT of ash decreased systematically with increasing root diameter.With increasing root order,the cumulative remaining rate of both species increased significantly(p＜0.05),resulting in a delay time of 136 d and 90 d of MRDT for higher order comparing with first order in larch and ash roots, respectively.The fine-root cumulative remaining rate of both species showed similar pattern among soil depths,which at 0～15 cm depth was significantly less than 15～30 cm and 30～45 cm (p＜0.05).The birth season influence the fine-root cumulative remaining rate of both species in the same way.The roots produced in spring(April to June)decayed more faster than the roots which produced in summer(July and August)and autumn(September and October).The cumulative remaining rate of the roots produced in autumn was higher significantly than that of the roots produced in the other two season(p＜0.05).The season of root death had a significant effect on root decomposition.For Larch,the roots dead in summer decayed more faster than the roots which dead in spring and autumn(p＜0.05);but for Ash,the roots which dead in spring has the shortest MRDT.The roots which dead in autumn of both ash and larch decayed more slower than the roots which dead in the other two season.N treatment confined the decomposition process of fine-root.The cumulative remaining rate of fine-roots in both species increased significantly as along as fertilization(p＜0.05).The MRDT of total fine-roots for larch and ash was delayed about 100 d and 76 d respectively by N treatment.The cumulative remaining rate of different diameter classes(except≤0.3 mm),both root order,different soil depth(except 30～45 cm for larch)and spring or summer birth cohorts all increased as along as fertilization.There were a few different for the degree of the response to N treatment between larch and ash.Generally,the response to N treatment of ash roots was more obviously than larch.N treatment accelerate the decomposition for the finest roots(≤0.3 mm)of both ash and larch,and shorten the MRDT of larch roots which distributing in the 30～45 cm soil depth.This study suggested that fine-root decomposition might be a species-specific process, internal factors(e.g.diameter)and external factors(i.e.soil depth)mutually control this process.Also,fine roots decomposition observation in situ via minirhizotron method indicated that decay of fine roots should be faster than previous reported.