The Relative Contributions Of Aboveground Litter And Fine Roots To Soil Organic Matter In Mangroves

In recent years,the carbon sequestration potential of mangroves has attracted much attention in the context of increasing atmospheric CO2 concentration and climate warming.At present,the production and decomposition dynamics of mangrove leaf litter has been extensively studied.However,as the mangrove environment's specifieness of long-term tidal soak and difficulty of mangrove root sampling and operability,there are few studies on the fine roots of mangrove plants,especially in the study of the fine root productivity,turnover,decomposition dynamics and the contribution to soil organic matter.In this study,Bruguiera sexangula,Rhizophora stylosa,Avicennia marina communities which are mono-specific stands,and Ceriops tagal community in two different stand types were selected as the research object,to observe the yield of aboveground litter?including leaves,branches,flowers and fruits?of these natural mangrove communities consecutively and systematically,and estimate the fine root productivity and turnover rate of mangroves with sequential coring and in growth core method to collect fine roots,through separating two kinds of fine root functional groups,lower order roots and higher order roots.The decomposition dynamics of litters and fine roots was studied,and the relative contribution of litter and fine roots to the accumulation of soil organic matter was evaluated,to reveal the key regulatory mechanism of mangrove soil carbon sequestration and provide data support and decision-making basis for mangrove ecosystem carbon sink-oriented management and management.?1?Annual biomass of aboveground litter in each natural mangrove communities were studied,the results showed that:The contribution of the leaf litter was far higher than the other parts.Annual leaf litter production of riverine Ceriops tagal,seashore Ceriops tagal,Bruguiera sexangula,Rhizophora stylosa,and Avicennia marina stands ranged between 256.95g m^-2 and 814.31 g m^-2,occupy 75.67%,72.74%,89.07%,86.37%and 56.57%of the annual total production of aboveground litters respectively.There are significant differences between the aboveground litter?leaves,branches,flowers and fruits?production of each natural mangrove communities,and all had obvious seasonal variations,but no certain rule of change.Annual root biomass of each natural mangrove communities were studied,the results showed that:The biomass of lower order fine roots in the distal two branching orders were ranged from 602.84 g·m^-2 to 5465.61 g·m^-2;of higher order fine roots were ranged from256.825 g·m^-2 to 2810.45 g·m^-2.The biomass of fine roots?<2mm?were ranged from 970.89g·m^-2 to7477.08 g·m^-2;of coarse roots were ranged from 821.04 g·m^-2 to13303.40 g·m^-2.The biomass of dead lower order fine roots were ranged from 1578.21 g·m^-2 to 7351.01 g·m^-2,of dead fine roots?<2mm?were ranged from 1578.21 g·m^-2 to 7351.01 g·m^-2.There were obvious seasonal variations in the biomass of lower order fine roots,thus higher order fine roots and coarse roots remained relatively stable as seasonal change,but no certain rule of change all.The annual biomass of lower order fine roots accounted for 43-83%of annual biomass of fine roots?<2mm?,but dead lower order fine root biomass accounted for 83-98%of dead fine roots?<2mm?biomass.This means that the turnover of the lower order fine roots is faster and the life is shorter.The lower order roots are the active unit in the root system of the mangrove,which is the main factor of the fine root turnover.On the contrary,the higher order roots and the coarse roots have the perennial characteristics and the low turnover rate.?2?The regularity of the fine roots biomass variation with soil depth in natural mangrove communities was compared,the results showed that:The fine roots biomass decreased with the increase of soil depth.In the 0-20cm soil layer,amount of fine roots biomass was the largest,accounted for 50%of the fine root biomass.In the 0-60cm soil layer it accounted for 80%,and90%in the 0-60cm soil layer.?3?The productivity and turnover rate of fine roots were estimated through collected fine roots by sequential coring method,the results showed that:the productivity of middle and lower order roots of each communities ranged from 571to 2838 g·m^-2,that of fine roots less than 2mm ranged from 576 to 3011 g·m^-2.Turnover rates of lower order fine roots was1.46-5.96 per year,and that of fine roots?<2mm?was 0.76-5.17 per year.The turnover of mangrove communities'fine roots are underestimated 13-48 percent,by using fine roots less than 2mm rather than lower order fine roots that is fast turnover.?4?The productivity of fine roots were estimated through collected fine roots by ingrowth core method,the results showed that:The productivity of the lower order fine roots of each communities ranged from 187.91 g·m^-2·a^-1 to 462.89 g·m^-2·a^-1;and that of fine roots less than2mm ranged from 239.67 g·m^-2·a^-1 to 653.15 g·m^-2·a^-1.The value of the lower order fine roots productivity estimated by sequential coring was 2.4-4.6 times higher than that obtained by ingrowth core;and of fine roots less than 2mm was 3.1-6.1times.Due to the significant interference and damage to the fine roots and the soil environment caused in the process of burying core with ingrowth core method,in addition,with sequential coring method roots biomass can be obtained in the year by direct sampling which is relatively convenient and rapid,thus it is considered that the sequential coring is more suitable for estimating the productivity of fine roots.?5?Correlation analysis between fine root biomass and soil nutrient of each natural mangrove communities were studied,the results showed that:annual fine root production was positively related to soil nutrients such as N and P concentrations,suggesting that mangrove fine root production is nutrient limited at our study site.Soil characteristics played a more important role than species composition in controlling fine root production,and the community and site condition are the main factors affecting the turnover rate of fine roots.?6?Field decomposition experiments on leaf litter and fine roots of each natural mangrove communities were carried out,the results showed that:leaf litter on soil surface decomposed more slowly than within soil,decomposed faster in the lower intertidal zone than upper intertidal zone.The decomposition rate of fine roots is lower than that of leaf litter.The effect of intertidal zone on fine root decomposition is not obvious.?7?The annual input of leaf litter and fine roots to soil organic matter were estimated by annual biomass and decomposition of litter,the results showed that:The annual productivity of fine roots,which was positively correlated with soil C concentration,is 1-3.5 times higher than that of aboveground litter,but the annual decomposition amount of fine roots is lower.This study indicates that the accumulation of soil organic matter in mangrove community mainly depends on the fine root productivity,thus the underground litter?fine roots?contributes much more to the soil organic matter accumulation than the aboveground litter.It is an effective way to promote carbon sequestration in mangrove soil through improving the fine roots productivity of mangroves.