| Chinese fir (Cunninghamia lanceolata) is one of the most important native fast-growing evergreen coniferous species in southern China. Due to an increasing demand for timber, monoculture Chinese fir plantations were widely planted, however concerns have been expressed about the declining timber yield and soil fertility degradation on successive rotations of Chinese fir plantations. At present, the fundamental reasons contributing to the long-term problems are not clear. As a result, how to further reveal inter-mechanism of declining timber yield of Chinese fir plantations has become the major task in current forestry career.The decline in Chinese fir productivity has been attributed to a combination of factors including site preparation methods, management intensity and silvicultural practices. However the role of the physiological characteristics of the species and nutrient cycling utilization in contributing to the decline in productivity is yet to be explored. Litterfall and fine root are important components of forest biogeochemical and nutrient cycling. Nutrient return and turnover through above-and belowground litterfall is an important pathway for self-fertilization in forests, which is a critical way to sustain the long-term productivity of Chinese-fir plantations. Although litterfall characteristics, fine root turnover and nutrient cycling utilization regarding to Chinese fir plantations has been studied, nutrient translocation and utilization efficiency was seldom studied and which has important theoretical and practical significance to understand the mechanism of productivity decline in Chinese fir plantations.In view of this, this study focus on the problems in the nutrient cycling in Chinese fir plantations, from the aspect of the nutrient retranslocation and utilization, aboveground litterfall production, fine roots production, internal nutrient cycling in senescent leaf and branches and fine roots and nutrient distribution and fluxes were investigated in three monospecific Chinese-fir plantations at different developmental stages (10-,22-and34years) using the chronosequence method in Sanming city, Fujian province. The study will have important theoretical significance to understand the mechanism of productivity decline, also will have an important practical value to improve nutrient utilization and maximize forest productivity. The main results are as follows:1. The biomass, productivity and nutrient utilization characteristic varied in different-stages Chinese fir plantations. Total biomass of the stands increased with the stage in the order of38.11,104.03and138.24t·hm-2respectively for the young, mature and over-mature stands. The total productivity also increased with stand stage, which were4.80,6.97and8.11t·hm-2·a-1for the10-,22, and34-year old stands respectively. Nutrient utilization efficiency in different developmental-staged Chinese fir plantation increased with the stage increasing. Producing dry matter per ton required macronutrient (N, P, K, Ca, Mg) of18.58,13.66and11.82kg respectively in young, mature and over-mature stage. Producing dry matter per ton required micronutrients (Fe, Mn, Cu, Zn) of0.608,0.490and0.490kg respectively in young, mature and over-mature stage. Nutrient utilization efficiency of macronutrient was in the order of P>Mg>Ca>K>N, and the micronutrient was in the order of Cu>Zn>Fe or Mn.In conclusion, the young stage of Chinese-fir plantation was characterized by high nutrient uptake from the soil, low nutrient return, short turnover time and low nutrient utilization rate. In contrast, the mature stand circulation coefficient, turnover time, and nutrient utilization rate all increased, which indicates a higher nutrient uptake from soil as well as higher nutrient return and higher nutrient depletion. However, the turnover time in the over-mature stage stand was high and was characterized by lower uptake nutrient from the soil, higher nutrient return and utilization. This implies that harvesting Chinese-fir plantation at young stage would lead to high nutrient loss, which may not be sustainable in maintaining the long-term productivity of the species. As a result, prolonging the rotation of Chinese fir plantations properly will have great significance of soil fertility recovery.2. For litterfall production, the height and layout mode of litter trap had no significant effect on litterfall collection with the results of50cm>25cm>100cm and "random" mold>"×"mold>"+" mold. The area of trap had significant effect on litterfall collection (p<0.05), with the order of0.5m2>2m2>1m2. The percentage of total trap area accounting for samples had significant effects on litterfall collection (p<0.05) in the order of1.5%>0.75%>0.5%. Annual litterfall production in different developmental-staged stands varied temporally. On average, annual litterfall production was in the order of over-mature stage (507.84g·m-2·a-1)> mature stage (458.79g·m-2·a-1)>young stage (348.52g·m-2·a-1). Reduced major axis analysis showed a significant positive correlation between litterfall fragments and total production. The allometric index between needles, branches, flowers and cones, other components and total litterfall production was1.36,1.41,1.52and1.31respectively. The allometric indexes between needles and branches, needles and flowers, branches and flowers, needles and others, branches and others, flowers and others were0.91,0.82,0.84,1.19,1.19and1.32respectively. With the exception of other components, needles accounted for the largest percentage of litterfall production (32.9%-44.4%), followed by branches (15.2%-17.2%), and flower and cones accounting for the smallest percentage (2.4%-10.8%). Monthly dynamics of litterfall production of Chinese fir plantations was obvious with three peaks of April-May, August and December. During2012-2013, the young and mature stage plantations had two peaks (April-May and August) in litterfall, however, monthly dynamics in the over-mature stage plantation was three peaks (April-May, August and January-February) in2012, and was one peak (April) in2013. Needles and branches had similar monthly dynamics as they dropped together. Seasonal dynamics of litterfall was in the order of spring> summer> autumn> winter. Leaves fell in spring, branches and flowers fell in summer.Litterfall characteristics varied among different-staged Chinese fir plantations. Macronutrient concentration in litter of Chinese fir plantations was in the order of N> Ca> Mg> K> P, and micronutrient concentration was in the order of Mn> Fe> Zn> Cu.. Nutrient concentration in different litterfall components has obvious monthly dynamics, and it will be higher or lower evaluation of annual nutrient return by using nutrient concentration in one month. Macronutrient return (N, P, K, Ca and Mg) through litterfall increased with stand age, in the order of over-mature stage (146.61±25.19kg·hm-2·a-1)>mature stage (131.46±24.36kg·hm-2·a-1)>young stage (96.63±14.57kg·hm-2·a-1) respectively. Micronutrient return (Fe, Mn, Cu, Zn) also increased with stand age, following the order of over-mature stage (12082.46g-hm"2)> mature stage (9087.19g·hm-2)>young stage (7796.22g·hm-2) plantations respectively. The order of annual nutrient return by litterfall was N> Ca> K or Mg> P and Fe> Mn> Zn> Cu. Nutrient return in different components was in the order of leaves or other components>branches> flowers and cone. Monthly dynamics of N nutrient return was similar to litterfall production recording two peaks in the young and mature plantations (May and August), and three peaks in the over-mature stand (January-February, April-May and August). Monthly dynamics of P and K return were there peaks, and monthly dynamics of Ca and Mg had one peak in August. Monthly dynamics of Fe return had muti-peaks (Februay, May, June and August). Meanwhile the monthly dynamics of Mn return in the young and mature stands was one peak (August), and two peaks in the over-mature plantation (May and August). Monthly dynamics of Cu and Zn return were two peaks (May and August).Pearson correlation showed significant relationship between C, K, Fe, Cu and Zn returned and litterfall production. The micronutrient returned through litterfall had a significant relationship with soil nutrient availability. N and C concentration was significantly correlated with soil organic matter. On the other hand P, C:N, cellulose content and lignin content had significantly negative correlation with soil organic matter. It can be concluded that the higher N content and the lower C:N and cellulose and lignin content is more advantageous to the mineralization of soil organic matter.3. Independent-T test showed that N, P, K, Cu, Ca and Mg nutrients in senesced components retranslocated to the fresh ones, and Fe, Mn and Zn nutrients did not retranslocate when the leaves and branches senesced. Nutrients translocation rate of senesced needles was in the order of K> P> Cu> N> Ca> Mg, and the order for branches was P>Cu>K>N>Ca> Mg. The P nutrient retranslocation rate in leaves was higher following the order of over-mature stage (66.0%)>mature stage (65.4%)>young stage (60.9%), and the P nutrient retranslocation rate in branches was highest to71.0%-73.8%. The nutrient retranslocation rates of N, K, Ca, and Mg in senesced branches and leaves were30%-40%,47%-70%,4%-40%and4%-20%respectively. Monthly dynamics of N, P, K and Cu in senescenced leaves and branches was similar. N and P retranslocation rate peaked in April and October. Cu retranslocation rate was lowest in April (16.0%-24.7%), and higher in other months. Pearson correlation analysis showed that there was no significant correlation between nutrient retranslocation rate and soil nutrient pool. However, nutrient retranslocation rate was more related to nutrient content in fresh and dead tissues, but not a result of simple migration progress from high concentration to low concentration.The nutrient retranslocation of N, P, K, Ca, Mg and Cu in leaves and branches increased with age in order of25.51,39.03, and47.25kg·hm-2·a-1for young, mature and over-mature stands respectively. The nutrient retranslocation amounts were18.94-37.61kg·hm-2·a-1in leaves and5.57-10.09kg-hm’^a"1in branches. Nutrient translocation amount through leaves and branches was considerable compared to nutrient return through litterfall, accounting for71.3%,57.9%and64.9%in different-staged Chinese fir plantations. Nutrient rethanslocation amount was in the order of N>K>Ca>Mg>P>Cu. Moreover seasonal dynamics of N, P, K and Cu retranslocation was apparent. The highest N and P retranslocation was recorded in summer (3.27-7.11kg·hm-2and0.21-0.59kg·hm-2) and the lowest was in autumn (1.33-4.59kg·hm-1); the highest K retranslocation amount was in spring in mature and over-mature stands and summer in young stand; the highest Cu retranslocation amount was in winter and summer in young and mature stands and winter in over-mature stand.4. Fine root production and distribution varied in different-staged Chinese fir plantations. The total root biomass production was in the order of mature stage (3.49t-hm’2)>over-mature stage (3.07t·hm-2)> young stage (2.80t·hm-2) stands. The production of live fine roots was in the order of mature stage (2.73t·hm-2)> young stage (2.49t·hm-2)> over-mature stage (2.13t·hm-2) stands. The vertical distribution of fine roots of Chinese fir was accumulated in surface soil layer (0-20cm). The fine roots biomass production in0-20cm accounted for40.4%-46.3%of the total fine roots, and47.4%-54.4%of live fine roots. There was seasonal variation in fine root biomass production. Total production of fine root biomass showed two peaks with dominant peak in April and minor-peak in October. The production of live fine roots was high in January and April, while the pattern of dead fine roots showed opposite trend with high biomass in January and April and low biomass recorded in July and August.Fine root nutrient characteristics varied in different-aged Chinese fir plantations. The macronutrient amount (N, P, K, Ca and Mg) of total fine roots for the young, mature and over-mature Chinese-fir stands were46.84,47.79, and45.29kg-hm’2respectively.The micronutrient concentration (Fe, Mn, Cu, Zn) of total fine roots were1197.61,1720.49and1726.35g·hm-2for the young, mature and over-mature stands respectively. The nutrient amount in live fine roots decreased with the stage increasing, while the nutrient amount in dead find roots increased with the stage increasing. The order of nutrient accumulation was in the order of N>K or Ca>Mg>P and Fe>Mn>Cu or Zn. Pearson correlation analysis showed that the concentration of N, P, K, Mg, Fe, Cu and Zn concentration in live fine roots decreased with root diameter increasing, while concentration of Ca in live fine roots showed increased trend with root diameter increasing. The concentration of N、P、K in dead fine roots increased with diameter increasing, while the other nutrients in dead fine roots showed different pattern. The concentration of N, P, K, Fe, Cu and Zn in live fine roots decreased with root order increasing. The concentration of N, P and K in dead fine roots decreased with root order increasing, while the concentration of Ca and Mg in dead fine roots increased with root order increasing.Independent-T test showed that K, Ca, Mg, Cu nutrients retranslocated from dead roots to live roots, and N, P and Fe did not existed retranslocation when fine root seneced in Chinese fir plantations. The nutrient retranslocation rate was in the order of Cu>Ca>Mg>Mg and in the order of young stage> over-mature stage> mature stage. Pearson correlation analysis showed that K, Mg and Zn nutrient retranslocation exsted in the diameter of0.5-1mm and in the first and second order of fine roots. |
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