Carbon Storage and Distribution of the Mature Pinus massoniana Plantation in Northwest Guangxi

Zhenge HUANG Minyang XIE Mingbao WEI Bin HE Shaozhuang MO Gang ZHOU Ji LIANG

Abstract [Objectives] This study was conducted to understand the carbon sequestration function of mature Pinus massoniana plantation in northwest Guangxi.

[Methods] The carbon storage and distribution in a 26 year old P. massoniana plantation were investigated through biomass harvesting in Shankou Forestry Farm of Nandan County， Guangxi Province.

[Results] The average carbon content of P. massoniana was 489.3 g/kg， and the carbon contents of different organs ranked from large to small as pine needles > trunks > trunk bark > roots > branches. The carbon contents of understory shrub layer， herb layer and litter layer were 453.0， 425.6 and 482.5 g/kg， respectively. The soil organic carbon content in forestland varied from 6.20 to 32.15 g/kg， decreasing with the depth of the soil layer. The carbon storage of the mature P. massoniana plantation ecosystem was 232.13 t/hm2， of which the tree layer， shrub layer， herb layer， litter layer and soil layer were 92.67， 1.36， 1.12 and 134.49 t/hm2， respectively， which accounted for 39.92%， 0.59%， 0.48%， 1.07% and 57.94% of the carbon storage of the whole ecosystem， respectively. The annual net productivity of the tree layer of the mature P. massoniana plantation was 10.36 t/（hm2·a）， the annual net carbon sequestration was 5.41 t/（hm2·a）， and the annual net CO 2 absorption was 19.83 t/（hm2·a）.

[Conclusions] This study provides basic data and scientific basis for rational evaluation of the ecological benefits of P. massoniana plantation in this area.

Key words Pinus massoniana; Mature forest; Carbon content; Carbon storage

Forests are the mainstay of terrestrial ecosystems and a major part of terrestrial carbon pools. They stored the organic carbon accounting for 76%-98% of the terrestrial ecosystems， and about two thirds of the earths annual biochar fixation comes from forest ecosystems[1]. Therefore， forest ecosystems play an extremely important role in the global carbon cycle and carbon balance and in maintaining and improving the global ecological environment. Plantations are an important part of forests. As the country with the largest area of artificial forests in the world， China has increased the carbon fixation in forest ecosystems through activities including afforestation， reforestation and forest management， to further improve the forests ability to balance atmospheric CO 2 and slow down the accumulation rate of CO 2 in the atmosphere， so plantations play an important role[2]. Since the 1990s， in order to reveal the carbon sink function and ecological benefits of forest  ecosystems ， Chinese scholars have conducted systematic studies on the carbon sink function of major plantations including Pinus massoniana[3]， Cunninghamia lanceolata[4]， Populus[5]， P. massoniana[6]， Eucalyptu[7] and Betula alnoides[8]， providing basic data for rational assessment of plantation carbon sink function and ecological benefits.

P. massoniana is one of the most important fast growing and high yield forest species in the subtropical region of China[3]. It is also the main plantation tree species in Guangxi， and plays a pivotal role in Chinas forestry production and timber strategic reserve base construction. At present， there have been many reports on the biological productivity and carbon sink function of main plantations in Guangxi[9-13]， but there is no report on the carbon storage of P. massoniana plantations in northwest Guangxi. In this study， based on field investigation and indoor chemical analysis， the carbon storage and distribution characteristics of the mature forest  ecosystem  of 26 year old P. massoniana in Nandan County， Guangxi Province， were systematically studied， so as to reveal the carbon sequestration capacity of P. massoniana plantation ecosystems in northwestern Guangxi. This study provides basic data and scientific basis for the reasonable evaluation of the ecological benefits of P. massoniana plantations in this area.

Materials and Methods

General situation of the experiment field

The study area is located in the Mushan branch of the Shankou Forest Farm in Nandan County， northwestern Guangxi. Nandan County is located at 107°1′-107°55E， 24°42-25°37′N. The climate belongs to the mid subtropical mountain climate with an average annual temperature of 16.9 ℃， annual rainfall of 1 498.2 mm and altitude of 600-1 100 m. This area has a low mountain landform. The soil parent material is dominated by sand shale entrained with gray green slate， and the soil is mountainous yellow soil. The thickness of the soil layer is over 80 cm and the thickness of the humus layer is about 15-23 cm.

The previous forest was a C. lanceolata plantation. The  P. massoniana  seedlings were raised from March 1991. The initial planting density was 2 500 plants/hm2， and the plant spacing was 2 m × 2 m. At the end of May 2017， the plantation was relatively neat. After natural thinning and intermediate cutting， the stand density， canopy density， average diameter at breast height and  average  tree height was 637 plants/hm2， 0.70， 26.5 cm and 19.4 m， respectively. The understory plants were mainly Rhus chinenesis， Maesa japonica， Mallotus barbatus， Urena lobata， Rubus alceaefolius， and Miscanthus floridulus and Dicranopteris dichotoma. The thickness of the litter layer was 2-3 cm.

Research methods

Stand biomass determination

On the basis of comprehensive investigation of the existing P. massoniana plantation， in May 2017， the area with basically the same site conditions （including slope direction， slope position， slope and altitude） was selected from the adjacent P. massoniana plantation and three 20 m ×  20 m  standard plots were set. The biomass of the leaves， branches， bark， stems， roots of the tree layer and the understory vegetation was determined according to reference [10].

Sample collection and carbon content analysis

Plant samples of different structural levels of the ecosystem were collected according to reference[10]. The collected samples were filled in plastic ziplock bags for testing after drying pulverization and  sieving.

At the same time， three representative sampling points were set in each standard plot， and 1 kg of soil was collected from each layer with a depth of 20 cm in the 0-80 cm soil layer， followed by mixing. The mixed soil samples were naturally dried， pulverized and filled in bottles for testing. The carbon content of plants and the content of organic carbon in soil were both determined by the potassium dichromate oxidation heating method[14].

Data processing and analysis

The carbon storage of the  P. massoniana  plantation ecosystem was calculated according to reference[10]. The annual net carbon fixation of the tree layer was calculated from the annual average carbon storage of each organ. The average net production of trunks， branches and roots was calculated according to the actual age of the stand. Because P. massoniana stands at different ages differ in the composition of pine needles and the age of pine needles， this study used 1.7 years as the pine needle age according to reference [15] to perform evaluation.

Regular data processing was performed using Excel2003 software， and the statistical analysis used SPSS17.0.

Results and Analysis

Carbon content in different structural levels of the mature  P. massoniana  plantation

It can be seen from Table 1 that the carbon contents of different structural levels in the mature P. massoniana plantation showed trend of decreasing with the decrease of plant height or the degree of tissue lignification. That is， the tree layer had the highest carbon content of 489.3 g/kg， followed by the shrub layer， which was 453.0 g/kg， and the herb layer showed the lowest value of 425.6 g/kg. Among the carbon contents in the organs of the mature P. massoniana plantation， the pine needles had the highest content of 511.2 g/kg， followed by the trunks， trunk bark and roots， of which the carbon contents were 501.8， 490.5 and 479.2 g/kg， respectively， and the lowest content was 463.4 g/kg in the branches.

The average soil organic carbon content of the mature  P. massoniana  plantation was 14.81 g/kg. Under the influence of the accumulation and decomposition of surface litter， the soil organic carbon content of the mature P. massoniana plantation showed a trend of decreasing with the increase of soil depth in the vertical distribution， exhibiting an obvious surface aggregation characteristic. The differences in organic carbon content between different soil layers reached a significant level （P<0.05）， and the organic carbon content of the 0-20 cm soil layer was significantly higher than that of other soil layers， and was 1.72， 4.00 and 5.18 times of the 20-40， 40-60 and 60-80 cm soil layers， respectively.

Carbon storage and its distribution in the mature P. massoniana plantation ecosystem

The carbon storage of the mature P. massoniana plantation  ecosystem  include the tree layer， shrub layer， herb layer， litter layer and soil layer carbon storage. It can be seen from Table 2 that the total carbon storage of the P. massoniana plantation ecosystem was 232.13 t/hm2， and the carbon storage of different ecosystem layers differed. The carbon storage of the soil layer was the largest， up to 134.49 t/hm2， accounting for 57.94% of the total carbon storage. The carbon storage was on the decrease with the increase of soil depth. The tree layer has the second largest carbon storage of 92.67 t/hm2， accounting for 39.92%. The carbon storage of the various organs of the tree layer differed greatly. The trunks showed the largest carbon storage of 58.42 t/hm2， accounting for 25.17% of the total carbon storage; the second was the roots of the trees， which had the carbon storage of 11.76 t/hm2， accounting for  5.07% ; the carbon storage of the branches was  11.35  t/hm2， accounting for 4.89%; and the carbon storage of the trunk bark was 7.79 t/hm2， accounting for 3.36%; and the pine needles exhibited the smallest carbon storage， only  3.35   t/hm2 ， accounting for 1.44%. The order of other structural levels of the stand was： litter layer （2.49 t/hm2）> shrub layer （1.36 t/hm2）> herb layer （1.12 t/hm2）.

Annual net carbon fixation of the tree layer in the mature  P. Massoniana  plantation

In this study， the annual net carbon fixation in the tree layer was used to estimate the capacity of the P.massoniana plantation ecosystem to assimilate CO 2. It can be seen from Table 3 that the annual net productivity of the P. massoniana plantation was 10.36  t/（hm2·a） ， the annual net carbon sequestration was  5.41   t/（hm2·a），  and the CO 2 absorption was 19.83 t/（hm2·a）.  Among  the annual net carbon sequestration of various organs of the plantation， the annual net carbon sequestration of the trunks was the largest， up to 2.25 t/（hm2·a）， accounting for 41.59% of the total annual net carbon fixation， followed by the pine needles， roots and branches， of which the annual net carbon sequestration of branches and branches were 1.97， 0.45 and 0.44 t/（hm2·a）， respectively， accounting for 36.41%， 8.32% and 8.13%， respectively， and the minimum was the trunk bark， of which the annual net carbon sequestration was 0.30 t/（hm2·a）， only accounting for 5.54%.

Conclusions and Discussion

The carbon contents of various organs of P. massoniana  varied  from 485.7 to 512.0 g/kg， and ranked as trunks （512.0 g/kg） > pine needles （502.1g/kg） > roots （499.2g/kg） > trunk bark （490.5g/kg） > branches （485.7g/kg）. The order of carbon content in different structural levels was： tree layer > litter layer > shrub layer > herb layer > soil layer. Affected by the return of litter and plant roots， the soil organic carbon content showed an obvious surface aggregation characteristic， and gradually decreased with the increase of soil depth. It is consistent with the vertical distribution characteristics of the soil organic carbon contents of the T. flousiana plantation [9]， C. Lanceolata[10] and B. alnoides in the same area[10].

The carbon storage of the 26 year old P. massoniana plantation ecosystem was 232.13 t/hm2， which was higher than the carbon storage of the 20 year old and 30 year old P. massoniana plantation ecosystems in Shunchang County， Fujian Province （152.87， 183.94 t/hm2）[3] and the carbon storage of the mature 32 year old P. massoniana plantation ecosystems in Hunan Province （187.29 t/hm2）[16]， in which the carbon storage of the tree layer was 92.67 t/hm2， accounting for 39.92% of the total carbon storage of the whole ecosystem. The differences in the carbon storage between the various components of the tree layer were very large. Specifically， the trunks had the largest carbon storage of 58.42 t/hm2， accounting for 25.17% of the total carbon storage; the second was the roots， which showed the carbon storage of  11.76  t/hm2， accounting for 5.07%; the branch carbon storage was 11.35 t/hm2， accounting for 4.89%; and the trunk bark exhibited the carbon storage of 7.79 t/hm2， accounting for 3.36%; and the pine needles showed the smallest carbon storage， which was only 3.35 t/hm2， accounting for 1.44%. The order of other structural levels was： litter layer （2.49 t/hm2） > shrub layer （1.36 t/hm2） > herb layer （1.12 t/hm2）. The carbon storage in the soil layer （0-80 cm） was 134.49 t/hm2， which was lower than the average carbon storage of the forest soil in China and the average carbon storage of the worlds soil， but slightly higher than the average carbon storage of the tropical forest soil in China[17].

In this study， the annual net carbon fixation of the 26 year old P. massoniana plantation was 5.41 t/（hm2·a）， and the annual net CO 2 absorption was 19.83 t/（hm2·a）. According to reports， the annual net carbon fixation of the tree layers of the  21 year old  and 32 year old P. massoniana plantations in Guangxi was 4.93 and 4.46 t/（hm2·a）[15]; the annual net carbon fixation of the tree layers of the 23 year old and 38 year old P. massoniana plantations in the hilly area of central Wuxuan County， Guangxi was 5.15 and 4.72 t/（hm2·a）[18] （the pine needle age was calculated according to 1.7 and 1.8 years， respectively）; and the 23 and 32 year old P. massoniana plantations in Hunan Province had the net carbon fixation of the tree layers of 3.77 and 3.62 t/（hm2·a）[9]， respectively. The annual net carbon fixation of the tree layers of the 20 year old and 28 year old 1 st ， 2 nd  and 3 rd  generation of C. lanceolatan was 5.55， 4.68， 3.92 t/（hm2·a） and 3.58， 3.25， 2.87 t/（hm2·a） was Jianou City， Fujian Province[17]. The average annual net carbon sequestration of  Chinese  forests is 5.54 t/（hm2·a）[17]. It can be seen that  P. massoniana  plantations in northwestern Guangxi have a high level of biological productivity and also have a high carbon sink function， thus playing an important role in regulating atmospheric CO 2， promoting forest carbon cycle and maintaining and improving the environment.

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