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Abstract [Objectives] This study was conducted to reveal the characteristics of nutrient absorption and accumulation in Pinus massoniana plantations in Northwestern Guangxi.
[Methods] Based on field investigation and indoor analysis， the contents， accumulation and annual net accumulation of five nutrient elements （N， P， K， Ca and Mg） in a mature P. massoniana plantation （26yearold） in Nandan County， Guangxi Province were studied.
[Results] The contents of nutrient elements in different organs of the mature P. massoniana plantation were the highest in the leaves， followed by the bark， branch and root， and the lowest in the stem. In general， among the contents of the five elements in different organs， N content was the highest， followed by K or Ca， and P and Mg were the lowest. The total accumulation of nutrient elements in the 26yearold mature P. massoniana plantation in northwestern Guangxi was 1 384.05 kg/hm2. Among the different structural levels of the stand， the tree layer had the highest accumulation of nutrient elements， which was 1 198.41 kg/hm2， accounting for 86.59% of the total accumulation of nutrients in the plantation， and the accumulation of nutrients in other layers from the largest to the smallest was the litter layer （91.97 kg/hm2）， herb layer （49.86 kg/hm2） and shrub layer （43.92 kg/hm2）， accounting for 3.17 %， 3.60% and 6.64% of the total nutrient accumulation of the plantation， respectively. The annual net accumulation of nutrient elements in the tree layer of the mature P. massoniana plantation was 46.09 kg/（hm2·a）， and the order of the annual net accumulation of different nutrient elements followed N>K>Ca>Mg>P; and the accumulation of 1 t of dry matter needed 6.37 kg of the five nutrients.
[Conclusions] This study provides a scientific basis for the rational management of P. massoniana plantations， especially forest soil management.
Key words Pinus massoniana; Mature forest; Nutrient elements; Accumulation; Distribution
The accumulation and distribution pattern of nutrient elements in forests are important indicators of forest productivity in the form of nutrients， which are of great significance to guide the management of forests， especially plantations， especially forest soil management， to carry out rational forest fertilization and tree species allocation， and to improve the utilization efficiency of forest nutrients and the production potential of forests[1-2]. Pinus massoniana is one of the most important fastgrowing and highyield forest species in the subtropical region of China， as well as the main plantation tree species in Guangxi， playing an important role in the forestry production and the construction of timber strategic reserve bases in China[3]. Northwestern Guangxi is an important cultivation area of P. massoniana plantations. At present， there are some reports on the research of P. massoniana plantations in this area， but they are mainly concentrated on the growth characteristics， biological productivity， soil fertility and understory plant diversity of forests[4-9]， and rarely on the characteristics of nutrients. To this end， based on field investigation and indoor chemical analysis， a systematic study was conducted on the nutrient content， accumulation and annual net accumulation of the 26yearold matureP. massoniana plantation in Danxian County， Guangxi， so as to reveal the characteristics of nutrient absorption and accumulation in P. massoniana plantations in this area. This study provides a scientific basis for the rational management of P. massoniana plantations， especially the soil management of forest land. Materials and Methods
General situation of the experiment field
The study area is located in the Shankou 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 midsubtropical mountain climate with an average annual temperature of 16.9 ℃， annual rainfall of 1 498.2mm 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 graygreen slate， and the soil is mountainous yellowred and yellow soil. The average thickness of the soil layer is over 80 cm. The soil （0-40 cm） had a pH value of 4.37， and the soil organic matter， total nitrogen and total phosphorus contents were 23.17， 1.02 and 0.38 g/kg， respectively. The soil hydrolyzable nitrogen， available phosphorus and available potassium contents were 85.2， 1.15 and 41.6 mg/kg， respectively.
The P. massoniana seedlings were raised from March 1991. The initial planting density was 1 667 plants/hm2， and the plant spacing was 2 m × 3 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， 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 26yearold P. massoniana plantation， in May 2017， a 20 m × 20 m standard plot was set up at the upslope， midslope and downslope， respectively. The biomass of each layer of leaves， branches， bark， trunks， roots and understory vegetation was determined[10].
Plant sample collection and nutrient element analysis
According to the method of reference [10]， samples of different organs of the tree layer， the shrub layer， the herb layer and the litter layer of the mature P. massoniana plantation were collected. After drying and pulverization， for the determination of N， P and K， the samples were first digested with concentrated H2SO4HClO4， and then the three elements were determined by Kjeldahl method[11]， molybdenum antimony colorimetry[11]and a flame photometer[11]， respectively. As to Ca and Mg， the samples were first digested by the HClO4HNO3 digestion method， and then determined by atomic absorption spectrometry[11]. Data processing and Analysis
The nutrient accumulation of the mature P. massoniana plantation was calculated according to the literature[10]. The annual net accumulation of nutrient elements in the tree layer was estimated as the annual net accumulation of nutrients in the plantation， the values of which were obtained by dividing the nutrient accumulation in the tree layer by the age of the plantation （26 years）.
Data processing was performed using Excel2003 software， and statistical analysis was performed using SPSS 17.0.
Results and Analysis
Nutrient element contents
It can be seen from Table 1 that the nutrient elements in different organs of the mature P. massoniana plantation were different due to their different physiological functions. The contents of nutrient elements in the assimilatory organs were the highest， and the nutrient contents in the nonassimilatory organs were the lowest. Overall， the nutrient contents in various organs ranked as lenves>branch>bark>root>stem. The five nutrient elements have different functions in the growth process of the trees， so there are some differences in the contents of different elements in the forest. In general， N content was the highest， followed by K or Ca， and P and Mg were the lowest.
Nutrient accumulation and distribution
It can be seen from Table 2 that the total accumulation of nutrient elements in the 26yearold mature P. massoniana plantation in northwestern Guangxi was 1 384.05 kg/hm2. Among the accumulation of different nutrient elements， the N accumulation was the highest， which was 527.16 kg/hm2， followed by K， Ca and Mg， which were 385.83， 319.84 and 104.44 kg/hm2， respectively， and P showed a minimum of 46.78 kg/hm2. Among the different structural levels of the stand， the tree layer had the highest accumulation of nutrient elements， which was 1 198.41 kg/hm2， accounting for 86.59% of the total accumulation of nutrients in the plantation， and the accumulation of nutrients in other layers from the largest to the smallest was the litter layer （91.97 kg/hm2）， herb layer （49.86 kg/hm2） and shrub layer （43.92 kg/hm2）， accounting for 3.17 %， 3.60% and 6.64% of the total nutrient accumulation of plantation， respectively.
As to the distribution of nutrient accumulation in different organsof the tree layer， the stem exhibited the largest accumulation of nutrients （321.36 kg/hm2）， accounting for 26.82% of the nutrient accumulation of the tree layer， followed by the branches （319.60 kg/hm2）， the roots （223.07 kg/hm2）and bark （175.04kg/hm2）， accounting for 26.67%， 18.61% and 14.61%， respectively， and the leaves had the smallest accumulation （159.34 kg/hm2）， accounting for only 13.30%. Among the accumulation of nutrient elements in the tree layer， the accumulation of N was the largest， at 448.21 kg/hm2， accounting for 36.98% of the accumulation of nutrient elements in the tree layer， followed by K， Ca and Mg， which were 333.26， 285.72 and 96.70 kg/hm2， respectively， accounting for 27.81%， 23.84% and 8.07%， respectively， and the P accumulation was the smallest， at 39.52kg/hm2，accounting for only 3.30%. Annual net accumulation of nutrient elements
It can be seen from Table 3 that the annual net accumulation of nutrient elements in the 26yearold mature P. massoniana plantation in northwestern Guangxi was 46.09 kg/（hm2·a）， of which 12.35 kg/（hm2·a） was the annual net accumulation of the trunks， which was the highest， followed by branches， roots and bark， which were 12.29， 8.58 and 6.74 kg/（hm2·a）， respectively， and the smallest was 6.13 kg/（hm2·a） of the leaves. Among the annual net accumulation of nutrient elements in the plantation， the accumulation of N was the largest， at 17.05 kg/（hm2·a）， followed by the elements K， Ca and Mg， which were 12.81， 10.99 and 3.72 kg/（hm2·a）， respectively， and the P accumulation was the smallest， only of 1.52 kg/（hm2·a）.
Nutrient utilization efficiency of elements in forest trees
The utilization efficiency of forest nutrient elements reflects the adaptation of forest trees to nutrient environment and their utilization. Chapin index is generally used as an indicator to reflect the utilization efficiency of forest nutrient elements. Its value is characterized by the ratio of plant nutrient accumulation to plant biomass[12]. It can be seen from Table 4 that the growth of 1 t of dry matter in the 26yearold mature P. massoniana plantation required 6.37 kg of the five nutrient elements， which was higher than the 28yearold Taiwania flousiana Gaussen plantation[12]and 23yearold P. massoniana plantation in the same or similar areas[13]. Among the five nutrients， the amount of N was 2.35 kg， and its nutrient utilization efficiency was the lowest; the amounts of K， Ca and Mg were 1.77， 1.52 and 0.51 kg， respectively， and they had higher nutrient use efficiency; and the amount of P was 0.21 kg， which was the smallest， and its nutrient utilization efficiency is also the highest.
Total17.051.5212.8110.993.7246.09
Conclusions and Discussion
The nutrient content of the mature P. massoniana plantation in northwestern Guangxi differed significantly according to different organs. The contents of various nutrients were highest in the needles， followed by branches， bark and roots， and the lowest in the trunks. Among the contents of different nutrient elements in the organs， the content of N was roughly the highest， followed by K， Ca and Mg， and P was the lowest. This order is quite different from those of T. flousiana and Cunninghamia lanceolata （Lamb.） Hook. plantations （Ca， N or K>Mg>P） in the same area， and is different from the order of the P. massoniana plantation （K>N>Ca>Mg>P） in the nearby area， Wuxuan County， Guangxi to a certain degree[12]， indicating that the contents of nutrient elements in forest trees depend mainly on the biological characteristics of the tree species itself， and are also affected by environmental conditions. The accumulation of five nutrient elements in the 26yearold mature P. massoniana plantation was 1 384.05 kg/hm2， of which 1 198.41 kg/hm2 attributed to the tree layer， accounting for 86.59%of the total nutrient accumulation of the stand， and 43.92， 49.86 and 91.97 kg/hm2 were of the shrub layer， herb layer and litter layer， respectively， accounting for 3.60%， 3.17% and 6.64% of the total accumulation of nutrient elements， respectively. The accumulation of nutrient elements in different organs of the tree layer ranked as stem>branch>root>bark>leaf， which is different from the 28yearold T. flousiana （stem>leaf>root>bark>branch）[13]， the 23yearold P. massoniana plantation in adjacent area， Wuxuan County， Guangxi （tree roots>branch>bark>stem>leaf）[12]and 34yearsold Mytilaria laosensis Lec. plantation in Ningming County， Guangxi （stem> root>branch>bark>leaf）[14]to certain degrees， reflecting the accumulation and distribution characteristics of nutrient elements in mature P. massoniana plantations in this area.
The annual net accumulation of nutrients in the tree layer of the 26yearold mature P. massoniana plantation was 46.09kg/（hm2·a）， and the annual net accumulation of different nutrient elements was highest in N， which was 17.05 kg/（hm2·a）， followed by K ， Ca and Mg， which were 12.81， 10.99 and 3.72 kg/（hm2·a）， respectively， and P was the smallest， being 1.52 kg/（hm2·a）. For each 1 t of dry matter， 6.37 kg of the five nutrient elements was required in total， which is slightly higher than the 23yearold P. massoniana plantation （6.07 kg） in Wuxuan County， Guangxi[12]， but far lower than the 28-32yearold P. tabulaeformis plantations in Zhengning County， Gansu Province （9.76 kg）[15]. It can be seen that the nutrient utilization efficiency of the mature P. massoniana plantation was higher， but because the soil available phosphorus content was poor （below 3.0 mg/kg）， rational application of phosphate fertilizer in future management of P. massoniana plantations may play a positive role in improving the phosphorus level of forest soil and promoting the growth of forest trees.
References
[1] ZHANG CS， LI K. Research status and advances of nutrient cycling of plantation[J]. World Forestry Research， 2005， 18（4）： 38-39. （in Chinese）
[2] HE B， QIN WM， YU HG， et al. Biological cycling of nutrients in different ages classes of Acacia mangium plantation[J]. Acta Ecologica Sinica， 2007， 27（12）： 5158-5167. （in Chinese） [3] TAO YH， FENG JC， MA LY， et al. Carbon storage of Cunninghamia lanceolate， Pinus massoniana and Eucalyptus grandis × E. urophylla and its dynamic change in Luocheng， Guangxi[J]. Ecology and Environment， 2011， 20（11）： 1608-1613. （in Chinese）
[4] HUANG CB， DENG SL， YU CJ， et al. Comparison of young growths of Betula luminfera and Pinus massoniana plantation in Northwest of Guangxi[J]. Journal of Guangxi Academy of Sciences， 2001（3）： 97-99， 103. （in Chinese）
[5] WEI XM. Investigation and study on vegetationeffect in different vegetation restoration patterns of returning land for farming to forestry in Guangxi[D]. Nanning： Guangxi University， 2008. （in Chinese）
[6] HUANG XR， HUANG CB， CEN GF， et al. A Study on species diversity of undergrowth vegetation in different age masson pine plantation[J]. Agricultural Research and Application， 2013（4）： 1-6. （in Chinese）
[7] QIN QY， CAO JZ， LI J， et al. Research on the variation of the soil fertility in the planted young Pinus massoniana forest and comprehensive evaluation of its soil fertility[J]. Journal of Central South University of Forestry & Technology， 2013， 33（3）： 64-69. （in Chinese）
[8] TAO YH， CAO SG， LONG WG， et al. Correlation between species richness and biomass of Pinus massoniana plantation understory in Luocheng of Guangxi[J]. Heilongjiang Agricultural Sciences， 2014（10）： 88-90. （in Chinese）
[9] MENG CJ， WEI J. Effects of different forest management techniques on the growth of Pinus massoniana plantation[J]. Journal of Green Science and Technology， 2018（1）： 38-39. （in Chinese）
[10] LIU K， WEI JG， HE B， et al. Productivity of Taiwania flousiana plantation and puccessive rotation plantation of Cunninghamia lanceolata in cutover forest land of C. Lanceolata[J]. Agricultural Biotechnology， 2018， 7（4）： 178-180， 186.
[11] LU RK. Soil argrochemistry analysis protocoe[M]. Beijing： China Agriculture Science and Technique Press， 1999， 317-322. （in Chinese）
[12] XIANG WH， TIAN DL. Nutrient cycling in Pinus massoniana stands of different age classes[J]. Chinese Journal of Plant Ecology， 2002， 26（1）： 89-95. （in Chinese）
[13] HE B， HUANG HC， HUANG CB， et al. Study on the content， accumulation and distribution characteristics of nutrient elements in the plantation of Taiwania flousiana Gaussen plantation[J]. Journal of Natural Resources， 2008， 23 （5）： 903-910. （in Chinese）
[14] CHEN ZH， LI YH， YANG WX， et al. Accumulation and distribution of nutrients in Mytilaria laosensis plantation[J]. China Forestry Science and Technology， 2015， 29（02）： 133-136. （in Chinese）
[15] ZHANG XB， SHANGGUAN ZP. Nutrient distributions and biocycle patterns in both natural and artificial Pinus tabulaeformis forests in Hilly Loess Regions[J]. Acta Ecologica Sinica， 2006， 26（2）： 373-382. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU
[Methods] Based on field investigation and indoor analysis， the contents， accumulation and annual net accumulation of five nutrient elements （N， P， K， Ca and Mg） in a mature P. massoniana plantation （26yearold） in Nandan County， Guangxi Province were studied.
[Results] The contents of nutrient elements in different organs of the mature P. massoniana plantation were the highest in the leaves， followed by the bark， branch and root， and the lowest in the stem. In general， among the contents of the five elements in different organs， N content was the highest， followed by K or Ca， and P and Mg were the lowest. The total accumulation of nutrient elements in the 26yearold mature P. massoniana plantation in northwestern Guangxi was 1 384.05 kg/hm2. Among the different structural levels of the stand， the tree layer had the highest accumulation of nutrient elements， which was 1 198.41 kg/hm2， accounting for 86.59% of the total accumulation of nutrients in the plantation， and the accumulation of nutrients in other layers from the largest to the smallest was the litter layer （91.97 kg/hm2）， herb layer （49.86 kg/hm2） and shrub layer （43.92 kg/hm2）， accounting for 3.17 %， 3.60% and 6.64% of the total nutrient accumulation of the plantation， respectively. The annual net accumulation of nutrient elements in the tree layer of the mature P. massoniana plantation was 46.09 kg/（hm2·a）， and the order of the annual net accumulation of different nutrient elements followed N>K>Ca>Mg>P; and the accumulation of 1 t of dry matter needed 6.37 kg of the five nutrients.
[Conclusions] This study provides a scientific basis for the rational management of P. massoniana plantations， especially forest soil management.
Key words Pinus massoniana; Mature forest; Nutrient elements; Accumulation; Distribution
The accumulation and distribution pattern of nutrient elements in forests are important indicators of forest productivity in the form of nutrients， which are of great significance to guide the management of forests， especially plantations， especially forest soil management， to carry out rational forest fertilization and tree species allocation， and to improve the utilization efficiency of forest nutrients and the production potential of forests[1-2]. Pinus massoniana is one of the most important fastgrowing and highyield forest species in the subtropical region of China， as well as the main plantation tree species in Guangxi， playing an important role in the forestry production and the construction of timber strategic reserve bases in China[3]. Northwestern Guangxi is an important cultivation area of P. massoniana plantations. At present， there are some reports on the research of P. massoniana plantations in this area， but they are mainly concentrated on the growth characteristics， biological productivity， soil fertility and understory plant diversity of forests[4-9]， and rarely on the characteristics of nutrients. To this end， based on field investigation and indoor chemical analysis， a systematic study was conducted on the nutrient content， accumulation and annual net accumulation of the 26yearold matureP. massoniana plantation in Danxian County， Guangxi， so as to reveal the characteristics of nutrient absorption and accumulation in P. massoniana plantations in this area. This study provides a scientific basis for the rational management of P. massoniana plantations， especially the soil management of forest land. Materials and Methods
General situation of the experiment field
The study area is located in the Shankou 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 midsubtropical mountain climate with an average annual temperature of 16.9 ℃， annual rainfall of 1 498.2mm 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 graygreen slate， and the soil is mountainous yellowred and yellow soil. The average thickness of the soil layer is over 80 cm. The soil （0-40 cm） had a pH value of 4.37， and the soil organic matter， total nitrogen and total phosphorus contents were 23.17， 1.02 and 0.38 g/kg， respectively. The soil hydrolyzable nitrogen， available phosphorus and available potassium contents were 85.2， 1.15 and 41.6 mg/kg， respectively.
The P. massoniana seedlings were raised from March 1991. The initial planting density was 1 667 plants/hm2， and the plant spacing was 2 m × 3 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， 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 26yearold P. massoniana plantation， in May 2017， a 20 m × 20 m standard plot was set up at the upslope， midslope and downslope， respectively. The biomass of each layer of leaves， branches， bark， trunks， roots and understory vegetation was determined[10].
Plant sample collection and nutrient element analysis
According to the method of reference [10]， samples of different organs of the tree layer， the shrub layer， the herb layer and the litter layer of the mature P. massoniana plantation were collected. After drying and pulverization， for the determination of N， P and K， the samples were first digested with concentrated H2SO4HClO4， and then the three elements were determined by Kjeldahl method[11]， molybdenum antimony colorimetry[11]and a flame photometer[11]， respectively. As to Ca and Mg， the samples were first digested by the HClO4HNO3 digestion method， and then determined by atomic absorption spectrometry[11]. Data processing and Analysis
The nutrient accumulation of the mature P. massoniana plantation was calculated according to the literature[10]. The annual net accumulation of nutrient elements in the tree layer was estimated as the annual net accumulation of nutrients in the plantation， the values of which were obtained by dividing the nutrient accumulation in the tree layer by the age of the plantation （26 years）.
Data processing was performed using Excel2003 software， and statistical analysis was performed using SPSS 17.0.
Results and Analysis
Nutrient element contents
It can be seen from Table 1 that the nutrient elements in different organs of the mature P. massoniana plantation were different due to their different physiological functions. The contents of nutrient elements in the assimilatory organs were the highest， and the nutrient contents in the nonassimilatory organs were the lowest. Overall， the nutrient contents in various organs ranked as lenves>branch>bark>root>stem. The five nutrient elements have different functions in the growth process of the trees， so there are some differences in the contents of different elements in the forest. In general， N content was the highest， followed by K or Ca， and P and Mg were the lowest.
Nutrient accumulation and distribution
It can be seen from Table 2 that the total accumulation of nutrient elements in the 26yearold mature P. massoniana plantation in northwestern Guangxi was 1 384.05 kg/hm2. Among the accumulation of different nutrient elements， the N accumulation was the highest， which was 527.16 kg/hm2， followed by K， Ca and Mg， which were 385.83， 319.84 and 104.44 kg/hm2， respectively， and P showed a minimum of 46.78 kg/hm2. Among the different structural levels of the stand， the tree layer had the highest accumulation of nutrient elements， which was 1 198.41 kg/hm2， accounting for 86.59% of the total accumulation of nutrients in the plantation， and the accumulation of nutrients in other layers from the largest to the smallest was the litter layer （91.97 kg/hm2）， herb layer （49.86 kg/hm2） and shrub layer （43.92 kg/hm2）， accounting for 3.17 %， 3.60% and 6.64% of the total nutrient accumulation of plantation， respectively.
As to the distribution of nutrient accumulation in different organsof the tree layer， the stem exhibited the largest accumulation of nutrients （321.36 kg/hm2）， accounting for 26.82% of the nutrient accumulation of the tree layer， followed by the branches （319.60 kg/hm2）， the roots （223.07 kg/hm2）and bark （175.04kg/hm2）， accounting for 26.67%， 18.61% and 14.61%， respectively， and the leaves had the smallest accumulation （159.34 kg/hm2）， accounting for only 13.30%. Among the accumulation of nutrient elements in the tree layer， the accumulation of N was the largest， at 448.21 kg/hm2， accounting for 36.98% of the accumulation of nutrient elements in the tree layer， followed by K， Ca and Mg， which were 333.26， 285.72 and 96.70 kg/hm2， respectively， accounting for 27.81%， 23.84% and 8.07%， respectively， and the P accumulation was the smallest， at 39.52kg/hm2，accounting for only 3.30%. Annual net accumulation of nutrient elements
It can be seen from Table 3 that the annual net accumulation of nutrient elements in the 26yearold mature P. massoniana plantation in northwestern Guangxi was 46.09 kg/（hm2·a）， of which 12.35 kg/（hm2·a） was the annual net accumulation of the trunks， which was the highest， followed by branches， roots and bark， which were 12.29， 8.58 and 6.74 kg/（hm2·a）， respectively， and the smallest was 6.13 kg/（hm2·a） of the leaves. Among the annual net accumulation of nutrient elements in the plantation， the accumulation of N was the largest， at 17.05 kg/（hm2·a）， followed by the elements K， Ca and Mg， which were 12.81， 10.99 and 3.72 kg/（hm2·a）， respectively， and the P accumulation was the smallest， only of 1.52 kg/（hm2·a）.
Nutrient utilization efficiency of elements in forest trees
The utilization efficiency of forest nutrient elements reflects the adaptation of forest trees to nutrient environment and their utilization. Chapin index is generally used as an indicator to reflect the utilization efficiency of forest nutrient elements. Its value is characterized by the ratio of plant nutrient accumulation to plant biomass[12]. It can be seen from Table 4 that the growth of 1 t of dry matter in the 26yearold mature P. massoniana plantation required 6.37 kg of the five nutrient elements， which was higher than the 28yearold Taiwania flousiana Gaussen plantation[12]and 23yearold P. massoniana plantation in the same or similar areas[13]. Among the five nutrients， the amount of N was 2.35 kg， and its nutrient utilization efficiency was the lowest; the amounts of K， Ca and Mg were 1.77， 1.52 and 0.51 kg， respectively， and they had higher nutrient use efficiency; and the amount of P was 0.21 kg， which was the smallest， and its nutrient utilization efficiency is also the highest.
Total17.051.5212.8110.993.7246.09
Conclusions and Discussion
The nutrient content of the mature P. massoniana plantation in northwestern Guangxi differed significantly according to different organs. The contents of various nutrients were highest in the needles， followed by branches， bark and roots， and the lowest in the trunks. Among the contents of different nutrient elements in the organs， the content of N was roughly the highest， followed by K， Ca and Mg， and P was the lowest. This order is quite different from those of T. flousiana and Cunninghamia lanceolata （Lamb.） Hook. plantations （Ca， N or K>Mg>P） in the same area， and is different from the order of the P. massoniana plantation （K>N>Ca>Mg>P） in the nearby area， Wuxuan County， Guangxi to a certain degree[12]， indicating that the contents of nutrient elements in forest trees depend mainly on the biological characteristics of the tree species itself， and are also affected by environmental conditions. The accumulation of five nutrient elements in the 26yearold mature P. massoniana plantation was 1 384.05 kg/hm2， of which 1 198.41 kg/hm2 attributed to the tree layer， accounting for 86.59%of the total nutrient accumulation of the stand， and 43.92， 49.86 and 91.97 kg/hm2 were of the shrub layer， herb layer and litter layer， respectively， accounting for 3.60%， 3.17% and 6.64% of the total accumulation of nutrient elements， respectively. The accumulation of nutrient elements in different organs of the tree layer ranked as stem>branch>root>bark>leaf， which is different from the 28yearold T. flousiana （stem>leaf>root>bark>branch）[13]， the 23yearold P. massoniana plantation in adjacent area， Wuxuan County， Guangxi （tree roots>branch>bark>stem>leaf）[12]and 34yearsold Mytilaria laosensis Lec. plantation in Ningming County， Guangxi （stem> root>branch>bark>leaf）[14]to certain degrees， reflecting the accumulation and distribution characteristics of nutrient elements in mature P. massoniana plantations in this area.
The annual net accumulation of nutrients in the tree layer of the 26yearold mature P. massoniana plantation was 46.09kg/（hm2·a）， and the annual net accumulation of different nutrient elements was highest in N， which was 17.05 kg/（hm2·a）， followed by K ， Ca and Mg， which were 12.81， 10.99 and 3.72 kg/（hm2·a）， respectively， and P was the smallest， being 1.52 kg/（hm2·a）. For each 1 t of dry matter， 6.37 kg of the five nutrient elements was required in total， which is slightly higher than the 23yearold P. massoniana plantation （6.07 kg） in Wuxuan County， Guangxi[12]， but far lower than the 28-32yearold P. tabulaeformis plantations in Zhengning County， Gansu Province （9.76 kg）[15]. It can be seen that the nutrient utilization efficiency of the mature P. massoniana plantation was higher， but because the soil available phosphorus content was poor （below 3.0 mg/kg）， rational application of phosphate fertilizer in future management of P. massoniana plantations may play a positive role in improving the phosphorus level of forest soil and promoting the growth of forest trees.
References
[1] ZHANG CS， LI K. Research status and advances of nutrient cycling of plantation[J]. World Forestry Research， 2005， 18（4）： 38-39. （in Chinese）
[2] HE B， QIN WM， YU HG， et al. Biological cycling of nutrients in different ages classes of Acacia mangium plantation[J]. Acta Ecologica Sinica， 2007， 27（12）： 5158-5167. （in Chinese） [3] TAO YH， FENG JC， MA LY， et al. Carbon storage of Cunninghamia lanceolate， Pinus massoniana and Eucalyptus grandis × E. urophylla and its dynamic change in Luocheng， Guangxi[J]. Ecology and Environment， 2011， 20（11）： 1608-1613. （in Chinese）
[4] HUANG CB， DENG SL， YU CJ， et al. Comparison of young growths of Betula luminfera and Pinus massoniana plantation in Northwest of Guangxi[J]. Journal of Guangxi Academy of Sciences， 2001（3）： 97-99， 103. （in Chinese）
[5] WEI XM. Investigation and study on vegetationeffect in different vegetation restoration patterns of returning land for farming to forestry in Guangxi[D]. Nanning： Guangxi University， 2008. （in Chinese）
[6] HUANG XR， HUANG CB， CEN GF， et al. A Study on species diversity of undergrowth vegetation in different age masson pine plantation[J]. Agricultural Research and Application， 2013（4）： 1-6. （in Chinese）
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Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU