1 Introduction

Forest is an enormous green ecological system on the land (Kimmins, 1987). The multiple benefits of forest are generally recognized by the world. With the economic development, more and more attention has been paid to the environmental issues. Up to now, due to over harvesting of natural forest, the area of natural forest in the world has been reduced in the past years. To meet the increasing demand for timber, the government of each country has strengthened the effort to establish plantations. Plantation area in China is 34.25 million hm², which makes up 26% of the total forest land in the world. Especially, from 1990 to 1995, the Chinese government established 1.38 million hm², of intensively managed plantations by US$ 300 million from World Bank Loan and RMB 1.7 billion yuan as counterpart fund from Chinese government. During the implementation of the project, the negative impact on environment would occur if attention were not paid to the environment protection. One of the main negative impacts is the degradation of soil fertility. In view of the existing negative impact, the environmental protection guideline of NAP was worked out and environmental monitoring plots were established. The article mainly analyzes the situations and causes of the degradation of soil fertility and elaborates the countermeasures to these issues.

2 Materials and methods

According to climatic factors and types of landscape etc., the project's areas were divided into tropical, subtropical and temperate zone (see Tab. 1). 110 plots in the project areas were set up to monitor the degradation of soil fertility, the occurrence of insect pests and diseases, soil and water erosion. The materials mainly came from the survey results of the environmental monitoring reports of Hebei, Liaoning, Anhui, Fujian, Jiangxi, Guangdong, Guizhou, Yunnan, Hainan provinces and Guangxi Zhuang Autonomous Region. The degradation of soil fertility was monitored based on the unified methods required by the monitoring plan of soil fertility. The dynamic situation of the majority soil nutrients such as available phosphate, available nitrogen, available potassium, organic matter, slowly available potassium, exchangeable magnesium and calcium etc. were also monitored.

3 Results and discussions 3.1 The degradation of forest land in the project areas

The fertility of forest land for plantation establishment was poor with unbalance nutrient in the project areas. The soil of forest land on hilly and low-mountainous areas situated at the southern part of China was short in phosphate and potassium with insufficient organic matter and nitrogen. The soil of forest land on the plain area in the northern part and center of China has poor productivity except rich potassium and was seriously scarce of phosphate and nitrogen, and especially, in recent years, the intensively managed plantations with shorter rotation have been developed on a large scale. Due to those plantations with the characteristics of high-yielding and fast-growing and shorter rotation, the nutrient of soil was over consumed and the degradation of forest land was obvious. For example, based on the research of I-214 poplar, if 1kg dry matter of poplar is produced, 4.59 g nitrogen, 5 g potassium, 1.4 g phosphate and 1.32 g calcium will be consumed. Meanwhile, the statistics from foreign countries showed fast-growing and high-yielding plantation with an area of one hm²·a^-1 consumes 40 kg nitrogen, 23 kg potassium, 25 kg calcium, 4 kg phosphate and 6 kg magnesium. For example, soil erosion, burning harvesting remnants and clear-cutting trees in forest land will loss 490~920 kg nitrogen per hm² and reduce the content of organic matter. Therefore, soil fertility easily decreases to establish the shorter rotation plantations. In order to solve the problems, fertilizers are applied in some counties to improve the soil fertility. For example: to increase the content of organic matter and prevent soil degradation, fertilizer is applied and harvesting remnants are left in original place in forest land.

The degradation of soil fertility was obvious in the project area, indicating the growth of Chinese Fir decreases generations by generations. Among these tree species of the plantations, the Chinese Fir accounts for 60%~80% of total area of plantations in some counties. Therefore, it is very important to avoid the degradation of soil fertility for managing plantation in the south parts of China. Based on the research of Chinese Fir in Huitong county by Chinese Academy of Forestry, the soil nutrients including total nitrogen, total phosphate, available nitrogen, available phosphate, available potassium for the third and the second generation of Chinese Fir have reduced to 23%, 14.6%, 1%~5.8%, 8.3%~16.6%, 5.5%~7% respectively compared with those of the first generation. The soil fertility and quantity of microorganism in the land of Chinese Fir plantation has decreased with the increasing age of trees. 19 years after planting, the nutrient of nitrogen, phosphate and potassium at the depth of 60 cm under the surface of the soil has decreased by 43.6%, 24.3% and 5.5% compared with that of the original soil. Otherwise, the growth of tree height for the second and the third generation has decreased by 7%~23% compared with that of the first generation. The site index of second generation of Chinese Fir has decreased by half index compared with that of the first generation. The site index of the third generation has degraded one index compared with that of the first generation. While reducing one index, the growth of standing volume has reduced by 0.1~0.15 cm·hm^-2a^-1. Therefore without effective measures to prevent the degradation of soil fertility, the increase of standing volume by improving genetic materials will be lost while management of the intensively managed plantation.

3.2 The cause of the degradation of forest land 3.2.1 The more coniferous species were used in plantation establishment.

Based on the recent statistic data of forest resources, the ratio of coniferous species is 53.8%, the ratio of broad-leaved tree species such as poplar, elm, black locust, paulownia and willow is very low, which distributed in mountainous areas, especially, in southern provinces. For example, the area of broad-leaved species takes 1.2% of total plantation area in Zhejiang Province, 4.2% in Fujian Province, 3.8% in Guangdong Province, 5.6% in Hunan Province. So, it indicates that the ratio of coniferous species is too high.

Capability of maintaining soil fertility of coniferous species is far less than that of broad-leaved species. In light of the research made by Zhou Conglian, the litter quantity of Chinese Fir and the broad-leaved species were 2 031 kg·hm^-2a^-1 and 4 581 kg·hm^-2a^-1 respectively. The litter of broad-leaved tree species is mostly decomposed each year and the nutrient of leaves reverts to the soil, but no more than half litter of coniferous species is decomposed.

During plantation establishment, more attention is not paid to the protection of natural broad-leaved tree species, up to now, some areas of natural forests have been cut down and the broad-leaved tree species have been replaced by coniferous tree species. The ratio of broad-leaved tree species is too low in some provinces. For example, the ratio of broad-leaved tree species is 13.5% in Zhejiang Province, 27.6 % in Jiangxi Province, 21.7% in Hunan Province, in this case, which not only causes the degradation and influence of the recovery of soil fertility but also deteriorates the ecological environment.

3.2.2 Unreasonable methods of plantation establishment worsening the physical property of soil

Site clearing with fire is broadly adopted on steep slope of the mountains and hills in the southern part of China during plantation establishment. Based on the research at home and abroad, site clearing with fire would lose organic matter and destroy the soil structure. Furthermore the ash stops up the soil porosity and influences water percolation, and causes the soil erosion. In light of the research results of Chinese Academy of Forestry, the quantity of run-off (the mother rock of soil is sandstone)is 2 743.49 cubic meter ·hm^-2a^-1 and the quantity of soil erosion was 24.8 ton ·hm^-2a^-1, which are respectively 11 and 88 times as much as those of the site clearing without fire. The available nutrient is rapidly leached and washed away. The loss of the organic matter, available nitrogen and available potassium on the site clearing with fire respectively is15, 24 and 5 times as much as that of site clearing without fire. Therefore, the method of site clearing with fire is unfavorable for maintaining soil fertility. Otherwise, the other irrational methods of plantation establishment are the main reasons causing soil erosion, for example, overall site-preparation and inter-cropping during Chinese Fir plantation establishment as well as unreasonable site-preparation and inter-cropping not only destroyed the aggregate structure of soil but also reduced soil fertility.

3.2.3 The structure of plant community is extremely simple

On one hand, the more coniferous tree species are planted during plantation establishment. On the other hand, the trees are planted in high density so as to acquire high productivity, which leads to the simplest structure of plant community and disadvantages for maintaining the soil fertility without protection of vegetation under crown canopy. In recent years, some research results indicate that the nutrient content of the vegetation under tree is higher than that of trees. The dead vegetation can be rapidly decomposed. Therefore, the vegetation plays an important role in maintaining the fertility of forest land.

3.2.4 Extensive management only output without input in forest land

The main reason to lose soil fertility is that we only harvest timbers without any fertilization in forest land during plantation management. For example, when Chinese Fir, Pine and Eucalyptus are planted, there is not any fertilizer application. Therefore, the degradation of forest land is increasing with each passing days. Local farmers in forest areas where population is dense and traffic condition is good and fuel wood is in shortage, which rapidly reduces the soil fertility, use the litter under forests as fuel wood.

3.3 The countermeasures to prevent the degradation of soil fertility are applied in National Afforestation Project

If there are not any measures to prevent the degradation of forest land in the establishment of intensively managed plantation on a large scale, the goals of fast growing and high yielding of intensively managed plantations can not be reached. To maintain the soil fertility and reach the target of sustainable development, the guideline of environmental protection on NAP has been mapped out and the appropriate measures of environmental protection have been implemented.

3.3.1 Restructuring the composition of tree species in plantation's areas through the development of broad-leaved plantation and conservation of existing broad-leaved forests

Since broad-leaved tree species have stronger capabilities of maintaining fertility than coniferous tree species. In the guideline of environmental protection some measures have been clearly identified such as suitable site for suitable tree species and mixture of broad-leaved with coniferous tree species as well as the ratio of broad-leaved tree species being not less than 20% for one county in coniferous trees species' areas. The area of plantation for one species is not more than 100 hm². in one place. For new plantation establishment, the original broad-leaved tree species should be conserved to avoid one species used in a large area. China is rich in good broad-leaved tree species. But only a few of them are used in plantation establishment, most of them have been used as fuel wood. Therefore, the public awareness of importance of broad-leaved tree species should be raised. From a long-term point of view, the development of broad-leaved tree species plantation can not only maintain the soil fertility and improve the ecological environment in forest areas but also can provide the sufficient supply of hardwood timber, for example plywood, decorative veneer etc. for future. The existing broad-leaved tree forest, except development of broad-leaved forest, should also be carefully protected. Especially, the broad-leaved forests with good cultivating target and distributed in poor site condition, on steep slope and the ridge of mountains or hill should be carefully protected and managed.

3.3.2 Improving the management system

To prevent soil and water erosion, the measures of site clearing without fire and hole-site-preparation, strip site-preparation should be taken and planting hole should be arranged in triangle forms. Overall site-preparation will be only allowed to operate in plain areas and on the slopes with below 15 degree and the areas of overall site-preparation must be as small as possible and not be on a large scale. If the length of slope is over than 200 meters, 3 meters of vegetation strip should be kept as an original place for each 100 meters of slope.

3.3.3 Improving the community of plantations

Forest ecological system is a complicated structure of community. Improving the structure of community has an good advantage of increasing the fertility of plantation land. Based on the investigation, the vegetation under Chinese Fire, Pine plantation can be recovered by nature through restructure of density. Recovering the vegetation under the plantation not only improves the nutrient cycle but accelerates the decomposition of the litter under plantations. Some vegetation can be cut and used as green manure. This method, compared with establishment of mixed plantations in improving soil fertility, plays the same role and is easily operated.

3.3.4 Improving management

Fertilizer application is broadly used in foreign countries. As there is not adequate chemical fertilizer in China, fertilizer application can only be used in small area of forest land. To improve and maintain the fertility of forest land, in the guideline of the environmental protection, a series of measures are identified such as planting green manure under plantations, mixing broad-leaved tree species with coniferous tree species, the alternation of culture, conversion the topsoil to the planting holes, through the tending and cutting the vegetation as green manure, furthermore, chemical fertilizer should be rationally applied in a right way. The varieties and quantity of fertilizer should be identified based on nutrient diagnosis of soil and leaves. Meanwhile, the vegetation and litter under plantations should be careful conserved. Establishment of fuel wood plantation can produce fuel-wood for local farmers to protect litter under forests.

3.4 The monitoring results of fertility on forest land after National Afforestation Project was implemented for 5 years

Based on monitoring results and the surveys of 300 plots, the growth of young trees had significantly increased after fertilizer application. The canopy of young growth after fertilizer application was closed 1~2 years earlier than that without fertilization application under the same site condition, and the tending cost of young growth would be saved. The increasing range of production for young growth after fertilizer application in a large area was 15%~25% for coniferous tree species and 20%~30% for broad-leaved tree species. Under general conditions, the production increment of native tree species was lower than that of tree species introduced from other countries, and the increment of coniferous tree species was lower than that of broad-leaved tree species. The production increment and economic benefit of eucalyptus was the highest among short-rotation tree species. The optimum quantity of fertilizer application was identified based on the results of monitoring and scientific research that was described in Table 2.

The methods and main varieties of chemical fertilizer are suitable for NAP. The results of research and monitoring indicated: (1) phosphate is an important factor to effect the growth of young tree. The varieties of phosphate fertilizers for production increment from the highest to the lowest are calcium magnesium phosphate, lime super-phosphate; di-ammonium orthophosphate, ground phosphate rock. Although the production increment of ground phosphate rock is the lowest, the Internal Rate of Return (IRR) of this kind of fertilizer is the highest for the cost of the fertilizer is the lowest. (2) the National Afforestation Project was mainly located in the southern part of China where most of forest lands were shortage of phosphate and the ratio of nutrient in soil is unbalance. For most of tree species, phosphate plays a significant role in production increment and its efficiency can last many years. Therefore, phosphate application combining with potassium, nitrogen should be taken into account. Fertilizer application on the forest land can prevent the degradation of soil fertility and increase fertility, improve the soil and ecological environment. Therefore, selection of suitable varieties of fertilizer is very important to raise the efficiency and utilization ratio of fertilizer and prevent the environmental pollution.

At the stage of young growth, the specific surface area of the roots in soil is small and their ratio of intercepting and capturing the fertilizer is low and the ratio of fertilizer utilization at this time is also low. At this stage, some slow-release fertilizers such as ground phosphate rock, lime super-phosphate, calcium magnesium phosphate and farmyard manure etc. should be applied, which not only significantly raise the ratio of fertilizer utilization but also produce good economic benefit. Meanwhile, some of rapidly available chemical fertilizer does not adopt to be applied because the application not only wastes a large quantity of fertilizer but also pollutes environment and water resource. Based on the economic analysis and research of fertilizer application on plantations by Japan and America, it was better to apply rapidly available fertilizer for longer rotation (20~30 a) of tree species in the last five years before harvesting, which can get the best economic benefit and output since the specific surface area of the roots in soil is the largest and ratios of intercepting and capturing the fertilizer, and fertilizer utilization are in the highest and the nutrient of soil at the time is seriously shortage. The rapidly available fertilizer mainly suites to be applied in short-rotation tree species such as eucalyptus, poplar and used as additional fertilizer for long rotation tree species. The main suitable methods for fertilizer application are: (1) mixture of chemical fertilizer with farm yard manure as the basal manure is applied in the planting holes, and (2) additional fertilizer should be applied around planting holes, then covered with soil after dressing to avoid losses.

3.4.1 The changes of soil fertility in forest land in tropical project areas

There are normally shortage of phosphate and potassium nutrients in the areas where the National Afforestation Project was located. To prevent the degradation of soil fertility, except that a series of environmental protection measures were taken, the fertilizer of nitrogen and phosphate were applied, which not only raised the growth of plantations but also improved soil fertility. The changes of each nutrient in tropical project area were showed in Tab. 3 and Fig. 1 since the project has been implemented for 5 years. In light of Tab. 3, only rapidly available nitrogen has decreased, the others increased with a slight degree. The main reason is that the vegetation on top soil has been destroyed at the first year after tree planting which has caused loss of rapidly available nitrogen of soil.

3.4.2 The changes of soil fertility in forest land of subtropical project areas

There were generally shortage of the nutrient of potassium and phosphate in the subtropical project's areas. The change of nutrient in the areas were showed in Tab. 4 and Fig. 2, which indicated that both available nitrogen and organic matter decreased, the other 7 kinds of nutrient increased in a small range.

3.4.3 The changes of soil fertility of forest land in the plain of project areas

There were serious shortage of phosphate, nitrogen and organic matter except of rich potassium in plain project's areas. The change of nutrient in the soil was showed in Tab. 5 and Fig. 3, which indicated the organic matter of soil was increased in a small range and the available nitrogen decreased but normally recovered at the fifth year after trees planted.

4 Conclusions

(1) Since the NAP was carried out, the degradation of soil fertility has been effectively avoided and all afforestation activities which are strictly conducted by following the guideline of environmental protection on NAP. The monitoring results indicate: the guideline of environmental protection is a very useful document that provides the practical measures to avoid the degradation of soil fertility. Though the project has been carried out for 5 years, the soil fertility has not been degraded but increased with a slight trend.

(2) Except that the available nitrogen and organic matter of the soil in subtropical and tropical project's areas have decreased, 7 kinds of nutrients such as total nitrogen, available phosphate, available potassium, slowly available potassium etc. in the soil have increased with a slight trend.

(3) The content of each kind of soil nutrients in plain project's areas has continually increased. The increasing rates of available phosphate, available potassium and exchangeable magnesium and calcium are higher.

Acknowledgements

The authors would heartily like to thank the following for the use of their data: The environmental monitoring reports of Hebei, Liaoning, Zhejiang, Anhui, Fujian, Jiangxi, Shandong, Henan, Hubei, Hunan, Guangdong, Hainan, Sichuan, Guizhou, Yunnan provinces and Guangxi, Zhuang Autonomous Region from 1991 to 1995, and the colleagues who participated the environment monitoring work, particularly Prof. Shen Weitong and Prof. Zhang Wanru who give me a great help.