| Energy flow is one of important functions of ecosystem. The zhifanggou watershed has been reconstructed for twenty years. Taking this watershed as case, based on the data of land use and social economy, and field survey, this paper studied the energy flow of cropland subsystem, grassland subsystem, woodland subsystem, stockbreeding subsystem, farmer subsystem and watershed ecosystem. The main results were as follows:1. The structure of supplementary energy of cropland subsystem has been made significant ckanges for more than 20 years. With the increase of the total supplementary energy of cropland, the input of industrial energy is on the rise year after year, especially chemical fertilizer and mechanical utility. The results show that cropland subsystem has become more and more dependent on the environment. Gross output -input ratio and economy output-input ratio are increasing from 1980s to 2000s. With output-input ratio analysis, the organic supplement energy has placed more and more important effect on the productivity of cropland. The currency speed of cropland subsystem was 0.77, which indicate most part of fixed energy in cropland has transformed to other subsystems and environment. The proportion of energy toward farmer subsystem is on the decline, but the proportion of energy toward environment is on the rise. The energy transformed to stockbreeding and the energy held in the cropland were relatively stable. The relationship between solar efficiency of cropland and supplementary energy can be expressed: y=0.1967x+0.002, y: solar efficiency (%), x: supplementary energy (109J/mu).2. The solar efficiency of grassland had been going up, especially in 1990-1994, and then growing slowly after 1995. However, the most part of energy of grassland didn't participate in energy flow of the agro-ecosystem, the currency speed was only 0.066. Supposed the utility efficiency of gross grassland energy was 50%, then there was 7549 X 109J can be used. Supposed the utility efficiency of grazing grass was 70%, then there was 3581 X 109J can be used to support 842 sheep. So it is an important measure to improve the utility of grassland energy and sustainable utility.3. In woodland subsystem, the solar efficiency of lack locus was 1.06%, the solar efficiency of caragana microphylia L. was 1.03%, the solar efficiency of the apple orchard was 0.04%. the currency speed of woodland was only 0.17. The fixed energy in woodland was large, but only eleven percent of gross energy had been put out. After analysis on economy benefits, the accumulation of timber was large potential advantage.4. During more than 20 years of reconstruction, the ratio of energy output to input of stockbreeding subsystem was growing up, and reached 0.57 in 2001.The supplementary energy was changing with the scope of stockbreeding subsystem. The best measure to improve the energy transformation efficiency was to modulate the construction of livestock. From 1980s to recent year, the efficiency of stockbreeding subsystem was above 0.70. With the decline of cropland, the energy transformed to cropland was shrinking. The energy distracted to farmer subsystem is on the rise, but the proportion was small.5. In farmer subsystem, as labor force, the people worked in cropland, stockbreeding subsystem, and so on. The mean labor energy distracted to cropland accounted for 57% in energy output. The energy transformed to stockbreeding subsystem was -waved with the size of stockbreeding subsystem, which account for 13% in the total energy output. Compared with the energy distracted to stockbreeding subsystem and cropland subsystem, the energy transformed to woodland was small. However, more and more labor force was distracted from cropland to outside of watershed. As consumer, the energy consumed from crop yield was descending. At the same time, the energy consumed from stockbreeding subsystem was growing up, expenditure of cropland subsystem was on the decline, while the expenditure from outside of the watershed was rising.6. The supplementary ener...
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