农业生态系统是一个养分大量输入和输出的系统。大量农、畜产品作为商品输出，使养分脱离系统，产品输出越多，被带走的养分越多。为维持农业生态系统的养分循环平衡，必须返回各种有机物质，并投入大量化学肥料。因此，农业生态系统物质循环的封闭性，远低于自然生态系统。但? Agro-ecosystems are a system of large-scale input and output of nutrients. A large number of agricultural and livestock products as a commodity output, so that nutrients from the system, the more the output of the product, the more nutrients are taken away. In order to maintain a balanced nutrient cycle in agro-ecosystems, it is imperative to return all kinds of organic matter and invest a large amount of chemical fertilizers. Therefore, the closed material circulation of agro-ecosystems is much lower than that of natural ecosystems. but?