中文摘要：

农地资源的正外部性直接或间接对人类福利产生影响，但由于市场机制的自身缺陷导致人们对其价值缺乏定量认识，致使农地资源得不到应有的保护，造成向城市的过度流转。以湖北省为例，在将农地资源正外部性划分为提供社会保障、保证粮食安全以及提供生态服务3个方面的基础上，以2006年为评估期日，尝试用市场价格评估的方法对其年价值和无限年期的总价值进行了估算。结果显示：2006年湖北省农地资源在提供社会保障方面的年价值及无限年期价值分别为874．60元／hm^2·a和34706．35元／hm^2；粮食安全保障功能的年价值及无限年期价值分别为322．09元／hm^2·a和12781．35元／hm^2；生态服务功能的年价值及无限年期价值分别为4348．55元／hm^2·a和172561．51元／hm^2。最终核算出湖北省正外部性的年价值总量及无限年期价值总量分别为5545．24元／hm^2·a和220049．21元／hm^2，这部分无法通过市场价格表现的价值大致与湖北省农地经济产出价值相当，甚至略高出5％～38％，无疑是农地价值中不容忽视的重要组成部分，应当引起土地管理者和社会决策者的充分重视。农地城市流转的失控和对农地保护的忽视实质上就是对农地正外部效用的抛弃和破坏，最终将阻碍社会经济的可持续发展。

英文摘要：

The positive externalities of agricultural land affect the welfare of human directly or indirectly. But their value is short of quantitative cognitions due to the limitation of marketing mechanism all along, which results in the insufficient protection of agricultural land and the excessive rural-urban land conversion. In this paper, via setting 2006 as the appraisal time and using the method of market price evaluation, the author takes Hubei Province as an example to assess the annual and infinite year value of agricultural land＇s positive externalities based on classifying them to following three facets： offering social insurance, guarantying the food security and providing ecosystem services. The results show that, in 2006 the annual and infinite year value of offering social insurance which pertains to agricultural land in Hubei Province were ￥874.60/hm^2 year and ￥34706.35/hm^2 , respectively. The Homothetic values of guarantying the food security were ￥322.09/hm^2 year and ￥ 12 781.35/hm^2 , respectively. And for providing ecosystem services, the values were￥4 348.55/hm^2 year and￥172 561.51/hm^2, respectively. At last, it figures out that the aggregate annual and infinite year values of the holistic positive externalities of agricultural land in Hubei Province are ~5 545.24/hm~ year and ~220 049.21/hmz. This part of value which canPt be revealed in the form of market price is approximately equivalent to the output values of agricultural land in this province, even to be much higher by 5%-38%. It is obviously that the value of the agricultural land＇ s positive externalities is a quite important portion of agricultural land＇s whole value which should attract adequately attentions of the land managers and social decision makers. The running away of rural-urban land conversion and the neglect of the rural land protection are abandonment and breakage to the external economy of rural land virtually which is to hamper the sustainable development of socio-economic finally.