论“稻田圈”在保护城乡生态环境中的功能 Ⅰ.稻田土壤磷素径流迁移流失的特征

ECOLOGICAL FUNCTION OF "PADDY FIELD RING" TO URBAN AND RURAL ENVIRONMENT Ⅰ. CHARACTERISTICS OF SOIL P LOSSES FROM PADDY FIELDS TO WATERBODIES WITH RUNOFF

太湖流域最近5年的研究表明:单位面积上径流迁移的土壤磷素是桑园>>菜园≥大田麦季>大田稻季;稻麦轮作田每年向水体排放的磷量为P 0.84 kg hm-2,占当年磷肥用量的2.5%,而菜园地5个月内土壤磷素流失量就达P 0.6 kg hm-2,桑园在4个月内高达P 1.1 kg hm-2。径流迁移的土壤磷素形态主要是颗粒态磷(PP),占总流失磷的70%-80%,可溶性磷(DP)仅占20%-30%。在径流携出的可溶性磷总量中,可溶性无机磷(DIP)占30%-40%,可溶性有机磷(DOP)占60%-70%。径流产生的机制与土地利用方式有关:稻田产生的是“机会径流”,蔬菜地等旱地是“开放径流”,而桑园等则是“强化径流”,不同的产流机制决定径流的次数、流量和强度并导致不同的磷素迁移量。太湖流域水稻土磷素向水体排放的警戒值(Break point)为有效磷(P)25-30 mg kg-1,目前该地区水稻土平均的土壤有效磷水平为12-15 mg kg-1;因此常规条件下,未来5-10a内稻田不会形成严重的磷素面源污染威胁。故在城镇郊区、桑园和蔬菜基地周边建立“稻田圈”是防治磷索面源污染有效的生态措施。

Results of field trials revealed that P losses from soil with runoff are closely related to land use patterns, which are in the order of： mulberry garden 〉〉 vegetable fields≥ paddy in wheat season 〉 paddy in rice season. The annual mean total P loss from paddy field under wheat - rice crop rotation was about P 0.84 kg hm^-2, which was equivalent to 2.5% of the total P applied. P loss from vegetable field reached P 0.6 kg hm^-2 in 5 months due to a very high content of soil available P, and from mulberry garden P 1.1 kg hm^-2 in 4 months. In terms of forms of P moved with runoff, about 70% - 80% was found to be Particle P （PP）, while the rest （20% - 30% ） was dissoluable P （DP）. Of the dissoluable, inorganic P （DIP） accounted for only 30% - 40% while organic P （DOP） for about 60% - 70% . Mechanisms of runoffs were closely related to land use patterns and determined strength, frequency and amount of runoffs. Flooded paddy field is surrounded by ridges, and embedded with a plowpan below the cultivation horizon developed after long term rice cultivation. Thus flooded paddy field is recognized as a closed system for runoff. But runoff would occur occasionally in case of heavy rainshowers that lead to overflow of the overlying water, and the runoff flow from flooded paddy field is defind as “occasional runoff ”. In the case of vegetable field or paddy under winter wheat, the field is open to free runoff as long as there is rain because it is not protected by ridges. This type of runoff is called “open runoff ”. As mulberry gardens in this area are usually located on upland with varying degrees of slope in the rural area or along both sides of canals or rivers, runoff in this case is accelerated by the gravity and could carry more particles with faster velocity from the upper to the bottom along the slope, which is defind as “intensified runoff”. The break point of P runoff losses from the two types of paddy fields tested was P 25 - 30 mg kg^-1（Olsen-P） . The present average soil available P in paddy soils in this region was around P 12 - 15 mg kg^- 1, which is still far below the break point under the current management practice in this region. As long as the regular fertilization regime is maintained, paddy fields will not pose a source of non-point source P pollution within the next 5 - 10 year. Therefore, setting up a “Paddy field Ring” around villages and/or town, vegetable bases/or mulberry gardens should be an ecological and valuable means for controlling the non point source P pollution with runoff.