稻菜轮作休闲期菜地土壤残留氮去向及其固持机制

Hainan is the largest production base for winter vegetables in China.Excessive application of nitrogen(N) fertilizer results in large amounts of fertilizer N remained in vegetable soils.Rice-vegetable rotation(RVR) is common in Hainan. Approximately 2 months of fallow period after harvesting winter vegetable is just rainy season. In addition, the property of soil in Hainan is low soil nutrient retaining capacity. Therefore, residual N in vegetable soils is most likely to lose during the fallow period.However,systematic research has not been reported. It is well known that catch crop plays a important role in reducing N loss of soil during the fallow period.However, the study on the effect of root activities on N recycling is lacking. Therefore,a consecutive three-year situ field experiment was conducted to systematically investigate dynamic characteristic of the leaching ,runoff and N20 emission of residual N in vegetable soils during the fallow period of RVR.The favorable catch crop was selected and then studies focused on N uptake of root and the difference of content of NH4+-N,Organic N and soil microbial biomass N in upper layer of soils and N2O emission between fallow and catch crop in simulated soil column and microplot by 15N -labeled fertilizer for two years. The results could reveal the loss mechanisms of residual N in vegetable soil and retention mechanisms of residual N for catch crop,which was significant in improving N recycling in soil-plant system and reducing enviroment pollution as well as promoting sustainable development of agriculture.

海南是我国最大的冬季瓜菜生产基地，过量施用氮肥导致大量肥料氮残留在土壤中。稻菜轮作是当地习惯模式，蔬菜收获后有2个月左右的休闲期，恰逢雨季，而且海南土壤保肥力低，因此菜地土壤残留氮易在休闲期损失，但系统研究未见报道。休闲期种植填闲作物可减少土壤氮淋洗，但机理研究很少关注其根系活动对土壤氮循环的影响。 本项目拟通过连续3年的田间原位试验，系统研究稻菜轮作休闲期菜地土壤残留氮的淋洗、径流损失以及N20排放动态规律；筛选出适宜的填闲作物，再进行连续2年的模拟土柱和田间微区试验，结合15N标记氮肥重点研究填闲作物根系对氮的吸收比例；与休闲相比，上层土壤的铵态氮、有机氮和微生物量氮含量以及N20排放通量的差异。研究结果可揭示热带稻菜轮作休闲期菜地土壤残留氮的损失规律，阐明填闲作物对残留氮的固持机理，这对提高稻菜轮作系统中的氮素循环利用，减少环境污染，促进农业可持续发展都有重要意义。

通过田间试验与15N标记氮肥的微区试验研究了不同氮肥用量对辣椒产量及其氮肥去向的影响，在休闲期原位收集淋洗液研究土壤残留氮的淋洗特征。通过田间试验从10种作物中筛选出适宜的填闲作物，再利用田间微区与模拟土柱结合15N标记氮肥研究填闲作物对土壤残留氮的固持机理，以及填闲作物秸秆还田对后茬水稻生长及其氮素吸收累积的影响。结果表明：1)相比农民习惯施氮肥（NT），减氮30%（NR）后线椒产量略增加，但差异不显著，辣椒收获后0-10cm土壤中硝态氮含量、铵态氮含量都是NT显著高于NR。15N标记肥料的微区试验结果表明不同氮肥用量处理的线椒整个生育期果实、茎和叶的氮素总累积量几乎相等，农民习惯施氮肥较多，主要增加了土壤中的0-40 cm残留量和表观损失量。2) 休闲期淋洗液中以硝态氮为主，占无机氮总量的91.3%-97.9%，铵态氮很少，仅占2.1%-8.7%。硝态氮淋洗量都是NT显著高于 NR与 CK，铵态氮淋洗量三个处理间差异不显著。3）如果休闲期在5周以内，高丹草适合作为填闲作物。休闲期延长至63d，田箐更适合作为填闲作物。4）田箐对土壤残留氮的固持机理主要是通过田箐吸收（根系的吸氮量不容忽视），通过减少淋洗液体积和淋洗液中硝态氮浓度减少淋洗，田箐将硝态氮转化为铵态氮，增加微生物量碳、氮。5）田菁秸秆还田促进水稻茎叶、根系生长，叶片叶绿素合成，氮素吸收累积，同时增加了15N标记氮肥在土壤中残留。因此稻菜轮作休闲期种植田箐提高了轮作系统中氮素的循环利用，减少环境污染，促进农业可持续发展。