压砂地土壤氮素转化及供应与西瓜需氮规律研究

The watermelon from gavel-mulched field is one of most important industry of Ningxia, and also is the main way of money-making for farmers on dry land. However, the sustainable production of gravel-mulched field was seriously affected in recent years. The reason is that soil nutrients became less and less and watermelon yield decreased with the increase of planting years because of difficult fertilization and then no fertilizer or a little fertilizer applying, or that soil nutrients was imbalance and watermelon's quality became bad caused by unknown watermelon's nutrients requirement regularity and then irrational fertilization. So, how to rational fertilizing and building up soil fertility is the most urgent problem for gravel-mulched field. The precondition of resolving this problem is to clear the transformation and release mechanism of soil nutrients and watermelon's nutrients requirement regularity,. By 4-year field plot experiments and soil-column incubation experiment, with 15N isotope tracer technique，this project is to study the change of soil nitrogen in the stable organic, labile organic matter（including microbial biomass, light fraction organic matter and dissolved organic matter）, and inorganic forms, and also to difinite nitrogen mineralization feature in soil from gravel-mulched fields. the objective is to investigate transformation regularity, release mechanisim and supplying capacity of soil nitrogen, to definite nitrogen-uptaking characteristic and requirement regularity of watermelon, and then to found the contradiction between soil nitrogen supplying and watermelon's requirement on gravel-mulched field. The results wound provide theoretical basis for making rational N-application system and improvement soil fertility methods, and then ensure wantermelon's high yield, high quality, and high ecnomical effect, and the sustainable utilization of gravel-mulched field.

压砂西瓜是宁夏的特色优势产业和干旱地区农民脱贫致富的主要途径。由于施肥难而不施肥导致压砂地土壤养分逐年减少和西瓜产量降低，或由于不明确西瓜需肥规律而盲目施肥导致土壤养分失衡和西瓜品质降低均会限制压砂地西瓜产业的可持续发展。因此，合理施肥与培肥是压砂西瓜生产中亟待解决的问题，而解决这一问题的前提是明确压砂地土壤的养分转化和释放机理及西瓜的需肥规律。本项目拟通过4年田间定位试验和土柱模拟试验相结合的方法，采用15N示踪技术，研究压砂地土壤中稳定有机态、活性有机态（微生物量、轻组有机质、可溶性有机质）和无机态氮的动态变化和矿化特征，探讨压砂地土壤氮素的转化规律、释放机理和供应能力；阐明压砂西瓜对氮素的吸收特点和需求规律；揭示出压砂地土壤氮素供应与西瓜需求之间的矛盾，为压砂地制定合理氮肥施用制度和培肥措施提供理论依据，为压砂西瓜高产高效优质生产和压砂地的可持续利用提供保障。

针对压砂西瓜氮肥施用不合理问题，通过4年田间试验和2次土柱模拟实验，研究了不同施氮水平对压砂西瓜生长、产量和品质的影响，分析了西瓜对氮素的吸收累积规律，并探索了压砂地土壤氮素转化及供应状况，所获数据和研究结果基本达到了预期研究目标，但数据还需进一步挖掘分析和补充完善。.目前主要研究结果主要有：（1）压砂地土壤中的硝态氮、铵态氮、可溶性有机氮和微生物量氮含量均随西瓜生育期而不断变化，在生长旺盛期（伸蔓期和座瓜期）含量较低，而苗期和膨瓜期相对较高，说明，四种形态氮素均可被作物吸收利用；（2）硝态氮受氮肥用量的影响较大，适量施氮可以维持和适当增加土壤的氮素肥力，过量的氮肥则使土壤中累积大量的硝态氮；铵态氮主要受土壤湿度和作物生长吸收的影响较大，氮肥用量对其影响比较短暂，土壤湿度较大时可抑制硝化作用，增加土壤氨态氮含量；可溶性有机氮主要受作物根系吸收的影响较大，在旺盛生长期含量较少，而生育前后期含量较多；微生物量氮的变化较为复杂，不但受生育期和氮水平的影响，还受到土壤水分状况的影响。（3）压砂西瓜茎、叶和果实中的氮素累积量随施氮水平的增加而增加，传统种植方式下，茎、叶中的氮素累积高峰值出现在移栽60天左右，滴灌种植下则出现在80天左右；两种方式茎和叶中氮素累积总量分别可达14.27 kg/ha（N4）和14.96 kg/ha（N3）。（4）适量施氮可促进压砂西瓜生长，提高产量和品质。西瓜主蔓长、茎粗、叶片数、SPAD值、产量和品质均随施氮量增加呈报酬递减式的抛物线变化趋势，适宜氮肥用量为80-120 kg/ha。