| The reseach, based on the question of overuse or irrational use nitrogen causing nitrateaccumulation in pursuit of higher crop productivity in dryland, explored the process of nitrateaccumulation and crop validity of virious depths' nitrate in soil profile with the 15N isotopetraced technique and long-term position experiment, and investigated the situation of nitrateaccumulation in the Loess Plateau with the method of point-region combination by collectingsoil profile samples in the representative regions. The results were followings:(1) By investigating soil nitrate in 0-300cm profile in the representative regions of LoessPlateau, nitrate accumulating amount was in 1.0-353 mg/kg, average 27 mg/kg, C.V. 2.1 in0-300cm soil profile of total samples coming from representative regions' orchard land andfarmland, in general orchard > farmland. Nitrate accumulating amount was in 1.2 – 54mg/kg,average 8.2 mg.kg, C.V. 1.3 in farmland and in 1.3 – 352 mg/kg,average 47 mg/kg,C.V. 1.7 in orchard, respectively. In the different land types, nitrate accumulation was seriousin highland, terraceland and plainland in orchard, and the amount would increase with thefertilization levels or planted years. In farmland, nitrate accumulating amount showed theorder of highland>damland>plainland>terraceland, but the differences were small. Becausethe water table in dam-land or plain-land is shallow, nitrogen rate-increase might bring soilnitrate accumulation as well as underground water pollution. The rainfall in Changwu countyis richer than one in others in Loess Plateau, so the result from Changwu showed the deepestdepth of nitrate accumulation was 240cm in farmland or 300cm in orchard land in LoessPlateau.(2) The result from long-term fertilization experiment showed that nitrate accumulationin soil was inevitable in long-term imbalance use or overuse nitrogen. Nitrate content,accumulating amount, accumulative ratio and accumulative depth in soil were correlated withtype of fertilizers' mixture, rate-applying, every item would increase with the rate of nitrogen,but decrease with the rate of phosphorus. Organic manure had the role of reducing nitrateleaching to deeper depth, but it was not able to decrease the amount of nitrate accumulation insoil profile. The accumulative ratio was in 8.5%-31.0% in 0-200cm soil profile withtreatments of NPM>N>NP>M. The most accumulative ratio was 44.1% in 0-400cm soil inonly use nitrogen of 180 kg/hm2.(3) 15N isotope traced results in field experiment in the south of Loess Plateau showedthat nitrogen would have aftereffect in evidence, the total recovery of five crops planted oneby one was 44.1%-46.5%, the remnant N in soil was 22.4%-28.3%. it showed that the greatremnant-N in soil was mostly reason for nitrate accumulation formation. Wheat, cornmulching planting did not have the role of improvement to nitrogen use efficiency(NUE) infirst crop, but increased obviously remnant-N in soil. The remnant-N could be useed byaftercrop, total NUE in two season wheats increased by 16.6%. The great remnant-N wouldmove down in soil profile by the way of mass-flow in action of rainfall, it was main reason ofnitrate accumulation in soil profile.(4) In rainfed farmland the nitrate leaching, movement and accumulation are impactedby some factors such as crops, fertilization, water and soil types. The crops with deep rootsand high yield might use nitrate in big extent and amount, soil nitrate accumulation mightdecrease. Nitrogen's form, type and rate-using would affect directly soil nitrate movingtspeed and accumulative ratio. When nitrogen was overused or mixed with not enough othernutrient elements (named imbalance fertilization), soil nitrate accumulation would be easy.Due to limited rainfall in the loess region, soil nitrate leaching and accumulation depth mightreach less to 300cm in rainfed farmland. Landscape and soil type might decide to waterdistribution in surface of land and impact nitrate leaching and accumulation in soil profile.(5) In Loess Plateau, soil nitrate leaching loss would be in existence for shallow-rootcrops in majority, even some deep-root crops. Results showed nitrate leaching out of 100cmwould account for more than 50%, thereinto 83% for orchard and 67% for farmland. Nitrateleaching out of 200cm, in average, would account for 46% in orchard and 17% in farmland.For different crops due to roots distribution and productivity difference, nitrate might beutilized in big difference. Rotations of deep-root crops and shallow-root crops or onlyplanting deep-root crops, such as the grain-alfalfa rotation, planting alfalfa, would avoid soilnitrate leaching to deep depth. Controling nitrogen rate-used, mixing fertilizers with manyelements and rotations of deep-root crops and shallow-root crops would be efficient practicesto prevent nitrate excessive accumulation in soil profile.(6) The results showed the rational crop yield and economic binefit could not attainunder the situation of soil nitrogen balance in a long-term fertilization experiment. Nitrogenbalance value was 16.8 kg/hm2, ratio of input to output was 1.12 under the rational economicbinefit on wheat in loess highland. It showed that overuse a limitted rate of nitrogen wasnecessary for keeping crop productivity and soil nitrogen supply. It was inescapable to have acertain range of nitrate accumulation in soil profile (less than 26 kg/hm2 in wheat land), evenit was obligatory.(7) The rsults from long-term fertilization test showed nitrate or mineral-N amount in0-120 cm soil profile was well correlation to wheat N-absorbing(1% level), the correlationcoefficient in 120-200cm profile reached to 5% level, and the correlation coefficient woulddecrease with soil depth's deeper. It showed nitrate or mineral-N in 0-200cm soil depth hadvery important roles for wheat nutrition, and nitrate in 0-120 cm, as an index of supplyingnitrogen ability, should be take into account in recommending fertilization. |
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