黄土区旱地夏季休闲期土壤剖面残留肥料氮的损失途径及机理

Summer fallow is a common practice to conserve water and renew soil fertility in dryland farming on Loess Plateau. In recent years,the nitrogen fertilizer application rate in this region increases rapidly; it results in high level of N fertilizer remained in soil after crop harvesting. The high temperature and moisture in summer fallow increases the loss risk of the residual N fertilizer in soil profile. However, there is little information available on the loss of residual N fertilizer during the summer fallow, including the loss mechanism of residual N fertilizer, and environmental factors that play key roles. Therefore, we'll use the different fertilization treatments from two long-term field trials located at the south edge of the Loess Plateau as our key field experiment. And the lysimeter and incubation methods, and the conventional and isotopic methods (15N and dual isotopic composition of nitrate) will use to study the mineralization and leaching loss of the residual N fertilizer in soil during the summer fallow. The N2O emission from the residual N fertilizer in soil during the summer fallow will also determined. The effects of different temperature and moisture on loss of the residual N fertilizer in soil will also evaluated. The aims of this study are to understand the amounts and mechanism of residual N fertilizer loss during the summer fallow. We hope our study would provide sound knowledge to reduce loss of the residual N fertilizer during the summer fallow.

夏季休闲是我国黄土区旱地常见的蓄水保墒、恢复地力的方式。近年来，随着氮肥用量的不断增加，黄土区旱地一季作物收获后土壤剖面累积了大量的肥料氮，夏季休闲期间的高温、多雨，无疑会增加残留肥料氮损失的风险，而关于夏季休闲期间残留肥料氮损失数量、机理及影响因素的研究尚少见报道。为此，本项目以在黄土高原南部进行的两个长期定位试验不同施肥处理为主要研究对象，辅以渗漏池试验与室内模拟试验，采取常规测定法与15N标记及硝态氮（NO3-）中的氮、氧稳定性同位素(δ15N及δ18 O丰度)相结合的方法，研究该区夏季休闲期间土壤残留肥料氮的矿化特性，残留肥料氮在土壤剖面的淋溶损失数量，残留肥料氮的气态损失（N2O）数量及动态变化，以及温度、水分等环境条件对残留肥料氮损失的定量影响，旨在揭示黄土区旱地夏季休闲期间土壤残留肥料氮的损失途径及机理，为采取有效措施减少夏季休闲期间肥料氮的损失提供理论依据。

夏季休闲是我国黄土区旱地常见的蓄水保墒、恢复地力的方式。近年来，随着氮肥用量的不断增加，该区旱地一季作物收获后土壤剖面累积了大量的肥料氮，夏季休闲期间的高温、多雨，无疑会增加残留肥料氮损失的风险，而关于夏季休闲期间残留肥料氮损失数量、机理及影响因素的研究，尚少见报道。.项目以位于黄土高原南部的长武、杨凌两地旱地为研究对象，连续三年研究了休闲前后不同田块及氮肥用量处理土壤剖面矿质氮含量及土壤氮素矿化特性；采用大型渗漏池（3 m×2.2 m×3 m）施用15N标记尿素研究了一季小麦收获后肥料氮在土壤的残留及连续3年的去向；采用在田间设置微区，给微区加入15N标记硝态氮的方法连续两年研究了标记硝态氮的去向。.结果表明，长武旱地一季小麦收获后土壤剖面（0-2 m）累积的硝态氮在97-328 kg hm-2之间，平均193 kg hm-2。15N试验表明，一季小麦收获后土壤0-100 cm残留肥料氮的比例占氮肥用量的37%，说明黄土高原旱地一季小麦收获后肥料氮残留量大。与休闲前相比，休闲后土壤氮素累积矿化量均不同程度降低，双组分一级动力学模型拟合表明，休闲主要降低了土壤易矿化氮组分含量。田间观测表明，夏季休闲期降水量是影响硝态氮在土壤剖面移动的关键因素；15N标记微区试验发现，即使降水量低的欠水年也会发生硝态氮在土壤剖面的淋溶；降水量高的丰水年, 硝态氮淋溶至1 m以下土层；夏季休闲期间也会发生硝态氮在土壤剖面的上移。随着降水量增加，土壤剖面硝态氮的淋溶深度增加。大型渗漏池试验表明，经过两个冬小麦-夏休闲季后，土壤残留的肥料氮已淋溶到120 cm及以下土壤深度。虽然休闲结束后淋溶到土壤剖面的肥料氮仍可以被下季小麦吸收利用，但降水量高处理小麦对肥料氮的吸收利用显著降低。本研究中15N标记微区试验加入的肥料氮为15NO3-，但休闲结束后0-100 cm土壤剖面以硝态氮存在的肥料氮仅占3%-47%，说明加入的硝态氮有相当一部分转化成其他形态氮素，相关机理值得研究。休闲结束后加入的15N标记肥料氮平均损失率为28%，损失机理也值得进一步研究。