Increasing crop nitrogen use efficiency (NUE) has important implications for food security and agricultural sustainability. Changes in nutrient availability due to stoichiometric imbalances in soil under long-term application of nitrogen (N) can limit crop NUE and yield. However, little is known about the linkages across stoichiometric balance, N fertilizer application, and N uptake. We investigated the changes in soil stoichiometry, microbial community, and crop NUE relative to N fertilizer application in a 16-year field experiment in which five levels (0, 70, 140, 210 and 280 kg N ha) of mineral N fertilizer treatment were applied to wheat and maize cropland. The results showed that the P storage decreased from 4.3 Mg P ha under the fertilizer dose of 0 kg N ha to 3.5 Mg P ha under the fertilizer dose of 280 kg N ha. Thus, long-term N application increased soil C/P and N/P ratios, in a marked decrease in the content of soil available P. The microbial community based on phospholipid fatty acids (PLFAs) increased with increasing N addition rates, from 0 to 140 kg N ha, but significantly decreased at application rates above 210 kg N ha. Thus, applying N at a rate of 140 kg ha resulted in the N threshold rate for microorganism survival being reached or exceeded. The crop NUE peak occurred at the urea application rate of 140 kg N ha (60.4 %), after which NUE declined. The soil elementary and enzymatic stoichiometric ratios under long-term N addition directly affected crop NUE in the topsoil. Soil elementary stoichiometric ratios affected crop NUE by indirectly altering microbial biomass of the subsoil. Overall, soil stoichiometric imbalances under long-term N addition were the key factors driving NUE across N gradients.

中文翻译：

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土壤和微生物化学计量与作物氮利用效率的联系：长期氮添加实验的证据

提高作物氮利用效率（NUE）对粮食安全和农业可持续发展具有重要影响。长期施氮 (N) 导致土壤化学计量失衡导致养分有效性变化，可能会限制作物的 NUE 和产量。然而，人们对化学计量平衡、氮肥施用和氮吸收之间的联系知之甚少。我们在为期 16 年的田间试验中研究了土壤化学计量、微生物群落和作物 NUE 相对于施氮肥的变化，其中五个水平（0、70、140、210 和 280 kg N·ha）的矿质氮肥处理分别为：适用于小麦、玉米田。结果表明，施肥量为280 kg N·ha时，磷储存量从0 kg N·ha下的4.3 Mg P·ha下降到3.5 Mg P·ha。因此，长期施氮增加了土壤 C/P 和 N/P 比率，土壤有效磷含量显着下降。基于磷脂脂肪酸 (PLFA) 的微生物群落随着施氮量的增加而增加，从 0至 140 千克氮·公顷，但在施用量高于 210 千克氮·公顷时显着下降。因此，以 140 kg ha 的施用量导致达到或超过微生物存活的氮阈值。作物NUE峰值出现在尿素施用量为140 kg N ha（60.4%）时，此后NUE下降。长期施氮条件下土壤元素和酶的化学计量比直接影响作物表土的氮肥利用效率。土壤元素化学计量比通过间接改变底土的微生物量来影响作物的氮肥利用效率。总体而言，长期施氮条件下的土壤化学计量失衡是驱动不同氮梯度NUE的关键因素。