ABSTRACT Plant nitrogen (N)-acquisition strategy affects soil N availability, community structure, and vegetation productivity. Cultivated grasslands are widely established to improve degraded pastures, but little information is available to evaluate the link between N uptake preference and forage crop biomass. Here an in-situ 15N labeling experiment was conducted in the four cultivated grasslands of Inner Mongolia, including two dicots (Medicago sativa and Brassica campestris) and two monocots (Bromus inermis and Leymus chinensis). Plant N uptake rate, shoot- and root biomass, and concentrations of soil inorganic-N and microbial biomass-N were measured. The results showed that the root/shoot ratios of the dicots were 2.6 to 16.4 fold those of the monocots. The shoot N concentrations of the dicots or legumes were 40.6% to 165% higher than those of the monocots or non-legumes. The four forage crops in the cultivated grassland preferred to uptake more NO3−-N than NH4+-N regardless of growth stages, and the NH4+/NO3− uptake ratios were significantly lower in the non-legumes than in the legumes (p < 0.05). Significant differences in the NH4+-N rather than NO3−-N uptake rate were observed among the four forages, related to plant functional types and growth stages. The NH4+ uptake rate in the perennial forages exponentially decreased with the increases in shoot-, root biomass, and root/shoot ratio. Also, the plant NH4+/NO3− uptake ratio was positively correlated with soil NH4+/NO3− ratio. Our results suggest that the major forage crops prefer to absorb soil NO3−-N, depending on soil inorganic N composition and belowground C allocation. The preferential uptake of NO3−-N by forages indicates that nitrate-N fertilizer could have a higher promotion on productivity than ammonium-N fertilizer in the semi-arid cultivated grassland.

中文翻译：

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内蒙古温带栽培草地土壤无机氮组成和植物功能类型决定牧草作物氮素吸收偏好

摘要 植物氮 (N) 获取策略影响土壤氮的有效性、群落结构和植被生产力。栽培草地被广泛建立以改善退化的牧场，但很少有信息可用于评估氮吸收偏好与饲料作物生物量之间的联系。在这里，在内蒙古的四个栽培草原上进行了原位 15N 标记实验，包括两种双子叶植物（紫花苜蓿和芸苔）和两种单子叶植物（无头无花果和羊草）。测量了植物吸氮率、地上部和根系生物量以及土壤无机氮和微生物生物量 N 的浓度。结果表明，双子叶植物的根茎比是单子叶植物的2.6～16.4倍。双子叶植物或豆科植物的枝条氮浓度为 40。比单子叶植物或非豆科植物高 6% 至 165%。栽培草地的4种牧草作物无论在哪个生长阶段都更愿意吸收比NH4+-N更多的NO3--N，且非豆科植物的NH4+/NO3-吸收比显着低于豆科植物（p < 0.05） . 在四种牧草中观察到 NH4+-N 而非 NO3--N 吸收率的显着差异，这与植物功能类型和生长阶段有关。多年生牧草中NH4+的吸收率随着地上部、根系生物量和根/地上部比的增加呈指数下降。此外，植物 NH4+/NO3− 吸收率与土壤 NH4+/NO3− 比率呈正相关。我们的结果表明，主要饲料作物更喜欢吸收土壤 NO3--N，这取决于土壤无机 N 组成和地下碳分配。