LAUSR

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.