Abstract: 【Objective】With the rapid development of animal husbandry in China and the increasing demand for forage, the adjustment of planting structure has become an inevitable trend. The objective of this study is to quantify the nitrogen (N) and phosphorus (P) spatial flow of cultivated forage, and to provide a scientific basis for increasing grassland productivity. 【Method】Taking alfalfa, ryegrass and oat grass as target forage species, the database of N and P input and output was established, the nutrient balance budgets, nutrient use efficiency and environmental emissions were quantified in cultivated grassland of China by using NUFER model. 【Result】 (1) In 2014, the total input (output) of N in alfalfa, ryegrass and oat grassland of China was 1 547, 236 and 67 Gg, respectively. The N input (output) of per unit area in alfalfa, ryegrass and oat grass grassland of China was 326, 427 and 217 kg N·hm^-2, respectively. The total input (output) of P in alfalfa, ryegrass and oat grass grassland of China was 323, 44 and 16 Gg and the P input (output) of per unit area was 49, 18 and 35 kg P·hm^-2, respectively. N fixation was the largest input item for alfalfa which accounted for 51% of the total N input into alfalfa grassland. N fertilizer was the largest input item for oat grass and ryegrass which accounted for 93% and 84% of the total N input into ryegrass and oat grass grassland, respectively. P fertilizer was the most important input of P in these kinds of forage. (2) In 2014, the nitrogen use efficiency (NUE) of alfalfa, ryegrass and oat grass grassland was 64%, 93% and 69%, respectively. The phosphorus use efficiency (PUE) was 28%, 77% and 34%, respectively. The NUE and pue varied greatly in different regions. (3) In 2014, the N loss of alfalfa, ryegrass and oat grass grassland per unit area was 23, 4.0 and 9.9 kg N·hm^-2 and the P loss of them was 2.6, 3.8 and 2.6 kg P·hm^-2, respectively. The N and P loss of alfalfa and ryegrass grassland in Southwest China was higher than that in other regions. The N and P loss of oat grass grassland in Western China was higher than that in other regions. (4) In 2014, the N and P accumulation of cultivated grassland were positive except for N accumulation in ryegrass grassland. The accumulation rate of alfalfa grassland in Western China was higher than that in Eastern China. There was no regular regional change of N and P accumulation for ryegrass grassland. The N and P accumulation rate of oat grass grassland in Tibet Plateau was higher than that in other regions. 【Conclusion】In 2014, there was a large gap of N and P input (output) for alfalfa, ryegrass and oat grass grassland in different regions of China. N fixation was the major way for alfalfa to acquire N nutrition, so alfalfa grassland management should reduce the input of N fertilizer. Oat grass and ryegrass mainly get the required nutrients by chemical fertilizer, so it should pay attention to fertilizer application for oat grass and ryegrass grassland. The NUE of target forage species was higher than 60%, but PUE was relatively low. Ryegrass grassland was deficient in N, and it is necessary to apply more nitrogen fertilizer. other cultivated grasslands had surplus of N and P in varying degrees, so the excessive application of fertilizer should be controlled.

Key words: cultivated grassland, nitrogen, phosphorus, nutrient flow, nutrient use efficiency, soil nutrient accumulation