| Due to the special terrain and arid climate characteristics in the Loess Plateau,the development of livestock husbandry is hindered,the soil is barren and the utilization efficiency is low.Planting cultivated grassland in the loess plateau can solve the problem of insufficient forage in winter and spring and improve soil fertility,and grazing on cultivated grassland is a way of efficient utilization of grassland.However,the suitable planting modes and nitrogen application level of grazing cultivated grassland in this area are still unclear.Therefore,a field experiment was conducted in Huan County,Qingyang City,and adopted split plot experimental design:grazing（G）and mowing（M）are set up in the main area,monoculture of Medicago sativa（A）,monoculture of Bromus inermis（B）and the mixture of these two forages（AB）are set up in the subplot,0 kg·hm-2（N1）,80 kg·hm-2（N2）and 160 kg·hm-2（N3）nitrogen application levels are set up in the re-split zone,then consider the effects of utilization modes,planting patterns and nitrogen application levels on forage yield,land equivalent ratio,soil physical and chemical properties and soil enzyme activity in cultivated grassland.The main findings are as follows:1.Under grazing utilization,the average yield of total fresh grass and total hay in two years was significantly higher than mowing utilization,which increased by 76.52%and 87.41%respectively.Compared with mowing,grazing in 2021 increased the content of water soluble salt in 0～10 cm soil layer and decreased the soil p H value,but there was no significant difference between the two utilization modes in the same soil layer in 2022,compared with mowing,grazing for two years increased soil water content,significantly increased soil nitrate nitrogen,organic carbon content in 0～10cm and 10～20 cm soil layers,and ammonium nitrogen content in 0～10 cm soil layers,but the contents of total nitrogen and available phosphorus in soil were not significantly different from those in mowing.In the 0～10 cm soil layer,the urease activity of the soil under grazing in 2021 and 2022 increased significantly by 31.43%and 24.07%respectively compared with mowing,while the sucrase activity of the soil under grazing in 2022 was significantly lower than mowing,there was no significant difference between the activities of catalase and acid phosphatase in mowing and grazing at the same soil layer for two years.The above results show that grazing can improve grass yield,soil urease activity,soil water content,nitrate nitrogen,ammonium nitrogen and organic carbon content.2.In 2022,the total fresh grass and hay yield of mixed sowing grassland were significantly higher than those of bromus inermis,but there was no significant difference between mixed sowing grassland and alfalfa monoculture,however,the total fresh grass yield of alfalfa unicast grassland in 2021 was significantly higher than that of mixed sowing grassland and bromus inermis.The land equivalent ratio of mixed grassland in two years was greater than 1,indicating that the resource utilization efficiency of mixed grassland was higher than that of monoculture.Compared with monoculture,mixed sowing grassland in two years increased soil water content,significantly increased soil organic carbon content in 0～10 cm and 10～20 cm soil layers,and significantly decreased soil p H value,water soluble salt and available phosphorus content,however,the total nitrogen content in soil was significantly lower than that in alfalfa monoculture.In 2021 and 2022,the content of soil nitrate nitrogen in 0-10 cm soil layer of mixed sowing grassland was significantly reduced by 23.09%and 23.44%respectively compared with alfalfa monoculture,while the soil sucrase activity was significantly increased by 32.14%and 39.68%respectively compared with alfalfa monoculture.Compared with bromus inermis,two-year mixed sowing grassland significantly increased soil urease activity in 0～10 cm and 10～20 cm soil layers,as well as soil catalase and acid phosphatase activity in 0～10 cm soil layers,among them,the activities of catalase and acid phosphatase in 0～10 cm soil layer increased by 23.14%and 80.32%respectively in 2021,and increased by 54.56%and 62.58%respectively in2022.3.Compared with no nitrogen application,nitrogen application for two years significantly increased the total fresh grass and total hay yield,and the total hay yield of 80 kg·hm-2 and 160 kg·hm-2 nitrogen application in 2021 significantly increased by42.52%and 41.43%respectively,there was no significant difference between the two nitrogen application treatments in 2022.In May,June,July,September of 2021 and May of 2022,the soil water content of 80 kg·hm-2 nitrogen application was significantly higher than that without nitrogen application,but there was no significant difference with nitrogen application of 160 kg·hm-2.There was no significant difference in soil water soluble salt and ammonium nitrogen content under different nitrogen application levels in two years,the content of organic carbon and the activities of urease,sucrase and catalase in soil treated with nitrogen fertilizer were significantly higher than those without nitrogen fertilizer,and there was no significant difference between the two treatments.In 2021 and 2022,the activity of acid phosphatase in 0～10 cm soil layer of 80 kg·hm-2 nitrogen application increased by 21.41%and 25.32%respectively compared with that without nitrogen application,however,there was no significant difference with nitrogen application of 160 kg·hm-2.The results showed that nitrogen application could increase forage yield,soil organic carbon content and soil urease,sucrase and catalase activities,however,excessive nitrogen application(160 kg·hm-2)could not further improve the above indexes.4.The results of structural equation model analysis show that 77.50%of forage yield can be explained by utilization modes,nitrogen application levels and soil enzyme activity.Nitrogen application levels and planting patterns directly affect soil enzyme activity and soil physical and chemical properties,and the path coefficients are 0.376,-0.661,0.173 and-0.312 respectively,soil enzyme activity and soil physical and chemical properties interact with each other,the path coefficient is 0.705.To sum up,nitrogen application of 80 kg·hm-2 in alfalfa and bromus inermis mixed grassland under grazing utilization can increase forage yield and improve soil environment,which is a management measure suitable for cultivating grassland in Loess Plateau. |
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