Spatiotemporal variability of soil nitrogen content of Medicago sativa and Poa pratensis rotation

The spatiotemporal variability of soil nitrogen content under alfalfa and Poa pratensis rotation systems was examined to explore the spatiotemporal distribution of soil nitrogen under different rotation systems. The experiment included four treatments, Poa pratensis→alfalfa (PA), alfalfa→Poa pratensis (AP), Poa pratensis→Poa pratensis (PP), and alfalfa→alfalfa (AA). Soil was sampled from depths of 0–20 and 20–40 cm each month, from April to October in the second year after planting. The total N and available N contents were measured. The results showed that, under the PA mode, dynamic changes in total N content at soil depths of 0–20 and 20–40 cm were observed, first decreasing and then increasing from April to October. Available N content at a soil depth of 0–20 centimeter soil first increased and then decreased, before increasing a second time; this up and down trend was observed for a soil depth of 20–40 cm. In the 0–20 cm layer, the soil total N content of PA was significantly higher than that of PP from April to October (except for August), while in the 20–40 cm soil layers, the PA mode was significantly higher than that of PP in all months except August and September. The soil available N content of PA was significantly higher than that of PP in different soil layers except for April(P < 0.05). Under the AP treatment, dynamic changes in total and available N content followed downward trends in different layers. In the 0–20 cm soil layers, the soil total N content under AP was significantly lower than that under AA in all months except April, May, and June (P < 0.05). In the 20–40 cm soil layers, there was no significant difference between AP and AA, except in October (P > 0.05). In the 0–20 cm soil layer, except in April, May, and June, the soil available N content of AP was significantly lower than that of AA in other months, and in the 20–40 cm soil layers,the AP mode was significantly lower than AA in October (P < 0.05). In summary, the Poa pratensis→alfalfa rotation system can significantly increase soil total and available N content, and the alfalfa→Poa pratensis rotation system can improve the utilization rate of soil total and available N content. Compared with alfalfa→Poa pratensis treatment, the Poa pratensis→alfalfa rotation system can effectively increase soil nitrogen content.