Study on greenhouse soil nitrogen absorption and soil layer transport of different summer catch crops with different planting density in North ChinaPei Zhiqiang, Lu Shuchang, Wang Xi, Hou Kun, Ya Zongjie, Zhang Yu, Wang Dafeng and Li XiawenE3S Web Conf., 143 (2020) 02023DOI: https://doi.org/10.1051/e3sconf/202014302023