COMPARATIVE STUDIES ON SEASONAL DYNAMICS OF SOIL ENZYMATIC ACTIVITIES AND SOIL NUTRIENT AVAILABILITY IN MONO- AND MIXED- CULTURE PLANT COMMUNITIES OF AGERATINA ADENOPHORA AND SETARIA SPHACLATA Research advances in plant competition in agroforestry systems Competitive effects between Medicago sativa and Achnatherum inebrians Response of root systems to soil heterogeneity and interspecific competition in Alhagi sparsifolia Effects of Achnatherum inebrians infected with Neotyphodium endophyte on accompanying species of Stipa capillata and Poa sphondylodes