13 SOIL MICROBIAL C, N AND P DYNAMICS IN TRADITIONAL AGROFORESTRY SYSTEM OF NORTHEAST INDIA S. TANGJANG, A. ARUNACHALAM*, K. ARUNACHALAM* AND A. K. SHUKLA Department of Botony, Rajiv Gandhi University, Rano Hills-791112, Arunachal Pradesh. *Restoration Ecology Laboratory, Department of Forestry, North Eastern Regional Institute of Science and Technology, Nirjuli-791109, Arunachal Pradesh. ABSTRACT Microbial biomass, as part of the active pool of soil organic matter, is critical in decomposition of organic matter, nutrient, and soil structure formation. The values of microbial C, varies between 47.5 and 1167.6 μg g -1 . The value was higher in Harmutty as compared to the sites in Arunachal Pradesh. Nevertheless, Microbial N and P had similar spatial variations varying their value from (Microbial N-2.7-147.1 μg g -1 and microbial P- 0.13 and 19.0 μg -1 respectively). Percentage contribution of microbial C to total soil organic C ranged from 0.36-7.15%; microbial N to soil total N ranged between 0.11 and 9.24% and microbial P to available P between 0.23-21.15 percent respectively. Ratio of microbial C/N varied between wide ranges of 1.07-44.92, 2.21-179.91 and 2.28-121.5 μg g -1 in Harmutty, Nirjuli and Doimukh respectively while, microbial C/P ratio was also very large (25.01-1633). INTRODUCTION Flow of C, N, and P through microbial biomass has been identified as the most useful property for assessing soil health in the ecosystems (Sparling, 1991). Soil microbial biomass can be an indicator of management-induced changes brought about by tillage practices, incorporation of crop residues, N fertilization, crop rotation sequence, and changes in soil moisture regimes in agricultural systems (Pankhurst, 1994). Soil microbial C is comparatively a small proportion of total organic C Pool (1-10%; Franzluebbers, 1999). It is also understood that the carbon fraction is used by soil microbial biomass as an energy source for metabolic activity, with nutrient availability and soil structure being altered in the process (Paul, 1991). The microbial biomass is considered to be dynamic source and sink of plant available nutrients in the soil (Singh et al., 1989) and a driving force behind SOM transformations. Microbial biomass is relatively a labile fraction of the soil organic matter and is central to P cycling (Brookes et al., 1984). The quantities and forms of P in soil depend on soil-forming factors and may modify C and N accumulation during pedogenesis (Cole and Heil, 1981). Further, Cole et al. (1977) estimated that 3-5 times more P was taken up by soil microorganisms than the plants in a simulation model of P cycling in semi-arid grassland. Direct measurement of the P content of the soil biomass is thus critical for a precise evaluation of the importance of the microbial biomass in P cycling and in crop nutrition. Although studies accentuating the role of microbial biomass in SOM and nutrient dynamics of soils is subjected to various disturbance regimes (Franzluebbers, 1999), the present study discusses the temporal and spatial dynamics of microbial C, N and P and its role in soil nutrient conservation in traditional agro forestry system in a humid tropical environment in the north- eastern India. Advances in Biotechnology and Microbiology eds P. R. Jatkar, P. Parihar & L. Parihar Published by: Agrobios India Ltd. Jodhpur (2010) pp. 13-22