Range Mgmt. & Agroforestry 41 (2) : 258-266, 2020 ISSN 0971-2070 Comparative study of soil aggregation, carbon and microbial dynamics under agro-forestry, grasslands, orchard and cultivated land use systems Bappa Paramanik 1 , Shovik Deb 2,* , Ashok Choudhury 2 , Somsubhra Chakraborty 3 , Parimal Panda 2 , Anarul Hoque 2 and Dibyendu Deb 4 1 Dakshin Dinajpur KVK, Uttar Banga Krishi Viswavidyalaya, Majhian-733133, India 2 Uttar Banga Krishi Viswavidyalaya, Pundibari-736165, India 3 Indian Institute of Technology, Kharagpur-721302, India 4 ICAR-Indian Grassland and Fodder Research Institute, Jhansi-284003, India *Corresponding author e-mail: shovik@ubkv.ac.in Received: 18 th July, 2019 Accepted: 20 th August, 2020 Abstract Land use has a profound impact on soil aggregate stability, carbon (C) sequestration and soil microbial activities. This study tried to explore the dynamics of soil C pools, aggregation, aggregate occluded C, microbial biomass and enzymatic activities within soil aggregates under four types of land uses viz. cultivated land, orchard, social/agro-forestry and grasslands. Outcomes indicated the maximum presence of total C in undisturbed soils of natural ecosystems like social/agro-forestry system (6.35 g kg -1 ), followed by orchard (4.05 g kg -1 ), grassland (4.03 g kg -1 ) and cultivated lands (3.07 g kg -1 ). Nevertheless, the proportion of water extractable C (WEC) + hot water extractable C (HWEC) per unit total C in soil was highest in tilled arable lands (0.128) and orchard (0.113) followed by social/agro-forestry (0.104) and grasslands (0.048). Although, land use showed no significant impact on soil microaggregates, macroaggregate quantity followed the order: social/agro-forestry (523.8 g kg -1 of total aggregate amount)> grassland (469.9 g kg -1 )> orchard (454.1 g kg - 1 )> cultivated lands (332.7 g kg -1 ). Macroaggregate associated C was also found highest in soils of social/ agro-forestry (61.5 g C kg 1 of macroaggregate), followed by grasslands (50.8 g kg 1 ), orchard (49.5 g kg -1 ) and cultivated lands (37.1 g kg -1 ). Analysis of microbial biomass C, present within the water stable aggregates, showed the same trend of macroaggregate quantity with land use change, while it was not so for the soil enzymatic activities. Analysis of both the dehydrogenase and β- glucosidase enzymes showed their presence in the following order: cultivated soils> orchard> social/agro- forestry> grasslands. It was postulated that these enzymatic activities were controlled by the proportionate presence of labile C (WEC and HWEC) in soils. Keywords: Aggregate occluded C, Enzymatic activities; Land use, Microbial biomass C, Soil aggregates Introduction Soil is the largest global terrestrial carbon (C) sink (Deb et al ., 2015). Carbon sequestration in soil largely depends on the aggregation as C is believed to be protected from microbial mineralization and enzymatic degradation within soil aggregates (Bronick and Lal, 2005). Earlier studies have indicated that soil aggregates can be of different sizes, which determines their C sequestration capacity (Deb et al ., 2018). In soil, attachment of organic molecule with clay and cations results in the formation of microaggregates, while several microaggregates join together by chelating agents to form macroaggregates (Deb et al., 2019). Consequently, microaggregate occluded C gets two-fold protection and are found to be older and less labile than the macroaggregate associated C (Six et al., 2000; Lichter et al., 2008). The small pore size of microaggregates restricts the free microbial movements and activity, which are further reduced by the water-filled pores and reduced oxygen diffusion within these microaggregates (Bronick and Lal, 2005; Govaerts et al., 2009). Land use and land cover influences soil aggregation, while soil properties also affect sustainability of different land forms (Singh et al., 2011, Deb et al., 2015). Devi et al. (2013) and Bangroo et al. (2013) indicated the impact of natural forests, plantation and agro-forestry on soil properties and on C sequestration. Studies have indicated that macroaggregate associated C is more vulnerable to changes in land use than the C present in microaggregates (Puget et al., 2000). Saha et al. (2012)