1 Introduction
The conversion of grasslands to cultivated croplands is a common occurrence in the agro-pastoral ecotone of northern China, which covers 6.2×105 km2 and is one of the major factors affecting the biodiversity and functioning of grassland ecosystems (Yang et al. 2015). Intensive farming have led to severe land degradation and have profound effects on soil physicochemical properties (Don et al. 2011; van der Gast et al. 2011; Wang et al. 2011; Kocyigit et al. 2012), nutrient turnover and microbial communities in soils (Lange et al. 2015), especially in topsoil. Most of previous studies investigating how grassland conversion affects soil properties change mainly focused on the top 30-cm depth (Poeplau et al. 2011), which is the depth recommended by the IPCC (2003). Nonetheless, these soil parameters were often studied separately and their combined effects to soil microbial community after grassland conversion remain unclear (Ying et al, 2013).
Soil microbes are essential to the maintenance of a large number of important ecosystem processes (Poeplau et al. 2011; Ying et al. 2013; Lange et al. 2015). Grassland cultivation replaces the original plant communities, which may exert great influence on soil microbes via regulating allocation of belowground photosynthates or root exudate (Ying et al. 2013). Plant harvest leads to lower input of above- and belowground biomass and plant cover (Wang et al. 2011; Poeplau et al. 2013) with vary in litter and their chemical composition (Hamer et al. 2008), which could decline carbon and nutrient accumulate in soil (Poeplau et al. 2011), change soil texture and increase wind and water erosion (Six et al. 2000). These changes in plant and soil will alter soil microbial growth, activity and community structure. Moreover, grassland cultivation influences soil physic-chemical properties (Jangid et al. 2011; Le Guillou et al. 2012; Baumann et al. 2013), leading to decreasing in content of soil nutrient and altering in soil pH (Don et al. 2011; van der Gast et al. 2011; Wang et al. 2011; Kocyigit et al. 2012). Consequently, microbial activities may be constrained because the reduced availability of soil substrates and the changed pH in soils (Jangid et al. 2011; Ma et al. 2015). Furthermore, grassland cultivation results in the breakdown of soil aggregates, thereby exposing protected soil organic carbon (SOC) to microbial decomposition and altering soil moisture and aeration (Kocyigit et al. 2012), which may be indirect effected the soil microbial community (Le Guillou et al. 2012; Ma et al. 2015). All of these edaphic factors were altered after grassland converted to cropland, but it is unclear which factors have the dominant influence on soil microbial communities.
In this study, we investigated the effect of converting grassland to cropland (30-60 year conversion) on microbial community and biomass by using a paired design with a soil sampling depth of 0-30 cm in agro-pastoral ecotone of Northeast China. We addressed the following three questions: (i) Do microbial community and biomass vary at the topsoil after grassland conversion to cropland? (ii) Can we explain any variation in the soil microbes based on variations in the soil physicochemical properties? (iii) What is the determine factors in affecting the soil microbial community?