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?