Abstract
Soil microbes are key to nutrient cycling and soil formation, yet the impact of soil properties on microbe biomass remains unclear. Using 240 soil cores of 0–15 cm depth, taken at random points across six cattle-grazed pastures on an undulating landscape, we evaluated the biomass of microbes in soil as affected by naturally occurring variation in soil organic carbon (SOC), clay content, and local topography. The study pastures varied in historic land-use for crops or forage seeding. SOC was found to be greater in topographically low areas. In contrast, clay content was not related to topography, and clay deposition possibly varies with glaciation legacy. Microbial biomass carbon (MBC) was correlated positively with SOC, increasing from 700 mg kg−1 MBC at 25 g kg−1 SOC to 2240 mg kg−1 MBC at 90 g kg−1 SOC. Most likely, SOC promotes MBC through the release of water-soluble organic carbon. However, the response of MBC to clay content was negative, decreasing from 1340 mg kg−1 MBC at 5% clay to 880 mg kg−1 MBC at 30% clay. Small voids in association with clay particles likely restrict the access of microbes to SOC. The relationship between SOC and MBC illustrates the important role of SOC for soil function, in terms of nutrient availability and development of soil structure via the contribution of microbes. Lastly, there was considerable spatial variability in MBC across the 65 ha site, highlighting the importance of land-use histories and gradients in environmental variables, to determine the biomass of microbes in soil.
Highlights
Data on soil biota is limited markedly compared to aboveground systems, and to build our understanding of functional relationships will require more information about soil organisms and their effects on ecosystem services [1]
Microbial biomass carbon (MBC) corresponds to only 1–3% [5] of soil organic carbon (SOC), microbes play a central role in aggregate formation during soil structural development [6]
SOC was higher in topographic depressions in the pastures of the prairie-pothole landscape at Brookdale Research Farm (BRF)
Summary
Data on soil biota is limited markedly compared to aboveground systems, and to build our understanding of functional relationships will require more information about soil organisms and their effects on ecosystem services [1]. Microbial biomass carbon (MBC) can be determined by measuring the flush of carbon released from soil, in response to chloroform fumigation, and it corresponds to the entirety of microorganisms therein [2]. MBC corresponds to only 1–3% [5] of soil organic carbon (SOC), microbes play a central role in aggregate formation during soil structural development [6]. Microaggregates of less than 250 μm diameter are formed from clays and other minerals, sesquioxides, SOC, and microbes [7]. It is well-established that SOC comprises a passive fraction of long-term stability and an active fraction of swift turnover; the latter concentrated in the surface soil [8], while
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