Effect of management on soil organic matter distribution near intra-aggregate pores as determined by µCT images

Michelle Quigley, Alexandra Kravchenko, and Mark Rivers
Department of Plant, Soil and Microbial Sciences, Michigan State University; Department of Geophysical Sciences and Center for Advanced Radiation Sources, The University of Chicago Chicago, IL.

Presented at the All Scientist Meeting (2015-04-15 to 2015-04-16 )

Soil organic matter (SOM) is an important soil quality that plays a major role in agricultural sustainability. SOM is known to positively affect soil aggregation, soil cation exchange, soil water holding capacity, and soil drainage in agricultural soils. In addition, SOM is the major sink of atmospheric CO2 in terrestrial settings. Agricultural management effects the amount of SOM in agricultural soils. Tillage is one of the main influences on SOM. Another major component influencing SOM is duration, variety, and amount of live vegetation. Kellogg Biological Station Long Term Ecological Research (KBS-LTER) provides a unique long term agricultural management site to compare the effect of tillage and vegetation on SOM. For this study, conventional tillage (T1), organic (T4), and primary succession (T7) managements were used. These managements were chosen as they represent three levels of tillage and vegetation management. T1 uses tillage and leaves fields exposed without cover for long periods of time, which allows more SOM decomposition by microorganisms to take place. Therefore, T1 has the lowest capacity of SOM storage. Management that utilized cover crops as part of the rotation (T4) uses cover year round, but still uses tillage, resulting in greater SOM storage capacity than T1. Primary succession (T7) contains the amount of SOM believe to be near or at maximum storage capacity of a soil. A lot of protection and storage of SOM is believed to take place within soil aggregates. However, detailed understanding of the mechanisms driving the intra-aggregate protection is still lacking. It is generally assumed that pore structure within aggregates is important for SOM storage as pores determine access to carbon sources for microorganisms. Different pore size distributions effect how microorganisms can travel through an aggregate and, therefore, access SOM sources. This, in turn, determines utilization and possibly distribution of SOM within a soil aggregate. Most techniques for soil analyses are destructive, however, the advent of computed microtomography (µCT) has allowed for in situ non-destructive analysis of soil aggregates that allows visualization of intact intra-aggregate pore structure and its features. Images obtained from µCT are 3D gray scale images in which the gray scale values (GVs) correlate to structural components of the studied material. For soil samples, the important components influencing the image GVs patterns are: differences in mineralogy, distribution of solid/void space, density, and presence and amount of SOM. Because SOM has a lower density than the mineral phase, it typically has lower GVs on the images. Our preliminary results indicated that intra-aggregate soil carbon is significantly correlated with image GVs. Here we would like to build on this relationship to explore spatial patterns in the GVs of intra-aggregate solid material in relation to positions and characteristics of soil pores. We assume that the GV patterns will serve as an indirect indicator of SOM patterns. The study objective is to examine the GV distribution around pores within aggregates from three different management types (T1, T4, and T7). Soil aggregates (n=32) had pores defined in the images using indicator kriging. Voxels at 13-78 μm, 78-143 μm and 143-205 μm away from defined pores were then analyzed for average GVs in 200 × 200 × 200 sections (n=96). Average GVs were compared to the average GVs of the solid material of the entire image. Results show that GVs tend to be lower nearer the pores, gets lighter at 13-78 μm and even lighter at 143-205 μm for all treatments. This is either from less SOM accumulation farther from the pores and/or less micropores farther from the image identified pores. More variation occurs in T4 and T7 aggregates than T1, possibly due to the more uniform mixing due to tillage in T1 verses stabilization due to plant root activity in T4 and T7.