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Title: ROOT-DERIVED CARBON AND THE FORMATION AND STABILIZATION OF AGGREGATES

Author
item GALE, W - IOWA STATE UNIVERSITY
item Cambardella, Cynthia
item BAILEY, T - IOWA STATE UNIVERSITY

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: The processes that occur within the soil are complex and often overlooked. Organic matter that is incorporated into the soil begins to undergo decay processes that lead to increases in different forms of organic matter within the soil. These decay products are necessary for the development of stable soil aggregates and attributes of the soil that are considered important to a healthy soil resource. There are sand-sized particles of organic matter that have been found to play a large role in soil processes; however, there is little understanding of how these are formed and the role of the plant root system in their formation and change over time. Our research project was designed to answer questions about the role of these particles of organic matter on soil aggregate development. We found that many of these large aggregates formed next to the root throughout the growing season due to the presence of sticky chemical compounds produced by ythe decomposition of organic matter by soil microbes. The biological activity within the soil varies throughout the year depending upon the availability of organic material from plant roots. These results help to develop a clearer picture of the biological and chemical dynamics that are ongoing within the soil and how we can promote better soil. This information can help producers begin to understand the type of responses they often observe within the soil in response to the tillage practices and crop rotations they select.

Technical Abstract: Particulate organic matter (POM) has been related to the stability of soil aggregates, but the mechanisms driving this relationship are not well understood. The objective of this study was to quantify the relationship between inputs of C into the POM fraction and the formation and stabilization of soil aggregates during the decomposition of in situ **14C-labeled roots. We sampled the experiment 5 times at 3 month intervals for 1 year. Two pretreatments (capillary wetted and slaked) were applied to the soil prior to isolation of aggregates with wet sieving. We characterized the unstable aggregate fraction by comparison of the two pretreatments. Macroaggregates contained large quantities of new intraaggregate POM (iPOM) C on day 0, but slaking disrupted these unstable aggregates and resulted in the release of this new-C into the free and released POM (frPOM) pool. The amount of new frPOM-C decreased with time until d 180 while the amount of new iPOM-C inside microaggregates increased. The data suggest that macroaggregates form around cores of new root-derived POM during plant growth; many of these newly-formed aggregates are unstable on d 0; the stability of these newly-formed macroaggregates reaches a maximum at d 180; and, microaggregates that are inside the newly-formed macroaggregates contain large amounts of new POM C at d 180. These data support the hypothesis that microaggregates form within existing macroaggregates and provide evidence that aggregate formation and stabilization are directly related to the dynamics of POM C in the soil.