A deeper look at the relationship between root carbon pools and the vertical distribution of the soil carbon pool

Ranae Dietzel,Matt Liebman,Sotirios Archontoulis

doi:10.5194/soil-3-139-2017

Abstract

Abstract. Plant root material makes a substantial contribution to the soil organic carbon (C) pool, but this contribution is disproportionate below 20 cm where 30 % of root mass and 50 % of soil organic C is found. Root carbon inputs changed drastically when native perennial plant systems were shifted to cultivated annual plant systems. We used the reconstruction of a native prairie and a continuous maize field to examine both the relationship between root carbon and soil carbon and the fundamental rooting system differences between the vegetation under which the soils developed versus the vegetation under which the soils continue to change. In all treatments we found that root C : N ratios increased with depth, and this plays a role in why an unexpectedly large proportion of soil organic C is found below 20 cm. Measured root C : N ratios and turnover times along with modeled root turnover dynamics showed that in the historical shift from prairie to maize, a large, structural-tissue-dominated root C pool with slow turnover concentrated at shallow depths was replaced by a small, nonstructural-tissue-dominated root C pool with fast turnover evenly distributed in the soil profile. These differences in rooting systems suggest that while prairie roots contribute more C to the soil than maize at shallow depths, maize may contribute more C to soil C stocks than prairies at deeper depths.

Highlights

Prairie-formed Mollisols support some of the world’s most productive agriculture, but declines in levels of soil organic matter threaten the reliability of this production
We distinguish between a root C pool defined as C found in any material that can still be visually identified as a root and a soil organic C pool defined as the rest of the soil organic C
Some studies suggest that root C pool size and soil organic C pool size have a direct relationship and that most soil organic matter is derived from roots (Balesdent and Balabane, 1996; Rasse et al, 2005; Kong and Six, 2010)

Summary

Introduction

Prairie-formed Mollisols support some of the world’s most productive agriculture, but declines in levels of soil organic matter threaten the reliability of this production. Some studies suggest that root C pool size and soil organic C pool size have a direct relationship and that most soil organic matter is derived from roots (Balesdent and Balabane, 1996; Rasse et al, 2005; Kong and Six, 2010). This would mean that a change in root inputs, such as that engendered by switching from annual to perennial systems, would have a direct impact on soil organic matter even deep into the soil profile. Few direct comparisons of annual and perennial rooting systems have been made, and our understanding of soil C dynamics decreases as soil depth increases

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Discussion

Conclusion