Rhizosphere spatial distribution: Plant root proliferation into patches of varying quality

Loecke, T.D. and G.P. Robertson

Presented at the ASM at Estes Park (2006-09-01 to 2012-09-23 )

Soil nutrients, microorganisms, and plant roots are non-uniformly distributed in most ecosystems. The rhizosphere, soil in direct contact with plant roots, sponsors a wider variety of microorganisms and higher rates of nutrient cycling than does the bulk soil. Understanding the spatial distribution of plant roots and soil nutrients is thus important for developing mechanistic interpretations of soil process rates. Plant roots are known to selectively proliferate into patches of high resource availability; however, less is known of plant root avoidance of low resource patches. Hence a selectively distributed rhizosphere may exert greater influence on soil processes in resource rich patches than in resource poor patches. We conducted two experiments examining root growth selectivity in response to the relative quality of resource patches. In the greenhouse, we planted Avena sativa into sand with a choice of two types of resource patches located in paired-opposite quarters of a 3L container. Four types of resource patches were paired for a total of six treatment combinations. The resource patches varied in N mineralization rate with patches of Trifolium pratense shoots mineralizing rapidly, T. pratense roots and green A. sativa leaves mineralizing at intermediate rates, and senesced A. sativa shoots mineralizing least. Twenty-eight days after seedlings were transplanted into containers the root length distribution varied with patch quality contrast. The highest patch quality contrast (T. pratense shoots versus senesced A. sativa shoots) resulted in 89% of total root length found in the high quality patches. Minimal contrast between patch qualities resulted in uniform root distribution. In an N fertilizer gradient experiment conducted on the KBS-LTER site, we compared Zea mays root proliferation in soil amended with T. pratense shoots against the background N fertilizer gradient. Root proliferation into the amended patch was 4 times greater than into unamended soil when no N fertilizer was applied, whereas at high N application rates root growth was similar in the amended and unamended soil. Root proliferation and aboveground biomass responded similarly to the N fertilizer gradient in that both plant responses were no longer sensitive to N availability at the same application level. We conclude that the potential for rhizosphere influences on patchy soil processes varies with contrast between patch qualities and overall nutrient availability.

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