Loecke, T.D. and G.P. Robertson
Presented at the All Scientist Poster Reception (2006-05-09 )
Plant root systems are often said to be efficient integrators of soil resource heterogeneity within their reach; however, this view may be too simplistic if processes controlling resource supply rate are altered by spatial heterogeneity. The spatial aggregation of labile organic matter in soil has potential to influence several interacting ecosystem processes including decomposition, nutrient mineralization, plant nutrient acquisition, and primary productivity. To investigate the effects of soil resource aggregation on C and N cycling, we experimentally manipulated the spatial distribution Trifolium pretense litter in soil into aggregation gradients. Our initial experiment followed seedling growth response of seven herbaceous plant species to an aggregation gradient with five levels: 2, 8, 32, and 128 patches or uniformly. The litter aggregation gradient influenced the aboveground growth of the two species with the greatest absolute growth rates. We also monitored the decomposition rate of the litter along this gradient in the absence of seedlings, and found that aggregating litter into a few large patches delayed the maximum decomposition rate by six days relative to the uniform distribution. We designed a subsequent field experiment to test the hypothesis that the synchrony between litter N mineralization rate and plant N uptake plays an important role in plant growth responses to soil resource spatial aggregation. We measured N mineralization and plant N uptake in response to 60g of 15N-labeled Trifolium pretense litter (21 g of N kg-1) distributed into 8 or 32 patches or uniformly surrounding individual Zea mays plants. Aboveground biomass was 14% greater (p<0.02) in response to aggregated litter than to uniformly distributed litter. Soil inorganic N varied across the aggregation gradient early in the growing season with the highest concentration in the most aggregated, intermediate with intermediate aggregation, and the lowest soil inorganic N concentration from uniformly distributed litter. 15N budgeting methodologies help to interpret these results. Nutrient supply rates in soils at the scale of individual plants are naturally heterogeneous. This research illustrates the importance of this heterogeneity on ecosystem scale processes.
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