Loecke, T. D. 2007. Soil resource heterogeneity and ecosystem processes: Effects of litter aggregation on soil microbial processes and plant root foraging. Ph.D. Dissertation, Michigan State University, East Lansing, Michigan, USA.

Citable PDF link: https://lter.kbs.msu.edu/pub/2307

Resource spatial distribution alone can alter ecosystem process rates. Soil resource aggregation within the scale of individual plants can potentially affect primary productivity, plant C allocation, plant N acquisition, decomposition, net N mineralization, N2O emissions, and ecosystem N retention. To understand how plant litter aggregation affects these processes I distributed Trifolium pratensis litter in soil across an aggregation gradient from uniformly distributed to highly aggregated. I examined the effects of this aggregation gradient on decomposition rates and N2O emissions with two laboratory studies and plant growth and N cycling with two field experiments.

Results show that litter aggregation in soil delayed decomposition for 5 to 7 days and that this delay was likely caused by insufficient O2 supply to the interior of the litter aggregates. In contrast, aggregated litter stimulated emissions of the greenhouse gas N2O 7-fold compared to uniformly distributed litter. Elevated N2O emissions in response to litter aggregation were found regardless if the litter was finely ground or chopped into 5 mm pieces.

Plant root systems can respond to litter aggregation by foraging into resource-rich microsites; however, the degree to which plants benefit from root foraging into microsites of varying quality is largely unknown. I examined whether root foraging into microsites of varying quality depended on plant growth. I found that Avena sativa L. root foraging was positively correlated with growth in response to pairwise choices of contrasting microsite qualities. In contrast, root foraging by Bromus inermis L. was not related to plant growth response. In addition, I found that plant N status plays an important role in regulating Zea mays L. root foraging under field conditions. These two results suggest that root foraging is only an important mechanism for plant N acquisition under heterogeneous conditions where N is limiting plant productivity.

To better understand the effects of litter aggregation on plant growth and N cycling I distributed 15N-labeled litter across an aggregation gradient and followed the fate of the litter-N into plant and soil N pools. Under N-limited conditions maize was 14% more productive in response to aggregated than uniformly distributed T. pratensis litter. In contrast, Secale cereale litter aggregation did not affect maize growth. Litter distribution did not affect root to shoot ratio; however, total belowground C allocation appeared to be greater in response to uniformly distributed than to aggregated T. pratensis litter. Plant N acquisition was greater in response to aggregated than uniformly distributed litter. Litter aggregation also increased litter-derived N mineralization by 20%, shoot N by 18%, and root N by 33% relative to uniformly distributed litter. I suggest that the spatial coupling of roots and litter aggregates is an important factor regulating C and N cycling in agricultural system with heterogeneous resource distributions and where N is limiting plant productivity.

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