Brown pathways in green systems: source, habitat and nutrient effects on organic matter breakdown in Mississippi Delta agricultural bayous.

Author(s): Testa III, S.; Taylor, J.; Lizotte, R.; Dillard, K.

Excess nutrient runoff can impact agricultural water bodies but little is known about the role of agriculturally derived organic matter in mediating water quality impacts associated with nutrient enrichment in agricultural landscapes. We compared source, habitat and nutrient enrichment effects on breakdown rates of agricultural field (corn residue) and riparian (willow oak) organic matter in natural bayou ecosystems and stream mesocosms. Field data indicated that breakdown rates for corn were high (k = 0.0321 ± 0.0027 d-1) compared to willow oak (k = 0.0109 ± 0.0033 d-1) in bayous, and, for both species, more hydrologically dynamic inflow sites had significantly lower breakdown rates than lentic sites. Mesocosm results indicated corn residue breakdown rates did not increase with phosphorus (P) enrichment, increased with nitrogen (N) enrichment, but were highest when mesocosms were enriched with N and P (N+P). Willow Oak had much lower breakdown rates than corn residue in mesocosms, confirming species effects observed at field sites. The highest oak breakdown rates were observed in mesocosms enriched with N and P which were significantly higher than control or P enriched streams. Oak breakdown rates in N enriched mesocosms were also significantly higher than control streams but intermediate between P enriched and N+P enriched mesocosms. Respiration rates for corn residue were double that of willow oak and enrichment effects varied with time, but there was clear experimental evidence that N + P enrichment increased respiration for both species. Our results demonstrate that changes from riparian species to more labile crop residue sources of organic matter, combined with nutrient runoff, may impact agricultural water bodies by increasing microbial respiration associated with faster organic matter breakdown rates. However, reductions in nutrient runoff from agricultural areas, as well as increasing buffers of natural vegetation through agricultural best management practices, have the potential to reduce high inputs of crop residue and decrease microbial respiration rates associated with corn residue and nutrient enrichment in bayous of alluvial plain agroecosystems.

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