Breakdown rates and associated nutrient cycling vary between novel crop-derived and natural riparian detritus in aquatic agroecosystems

Authors: Taylor, Jason; Lizotte, Richard; Testa Iii, Sam

Submitted to: Hydrobiologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2018
Publication Date: 10/8/2018
Citation: Taylor, J.M., Lizotte Jr, R.E., Testa III, S. 2018. Breakdown rates and associated nutrient cycling vary between novel crop-derived and natural riparian detritus in aquatic agroecosystems. Hydrobiologia. 827:211–224. https://doi.org/10.1007/s10750-018-3766-x.
DOI: https://doi.org/10.1007/s10750-018-3766-x

Interpretive Summary: Oxbow lakes and bayous are common sources of freshwater within the Lower Mississippi River Basin (LMRB) and floodplains of large low gradient rivers throughout the world. Changes in the type of leaf matter combined with agricultural pollutants entering these systems may significantly alter their function. This study compared the breakdown of leaves from crop residues (corn, cotton, and soybean) with that of native riparian species (willow oak, sycamore, and cottonwood) to assess how landscape-scale shifts in organic matter sources may influence small lakes within the LMRB. Central findings of this study were: 1) All crop residues were highly labile and decomposed more rapidly than leaves from native riparian species; and 2) differences in tissue chemistry may influence breakdown rates. These results support establishing more extensive forested areas to not only intercept and process excess nutrients in agricultural runoff, but also modulate the timing, quality, and composition of leaf inputs. This will make leaf-based food resources more available for dependent wildlife through the year and may reduce the magnitude and frequency of low oxygen events potentially associated with large pulses of labile crop material entering LMRB lakes.

Technical Abstract: Freshwater ecosystem function within agricultural landscapes may be altered by potential differences in storage and processing of organic matter (OM) detritus entering freshwater habitats. We compared decomposition rates of three crop residues; maize, cotton and soybean, with three native riparian species: willow oak, American sycamore and cottonwood from bayous located within the Lower Mississippi River Basin. We observed an OM processing continuum among the six species with the highest and lowest breakdown rates represented by crop residues (¯X k d-1 = 0.00695-0.01128) and riparian species (¯X k d-1 = 0.002914-0.00537), respectively. OM nutrient content varied widely across the six species. OM C:N ratios declined with time for all species except cotton. In contrast, cotton C:P ratios increased, maize decreased and cottonwood ratios did not change during the decomposition process while soybean, sycamore, and willow oak C:P ratios all initially increased before declining throughout the rest of the decomposition process. Decomposition rates was negatively related to increasing C:N ratios of OM at the end of the study but the slope of the relationship was significantly greater for riparian compared to crop species. Historic shifts in landscape-scale OM sources from diverse bottomland tree assemblages to crop residues has likely altered LMRB bayou and oxbow ecosystems by shifting both the timing and lability of OM pulses. This potentially affects these important habitats by increasing ecosystem respiration during low-flow conditions and decreasing long-term storage of OM, an important habitat and food resource for aquatic communities.