Decomposition of maize leaves and grasses in restored agricultural streams

Abstract

Headwater streams draining row-crop agriculture receive allochthonous inputs of maize detritus and grasses, but organic matter (OM) processing is not well studied in agricultural streams. Agricultural streams in the midwestern USA have incised, trapezoidal channels that retain less particulate OM than forested streams. The 2-stage ditch is a restoration strategy in which small floodplains are constructed and connected to stream channels to increase channel stability and decrease erosion. Microbial decomposition may be higher on restored floodplains because water residence times are longer than on the steep banks of trapezoidal streams. We examined decomposition of maize leaves (Zea mays), native rice cutgrass (Leersia oryzoides), and invasive reed canary grass (Phalaris arundinacea) in 4 restored streams. We measured breakdown rates in the main channel of upstream control reaches (incised, trapezoidal channel), the main channel of downstream treatment reaches (restored with constructed floodplains), steep control banks, and treatment floodplains. OM decomposed faster in channels than on banks and floodplains, and maize decomposed faster (stream k = 0.0160/d, riparian k = 0.0040/d) than rice cutgrass (stream k = 0.0065/d, riparian k = 0.0018/d) and reed canary grass (stream k = 0.0036/d, riparian k = 0.0014/d) probably because lignin and N content differed. Breakdown rates varied among streams because of differences in shredder density (primarily Isopoda: Lirceus and Caecidotea) and water temperature. Floodplain restoration did not affect breakdown rates. Floodplains of 3 streams were inundated longer than steep banks in upstream control reaches, but breakdown rate and inundation duration were not related. However, OM must be retained within the stream to be available for decomposition. Thus, the floodplains may promote the retention of OM, and ultimately, incorporation of maize and grasses into headwater-stream food webs.