Influences of leaf toughness and nitrogen content on in-stream processing and nutrient uptake by litter in a Waikato, New Zealand, pasture stream and streamside channels

The factors influencing in-stream disintegration and uptake of dissolved nutrients by leaves were investigated to improve the basis for selecting plants for riparian management. Leaves from five introduced and five New Zealand native plants were studied in 12 streamside channels and a natural stream. Fallen, air-dried leaves were tethered in packs attached to tiles in the channels and stream, and measured for mass loss, nitrogen and phosphorus uptake, toughness (by penetrometry), respiration rates, carbon : nitrogen (C:N) ratios, and macroinvertebrate communities after 7, 15, 28, 56, 126, and 226 days. Initial leaching over 2 days reduced mass by 8-32%, and microbial respiration could account for up to 23-93% (median = 43%) of dry mass (DM) lost over the first 56-126 days. First order mass loss rates (k, day(-1)) varied 14-fold, from -0.0028 day(-1) for Knightia excelsa to -0.039 day(-1) for Juglans nigra, and were correlated most strongly with leaf toughness after 7 days in stream water (r = -0.84). Leaf packs in Mangaotama Stream, that were more rapidly colonised by invertebrates and exposed to frequent now disturbances, had 3- to 5-fold higher mass loss rates than those in the streamside channels. The 12-fold variation amongst leaf species in mean uptake rate of dissolved reactive phosphorus (DRP), and the 6-fold variation in mean nitrate uptake were correlated with k (r = -0.78 and -0.92 for DRP and NO3-N, respectively), respiration after 7 days (r = 0.80 and 0.84) and toughness after 2 days of leaching (r = -0.77 and -0.82). These findings improve the basis for selecting riparian plant species to optimise in-stream nutrient retention and invertebrate food supply in streams with different abilities to retain litter fall. For example, riparian plants with soft, N-rich, leaves are recommended to enhance the detrital energy base of streams where frequent spates flush leaf litter down stream, whereas a wider variety of plants and leaf types will be suitable for riparian planting for this purpose along highly retentive streams.