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

The factors influencing in-stream disintegration and uptake of dissolved nu trients by leaves were investigated to improve the basis for selecting plan ts for riparian management. Leaves from five introduced and five New Zealan d 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 up take, 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 a fter 7 days in stream water (r = -0.84). Leaf packs in Mangaotama Stream, t hat were more rapidly colonised by invertebrates and exposed to frequent no w disturbances, had 3- to 5-fold higher mass loss rates than those in the s treamside channels. The 12-fold variation amongst leaf species in mean upta ke 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 an d NO3-N, respectively), respiration after 7 days (r = 0.80 and 0.84) and to ughness after 2 days of leaching (r = -0.77 and -0.82). These findings impr ove the basis for selecting riparian plant species to optimise in-stream nu trient retention and invertebrate food supply in streams with different abi lities to retain litter fall. For example, riparian plants with soft, N-ric h, leaves are recommended to enhance the detrital energy base of streams wh ere frequent spates flush leaf litter down stream, whereas a wider variety of plants and leaf types will be suitable for riparian planting for this pu rpose along highly retentive streams.