Understanding the characteristics of riparian zones in low relief, sandy catchments that affect their nutrient removal potential

• Published in: Agriculture, ecosystems & environment ecosystems & environment, 2018-04, Vol.258, p.182-196
• Main Authors: O’Toole, P., Chambers, J.M., Bell, R.W.
• Format: Article
• Language: English
• Subjects: Agricultural land; Catchments; Confining; Denitrification; Dissolved organic carbon; Groundwater; Hydrology; Litter fall; Nutrient dynamics; Nutrient removal; Nutrients; Piezometers; Riparian environments; Riparian land; Riparian vegetation; Root zone; Sand; Soil; Soil dynamics; Soil improvement; Soil layers; Soil porosity; Soil profiles; Soil properties; Soils; Streams; Vegetation; Watersheds; Waterways
• ISSN: 0167-8809; 1873-2305
• DOI: 10.1016/j.agee.2018.02.020

•Little phosphorus retained by riparian zones in sandy, low relief catchments.•Hydrology and nutrient dynamics different in these riparian zones.•Riparian zone processed nutrient-enriched stream water.•Organic matter from litterfall improved nutrient retention by riparian soils. Riparian zones are considered to improve stream condition by providing a buffer between waterways and agricultural land that can intercept nutrients, but is their efficacy universal? This paper develops a conceptual model comparing the interactions of slope, soil, hydrology, vegetation and nutrient dynamics between 1) the riparian zone of an intermittent stream in a low-relief (1.6%) catchment with deep sands of low reactivity (Bingham Creek) and 2) a perennial stream in a sloped (10%) catchment with reactive soils over an impermeable layer (Lennard Brook), with a view to compare and contrast their riparian functionality. This study compared the attributes of groundwater (three rows of nested piezometers) (0.5 m,1.5 m and 2.5 m depth), stream, soil and vegetation across a transect from the stream, through the riparian zone to agricultural paddocks. In the low-relief catchment, water did not flow through the riparian zone as in a sloped catchment. Porous soils, together with a lack of slope or a confining layer meant water oscillated vertically through the soil profile over the season, with minimal horizontal movement and limited interaction with the active root zone of riparian vegetation; the intermittent stream discharged P-rich water into the riparian zone during the first flush of winter rains. The highly unreactive sands resulted in trivial P or C uptake resulting in high dissolved concentrations in adjacent streams (0.6–0.9 vs 0.001–0.002 mg/L TP, 58 vs 3 mg/L DOC for flat vs sloped catchments respectively). The high DOC in slow-moving groundwater resulted in highly reducing conditions, promoting P solubility and potentially denitrification. Litterfall from vegetation marginally improved riparian soils with better P retention relative to the adjacent paddock (3620–268 kg TP/ha storage) and reduced FRP in the groundwater relative to the stream (27 vs 80%). The conceptual model developed highlights an alternative functionality of riparian zones for low-relief catchments that challenges the assumption of riparian efficacy.