Land Use Types Influence Diffuse N and P Entering a River in an Agricultural Watershed: Evidence from Compound-Specific Stable Isotope

Abstract

The increase in sediment and nutrient losses to freshwater has resulted in ecosystem
degradation and non-point source (NPS) pollution worldwide. Using stable isotopes
and direct monitoring techniques to quantify the losses has been effective, but it has
received little scientific attention. This study identified and quantified the sources of
sediment, particulate nitrogen (PN), and phosphorus (PP) entering rivers from land use
types, and clarified their influencing factors. Compound-specific stable isotope (CSSI),
using the carbon-13 (δ13C) isotope signature of fatty acids, was applied in June and
September 2019. Four sediment sources: eucalyptus, road, sugarcane, and stream
channel, were identified in the watershed. The δ13C of the sediment mixtures was -
16.80 and 17.01 ‰ in June and September, respectively. The soil proportions from
different sources in June and September were in the order of road (37.1%) >sugarcane
(32.4%) >stream channel (27.5%) >eucalyptus (3.1%), and sugarcane (40.5%) >stream
channel (29.2%) >road (27.6%) >eucalyptus (2.7%), respectively. Ground cover, soil
organic matter (SOM), bulk density, and slope gradient significantly (P <0.05)
influenced sediment, PN, and PP entering the river. The combined application of CSSI
and Bayonet monitoring techniques could provide a better understanding of sediment
and nutrient redistribution in agricultural watersheds.