Improving litter nutrients import/export modeling in a forested watershed through enhanced forest growth representation

Abstract

Nutrient release from litter significantly contributes to the global nutrient cycle and the nutritional status of river systems. Simulating the diffuse nutrients from litterfall at the watershed scale is extremely uncertain, owing partly to a lack of measurements covering a variety of features and underlying terrain and insufficient modeling methods. We quantified annual litter production (ALP) simulation at the basin and sub-basin scales by improving the Soil and Water Assessment Tool (SWAT) for forest nutrient cycling, including the forest growth module, tree biomass accumulation, and parametrization scheme. The modified model was comprehensively constrained and calibrated for tree biomass, ALP, monthly dynamics of nutrient mineralization, monthly stream flow, sediment yield, nitrate nitrogen (NO3-N), dissolved organic nitrogen (DON), and total phosphorus (TP) in the Tumen River Basin (TRB). Results showed that the modified SWAT increases the ALP simulation from 132.13 ± 21.73 g/m2 to 351.88 ± 18.28 g/m2 and the relative error of spatial ALP decreased from −50.65 %∼66.59 % to −11.18 %∼27.62 %. The accuracy of nitrate improved notably (coefficient of determination (R2) increased by 0.2–0.25, Nash-Sutcliffe Efficiency (NSE) increased by 0.09–0.19), and that of runoff simulation improved modestly (R2 increased by 0.05 and NSE increased by 0.08 for validation period). A comparison with the original simulation of nutrient dynamics also indicated that the modified SWAT model could reasonably identify the dynamics and interactions between litter nutrients and climate factors. Increased mineralization caused by warming and snow melting in spring is the main factor contributing to nitrate accumulation and soluble phosphorus (soluble P) loss in the soil. Improved forest nutrient cycles are expected to help enhance future applications of SWAT for investigating the biogeochemical consequences of climate change.