GDNDC (v2.0): Modelling long-term soil acidification of cropland under different fertilization scenarios

Soil pH is one of the most important properties for soil health, affecting the microbial activities, aggregate structure, nutrient availability and soil toxicity. For croplands in China, intensive application of ammonium-based fertilizer, as well as increased rate of nitrogen deposition, are inducing significant soil acidification in the long term and therefore threating agricultural sustainability. However, in almost all process-based biogeochemical models, soil pH is used as model input (constant) but its dynamic (especially at decadal scale) has not been simulated properly. In this study, we developed the new soil pH module in GDNDC (Gridded version of DeNitrification and DeComposition model) model to simulate the evolution of soil acidification processes within 0-40cm depth and its effect on crop growth. Using charge balance as the principle, different equations based on the chemical equilibrium between H⁺, NH₄⁺, NO₃^-, Al³⁺, base cations (e.g. Mg²⁺, Ca²⁺ and K⁺), organic anion (Org^-) and CO₂ were integrated into the new model and then numerically solved at daily step. Over 20-year field observations (e.g. soil nitrogen content, soil pH, crop yield, etc) under different fertilization scenarios (including non-fertilizer, inorganic NPK only, organic manure only, and inorganic NPK + organic manure) from both Qiyang and Jinxian sites in China were used to validate the accuracy of model’s prediction. By comparing the model outputs with field measurements, we found the GDNDC (v2.0) could effectively capture the unique trend of soil pH evolution under different fertilization scenarios at decadal scale, for example, the accelerated soil acidification under NPK and the buffering effect of organic manure. The difference of crops yield under different fertilization scenarios was also predicted precisely. As a result, our model has the capacity to simulate the dynamic of soil pH under various fertilization schemes and it can make a great contribution to long-term policy making on improved fertilization for agricultural sustainability.