Nitrogen (N) and phosphorus (P) are essential nutrients for agriculture and are inextricably linked with each other in living things, including crops. Inputs of N and P have helped China’s agriculture to maintain high crop productivity but also caused damaging eutrophication in aquatic ecosystems. Several studies have addressed the pathways and magnitude of these nutrient flows through agricultural systems. However, the coupling mechanisms of N and P flows are still not well-known. Quantifying this biogeochemical coupling can improve our understanding of the interactions between N and P, which is necessary to achieve a sustainable agriculture as well as to maintain ecosystem services. In this study, we evaluated the temporal dynamics of N and P cycling in China’s agricultural system with a mass balance approach for the period of 1980-2015. Molar N to P (N:P) ratios were quantified to show changes of N and P coupling over time. Agricultural and food data sources mainly involved national and provincial statistics while N- and P-associated parameters came from the literature.
Results/Conclusions
Both total N and P inflows of China’s agricultural system increased from 20 to 47 Mt-N/yr and 2 to 9 Mt-P/yr, respectively, with an N:P ratio of 11-19 (molar). Considerable imbalances existed between agronomic N and P inputs and harvested crops, with N use efficiency gradually improving from 24% to 29% and P use efficiency decreasing from 56% to 40% during 1980-2015. Mineral fertilizer was the dominant contributor to N and P inputs, of which the N:P ratio decreased from 17 to 10 over time. However, the N:P ratio of crops did not show significant changes, always being maintained around 8-9. In 2015, 3.3 Mt-N and 0.4 Mt-P was transferred to surface water and groundwater (N:P ~17 (molar)), consistent with a P-limited or N/P co-limited state for aquatic primary production. In contrast, disproportionate accumulation of P relative to N (N:P ~4 (molar)) in agricultural soils indicates the formation of a more N-limited status in soils. This contrast may be caused by the overapplication of P fertilizers and lower mobility of P in soils. These shifts may induce dramatic alterations in structure and function of ecosystems by shifting the N:P stoichiometry of nutrient supplies to soils and waters.