Uncovering the spatio-temporal dynamics of crop-specific nutrient budgets in China.

Nitrogen (N) and phosphorus (P) are two critical nutrients for agroecosystems. In meeting food demands, human use of both nutrients has crossed planetary boundaries for sustainability. Further, there has been a dramatic shift in their relative inputs and outputs, which may generate strong N:P imbalances. Despite enormous efforts on agronomic N and P budgets, the spatio-temporal characteristics of different crop types in using nutrients are unknown as are patterns in the stoichiometric coupling of these nutrients. Thus, we analyzed the annual crop-specific N and P budgets and their stoichiometric relations for producing ten major crops at the provincial level of China during 2004-2018. Results show that, China has generally witnessed excessive N and P input over the past 15 years, with the N balance remaining stable while the P balance increasing by more than 170%, thus resulting in a decline in the N:P mass ratios from 10.9 in 2004 to 3.8 in 2018. Crop-aggregated nutrient use efficiency (NUE) of N has increased by 10% in these years while most crops have shown a decreasing trend of this indicator for P, which reduced NUE of P from 75% to 61% during this period. At the provincial level, the nutrient fluxes of Beijing and Shanghai have obviously declined, while the nutrient fluxes of provinces such as Xinjiang and Inner Mongolia have increased significantly. Although N management has made progress, P management should be further explored in the future due to eutrophication concerns. More importantly, N and P management strategies for sustainable agriculture in China should take account of not only the absolute nutrient use, but also their stoichiometric balance for different crops in different locations.