Keeping Nitrogen Use in China within the Planetary Boundary Using a Spatially Explicit Approach

Abstract

Nitrogen (N) supports food production, but its excess causes water pollution. We lack an understanding of the boundary of N for water quality while considering complex relationships between N inputs and in-stream N concentrations. Our knowledge is limited to regional reduction targets to secure food production. Here, we aim to derive a spatially explicit boundary of N inputs to rivers for surface water quality using a bottom-up approach and to explore ways to meet the derived N boundary while considering the associated impacts on both surface water quality and food production in China. We modified a multiscale nutrient modeling system simulating around 6.5 Tg of N inputs to rivers that are allowed for whole of China in 2012. Maximum allowed N inputs to rivers are higher for intensive food production regions and lower for highly urbanized regions. When fertilizer and manure use is reduced, 45-76% of the streams could meet the N water quality threshold under different scenarios. A comparison of "water quality first" and "food production first" scenarios indicates that trade-offs between water quality and food production exist in 2-8% of the streams, which may put 7-28% of crop production at stake. Our insights could support region-specific policies for improving water quality.

Keywords: food production; nitrogen; planetary boundary; spatially explicit boundary; water quality.

Figure 1

A graphical scheme of the bottom-up approach for calculating the regional nitrogen (N) boundary for inputs of N to rivers based on the surface water quality threshold. The descriptions of the parameters and variables between brackets refer to explanations of the associated equations in the Supporting Information. The first step is to quantify the gap of accumulated load (in-stream load, kton/year) based on the gap of in-stream N concentrations and N threshold by multiplying it with the water discharge for the grid cell. The second step is routing the calculated gap of accumulated N load along the river network to correct the upstream-to-downstream influence and thus quantify the actual required reduction of in-stream load (kton/year) from local grid cells. The third step is to quantify the required reduction of N inputs to rivers (kton/year) based on the required reduction of in-stream N load. The final step is to quantify the regional N boundary (kton/year) by using current N inputs to rivers minus the required reduction of N inputs to rivers. The associated quantifications of step 1 to step 4 are described in detail in Section S1.4.

Figure 2

Schematic illustration of the two reference scenarios including the Baseline for the year 2012 and Whole Food Chain (WFC) management scenarios based on the year 2012. WWTPs refer to wastewater treatment plants. (S1) refers to management strategy 1, i.e., increasing soil fertility; (S2) refers to management strategy 2, i.e., abandoning discharge of manure and increasing recycling of manure; (S3) refers to improved livestock manure management with low ammonia emission; and (S4) is to include new systems to recycle human excretion and food waste. The red arrow refers to the increase or decrease of N sources in WFC compared to the Baseline scenario.

Figure 3

Calculated maximum levels of N inputs to rivers (kton/year) as China’s share in planetary boundaries (PB) to ensure that N concentrations do not exceed the PB threshold (1 mg/L) for the Baseline-reqN and WFC-reqN scenarios: (A). N boundaries for sub-basins for the 2012 Baseline; (B). N boundaries for counties for the 2012 Baseline; (C). N boundaries for sub-basins for the Whole Food Chain (WFC) management scenario; and (D). N Boundaries for counties for the WFC scenario. “reqN” refers to scenarios that quantify the regional N boundary by applying bottom-up approach (Section 2). The study area of subbasins is presented in Figure S3.

Figure 4

Results for different scenarios. (A) The red bars show the percentage of polluted streams (above the threshold of 1 mg/L), and the orange line shows the number of people living in these polluted watersheds (in billion). The green bars show the required reductions in synthetic fertilizers and animal manure (%) for each scenario compared to the total synthetic fertilizer and animal manure of the Baseline or Whole Food Chain (WFC) scenarios. (B) N is required to maintain current yield levels (dashed lines) and N inputs to agricultural land (bars) under the Baseline and WFC and their associated bottom-up scenarios. Note: Blue and black dashed line refers to the required N to maintain the current crop yield under Baseline and WFC scenario. *Recycled N input includes N recycled from straw, livestock manure, and human excretions, which were corrected by synthetic fertilizer value and N deposition. **N fixation refers to biological N fixation. Baseline refers to the situation of N sources in 2012; WFC refers to Whole Food Chain nutrient management and recycling. Baseline-wq1st and WFC-wq1st refer to the scenarios that first ensure water quality without considering crop needs (details in Section 2). Baseline-food1st and WFC-food1st refer to the scenarios that first ensure crop needs and then reduce the extra fertilizer and manure for meeting the water quality threshold.

Figure 5

Surface water quality (in concentration of dissolved inorganic nitrogen, mg/L) under “food security first” (food1st) scenarios (a,c) and “water quality first” (wq1st) scenarios (b,d). Baseline refers to the baseline for the year 2012. WFC refers to Whole Food Chain management based on the 2012 scenario. Baseline-food1st and WFC-food1st refer to the scenarios that first ensure crop needs and then reduce the extra synthetic fertilizer and animal manure for meeting water quality threshold. Baseline-wq1st and WFC-wq1st refer to the scenarios that first ensure water quality without considering crop needs (details in Section 2). Figure S3 indicates the dessert areas, of which the interpretation of the presented concentrations should particularly consider the uncertainties of the hydrological outputs in these very dry areas.

Figure 6

Calculated maximum levels of N inputs to rivers (kton/year) as China’s share in a planetary boundary (PB) aimed at water quality levels not exceeding thresholds in the Baseline-reqN and WFC-reqN scenarios. The chosen thresholds are 0.5, 1, and 2.5 mg/L for DIN concentrations. Baseline refers to the situation in 2012; WFC refers to Whole Food Chain nutrient management and recycling. The “reqN” scenarios refer to scenarios that quantify the regional N boundaries by applying our bottom-up approach (Section 2).