Negative global phosphorus budgets challenge sustainable intensification of grasslands

Abstract

Grasslands provide grass and fodder to sustain the growing need for ruminant meat and milk. Soil nutrients in grasslands are removed through withdrawal in these livestock products and through animal manure that originates from grasslands and is spread in croplands. This leads to loss of soil fertility, because globally most grasslands receive no mineral fertilizer. Here we show that phosphorus (P) inputs (mineral and organic) in global grasslands will have to increase more than fourfold in 2050 relative to 2005 to achieve an anticipated 80% increase in grass production (for milk and meat), while maintaining the soil P status. Combined with requirements for cropland, we estimate that mineral P fertilizer use must double by 2050 to sustain future crop and grassland production. Our findings point to the need to better understand the role of grasslands and their soil P status and their importance for global food security.

Figure 1. Phosphorus budget model for the grassland system.

Phosphorus flows within grassland and between the grassland and cropland systems. The grassland system comprises four compartments (grassland-based livestock population, grassland-based livestock manure, soil including weathering supplying P from soil minerals and grass) and six compartments outside the grassland boundaries (products, other uses of manure, erosion and atmospheric deposition, fertilizer (only in the intensive system) and non-grassland-based livestock manure). Next to the grassland system budget, we also consider the agronomic soil P budget, with the soil surface as boundary; grass P uptake (that is, the P withdrawal by grass harvest or grazing) is considered as output, and mineral fertilizer and manure (internal and external manure inputs) as inputs. We assume that grass P uptake equals livestock P intake. Thus, this approach ignores any P losses during mowing, transporting or stall-feeding of grass. P transfers between grasslands and croplands are represented by the flows (thick arrows) ‘Livestock feed' (imported P from cropland to grassland as livestock feed) and ‘Spreading in cropland' (exported P, the manure P that is deposited in grasslands, but is transferred to croplands for spreading). The bold, dashed arrow represents the P flow leaving the grassland systems via animal products, mainly meat and milk.

Figure 2. Agronomic soil P budget.

Historical trends of annual P application and P uptake in grassland systems (intensive and pastoral) for the period 1970–2005 in (a) Globe, (b) Africa, (c) Asia, (d) Eastern Europe, (e) Latin America, (f) North America, (g) Oceania and (h) Western Europe. These regions were also used in a previous study on residual P in cropland. Dashed and solid lines represent P application and P uptake, respectively. P application represents the P inputs from manure plus mineral fertilizer to the grassland soils and P uptake refers to grass P uptake.

Figure 3. Manure allocation framework.

The figure illustrates the manure allocation framework and the global data for 2005. All flows are shown in Tg of P per year. ‘Other uses' represents the use of manure for non-agricultural purposes such as fuel. ‘Grazing' and ‘Application grassland' represent the amount of manure deposited as animals graze and the amount that is spread as organic fertilizer in grasslands, respectively. Both are accounted as grassland soil P inputs. ‘Application cropland' is the amount of manure used as organic fertilizer in croplands.

Figure 4. Phosphorus transfers between grasslands and croplands.

Historical trends of annual P import to and export from grasslands (intensive and pastoral) as livestock feed and manure spread in croplands, respectively, for the period 1970-2005 in (a) Globe, (b) Africa, (c) Asia, (d) Eastern Europe, (e) Latin America, (f) North America, (g) Oceania and (h) Western Europe. Dashed and solid lines represent imported P (feed) and exported P (manure), respectively. Imported P stands for the feed produced in croplands that is consumed by livestock on grasslands, and exported P stands for the manure P that originates from grasslands (intensive and pastoral), but is transferred to croplands.

Figure 5. Trends of global annual P application and grass P uptake in grassland for the period 1970–2050; the 2050 target uptake was derived from the Rio+20 scenarios.

The uncertainty in the target 2050 uptake of 8.3 kg P per ha may amount to ±40% (based on the difference between the four Millennium Ecosystem Assessment (MEA) scenarios—shading). According to the model sensitivities, the variation in the simulated uptake as a result of variation of all the parameters for 2050 results in −26 to +17% (2.5 and 97.5 percentiles) around the median of 8.3 kg ha⁻¹ (dot shading). Markers and lines illustrate long-term historical data and simulation results, respectively. Circles and triangles refer to P application and P uptake rates, respectively. Dashed red and solid green lines refer to P application and P uptake rates, respectively. P application stands for P inputs into the soil including internal manure, external manure and fertilizer. P uptake stands for the grass P uptake. The R² value for calculated (model) versus observed (data) P uptake (1970–2005) is 0.65.