Closing Pakistan's Yield Gaps Through Nutrient Recycling

Abstract

Achieving food security will require closing yield gaps in many regions, including Pakistan. Although fertilizer subsidies have facilitated increased nitrogen (N) application rates, many staple crop yields have yet to reach their maximum potential. Considering that current animal manure and human excreta (bio-supply) recycling rates are low, there is substantial potential to increase the reuse of nutrients in bio-supply. We quantified 2010 crop N, phosphorus (P), and potassium (K) needs along with bio-supply nutrient availability for Pakistani districts, and compared these values to synthetic fertilizer use and costs. We found that synthetic fertilizer use combined with low bio-supply recycling resulted in a substantial gap between nutrient supply and P and K crop needs, which would cost 3 billion USD to fill with synthetic fertilizers. If all bio-supply was recycled, it could eliminate K synthetic fertilizer needs and decrease N synthetic fertilizer needs to 43% of what was purchased in 2010. Under a full recycling scenario, farmers would still require an additional 0.28 million tons of synthetic P fertilizers, costing 2.77 billion USD. However, it may not be prohibitively expensive to correct P deficiencies. Pakistan already spends this amount of money on fertilizers. If funds used for synthetic N were reallocated to synthetic P purchases in a full bio-supply recycling scenario, crop needs could be met. Most recycling could happen within districts, with only 6% of bio-supply requiring between-district transport when optimized to meet national N crop needs. Increased recycling in Pakistan could be a viable way to decrease yield gaps.

Optimizing Nutrient Recycling From Excreta in Sweden and Pakistan: Higher Spatial Resolution Makes Transportation More Attractive

“…After calculating the nutrient balance of grids, we let the set n + = g ∈ G : B n g > 0 represent all supply nodes (supply grids) and the set n − = g ∈ G : B n g < 0 represent all crop nutrient need nodes (grids with deficits in nutrient n) for each grid-resolution in each country. In Pakistan we chose to optimize transportation of excess excreta between grids with respect to nutrient N, because we were interested in comparing it with the optimization of the political resolution study in Pakistan (Akram et al, 2018), while we optimized for nutrient P in Sweden to compare it to municipal resolution results (Akram et al, in press). The choice of N in Pakistan and P in Sweden were originally motivated by local conditions, notably that it is possible for Pakistan to decrease synthetic N fertilizer use and that in Sweden P is now coming to the forefront of management decisions.…”

“…In Sweden it was the opposite, a larger proportion of P needs could be met (45% N vs. 88% P, Figure 5). As such it made sense to optimize transportation for different nutrients in each country (Akram et al, 2018;Akram et al, in press).…”

“…One way to look at the cost-effectiveness of recycling between countries is to compare the cost of transport to the value of fertilizers being transported as excreta (expressed as a ratio in Table 3). In Pakistan transporting excess excreta seemed economically favorable even at the political resolution with a ratio value below 1 (Akram et al, 2018), but dropped even further at the decimal resolution; 13% of the fertilizer value being transported could cover the entire cost of transportation ( Table 3, Figure 4). For Sweden, transportation at all resolutions seemed unfavorable, but the ratios actually increased from political to decimal resolution, and then decreased overall at the 5 km resolution (Table 3, Figure 4).…”

“…For human excreta, which may be in the form of sludge or biosolids after treatment in a wastewater treatment plant, acceptable transport distances can vary from 0 (i.e., no recycling) to cross-country travel to find a suitable recipient (e.g., 2,000 km from Boston to Florida in the USA Bergendahl et al, 2018); but transportation is only one of the many factors that drive or inhibit recycling, including potential contaminants and social acceptance (Englande and Reimers, 2001;Metson et al, 2018;Wadsworth et al, 2018). Although the ensemble of these distance ranges is broad, for the most part the distances are shorter than what would be required to ensure total recycling within a country (e.g., Metson et al, 2016;Akram et al, 2018;Akram et al, in press). Finding ways to minimize transportation distances will thus be a critical part of moving toward a more circular organic waste economy regardless of location.…”

“…increasing access to affordable fertilizers is necessary to increase yields and achieve food security (Solaiman and Ahmed, 2006;Akram et al, 2018). We use these cases to answer the following questions:…”