Agricultural legacies, food production and its environmental consequences.

N ear the end of Proust’s famous novel Remembrance of Things Past, the protagonist at last comes to understand that a person’s present can only be understood in the context of their past. In PNAS, Sattari et al. (1) remind us the same is true for ecosystems. Focusing on the present and future of agriculture, the authors argue that a legacy of excess fertilizer use has caused soil phosphorus (P) to accumulate in the world’s traditional breadbaskets and enhance recent crop yields. Further, they suggest those past P deposits can delay a potential crisis in global P supply (2). Their work is the latest contribution to a debate about one of society’s fundamental challenges: how do we feed a growing population while minimizing agriculture’s collateral damage on finite resources and the environment (3, 4)? It is a daunting question. In a single human lifetime, agriculture has more than doubled the amount of P and nitrogen (N) cycling in terrestrial systems (5–7). Past fertilizer excesses may subsidize future crop growth but are also responsible for a litany of environmental ills: P runoff has caused widespread freshwater eutrophication (6), whereas N’s “rap sheet” includes climate change, marine eutrophication, biodiversity loss, and air pollution (7). From a food security perspective, however, there is a critical difference between N and P. Although inequitable global distribution of mineral fertilizers perpetuates malnourishment in some world regions (8), our reliance on atmospheric N2 for fertilizer means we will never run out of N. The same cannot be said for P. Currently, world agricultural output relies on P fertilizers extracted from a few mineral deposits that—much like fossil fuels—took millions of years to form (9). Although the extent of Earth’s P capital remains debated, absent the discovery of new reserves and better recycling, P supply may eventually limit food production (9, 10). Hence the importance of Sattari et al.’s “residual” soil P pools (1). As early as 1942, Coleman (11) argued that residual P—that missed by simple soil extractions—might be more available to plants than originally thought, a hypothesis supported by multiple recent studies (e.g., ref. 12). Sattari et al.’s analysis creates optimism that present and future crops can recoup some benefits from the mistakes of our past. Yet while information on accumulated soil P ought to inform decisions about fertilizer needs, there are several reasons to temper our optimism. History also teaches us that, in addition to crop requirements, a web of biophysical and socioeconomic factors drives agricultural decisions and outcomes (Fig. 1). For example, even if residual P analyses inform fertilizer requirements, those needs are often targeted to maximum potential yields. In some ways, that makes intuitive sense: farmers do not want nutrient shortages to cause lower economic returns, thus fertilizer is often viewed as cheap insurance. However, getting the most out of a crop is a rare event. Climate variability alone causes most years to fall below maximum potential yields (13), and disturbances like pest outbreaks or ground-level ozone damage drive additional gaps between realized and maximum potential harvests (14). Thus, although overall balances are improving in some world regions (5), poor nutrient use efficiency remains common. In the case of N, a gap between supply and demand typically means a proportionate loss of N to the environment (15). The consequences of low P use efficiency Fig. 1. Expansion of P-intensive soy agriculture in Mato Grosso, Brazil has led to soil P storage, a legacy that may lower future P fertilizer requirements (1). However, agricultural P demand and efficiency may depend as much on climatic variability and socioeconomic drivers as on soil biogeochemical conditions. Ideally, P inputs mirror crop outputs, creating high PUE, which reduces pressure on finite P fertilizer reserves and minimizes P loss to the surrounding environment. However, high PUE is rare because suboptimal climate variation and other disturbances typically suppress crop yields below their maximum potential. Society’s overall P use efficiency is also low for reasons that range from P-intensive dietary choices to minimal P recycling. Reducing P-driven environmental damages and world fertilizer demand, while simultaneously meeting food security challenges, will require management and policy changes that increase PUE along the entire path from crop growth to human consumption. Photo credit: Chris Neill.