Constructed Climates

Figure 1.6: Top left plot shows the yields of many corn hybrids, bred for high performance at either low or high nitrogen levels, grown in low and high nitrogen conditions (after Presterl et al. 2002). Selective breeding certainly helps increase yields, but adding nitrogen increased yields from about 500 g/m² to about 850 g/m². Switchgrass biomass, a much touted biofuel, also increases with applied nitrogen (after Vogel et al. 2002). Artificial production of reactive nitrogen increased greatly over the last century (after Galloway et al. 2003), enabling the large crop yields shown in Figure 1.5.

We owe increased yields, in large part, to nitrogen, and we owe nitrogen to fossil fuels. Plant growth increases with available warm water, but many things can limit plant growth, nitrogen included. Amino acids, the building block of all proteins, have an NH₂ group attached to a carbon atom attached to everything else, so without nitrogen, growing plants have a problem.

Nitrogen limitation happens, in part, by harvesting crops and shipping them to city dwellers, taking available nitrogen away from fields, ultimately getting flushed down toilets rather than entering the local nitrogen cycle.[22] Nitrogen enters the biosphere naturally when fixation takes place in some free-living bacteria, as well as the mutualistic root nodules of some plants, importantly the legumes.

Nitrogen limits biomass production; 2,000 years ago farmers knew that planting legumes reconditions soils.[23] In Figure 1.6 I show two far more recent studies, one involving corn and the other switchgrass, a forage crop recently touted as a biofuel source.[24] Using sets of northern European corn hybrids selected for high yields at low and high nitrogen levels, one plot compares yields at low and high nitrogen levels.[25] Plant breeding helps: Hybrids bred for performance at low nitrogen had 10% greater yields at low nitrogen. Yet, independent of breeding, nitrogen limitation greatly reduces yields. Between these low and high nitrogen fertilizer treatments, available nitrogen slightly more than doubled, and yields increased by some 80%. Likewise, switchgrass crops grown in Ames, Iowa, and Mead, Nebraska, show clear increases in biomass[26] with more applied nitrogen, somewhere around a 50% increase.[27]

Lots of people means growing lots of food, and we just can’t get around that fact. Does adding nitrogen make economic sense to farmers? Fertilizer prices for agriculture over the last few years were around $400 per ton (or 900 kg). In the corn experiments the high N treatment hit 200 kg/hectare (179 lb/acre) meaning a ton covers 4.5 hectares (about 11 acres) at a cost of $36 per acre (ignoring application costs).[28] This level provided about 400 g/m² yield gain, or 63 bushels/acre. At $3/bushel for corn, the economic benefit reaches $190/acre, a return of about $5 for every $1 spent. A farmer planting 1,000 acres of corn needs that $200,000.

The bottom plot of Figure 1.6 shows total nitrogen use since 1850: Just since 1950 it increased sixfold, as did the crop yields in Figure 1.5. Some nitrogen comes from fossil fuel use — think emissions — and some from planting legumes[29], but most comes from commercial fertilizers produced via the energy-intensive Haber– Bosch process, invented about 100 years ago.[30] The resulting high-efficiency farming methods prompted the land-use changes shown next.

——————————————-

[22]Herbivory and/or plant death happens, and decomposition leads to ammonium ions, NH₄⁺, and ammonia, NH₃. Nitrification, called the oxidation of ammonium, transforms these chemicals to the nitrate ion, NO₃^-, and many bacteria make their living using the energy released in this transformation. Plants take in nitrate ions (and some ammonium, too), then put the energy back into these molecules to make ammonium ions. This process is called reduction. Denitrification transforms nitrate, NO₃^-, into nitrogen gas, N₂, and nitrous oxide, N₂O, releasing nitrogen to the atmosphere.

[23]Citations to ancient Greece and Roman agricultural practices are found in White (1970).

[24]Schmer et al. (2008) report that real farms growing switchgrass produce five times more energy than consumed, making switchgrass a potentially efficient energy crop.

[25]Presterl et al. (2002) studied nitrogen limitation in corn hybrids, comparing low and high nitrogen hybrids at low and high nitrogen levels.

[26]Considering switchgrass, note that in Figure 1.6 10 Mg/hectare biomass equals 1 kg/m², right around the values shown for plant growth in Figure 1.2 (except carbon versus dry biomass — multiply by 0.44 to convert to carbon [Dan Walters, pers. comm.] makes it spot on), and just over the grain produced by corn (ignoring stalks and such). Switchgrass isn’t a miracle plant, but grows without much cultivation and might not take land for food production out of commission.

[27]Vogel et al. (2002) studied switchgrass biomass production with increasing nitrogen.

[28]The high nitrogen treatment added enough fertilizer to get to a level of 200 kg/hectare (179 lbs/acre) of available nitrogen. This number is agriculturally relevant: Corn growers apply nitrogen fertilizers at about 140 lbs per acre in Minnesota.

[29]A plowed-under crop of alfalfa can add up to 300 kgs of nitrogen per hectare, but, of course, at the cost of one year’s crop. Crop rotation with soybeans helps soil quality while also producing a marketable crop. Nitrogen can fall from the sky when nitrogen gas gets converted to a useful form, but the amount is minimal. Rain brings in about 7 kgs of available nitrogen per hectare per year, or about 6 lbs per acre. In contrast, the air above an acre holds about 35,000 tons of nitrogen gas, N₂, essentially unavailable to all but a few biological processes.

[30]Galloway et al. (2003) document the growth of nitrogen use over the last century.