Putting it all together

For simplicity, we often discuss methods of reducing emissions for each gas separately. But the C and N cycles are tightly interwoven; a change in farming practice that reduces emission of one gas almost always affects another. Whether or not a new practice helps alleviate the greenhouse effect depends on the net effect on emission of all gases and the relative warming potential of each. A few examples may help to illustrate some of the complex interactions.

One of the ways to reduce CO₂ emissions is to farm more intensively: to eliminate summer fallow, to use higher-yielding varieties, and to aim for higher productivity. Such practices can increase stored C by producing higher amounts of residue that become soil organic matter. At the same time, however, the new, more-intensive system may require higher inputs, including fertilizers, to maximize yields. And those higher inputs of fertilizer may increase N₂O emissions. The overall effect of the new practice must therefore take into account the change in soil C, the CO₂ cost of making the added inputs, and any increase in N₂O emission. Because N₂O is such a potent greenhouse gas, a small increase in emission rate (say 1 kg N per hectare per year), will offset a comparatively high rate of soil C accumulation (~130 kg C per hectare per year).

The evaluation becomes even more complex if we include animals. Suppose, for example, we opt to allocate greater land area to producing forages. This effect would have pronounced benefits for storing soil C. Furthermore, it would reduce fertilizer requirements (and N₂O emissions from that fertilizer), because nutrients are effectively recycled back to the soil as manure. On the other hand, much of the C in that system would be fed to animals, and a portion would be released as CH₄. Furthermore, some CH₄ and N₂O would be produced from manure. Thus, with one management change, we have affected emission of all three gases, sometimes both negatively and positively. And to know the net effect of the practice, we must consider all three and their relative warming potentials.

We cannot yet grasp all the interactions among gases, nor are our models sophisticated enough to predict them. At present, however, it may be sufficient to recognize that all are part of a complex web, and any attempt to reduce emissions of one may affect the others. Often, the net effect may still be overwhelmingly positive; for example, it may be that the increased soil C from a livestock-based system more than offsets any increase in CH₄ emission. Indeed, sometimes the effects may even be mutually positive; no-till, for example may increase soil C, reduce CO₂ from fossil fuel, and perhaps even reduce N₂O emissions. Similarly, more efficient use of manures, can almost certainly reduce N₂O and CH₄ emissions, while reducing CO₂ costs of fertilizer manufacture.

A final consideration is that the various practices aimed at reducing greenhouse gas emissions may work over different periods. For example, efforts to increase soil C gains may show largest response in the short term, say one or several decades, but rates of C gain may diminish thereafter because each new increment of C becomes harder and harder to achieve. In comparison, efforts to reduce CH₄ emission from ruminants, N₂O emission from soils, or CO₂ emission from fossil fuels may have only small effects in the short term but achieve highest effect over many decades because the benefits accrue indefinitely.