Chapter 5: Agriculture

Overview/Main Issues

This chapter examines the role of agriculture in the carbon cycle. It quite justifiably emphasizes the role of below-ground processes, particularly changes in soil organic carbon (SOC). This chapter also deeply examines how human decision making affects carbon cycling in agro-ecosystems, including trends in food production, management, economic drivers, and dietary choices. There is also a good discussion of methane emissions associated with agriculture, especially animal husbandry.

The potential for agro-ecosystems to mitigate atmospheric carbon is addressed, with an emphasis on managing methane emissions from livestock. Climate change effects on agro-ecosystems are discussed. In this context, the stimulation of carbon mineralization in soils by elevated temperature is an important feedback to the climate system. The authors acknowledge the significant co-benefits of managing agro-ecosystems for increasing SOC, which include improved water holding capacity and nutrient status.

The chapter is generally well-constructed and the major conclusions are amply supported by cited references; however, some improvements could be made. For instance, consideration of role of Mexican and Canadian agro-ecosystems in the carbon cycle are given short shrift, and there are a few statements that, if taken out of context, could be misleading.

Statement of Task Questions

• Does the report accurately reflect the scientific literature? Are there any critical content areas missing from the report?

There are a few statements in this chapter that are not entirely consistent with common understanding of carbon cycling in agro-ecosystems and may be misleading. For example, the authors claim that leaving land fallow tends to lead to carbon losses (p.218, line 15). Relative to land under intensive tillage, fallow land (particularly if occupied by perennial vegetation) tends to build SOC. If the authors disagree, they should strengthen their case with proper references. The authors also claim that crops are carbon neutral because after they are harvested, they grow again in the subsequent year (p.227, line 7). This is inconsistent with common understanding that intensive row-crop agriculture, by stimulating soil carbon mineralization, is a net source of carbon to the atmosphere. The authors should reconsider these statements or provide additional explanation.

Changes in soil carbon in agriculture is in part controlled by inputs from above- and below-ground biomass. From the 1930s to today, there have been enormous increases in crop yields, with relatively small changes over this time period in the allocation of biomass within a crop between grain and aboveground biomass. Consequently, biomass inputs have gone up at same time that yields have increased, and both yield and biomass are projected to increase well into the future. High yields today already are producing more stover than can be assimilated into the soil. It would be useful if the authors gave some consideration of how projected increases in yield and biomass may affect SOC.

A major driver of changes in SOC, particular in the rain-fed Midwest U.S., is soil drainage. Expansive areas of the corn belt are underlayed by tile drains, and the installation of these drains, along with intensive tillage, were responsible for dramatic losses of SOC beginning in the 19^th century. Some

discussion of how the interaction between changing precipitation regimes and tile drainage affect SOC in the future would be interesting.

• Are the findings documented in a consistent, transparent and credible way? Are the report’s key messages and graphics clear and appropriate? Specifically, do they reflect supporting evidence, include an assessment of likelihood, and communicate effectively?

Yes on both

• Are the research needs identified in the report appropriate?

While this chapter addresses “gaps” and “uncertainties”, the authors do not clearly articulate future research needs in this chapter.

• Are the data and analyses handled in a competent manner? Are statistical methods applied appropriately?

This chapter is largely synthetic and as such, relies on statistical treatment of data from the primary literature, rather than applying de novo statistical tests, such as meta-analysis.

• What other significant improvements, if any, might be made in the document?

The discussion of the role of no-till agriculture (p.225) as a modulator of SOC does not seem well balanced. Measuring changes in SOC over relatively short periods of time—a decade or less—is problematic. Spatial variation often swamps temporal trends in SOC. The best indicator of how management/crop type/land use change effects SOC may be eddy covariance—a direct measurement of Net Ecosystem Exchange. One study of how cropping affects SOC (Bernacchi et al., 2005) demonstrated rather convincingly that no-till agriculture and clay-rich mollisols builds SOC.

See also the line-specific suggestions below.

Line-Specific Comments

P214, Line 21-25

Would be interested to learn more about temp effects on SOC.

P217, Line 3-17

Include references from Bernacchi and Robertson

P217, Line 17

Include references by Bernacchi

P217, Line 21

What is forage productivity?

P218, Line1

Drainage has a major impact on SOC

P218, Line 1-2

Extent and efficiency of drainage?

P218, Line 15-18

How? Not sure this is believable.

P223

More discussion of how expected increases in temperature will affect SOC would be useful. See a recent paper by Black et al. (2017).

P224, Line 20-22

Why add on for Canada and not Mexico? Scope?

P225, Line 13-14

Two is not numerous

P225, Line13

Delete “Numerous authors and models have reported that…”

P225, Line 41-42

Very difficult to measure SOC changes directly; preferable is data from eddy covarience. (See Bernacchi et al., 2005)

P226, Line 1-20

The authors may wish to acknowledge that while perennial, cellulosic biofuel feedstocks still suffer from high costs of conversion, they have enormous potential to build SOC. Some of these crops increase SOC by as much as 1 Mg/yr after removal of aboveground biomass.

P226, Line 14-20

There is enormous potential of perennial bioenergy crops to restore SOC and reduce N20

P227, Line 7-8

Not consistent w/effects of intensive agriculture on SOC

P227 Line 33-34

Remove “such as nitrates” from the end of the sentence and place before “also”

P233, Line 37

Delete the word “managing”

P248, Figure 5.1.

The axes are unclear—both left and right y-axes state million acres. Also, an additional y-axis labelled with hectares would be helpful, since the text uses hectares.

P251, Figure 5.4.

The authors should be explicit in the legend that negative values represent a flux of carbon from the atmosphere to soil, and positive values represent the opposite.

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