Technical Comments

Comment on "Managing Soil Carbon" (I)

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Science  10 Sep 2004:
Vol. 305, Issue 5690, pp. 1567
DOI: 10.1126/science.1100273

Lal et al. (1) recently argued that no-till agriculture is a viable strategy for restoring on-site soil carbon, for reducing soil erosion and sediment yields, and, consequently, for enhancing soil quality. We believe, however, that their statements regarding the relation between soil erosion and increases in atmospheric CO2 do not take into account all relevant aspects of agricultural soil erosion. Failure to correctly assess the role in the carbon cycle of soil erosion—in particular, tillage erosion—on arable land may lead to an overoptimistic view of the potential benefits of no-till farming.

The first issue that requires attention is the fate of eroded soil carbon and rapid carbon replacement at eroded sites. Lal et al. (1) identified soil erosion by water as an important source of atmospheric CO2, on the order of 1 gigaton (Gt) C/year. This assessment assumes that 20% of the C that is displaced by water erosion is emitted into the atmosphere, mostly due to the breakdown of aggregates and subsequent C mineralization during transport by overland flow (2). However, C mineralization during transport is not the only process that affects the C balance of the water erosion process. The admixture of carbon-poor subsoil at eroding, carbon-depleted sites leads to rapid C replacement through roots and litter input in the soil, whereas the carbon that is buried at depositional sites is slowly mineralized. Thus, ultimately, soil erosion and deposition may lead to carbon sequestration. Some authors estimate that 0.6 to 1.5 Gt C/year may be sequestered globally through deposition in terrestrial environments (3). The mobilization of terrestrial C during erosion events may indeed have a significant effect on the global carbon budget; however, whether that erosion creates an atmospheric sink or source is still highly uncertain, as the various fates of eroded soil organic carbon (SOC) are poorly understood.

The second issue that demands attention is that Lal et al. (1) did not consider carbon storage due to tillage-induced soil redistribution. Over the past decade, a paradigm shift in erosion research has occurred with the identification and growing acceptance of the dominant role of tillage in redistributing soil within rolling arable fields (4). Tillage erosion redistributes soil in amounts that often dwarf the effect of water erosion at the field scale, and the process is now identified as a major contributor to the formation of colluvial deposits in agricultural landscapes. In contrast to the sediment mobilized by water erosion, the soil eroded by tillage erosion is deposited within the same field and no transport-related mineralization of organic matter occurs. Therefore, tillage erosion results in high carbon inventories at depositional sites (Fig. 1). Because part of the eroded carbon is dynamically replaced at eroding sites as a result of increased humification, tillage erosion and deposition lead to carbon sequestration on arable land. For example, assuming a tillage erosion rate of 10 Mg/ha per year, a carbon content of 2%, and that only 50% of the eroded carbon is replaced at eroding sites (3), tillage erosion leads to an annual carbon sequestration rate of 10 g C/m2.

Fig. 1.

Spatial differences in carbon storage due to tillage erosion. Soil carbon inventories (g C/m2) for the 0 to 0.45 m soil layer were measured at an eroding agricultural field in Denmark (57°20'N, 10°31'E). Erosion classes are based on the measured 137Cs activity; values significantly lower or higher than the 137Cs reference value (at the 0.05 level) were classified as erosion and deposition, respectively. The 137Cs reference value was determined by sampling an uneroded site in the area. n indicates the number of samples in each class. Error bars represent 1 SD. There is a significant difference in the measured SOC inventories between eroding, stable, and aggrading sites (F = 29.46, P < 0.0001 in one-way analysis of variance).

Recognizing and understanding the magnitude and dynamics of the tillage erosion–induced carbon sink is crucial, given that the sink would be lost with a change to no-till agriculture. In the United States and Europe, conversion from conventional tillage to no-till or minimal-tillage agriculture is considered to be the practice with the highest carbon sequestration potential for arable land (5,6). Yet tillage erosion leads to C sequestration rates that are of the same order of magnitude as the projected annual potential carbon sequestration rate of 10 to 40 g C/m2 from the conversion of agricultural land to no-till (6, 7). The carbon sequestration benefit of no-till practices on sloping land may, therefore, be considerably less than expected.

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