Reduced tillage in annual field crops

In this last section, we look at reduced tillage as a land-management practice currently seen as a possible way to withdraw some CO2 from the atmosphere and store it in the form of organic C in soil. Reduced tillage is, in principle, applicable to all types of annual cropping in any part of the world. Initially, cultivation methods with reduced tillage were developed for purely agronomic reasons, such as the preservation of soil moisture, for reducing wind erosion, or for reducing the energy demand in field preparations. Commonly these methods are termed 'no-till', although soil disturbance cannot be completely avoided, for example during the seeding of crops. No-till also tends to increase soil organic C contents, so additional interest in promoting this practice has been raised in the light of climate change.

Data in Smith et al (1998) indicate a potential for C sequestration through a change to no-till systems of about 350kg C ha-1 yr-1. Results of 76 long-term experiments in the US provided an almost identical number (337kg C ha-1 yr-1) (West and Marland, 2002). Tan et al (2006) estimate that if all croplands in the east-central US under conventional tillage in 1992 were converted to no-till, the soil organic C pool would increase by 16.8 per cent by 2012, equivalent to the sequestration of 0.5Pg CO2 over 20 years. The scale and permanence of such measures are debatable but the short-term effect on atmospheric CO2 concentrations is certainly welcome. However, concerning the effect of reduced tillage on N2O emissions, we have to look carefully at different soil and environmental conditions.

Initially, we remain with the cultivated organic soils discussed in the previous section. Kasimir Klemedtsson et al (2009) observed large N2O emission peaks, the largest >15.9mg N2O m-2 hour-1, after ploughing and harrowing of a highly fertile drained organic soil in southern Sweden. Elevated emissions continued for about four days before returning to previous levels. They presumed the mechanical perturbation had created and exposed new soil surfaces where rapid mineralization could take place. Since there were no growing plants at the time to provide a sink for released mineral N, the latter was entirely available to the nitrifier and denitrifier communities. Tillage was timed to coincide with soil conditions being favourable to sowing the crop. The same conditions, warm and moist, are also favourable for N2O production. To reduce emissions, Kasimir Klemedtsson et al (2009) suggested that soil perturbations should be minimized. Increasing the water table was seen as an additional option, although annual crops may no longer be cultivated under such conditions. Consequently, annual crops would need to be replaced with long-standing grassland and tillage would cease completely. On organic soil in north-central Ohio, Elder and Lal (2008) investigated whether minimizing soil perturbation may indeed reduce N2O emissions from cultivated organic soils. They compared mouldboard ploughing with direct seeding or planting of maize, associated with minimal soil disturbance (no-till). As usual in N2O studies, observed emissions were notoriously variable. However, differences between no-till and ploughed cultivation were statistically significant; no-till soil management resulted in a reduction of N2O emission by approximately 63 per cent. Assuming all cultivated organic soils in the US are currently being ploughed, Elder and Lal (2008) estimated a potential reduction in N2O emissions by introducing no-till to all organic soils in the US of equivalent to 1.1Tg CO2 yr-1.

In a number of studies on mineral soils, the contrary has been observed. Here, no-till may increase N2O emissions (Smith and Conen, 2004; Rochette, 2008). Available oxygen often limits denitrification in mineral soils (Smith and Tiedje, 1979). Since no-till commonly leads to a reduced soil-pore volume, the fraction of water-filled pore space (WFPS) tends to increase when tillage ceases. Consequently, conditions for denitrification improve and more N2O can be produced and emitted (Smith et al, 2003). Thus reduced aeration of soil through no-till has two counteracting effects on N2O emissions, and it is a shift in the balance between these effects that determines whether more or less N2O is emitted under the new cultivation practice. In organic soils, reduced aeration seems to have a greater effect on limiting mineralization and supply of mineral N than it does on improving conditions for denitrification by restricting oxygen supply. In mineral soils with an already poor aeration under tilled conditions, it is the other way round. Where aeration is good, little difference is caused by a change in cultivation. Rochette (2008) has summarized results of 25 studies on N2O emissions (approximately 45 site-years of data) involving comparisons of tilled and no-till soils at the same site (Figure 7.4). He grouped the studies into three soil aeration categories, which we now analyse.

Figure 7.4 Relative N2O emission from no-till compared to tilled cultivation on (white bar) organic soil; and on mineral soils with (hatched bars from left to right) poor, intermediate and good aeration status

Source: Data for white bar from Elder and Lal (2008); data for hatched bars from Rochette (2008)

Figure 7.4 Relative N2O emission from no-till compared to tilled cultivation on (white bar) organic soil; and on mineral soils with (hatched bars from left to right) poor, intermediate and good aeration status

Source: Data for white bar from Elder and Lal (2008); data for hatched bars from Rochette (2008)

On poorly aerated sites, no-till resulted in annual N2O emissions being enhanced by 2.00kg N2O-N (equivalent to 3.14kg N2O). Let us assume an average CO2 sequestration of 350kg C ha-1 yr-1 (equivalent to 1283kg CO2 ha-1 yr-1) and consider that 1kg of N2O has 310 times the global warming potential (GWP) of 1kg of CO 2. Consequently, three-quarters of the benefit from C sequestration would be offset by the enhanced N2O emissions on poorly aerated soils. Soils with a medium aeration status would have on average only about 5 per cent offset by additional N2O emissions. On well-aerated soils, N2O emissions from no-till may on average (difference between geometric means) be even lower than from tilled plots and increase the effect of C sequestration by another 2 per cent in terms of GWP. While rates of C sequestration will decline within a few decades and eventually approach zero, emissions of N2O from poorly aerated mineral soils may continue to be enhanced. If so, the cumulative offset would grow with time and the greenhouse gas balance could become negative in the longer term.

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