| 英文摘要: | The Emissions Gap Report 2013 from the United Nations Environment Programme restates the claim that changing to no-till practices in agriculture, as an alternative to conventional tillage, causes an accumulation of organic carbon in soil, thus mitigating climate change through carbon sequestration. But these claims ignore a large body of experimental evidence showing that the quantity of additional organic carbon in soil under no-till is relatively small: in large part apparent increases result from an altered depth distribution. The larger concentration near the surface in no-till is generally beneficial for soil properties that often, though not always, translate into improved crop growth. In many regions where no-till is practised it is common for soil to be cultivated conventionally every few years for a range of agronomic reasons, so any soil carbon benefit is then lost. We argue that no-till is beneficial for soil quality and adaptation of agriculture to climate change, but its role in mitigation is widely overstated.
The recent Emissions Gap Report 20131 makes bold statements about agriculture's potential to reduce greenhouse gas (GHG) emissions. The authors of the chapter on 'Policies for Reducing Emissions from Agriculture' estimate that at a marginal cost of less than US$50–100 per tonne of CO2 equivalent (CO2e), direct emissions from agriculture could be reduced by 1.1 to 4.3 Gt CO2e yr−1 by 2020. They claim that 89% of this potential could be realized through improved management practices including conversion to no-tillage land preparation (Box 1), more efficient use of water and fertilizers and addition of biochar to soil.
Box 1: What is no-till?
No-till means reduced soil disturbance as an alternative to traditional cultivation by ploughing or discing, in which the soil is broken and then further cultivated to prepare a seedbed for planting crops. In large-scale mechanized farms tillage operations are performed with heavy machinery pulled by tractor; in smallholder agriculture in less developed regions it is generally achieved using a small animal-drawn implement, or hand-held tools. Where conventional cultivation is eliminated seeds are sown in a slot cut in the soil, causing minimum soil disturbance. Large-scale tractor drawn no-till seeders are widely used, but small-scale no-till seeders are increasingly available for use with either animal traction or small tractors. In Subsaharan Africa no till planting may also be achieved by making a hole for individual seeds, such as those of maize, with a 'dibble stick'. Although complete absence of tillage is called no-till or zero till, reduced tillage or minimum tillage practices are also used whereby there is an intermediate amount of soil disturbance. No-till and reduced till sometimes form a component of a suite of practices termed conservation agriculture (CA), comprising retention of crop residues on the soil surface and diversification of cropping systems in addition to reduced or no-till. Here we specifically address no-till agriculture rather than the complete CA package because this was the focus of the UNEP report with which we take issue, though in a few instances we refer to published data for the full set of CA practices where this is relevant or data is more readily available. For simplicity we use the term 'no-till' throughout to include the range of reduced till practices, from no-till to minimum till. The term 'conservation tillage' is used by some authors but we avoid this as it can be ambiguous, either meaning no-till/reduced till or, depending on the context, it may refer to the no-till component of CA.
Overall the United Nations Environment Programme (UNEP) report1 is helpful: it demonstrates that current global efforts to decrease emissions are far below what is necessary to avoid dangerous climate change2 and it attempts to quantify opportunities for further reductions in different sectors. However, we have substantial concerns that the report overstates the potential for climate change mitigation in agriculture due to over-optimistic assumptions concerning the impact of no-till practices (Box 1 and Fig. 1).
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