I wish I had time to go into the details, but alas I do not. However, I do want to state that many soil have been degraded by cultivation, and that increasing organic matter contents of these soils to levels close to what they were under natural vegetation represents a major potential contribution to the improving the global C imbalance over a period of 25 to 50 years. Beyond that time, the new soil organic matter levels would be about as high as they are going to get, so no further net C sequestration would be possible in that soil.
Soil organic matter is not a trivial part of the global C cycle. In fact soil organic matter accounts for about 3 times as much C (1500 to 2400 Pg) as found in ALL of the worlds forest trees, grass, marches, crops and all other vegetation (total of about 550 Pg). Some of the C added to soils will remain in the soil for only a few years before it is respired and returned to the atmosphere, but some will be humified or protected in soil aggregates so that it lasts in the soil for centuries. Forest tree growth can also make a contribution to C sequestration, but again only over a few decades while the forest is regenerating. Once it has matured, C losses pretty much equal the gains in new growth, so there is little further net
There can be no question that soil and land management, along with fossil fuel burning, have played a big role in increasing the atmosphere's CO2 concentration during the past century or so. The big culprits have been the burning of forest land, mainly to clear it for agriculture, and cultivation of agricultural lands by methods requiring soil tillage. Fruits and vegetable production plays a minor role in this balance globally, because so little land is devoted to these crops. But the production of grains, staple foods and forages involves huge expanses of land, and so the management of these enterprises is of material importance to the global warming equation.
Generally, less tillage is better than more tillage. Better for the soil and for the environment (global warming, erosion, etc.). And, by the way, many studies have clearly shown that reducing tillage reduces the amounts of fossil fuel energy used in farming. Reduced tillage, even no-tillage, is most beneficial with complex rotations, good integration of animal agriculture and the use of diverse cover crops or intercrops. It can be done without need for nitrogen fertilizer, with only small requirements for other fertilizers to prevent soil mining, and with less requirement for pesticides than farming methods that involve extensive tillage. It can be done with or without machinery (dibble stick, anyone?), and with or without herbicides.
However, for large scale use, no-tillage without using any herbicide is very challenging and labor intensive, indeed. Perhaps, someday we'll get our staple food from perennial grains and nut trees in systems that mimic nature (ala Wes jackson) in more ways than just lack of tillage, but until then, I take my hat off to those who can adapt no tillage systems to the needs of their landscape and their lifestyles.
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