Conversion of forest land to agricultural land or urban use can result in changes in emissions of soil C as CO2. Conversely, net additions of forest and crop biomass can result in soil acting as a sink for CO2 (Raich and Potter, 1995). Agriculture and intensive tillage have caused a decrease of between 30 and 50% in soil C since many soils were brought into cultivation more than 100 years ago (Schlesinger, 1986; Houghton, 1995). There needs to be a better understanding of tillage processes, the mechanisms leading to C loss and how this C loss can be linked to soil productivity, soil quality, C sequestration, and ultimately to crop production. Long-term studies of soil C point to the role of intensive tillage and residue management in soil C losses (Lal, 1997; Paul et al., 1997; Paustian et al., 1997b); however, extrapolation of these data to a global value is complicated by uncertainties in soil C quantities and distribution across the landscape. Paustian et al. (1998) estimated that better global management of agricultural soils, restoring degraded soils, permanent set-aside of surplus land and restoration of some wetlands now used for agriculture could sequester between 400 and 900 MMTCE per year in the soil. They caution that soils have a finite capacity to store additional C which likely will be realized within 50—100 years. The potential for improved management offers hope that agriculture can decrease GHG emissions.
Mineral soils generally have fairly shallow organic layers and, therefore, have low organic C content relative to organic soils (Lal et al., 1997a; Paustian et al., 1997b). Consequently, it is possible to deplete the C stock of a mineral soil within the first 10-20 years of tillage, depending on type of disturbance, climate and soil type. Once the majority of native C stocks have been depleted, an equilibrium is reached that reflects a balance between accumulation from plant residues and loss of C through decomposition. Lal (1997) calculates that if 15% of the C in crop residues is converted to passive soil organic C (SOC), it may lead to C sequestration at the rate of 200 MMTCE year-1 when used with less intensive tillage. If the current changes in improved residue management and conversion from conventional tillage to conservation tillage in mineral soils continue as they have in the recent decade, these changes may lead to cumulative global C sequestration that ranges from 1500 to 4900 MMTCE by the year 2020 (Lal, 1997). In addition to increasing SOM, combined ecological and economic benefits of conservation tillage also accrue from decreased soil erosion, lower energy costs, water conservation and quality improvements, soil temperature regulation and improved soil structure. These all contribute to enhanced environmental quality and increased crop production.
One example of what intensive tillage in agricultural production systems has done to soil organic C is illustrated in Fig. 3.2. These data illustrate the long-term trends in soil C at the Morrow plots in Champaign, Illinois (Peck, 1989), and Sanborn Field at the University of Missouri, Columbia, Missouri. (Wagner, 1989). Both locations show similar decreases in SOC over the last 100 years. The only experimental parameter or factor common to the two locations was use of a mouldboard plough to till the experimental plots. Different cropping systems or rotations yielded a difference in soil C, which shows that management options exist for controlling SOM and improving soil C levels. The large decline in soil C was a result of tillage-induced soil C losses caused by use of the mouldboard plough and disk harrow, and a change to annual species. Other work around the world shows similar trends (Lal, 1997; Paul et al, 1997; Paustian et al, 1997b) and supports the need for conservation tillage with improved residue management. The significant 'flush' of CO2 immediately after tillage reported by Reicosky and Lindstrom (1993, 1995) partially explains the long-term role of tillage in affecting C flow within agricultural production systems. Tillage, particularly mouldboard ploughing, resulted in a loss of CO2 within minutes of tillage. Nineteen days after mouldboard ploughing, C lost as CO2 accounted for 134% of the C in the
previous wheat residue. Mouldboard ploughing, one of the most disruptive types of tillage, appears to have two major effects: (i) to loosen and invert the soil, allowing rapid CO2 loss and O2 entry into the soil; and (ii) to incorporate/mix the crop residues, thus enhancing microbial attack. Tillage perturbs the soil system and causes a shift in the gaseous equilibrium by releasing CO2 that enhances oxidation of soil C and organic matter loss. Conservation tillage, or any form of less intensive tillage, can minimize this tillage-induced C loss (Lal, 1997; Paustian et al., 1997b).
Sustainable agriculture requires new technologies for efficient biomass C utilization. Crop stover or residue is an important and renewable resource that is manageable and serves as the primary input for soil C sequestration. Lal (1997) has estimated that the global arable land mass of about 1.4 x 109ha annually produces 3.44 x 109 Mg of crop residue. At mean C content of 45 g kg-1 residue, the total global C assimilation is about 1500 MMTCE year-1. While a large portion of crop residue C is recycled to CO2 through microbial decomposition when the residue is mixed with soil by tillage, a small portion remains as humus that contributes to long-term sequestration in soil. The C from agricultural crop residues is only a small fraction (1%) of the estimated total global C fixed in photosynthesis; however, it is one amenable to management.
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