Effects of Convectional Tillage and Minimum Tillage on Carbon Stocks - Literature review Example

Running Head: Tillage Tillage Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Name Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Course Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Lecture Xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Date Introduction The storage of carbon in the soil has been suggested as a possible way of minimizing the amount of carbon in the atmosphere. Greater amount of Carbon can be taken from the atmosphere using a no-tillage or a minimum tillage cropland management system. In cases, where the land has never been tilled or has been tilled for a short, the amount of carbon stored is greater than that stored in tilled lands. This paper compares and contrasts the potential and effects of convectional tillage and minimum tillage on Carbon stocks. Potential of Carbon Storage for conventional tillage vs minimum tillage In the soil, carbon is in both organic and inorganic forms. This paper concentrates on the possible storage of Organic C in the collective form of Soil Organic Matter (SOM). Most of the soil organic Carbon is stored as living and non-living matter in the soil. According to Lal (2004a) between 900-1700 Pg of inorganic C and 1500 Pg of organic C are stored in global soils with 60 Pg grams exchanged with the atmosphere each year. According to Lal (2004a) the potential of Soils to store carbon comes from the size of the soil Carbon pool and the amount of Carbon exchanged between the soil and the atmosphere each year. According to Smith et al. (2008), conventional tillage leads to losses of 78 Pg of C every year and other associated process on the soil including mineralization lead to 26 Pg and 52 Pg loss respectively. According to Guo and Gifford (2002), the conversion of land from the no-tillage to the conventional tillage system has resulted to between 42 per cent and 59 per cent loss in Soil organic carbon stocks. This indicates that under conventional tillage more carbon is lost to the environment than in no-tillage land management system. Therefore, most experts believe a minimum tillage system is a middle ground between the lower carbon loss in no-tillage system and the great amounts of Carbon lost in conventional tillage systems. The processes that lead to Carbon fixation and loss in the soil help explain the potential of agricultural soils to retain Carbon under the minimum and conventional tillage methods of farmland management. One of the factors that lead to higher losses of Carbon in land under conventional tillage is harvesting. In the views of Robertson and Thorburn (2007) a substantial amount of fixed Carbon in the soil is removed by the 30 or 50% of crop mass removed in the harvesting of cereal crops. The rest of the fixed Carbon remains above ground residue and residue below the ground that is contained in the root biomass. In minimum tillage system like pasture the Carbon stored in the root mass is not disturbed meaning losses are minimized. In contrast, Conventional tillage the below ground root biomass is ploughed leading to greater losses of Carbon into the atmosphere. To increase the potential of soil to retain aboveground residue Carbon for longer lengths of time the material has to be incorporated into the mineral soil. In convectional system, residue is mixed with mineral soil through tillage. While in minimum tillage system, the natural system of mixing above ground residue with mineral soils is more impactful. According to Paoletti (1999), the soil fauna including litter arthropods and earthworms is more effective in mixing surface residue with mineral soils. Through leaching of organic materials 30% of the mass of the material is lost. Due to heterotrophic respiration a fraction of this solubilised organic carbon is immediately lost. The remainder of the carbon enters the soil. According to Paoletti (1999) when above ground residue is converted into below ground residue it increases the potential of soil to hold Carbon stocks. The fact that this conversion ratio is greater in minimum tillage systems than in conventional tillage systems means the potential of the soil to hold on to this Carbon is greater. The association between root distribution and Soil organic Carbon is important in determining the potential of the soil to retain carbon. Where root mass is more, the potential for retention of Carbon is higher. The fact that roots decay slower than other organic matter means that their potential to release stored Carbon is lower. Research on the long term effect of carbon storage in the soil due to absorption of minimum or convectional tillage is scarce. Few studies have studied the ability of soil to store carbon after a number of years. In Heenan et al (1995) it was found that there was no difference on Carbon retention for minimum and convectional tillage after 14 years. However, measurement using a single point of reference found that minimum tillage would lead to 31% higher rate of Carbon retention than convectional storage. In Halugalle et al (2002) Soil Organic Carbon was found to be higher for a piece of land managed used minimum tillage for 3 years after it had been tilled conventionally for 15 years. However, by year 6 of the trials the gains in Carbon retention were lost. This difficult in determining the effect of tillage practice comes from the fact that it is difficult to tell the direction of changes whether it is due to greater Carbon retention capacity or a reduction in losses. According to Sanderman , Farquharson and Baldock (2009), the formation of aggregate structure is slow thus Carbon accumulation is slower than the decomposition of tillage. In Pankhurst et al (2002) rapid losses in Carbon above the soil depth of 10 cm occurred after a piece of land managed under no tillage was switched to convectional tillage for 3 years. On the other hand, a piece of land that had been under convectional tillage for 14 years showed no significant improvement in Carbon retention after 3 years of minimal tillage management. Secondly, the rate of soil erosion and runoff is higher in convectional tillage than in minimum tillage and may be up to 75% lower. According to Paustian et al (2000),the improved soil health due to minimum tillage results in higher Carbon retention capability for soil managed under this system. However, where minimum tillage is just a reduction in the number of plow passes, the Carbon levels are found to be the same as those of convectional tillage. According to White (1990) the rate of Carbon loss in convectional and reduced tillage management system is twice that of minimum tillage with direct drill. Likely storage potential (tonnes per hectare ?) In a long-term experiment in Carbon storage, West and Post (2002) found that changing management practice from Convectional tillage to minimal tillage led to mean relative sequestration rate of 0.57 ± 0.14 Mg C ha-1 per year. In 15-20 years the soils converted to minimum tillage had higher stocks of Carbon. However, this tillage results were disputed by Baker et al (2007) who insisted the sampling depth led to biased results. In Baker et al (2007), a sampling depth of 30 reported depletion of Carbon stock. In contrast, minimal tillage system were found to contribute little to GHG. Angers and Eriksen-Hamel (2008), support the view that the minimal tillage Carbon stock shows minimal changes or decreases when compared with convectional tillage relative to increase in sampling depth. In the views of Blanco-Canqui and Lal (2008), minimum tillage results in gains in Carbon retention on the topmost layer of the soil. Here, the carbon retained is most likely to be lost rapidly, bringing into question whether the gains can be stored permanently. A comparison of Carbon stock between land managed under minimal tillage and conventional tillage, show higher Carbon stocks in minimal tillage soils 100.3 Mg C ha-1 to 95.4 Mg C ha-1 in Convectional tillage (Angers and Eriksen-Hamel,2008). However, these gains were found in the upper 10 cm of the soil while deeper in the soil there were significantly more Carbon stocks in the soil under Convectional tillage. A conversion of sugarcane farming from convectional tillage to minimal tillage with stubble should in theory result in higher Carbon stocks. However, in trials in Queensland it was found that this conversion led to an insignificant increase in Carbon stock at a soil depth above 10 cm. This was despite the increase in above ground residue due to the change in stubble retention and the extraordinarily fast litter decomposition rate in the warm and humid, Queensland weather. In the views of Chan (2009), the lower Carbon stocks are due to the 100 Mg d.m. ha-1 that is lost at harvest time. According to Graham et al (2002) quoting results of a 60-year trial in South Africa asserts that gains in Carbon stocks remain confined to the topmost soil layer even after 60 years. Feasibility in the Australian Context The results of the studies on Carbon sequestration discussed in this paper may not be applicable to Australian soils. The reason for this is that climate and physicochemical soil properties influence the retention capability and capacity of a given soil. In Australia these extremes in climate and soil physicochemical characteristics are more pronounced. In other cases the prevailing weather conditions may mask the effects of minimal or convectional tillage management effect. For instance in Bradford (2001), a semiarid grazing land was found to have absorbed 0.5 Mg C ha-1 in wet year. Ogle et al (2005) shows that the tillage management practice is less likely to have a positive effect in a drier region. The changes in carbon stock between minimal tillage and convectional tillage are more insignificant in the drier regions of Australia. In Chan et al (2003), putting light textured Australian soils under different tillage management practices only showed an identifiable difference in places where rainfall was above 500mm yr. The perception that Carbon stocks in the soil are affected by plant productivity has also been discredited. The results that suggest that the use of minimal tillage is not effective is disappointing to farmers in dry areas. Most of Australia is range land, showing the wide adoption of no-tillage or minimal tillage is ineffective in increasing carbon storage in the soil. Furthermore, most of Australia’s rangeland is used for grazing making it stubble retention which we have seen earlier promote Carbon retention almost impossible. However, Wang and Dalal (2006) notes that minimal tillage in combination with stubble retention and N fertilization is more likely to lead to higher stocks of Carbon than convectional tillage. Conclusion As seen in the discussion above the minimal tillage system is likely to lead to higher carbon stock in the topmost layer of the soil. However, this storage of Carbon in the upper layer of the soil is lost after a few years. A number of studies have also found that minimum tillage may have little effect on the carbon storage potential and in some cases of reduced tillage higher levels of Carbon loss were recorded in comparison to those of convectional storage. The application of minimal tillage over convectional storage has questionable Carbon storage benefits especially in Australia where the dominant dry landscape will minimize the positive effect of the management practice on Carbon stocks. Despite this minimal tillage was found to have other benefits like reduced costs and higher crop productivity. Therefore I would recommend combination of minimal tillage, stubble retention and N fertilization as a land management practice for Australian farmers which would lead to increased productivity and higher levels of Carbon storage in the soil. From the discussion above no apparent advantage of using minimum storage over convectional tillage were found in terms of Carbon Storage. However, farmers can gain from the increased productivity of their fields if they use the minimum tillage method to manage their farm. In conclusion, the ability of a farm management system to store carbon should not be an important consideration in choosing whether to adopt the minimum or convectional tillage method of farm management. References Angers DA, Eriksen-Hamel NS (2008) Full-inversion tillage and organic carbon distribution in soil profiles: A meta-analysis. Soil Science Society of America Journal 72, 1370-1374. Baker JM, Ochsner TE, Venterea RT, Griffis TJ (2007) Tillage and soil carbon sequestration - What do we really know? Agriculture Ecosystems & Environment 118, 1-5. Blanco-Canqui H, Lal R (2008) No-tillage and soil-profile carbon sequestration: An on-farm assessment. Soil Science Society of America Journal 72, 693-701. Bradford JB, Hicke JA, Lauenroth WK (2005) The relative importance of light-use efficiency modifications from environmental conditions and cultivation for estimation of large-scale net primary productivity. Remote Sensing of Environment 96, 246-255. 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