Is Conservation Tillage The Answer To Global Warming

IS CONSERVATION TILLAGE THE ANSWER TO GLOBAL WARMING?

Daniel Walters Institute of Agriculture and Natural Resources University of Nebraska – Lincoln U.S.A.

SOILS, AGRICULTURE AND GLOBAL WARMING

During the past 150 years, the conversion of native ecosystems to agricultural lands in both temperate and tropical regions represents the most extensive ecological disturbance experienced in recorded history. A consequence of this disturbance has been the loss of enormous quantities of carbon and nitrogen to the atmosphere from soils and native biomass. The net loss C from changes in land use in the past 150 years is estimated at 120 Pg (1.2x1017 g), one-third of which came from the oxidation of soil carbon (8). Atmospheric gases such as CFC's, 17% carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) are called greenhouse gases because of the effect they have on preventing the emission of longwave radiationOther from Industrial, the 3% earth. Agricultural activity represents an estimated 26, 65 and 92 percent of anthropogenic emissions of these three gases, respectively, and contributes to an estimated 23% of global warming (5) (Fig. 1). Energy Use Agricultural Based upon meanPractices, lifetime 15% and radiative forcing 57%potential, per molecule, CH4 has 21 times and N2O 290 times more global warming potential than CO2 (17). As soils represent a sizable sink for carbon and nitrogen, there has been considerable interest in sequestration of soil C (and N) through conservation tillage. This paper willLand Usepresent a brief overview of agriculture’s impact on carbon Modification, 8% losses and the potential for conservation tillage to reverse global warming trends.

Figure 1. Contribution of anthropogenic activities to global warming (5, 20). U.N. WORLD POPULATION PROJECTIONS

) 60 s n

o 40 i l l i HIGH b (

20 MEDIUM

N LOW O I CURRENT INVENTORIES T 10

A 8 L 6 World PopulationU and Grain Demand: The major driving force behind agricultural P 4

expansion hasO been unprecedented population growth in the past several P decades. According to the United Nations Department of Economic and Social D 2 Affairs, world populationL currently stands at 6 billion persons and is growing at a R

rate of 1.33 percentO per year (19 ). The best estimate of long-range population 1 increase projectsW a stabilization of growth rate at replacement level around 2050 with a maximum population1850 1900 of 101950 billion2000 by 2150-2200.2050 2100 (Fig2150 2). YEAR

Figure 2. United Nations long-term population projections. Stabilization @ 8.9 million in 2050 with: Medium = fertility @ replacement level; High = fertility ½ a child higher than medium; Low = fertility ½ a child lower than medium ( UN ).

If the medium population growth scenario is realized, the demand for major cereals will increase at a rate of about 1.2 percent per annum for wheat and rice and 1.5 percent for maize (6, 14). The projected demand for cereal production in the next 30 years stands at 43% for rice, 44% for wheat and 56% for maize above current production levels (2 ) (Table 1). Table 1. World wide harvested area of the major cereals in 1967 and 1997, average growth rate in production since 1967, projected annual growth rate in cereal demand, and projected production increase for UN medium population growth scenario. (2 ) Annual growth rate, % Harvest area, ha x 106 Grain Increase in production demand to meet 2030 demand, 1967-97 projected % of present Production Crop 1967 1997 to 2030

Wheat 219 229 2.5 1.22 44

Rice 128 150 2.5 1.19 43

Maize 112 142 2.6 1.49 56

Arable Land: Increases in agricultural production have kept pace with population growth over the past 100 years primarily from the expansion of agriculture into virgin territory. Worldwide area in agriculture (cropland) exceeds 1.5 x 109 ha (6 ) yet the rate of land conversion to agricultural production has declined since 1950 owing to a decline in land area that is suitable for agriculture (7, 13) (Fig.3). The majority of land clearing today occurs from tropical deforestation. Between 1850 and1980, 60% of the 900 million acres cleared for agricultural production has been in tropical regions (8). Although these statistics are formidable, it is surprising that the FAO attributes only a fraction of total deforestation to the establishment of permanent agricultural land. Estimates put the expansion of agricultural lands at only 27% of total deforestation between 1980 and 1985 (8), with a large percentage classed as abandoned “other land” (6). Figure 3. Worldwide area in agriculture (- - -) and rate of land clearing ( ), (7).

Global Carbon Pools and Fluxes: The earth contains an estimated 1023 g (108 Pg) of carbon. Only 0.04 percent of this carbon is involved in relatively “active” biogeochemical exchange (18). The largest pool of “active” carbon resides as dissolved carbonates in the ocean and fluxes to the atmosphere from the ocean are approximately 175% greater than the flux of CO2 from soil to the atmosphere (16). Soil comprises the largest biologically active terrestrial C pool and stores of soil C are estimated to be approximately 1500 Pg (1.5 x 108 g) with a flux to the atmosphere of approximately 55 to 60 Pg yr -1 (16 ) (Fig 4). Prior to 1960, the principal source of CO2 enrichment came from the oxidation of soil and plant biomass due to land use change (agriculture and deforestation) (7), (Fig. 5). Current estimates of anthropogenic CO2 flux from terrestrial land management indicate that it represents approximately 25% of the loss generated from fossil fuel use (15) (Fig 6). Of the current 1.6 Pg yr-1 of C loss from soil and deforestation, 90 percent is generated from land use changes in the tropics (7) (Fig 7). Most C losses from land use changes are associated with the oxidation of plant material with approximately one third of the 120 Pg lost in the past 150 years coming from soil (8). At the current rate of fossil fuel consumption, it takes only eight years to equal the loss of C from soil over the past 150 years. GLOBAL CARBON POOLS

Oceans, 38,000 Pg

Atmosphere, 750 Pg Miscellaneous, 150 Pg

Terretrial, 2100 Pg (Soils, 1550 Pg & Biota, 555 Pg)

Figure 4. “Active” global carbon pools. (Pg = petagram = 1 x 1015 grams). (10) SOURCES SINKS

Deforestation & Landuse Unaccounted for, 1.6 Pg/Yr 1.8 Pg Yr

Atmosphere 3.2 Pg/Yr Fossil Fuel, 5.4 Pg/Yr Figure 5. Net flux of carbon to the atmosphere Oceans, as a result of land-use change 2.0 Pg/Yr and deforestation () and as a result of the combustion of fossil fuel (----). (7).

Figure 6. Principal anthropogenic CO2 sources and global CO2 sinks. (10) Figure 7. Net flux of carbon to the atmosphere as a result of land-use change and deforestation in Latin America, Tropical Africa, North America and Europe. (7).

POTENTIAL FOR CONSERVATION TILLAGE TO ENHANCE SOIL CARBON SEQUESTRATION

Tillage affects soil C in a number of ways. The combination of reduced residue decomposition rate and less soil mixing under conservation tillage tends to reduce overall soil-C oxidation rate and lower soil temperatures under CT tend to retard residue decomposition rate. Evidence of enhanced soil C sequestration through reduced tillage has been mixed. In cases where reduced tillage has limited crop productivity and therefore residue-C input, there has been little change in soil C over time (11). Intensification of cropping in traditionally wheat- fallow regions has been shown to significantly favor reduced tillage systems in both grain yield and increased soil C (3). Generally, high surface residue systems result in higher C sequestration when compared to intensive tillage systems when water is limiting and temperature or drainage does not inhibit crop productivity (12).

Much of the work reporting tillage impacts on global C budgets relies on the use of process models that simulate the effects of edaphic, climatic and biological factors as well as C inputs on soil C dynamics. Soil C enhancement is a relatively long-term process. Given the fact that CT has been practiced for a relatively short time, there are few tillage studies, per-se, that have been in place long enough to study organic matter build-up from reduced tillage. On the other hand, there is a larger world laboratory of long-term studies and sites that have recorded the decline of soil C from agricultural practices. These have been largely used to test and validate process models.

One such study attempted an assessment of alternative management practices affecting soil C in agroecosystems of the central United States (4). Simulations were run using the Century model over a database of soils that represented 60% of the agricultural land area of the continental U.S. The Century model simulates biomass production and conversion to soil organic matter under defined climates, soils and management practices. Soil organic matter pools are divided into active, passive and stabilized fractions with turnover times representing short (1- 5 yr), slow (20–40 yr) and recalcitrant (200 to 1500 yrs). As Century is driven by C inputs, prediction of future trends in soil C input in this study were simulated with biomass increasing at a rate of 0.5, 1 or 1.5% yr-1.

The Century model performed quite well and predicted the documented decline in soil organic C to approximately 53% of original 1907 level (Fig. 8). With the onset of improved crops and fertilizer use in the1960’s, the rate of SOC increased to 61% of original levels by 1990 consistent with most published observations. With increased conservation tillage after 1990, the projections were for an almost linear increase in SOC at rates of 0.6, 0.9 and 1.2% yr -1. These projected scenarios would result in a sequestration of 1.0, 1.36 and 1.8 Pg C for the study area by 2030. These values were compared to a more empirical simulation made by Kern and Johnson (9). The Century estimates were found to be within the limits or slightly lower than Kern and Johnson’s empirical estimates.

In the Century simulation, the maximum C sequestration potential had the capacity to fix only 0.1% of the total global CO2 emissions from fossil fuel. Kerr and Johnson’s best estimate (@76% no-till nationwide) would only sequester 0.5 % of global fossil fuel emissions. These projections become somewhat less favorable if one considers the added fuel cost associated with increased nitrogen fertilization necessary for high levels of crop productivity and the potential for greater N2O emissions under conservation tillage (1).

Many other factors have not been adequately considered in evaluating the impact of increased C sequestration from the practice of CT. The reduction in total farm fuel consumption and the potential for reduced fertilizer N consumption as soils become more biologically stable under CT may significantly alter conclusions from these previous scenarios. The impact of “C fertilization” from elevated atmospheric CO2 on crop biomass production, nutrient use efficiency and soil C sequestration potential is unclear. It seems reasonable, however, to promote integrated soil conservation systems designed to remove marginal land from crop production to forest or biomass fuel production. This would have considerably greater impact on CO2 emissions than wholesale conversion of all cultivated lands to CT. A program for global reforestation of degraded lands offers the fastest means to sequestration of atmospheric CO2 but it will require controls on population growth, reduction in the current rate of deforestation for agricultural purposes and a concerted energy conservation policy especially by the industrialized nations.

Figure 8. Simulated total soil carbon from 1907 to 2030 for historic (<1990) and three alternative levels of future crop yield increases. Future projections assume 27% reduced till and 3% no-till for the region.( 4)

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