Management practices used on croplands to enhance crop yields and quality can contribute about 10–20% of global greenhouse gases (GHGs: carbon dioxide [CO2], nitrous oxide [N2O], and methane [CH4]). Some of these practices are tillage, cropping systems, N fertilization, organic fertilizer application, cover cropping, fallowing, liming, etc. The impact of these practices on GHGs in radiative forcing in the earth’s atmosphere is quantitatively estimated by calculating net global warming potential (GWP) which accounts for all sources and sinks of CO2 equivalents from farm operations, chemical inputs, soil carbon sequestration, and N2O and CH4 emissions. Net GWP for a crop production system is expressed as kg CO2 eq. ha−1 year.−1 Net GWP can also be expressed in terms of crop yield (kg CO2 eq. kg−1 grain or biomass yield) which is referred to as net greenhouse gas intensity (GHGI) or yield-scaled GWP and is calculated by dividing net GWP by crop yield. This article discusses the literature review of the effects of various management practices on GWP and GHGI from croplands as well as different methods used to calculate net GWP and GHGI. The paper also discusses novel management techniques to mitigate net CO2 emissions from croplands to the atmosphere. This information will be used to address the state of global carbon cycle.
- crop yield
- greenhouse gas
- global warming
- management practice
- soil carbon sequestration
Management practices on croplands can contribute about 10–20% of global greenhouse gases (GHGs: carbon dioxide [CO2], nitrous oxide [N2O], and methane [CH4]) [1, 2]. Quantitative estimate of the impact of these GHGs in radiative forcing in the earth’s atmosphere is done by calculating net global warming potential (GWP) which accounts for all sources and sinks of CO2 equivalents from farm operations, chemical inputs, soil carbon (C) sequestration, and N2O and CH4 emissions [3, 4]. Net GWP for a crop production system is expressed as kg CO2 eq. ha−1 year.−1 Net GWP can also be expressed in terms of crop yield (kg CO2 eq. kg−1 grain or biomass yield) which is referred to as net greenhouse gas intensity (GHGI) or yield-scaled GWP and is calculated by dividing net GWP by crop yield . These values can be affected both by net GHG emissions and crop yields. Sources of GHGs in agroecosystems include N2O and CH4 emissions (or CH4 uptake) as well as CO2 emissions associated with farm machinery used for tillage, planting, harvesting, and manufacture, transportation, and applications of chemical inputs, such as fertilizers, herbicides, and pesticides, while soil carbon sequestration rate can be either a sink or source of CO2 [4, 5, 6]. In the calculations of net GWP and GHGI, emissions of N2O and CH4 are converted into their CO2 equivalents of global warming potentials which are 265 and 28, respectively, for a time horizon of 100 year . The balance between soil carbon sequestration rate, N2O and CH4 emissions (or CH4 uptake), and crop yield typically controls net GWP and GHGI [3, 4, 7].
Some of the improved management practices used for reducing net GWP and GHGI from croplands are no-till, increased cropping intensity, diversified crop rotation, cover cropping, and reduced N fertilization rates [3, 4, 7, 8, 9, 10]. Soil organic carbon can usually be increased by adopting no-tillage practice which decreases microbial activity and CO2 emissions as a result of reduced soil disturbance and residue incorporation compared with conventional tillage practice [3, 11]. Tillage, however, can interact with crop residue carbon input on soil carbon sequestration which varies by region . When carbon input is <15% due to reduced crop yields, soil organic carbon is often lower with no-tillage than conventional tillage in regions with wetter and cooler climate, such as in eastern USA and Canada . In such regions, tillage often redistributes crop residues in the soil profile, resulting in lower soil organic carbon in the surface soil and greater in the subsurface soil, with overall lower soil profile carbon is greater in conventional tillage than no-tillage [13, 14]. The reverse is true when carbon input is >15% . With double rather than single crop in a year, Luo et al.  found that no-tillage increased soil organic carbon in the soil profile compared with conventional tillage. In regions with subtropical humid and semiarid climates, such as in southern USA and northern Great Plains, no-tillage can increase soil organic carbon compared with conventional tillage by increasing carbon input as well as reducing soil organic matter mineralization .
Soil organic carbon can also be increased by increasing the quality and quantity of crop residue returned to the soil due to diversified cropping systems, such as intensive cropping, crop rotation, and cover cropping, compared with non-diversified systems, such as crop-fallow, monocropping, and no cover crop [3, 15]. Crop rotation can increase soil organic carbon by increasing carbon input through increased crop yield compared with monocropping [3, 15, 16]. Similarly, cover cropping can increasing soil organic carbon by increasing the amount of crop residue returned to the soil . In contrast, fallowing can reduce soil organic carbon by reducing carbon input and by increasing soil organic matter mineralization as a result of increased soil temperature and water content [16, 18]. The effect of nitrogen fertilization on soil organic carbon is variable [18, 19, 20].
Nitrogen is usually required in large amounts to sustain crop yield and quality compared with other nutrients, such as phosphorus and potassium. Nitrogen fertilization typically stimulates N2O emissions when the amount of applied nitrogen exceeds crop nitrogen demand [3, 8, 9, 10, 21]. Nitrogen fertilization, however, can have a variable effect on CO2 and CH4 emissions [15, 22]. Because N2O emissions has a large effect on net GWP and GHGI, practices that can reduce N fertilization rates without influencing crop yields can substantially reduce net GHG emissions [3, 4]. Other factors that can influence N2O emissions are type, placement, and method of application of nitrogen fertilizers. Applying nitrogen fertilizer in the spring compared with autumn and using split application compared with one single application at planting can reduce N2O emissions in some cases [23, 24, 25]. Applying N fertilizer at various depths can have variable effects on N2O emissions [26, 27, 28, 29]. Anhydrous ammonia can increase N2O emissions compared with urea [27, 30, 31]. Similarly, chemical additives to reduce nitrification from nitrogen fertilizers, such as polymer-coated urea and nitrification inhibitors, can substantially reduce N2O emissions compared with ordinary urea and non-nitrification inhibitors fertilizers [32, 33, 34]. Some nitrogen fertilizers, such as urea, emit both CO2 and N2O. Nitrogen fertilizers also indirectly emit N2O through ammonia volatilization and nitrate leaching .
Some management practices used for reducing GHG emissions can have adverse effects. Examples of such practices are no-tillage systems where denitrification resulting from higher soil water content can increase N2O emissions compared with conventional tillage systems in humid regions, thereby reducing the GHG mitigation potential . In contrast, N2O emissions can be similar  or lower [3, 37] with no-tillage compared with conventional tillage in semiarid and arid regions. Sainju et al.  reported that crop rotation had no effect on N2O emissions, but CH4 uptake was greater with barley-pea rotation than continuous barley in the semiarid region. Cover crops also have variable effect on N2O emissions . Legume cover crops can increase N2O emissions compared with nonlegume cover crops during their growth, but emissions can be similar among cover crops when measured over the entire year . Similarly, root respiration and mineralization of crop residue and soil organic carbon can have negative impacts on GHG mitigation, although greater root biomass and distribution can increase carbon sequestration [15, 39]. Therefore, while calculating net GWP and GHGI, all of these factors should be accounted for, regardless of management practices used [3, 4, 40].
Several methods have been used to calculate net GWP and GHGI. Some have used the sum of CO2 equivalents of N2O and CH4 emissions [21, 41, 42], while others [43, 44] have included CO2 equivalents of all three GHGs. Still others have used CO2 equivalents of N2O and CH4 emissions and soil carbon sequestration rate [45, 46, 47]. A full accounting of all sources and sinks of CO2 emissions to calculate net GWP and GHGI includes CO2 equivalents from farm operations, N fertilization, and other inputs in addition to above parameters [3, 7, 9, 10, 40, 48, 49, 50, 51]. Several researchers have used DAYCENT and GREET models to estimate GWP and GHGI [52, 53]. Some have excluded N2O and CH4 emissions, but used CO2 equivalents of all other sources and sinks . An alternative method of calculating net GWP and GHGI includes substituting soil carbon sequestration rate by soil respiration and the amount of previous year’s crop residue returned to the soil [3, 9, 10, 50, 51, 54]. Each method has its own advantages and drawbacks.
2. Impact of management practices
Various studies have shown that both net GWP and GHGI were lower with no-tillage than conventional tillage, regardless of soil and climatic conditions, cropping systems, and methods of calculations [3, 7, 44, 47, 49, 55]; Sainju  observed that reductions in net GWP and GHGI due to no-tillage vs. conventional tillage vary among regions with various soil and climatic conditions, but largest difference occurred in sandy soil under moderate annual precipitation (900 mm). Net GWP values, however, increased in regions with higher air temperature. A meta-analysis of nine experiments by the same author on the effect of tillage showed that no-tillage reduced net GWP by 55% and net GHGI by 58% compared with conventional tillage when all sources and sinks of CO2 were accounted for. With the partial accounting of sources and sinks, the reductions in net GWP and GHGI due to no-tillage vs. conventional tillage were 81 and 73%, respectively, indicating that partial accounting can inflate net GWP and GHGI values . Differences in crop yields among cropping systems and regions resulted in different proportion of reductions in net GWP and GHGI due to no-tillage vs. conventional tillage . Increased soil carbon sequestration rate due to reduced soil disturbance and carbon mineralization reduces net GWP and GHGI in no-tillage [4, 40, 57]. In contrast, increased crop residue incorporation and aeration increases microbial activity which reduces carbon sequestration, thereby reducing net GWP and GHGI in conventional tillage [3, 7, 9]. Reduction in tillage intensity can also reduce net GWP and GHGI .
The duration of study can also have a profound influence on net GWP and GHGI with no-tillage vs. conventional tillage. Under corn-soybean rotation in clay loam soil in Colorado, Mosier et al. [3, 7] found that net GWP with no-tillage vs. conventional tillage was lower after 1 year than after 3 year due to differences in soil carbon sequestration rates. In contrast, Six et al.  reported that reduction in net GWP with no-tillage vs. conventional tillage was realized only after 10 year in the humid region and 20 year in the dry region due to increased soil aggregation, reduced aeration, and increased soil carbon sequestration. In a meta-analysis of nine experiments, Sainju  found that changes in net GWP and GHGI due to no-tillage vs. conventional tillage increased with the duration of the experiment, regardless of the method used for calculation. When soil and climatic conditions, such as soil texture, annual precipitation, and average air temperature of the experimental sites were included in the multiple linear regressions, the relationships were further improved. While air temperature had a negative effect on net GWP and GHGI, the effect of soil texture varied. This could be explained by several factors: (1) no-till can some time increases N2O emissions due to increased soil water content and denitrification compared with conventional till, especially in the humid region, thereby reducing net GWP and GHGI [4, 40, 57], (2) the potential for soil carbon sequestration using no-tillage decreases and reaches a steady state as the duration of the experiment increases [57, 59], and (3) there is a high uncertainty in spatial and temporal variability in GHG emissions within and among regions due to variations in soil and climatic conditions and management practices [7, 9, 10, 40]. Nevertheless, more long-term experiments are needed to relate the effect of tillage with the duration of experiment on net GWP and GHGI.
2.2. Cropping system
Crop type and cropping systems can affect net GWP and GHGI. Various researchers [3, 7, 48, 49] reported that both net GWP and GHGI were lower with continuous corn than corn-soybean rotation, but net GHGI was lower with soybean than corn when grown alone . Increased soil carbon sequestration due to greater amount of crop residue returned to the soil reduced net GWP and GHGI under continuous corn compared with corn-soybean rotation, although nitrogen fertilization rate to produce sustainable yield was higher in continuous corn [3, 7, 48, 49]. In contrast, greater N2O emissions following soybean increased net GWP and GHGI in corn-soybean rotation [3, 7, 48, 49]. Under small grain crops, however, several researchers [9, 10, 60, 61] have found that including legumes, such as pea and lentil, in rotation with nonlegumes, such as wheat and barley, reduced net GWP and GHGI compared with continuous nonlegumes. They observed this because (1) no nitrogen fertilizer was applied to legumes compared with nonlegumes which required large amount of nitrogen fertilizers to sustain yields, as nitrogen fertilizer stimulates N2O emissions and (2) legumes supplied greater amount of nitrogen to succeeding crops due to higher nitrogen concentration when above- and belowground residues were returned to the soil and reduced nitrogen fertilization rate compared with nonlegumes. Sainju et al. [9, 10] also found that legume-nonlegume rotation increased soil carbon sequestration because of increased turnover rate of plant carbon to soil carbon compared to continuous nonlegume.
In a meta-analysis of 11 experiments on the effect of crop rotation containing small and large grain crops on net GWP and GHGI, Sainju  reported that crop rotation increased net GWP by 46% and net GHGI by 41% compared with monocropping. This was especially true for large grain crops, such as corn and soybean where net GWP and GHGI were 215 and 325%, respectively, greater under corn-soybean than continuous corn. In contrast, for small grain crops, such as barley and pea, net GWP was 22% lower under barley-pea than continuous barley. Both net GWP and GHGI were 168 and 215%, respectively, lower with perennial than annual crops. Greater number of experiments and magnitude of changes, however, resulted in higher net GWP and GHGI in monocropping than crop rotation under large than small grain crops when values were averaged across experiments during data analysis .
As cropping intensity increased, net GWP and GHGI reduced . Greater amount of crop residue returned to the soil and increased carbon sequestration reduced net GWP and GHGI when cropping intensity was increased [9, 50]. Increased carbon sequestration with increased cropping intensity in the semiarid regions with limited precipitation is well known [18, 62]. Several researchers [7, 9, 50] have found that fallowing or crop-fallow rotation increased GHG emissions and therefore net GWP and GHGI compared with continuous cropping due to increased soil temperature and water content that enhanced microbial activity and absence of crops to utilize mineralized nitrogen during fallow. Using partial accounting of CO2 sources and sinks, Liebig et al. , however, did not found significant difference in net GWP between alternate-year fallow and continuous cropping in North Dakota. Perennial crops can reduce net GWP and GHGI compared with annual crops [7, 44, 50] due to higher root biomass production [64, 65] and increased soil carbon sequestration . Because land under perennial crops is not tilled and perennial crops are not applied with fertilizers, herbicides, and pesticides, GHG emissions are usually lower with perennial than annual crops .
Sainju  found that changes in net GWP and GHGI due to crop rotation vs. monocrop and corn-soybean vs. continuous corn decreased with increased duration of experiment, but increased due to annual vs. perennial cropping systems. The relationships were further improved when soil and climatic conditions were accounted for in the multiple linear regressions of net GWP and GHGI with the duration of the experiment. He observed that soil texture had a positive effect on net GWP and GHGI for cropping intensity, but negative effect on net GWP for crop rotation vs. monocrop and perennial vs. annual crop. The trend was opposite for mean air temperature while annual precipitation had small effect. Because the magnitude of carbon sequestration rate is lower and time for carbon saturation is longer for the effect of cropping systems than for tillage systems [57, 59], reduced net GWP and GHGI for increased cropping intensity and crop rotation vs. monocrop with increased duration of experiment was due to increased carbon sequestration. Sainju  reported that crop rotation had a greater potential to reduce net GWP and GHGI compared with monocropping in the long run, but the potential can vary for perennial vs. annual cropping systems.
2.3. Nitrogen fertilization
Nitrogen fertilizer application rate, source, and timing and method of application can influence net GWP and GHGI. Increased nitrogen fertilization rate enhanced net GWP and GHGI due to increased N2O emissions and CO2 emissions associated with manufacture, transport, and application of nitrogen fertilizers, regardless of cropping systems and methods of calculations [3, 7, 21, 33, 42, 55]. In a meta-analysis of 12 experiments, Sainju , after accounting for all sources and sinks of CO2 emissions, reported that net GWP decreased from 0 to ≤45 kg N ha−1 and net GHGI from 0 to ≤145 kg N ha−1 and then increased with increased nitrogen fertilization rate. Using partial accounting, net GWP decreased from 0 to 88 kg N ha−1 and net GHGI from 0 to ≤213 kg N ha−1 and then increased with increased nitrogen rate. These nitrogen rates probably corresponded to crop nitrogen demand when crops used most of the soil available nitrogen, leaving little residual nitrogen in the soil that reduced N2O emissions and therefore net GWP and GHGI. When nitrogen rates exceeded crop nitrogen demand, net GWP and GHGI increased linearly , suggesting that excessive application of nitrogen fertilizers can induce net GHG emissions. Similar results have been reported by several researchers [8, 66, 67]. Therefore, nitrogen fertilizers should be applied at optimum rates to reduce net GWP and GHGI while sustaining crop yields. The optimum nitrogen rates, however, depended on net GWP measured either per unit area or per unit crop yield.
Sainju  observed that the relationships between net GWP, net GHGI, and nitrogen rate were further improved when the duration of the experiment and soil and climatic conditions were taken into account in the multiple linear regressions. Duration of experiment and annual precipitation had positive effects, but air temperature and soil texture had negative effects on net GWP when all sources and sinks of CO2 emissions were accounted for. With partial accounting, only air temperature had positive effect on net GWP, but other factors had negative effects. For net GHGI, the factors having negative effects were air temperature using the complete accounting of CO2 emissions and annual precipitation and soil texture using the partial accounting.
Alder et al.  reported that anhydrous ammonia reduced net GHGI compared with urea, urea ammonium nitrate, and polymer-coated urea under corn, wheat and switchgrass due to lower energy requirement for fertilizer production. They found that polymer-coated urea reduced net GHGI by slowly releasing nitrogen to the soil and reducing indirect N2O emissions compared with urea ammonium nitrate. Little is known about the placement and methods of nitrogen fertilizer applications on net GWP and GHGI, although various results have been reported on N2O emissions using these practices [23, 24, 25, 26, 27, 28, 29]. More research is needed about the effects of source, placement, and timing of nitrogen fertilizer application on net GWP and GHGI.
2.4. Other fertilizers
Application of combination of nitrogen, phosphorus, and potassium increased net GWP compared with no application and net GWP further increased as these nutrients were applied with a combination of inorganic fertilizer, green manure, and farmyard manure, although total amount of nutrients applied from various sources were similar under rice in China and India [43, 46]. They found that increased substrate availability from fertilizers and organic amendments increased N2O and CH4 emissions and therefore net GWP. Shang et al. , however, found lower net GHGI with these nutrient applications than without due to increased crop yield. Adviento-Borbe et al.  also observed increased net GWP and GHGI with combined application of nitrogen, phosphorus, and potassium compared with no application under corn in Nebraska.
2.5. Miscellaneous practices
Burning of crop residue increased net GWP and GHGI compared with residue retained in the soil due to reduced carbon input and soil carbon sequestration in upland crop production [40, 47]. Sainju et al.  found that irrigation increased net GWP and GHGI compared with no irrigation due to lower soil carbon sequestration as a result of increased carbon mineralization and loss of water soluble carbon from increased soil water availability. Under lowland rice, Li et al.  found that midseason and shallow drainage reduced net GWP and GHGI by 21–205% compared with continuous flooding. Under upland rice where flooding is minimized, they found that drainage reduced net GWP and GHGI from 17 to 322% compared with no drainage. They also found that application of nitrogen fertilizer and straw in flooded rice reduced net GWP and GHGI from 16 to 91% compared with no application, but net GWP increased by 18% by using slow N release fertilizer compared with normal nitrogen fertilizer.
2.6. Combined management practices
Using combined effects of tillage, crop rotation, and nitrogen fertilization rates, various researchers [3, 7, 49] found that net GWP and GHGI were lower with no-tillage continuous corn with reduced nitrogen rate than conventional tillage corn-soybean rotation with recommended nitrogen rate. They attributed this to increased soil carbon sequestration and reduced N2O emissions, as corn used most of nitrogen during growth, leaving little soil residual nitrogen. They found that soybean increased N2O emissions compared with corn, thereby increasing net GWP and GHGI with corn-soybean compared with corn. Similarly, Adviento-Borbe et al.  reported that net GWP and GHGI were lower with lower rates of nitrogen, phosphorus, and potassium applied to continuous corn than lower or higher rates applied to corn-soybean. Johnson et al.  reported that minimum till diversified crop rotation with appropriate rates of nitrogen, phosphorus, and potassium reduced net GWP and GHGI compared with conventional tillage with less diversified crop rotation and high rates of nutrients. In small grain cropping systems, Sainju et al. [9, 55] observed that net GWP and GHGI were lower with no-tillage malt-barley pea with reduced nitrogen fertilization rate than conventional tillage continuous malt barley or malt barley-fallow with recommended nitrogen rate. They attributed this to increased soil carbon sequestration, reduced N2O emissions, and sustained crop yields.
Using a meta-analysis of nine experiments, Sainju  reported that the improved combined management practice that included no-tillage, diversified cropping system (crop rotation, increased cropping intensity, cover crop, and perennial cropping system) and reduced nitrogen rate reduced net GWP and GHGI by 70–88% compared with the traditional combined practice that included conventional till, less diversified cropping system (monocropping, crop-fallow, no cover crop, and annual cropping system) and recommended nitrogen rate. He also found that combined management practice further reduced net GWP and GHGI compared with individual management practices. He found that changes in net GWP and GHGI due to improved vs. traditional combined management practice increased with the duration of the experiment. The relationships were further improved by including soil and climatic factors in the multiple linear regressions. Some of the possible reasons for increased net GWP and GHGI for improved vs. traditional combined management with increased duration of the experiment are: (1) high spatial and temporal variations of GHG emissions due to differences in soil and climatic conditions and management practices, (2) reduced potential for soil C sequestration with increasing duration of the experiment, (3) use of full or partial accounting of sources and sinks of GHG emissions, and (4) uncertainty in the methods of measuring GHG emissions, such as variations in type and size of static chambers, placement of chamber in the plot (row vs. inter-row or including vs. excluding plants in the chamber), time of GHG measurement during the day, and calculation of GHG fluxes (linear or nonlinear emissions with time).
When crop residue was burned compared with residue retained in the soil under wheat applied with or without nitrogen fertilizer with various tillage practices, Wang et al.  found that net GWP and GHGI were lower in conventional tillage wheat without nitrogen fertilizer where residue was burned than conventional tillage or no-tillage wheat with nitrogen fertilizer where reside was either burned or retained in the soil. They found that the larger impact of N2O emissions than soil carbon sequestration on global warming potential increased net GWP and GHGI with N fertilization than without.
Using an alternative method where soil respiration and previous year’s crop residue returned to the soil are used in place of soil carbon sequestration rate to calculate net GWP and GHGI, Mosier et al.  observed that no-tillage continuous corn with reduced nitrogen fertilization rate reduced net GWP and GHGI compared with conventional tillage corn-soybean with recommended N rate, a case similar to that calculated by the regular method above. They attributed this to increased amount of crop residue returned to the soil and grain yield. Similarly, using this method, Sainju et al.  found lower net GHGI in nonirrigated no-tillage barley-pea with nitrogen fertilizer than conventional tillage continuous barley with nitrogen fertilizer, a case similar to that obtained for the regular method. They, however, observed different trends for net GWP. Similarly, using the alternative method, Johnson et al.  found lower net GWP and GHGI in conventional tillage corn-soybean with nitrogen and phosphorus fertilizers than no-tillage continuous corn with the same fertilizers, a case different to that obtained by using the regular method. Popp et al.  using the alternative method, found that net GWP was lower with nonirrigated corn than irrigated and nonirrigated cotton, soybean, sorghum, irrigated rice, and nonirrigated wheat. The magnitude of net GWP and GHGI obtained by two methods can be different, but both methods showed that no-till with continuous cropping produced lower net GWP and GHGI compared with conventional tillage with crop fallow [3, 9].
2.7. Implications of management practices
These studies showed that no-tillage systems, in general, can reduce net GWP and GHGI compared with conventional tillage systems. Perennial crops can reduce net GWP and GHGI compared with annual crops and wheat can reduce net GWP and GHGI compared with rice and corn. Inclusion of legumes in rotation with nonlegumes has variable effects on net GWP and GHGI compared with continuous nonlegumes. Inclusion of fallow in the crop rotation, however, can increase net GWP and GHGI compared with continuous cropping. Crops adequately fertilized with nitrogen, phosphorus, and potassium fertilizers can reduce net GWP and GHGI compared with no fertilized treatments, but excessive nitrogen fertilization beyond crop nitrogen demand can increase net GWP and GHGI. Burning of crop residue slightly can increase net GWP and GHGI compared with residue retained in the soil, but irrigation has minor effect compared with non-irrigation. Improving drainage or using shallow flooding in rice can lower net GWP and GHGI compared with continuous flooding. Values of net GWP and GHGI measured by the regular and alternative methods are variable, depending on soil and climatic conditions and management practices. Both methods, however, showed that the improved management practice can reduce net GHG emissions compared with the traditional management practice. Changes in net GWP and GHGI due to improved vs. traditional management varied with duration of the experiment and inclusion of soil and climatic factors improved their relationships. Also, combined management practice can lower net GWP and GHGI compared with the individual practice. Net GWP and GHGI values can be more reliable by accounting full than partial sources and sinks of CO2 emissions. Because of the limited data, further studies are needed to evaluate the effects of management practices on net GWP and GHGI.