Effect of cover crop and N fertilization rate on yield and N uptake by cotton lint, sorghum grain, and their biomass (stems + leaves) from 2000 to 2002 in central Georgia, USA .
Nitrogen (N) is a major limiting nutrient to sustain crop yields and quality. As a result, N fertilizer is usually applied in large quantity to increase crop production throughout the world. Application of N fertilizers has increased crop yields and resulted in achievement of self-sufficiency in food production in many developing countries. Excessive application of N fertilizers beyond crops’ demand, however, has resulted in undesirable consequences of degradation in soil, water, and air quality. These include soil acidification, N leaching in groundwater, and emissions of nitrous oxide (N2O), a potent greenhouse gas that contributes to global warming. Long-term application of ammonia-based N fertilizers, such as urea, has increased soil acidity which rendered to soil infertility where crops fail to respond with further application of N fertilizers. Another problem is the groundwater contamination of nitrate-N (NO3-N) which can be a health hazard to human and livestock if its concentration goes above 10 mg L−1 in drinking water. The third problem is emissions of N2O gas which is 300 times more powerful than carbon dioxide in terms of global warming potential. This chapter examines the effect of N fertilization on soil and environmental quality and crop yields.
- crop yields
- environmental quality
- management practices
- nitrogen fertilizer
- nitrogen-use efficiency
- soil quality
Nitrogen (N) is a major limiting factor for sustainable and profitable crop production. However, excessive N application through fertilizers and manures can degrade soil and environmental quality by increasing soil acidification, N leaching, and emissions of ammonia (NH3) and nitrogen oxide (NO, N2O, and NO2) gases, out of which N2O is considered a highly potent greenhouse gas that contributes to global warming [1, 2]. Nitrogen application more than crop’s need can also result in reduced yield . Additional N inputs include dry and wet (snow and rain) depositions from the atmosphere, biological N fixation, and irrigation water. Because crops can remove about 40–60% of applied N, the soil residual N (nitrate-N [NO3-N] + ammonium-N [NH4-N]) after crop harvest can be lost to the environment through leaching, denitrification, volatilization, surface runoff, soil erosion, and N2O emissions [3, 4]. One option to reduce soil residual N is to increase N-use efficiency. Nitrogen-use efficiency for crops, however, can be lower at high N fertilization rates . Improved management practices can increase N-use efficiency, enhance soil N storage, and reduce N fertilizer application which reduce N losses to the environment . An account of N inputs, outputs, and retention in the soil provides N balance and helps to identify dominant processes of N flow in the agroecosystem .
Economically profitable crop yields could be achieved by recommended N fertilization rates . However, such a yield potential for a crop varies with soil and climatic conditions, crop species, variety, nutrient cycling, and competitions with weeds and pests . Crop production can be optimized and potential for N losses minimized by adjusting N fertilization rates using soil residual and potentially mineralizable N values. Studies show that ∼1–2% of soil organic N in the 0–30 cm depth is mineralized every year . Measuring the actual amount of N mineralized is a time taking process. A commonly used method for measuring soil available N and determining nitrogen rates for crops in semiarid regions of northern Great Plains, USA is based on testing NO3-N content in soils to a depth of 60 cm after crop harvest in the fall season of the previous year and deduct the value from recommended N rates for the current crop year [7, 8]. In semiarid regions such as Great Plains of USA, N losses to the environment due to N leaching, volatilization, and denitrification during the winter are considered minimal due to cold weather and limited precipitation in the region.
Nitrogen fertilizers are being increasingly applied to crops to enhance their yield and quality in South Asia, where land available for crop production is limited, the proportion of cultivated land to population is low, and the pressure to increase crop yields to meet the demand for growing population is high. Continuous application of N fertilizers to nonlegume crops and excessive application rates in some places have led to undesirable consequences, such as reduced crop yields and degraded soil and environmental quality from soil acidification, N leaching, and greenhouse gas (N2O) emissions. In this chapter, we discuss the consequences of N fertilization to crop yields and soil and environmental quality.
2. Crop yields, nitrogen uptake, and nitrogen-use efficiency
Nitrogen fertilization can increase crop yields and N uptake compared with no N fertilization. This has been documented for malt barley (Hordeum vulgare L.), cotton (Gossypium hirsutum L.), and sorghum (Sorghum bicolor [L.] Moench) (Figures 1 and 2, Table 1) by various researchers in Georgia and Montana, USA [9, 10, 14]. It is not unusual to achieve higher crop yield with increased N fertilization rate due to increased soil N availability . Crop yields, however, can remain at similar level or decline with further increase in N rates after reaching the maximum yield. Sainju  observed that annualized grain and biomass yields of barley and pea (Pisum sativum L.) and their C content maximized at 80 kg N ha−1 and then declined, as N rate increased to 120 kg N ha−1 (Figure 1). Similarly, Sainju et al.  reported that malt barley yield and N uptake increased from 0 to 40 kg N ha−1 and then declined with further increase in N rates in no-till and conventional till malt barley-fallow rotation (Figure 2). In no-till continuous malt barley and malt barley-pea rotation, they found that increased N rate from 0 to 120 kg N ha−1 continued to increase malt barley yield and N uptake. Increased soil residual N due to fallow as a result of enhanced soil N mineralization from increased soil temperature and water content resulted in a reduced response of malt barley yield and N uptake with N fertilization in no-till and conventional till malt barley-fallow rotation. A study reported a need of 27 kg of total soil and fertilizer N to produce 1 Mg of malt barley grain in irrigated no-till field in Colorado, USA .
|2000 cotton lint (kg ha−1)||2000 cotton biomass (kg ha−1)||2001 sorghum grain (kg ha−1)||2001 sorghum biomass (kg ha−1)||2002 cotton lint (kg ha−1)||2002 cotton biomass (kg ha−1)|
|Treatment||Yield||N uptake||Yield||N uptake||Yield||N uptake||Yield||N uptake||Yield||N uptake||Yield||N uptake|
|N fertilization rate (kg N ha−1)|
Increased N fertilization rate can also increase grain quality, such as protein concentration [10, 11]. Increased N fertilization rates increased malt barley grain yield and protein concentration, but reduced kernel plumpness in Canada . While some studies reported malt barley grain protein concentration of <130 g kg−1 with N rate of 168–200 kg ha−1 (e.g., ) others, observed an increase in protein concentration even with N rates <150 kg N ha−1 (e.g., ). Grain protein and kernel plumpness are important characteristics of malt barley that need to be maintained at critical levels (grain protein ≤129 g kg−1, kernel plumpness ≥850 g kg−1) for beer production . Therefore, appropriate N fertilization rates are required to malt barley to achieve a balance between optimum grain yield, kernel plumpness, and protein concentration .
Sainju et al.  evaluated the effect of N fertilization on cotton and sorghum yields and N uptake from 2000 to 2002 in central Georgia, USA (Table 1). They found that cotton lint, sorghum grain, and cotton and sorghum biomass yields and N uptake increased from 0 to 60–65 kg N ha−1 and then remained either at a similar level or slightly increased at 120–130 kg N ha−1. The response of cotton yield to N fertilization, however, depended on climatic condition, as cotton lint and biomass yields were greater in 2000 than 2002 when the growing season precipitation was below the average. The N fertilizer required for optimizing cotton and sorghum yields varied with the type of tillage and cover crop . Boquet et al.  reported that cotton lint yield was lower with no-tillage than surface tillage without applied N, but at optimum N rate, yields were higher with no-tillage. They also found that additional N was required to optimize cotton yield following wheat (Triticum aestivum L.) in no-tillage and surface tillage systems without cover cropping, but no N rate was required following hairy vetch cover crop in either tillage practices. Similarly, N fertilization rates to cotton and sorghum can be reduced or eliminated by using legume cover crops, such as red clover (Trifolium incarnatum L.) and hairy vetch (Vicia villosa Roth), regardless of tillage practices . The high rate of N fertilization can produce excessive vegetative growth that delays maturity and harvest and reduces cotton lint yield and N uptake .
Nitrogen-use efficiency, defined as crop yield or N uptake per unit applied N fertilizer, is a useful measurement of the efficiency of N fertilization to crop yields . Enhancing N-use efficiency can maximize crop yield and N uptake with limited use of fertilizer N while reducing N rate and sustaining the environment . Nitrogen-use efficiency, however, can decrease with increased N fertilization rate due to the inability of crops to utilize N efficiently . Sainju et al.  found that N-use efficiency by malt barley decreased curvilinearly with increased N fertilization rate (Figure 2). Varvel and Peterson  reported that N removed by corn and sorghum grain was 50% of the applied N at low N rates and at least 20–30% at high N rates.
Nitrogen fertilization can also increase aboveground biomass yield of perennial grasses used for feedstock or bioenergy production. Sainju et al.  observed that yields of intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey), switchgrass (Panicum virgatum L.), and smooth bromegrass (Bromus inermis L.) increased linearly or curvilinearly with increased N fertilization rate in 2011 and 2013 (Figure 3) when the annual precipitation was near or above the average. Biomass yield, however, did not respond to N fertilization in 2012 when the annual precipitation was below the average. Several researchers [21, 22] reported that maximum switchgrass shoot biomass yield reached at 120–140 kg N ha−1 in Iowa and Nebraska, USA, which had 2.5 and 2.2 times, respectively, more annual precipitation than in eastern Montana, USA. Power  also observed increased shoot biomass yield with increased N rate for smooth bromegrass in North Dakota, USA.
3. Soil acidification
Application of NH4-based N fertilizers can increase soil acidity due to the release of H ions during hydrolysis . Increased soil acidity following the application of N fertilizers leads to the development of infertile soils that do not respond well to crop yields with further application of N fertilizers [2, 25], thereby resulting in inefficient use of fertilizers . Sainju et al.  reported that, after 30 years of tillage and cropping sequence, continuous application of N fertilizers reduced soil pH at the 0–7.5 cm depth from 6.30 at the initiation of the experiment to 5.73 in spring till spring wheat-fallow (STW-F) and to 5.02 in fall and spring till continuous spring wheat (FSTCW) under rainfed condition in eastern Montana, USA (Table 2). A similar decline in soil pH at 7.5–15.0 cm was observed from 6.75 at the initiation of the experiment to 6.15 in spring till continuous spring wheat (STCW). Buffer pH, the buffering capacity of the soil to resist changes in pH and is used to measure lime requirement, also similarly decreased with continuous N fertilization in all treatments. Both pH and buffer pH, however, did not change below 15 cm with N fertilization. Because spring wheat was grown once in 2 years in spring wheat-fallow rotation where N fertilizer was applied only to spring wheat, soil pH was less declined in this treatment than continuous spring wheat where N fertilizer was applied every year. From the same experiment, Aase et al.  reported an average decline of pH at 0–7.5 cm from 6.3 to 5.7 after 10 years due to continuous N fertilization.
|Tillage and cropping sequencea||Soil depth|
|0–7.5 cm||7.5–15 cm||15–30 cm||30–60 cm||60–90 cm||90–120 cm|
|NT vs. T||0.29||0.26||−0.09||0.08||−0.08||0.04|
|CW vs. W-F||−0.68***||−0.88**||−0.08||0.01||0.13||0.04|
|CW vs. W-B/P||−0.43*||−0.11||0.20||0.15||0.16||0.14|
|NT vs. T||0.05||0.08||−0.01||0.01||0.01||0.01|
|CW vs. W-F||−0.43***||−0.24**||−0.01||−0.01||0.01||−0.01|
|CW vs. W-B/P||−0.24*||−0.08||−0.01||0.03||0.01||0.03|
Ghimire et al.  found that soil pH at 0–10 cm after 70 years of N fertilization was 5.70 with 0 kg N ha−1 and 5.0 with 135–180 kg N ha−1 under winter wheat-fallow in eastern Oregon, USA (Figure 4). Reduction in pH with N fertilization decreased with depth, with no significant effect below 30 cm. A study in China, where intensive farming and high rate of N fertilizer was applied for 20 years, showed that soil pH was dropped by 0.30–0.80 units from the original level . In eastern Oregon, USA, application of total N fertilizer at 2.25 Mg N ha−1 over the 43-year period lowered soil pH by 0.60 units . Liebig et al.  reported that, in North Dakota, USA, soil pH at 0–7.6 cm was lower under continuous corn than corn rotated with legume and other nonlegume crops because of the increased amount of N fertilizer applied. They recommended that soil samples be collected to a depth of 15 cm for measuring changes in soil pH due to N fertilization.
No-till (NT) system can increase soil acidity more than the conventional till (CT) system . This is due to differences in the amount and placement of N fertilizers in the soil and removal of basic cations through grain and biomass removal between the two tillage systems . Nitrogen fertilizers are usually placed at the soil surface, and N rates are usually higher in NT due to the accumulation of surface residue that partly immobilizes N than CT where fertilizers are incorporated into the soil due to tillage . Because of enhanced soil water conservation, crop yields are higher in NT than CT, especially in dryland cropping systems . As a result, crops remove more basic cations, resulting in increased acidity with NT compared with CT . In contrast, Ghimire et al.  reported that soil pH decreased with increased N rate, as tillage intensity increased.
Source of N fertilizer can also have a varying effect on soil acidity. Chen et al. found that soil acidity from N fertilizer sources was in the order (NH4)2SO4 > NH4Cl > NH4NO3 > anhydrous NH3 > urea. Similarly, Schroder et al.  reported that anhydrous NH3 produce more acidity than urea. Others , however, observed no significant differences in acidity among (NH4)2SO4, NH4NO3, anhydrous NH3, urea, and urea-NH4NO3.
4. Soil organic matter
Soil organic matter refers to soil organic C and N and is a crucial component of soil health and quality [36, 37]. Nitrogen fertilization can increase soil organic C and N by increasing crop biomass yield, and the amount of residue returned to the soil . Russell et al. , however, reported no difference in soil organic C with N fertilization rate. Sainju et al.  reported that 3 years of N fertilization to cotton and sorghum produced various results on soil organic C at the 0–30 cm depth in strip-tilled and chisel-tilled soils in central Georgia, USA (Table 3). Soil organic C at 0–10 and 10–30 cm varied with N fertilization rates in strip-tilled soil, but increased in chisel-tilled soil due to differences in tillage intensity. In strip tillage, only crop rows are tilled, leaving the area between rows undisturbed, and N fertilizer is applied in crop rows. In contrast, the land is tilled using discs in chisel tillage after N fertilizer is broadcast. Differences in N fertilization methods between tillage practices probably affected soil organic C due to N fertilization rates.
|N rate (kg N ha−1)||Soil organic C (Mg C ha−1)|
|0–10 cm||10–30 cm||30–60 cm||60–90 cm||90–120 cm|
Sainju  observed different trends of soil organic C at the 0–120 cm depth with 6 years of N fertilization rates in various cropping systems in eastern Montana, USA (Figure 5). Soil organic C at 0–5 and 5–10 cm peaked at 40 kg N ha−1 and then declined with further increase in N rates in no-till malt barley-pea (NTB-P) and continuous no-till barley (NTCB). In no-till malt barley-fallow (NTB-F) and conventional till malt barley-fallow (CTB-F), the trend of soil organic C with N rates varied at various depths. Soil organic C at these depths was greater with NTB-P and NTCB than other treatments at most N rates due to greater amount of crop residue returned to the soil. Soil organic C at 5–10, 30–60, and 60–90 cm were greater with 40 kg N ha−1 than other N rates. Sainju  also found that C sequestration rate at 0–10 cm was 83 kg C ha−1 year−1 with 40 kg N ha−1 that was close to 94 kg C ha−1 year−1 at 0–15 cm with 45 kg N ha−1 for dryland cropping systems in Colorado .
Under perennial grasses, several researchers [40, 41] did not find a significant effect of N fertilization on soil organic C at 0–30 cm after 2–5 years in Alabama and Colorado, USA. Only after 4–12 years, N fertilization increased soil organic C at 0–90 cm by 0.5–2.4 Mg C ha−1 year−1 compared with no N fertilization under switchgrass in USA and Canada [42, 43]. Rice et al.  reported that N fertilization to cool-season grasses increased C sequestration rate at 0–30 cm by 1.6 Mg C ha−1 year−1 compared with no N fertilization after 5 years in Kansas, USA. In Alberta, Canada, Bremer et al.  observed that N fertilization to perennial grasses increased C sequestration rate at 0–5 cm by 0.5 Mg C ha−1 year−1 compared with no N fertilization after 6–12 years. In South Dakota, USA, Li et al.  noted C sequestration rate of 2.4 Mg C ha−1 year−1 at 0–90 cm under switchgrass after 4 years. Sainju et al.  found increasing trend of soil total C at 30–60 cm with increased N rate under intermediate wheatgrass and smooth bromegrass and a declining trend with switchgrass after 5 years in eastern Montana (Figure 6). At 60–90 cm, the trend reversed with grasses. They suggested that longer than 5 years is needed to observe the effect of N fertilization on soil total C under perennial grasses.
Nitrogen fertilization has less impact on soil total N than soil organic C. Sainju and Singh  reported that soil total N at 0–15 cm under cotton and sorghum was greater with 60–65 than 0 kg N ha−1, but not at lower depths in the chisel-tilled soil in central Georgia, USA (Figure 7). Ghimire et al.  observed that soil total N at 10–20 cm increased with increased N rates after 70 years of N fertilization to winter wheat, but the trend varied with different tillage practices at higher N rates in eastern Oregon, USA (Figure 8). At 0–45 kg N ha−1, soil total N was greater with subsurface sweep than a moldboard plow. At 90–180 kg N ha−1, soil total N was lower with disc plow than other tillage practices. Increased N substrate availability due to N fertilization along with tillage may have increased microbial activity and N mineralization and therefore reduced soil total N over time.
5. Soil residual nitrogen and nitrogen leaching
Soil residual N refers to inorganic N (NH4-N + NO3-N) accumulated in the soil profile after crop harvest. This occurs because crops cannot take up all applied N fertilizer from the soil [5, 47]. Accumulation of soil NO3-N increases with depth and is directly related to N fertilization rate [47, 48]. Deep accumulation of NO3-N in the soil profile increases the potential for N leaching to shallow water tables . Nitrogen fertilization rates that exceed crop requirement can increase NO3-N accumulation in the soil profile and N leaching .
One of the ways to reduce N fertilization rates to crops while maintaining yield goals is to account for N mineralized from soil organic matter during the crop growing season and soil residual N at crop planting . Since the measurement of N mineralization requires a long time, N fertilization rates to dryland crops are adjusted by deducting soil NO3 content to a depth of 60 cm after crop harvest in the previous year or at planting of the current year from recommended N rates . Producers are increasingly interested in reducing the amount of N fertilizer applied to crops because of the higher cost of N fertilization and the associated environmental degradation.
Nitrogen fertilization rates to crops can be higher in the no-till than the conventional till system due to greater accumulation of surface crop residue that can enhance N immobilization . On the other hand, N rates can be reduced in crop rotations containing legumes compared to monoculture nonlegume cropping systems . Nonlegume monocropping can have higher soil residual NO3-N content than legume-based crop rotations due to increased N fertilization rate [5, 27]. Increased cropping intensity can reduce soil profile NO3-N content due to greater N immobilization, less summer fallow, and a greater amount of N removed by crops . Sainju et al.  and Sainju  found that both soil NH4-N and NO3-N contents increased with N rates and depths (Tables 4–6).
|Soil inorganic N|
|Treatment||0–10 cm||10–30 cm||0–30 cm|
|(kg N ha−1)|
|N fertilization rate (kg N ha−1)|
|N fertilization rate||NH4-N content at the soil depth|
|0–5 cm||5–10 cm||10–30 cm||30–60 cm||60–90 cm||90–120 cm||0–10 cm||0–30 cm||0–60 cm||0–90 cm||0–120 cm|
|kg N ha−1||kg N ha−1|
|N fertilization rate||NO3-N content at the soil depth|
|0–5 cm||5–10 cm||10–30 cm||30–60 cm||60–90 cm||90–120 cm||0–10 cm||0–30 cm||0–60 cm||0–90 cm||0–120 cm|
|kg N ha−1||kg N ha−1|
It is well known that excessive N fertilizer application can increase N leaching in the groundwater, which is a major environmental concern . Nitrate-N concentration >10 mg L−1 in the drinking water poses a serious threat to human and animal health . Nitrate-N is soluble in water and moves down the soil profile with percolating water [47, 57]. Increased application of N fertilizer to crops during the last several decades has increased NO3-N contamination of groundwater . This occurs because of excessive NO3-N accumulation in the soil profile  due to N fertilization rates that exceed crop requirements, accompanied by poor soil and crop management practices . Nitrate-N accumulation and movement in the soil profile depend on soil properties, climatic conditions, and management practices . For example, N leaching is greater in sandy than clayey soils due to the presence of a large number of macropores and leaching is higher in the humid than arid and semiarid regions due to differences in annual precipitation [56, 58]. Nitrate-N leaching occurs mostly in the fall, winter, and spring seasons in the northern hemisphere when evapotranspiration is low, crops are absent to uptake soil N, and precipitation exceeds the water holding capacity of the soil .
6. Greenhouse gas emissions and global warming potential
Management practices on croplands can contribute about 10–20% of global greenhouse gases (GHGs: carbon dioxide [CO2], nitrous oxide [N2O], and methane [CH4]) . Quantitative estimate of the impact of the GHGs to global radiative forcing is done by calculating net global warming potential (GWP) which accounts for all sources and sinks of CO2 equivalents from farm inputs, farm operations, soil C sequestration, and N2O and CH4 emissions [61, 62]. The net GWP for a crop production system is expressed as kg CO2 eq. ha−1 year−1. Net GWP is also expressed as net greenhouse gas intensity (GHGI) or yield-scaled GWP, which 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 C sequestration rate can be either a sink or source of CO2 [62, 63]. 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 310 and 28, respectively, for a time horizon of 100 years . The balance between soil C sequestration rate, N2O and CH4 emissions (or CH4 uptake), and crop yield typically controls net GWP and GHGI [61, 62].
Nitrogen fertilization typically stimulates N2O emissions when the amount of applied N exceeds crop N demand [51, 61]. Nitrogen fertilization, however, can have a variable effect on emissions of other GHGs, such as CO2 and CH4 [64, 65]. Sainju et al.  found that the application of 80 kg N ha−1 to dryland malt barley increased CO2 emissions, but not N2O and CH4 emissions (Table 7). 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 [61, 62]. Other factors that can influence N2O emissions are the type, placement, time, and method of application of N fertilizers. Applying N 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 . Applying N fertilizer at various depths can have a variable effect on N2O emissions . Anhydrous ammonia can increase N2O emissions compared with urea [67, 68]. Similarly, chemical additives to reduce nitrification from N fertilizers, such as polymer-coated urea and nitrification inhibitors, can substantially reduce N2O emissions compared with ordinary urea and non-nitrification inhibiting fertilizers . Some N fertilizers, such as urea, emit both CO2 and N2O. Nitrogen fertilizers also indirectly emit N2O through NH3 volatilization and NO3-N leaching .
|N fertilization||CO2 flux||N2O flux||CH4 flux|
|kg N ha−1||Mg C ha−1||g N ha−1||g C ha−1|
Increased N fertilization rate can enhance net GWP and GHGI due to increased N2O and CO2 emissions associated with the manufacture, transport, and application of N fertilizers, regardless of cropping systems and calculation methods [61, 70]. 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 N fertilization rate (Figure 9). 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 N rate. These N rates probably corresponded to crop N demand when crops used most of the soil available N. The cropping systems that left little residual N in the soil reduced N2O emissions, and therefore net GWP and GHGI, whereas net GWP and GHGI increased linearly with increase in N application rates that exceeded crop N demand, suggesting that excessive N fertilizer applications can induce global warming. Similar results have been reported by Li et al. . Therefore, N fertilizers should be applied at optimum rates to reduce net GWP and GHGI while sustaining crop yields. The optimum N 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 N 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 a 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. Sainju et al.  reported that net GWP and GHGI calculated from soil respiration and soil C sequestration methods were lower with 80 than 0 kg N ha−1 (Table 8). They noted that, although CO2 equivalents from N fertilization and soil respiration were higher with 80 kg N ha−1, the amount of plant residue returned to the soil, soil C sequestration rate, and grain yields were greater with 80 than 0 kg N ha−1, thereby resulting in lower net GWP and GHGI with N fertilization than without, regardless of the method used for calculation.
|Cropping sequencea||N rate||Farm operation (A)||N fertilizer (B)b||Soil respiration (C)||N2O flux (D)||CH4 flux (E)||Annualized crop residue (F)c||SOC (G)d||GWPR (H)e||GWPC (I)f||Annualized grain yield (J)||GHGIR (K)g||GHGIC (L)h|
|kg N ha−1||kg CO2 equivalent ha−1 year−1||kg ha−1||kg CO2 kg−1 grain yield|
Nitrogen fertilization is one of the most commonly used practice to increase crop yields throughout the world because of abundant availability of N fertilizers and their great effectiveness to increase yields compared with other organic fertilizers, such as manure and compost. Excessive application of N fertilizers in the last several decades, however, has resulted in undesirable consequences of soil and environmental degradations, such as soil acidification, N leaching to the groundwater, and greenhouse gas (N2O) emissions. Crop yields have declined in places where soil acidification is high due to unavailability of major nutrients and basic cations and toxic effect of acidic cations. Other disadvantages of excessive N fertilization include increased cost of fertilization, reduced N-use efficiency, and negative impact on human and livestock health. To reduce excessive N fertilization, composited soil sample to a depth of 60 cm should be conducted for NO3-N test prior to crop planting and N fertilization rate be adjusted by deducting soil NO3-N content from the desirable N rate.