Residue lignin content and biochemistry are important properties influencing residue decomposition dynamics and native soil C loss through priming. The relative contribution of high lignin residues ...to soil organic matter (SOM) may be less than previously believed, be more sensitive to soil N status, and may be more sensitive to increased temperature. We examined the role of residue biochemistry, temperature, and soil N on the decomposition dynamics of five crop residues varying in lignin content and composition (corn, sorghum, soybean, sunflower and wheat). We used natural abundance δ¹³CO₂to quantify residue decomposition and soil priming from a soil previously cropped to wheat-fallow or to corn-millet-wheat at 20 and 30 °C in a laboratory incubation. High lignin residues decomposed more completely than low lignin residues, supporting a new model of SOM formation suggesting high lignin residues have a lower efficiency for stabilizing SOM due to inefficient microbial processing. However, residues with lower residue respiration had greater soil C respiration (soil priming). Residue SG lignin was positively related to residue C respired and H-lignin positively related to soil C respired in all soils and temperatures, resulting in no net lignin chemistry effect on the combined total C respired. Effects of lignin on residue decomposition were most apparent in treatments with lower soil N contents indicating N limitation. Measuring both residue and soil respiration and considering soil N status is important to accurately assess the effects of residue biochemistry on soil organic carbon.
Enhanced‐efficiency N fertilizers (EENFs) have potential for mitigating N2O emissions from N‐fertilized cropping systems. Stabilized EENFs contain nitrification and/or urease inhibitors. Slow‐release ...EENFs contain N components that are slowly released with variable release rates. Controlled‐release EENFs release N at more predictable rates. The effectiveness of several EENFs in reducing soil N2O emissions from a clay loam soil under irrigated, corn (Zea mays L.)‐based production systems in Colorado (2002–2012) was investigated. A controlled‐release, polymer‐coated urea, ESN, reduced N2O emissions by 42% compared with urea and 14% compared with urea–NH4NO3 solution (UAN) in no‐till and strip‐till environments, but had no effect in a conventional tillage environment. A stabilized urea source, SuperU, reduced N2O emissions by 46% compared with urea and 21% compared with UAN. A stabilized UAN source, UAN + AgrotainPlus, reduced N2O emissions by 61% compared with urea and 41% compared with UAN alone. A slow‐release UAN source, UAN + Nfusion, reduced N2O emissions by 57% compared with urea and 28% compared with UAN. Urea–NH4NO3 reduced N2O emissions by 35% compared with urea. A linear increase in N2O emissions with increasing N rate was observed for untreated urea and UAN. Developers of management protocols to reduce N2O emissions from irrigated cropping systems in semiarid areas can use this information to estimate reductions in N2O emissions when EENFs are used. Policymakers can use this information to help determine financial credits needed to encourage producers to use these technologies in their crop production systems.
Few studies have assessed the common, yet unproven, hypothesis that an increase of plant nitrogen (N) uptake and/or recovery efficiency (NRE) will reduce nitrous oxide (N
O) emission during crop ...production. Understanding the relationships between N
O emissions and crop N uptake and use efficiency parameters can help inform crop N management recommendations for both efficiency and environmental goals. Analyses were conducted to determine which of several commonly used crop N uptake-derived parameters related most strongly to growing season N
O emissions under varying N management practices in North American maize systems. Nitrogen uptake-derived variables included total aboveground N uptake (TNU), grain N uptake (GNU), N recovery efficiency (NRE), net N balance (NNB) in relation to GNU NNB
and TNU NNB
, and surplus N (SN). The relationship between N
O and N application rate was sigmoidal with relatively small emissions for N rates <130 kg ha
, and a sharp increase for N rates from 130 to 220 kg ha
; on average, N
O increased linearly by about 5 g N per kg of N applied for rates up to 220 kg ha
. Fairly strong and significant negative relationships existed between N
O and NRE when management focused on N application rate (
= 0.52) or rate and timing combinations (
= 0.65). For every percentage point increase, N
O decreased by 13 g N ha
in response to N rates, and by 20 g N ha
for NRE changes in response to rate-by-timing treatments. However, more consistent positive relationships (
= 0.73-0.77) existed between N
O and NNB
, NNB
, and SN, regardless of rate and timing of N application; on average N
O emission increased by about 5, 7, and 8 g N, respectively, per kg increase of NNB
, NNB
, and SN. Neither N source nor placement influenced the relationship between N
O and NRE. Overall, our analysis indicated that a careful selection of appropriate N rate applied at the right time can both increase NRE and reduce N
O. However, N
O reduction benefits of optimum N rate-by-timing practices were achieved most consistently with management systems that reduced NNB through an increase of grain N removal or total plant N uptake relative to the total fertilizer N applied to maize. Future research assessing crop or N management effects on N
O should include N uptake parameter measurements to better understand N
O emission relationships to plant NRE and N uptake.
The impact of management on global warming potential (GWP), crop production, and greenhouse gas intensity (GHGI) in irrigated agriculture is not well documented. A no-till (NT) cropping systems study ...initiated in 1999 to evaluate soil organic carbon (SOC) sequestration potential in irrigated agriculture was used in this study to make trace gas flux measurements for 3 yr to facilitate a complete greenhouse gas accounting of GWP and GHGI. Fluxes of C(O)2, CH4, and N(2)O were measured using static, vented chambers, one to three times per week, year round, from April 2002 through October 2004 within conventional-till continuous corn (CT-CC) and NT continuous corn (NT-CC) plots and in NT corn-soybean rotation (NT-CB) plots. Nitrogen fertilizer rates ranged from 0 to 224 kg N ha-1. Methane fluxes were small and did not differ between tillage systems. Nitrous oxide fluxes increased linearly with increasing N fertilizer rate each year, but emission rates varied with years. Carbon dioxide efflux was higher in CT compared to NT in 2002 but was not different by tillage in 2003 or 2004. Based on soil respiration and residue C inputs, NT soils were net sinks of GWP when adequate fertilizer was added to maintain crop production. The CT soils were smaller net sinks for GWP than NT soils. The determinant for the net GWP relationship was a balance between soil respiration and N(2)O emissions. Based on soil C sequestration, only NT soils were net sinks for GWP. Both estimates of GWP and GHGI indicate that when appropriate crop production levels are achieved, net C(O)2 emissions are reduced. The results suggest that economic viability and environmental conservation can be achieved by minimizing tillage and utilizing appropriate levels of fertilizer.
Nitrogen fertilization is essential for optimizing crop yields; however, it increases N2O emissions. The study objective was to compare N2O emissions resulting from application of commercially ...available enhanced-efficiency N fertilizers with emissions from conventional dry granular urea in irrigated cropping systems. Nitrous oxide emissions were monitored from corn (Zea mays L.) based rotations receiving fertilizer rates of 246 kg N ha–1 when in corn, 56 kg N ha–1 when in dry bean (Phaseolus vulgaris L.), and 157 kg N ha–1 when in barley (Hordeum vulgare L. ssp. vulgare). Cropping systems included conventional-till continuous corn (CT-CC), no-till continuous corn (NT-CC), no-till corn–dry bean (NT-CDb), and no-till corn–barley (NT-CB). In the NT-CC and CT-CC systems, a controlled-release, polymer-coated urea (ESN) and dry granular urea were compared. In the NT-CDb and NT-CB rotations, a stabilized urea source (SuperU) was compared with urea. Nitrous oxide fluxes were measured during two growing seasons using static, vented chambers and a gas chromatograph analyzer. Cumulative growing season N2O emissions from urea and ESN application were not different under CT-CC, but ESN reduced N2O emissions 49% compared with urea under NT-CC. Compared with urea, SuperU reduced N2O emissions by 27% in dry bean and 54% in corn in the NT-CDb rotation and by 19% in barley and 51% in corn in the NT-CB rotation. This work shows that the use of no-till and enhanced-efficiency N fertilizers can potentially reduce N2O emissions from irrigated systems
Limited information is available on how N fertilizer placement affects soil nitrous oxide (N2O) emissions under irrigated conditions in the semiarid western United States. Our objective was to ...compare surface banding near corn row and broadcasting of three N sources (urea, polymer‐coated urea PCU, and stabilized urea SU containing urease and nitrification inhibitors) on N2O emissions from a clay loam soil under sprinkler‐irrigated continuous corn production. The N fertilizers were applied at a rate of 202 kg N ha−1 to strip‐till (2010 and 2011) and no‐till (2011) corn at crop emergence, with ∼19 mm irrigation water applied the next day. Band‐applied N had a 1.46‐fold greater N2O emission than broadcast N averaged over N sources and three studies. Soil N2O–N emissions from urea were 1.48‐ and 1.74‐fold greater than from PCU and SU, respectively, when averaged over N placement and studies. The N placement × source interaction was not significant. Averaged across studies, grain yield and N uptake did not vary with N placement, whereas grain yields were greater for SU than PCU but were not different from urea. Nitrous oxide emissions per unit of N applied, per unit of grain yield, and per unit N uptake were 59, 49, and 47% greater, respectively, with banded than with broadcast N fertilizer. These studies show that N placement and N source selection are important manageable factors that can affect N2O emissions and need to be considered when developing N2O mitigation practices in irrigated cropping systems in the semiarid western United States.
Nitrogen rate studies were conducted under furrow‐irrigated corn (Zea mays L.) production on a silty clay soil to compare granular urea with polymer‐coated urea (PCU) and stabilized urea (SU, ...contains urease and nitrification inhibitors) effects on corn yields, plant N uptake, and N use efficiency. Polymer‐coated urea had a yield advantage over urea 2 (continuous corn) of 3 yr at N rates below maximum yield, which resulted in greater economic returns with PCU (4–14%) at N rates from 168 to 280 kg N ha−1. The SU fertilizer had no yield or economic advantage over urea. Grain and stover yields and N uptake increased with increasing N rate for all N sources. Expressing grain yields from all N sources as a linear‐plateau function of N rate showed that yields were maximized at 14.3 Mg ha−1 at an N rate of 254 kg N ha−1 or available N level (soil NO3–N plus fertilizer N) of 295 kg N ha−1. Nitrogen recovery efficiency (RE) tended not to vary with N rate, with no differences between SU and urea but greater RE (19%) with PCU than urea under continuous corn. Fertilizer N use efficiency did not vary with N rate but was greater for PCU (36%) than for urea (32%) under continuous corn. In contrast to SU, PCU provided grain yield and potential economic advantages over urea under continuous corn production at N rates below those needed with urea for maximum grain yield.
We evaluated the effects of irrigated crop management practices on nitrous oxide (N2O) emissions from soil. Emissions were monitored from several irrigated cropping systems receiving N fertilizer ...rates ranging from 0 to 246 kg N ha−1 during the 2005 and 2006 growing seasons. Cropping systems included conventional‐till (CT) continuous corn (Zea mays L.), no‐till (NT) continuous corn, NT corn–dry bean (Phaseolus vulgaris L.) (NT‐CDb), and NT corn–barley (Hordeum distichon L.) (NT‐CB). In 2005, half the N was subsurface band applied as urea‐ammonium nitrate (UAN) at planting to all corn plots, with the rest of the N applied surface broadcast as a polymer‐coated urea (PCU) in mid‐June. The entire N rate was applied as UAN at barley and dry bean planting in the NT‐CB and NT‐CDb plots in 2005. All plots were in corn in 2006, with PCU being applied at half the N rate at corn emergence and a second N application as dry urea in mid‐June followed by irrigation, both banded on the soil surface in the corn row. Nitrous oxide fluxes were measured during the growing season using static, vented chambers (1–3 times wk−1) and a gas chromatograph analyzer. Linear increases in N2O emissions were observed with increasing N‐fertilizer rate, but emission amounts varied with growing season. Growing season N2O emissions were greater from the NT‐CDb system during the corn phase of the rotation than from the other cropping systems. Crop rotation and N rate had more effect than tillage system on N2O emissions. Nitrous oxide emissions from N application ranged from 0.30 to 0.75% of N applied. Spikes in N2O emissions after N fertilizer application were greater with UAN and urea than with PCU fertilizer. The PCU showed potential for reducing N2O emissions from irrigated cropping systems.
We conducted a long-term nitrogen (N) fertilizer study in an irrigated no-till continuous corn system to assess the effects of N rates on N losses, N use efficiency (NUE), and changes in soil N ...content. The NUE of the harvested grain (NUE
HG
), calculated using the difference method (difference between fertilized and non-fertilized control experimental units), was 49, 46, 35, and 29% for the annual N fertilizer applications of 67, 132, 196, and 246 kg N ha
−1
y
−1
, respectively during the 2006 to 2018 growing seasons. The system N loss (difference between N inputs and outputs) from N-fertilized treatments, calculated using an N balance from fall 2005 to fall 2018 that included N changes in the 0 to 120 cm soil depths, ranged from 19% with the 67 kg N ha
−1
y
−1
rate to 57% with the 246 kg N ha
−1
y
−1
rate. The data suggest that nitrate (NO
3
–N) leaching and atmospheric emissions could be pathways for these losses. The changes in soil N content were negative, with an average system N loss across fertilized treatments of 15 kg N ha
−1
y
−1
of soil organic N (SON) in the top 30 cm of soil of the fertilized and non-fertilized treatments. These results show that even no-till systems could potentially have significant net N loss, including a reduction in SON and particulate organic N (POM–N), and that higher N use efficiencies found using the system N balance (NUE
Sys
) approach indicate that the NUE
HG
does not accurately assess the N lost to the environment.
Nitrogen fertilization is essential for optimizing crop yields; however, it may potentially increase nitrous oxide (N2O) emissions. The study objective was to assess the ability of commercially ...available enhanced-efficiency N fertilizers to reduce N2O emissions following their application in comparison with conventional dry granular urea and liquid urea-ammonium nitrate (UAN) fertilizers in an irrigated no-till (NT) corn (Zea mays L.) production system. Four enhanced-efficiency fertilizers were evaluated: two polymer-coated urea products (ESN and Duration III) and two fertilizers containing nitrification and urease inhibitors (SuperU and UAN+AgrotainPlus). Nitrous oxide fluxes were measured during two growing seasons using static, vented chambers and a gas chromatograph analyzer. Enhanced-efficiency fertilizers significantly reduced growing-season N2O-N emissions in comparison with urea, including UAN. SuperU and UAN+AgrotainPlus had significantly lower N2O-N emissions than UAN. Compared with urea, SuperU reduced N2O-N emissions 48%, ESN 34%, Duration III 31%, UAN 27%, and UAN+AgrotainPlus 53% averaged over 2 yr. Compared with UAN, UAN+AgrotainPlus reduced N2O emissions 35% and SuperU 29% averaged over 2 yr. The N2O-N loss as a percentage of N applied was 0.3% for urea, with all other N sources having significantly lower losses. Grain production was not reduced by the use of alternative N sources. This work shows that enhanced-efficiency N fertilizers can potentially reduce N2O-N emissions without affecting yields from irrigated NT corn systems in the semiarid central Great Plains.
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