چرخش محصولات کشاورزی در دراز مدت و اثرات خاک ورزی در جذب گاز گلخانه ای خاک و تولید محصولات کشاورزی در آمریکا Long-term crop rotation and tillage effects on soil greenhouse gas emissions and crop production in Illinois, USA

1. Introduction The agricultural sector produces food, fuel, and fiber but is also an important source of greenhouse gas (GHG) emissions. Agriculture contributes around 9% of total United States GHG emissions, with carbon dioxide (CO2) making up the majority (81%), followed by methane (CH4) (11%) and nitrous oxide (N2O) (6%) (EPA, 2016). The global warming potential (GWP) of N2O and CH4 is 298 and 25 times greater than that of CO2, respectively. Global warming potential is a measure of the amount of energy one kilogram of a certain GHG will absorb over a given time period, usually 100 years, relative to CO2 (EPA, 2016). Agricultural soil management which includes synthetic fertilizer application and use, tillage practices, and crop rotation systems accounts for around 80% of total N2O emissions in the U.S. annually (EPA, 2016) (Venterea et al., 2011). Nitrous oxide emissions are directly affected by N application rate as well as fertilizer source and crop type (Eichner, 1990; FAO, 2001). Likewise, fertilizer application technique and timing, use of other chemicals, irrigation, and residual N and C from previous crops and fertilizer all affect N2O emissions (Eichner, 1990). Application of N fertilizer stimulates N2O production by providing a substrate for microbial N conversion through nitrification and denitrification (Venterea et al., 2005; Norton, 2008). Nitrification occurs when ammonium is either added to the soil in the form of fertilizers, as N fixation by legumes, or as mineralized soil organic matter (SOM) (Paustian et al., 2016). During this microbial process, ammonium is converted to nitrite and eventually to nitrate, yet small quantities can be lost as N2O (Snyder et al., 2009). Likewise, in conditions olow soil oxygen, denitrifiers use nitrate as a terminal electron acceptor and N2O is an intermediate step in complete denitrification to N2 gas (Aulakh et al., 1992; Robertson et al., 2007; Paustian et al., 2016). Since spring fertilizer application in the United States Corn Belt (Illinois, Iowa, Indiana, Ohio southern and western Minnesota, and eastern Nebraska) occurs when saturating rains are common, the soil may easily become water-logged, promoting large denitrification events wherein a large proportion of annual N2O flux can occur over short time scales (Venterea et al., 2012). Tillage studies often have mixed results with no-till (NT) or reduced till having less, more, or no effect on N2O emissions compared to conventional tillage systems (T) (Venterea et al., 2005; Rochette et al., 2008; Snyder et al., 2009). Snyder et al. (2009) compared various cropping rotation studies and found that continuous corn (Zea mays L.)- (CCC) had higher yields compared to a corn-soybean [Glycine max (L.) Merr.]-wheat (Triticum aestivum L.) (CSW) rotation. While CCC resulted in a two to three times higher N2O emissions, it produced four to five times the food yield in caloric value compared to the CSW rotation. Parkin and Kaspar (2006) observed that a corn-soybean (CS) rotation did not differ in N2O emissions between T and NT, but corn in the rotation emitted more N2O than did soybeans. In a meta-analysis by Pittelkow et al. (2015) studying the long-term effects of no-till on yield in several agroecosystems, the authors found that after 5+ years of notill, soybean and wheat yields matched that of conventional tillage; however, corn yields did not improve over time compared to conventional tillage. Relatively few studies have compared side-by-side crop rotation effects as influenced by tillage, and since both of these practices tend to influence soil properties more over time, long-term assessments are needed which allow for soils to stabilize.