Soil organic matter, greenhouse gases and net global warming potential of irrigated conventional, reduced-tillage and organic cropping systems

Nutr Cycl Agroecosyst DOI 10.1007/s10705-016-9811-0 ORIGINAL ARTICLE Soil organic matter, greenhouse gases and net global warming potential of irrigated conventional, reduced-tillage and organic cropping systems Rajan Ghimire . Urszula Norton . Prakriti Bista . Augustine K. Obour . Jay B. Norton Received: 7 March 2016 / Accepted: 5 November 2016 Ó Springer Science+Business Media Dordrecht 2016 Abstract Reducing tillage intensity and diversifying crop rotations may improve the sustainability of irrigated cropping systems in semi-arid regions. The objective of this study was to compare the greenhouse gas (GHG) emissions, soil organic matter, and net global warming potential (net GWP) of a sugar beet (Beta vulgaris L.)-corn (Zea mays L,) rotation under conventional (CT) and reduced-tillage (RT) and a corn-dry bean (Phaseolus vulgaris L.) rotation under organic (OR) management during the third and fourth years of 4-year crop rotations. The gas and soil samples were collected during April 2011–March 2013, and were analyzed for carbon dioxide (CO2), R. Ghimire
J. B. Norton Department of Ecosystem Science and Management, University of Wyoming, Laramie, WY, USA U. Norton
P. Bista
A. K. Obour Department of Plant Sciences, University of Wyoming, Laramie, WY, USA U. Norton Program in Ecology, University of Wyoming, Laramie, WY, USA R. Ghimire (&) Agricultural Science Center, New Mexico State University, Clovis, NM, USA e-mail: rghimire@nmsu.edu A. K. Obour Agricultural Research Center-Hays, Kansas State University, Hays, KS, USA methane (CH4), and nitrous oxide (N2O) emissions, water-filled pore space (WFPS), soil nitrate (NO3-–N) and ammonium (NH4?–N) concentrations, soil organic carbon (SOC) and total nitrogen (TN), and net global warming potential (net GWP). Soils under RT had 26% lower CO2 emissions compared to 10.2 kg C ha-1 day-1 and 43% lower N2O emissions compared to 17.5 g N ha-1 day-1 in CT during cropping season 2011, and no difference in CO2 and N2O emissions during cropping season 2012. The OR emitted 31% less N2O, but 74% more CO2 than CT during crop season 2011. The RT had 34% higher SOC content than CT (17.9 Mg ha-1) while OR was comparable with CT. Net GWP was negative for RT and OR and positive for CT. The RT and OR can increase SOC sequestration, mitigate GWP and thereby support in the development of sustainable cropping systems in semiarid agroecosystems. Keywords Carbon dioxide (CO2)
Crop rotation
Methane (CH4)
Net global warming potential (net GWP)
Nitrous oxide (N2O)
Soil organic carbon (SOC) Introduction Our warming and variable climate is causing increased soil mineralization rates that result in loss of SOM, higher levels of soil reactive N, and compromised ecological resiliency of agroecosystems, making it 123 Nutr Cycl Agroecosyst crucial that we rapidly develop practical and profitable cropping systems that restore the ability of soils to store organic C and mitigate GHG emissions (Paustian et al. 2016). Improved cropland management, particularly in areas with depleted soils, is one of the most important pathways toward GHG mitigation because of its large extent and potential for C sequestration. Areas such as the US High Plains have cold winters, hot and dry summers, and irrigationdriven wetting–drying cycles (Ghimire et al. 2014a). Combined with intensive tillage for cereal crop production, these conditions accelerate microbial mineralization and have resulted in very low SOM levels (Peterson et al. 1998). Reducing tillage intensity and incorporating legume crops in irrigated rotations are known to increase SOM and reduce GHG emissions (Robertson et al. 2000; Saenger et al. 2011). These practices may be of particular benefit in semi-arid regions, such as the US High Plains where low precipitation and cold climate are often limiting factors to successful crop production (USDA-NRCS 2006). The most common irrigated cash crop in this region is corn (Zea mays L.), but other crops such as dry beans (Phaseolus vulgaris L.), sugar beet (Beta vulgaris L.) and alfalfa (Medicago sativa L.) for hay are also produced. Typical crop management practices involve frequent tillage and high rates of synthetic fertilizer inputs. The longterm use of intensive tillage has depleted soil fertility and environmental quality (Ghimire et al. 2013; Hurisso et al. 2016). Transition from CT continuous corn to crop rotations with reduced-tillage (RT) or inputs of exogenous organic materials as in certified organic management (OR), can improve agroecosystem sustainability, including yields and profits (Robertson et al. 2000; Delate and Cambardella 2004; Halvorson et al. 2016). The time lag before benefits are realized is not well defined for semiarid irrigated systems, however, farmers are slow to adopt alternative management strategies (Kong et al. 2009). While improvements in soil quality and crop production are often attributed to residue retention and diversity (Halvorson et al. 2016), abundant fine roots under RT and OR can also contribute microbially available substrates that support diverse soil microbial communities and rapidly increase SOM content after transition (Ghimire et al. 2014a, b). The three GHG species of interest (CO2, N2O, and CH4) are biogenically produced and hence, 123 changes in the magnitude of their emissions along with soil inorganic N can be important indicators of belowground biogeochemical processes during the transition. Frequent tillage is known to increase decomposition of crop residues and SOM mineralization, consequently releasing CO2 and N2O (Bista et al. 2015; West and Post 2002). In contrast, reducing tillage can help sequester SOM, and reduce GHG emissions (Six et al. 2004). A study in northern Colorado, USA, showed lower N2O emissions during the first two years after transition from CT continuous corn to no-tillage irrigated corn-dry bean rotation (Halvorson et al. 2010). Effects of OR, specifically, use of manure, on GHG emissions is not well documented. While some studies show that manure application increases GHG emission (e.g. Rochette and Gregorich 1998; Thangarajan et al. 2013), others in which manure is incorporated and crop rotations are diversified, shown to improve corn yields, mitigate GHG emissions, and improve SOC accumulation compared to the conventional system (e.g. Delate and Cambardella 2004; Thelen et al. 2010). These practices reduce N loss to leaching by more effective bonding of N to stable SOM (Drinkwater et al. 1995). The net GWP is a measure of the cumulative radiative forcing of various GHG species and SOC sequestration relative to CO2 emissions, over a specific period of time. Assessing the early changes to GHG emissions, net GWP, and SOM after transition to conservation-oriented cropping systems can provide important information on climate change mitigation strategies and potential in cropping systems. Agriculture sector contributes 25% of total anthropogenic GHG emissions (Guzman et al. 2015). In the U.S. alone, agriculture is responsible for 8% of total anthropogenic GHG emissions and 74% of N2O emissions (USEPA 2015). While agriculture remains one of the most important sources of GHGs, improved soil management has great potential to mitigate emissions by increasing SOM, which also supports agricultural systems that are resilient to the impacts of climate change (Paustian et al. 2016). The magnitude and dynamics of GHG emissions after transition to more sustainable systems are important early indicators of changes in soil health and may provide additional incentives to farmers. The main objective of this study was to compare GHG emissions, SOM and net GWP among three Nutr Cycl Agroecosyst cropping systems during the third and fourth years after transition from CT continuous corn to corn-sugar beet-corn-dry bean rotations under CT and RT, and alfalfa–alfalfa-corn-dry bean rotation under OR. We compared fundamentally different cropping systems because in the study region sugar beet is essential for profitable conventional systems, but does not have an organic market, while alfalfa production is considered to be necessary for N fixation in organic systems. The whole-cropping-system comparison was designed to provide information about soil processes and sustainable crop production during and after transitioning to alternative management systems. We hypothesized that RT and OR would increase SOM and reduce net GWP compared to CT as early as three to four years of transition from continuous corn to crop rotations under alternative management systems, which are important indications of positive change towards more sustainable crop production. Materials and methods Experimental site and treatments The study was located at the University of Wyoming Sustainable Agriculture Research and Extension Center (SAREC) near Lingle, WY (42°70 15.0300 N and 104°230 13.4600 W). Soils at the study site are classified as coarse-silty, mixed, active, mesic Ustic Torriorthents with loamy texture (41% sand, 41% silt and 18% clay), slightly alkaline pH, and low SOM content (\1%) (Soil Survey Staff 2015). The area is characterized by a short growing season with an average frost-free period of 125 days, cool temperatures (average maximum 17.8 °C and minimum 0.06 °C) and 332 mm annual precipitation (Western Regional Climate Center 2015). More detailed information on weather conditions was obtained from the SAREC weather station (Campbell Scientific, Logan UT, USA) and presented in Fig. 1. The site received irrigation based on soil evaporative demand and antecedent precipitation. The study was established in 2011 as a part of the experiment initiated in 2009 and designed to assess the effects of the transition from continuous corn to crop rotations managed with CT, RT, and OR on soil quality of diversified corn-based crop rotations. Treatments (management systems) were designed to represent the best tillage strategies and cropping rotations for the local conditions as suggested by the advisory board composed of local farmers, researchers, and SAREC employees. The experiment had a randomized complete block design with three treatments and six replications. Individual plot size was 4 m 9 6 m. The CT and RT systems had a four-year crop rotation of dry bean-cornsugar beet-corn while the OR system had a four-year rotation of alfalfa–alfalfa-corn-bean/winter wheat rotation. More information on crop rotations, soil fertility management history, and tillage strategies in the cropping systems study can be found in Ghimire et al. (2014a). In spring 2011, CT and RT were planted to sugar beets followed by corn in 2012. After corn harvest, plots remained fallow until March 2013. The OR was planted to corn in 2011 followed by beans in 2012. After bean harvest, plots were planted to winter wheat until spring 2013 (Table 1). OR rotations are fundamentally different from rotations managed with CT or RT. Sugar beets are common in CT and RT rotations in this region due to large profits and close vicinity to the processing facility. However, sugar beet was not the part of the OR crop rotation because there is no local market or processing facility for organic sugar beets in the region. Crop rotations for OR start with growing a legume hay crop for at least 2 years to supply N for a subsequent high-value cash crop such as corn. Starting the OR rotation with 2 years of alfalfa was also desired by local producers because it provides effective weed control. While using a different crop rotation in OR than in the other two systems prevented some analyses of crop effects in the crop rotation on soil health and GHG emissions during the transition, identical organic and non-organic cropping systems were deemed to be of limited value by the local advisory committee. The CT and RT systems were amended with chemical fertilizers while OR received steer manure in spring 2011 and 2012 (Table 1). Fertilizer and manure applications were based on annual soil testing. Tillage in CT and OR was designed to loosen the soil and incorporate crop residues to prepare a good seedbed for crop production. It included moldboard plowing in the spring followed by disking, and harrowing. Tillage in RT was designed to loosen the surface without inverting the soil. This system leaves [15% crop residue on the surface as suggested by the Conservation Technology Information Center (CTIC 2016). Sugar beet in CT was planted with moldboard 123 Nutr Cycl Agroecosyst 60 a T-Max T-Min Temperature (oC) 40 20 0 -20 70 Precipitation / irrigation (mm) b Irrigation Precipitation 60 50 40 30 20 10 0 03/01/11 07/01/11 11/01/11 03/01/12 07/01/12 11/01/12 03/01/13 Date Fig. 1 a Daily maximum (T-Max) and minimum (T-Min) temperatures and b biweekly cumulative precipitation and the amount of irrigation water between 2011 and 2013 Table 1 Crops planted, inorganic fertilizer (expressed as a nitrogen-to-phosphorous-to sulfur ratio; N:P:S) and manure application rates in conventional (CT), reduced-tillage (RT), and organic (OR) management systems during 2011 and 2012 cropping seasons System CT RT ORa a Year Fertilizer (kg ha-1) Crop 2011 Sugar beet N:P:S 275:179:58 2012 Corn N:P:S 67:112:23 2011 Sugarbeet N:P:S 275:114:45 2012 Corn N:P:S 112:112:23 2011 Corn Manure 67 Mg ha-1 2012 Pinto bean-winter wheat Manure 56 Mg ha-1 Manure used in OR had dry matter 29.2% and C:N:P:S = 21.3:1.42:0.35:0.40% plowing followed by disking and packing and in RT with one pass of strip tillage (0–10 cm). Corn was planted in CT with moldboard plowing, disking and harrowing and in RT with one pass of a Landstar (Kuhn Krause, Inc., Hutchinson, KS), a five-step tillage system that discs, cultivates, and uniformly 123 distributes residues in the seedbed (0–20 cm) and on the soil surface. Corn in OR was planted using the same tillage management (moldboard plowing followed by disking, and harrowing) as in CT in 2011, and bean was planted in 2012 with one pass of Landstar (0–20 cm). Wheat in fall 2012 was drilled in Nutr Cycl Agroecosyst after tillage with plow (0–20 cm) and cultivator (0–10 cm) in OR. The OR system used additional tillage for weed control as needed, all in compliance with the United States Department of Agriculture National Organic Program standards (http://www. ams.usda.gov/AMSv1.0/nop). All plots were uniformly irrigated to *60% of field capacity with an overhead sprinkler irrigation system. Field sampling Greenhouse gas emissions were measured using the method of Hutchinson and Mosier (1981). Specifically, in April 2011, polyvinyl chloride (PVC) rings (25-cm inside diameter 9 10 cm tall) were installed to a 7-cm depth at the center of each plot. The PVC rings were occasionally removed for field operations and reinstalled immediately afterward. Chamber locations were recorded using GPS (Trimble GeoXT, Sunnyvale, CA) to ensure precise location of chambers for reinstallation. Gas sampling occurred in midmorning and at a minimum of 24 h after irrigation, rainfall or chamber reinstallation. Plants inside the chamber bases were clipped and removed before each sampling to avoid GHG contribution from aboveground plant respiration. Therefore, GHG emissions from soil processes only were considered in this study. Sampling occurred every other week from April to October (cropping season) and at 3- to 4-week intervals from November to March (fallow period) of each year. More frequent sampling and intensive samplings after disturbance events (irrigation, rainfall, and tillage) would give a better estimated inventory of GHG emissions, mainly for capturing N2O fluxes (Parkin 2008; Reeves and Wang 2015). In this study, we compared the GHG emissions and SOC in alternative management systems. The low sampling frequency may reduce the precision of the GWP estimates, but evaluation of emissions at the same time under equivalent conditions still allows comparison between management systems. During each sampling event, chamber tops were deployed on top of the bases for 30 min and sealed with rubber gaskets. Gas samples were collected from each chamber headspace using disposable 60-ml polypropylene syringes attached with stopcocks and 25-gauge needles. Gas samples were collected at 0, 15, and 30 min after chamber tops were sealed on chamber bases. The syringe was drawn full with headspace air and then injected back to mix the air in chamber headspace. This was repeated a total of three times before collecting any samples. At the end, a 60-mL aliquot of gas was drawn, 30-mL was flushed out of the syringe, and the remaining 30-mL was injected into 12.5-mL previously evacuated Labco Exetainer glass vial sealed with butyl rubber septa (Labco Limited, Buckinghamshire, UK). Concurrently with GHG sampling, soil samples (0–10 cm) were also collected at random locations approximately 0.5 m away from GHG chambers. The 0–10 cm depth was considered to be sufficient because the focus was on near-surface soil processes that influences GHG emission and global warming. Studies show that the greatest response of soil processes are observed in 0–5 or 0–10 cm soil depths, which influence SOC and nutrient dynamics and GHG emissions (Fierer et al. 2003; Hurisso et al. 2016). Five soil cores were collected from each plot using a soil probe (i.d. 3.2 cm), composited, homogenized, bagged, and placed on ice for transport to the laboratory. A separate set of five samples was collected from 0 to 10 cm depth of each treatment using a bulk density probe (i.d. 2.1 cm) and composited for determination of soil bulk density. Laboratory analyses Concentrations of CO2, CH4, and N2O were determined using a gas chromatograph (Shimadzu GC2014, Shimadzu Scientific Instruments, Kyoto, Japan) equipped with AOC 5000 Auto-Injector, flame ionization detector (FID), thermal conductivity detector (TCD) and electron capture detector (ECD) to analyze CO2, CH4, and N2O, respectively. Gas emissions were calculated using best estimates derived from a linear or logarithmic response of an individual gas that reflected concentration change in a chamber headspace at 0, 15, and 30 min after a chamber was sealed (Hutchinson and Mosier 1981). Gas emission (F) was calculated using the following equation: F ¼ DC  ðV  M Þ ðA  Vmol Þ ð1Þ where DC represents rate of concentration change over time for a gas X, V is the chamber volume, M is the molecular weight of a gas X, A is the soil area inside the chamber and Vmol is the volume of a mole of a gas X. Results were converted from lmol of a gas 123 Nutr Cycl Agroecosyst X min-1 cm-2 to a kg of gas X day-1 ha-1. Air temperatures at the experimental field at the time of air sampling were also monitored using a soil thermometer (Forestry Suppliers Inc.), and were used in the calculation of GHG emissions. Soil samples were stored at 4 °C and processed within 24 h of sampling. Ten grams of field moist soil was extracted in 0.5 M potassium sulfate (K2SO4), put on a shaker for 30 min and gravity filtered through Q5 filter paper (Fisher Scientific, Inc.). The extract was analyzed for nitrate (NO3-–N) (Doane and Horwath 2003) and ammonium (NH4?–N) (Weatherburn 1967) using a BioTek microplate spectrophotometer (BioTek Inc. Winooski, VT). Gravimetric soil water content was determined by oven drying 10 g of field moist soil at 105 °C for 48 h (Black 1965). The WFPS was calculated as suggested by Linn and Doran (1984). For this calculation, soil bulk density was determined by core method (Blake and Hartge 1986) and soil particle density value of 2.65 was used. Soil samples collected from 0 to 10 cm depth during the first sampling campaign (April 2011) and the last sampling campaign (March 2013) were analyzed for total C and N (TN) contents by dry combustion (EA1100 Soil C/N analyzer, Carlo Erba Instruments, Milan, Italy) and inorganic C concentration by digestion of carbonates in HCl and measurement of CO2 emitted using a pressure-calcimeter (Sherrod et al. 2002). The inorganic C content was \1% of the total carbon. The SOC content was determined by subtracting inorganic C from total C. Soil properties such as soil pH and EC (Thomas 1996), and particle-size distribution (Day 1965) were measured at the beginning of the cropping systems experiment in 2009. Net global warming potential estimates The GHG emissions, mainly CH4 and N2O exchange, and SOC change data were used to derive net GWP of different management systems (Robertson et al. 2000). The GWP of CH4 and N2O were calculated using CO2–C equivalents of 28.5 for CH4 and 264.5 for N2O using an IPCC (2014) 100-year time scale. The CO2 equivalent of SOC change was calculated by using annual rate of SOC accrual or loss from a system, and converting the number into CO2 equivalent (Robertson et al. 2000). Net GWP (kg CO2 eq ha-1) for each 123 management system was calculated using the following equation (Robertson et al. 2000): Net GWP ¼ GWPinputs: þ GWPCH4 þ GWPN2 O þ GWPSOC Þ ð2Þ where, GWPinputs were calculated as the sum of CO2 equivalent for irrigation, farm operation, and fertilizer application based on West and Marland (2002). Specifically, CO2 equivalent for irrigation was estimated by using energy equivalent of 598 kg C ha-1 m-1. The CO2 equivalent for manure during storage and from intrinsic fermentation was based on Pattey et al. (2005). In this calculation, a 90-day manure storage period was assumed, which was consistent with farming practices in the surrounding area. The GWPSOC was calculated by using the CO2 equivalent of SOC lost (as positive GWP) or accrued (as negative GWP) over the study period (Thelen et al. 2010). Net GWP values of N2O and CH4 released from manure during 90-day storage as well as from enteric fermentation were obtained from Pattey et al. (2005). Any germinating plants inside the chamber bases were clipped and removed before each sampling. Thus, the net GWP calculation did not account for plant CO2 exchange. Statistical analyses Soil and GHG data were statistically analyzed using a mixed procedure (PROC MIXED) of the Statistical Analysis System (SAS v9.3, SAS Institute, Cary, NC) for completely randomized experiments. This analysis considered management system as a fixed effect, sampling date as a repeated observation, and replication as a random term in the model (Littell et al. 2002). The SOC and TN contents from April 2011 and March 2013 were also analyzed using the PROC MIXED procedure. GHG emissions, soil WFPS, and soil N were averaged across crop growing season (April– October) and fallow or no-crop period (November – March) of each year and analyzed using one-way analysis of variance. Because year factor was indicative of the length of the transition and different crops were planted in 2011 and 2012, each time-period was analyzed separately, and no statistical comparisons were made between years or between the cropping seasons and fallow periods. Baseline soil properties Nutr Cycl Agroecosyst were analyzed using one-way ANOVA. The GWP for farm inputs were not analyzed statistically because the inputs were same for all replications. The GWPCH4, GWPN2O, GWPSOC and net GWP were analyzed using a one-way analysis of variance. Treatment means differing in F test (P B 0.05) were separated by using a mean comparison test (PDIFF) in SAS. Results Air temperature and precipitation Cumulative precipitation between April 2011 and March 2012 was 29% greater than the 60-year average precipitation (332 mm) mainly due to high snowfall and rainfall during early spring of 2012 (Fig. 1; Western Regional Climate Center 2015). The below average precipitation started in April 2012 resulting in 54% less cumulative precipitation between April 2012 and March 2013 compared to the 60-year average. Consequently, crops were irrigated with 388 mm of water during the 2012 cropping season, which was ten times [39 mm of irrigation water applied during the 2011 cropping season. Carbon dioxide, methane, and nitrous oxide emissions The CO2 emissions were consistently higher under OR than under CT and RT (Fig. 2a). The average CO2 emissions of 13.3 kg ha-1 day-1 in 2011 cropping season under corn and 14.4 kg ha-1 day-1 in 2012 cropping season under bean in OR, were 30 and 74% greater than CT under sugarbeet in 2011 and corn in 2012, respectively (Table 2). The RT system that had same crop rotation as CT had 26% lower CO2 emissions than CT in the 2011 cropping season, but no differences were observed between CT and NT in 2012 cropping season. The CO2 emissions were comparable between management systems during winter fallows, with values ranging between 2 and 4.26 kg ha-1 day-1 during fallow 1 and 1.53 and 2.1226 kg ha-1 day-1 during fallow 2. Despite variation in CH4 assimilation in cropping seasons and fallow periods (Fig. 2b), it was not significantly different between management systems in any of the time periods (Table 2). The amount of CH4 assimilated was generally low and ranged between -0.64 and -1.21 g ha-1 day-1 during cropping season and between -0.60 and -1.19 g ha-1 day-1 during fallow. The N2O emissions varied between management system in cropping seasons and fallow periods (Fig. 2c). In cropping season 2011, average N2O emissions were 43% lower in RT and 31% lower in OR than in CT (Table 2). They were 44% lower in RT and 26% lower in OR than in CT during fallow 1. In contrast, average N2O emissions were comparable between management systems during cropping season 2012 and fallow 2. The values ranged between 18.7 and 25.7 g ha-1 day-1 during cropping season 2012 and between 3.59 and 5.04 g ha-1 day-1 during fallow 2. Soil properties Soil WFPS varied between management systems in cropping seasons and fallows (Fig. 3). The WFPS averaged for cropping seasons and fallows differed significantly between management systems during cropping season 2012 and fallow 2, and it was comparable between management systems during cropping season 2011 and fallow 1 (Table 2). The values ranged between 56 and 61% during cropping season 2011 and 44–47% during fallow 1. The WFPS in RT was 11% greater than in CT and 14% greater than in OR during cropping season 2012, and it was 43% greater than in CT and 49% greater than in OR during fallow 2. Soil NO3-–N and NH4?–N concentrations were different between management systems in cropping seasons and fallows (Fig. 3). Soil NO3-–N concentrations averaged for cropping seasons and fallows were comparable between management systems during cropping season 2011 as well as 2012 (Table 2), but was three to five times higher in cropping season 2012 than cropping season 2011. Soil NO3-–N concentrations were not significantly different between management systems in fallow 1 and ranged between 30.0 and 33.2 mg NO3-–N kg-1 soil (Table 2). In fallow 2, however, NO3-–N concentrations differed between management systems such that 26.9 mg NO3-–N kg-1 soil in CT was 26% lower than OR and 14% greater than in RT. Soil NH4?–N concentration was 48% lower under OR than under CT and no differences between CT and RT during cropping season 2011 (Table 2). Concentrations were not significantly different between management systems in fallow 1 and ranged between 123 Nutr Cycl Agroecosyst 40 CO2 flux (kg C ha-1 day-1) a CT RT OR 30 20 10 0 4 b CH4 (g C ha-1day-1) 2 0 -2 -4 140 N2O (g N ha-1 day-1) 120 c Crop season Fallow 1 Crop season Fallow 2 100 80 60 40 20 0 03/01/11 07/01/11 11/01/11 03/01/12 07/01/12 11/01/12 03/01/13 Date Fig. 2 a Carbon dioxide (CO2), b Methane (CH4), and c nitrous oxide (N2O) emissions in conventional (CT), reduced-tillage (RT), and organic (OR) systems. Error bars indicate standard error (n = 6) 2.55 and 2.76 mg NH4?–N kg-1 soil. They also did not differ between management systems in cropping season 2012 and ranged between 1.39 and 1.81 mg NH4?–N kg-1 soil. Soil NH4?–N concentration beneath OR was 128% greater than CT and 71% greater than RT during fallow 2. The SOC contents did not differ between management systems in spring 2011 (the beginning of the 123 GHG study) but differed in spring 2013 (the end of GHG study). There was 34% more SOC beneath RT than CT and SOC content was comparable between CT and OR (Table 3). Over time, SOC content beneath RT was 18.3% greater in spring 2013 than in spring 2011. The SOC content did not change over time beneath CT and OR. Soil TN contents did not differ between management systems and years. The Nutr Cycl Agroecosyst Table 2 Average carbon dioxide (CO2); methane (CH4) and nitrous oxide (N2O) emissions, and soil water filled pore space (WFPS), nitrate (NO3-–N) and ammonium (NH4?–N) contents Cropping season 2011a in 0–10 cm depth of conventional (CT), reduced-tillage (RT), and organic (OR) systems for 2011 and 2012 cropping seasons, fallow 1, and fallow 2 Cropping season 2012a Parameter System CO2 (kg C ha-1 day-1) CT 10.2 (0.73)b 3.69 (0.78) 8.29 (0.75)b 1.53 (0.24) RT 7.51 (0.55)c 2.26 (0.32) 8.48 (0.55)b 1.82 (0.27) OR CH4 (g C ha-1 day-1) N2O (g N ha-1 day-1) WFPS (%) NO3-–N (mg kg-1 soil) NH4?–N (mg kg-1 soil) 13.3 (0.96)a Fallow 1 4.53 (0.99) 14.4 (1.22)a Fallow 2 2.12 (0.34) CT -1.01 (0.13) -1.07 (0.40) -0.99 (0.12) -0.60 (0.12) RT -0.64 (0.14) -0.71 (0.35) -0.83 (0.09) -0.63 (0.15) -1.19 (0.28) OR -0.83 (0.10) -1.21 (0.17) -0.76 (0.19) CT 17.50 (2.44)a 6.16 (0.61)a 18.7 (2.50) 5.04 (0.63) RT 10.00 (1.33)b 3.45 (0.45)b 25.7 (3.81) 3.85 (0.58) OR 12.00 (1.37)b 4.54 (0.47)b 19.6 (2.99) 3.59 (0.43) CT 55.6 (3.84) 47.1 (8.35) 55.0 (6.18)b 37.6 (6.11)b RT OR 58.6 (3.96) 58.8 (4.87) 43.5 (7.16) 46.6 (6.63) 61.3 (8.64)a 54.0 (6.82)b 53.8 (8.02)a 36.2 (5.15)b CT 13.9 (1.96) 33.2 (2.62) 50.9 (6.62) 26.9 (3.17)ab RT 14.5 (2.16) 31.6 (2.43) 46.2 (5.79) 23.0 (2.54)b OR 12.9 (2.14) 30.0 (2.36) 39.5 (5.06) 33.9 (3.17)a CT 2.14 (0.41)a 2.57 (0.28) 1.81 (0.30) 0.93 (0.22)b RT 2.03 (0.37)a 2.55 (0.33) 1.54 (0.31) 1.24 (0.22)ab OR 1.11 (0.11)b 2.76 (0.46) 1.39 (0.17) 2.12 (0.52)a a Values in parenthesis indicate standard error (n = 6). Value followed by a same letter within a column indicates no significant difference between management systems (p B 0.05) TN content ranged between 0.96 and 1.14 Mg TN ha-1 in spring 2011 and between 0.96 and 1.37 Mg TN ha-1 in spring 2013. Global warming potential The GWP associated with farm operations, fertilizer and manure additions and net SOC changes influenced net GWP under alternative management systems (Table 4). The farm operations contributed 49 and 12% less in GWP under RT and OR management, respectively, than under CT. The GWP of manure addition in OR was only 29% of that of chemical fertilizer applications in CT. The contribution of fertilizer application on GWP was 8% greater under RT than under CT. The GWP of N2O and CH4 emissions were not statistically different between management systems, and the GWP of SOC was negative for RT and OR, and it was positive for CT. The net GWP followed the trend of changes in SOC, and it was net negative in OR and RT while it was positive in CT. The net GWP was lower in RT than in OR. Discussion Results from this study support the main hypothesis that RT would increase SOC and reduce global warming compared to CT as early as the fourth year in transition from continuous corn. Lower net GWP and greater SOC and TN contents under RT than under CT are in agreement with observations by Mosier et al. (2006) in irrigated corn-based rotations in northern Colorado. In CT system, tillage incorporates crop residue and manure into the soil, increases soil-to-crop residue contact, and breaks soil aggregates, leading to loss of SOC and N as CO2 and N2O (Ogle et al. 2005; Snyder et al. 2009). The RT system was less disturbed (1–2 tillage passes per year) than CT (4–5 tillage passes per year). This may have led to smaller CO2 and N2O emissions along with greater SOC content in RT than in CT. Reduced disturbance accumulates SOC, improves soil aggregation, and increases soil porosity and WFPS (Linn and Doran 1984; Ogle et al. 2005). Aggregated soils with higher air–water exchange facilitate SOC storage and mitigate GHG emissions (Six et al. 2004). Greater microbial biomass together 123 Water Filled Pore Space (%) Nutr Cycl Agroecosyst 100 a CT RT OR 80 60 40 20 1800 -1 NO3 - N (mg kg soil) 160 b 140 120 100 80 60 40 20 0 -1 NH4 - N (Mg kg soil) 12 c Crop season Fallow 1 Crop season Fallow 2 10 8 6 4 2 0 03/01/11 07/01/11 11/01/11 03/01/12 07/01/12 11/01/12 03/01/13 Date Fig. 3 a Soil water filled pore space (WFPS, %), b soil nitrate (NO3-–N), and c ammonium (NH4?–N) (c) contents in conventional (CT), reduced-tillage (RT), and organic (OR) systems during 2011–2013. Error bars indicate standard error (n = 6) with a shift in microbial community composition is also known to facilitate SOC accumulation and stabilization (Johnson et al. 2010). The RT management increased microbial biomass, diversified microbial community, and supported fungal growth in alternative cropping systems in a separate study at the same study site (Ghimire et al. 2014a). The changes in SOC content and microbial properties illustrate a potential of RT management for SOC accumulation and GWP mitigation. 123 Lower net GWP in OR than CT despite the greater CO2 emissions is mainly due to lower N2O emission, improved N use efficiency, and SOC accumulation. The SOC content was not significantly different between CT and OR however, the CO2 equivalent of 3117 kg ha-1 year-1 for the rate of SOC accumulation in OR during the 2-year study period (0.85 Mg ha-1 year-1) significantly contributed to GWP mitigation. A wise soil management strategy that diversifies crop rotations, reduces tillage intensity Nutr Cycl Agroecosyst Table 3 Soil organic carbon (SOC) and total nitrogen (TN) in 0–10 cm depth of conventional (CT), reduced-tillage (RT), and organic (OR) April 2011–March 2013 System SOCa TN 2011 Mg ha 2013 -1 DSOC 2011 -1 % Mg ha year -1 2013 Mg ha -1 DTN % 0.00 Mg ha-1 year-1 CT 19.0 (0.57)a 17.9 (0.88)b -5.79 -0.55 0.96 (0.05) 0.96 (0.04) RT 20.2 (0.87)a 23.9 (0.99)a 18.3* 1.85 1.14 (0.07) 1.37 (0.10) 20.2 0.00 0.115 OR 18.3 (1.49)a 20.0 (1.60)b 9.29 0.85 1.00 (0.05) 1.11 (0.08) 11.0 0.055 a Values in parenthesis indicate standard error of the mean (n = 6). Value followed by a same letter within a column indicates no significant difference between management systems (p B 0.05) * indicates significant difference between years Table 4 Net global warming potential (net GWP) and contributing global warming potential (GWP) calculated for irrigation, farm operations, fertilizer and manure application, System CT greenhouse gas emissions (N2O, CH4), and soil organic carbon (SOC) sequestration during 2011–2013 under conventional (CT), reduced-tillage (RT), and organic (OR) systems GWPa Net GWP Irrigation Farm operations kg CO2 equivalent ha-1 year-1 Fertilizer/manure N2O CH4 SOC 51.7 626 3.14 -0.05 2017a 339 3037a RT 51.7 174 677 3.39 -0.04 -6783c -5877c OR 51.7 300 184 2.75 -0.06 -3117b -2579b The GWP for irrigation, farm operation and fertilizer N and P application was calculated based on West and Marland (2002) and for manure was based on Pattey et al. (2005), GWPSOC was calculated from annualized rate of SOC accrual or loss (Mosier et al. 2006). Different lower case letters within each column indicate significant difference between management systems (p B 0.05) and frequency, and uses organic soil amendments can increase SOC sequestration, improve N cycling, and mitigate N2O emissions (Paustian et al. 2016). Nonlegume crops after legume crops in the OR rotation may have improved N use efficiency and contributed to low net GWP. The finding of lower N2O emissions from OR than CT contradict results of many other studies reporting larger N2O emissions from legume cropping than non-legume cropping (IPCC 2007) potentially because legume crops in our study were in rotation with non-legume crops. Studies show that cropping systems that use diversified rotations and incorporate manure are effective in mitigating GHG emissions, improving N use efficiency and SOC accrual, and increasing crop yield (e.g. Delate and Cambardella 2004; Thelen et al. 2010) without increasing N2O emissions (Robertson et al. 2000; Rochette and Janzen 2005). Similarly, use of organic soil amendments such as manure and compost offsets N2O emission by slowing down the rate of N mineralization and mitigating N loss during nitrification and denitrification (Drinkwater et al. 1995). Slow release of nutrients improve nutrient use efficiency and increase crop production (Delate and Cambardella 2004). The OR system had greater CO2 emissions than CT at the same time SOC was accumulated and the net CO2 equivalent was significantly lower in OR than that of CT. Therefore, SOC accumulation combined with N2O mitigation contributed to low net GWP in OR system. The net GWP estimation allows comparison of the relative radiative forcing of alternative systems and accounts the energy equivalent of SOC accumulation as well as GHG emissions relative to the emissions of one ton of CO2. In the present study, both RT and OR management had smaller net GWP compared to CT indicating that the alternative systems can rapidly improve agroecosystem performance in highly depleted soils such as in the central High Plains region. The GWP mitigation was mainly contributed from SOC accumulation. The observed rate of SOC gain by 1.85 Mg ha-1 year-1 in RT and 0.85 Mg ha-1 year-1 in OR was greater than the rate of SOC accumulation in other agroecosystems (e.g. West and Post 2002; Baker 123 Nutr Cycl Agroecosyst et al. 2007), but not unexpected because SOC was measured only from 0 to 10 cm depth. There is an argument that benefits of SOC accrual in the surface soil during transitioning to alternative systems are balanced with SOC loss from the deeper profile (Powlson et al. 2014). Monitoring of SOC to the deeper depth for a longer period may help in estimating the SOC stock, and the C sequestration benefits of the alternative management systems. The benefits of deep profile SOC storage, however, is questioned as studies have noted a significant variation in deep profile SOC storage and lack of statistical power in such comparison (Kravchenko and Robertson 2011; Paustian et al. 2016). Moreover, the greatest response of soil processes that influence SOC and nutrient dynamics related to GHG emissions are observed in top 10-cm depth (Fierer et al. 2003; Hurisso et al. 2016). Observation of early indicators of changes in agroecosystem performance such as GHG emissions and GWP will help in rapidly developing the practical and profitable cropping systems. The GWP mitigation through a change in SOC accrual in CT, RT and OR management were equivalent to -2017, 6783 and 3117 kg CO2 ha-1, respectively, which suggested the potential of improving agroecosystem sustainability through RT and OR management. While overall effects of alternative systems on GWP mitigation and SOC accrual are crucial for improving agroecosystem performance, evaluating individual components (tillage, crop rotations, and soil amendments) will help in improving our understanding of the relative contribution of each component in designing a best-management practice. One of the challenges we faced in comparing OR with other systems is that OR system uses different crop rotation, fertility, and pest management strategies than CT and RT. A review of integrated cropping systems researches that compared two approaches in farming revealed that system level comparison considers multiple factors and processes within the alternative systems (Ghimire et al. 2013). Such a system comparison complicates estimation of the contributions of individual components, however, provides valuable information regarding system sustainability and profitability. Therefore, alternative management strategies that are agronomically, economically, and environmentally feasible should be pursued to develop more resilient cropping system in the face of climate change (Paustian et al. 2016). This is particularly important in the semiarid High Plains 123 region because the OR system differ fundamentally from the CT rotations as they respond to different needs for agricultural commodities. The present study provides information on the pace the transition takes place for these three alternatives commonly practiced in this region. Despite variation in GHG emissions under different cropping systems and soil management practices, improved managment can mitigate GHG emissions and net GWP. There was seasonal and interannual variation in GHG emissions and soil N dynamics potentially influenced by seasonal weather pattern, crops in rotation, and fertility management practices. Greater CO2 and N2O emissions during the crop growing season than during the fallow were observed across all management systems suggesting the influence of high soil temperatures, abundant plant available water, and high plant biomass production on soil C and N cycling. Higher soil temperature supports greater soil microbial activity and thereby larger GHG fluxes during crop growing season (April–October) compared to fallow (November–March). Besides, crop growth stimulates soil microbial activity as they release root exudates, and results in larger CO2 and N2O emissions during cropping seasons than during fallow (Johnson et al. 2010). The present study did not quantify root and microbial respiration during the cropping season, and low sampling frequency may have reduced the precision of the GWP estimates. Evaluation of GHG emissions at the same time under equivalent conditions allowed us to compare the soil processes, GHG emissions, and GWP of alternative management systems. The interannual variation in GHG emissions, especially larger N2O emissions in 2012 than in 2011 cropping season across all management systems, corresponded with high soil inorganic N content in 2012. Inorganic N in the 2012 cropping season in CT and RT was probably carried over from N applied to sugar beet in 2011, and that in OR might have been derived from the current year’s bean crop (Table 1). Previous studies show that irrigation pulses increase N mineralization and GHG emissions (e.g. Sainju et al. 2012). The experimental site received less precipitation in 2012 and crops were irrigated frequently compared to irrigation in 2011, which may have accelerated N turnover and N2O emissions in 2012 compared to 2011. Nevertheless, current observations in the third and fourth years of four-year rotations revealed a great potential of GWP Nutr Cycl Agroecosyst mitigation through improved cropping system management strategies. Conclusions This comparative study revealed that OR and RT systems improve SOC accrual and mitigate GWP within four years of transition from continuous CT corn to alternative crop rotations. Fewer disturbances in the RT system than in CT contributed to smaller CO2 and N2O emissions, greater SOC content, and smaller net GWP. The OR system had lower net GWP mainly due to SOC accumulation and lower N2O emissions. Changes in GWP were largely derived from near-surface SOC and N dynamics under CT, RT and OR indicated that agroecosystems respond quickly to changes in management, especially in highly depleted soils such as in eastern Wyoming. Further research that includes deeper soil depths and more intensive GHG monitoring may confirm the effects of alternative management systems to improve the sustainability of irrigated crop production in low fertility soils. This study suggests that reducing disturbance, including legumes, and applying organic amendments positively impacts near-surface soil processes in ways that enhance SOC accumulation and GWP mitigation even during a short period (\5 years) during the transition from CT to OR and RT systems. The RT system used minimum soil disturbance approach, and the OR system used organic inputs and legume crops in rotation to improve agroecosystem performance. 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