Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Late Spring 2013
7 The Fluid Journal Late Spring 2013 energy balance (average net energy yield [HEY]) and net energy ratio [NER] of 159 GJha-1 and 6.6 respectively) with substantial variation across site-years (Figure 2 A-C and Figure 3 A and C). The largest fossil fuel inputs came from embodied energy in N fertilizer and from fuel use for irrigation pumping, which represented 32 and 42% of total seed energy inputs, respectively (Table 1). Average energy inputs for irrigated maize production in the Tri-Basin NRD were much higher than previously reported energy inputs of US maize systems that were based mostly on rain-fed production (Figure 3A and Table 1). Hence, previous studies included little or no energy inputs associated with irrigation pumping and much less energy associated with N fertilizer because of lower fertilizer rates in rain-fed systems. Average NEY of irrigated maize in Tri-Basin NRD was the highest among published studies, whereas NER was equal to or higher than published Table 1. Average 3-y (2005 - 2007) applied inputs (and percentage of total energy input), total fossil-fuel energy input, grain yield, and interannual coefficient of variation, fertilizer nitrogen- use efficiency, water productivity, and conversion efficiency from solar radiation into grain or total biomass based on data collected from 123 irrigated maize fields in Tri-Basin NRD. Inputs Rate (per ha) N fertilizer, kg of N 183 (32%) P fertilizer, kg of P2O5 43(1%) K fertilizer, kg of K2O 11 (<1%) Herbicides, kg of a.i. 2.4 (3%) Insecticides, kg of a.i. 0.3 (<1%) Seed, kg 25 (1%) Machinery, MJ 464 (2%) Fuel use for on-farm operations,* L Field operations 63 (9%) Irrigation pumping** 324 (42%) Grain drying 61 (9%) Energy inputs, GJ.ha-1 30 Grain yield, Mg.ha-1 13.2 (CV = 3%) NUE,*** kg of grain, kg-1 of N fertilizer 73 WP,**** kg of grain.mm-1 of water supply 14 PAR cpmversopm efficiency, ***** % Grain 1.4 Total dry matter 3.3 a.i., active ingredient; CV, coefficient of variation; NUE, fertilizer nitrogen-use efficiency; WP, water productivity. *Expressed as diesel equivalents (S3). **Average 3-y (2005-2007) annual applied irrigation amount was 272 mm. ***Ratio of grain yield to applied N fertilizer. ****Ratio of grain yield to total water supply. Total water supply includes plant available soil water at planting and in-season rainfall plus applied irrigation water. ***** Ratio of embodied energy in grain or total dry matter to total incident photosynthetically active solar radiation (PAR) from sowing-to-maturity. values except for two of eleven cases. Despite relatively large fossil-fuel energy inputs, irrigated maize exhibited low GWPi (Figure 2D). On average, CO2, N2O, and CH4 emissions, expressed as CO2 equivalents (CO2e), accounted for 63%, 36%, and 1% of GWP in these irrigated maize fields (mean ± SE = 3,001 ± 67 kg of CO2eha-1). The largest impact on GWP came from soil N2O emissions associated with applied N fertilizer (34%), fuel use for irrigation (29%), manufacture and transportation of N fertilizer (17%), and fuel use for grain drying and field operations (13%). Frequency distribution of GWPi deviated significantly from normality as a result of exponential increase in N2O emissions at N surplus values >50 kg of N ha-1 (Figure 1B). Although GWP per unit area of irrigated maize in the Tri-Basin NRD was within the upper range of published values for maize systems, average GWP of 231 kg of CO2e.Mg-1 of grain and GWP per unit energy input of 103 kg of CO2eGJ-1 was the lowest among published values for US maize systems (Figure 3B and Table 1). Using the IPCC N-input approach to calculate N2O emissions gave GWP and GWPi 28% higher values than based on N2O emissions with the N-surplus method (Figure 1 and Table 1). Management impact. Energy balance and GWP were calculated for irrigated maize and different combinations of irrigation systems, tillage method, and crop rotation based on actual reported values in the Tri-Basin NRD dataset (Figure 4). Energy inputs in fields under pivot irrigation and some form of reduced tillage (no-till, ridge-till, or strip-till, which are also called conservation tillage methods) were lower than in fields under surface irrigation and conventional disk tillage, respectively, mostly because of energy savings from irrigation. Applied irrigation was 41% and 20% less in fields under pivot irrigation and reduced tillage, respectively, compared with their counterparts under surface irrigation and conventional tillage. Apparent advantage of fewer tillage operations was partially counterbalanced by extra fuel use for other field operations such as herbicide application. Although applied N was 21 kg of N.ha-1 less in maize-soybean rotations than under continuous maize, the associated rotation benefit on energy saving was not significant (P = 0.90) and small compared with the energy savings achieved with pivot irrigation or reduced tillage. Of interest was the observation that management systems with the highest grain yield (NER, and NEY) also had the lowest GWPi (i.e., pivot irrigation under soybean maize rotation and reduced-till). Differences in NEY due to crop rotation x tillage interactions were explained by variations in grain yield (Figure 4). Whereas crop rotation had no detectable impact on NEY in conventional-tilled fields, NEY of maize after soybeans was 7% higher than maize after maize in fields in which reduced tillage was practiced. On the average, NER was 23% and 5% higher in fields under pivot and reduced tillage than under surface irrigation and conventional tillage, respectively. GWPi was 7% and 14% smaller on fields in a maize-soybean rotation as well as fields under pivot irrigation (respectively), compared with their counterparts under continuous maize and surface irrigation. Reducing emissions. A large decrease in GHG emissions per hectare of crop production would result from converting
Early Spring 2013