Sign up for email alerts of new Fluid Journal issues!
Fluid Journal : Late Spring 2013
6 The Fluid Journal Late Spring 2013 and N in applied manure, which account for 81%, 15%, and 4%, respectively, of total N input. With few exceptions, estimated N2O emissions were consistently larger using the N-input approach across the range of N fertilizer rates applied to irrigated maize fields in the Tri-Basin NRD (Figure 1A). In a small number of fields that received >225 kg of N/ha-1, however, greater emissions were estimated by the N surplus approach. However, despite a high average rate of N fertilization, 76% of the fields had an N surplus <50 kg/ha-1 so that N2O emissions by the N surplus method were smaller than emissions estimated with the N-input approach (Figure 1B). Large N surplus (>50 kg of N ha-1) resulted from a combination of large N inputs and relatively low grain yields. Although there was a positive correlation between N surplus and the level of N input, large variation in N surplus was observed at any level of applied N input due to variation across fields and years in N use efficiency (NUE, kg of grain per kg applied N, also called partial factor productivity for N fertilizer; ref. 12) shown in Figure 1B, (Inset). Median values for direct N2O emissions from irrigated maize in this study were 1.6 and 3.3 kg N2O-N ha-1 when using the N-surplus and N-input approach, respectively. The N-surplus approach median value is similar to annual direct N2O emissions of 1.9 kg N2O-N/ha-1 measured in a well-managed irrigated continuous maize system in Nebraska that achieved grain yields similar to those in the Tri-Basin NRD. The proposition that N losses from applied fertilizer tend to be small when the N supply is balanced by crop uptake is scientifically robust and supported by published data. Hence, reported GWP in the following sections was calculated based on N2O emissions estimated by the N surplus approach unless stated otherwise. Energy/emissions. Large energy inputs to irrigated maize in the study area were associated with high and stable grain yields (Table 1). Irrigated maize yield was 2.2-fold greater and much less variable across years than lower yielding less intensively managed rain-fed maize in the same region (mean rain-fed yield ± SE = 5.9±0.8 Mg/ha-1; inter-annual coefficient of variation (CV) = 23%). Moreover, irrigated maize in the Tri-Basin NRD achieved, on average, 89% of its estimated yield potential as documented in a previous study. Although N fertilizer inputs were well above N rates reported in previous studies of energy balance and GWP in US maize systems, NUE achieved by irrigated maize products in the current study was much higher than previous published values (Table 1). Likewise, although total water supply was 41% greater with irrigation compared with rain-fed maize in the Tri-Basin NRD, water productivity of irrigated maize was 60% higher (14.0 vs. 8.8 kg/ha-mm-1, respectively). Remarkably, conversion efficiency from solar radiation to total dry matter of 3.3%, estimated for irrigated maize in the Tri- Basin NRD, compares well with highest observed conversion efficiencies (range: 3.9 to 5.2%) for field-grown irrigated maize grown with optimal management practices. Irrigated maize received relatively large fossil-fuel energy inputs (mean: 30.0 GJha-1) and also achieved a large positive Figure 2: Frequency distribution of fossil-fuel energy input (A), net energy yield (B), net energy ratio (C), and global warming potential intensity (GWPi)(D) based on data from 123 irrigated maize fields. Figure 3: Maize grain yield plotted against fossil-fuel energy inputs (A) and GWP (B). Lines indicate average 3-y median (solid line) and fifth and 95th percentiles (dashed line) for net NER and GWPi calculated for irrigated maize in Tri-Basin NRD. Relationship (C) between GWPi and net energy yield for irrigated maize in Tri-Basin NRD. De Oliveira MED, Vaughan BE, Rykiel EJ, Jr. (2005)
Early Spring 2013