Cost-benefit analysis of agroecosystems requires crop productivity be supplemented by a complete accounting of the system’s environmental footprint. Between 1998 and 2006, we quantified impacts of common cropping systems on greenhouse gas and leaching losses of nutrients and soil C and N pools. Systems included continuous maize (CM; Zea mays L.) or maize rotated with soybean (SM; Glycine max L. Merr.) with nutrients applied either in manure (fall or spring application) or inorganic (spring application only) forms. An unfertilized, restored prairie dominated by Andropogon served as a control. Plots (48; 10.8 m x 48 m) were arranged in a randomized complete-block design replicated four times. Plots contained 9 m x 24 m drainage lysimeters that permitted quantitative characterization of mass loss of soil constituents in leachate. Supplemental instrumentation permitted characterization of methane, carbon dioxide and nitrous oxide (N2O) emissions from the soil surface.
Results/Conclusions
Atmospheric losses of N2O were greatest in CM receiving either inorganic N or spring-applied manure (6.4 to 8.2 kg ha-1 yr-1). Fall-applied manure application to CM or spring N applications to MS rotations resulted in significantly lower N2O losses. Mass loss of N2O from the restored prairie was negligible (0.24 kg ha-1 yr-1). Highest mean NO3-N leachate losses (20.1 kg ha-1 yr-1) occurred in CM with fall-applied manure application and these losses were two-fold those occurring in any other agro-ecosystem including CM receiving spring-applied manure. Nitrate leachate losses from prairie averaged only 2.5 kg ha-1 yr-1. Likewise, leaching losses of ortho-P and NH4-N were minimal in prairie, but also in all agro-ecosystems where manure was not introduced. In contrast, fall-applied manure significantly raised mass loss of ortho-P relative to all other treatments (> 0.4 kg ha-1 yr-1), while spring-applied manure resulted in significant mass loss of NH4-N (~0.7 kg ha-1 yr-1). Temporal patterns of nutrient losses were associated with timing and duration of rainfall events relative to management operations with significant percent losses occurring in discretel events. Analysis of soil and plant tissue C and N pools revealed that prairie was strongly N limited as C:N ratios in fine particulate organic matter and aboveground biomass were markedly greater, while total biomass yield was reduced in prairie when compared to all other cropping systems. However, increasing prairie productivity to levels comparable to other system with additional N will increase edge-of-field nutrient losses. Methodology for converting disparate nutrient losses to a common base are needed for relevant life cycle analyses.
