The Green Revolution of the 20th Century generated unparalleled levels of agricultural productivity based on advances in crop breeding – and ample inputs of inexpensive nitrogen (N) fertilizer. Unintended losses of N from production agriculture contribute to increases in algal biomass in estuaries and marine waters leading to loss of fisheries and spawning habitats and creation of multitudes of hypoxic “dead zones” across the globe (Conley et al. 2009; Howarth et al. 2000). Providing food security for a global population that is projected to exceed 9 billion by 2050 (U.N. Population Division 2007) is likely to include even more intensive use of N fertilizer – with great implications for coastal waters. In addition, reactive N can undergo transformations that generate nitrous oxide (N2O) – a potent greenhouse gas – leading the U.S. National Academy of Engineering to declare managing the N cycle as one of the Grand Challenges of the 21st Century (http://www.engineeringchallenges.org/cms/challenges.aspx) – and the Stockholm Resilience Center to proclaim the N cycle as one of three planetary boundaries that has been exceeded globally (Rockstrom et al. 2009).Improved crop varieties, cropping systems, precision management, and soil and plant testing hold promise for greater N use efficiency at the field scale (Cassman et al. 2002). However, reactive N is notoriously leaky, suggesting that additional control measures are needed after reactive N leaves the field and begins to flow through a catchment.Predictive geospatial tools, such as the USGS SPARROW model (Preston et al. 2009), have targeted source locations within a growing number of watersheds that have high potential for delivery of waterborne N to coastal estuaries (e.g., the Mississippi Basin, Alexander et al. 2008; Chesapeake Bay, Preston and Brakebill 1999; the Southeast U.S., Hoos and McMahon 2009). These tools recognize that certain areas of the landscape function as removal sites (i.e., sinks) for waterborne N (National Research Council 1993). In these N sink areas, denitrification converts soluble nitrate to N gas, and plant and microbial biomass retains N. Nitrogen sinks include riparian wetlands, reservoirs and lower-order (headwater) stream reaches. These locations are characterized by extended retention times and flow paths that enhance interaction of nitrate-enriched waters with labile organic matter (Groffman et al. 2009). However, in many areas of the Mississippi River Basin nitrate sink areas have been removed and/or bypassed by tile drainage, which is important in transporting nitrate from fields to streams (David et al. 2010).Where natural sinks are absent, artificial sinks, such as constructed wetlands (Tanner et al. 2005) or carbon bioreactors – simple, wood-chip filled trenches (Schipper et al. 2010a) – hold great promise for reducing edge-of-field N losses. These artificial sinks are positioned to intercept and promote denitrification in drainage waters or N-rich ground water, particularly in settings where edge-of-field N losses are dominated by nitrate-N (Gentry et al. 1998; Goolsby et al. 1999; Nolan 2001). Artificial sinks are now starting to be employed in an array of climatic, geophysical and agricultural settings. Some practices are eligible for USDA EQIP support in select states (e.g., Iowa, Arkansas, and Illinois), and more widespread adoption could occur as research advances on seasonal performance and design criteria.Field testing of bioreactors and constructed wetlands has shown that these systems can remove nitrate-N in a range of climatic conditions and be integrated into different land uses if designed to meet site-specific conditions. There are emerging general design principles for both systems that can foster the development of best management practice (BMP) guidelines for specific spatial or geographic conditions, climate regimes, and agricultural practices. Research will play a critical role for these guidelines to address the current l
展开▼
