Perennial Filter Strips Reduce Nitrate Levels in Soil and Shallow Groundwater after Grassland‐to‐Cropland Conversion

Abstract

Many croplands planted to perennial grasses under the Conservation Reserve Program are being returned to crop production, and with potential consequences for water quality. The objective of this study was to quantify the impact of grassland‐to‐cropland conversion on nitrate‐nitrogen (NO3–N) concentrations in soil and shallow groundwater and to assess the potential for perennial filter strips (PFS) to mitigate increases in NO3–N levels. The study, conducted at the Neal Smith National Wildlife Refuge (NSNWR) in central Iowa, consisted of a balanced incomplete block design with 12 watersheds and four watershed‐scale treatments having different proportions and topographic positions of PFS planted in native prairie grasses: 100% rowcrop, 10% PFS (toeslope position), 10% PFS (distributed on toe and as contour strips), and 20% PFS (distributed on toe and as contour strips). All treatments were established in fall 2006 on watersheds that were under bromegrass (Bromus L.) cover for at least 10 yr. Nonperennial areas were maintained under a no‐till 2‐yr corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] rotation since spring 2007. Suction lysimeter and shallow groundwater wells located at upslope and toeslope positions were sampled monthly during the growing season to determine NO3–N concentration from 2005 to 2008. The results indicated significant increases in NO3–N concentration in soil and groundwater following grassland‐to‐cropland conversion. Nitrate‐nitrogen levels in the vadose zone and groundwater under PFS were lower compared with 100% cropland, with the most significant differences occurring at the toeslope position. During the years following conversion, PFS mitigated increases in subsurface nitrate, but long‐term monitoring is needed to observe and understand the full response to land‐use conversion.

Nutrient removal by prairie filter strips in agricultural landscapes

“…The application of N fertilizers and wet field conditions likely contributed to the high NO 3 -N concentration in the spring of the corn years (2008 and 2010). High groundwater table (Zhou et al 2010). Return flows of shallow groundwater to the surface can dominate the late recession of a runoff hydrograph.…”

“…The two watersheds were within 3 km (1 mi) of the nearest study watersheds and were 4.2 and 5.1 ha (10.4 and 12.6 ac) in size (noted as Cabbage on figure 1). Native prairie was planted in 2004 in the two watersheds as described by Tomer et al (2010), providing a 100% prairie restoration reference comparison to the 12 agricultural watersheds for the years of 2010 and 2011. These prairie watersheds were not part of the original experimental design and were sampled only from 2010.…”

Targeting perennial vegetation in agricultural landscapes for enhancing ecosystem services

“…Studies based on both modeling 64,65 and empirical measurements 66,67 suggest a nonlinear relationship between the amount of perennial cover and different hydrologic services, resulting in diminishing returns beyond a certain amount. Such nonlinear relationships have been demonstrated for sediment trapping efficiency 61,63,68,69 , retention of nitrogen (N), phosphorus (P) 68,70,71 and pesticides 72 , and runoff reduction 55,56 . Combined, these findings underscore the notion that relatively small amounts of perennial cover may produce significant ES benefits, while minimizing negative impacts on crop production (Fig.…”

“…Possible approaches to developing perennial grain crops include using related perennial species to add genes into annual crops such as wheat, and domesticating existing perennial species, such as intermediate wheatgrass (Thinopyrum intermedium), Maximilian sunflower (Helianthus maximiliani) and Illinois bundleflower (Desmanthus illinoensis), through selection for higher seed production and desirable agronomic and quality factors. Important biological and technical challenges exist for both approaches, but these might be surmountable in much less time than the millennia that were required to domesticate wheat, maize, rice and other grains 71 . Perennial grain crops may have a lower yield potential than annuals due to trade-offs between seed productivity and longevity: resources that might be allocated to seeds may instead have to be allocated to non harvested perennating structures to maintain survival from year to year 275 .…”

A Scale-Explicit Framework for Conceptualizing the Environmental Impacts of Agricultural Land Use Changes

“…For example, though Farms B and E share the same production practices, Farm B reduces the amount of N entering its adjacent headwaters through enhanced on-farm ecosystem services (e.g., 90% removal due to riparian buffers, swales, holding ponds, hedgerows, etc. [28]), as compared to Farm E. Once the runoff N leaves a farm boundary and enters surface waters, the measurable impact drops rapidly due to dilution (i.e., a rapid increase in drainage area) and continues to fall due to both continued dilution and accumulated aquatic ecosystem services [29]. Again, while this aquatic ecosystem service could be assessed relative to some criteria at any scale, Figure 3 illustrates its assessment along the main branch, up to the watershed mouth.…”