How does the first year tilling a long-term no-tillage field impact soluble nutrient losses in runoff?

Abstract

Conservation tillage practices are commonly used to reduce erosion; however, in fields that have been in no-tillage (NT) for long periods, compaction from traffic can restrict infiltration. Rotational tillage (RT) is a common practice that producers use in the central corn-belt of the United States, and could potentially reduce soluble nutrient loads to surface waters. The objectives of this study were to determine the first year impacts of converting from long-term NT to (RT) on N and P losses through runoff. Plots (2 m × 1 m) were constructed in two fields that had been in NT corn–soybean rotation for the previous 15 years. One field remained in NT management, while RT was initiated prior to planting corn in the other field using a soil finisher. Variable-intensity rainfall simulations occurred before and after fertilization with urea (224 kg N ha⁻¹) and triple superphosphate (112 kg P ha⁻¹). Rainfall was simulated at (1) 50 mm h⁻¹ for 50 min; (2) 75 mm h⁻¹ for 15 min; (3) 25 mm h⁻¹ for 15 min; (4) 100 mm h⁻¹ for 15 min. Runoff volumes and nutrient (NH₄-N, NO₃-N and dissolved P [DP]) concentrations were greater from the NT field than the RT field before and after fertilization.

Dissolved P concentrations in runoff prior to fertilization were greater during the 50 mm h⁻¹ rainfall period (0.09 mg L⁻¹) compared to the other periods (0.03 mg L⁻¹). Nutrient concentrations increased by 10–100-fold when comparing samples taken after fertilization to those taken prior to fertilization. Nutrient loads were greater prior to and after fertilization from the NT treatment. Prior to fertilization, NT resulted in 83 g ha⁻¹ greater NH₄-N and 32.4 g ha⁻¹ greater dissolved P losses than RT treatment. After fertilization, NT was observed to lose 5.3 kg ha⁻¹ more NH₄-N, 1.3 kg ha⁻¹ more NO₃-N, and 2.4 kg ha⁻¹ more dissolved P than RT. It is typically difficult to manage land to minimize P and N losses simultaneously; however, in the short term, tillage following long-term NT resulted in lowering the risk of transport of soluble N and P to surface water.

Introduction

Agriculture has been defined as a primary contributor to non-point source pollution of U.S. surface waters (USEPA, 1996). Specifically, nitrogen losses from cropland in the Midwestern U.S. have been cited as a major source of the hypoxic zone in the Gulf of Mexico (Goolsby et al., 2001). Eutrophication of drinking water reservoirs, and estuaries, has been blamed on phosphorus losses from agricultural sources (Schindler, 1977).

Tillage practices can affect nutrient losses to the environment. No-tillage (NT) has been shown to result in greater NH₃ volatilization and N₂O emissions than conventional tillage (Palma et al., 1998, Venterea et al., 2005). Leaching of NO₃ has been shown to be greater from NT than conventionally tilled (CT) soil (Brye et al., 2001, Stoddard et al., 2005), however, these results are not consistent in all studies (Stoddard et al., 2005, Thoma et al., 2005). Fertilizer N has been shown to be lost at greater rates in runoff from NT than CT plots (Soileau et al., 1994, Torbert et al., 1996, Francia Martinez et al., 2006). Zhao et al. (2001) observed that practices that produced greater levels of incorporation resulted in lower losses of soluble nutrients. However, in some systems, NT can reduce N losses in runoff (McDowell and McGregor, 1984, Yoder et al., 2005). In a review, Power et al. (2001) concluded that results from NT practices for N were inconclusive.

Incorporation of manure with tillage practices has been shown to be an effective strategy to reduce P losses through runoff (Schreiber and Cullum, 1998, Bundy et al., 2001, Zhao et al., 2001, Little et al., 2005). This observation is likely made because systems without tillage can result in P stratification with high concentrations near the surface, and thus increase the potential for P losses during runoff events (Simard et al., 2000, Sharpley, 2003). On the other hand, at the watershed scale, dissolved P (DP) concentrations in streams were greater with decreasing levels of tillage (Moog and Whiting, 2002). No-tillage has also been shown to result in greater DP concentrations and loads than CT at the plot scale (Daverede et al., 2003). Other studies have shown that CT can increase erosion, and thus total P losses (Torbert et al., 1996, Chambers et al., 2000). One review indicated that conservation tillage practices can result in reducing P losses by 6.1 kg ha⁻¹ year⁻¹, and was the most cost effective practice to reduce P losses (Sharpley et al., 2001). Buchanan and King (1993) demonstrated that P losses from plant residues can be increased when they are incorporated into soil instead of left on the surface, such as is the case with NT management.

Most published reports compare the effects of long-term NT or CT systems. However, rotational tillage (RT) management, in which fields are tilled, but not every year, appear to be more a more common practice in the northern corn-belt. Hill (2001) reported that the mean time in continuous NT as of 1999 was 2.4 years in Illinois and 2.3 years for Indiana. Little information has been gathered about how RT or changing from long-term NT to RT impacts nutrient transport. The objectives of this study were to determine the short-term impacts during the first year of converting from long-term NT to RT on soluble N and P losses through runoff. The testable hypothesis used to evaluate these objectives was: There will be no significant difference in soluble N or P transport through runoff from long-term plots in NT or plots recently converted to RT from NT.