Cover Crops and Fertilization Alter Nitrogen Loss in Organic and Conventional Conservation Agriculture Systems

Abstract

Agroecosystem nitrogen (N) loss produces greenhouse gases, induces eutrophication, and is costly for farmers; therefore, conservation agricultural management practices aimed at reducing N loss are increasingly adopted. However, the ecosystem consequences of these practices have not been well-studied. We quantified N loss via leaching, NH3 volatilization, N2O emissions, and N retention in plant and soil pools of corn conservation agroecosystems in Kentucky, USA. Three systems were evaluated: (1) an unfertilized, organic system with cover crops hairy vetch (Vicia villosa), winter wheat (Triticum aestivum), or a mix of the two (bi-culture); (2) an organic system with a hairy vetch cover crop employing three fertilization schemes (0 N, organic N, or a fertilizer N-credit approach); and (3) a conventional system with a winter wheat cover crop and three fertilization schemes (0 N, urea N, or organic N). In the unfertilized organic system, cover crop species affected NO3-N leaching (vetch > bi-culture > wheat) and N2O-N emissions and yield during corn growth (vetch, bi-culture > wheat). Fertilization increased soil inorganic N, gaseous N loss, N leaching, and yield in the organic vetch and conventional wheat systems. Fertilizer scheme affected the magnitude of growing season N2O-N loss in the organic vetch system (organic N > fertilizer N-credit) and the timing of loss (organic N delayed N2O-N loss vs. urea) and NO3-N leaching (urea >> organic N) in the conventional wheat system, but had no effect on yield. Cover crop selection and N fertilization techniques can reduce N leaching and greenhouse gas emissions without sacrificing yield, thereby enhancing N conservation in both organic and conventional conservation agriculture systems.

Reviews and syntheses: Review of causes and sources of N<sub>2</sub>O emissions and NO<sub>3</sub> leaching from organic arable crop rotations

“…Previous reviews have compared the net N 2 O emissions and NO 3 leaching from these systems. Lower areascaled, but roughly similar yield-scaled emissions (slightly higher, similar or slightly lower) are commonly observed for N 2 O emissions (Tuomisto et al, 2012;Skinner et al, 2014Skinner et al, , 2019 and NO 3 leaching (Kirchmann and Bergström, 2001;Stopes et al, 2002;Aronsson et al, 2007;Tuomisto et al, 2012;Benoit et al, 2014) from organic versus non-organic arable crop production.…”

“…Weier et al, 1992;Li et al, 2005), we address how the supply of N and carbon (C) via organic inputs drive N 2 O emissions in organic arable rotations. Based on the IPCC (2006), most inventories and farm models assume that 1 % of total N-input by fertilisers, manure and plant residues are emitted as N 2 O-N. Skinner et al (2014) found no correlation between total N-input and N 2 O emission in organic systems. If this is a general trend, the total N-input cannot be used to estimate N 2 O emissions from organic crop rotations.…”

“…Pappa et al, 2011;Nadeem et al, 2012;Brozyna et al, 2013;Krauss et al, 2017b;Skinner et al, 2019). Because of the enhanced content of SOM and thus the larger impact of background emissions of N 2 O in organic versus non-organic cropping systems (Skinner et al, 2014), increased background emissions are likely to have a major impact in organic-crop rotations. In order to design good mitigation strategies, it is useful to know the relative importance of these two sources of N 2 O emissions.…”

“…Skinner et al (2014) concluded in a review that soil characteristics (soil N content) had a greater impact on N 2 O emissions from organic production than the total N-input by fertilisation. Lack of correlation between N 2 O emissions and N fertilisation in organic production corresponds to the more recent findings ofKrauss et al (2017b) and Pugesgaard et al (2017).…”

Particle Size Affects Nitrogen and Carbon Estimate of a Wheat Cover Crop

“…An ignored yet important management factor that can influence cover crop tissue C and N estimation is sample‐grinding size. A broad range of sample‐grinding size, ranging anywhere from >2 mm to pulverized have been reported in the literature (Jahanzad et al, 2016; Etemadi et al, 2017; Shelton et al, 2018; Sievers and Cook, 2018), but no agreement about which sampling size is the most appropriate has been reached to present. In a literature review, we randomly assessed 36 peer‐reviewed journal articles, which reported cover crop C and N concentrations (Table 1; in addition, see online version of article for supplemental material).…”