Landscape control of nitrous oxide emissions during the transition from conservation reserve program to perennial grasses for bioenergy

Abstract

Future liquid fuel demand from renewable sources may, in part, be met by converting the seasonally wet portions of the landscape currently managed for soil and water conservation to perennial energy crops. However, this shift may increase nitrous oxide (N 2 O) emissions, thus limiting the carbon (C) benefits of energy crops. Particularly high emissions may occur during the transition period when the soil is disturbed, plants are establishing, and nitrate and water accumulation may favor emissions. We measured N 2 O emissions and associated environmental drivers during the transition of perennial grassland in a Conservation Reserve Program (CRP) to switchgrass (Panicum virgatum L.) and Miscanthus x giganteus in the bottom 3-ha of a watershed in the Ridge and Valley ecoregion of the northeastern United States. Replicated treatments of CRP (unconverted), unfertilized switchgrass (switchgrass), nitrogen (N) fertilized switchgrass (switchgrass-N), and Miscanthus were randomized in four blocks. Each plot was divided into shoulder, backslope, and footslope positions based on the slope and moisture gradient. Soil N 2 O flux, soil moisture, and soil mineral nitrogen availability were monitored during the growing season of 2013, the year after the land conversion. Growing season N 2 O flux showed a significant vegetation-bylandscape position interaction (P < 0.009). Switchgrass-N and Miscanthus treatments had 3 and 6-times higher cumulative flux respectively than the CRP in the footslope, but at other landscape positions fluxes were similar among land uses. A peak N 2 O emission event, contributing 26% of the cumulative flux, occurred after a 10.8-cm of rain during early June. Prolonged subsoil saturation coinciding with high mineral N concentration fueled N 2 O emission hot spots in the footslopes under energy crops. Our results suggest that mitigating N 2 O emissions during the transition of CRP to energy crops would mostly require a site-specific management of the footslopes.

Real‐time monitoring of greenhouse gas emissions with tall chambers reveals diurnal N2O variation and increased emissions of CO2and N2O fromMiscanthusfollowing compost addition

“…Similar N 2 O emissions events have been previously reported after rainfall (e.g. Breuer, Papen, & Butterbach‐Bahl, ; Mummey, Smith, & Bolton, ; Saha et al, ), and we suggest that the driver of the event reported in this paper was that nitrate (NO 3 ) built up in the soil during the preceding dry weeks, which was rapidly denitrified as the soil became anaerobic following the rainfall, a process experimentally described elsewhere (Krichels, DeLucia, Sanford, Chee‐Sanford, & Yang, ). This event reinforces the necessity to measure GHG fluxes at a temporal resolution appropriate to detect such emission events.…”

Lorenz Curve and Gini Coefficient Reveal Hot Spots and Hot Moments for Nitrous Oxide Emissions

“…At each monitoring location and soil depth, θ A was calculated from half‐hourly volumetric soil moisture measurements, soil porosity, and rock volume (see Saha et al, ). Since Saha et al () showed that a regression tree predicted higher N 2 O emissions in the same experimental site when θ A < 0.03 m 3 m −3 and when soil NO 3 availability was not limiting, we assumed θ A = 0.03 m 3 m −3 to be a threshold below which emissions unusually increase (Saha et al, ). The cumulative frequency (i.e., counts) of θ A < 0.03 m 3 m −3 was natural log transformed (for visual scaling), spatially interpolated, and overlaid with the temporal stability of growing season N 2 O flux.…”

“…Emissions of the greenhouse gas nitrous oxide (N 2 O) from soils often exhibit high spatial and temporal variability (Izaurralde et al, 2004;Pennock et al, 1992;Saha et al, 2017;van Kessel et al, 1993;Vilain et al, 2010). This variability emerges from the collective forcing by hydrologic and biogeochemical factors that control soil nitrogen (N) transformations, which are in turn spatially and temporally variable.…”

“…Topography interacts with landscape hydrology to influence carbon (C) and N biogeochemical cycling. Hot spots of N 2 O emissions have been observed in convergent hydrological flow paths due to high moisture and low O 2 concentration in the soil, favoring N 2 O production via denitrification (Enanga et al, 2016;Izaurralde et al, 2004;Pennock et al, 1992;Saha et al, 2017;Vilain et al, 2010). The hot moments are brief temporal windows with emissions that are substantially higher than the background level emission (Groffman et al, 2009;McClain et al, 2003;Molodovskaya et al, 2012).…”

Soil nitrous oxide flux following land‐use reversion from Miscanthus and SRC willow to perennial ryegrass

“…Significant progress has been made in quantifying growing cycle N 2 O emissions from biomass production but the start and end of crop life are still poorly quantified, despite these being the periods of greatest soil perturbation and C and N inputs (Whitaker et al, 2017). Although there have been some studies investigating GHG fluxes during conversion periods to energy crops (e.g., Nikièma, Rothstein, & Miller, 2012;Oates et al, 2015;Palmer et al, 2014;Roth et al, 2013;Saha et al, 2017) and others investigating the impacts of fertiliser additions on N 2 O emissions (Behnke et al, 2012;Drewer, Finch, Lloyd, Baggs, & Skiba, 2012;Duran, Duncan, Oates, Kucharik, & Jackson, 2016;Gauder, Butterbach-Bahl, Graeff-Honninger, Claupein, & Wiegel, 2012;Hellebrand, Scholz, Kern, & Kavdir, 2005;Jørgensen, Jorgensen, Nielsen, Maag, & Lind, 1997;Roth et al, 2015;Ruan, Bhardwaj, Hamilton, & Robertson, 2016), surprisingly, little work has considered GHG dynamics at the end of cropping cycles with reversion back to more typical agricultural systems. One study looked into the reversion of a 20-year-old Miscanthus x giganteus plantation into wheat production and set-aside (Drewer, Dufossé, Skiba, & Gabrielle, 2014;Dufossé, Drewer, Gabrielle, & Drouet, 2014).…”