Impact of climate change on greenhouse gas emissions and water balance in a dryland-cropping region with variable precipitation

Abstract

Wheat covers a significant fraction of the US Pacific Northwest (PNW) dryland agriculture. Past studies have suggested that management practices can differentially affect productivity and emission of greenhouse gases (GHGs) across the different agro-ecological Zones (AEZs) in PNW. In this study we used CropSyst, a biophysically-based cropping systems model that simulates crop processes and water and nitrogen cycles, with the purpose of evaluating relevant scenarios and contributing analyses to inform adaptation and mitigation strategies aimed at reducing and managing the risks of climate change. We compared the baseline historical period of 1980–2010 with three future periods: 2015–2045 (2030s), 2035–2065 (2050s), and 2055–2085 (2070s). The uncertainty of the future climate was captured using 12 general circulation models (GCMs) forced with two representative carbon dioxide concentration pathways (RCP 4.5 and 8.5). The study region was divided into three AEZs: crop-fallow (CF), continuous cropping to fallow transition (CCF), and continuous cropping (CC). The results indicated that areas with higher precipitation, N fertilization, and mineralization produced more N2O emissions during both baseline and future periods. The average annual N2O emission during the baseline period was between 1.8 and 4.1 kg ha−1 depending on AEZ. The overall N2O emission showed decreasing future trends from 2030s to 2070s which resulted from a higher proportion of N used by crops. From 2015 to 2085 under RCP 4.5, the average N2O emission was between 1.8 and 4.4 kg ha−1 year−1. They are slightly higher under RCP 8.5 since it is a warmer scenario. The soil organic carbon (SOC) content decreased during the baseline period while SOC did not reach equilibrium with the cropping systems considered in the study. SOC decreased during the future periods as well, with rate of change ranging from −146 to −352 kg ha−1year−1 depending on AEZ and RCP. Warming increased SOC oxidation in future scenarios, but after an initial increase of SOC losses during the 2030s period, the rate of SOC losses decreased in the 2050s, and more so in the 2070s as SOC and carbon input reached equilibrium with losses. Higher carbon input resulted from higher biomass production under elevated CO2 scenarios. The total GHG emissions were 1.95, 3.16 and 4.84 Mg CO2-equivalent ha−1year−1 under RCP 4.5, and 1.99, 3.43 and 5.49 Mg CO2-equivalent ha−1year−1 under RCP 8.5 during 2070s in CF, CCF and CC respectively, with N2O accounting for about 81% of total GHG emissions.