Potential of increasing yield while mitigating climate change in Australian wheat systems: a simulation study

Abstract

Sustainable management of agroecosystems is required in order to produce more food to feed the growing global population with less environmental costs. The management strategies have to be developed based on local soil and climatic conditions and for specific stakeholders' interests. In this study, we developed a framework to analyse optimal nitrogen (N) requirement for targeting different levels of crop yield and/or climate change mitigation. The Agricultural Production Systems sIMulator (APSIM) was applied at near- uniformly distributed multiple sites to simulate wheat yield in response to N applications, with the aggregated results tested against the national-level data in Australia. The calibrated model was applied to identify the optimal N management for different targets. Thereafter, correlation analysis was performed to quantify the relationships between N demand, yield and climate change mitigation potential (CCMP) and soil and climate variables. The CCMP was defined as the net soil carbon (C) change minus nitrous oxide (N2O) emissions in CO2 equivalents (Mg CO2-eq ha -1 yr -1 ). At the national scale, the aggregated modelling results could well predict the average wheat yield and soil C change under the current N fertilizer input level. Simulation results with different N application rates indicated that wheat yield could be considerably increased by up to 70% (from 1.8 to 3.0 Mg ha -1 ) by increasing N fertilizer input comparing to the current practice. To achieve 90% of the maximum yield, an average N input of 110 kg ha -1 yr -1 was required across the wheat growing regions, which was ~80 kg ha -1 yr -1 higher than the current N fertilizer application rates. If we targeted the maximum CCMP per unit area and per unit yield, the simulation results also showed a requirement of increased N input compared to the current level, which could result in a wheat yield level similar to or higher than 90% of the maximum yield. Importantly, such increased N input also led to a substantial decrease in net GHG emissions as compared to the current N management. The crop N demand, yield increase and climate change mitigation potential correlated strongly with site- specific precipitation, temperature, soil water holding capacity, and antecedent soil carbon content. Across the study area, the N demand, yield and CCMP varied widely under all four targets (maintaining current crop production, targeting the 90% of the maximum crop production, targeting the minimum net emissions, targeting the minimum net emissions per unit production). In general, rainfall had significant positive effect on simulated N demand and yield, and the degree of the effect varied across targets. The plant available water capacity (PAWC) also had significant positive effect on N demand and yield. However, rainfall had negative effect on CCMP. Compared with rainfall, the effects of temperature on N demand and yield were relatively neutral. Soil organic C content had significant positive effect on yield. For CCMP, soil organic C content had a negative effect. The effects of time period on N demand, yield and CCMP were negligible under current N management The results highlight the opportunity with well-managed intensification to simultaneously increase crop production and mitigate climate change by reducing net GHG emissions in Australian wheat systems. The same opportunity may be present in other low-input dryland cropping systems. The 'win-win' N management recommendations should and can be specified according to local climate and soil conditions by considering the target crop yield and/or climate change mitigation objectives.