Abstract
Nitrogen (N) is an essential nutrient for plant growth and maintaining crop productivity, although excess N in agroecosystem can be detrimental to soil health, lead to environmental degradation, and represents lost farm income when it results from overfertilization. Diversifying crop rotations to support N cycling and retention can improve crop production efficiency and soil health, and has potential to reduce excess N if coupled with guidance to better integrate plant-derived nutrients and reduce fertilizer applications. This study investigated winter pea grain and mixed-species forages integrated into dryland wheat cropping systems in the inland Pacific Northwest (iPNW) as a regionally appropriate strategy to improve crop production efficiency. To address the issue of excess N (Chapter 2), a practical framework for evaluating current N fertilization practices for winter wheat was developed with a specific focus on methods to utilize plant-derived N sources in combination with external fertilizers. The approach uses N use efficiency (NUE) concepts and common soil and crop N sampling techniques to track N pool storage, quantify plant available N, and assess N loss. We outline a procedure for developing field-scale targeted production goals, and improve crop N demand and N requirement thresholds to inform decision support tools for producers.
The capacity for winter pea grain and mixed-species forages to support crop productivity while improving NUE, water use efficiency (WUE) and N and water storage in iPNW winter wheat systems with continuous annual cropping (AC) and in rotations with summer fallow (Fallow-Transition, FT) was assessed in Chapter 3. The alternative rotations were compared to business-as-usual (BAU) rotations in fully replicated on-farm trials over 4 years (2018-2021) at two sites. Winter pea integration increased rotational WUE by 26 kg ha-1 cm water-1 compared to BAU at the AC (p<0.01) and the FT (p=0.08). Aboveground crop biomass C:N averaged across 3-yr rotations was 39 and 44% lower in grain-forage cropping system and 25 and 50% lower in crop rotations with winter pea compared to BAU systems at the AC and FT sites, respectively. Aboveground crop residue N retained in-field, summed over full crop rotations, was higher in winter pea (99, 115 kg ha-1) than BAU (64, 48 kg ha-1) at the AC (p<0.01) and FT sites (p<0.01), respectively. At the FT site, winter wheat yield decreased in some years, however, rotational grain yield was not impacted compared to BAU rotations. Fallow replacement decreased soil water in 0-30 cm soil at winter wheat seeding, though in years with late spring rains, this did not correspond to lower yields. At the AC site, there was no impact on winter wheat yield, but rotational N retention efficiency and crop N uptake efficiency were increased with forage and winter pea compared to the BAU cropping system. Positive crop rotational N-balances at both sites indicate all cropping systems had excess N, and were highest in the winter pea system. Weather varied greatly over this four-year study, resulting in high annual variation in soil and crop response variables.
Crop-livestock integration in iPNW wheat systems was investigated in large-scale field plots with cattle-grazed forages compared to rotations with winter peas or BAU management in the AC and FT regions of the iPNW (Chapter 3). Plots were large enough (25 ha) to allow evaluation with satellite-based remote sensing tools to track differences in crop N uptake due to crop rotation. Crop N uptake, yield, soil nitrogen and soil water availability were measured in spring and fall for four years. Additional samples were collected in May and June during winter wheat crop growth to assess the impact of preceding alternative crops on N availability, uptake, and to evaluation methods with normalized difference red-edge (NDRE) reflectance data. Field collected soil and crop N data were highly variability during spring growing months of winter wheat following different rotations, although there was no impact on yield due to crop rotation treatments. Simple linear models produced significant relationships between NDRE and crop N uptake, which varied across site-years, but when combined explained 42% of the variation. Additionally, field maps of NDRE data show clear relative differences in crop N uptake temporal patterns following crop rotation treatments. While predictive models could be improved, this study suggests that currently available tools for using the NDRE index to assess crop N uptake are sensitive to management, could support adoption of alternative crops and assist growers with better integrating plant-derived N, or other biological sources, into fertility management.