Understanding soil health across greenhouse gas emissions and soil characteristics

Decades of intensive agricultural production, consisting of monoculture crops like corn (Zea mays), has led to a drastic decline in soil health, indicated by a reduction in soil carbon, nutrient holding and water holding capacity. Shifting management from monoculture crops to perennial crops could improve these soil characteristics and boost the resilience of agricultural systems to climate change. Furthermore, dairy livestock production systems are major greenhouse gas (GHG) emitters. GHGs from crop-livestock systems originate from enteric fermentation, manure storage, soils, and farm energy use. Nevertheless, US dairy herd sizes have not changed significantly in recent decades, suggesting that annual enteric fermentation emissions remained constant, while manure and soil emissions (i.e., CO₂, N₂O, CH₄) increased from the intensive management, including tillage and the application of agricultural chemicals. However, soil emissions such as CO₂ efflux (efflux) also consists of natural biogenic emissions from plant and microbial activity. Hence, an efflux may indicate greater soil health, suggesting root activity and high soil microbial abundance. Similarly, less variability in soil moisture and temperature could indicate high compaction and inferior soil structure. Understanding the multiple responses of soils to agricultural management is critical for developing strategies to improve soil health.

Here we established a nested treatment experiment with four block replications and three replicated plots per block (30 by 30 feet) using six different cropping systems (corn, corn with cover crop, corn intercropped alfalfa (Medicago sativa), alfalfa, intermediate wheatgrass (IWG, Thinopyrum intermedium), and a five species mixture of pasture grasses and forbs), to understand the system tradeoffs among soil health, forage quality and milk production in a dairy agricultural system. To quantify changes in soil health, structure, and soil GHG emissions, we planted corn on all plots in year 1 (2020) before planting other treatments in year 2 (2021). We collected data on soil nutrients and carbon content, soil microbial abundance and diversity, soil CO₂ efflux, soil moisture and temperature, as well as forage samples and multispectral drone flights to assess forage quality.

Corn plots (monocultures and intercropped) had lower variability in environmental characteristics like soil moisture and temperature, while their magnitudes were elevated, indicating a more compacted and less aerated soil compared to plots with greater root density and lower bulk density (i.e., pasture plots). Similarly, corn plots respired significantly less CO₂, both in years 1 and 2, compared to perennial crop plots, conforming with soil microbial data, which indicated lower microbial diversity in corn plots compared to pasture plots. While corn biomass was greater at the time of harvest compared to other crops, pasture and alfalfa plots accumulated half of the corn biomass throughout three harvest cycles and showed to have lower variability in yield, while also having higher nutritious value compared to corn silage, with implications for milk quality. Our findings suggest that efforts to make dairy operations more resilient to climate change and weather extremes should focus on more variables than just GHG emissions and soil carbon storage, to sustain agricultural production, human nutrition, and biogenic nutrient recycling into the future.