Improving semi-arid agroecosystem services with cover crop mixes

Abstract

Winter wheat (Triticum aestivum, L.) production in the semi-arid US Northern High Plains (NHP) is challenged by frequent droughts and water-limited, low fertility soils. Composted cattle manure (compost) and cover crops (CC) are known to provide agroecosystem services such as improved soil health, and in the CC case, increased plant diversity, and competition with weedy species. The main concern of planting CC in winter wheat fallow rotation in regions that are more productive than the NHP, however, is the soil moisture depletion. It is unknown however, whether addition of CC to compost-amended soils in the NHP will improve soil properties and agroecosystem health without compromising already low soil water content. The main objective of this study was to assess the effects of four CC treatments amended with compost (45 Mg ha-1) or inorganic fertilizer (IF) (.09 Mg ha-1 mono-ammonium phosphate, 11-52-0 and 1.2 Mg ha-1ammonium sulfate, 21-0-0) on the presence of weeds, soil and plant total carbon (C), nitrogen (N), and biological dinitrogen (N2) fixation (BNF). Mycorrhizal Mix (MM), Nitrogen Fixer Mix (NF), Soil Building Mix (SB), a monoculture of phacelia (Phacelia tanacetifolia Benth L.) (PH), and a no CC control (no CC) were grown in native soil kept at 7% soil moisture in a greenhouse for a period of nine weeks. When amended with compost, MM was the most beneficial (48 g m-2 BNF and 1.7% soil C increase). SB had the highest germination, aboveground biomass, and decreased weed biomass by 60%. It also demonstrated the second highest amount of BNF (40 g m-2) and soil C increase by 1.5%. On contrary, IF hindered BNF by almost 70% in all legume-containing CC treatments and reduced soil C by 15%.

Fig 1. Greenhouse experimental layout.

Treatments include control (CON), phacelia (PH), soil building mix (SB), nitrogen fixer mix (NF) and mycorrhizal mix (MM). Soil additives include no soil additive (yellow), composted cattle manure (green) and inorganic fertilizer (blue). Boxes labeled “fans” represent cooling fans, box labeled “reference pot” represents pot filled with the same soil but no plants used as a reference for soil water adjustments. Pots were rotated weekly.

Fig 2. Cover crop and weedy species aboveground plant biomass in control (CON), phacelia (PH), soil building mix (SB), nitrogen fixing mix (NF), and mycorrhizal mix (MM) treatments.

Upper-case letters demonstrate treatments differences at p ≤ .05.

Fig 3

Biological nitrogen fixation (BNF) for cover crop treatments (no cover crop (CON), Phacelia (PH), Soil building mix (SB), Nitrogen fixer mix (NF) and Mycorrhizal mix (MM)) in control (A), compost (B), and inorganic fertilizer (C). BNF was calculated using weedy species of control treatment for each soil treatment as a reference. Different upper-case letters are different across cc treatments at p ≤ .05 and provide a stepwise statistical comparison of each parameter by cover crop mix treatment.

Fig 4

Phacelia (PH), Soil building mix (SB), Nitrogen fixer mix (NF), Mycorrhizal mix (MM) and weedy species natural ¹⁵N in soil (A), compost (B), and inorganic fertilizer (C). Blue lines indicate average soil natural ¹⁵N abundance. Red line indicates natural ¹⁵N abundance of the compost and black line indicates average natural ¹⁵N abundance of inorganic fertilizer [28].

Fig 5

Soil organic carbon among cover crop treatments (Control (CON), Phacelia (PH), Soil building mix (SB), Nitrogen fixer mix (NF) and Mycorrhizal mix (MM)) in control (A), compost (B), and inorganic fertilizer (C). Different upper-case letters are different across cc treatments at p ≤ .05.