Assessing Nitrogen and Carbon Pools in a Perennial Biomass Alley Cropping System in Minnesota U.S.A.

View the project final report

• 2014 annual report

Commodities

• Agronomic: grass (misc. perennial), hay
• Additional Plants: native plants, trees

Practices

• Animal Production: feed/forage
• Crop Production: windbreaks
• Education and Training: on-farm/ranch research
• Energy: bioenergy and biofuels
• Natural Resources/Environment: carbon sequestration, hedgerows, hedges - woody, riparian buffers
• Production Systems: permaculture
• Soil Management: organic matter

In the first study, we measured biomass yield and nutrient uptake in alley cropping agroforestry systems consisting of four herbaceous perennial crops and two short-rotation woody crops (SRWC) at Empire, MN, a site with a history of biosolids application.  Alley systems consisted of herbaceous perennials switchgrass, prairie cordgrass, ‘Rush’ intermediate wheatgrass, and an 11-species native polyculture planted between rows of ‘NM6’ poplar and ‘Fish Creek’ willow.  Alley cropping with poplar maximized biomass yields (13.5 Mg ha^-1 yr^-1), irrespective of herbaceous crop.  Poplar – intermediate wheatgrass systems had among the highest N, P, and K uptake (477, 62, and 301 kg ha^-1, respectively).  NM6 poplar outperformed Fish Creek willow over the first four years.  Intermediate wheatgrass showed the greatest potential for N, P, and K uptake in alley cropping systems, though the native polyculture and prairie cordgrass also performed well and are better choices for sites with occasional flooding. 

In the second study, we evaluated changes in SOC, and C sequestration and nutrient accumulation in root biomass in perennial alley cropping systems over four years at Empire, Minnesota.  Accumulation of C and N was greater in poplar than willow alley cropping systems, and greater in alleys with prairie cordgrass than any other herbaceous crop.  Intermediate wheatgrass and prairie cordgrass fine root biomass, C sequestration, and nutrient accumulation were generally greater in willow than poplar alleys.  The majority of C sequestered and nutrients accumulated were in SRWC, with the largest pool being fine roots.  SRWC and herbaceous crop selection did not influence SOC, though SOC declined from 2010 to 2013.  The NM6 poplar – prairie cordgrass system had among the highest C sequestration and nutrient accumulation.

In the third study, we found that competition for light and soil water substantially reduced native polyculture and prairie cordgrass yield up to 2.4 m from willow rows at Empire, but competition for soil nitrate was not evident. Competition for soil moisture occurred up to at least 2.4 m from tree rows in these alley systems, depending on seasonal water availability.  The relative impact of competition was similar for prairie cordgrass and a native polyculture, though the former had slightly higher yields overall. 

Over the first four years of study, NM6 poplar – intermediate wheatgrass and NM6 poplar – prairie cordgrass alley cropping systems performed best overall in terms of biomass yield; nutrient accumulation and removal; and C sequestration in belowground biomass.  NM6 poplar – native polyculture systems performed nearly as well, except that they sequestered less C.

Introduction:

Recently, increasing the “perennialization” of agriculture in the US Midwest has become an issue of national attention due to a variety of concerns such as water quality and regulation, decline of pollinators, control of pests and pathogens, C emissions, and resilience to climate change (Boody et al. 2005; Jordan and Warner 2010; Asbjornsen et al. 2013).  Agroforestry, the intentional integration of woody perennials with crops or livestock,  has been promoted as one approach to perennialization that can help address environmental concerns and provide a wide range of goods and services to society, including C sequestration, nutrient retention, and lignocellulosic feedstocks for bioindustrial applications (Holzmueller and Jose 2012; Bardhan and Jose 2012; Ehret et al. 2015).   Agroforestry systems have shown potential for production of C neutral or negative biomass feedstocks for energy and fuel production using perennial herbaceous and short rotation woody crops (SRWC) (Gamble et al. 2014; Ehret et al. 2015; Lamerre et al. 2015; Gamble et al. 2016).   Energy and fuels derived from such systems could displace more C-intensive fossil fuels (Holzmueller and Jose 2012), and the combined effects of C sequestration and fuel substitution could substantially improve GHG mitigation and offsets relative to other land use options (Jose and Bardhan 2012). 

In the US Midwest, agroforestry systems such as alley cropping and riparian buffers show particular promise for biomass feedstock production due to their potential adaptability,  both spatially and logistically, to modern farming systems (Holzmueller and Jose 2012; Jose and Bardhan 2012).  Widely spaced tree rows in alley cropping systems lend well to the high degree of mechanization present in modern farming, while perennial biomass crop-based buffers strips could fit strategically into lowland or floodplain agricultural sites to protect waterways.  However, little is known about how spatially integrating woody and herbaceous perennial crops will affect biomass production, C sequestration, and nutrient accumulation in such systems.

Riparian buffers composed of woody and herbaceous perennial vegetation can sequester substantial nutrient loads  (Schultz et al. 1995; Lee et al. 2003; Schoonover et al. 2005).  However, nutrients sequestered in riparian vegetation are, over time, mineralized through decomposition and could enter waterways (Vanek 1991; Jaynes and Isenhart 2014).  Therefore, the efficiency of nutrient removal could be enhanced through repeated harvest of aboveground vegetation.  Certain perennial biomass crops could be well suited to this niche due to their rapid growth rates, high productivity in lowland or floodplain agricultural sites, and nutrient export offsite with repeated harvesting (Pallardy et al. 2003; Tufekcioglu et al. 2003; Lee et al. 2009; Thelemann et al. 2010; Fortier et al. 2010b; Wilson et al. 2014; Zilverberg et al. 2014).  

Short rotation woody crops (SRWC) such as hybrid poplar (Populus spp.) and shrub willow (Salix spp.) are ideal for biomass production and nutrient sequestration in temperate regions because they are easily propagated, quick to establish, fast growing, high yielding, and can be harvested many times before replanting.   Poplars and willows have been used to remediate nutrients in municipal wastewater (Holm and Heinsoo 2013), biosolids (Heller et al. 2003; Börjesson and Berndes 2006; Felix et al. 2008), landfill leachate (Zalesny et al. 2008),  and have been used in riparian buffers to intercept nutrient flows from upland sources (Tufekcioglu et al. 2003; Lee et al. 2003; Young and Briggs 2005; Young and Briggs 2007; Fortier et al. 2010a; Jaynes and Isenhart 2014).  Herbaceous perennial crops such as native grasses and native grass-forb-legume polycultures have also been promoted as suitable biomass crops for lowland or marginal sites (Wilson et al. 2014; Zilverberg et al. 2014), though less is known about their nutrient sequestration potential since they are often promoted and utilized as low-input crops.  However, data on nutrient removal from harvested plant material is important for effective design and management of any cropping system, including biomass systems.  In buffer strips that are used to manage nutrients from agricultural fields, or systems designed to manage wastes like biosolids or manure, crops with low nutrient use efficiency and high nutrient removal are desirable.

Few studies have assessed the potential of perennial biomass crops as C and nutrient sinks in the riparian zone.  Fortier et al. (2010a) assessed nutrient accumulation in a variety of poplar clones in southern Quebec, while Tufekcioglu et al. (2003) assessed N accumulation in poplar and switchgrass buffers in Iowa.  In both cases, the buffers were not explicitly managed as biomass production systems, though each showed greater potential for nutrient accumulation in poplar or switchgrass buffers relative to cool-season grass or unmanaged buffers.  To our knowledge, no studies have assessed the biomass production, C sequestration, and nutrient uptake potential of integrated woody and herbaceous perennial biomass cropping systems.  Little is known about appropriate species combinations that optimize productive potential of SRWC and herbaceous biomass crops when grown in agroforestry configurations.

Roots play an important role as C and nutrient sinks in perennial bioenergy systems.  In shrub willow bioenergy systems in New York,  Pacaldo et al., (2013) found that root C accounted for 69% of total C sequestered.  In a switchgrass bioenergy production system in Washington, root C accounted for 47% of total C sequestered (Collins et al. 2010).  In addition to C, a substantial portion of the nutrients in bioenergy crops can be found in the roots.  Translocation of nutrients from aboveground to belowground plant components during the dormant season influences nutrient harvest, which has implications for nutrient cycling, fertilizer requirements, long term productivity, and C sequestration.   Quantifying nutrient accumulation in the belowground biomass that remains in-field following aboveground biomass harvest is important for understanding plant nutrient cycling and will provide insights as to the ability of various perennial biomass crops to act as nutrient sinks. 

The objectives of the first study were to determine the best pairing of perennial woody and herbaceous crops to maximize biomass yield and nutrient uptake in alley cropping systems at the Empire field site.  The objectives of the second study were to: 1) quantify root C, and root nutrient accumulation of alley cropped SRWC and herbaceous perennial crops; 2) determine the spatial distribution and relative allocation of belowground biomass C within alley systems; and 3) quantify changes in SOC and total system C (belowground biomass C + SOC) associated with alley cropped SRWC and herbaceous perennial crops.  The objective of the third study was to evaluate the spatiotemporal variation in PAR, soil water potential, and soil NO₃ in these alley cropping systems, and to use these data to explain observed patterns in herbaceous crop yield