Assessing Nitrogen and Carbon Pools in a Perennial Biomass Alley Cropping System in Minnesota U.S.A.
Agroforestry, the intentional integration of woody perennials with crop or livestock production, is a polyculture cropping strategy that has been proposed as a sustainable means of feedstock production for livestock and emerging bioenergy sectors. In addition, agroforestry systems may be an effective means to sequester atmospheric C in soil and perennial roots while maintaining or even improving agricultural productivity. However, little is known about appropriate species combinations, planting arrangements, rotation lengths, and long-term production potential for biomass crops in agroforestry systems in the North Central Region of the United States. Understanding these factors is critical because improper species selection and crop spatial arrangement can result in poor crop establishment, reduced overall productivity, and ultimately crop failure due to competition for resources between trees and crops. The overall objective of this research is to understand the long-term production potential and ecosystem benefits of alley cropping with short-rotation woody crops and perennial herbaceous biomass crops. Multiple on-farm experiments are being conducted to understand what factors affect long term crop persistence and productivity in these structurally and functionally diverse systems. For this proposal, the specific research objective is to understand the magnitude and partitioning of biomass, carbon, and nitrogen pools in a biomass crop- based alley cropping system.
The study was established in May and June 2010 at three privately owned farm sites in Minnesota. Sites were located on floodplains near Granada (43° 45’ 28” N; 94°20’48” W) and Fairmont, MN (43° 45′ 10″ N; -94° 29′ 6″ W), and on a stream terrace near Empire, MN (44° 39’ 59” N; 93° 06’ 39” W). Soils at Granada and Fairmont are very deep, poorly to somewhat poorly drained, formed in alluvium, and consist of the Coland (Fine-loamy, mixed, superactive, mesic Cumulic Endoaquolls) soil series. Soils at Empire are very deep, somewhat poorly drained, formed in loamy alluvium overlying sand and gravel outwash and are of the Cylinder (Fine-loamy over sandy or sandy-skeletal, mixed, superactive, mesic Aquic Hapludolls) soil series. Slopes at all sites range are less than 3%. Average annual temperature at Granada and Fairmont is 7.4 °C and at Empire 6.4 °C. Average annual rainfall at Granada and Fairmont is 79 cm and at Empire 88 cm.
Alley cropping systems consisted of four herbaceous biomass crops and two short-rotation woody crops (SRWC). Woody crops were established in two hedgerows, retaining a 15.2 m alley way between hedgerows and a 1.5 m unsown buffer between trees and crops on each side of the alley. Woody crops were poplar hybrid ‘NM6’ (Populus maximowiczii x P. nigra) and willow cultivar ‘Fish Creek’ (Salix purpurea). Herbaceous crops were switchgrass (Panicum virgatum L.), prairie cordgrass (Spartina pectinata Bosc ex Link), a mixture of Pioneer Brand ‘54V48’ alfalfa (Medicago sativa L.) and ‘Rush’ intermediate wheatgrass (Thinopyrum intermedium [Host] Barkworth and Dewey), and an eleven species native tallgrass-forb-legume polyculture.
While part of a larger research project that is attempting to understand spatiotemporal dynamics of light, N, and soil water in the alley cropping system, this project will specifically focus on the accumulation and partitioning of C and N. The goals of this project are to:
- Determine the productivity of various tree-crop combinations in alley cropping systems
- Determine the C sequestration potential of tree-crop combinations in alley cropping systems
- Determine to what extent trees and crops are competing for N in the alley cropping systems
To accomplish these goals, the project will be divided into the following activities:
- Quantify the amount, C and N content, and spatial distribution of tree and crop roots, aboveground biomass, and annual tree leaf litter inputs into the soil.
- Quantify soil organic C and fall residual soil nitrate-N within the alley.
- Utilize the data collected in Tasks 1- 2, along with productivity data, to determine the net impact of trees on herbaceous crop N use (competitive, neutral, facilitative)
In fall 2013 and fall 2014, woody crop productivity was assessed by collecting poplar and willow plant heights, diameters, and stem counts following plant senescence and calculating individual tree basal area (BA, mm2 tree-1) and stand basal area (SBA, m2 ha-1). Stem diameters were measured at a height of 30 cm for willow and at 140 cm for poplar. Willow data were collected in 2.8 m2 sampling areas (2 rows of three trees each) along the tree-crop interface (edge rows) and in the center of the plot (center rows) on each side of the alley. Poplar data were collected for 3 trees in each of the first (edge), second, and third (center) rows from the tree crop interface on each side of the alley. Following senescence in late fall 2013, woody plant samples were hand-harvested from each sampling area. Samples were immediately chipped, weighed, and dried to a constant weight at 45 ºC to determine aboveground leafless biomass yield on a dry weight basis.
Herbaceous biomass yield was measured in fall following a killing frost (0º C) or plant senescence. Samples of the herbage were harvested in each sub-plot within the alley in the fall to determine dry matter production. All plant material in a 2.78 square meter area (0.91 m x 3.05 m) was mechanically harvested with a Carter flail-type forage harvester (Carter Mfg Co, Brookston, IN) to a 10 cm stubble height at five distances from the tree row. Randomly collected sub-samples of at least 1000 g were dried in a 60º C oven to a constant weight and weighed again to obtain dry matter yield and moisture content. Weed biomass was manually separated from crop biomass in dried sub-samples to obtain weed free biomass yield estimates.
In 2013, leaf litter traps were constructed of a wood frame and fiberglass mesh screen. Leaf trap openings were 53 cm x 53 cm for an effective collection area of 0.2 m2 and were staked so that the top of the trap was 76 cm above the ground surface. Traps were placed at four locations: Within the tree rows, and at 1 m, 3.5 m, and 6 m from the tree rows in the grassed alley, resulting in 24 traps per site (2 tree types x 4 locations x 3 replicates). Traps were placed in the field in early August 2013 and litter was collected biweekly thereafter until all leaves had fallen from trees. Leaves were dried to a constant weight at 35º C and weighed to determine dry matter biomass production, which is expressed on an area basis. Dried leaf litter was ground with a Wiley mill (Thomas-Wiley Mill Co., Philadelphia, PA, USA) to pass a 1 mm screen, and then reground with a cyclone mill. Root mineral concentrations (P, K, S, Ca, Mg) were determined with inductively coupled plasma (ICP) mass spectroscopy following digestion with HNO3 and H2O2 except for N, which was determined via dry combustion and a Perkin- Elmer 2400 CHNS analyzer (Perkin – Elmer Inc., Waltham, MA, USA) by Brookside Laboratories, Inc. (New Bremen, OH, USA).
Soil cores were collected prior to initiation of the study in May 2010, and again in November 2013. Cores were collected in four depth increments (0 – 15 cm, 15 – 30 cm, 30 – 60 cm, and 60 – 90 cm), except at Empire, where a gravel layer prevented collection of samples in the 60 – 90 cm increment. Cores were collected with a hydraulic probe truck equipped with a high relief bit to minimize soil compaction (40.8 mm ID probe tip). In each sub-plot, cores were collected at 0 m (immediately adjacent to trees), 1 m, 3.5 m, and 6 m from the tree row. At each sampling location, plant residue was brushed aside to expose mineral soils, and three cores were collected and composited by depth increment. Subsamples of each composite were passed through a 2 mm sieve to remove fine roots, which were then returned to the composite sample. The subsamples were air-dried, ground using a mechanical grinder and ball mill, and analyzed for total C and N content using an elemental analyzer (Model NA 1500 NC, Carlo Erba/Fisons Instruments, Milan, Italy) and N–NO3, pH, organic matter, P, and cation exchange capacity by Agvise Laboratories (Benson, MN, USA).
The composite samples were washed with a hydropneumatic elutriation system to remove soil particles from roots. Roots were dried to a constant weight at 35ºC, and dry weights were obtained for each composite. Dried roots were ground with a Wiley mill (Thomas-Wiley Mill Co., Philadelphia, PA, USA) to pass a 1 mm screen, and then reground with a cyclone mill. Root mineral concentrations (P, K, S, Ca, Mg) were determined with inductively coupled plasma (ICP) mass spectroscopy following digestion with HNO3 and H2O2 except for N, which was determined via dry combustion and a Perkin- Elmer 2400 CHNS analyzer (Perkin – Elmer Inc., Waltham, MA, USA) by Brookside Laboratories, Inc. (New Bremen, OH, USA). An additional set of cores was collected from the center of each plot to calculate soil bulk density. The cores were dried to a constant weight at 35ºC. Dry weights were determined and soil bulk density was calculated as the ratio of the oven dried mass of soil to the core volume.
Impacts and Contributions/Outcomes
Data from this research is currently being analyzed and incorporated into four manuscripts for publication. Preliminary findings from the research have been incoporated with data from 2010 – 2012 and presented at five outreach events by the project coordinator:
- Integrating herbaceous and woody perennials for nutrient treatment: a bioenergy approach. Rural Advantage / UMN Extension Workshop: Addressing Water Quality Issues in Elm Creek – Farmer Led Solutions. Knights of Columbus Hall, Fairmont, MN. February 5, 2015
- Alley cropping with perennial biomass crops. Rural Advantage 3rd Crop Walk-N-Talk: Third crops for energy, food production, and wind protection. Southern Research and Outreach Center, Waseca, MN. September 16, 2014
- Integrating herbaceous and woody perennials for production and conservation benefits on the farm. University of Minnesota Extension workshop: Using Perennial Plantings to Improve Water Quality. Knights of Columbus Hall, Fairmont, MN. June 24, 2014
- Riparian alley cropping for perennial biomass production in the Elm Creek Watershed. Elm Creek Agricultural Meeting sponsored by Rural Advantage and University of Minnesota. Knights of Columbus Hall, Fairmont, MN. March 11, 2014
- Herbaceous and woody perennials in agroforestry. Rural Advantage / UMN Extension Field Day Empire, MN, speaker / presenter. August 26, 2013
University of Minnesota
411 Borlaug Hall
1991 Upper Buford Circle
Saint Paul, MN 55108
Office Phone: 6126257224