Abstract
Agroforestry systems that integrate useful long-lived trees have been recognized for their potential in mitigating the accumulation of atmospheric fossil fuel-derived carbon (C). Black walnut (Juglans nigra) is frequently planted and cultivated in North America for its valuable lumber and edible nuts, and is highly amenable to the integration of understory crops or livestock in agroforestry systems. However, little is known about C content in black walnut trees, including the amounts of C assimilated into lignocellulosic tissues within different tree compartments. Therefore, allometric equations for above- and below-ground compartments of 10-year-old black walnut trees across diverse locations were developed. Ten grafted black walnut trees from each of four sites across the midwestern USA were destructively harvested for above- and below-ground biomass, and dry biomass weight (DWw), C (Cw) and nitrogen (N; Nw) stocks were quantified. Soils surrounding the harvested trees were sampled and analyzed for soil organic C (SOC) and total N (TN). Total DWw ranged from 27 to 54 kg tree−1, with woody tissues containing an average of 467 g kg−1 C and 3.5 g kg−1 N. Woody tissues differed in Cw and Nw across location, and above-ground sections contained more C and less N compared with most root tissues. The slopes of the allometric equations did not differ significantly among locations, while intercepts did, indicating that trees only differed in initial size across locations. SOC and TN did not vary in distance from the trees, likely because the trees were not yet old enough to have impacted the surrounding soils. Our results establish a foundation for quantifying C and N stocks in newly established black walnut alley cropping systems across diverse environments.
Highlights
While numerous studies have estimated the carbon (C) accumulation potential for forests (e.g., Harmon 2001; Nepal et al 2012; Woodbury et al 2007), Nair et al (2009) proposed agroforestry as a strategy for impactful C assimilation; this is further supported by data from Cardinael et al (2018a, b)
The above- and belowground DWw varied among sites, with average aboveground DWw ranging from 10.2 kg tree-1 at FV to 36.0 kg tree-1 at NF, and below-ground DWw 9.6 kg tree-1 at MV and 18.0 kg tree-1 at BN
The dry weight (DW):fresh weight (FW) ratio (r) was higher for above-ground tissues compared with below-ground tissues
Summary
While numerous studies have estimated the carbon (C) accumulation potential for forests (e.g., Harmon 2001; Nepal et al 2012; Woodbury et al 2007), Nair et al (2009) proposed agroforestry as a strategy for impactful C assimilation; this is further supported by data from Cardinael et al (2018a, b). Agroforestry systems that integrate woody species with other crops or livestock can provide a high marginal rate of return for producers (Benjamin et al 2000), including the potential sale of C credits as such markets emerge (Udawatta and Jose 2012). Detailed studies are necessary to accurately quantify the amounts and rates of C assimilation in lignocellulosic biomass and in soils associated with various agroforestry species and practices to encourage adoption of such systems (Dollinger and Jose 2018; Udawatta and Jose 2012). Often released from agricultural systems as a result of excessive N fertilization or underutilization of N fertilizers by crops, has been
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