Nitrogen dynamics in woody plant ecosystems: Almond orchards, winegrape vineyards, and pinyon-juniper woodlands

Nitrogen is the most commonly limiting nutrient in natural and agricultural ecosystems throughout the world. The long-term sustainability of these ecosystems, for food production as well as conservation, may rely on a better understanding of the dynamics of biologically fixed nitrogen within these systems. In biologically integrated almond orchards and winegrape vineyards this nitrogen comes from leguminous cover crops planted in the understory and row middles. In lysimeter studies of these systems, I measured release and uptake of cover crop-derived nitrogen by labeling cover crops with 15N and tracing it from decomposing cover crop residue into the soil and ultimately into the tree or vine. In both systems, uptake of cover crop-released nitrogen was detected in the leaves of the almond tree or grapevine within two weeks and remained high for the duration of the experiments. The recovery of cover crop-released nitrogen within the tree or vine differed between the systems. Cover crop-derived nitrogen was almost evenly divided between leaves, stems, and roots of the almond trees with slightly more in the leaves. Grapevine leaves and roots, on the other hand, accounted for substantially more cover crop-derived nitrogen than did the stems and shoots. Almond lysimeter soil accounted for as much of the cover crop-released N as the tree, while grapevine soil contained less than one-third as much of the cover crop-released N as was recovered in the grapevine. Pinyon juniper woodlands in Mesa Verde National Park, Colorado contain the nitrogen fixing shrub, bitterbrush (Purshia tridentata (Pursh) DC), in the understory. Nitrogen fixation was measured in this environment using 15N-dilution methods and 82--100% of bitterbrush leaf nitrogen was derived from fixation in a chronosequence of four burns. Bitterbrush litter decomposition in the chronosequence sites was estimated to provide 8--65 kg N ha-1 yr -1 to the sites, depending on bitterbrush density. Nitrogen mineralization was more related to drought-dependent precipitation patterns than to seasonal averages, with the greatest ammonification rates occurring during the driest spring. Pinyon, juniper, and bitterbrush leaf delta 15N patterns indicated a strong contribution of bitterbrush-fixed N to the ecosystem as a whole.