Whole-tree dormant season nitrogen pools for different species receiving combinations of fertilization and irrigation after one short rotation

Our understanding of the accretion and distribution of tree tissue nitrogen (N) pools across a variety of species and genotypes suitable for short-rotation woody crop (SRWC) production in response to water and N availability remains limited. We measured dormant-season, rotation-length, whole-tree N pools for five tree genotypes from four species (two eastern cottonwood, Populus deltoides Bartr., genotypes; American sycamore, Platanus occidentalis L.; American sweetgum, Liquidambar styraciflua L.; and loblolly pine, Pinus taeda L.) receiving irrigation (I), fertilization (F), their combination (IF), or no resources manipulation (C). Our results demonstrate that foliar nitrogen concentration [N] responded to fertilization but was constrained within genotype-specific ranges and varied temporally. Tree genotypes differed in their composite and component tissue N content (NC), and these differences mostly reflected tissue mass (i.e., larger components and trees resulted in higher NC). Resource amendments (I, F, IF) resulted in up to 3.8-fold increases in NC compared with C, which were most pronounced for sycamore and sweetgum with F and IF treatments, respectively. By the end of the rotation, forest stands accumulated 73 to 452 kg N ha-1 in tree tissues with 40 - 78 % distributed aboveground and 22 - 60 % distributed belowground. A critical difference between genotypes was that all hardwoods exhibited larger belowground N distributions than the evergreen conifer. Our results stress the importance of belowground N pools and highlight differences among genotypes. Our study underscores valuable information about N pools across genotypes suitable for SRWC production, which can be leveraged to inform fertilization plans and devise sustainable nutrient management as production expands across marginal lands.