Soil phosphorus functional fractions and tree tissue nutrient concentrations influenced by stand density in subtropical Chinese fir plantation forests

October Soil phosphorus functional fractions and tree tissue nutrient concentrations influenced by stand density in subtropical Chinese fir plantation forests Xiang-Min Fang 0 1 Xiu-Lan Zhang 0 1 Ying-Ying Zong 0 1 Yang Zhang 0 1 Song-Ze Wan 0 1 Wen- Sheng Bu 0 1 Fu-Sheng Chen 0 1 0 Jiangxi Provincial Key Laboratory of Silviculture, College of Forestry, Jiangxi Agricultural University , Nanchang , China , 2 Jiulianshan National Observation and Research Station of Chinese Forest Ecosystem, 2011 Collaborative Innovation Center of Jiangxi Typical Trees Cultivation and Utilization, Jiangxi Agricultural University , Nanchang , China 1 Editor: Jorge Paz-Ferreiro, RMIT University , AUSTRALIA Stand density regulation is an important measure of plantation forest management, and phosphorus (P) is often the limiting factor of tree productivity, especially in the subtropics and tropics. However, the stand density influence on ecosystem P cycling is unclear in Chinese fir (Cunninghamia lanceolata) plantations of subtropical China. We collected rhizosphere and bulk soils, leaves and twigs with different ages and roots with different orders to measure P and nitrogen (N) variables in Chinese fir plantations with low density (LDCF) and high density (HDCF) at Fujian and Hunan provinces of subtropical China. Rhizosphere soil labile P, slow P, occluded P and extractable P were higher in LDCF than HDCF at two sites. Meanwhile, P and N concentrations of 1-year-old leaves and twigs were higher in LDCF than HDCF and leaf N/P ratio generally increased with increasing leaf age at two sites. Rhizosphere vs. bulk soil labile P and occluded P were greater in LDCF than HDCF at Fujian. Nitrogen resorption efficiencies (NRE) of leaves and twigs were higher in LDCF than HDCF at Fujian, while their P resorption efficiencies (PRE) were not different between two densities at two sites. The average NRE of leaves (41.7%) and twigs (65.6%) were lower than the corresponding PRE (67.8% and 78.0%, respectively). Our results suggest that reducing stem density in Chinese fir plantations might be helpful to increase soil active P supplies and meet tree nutrient requirements. - Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Introduction Stand density regulation and its ecological effects represent important measures of forest management that have been widely studied [ 1,2 ]. Decrease in stand density usually reduces the competition for soil nutrients, such as phosphorus (P) and nitrogen (N), between trees and promotes the growth of remaining plants [3]. Meanwhile, the variation in light, water, temperature and forest microenvironments prompted by stand density directly affects plant growth [ 4 ]. Moreover, the nutrient status of a tree may be indirectly influenced by stand density, since the litterfall biomass [ 5 ], litter decomposition rate, photosynthate distribution and other ecological processes that influence soil nutrient supply vary with the stand density [ 6,7 ]. Currently, the majority of studies of stand density have focused on its effect on tree radial growth and understory plant diversity [ 8,9 ], but the precise role of stand density in soil nutrient supply and plant nutrient distribution remains to be determined. Soil P is one of the most important elements limiting plant growth, especially in subtropical and tropical regions [ 10,11 ]. In natural systems, soil P mainly originates from the slow weathering of soil minerals with low inputs to soil, coupled with the adsorption and chemical fixation on clay and other soil components, such as amorphous iron and aluminum, which results in a low P availability and increases the limitation of P on plant growth [ 12,13 ]. In soil solution, P exists in a variety of forms with different ecological functions, but only a small portion of the inorganic P can be directly absorbed by plants [ 14 ]. Therefore, accurate estimate of the concentration and change of P forms is critical for understanding the ecological functions of soil P [ 15 ]. The Hedley fractionation method recognizes plant-available forms and refractory forms of soil P and has been widely used in research into the P dynamics of natural and managed ecosystems [ 16,17,18 ]. Although the impact of stand density on soil P dynamics has previously been investigated [ 19 ], the response of soil P functional fractions associated with plant availability is not yet fully understood. The rhizosphere encompasses the millimeters of soil surrounding a plant root and soil microbes, invertebrates and root systems of competitors [ 20 ]. In the rhizosphere, the amounts of root exudates and soil microbes are strongly different from the bulk soil, mediating the biological interactions and ecological processes, which is known as the rhizosphere effect [ 20,21 ]. The rhizosphere contains more micro-organisms feeded by the organic substrates from the root. In turn, th (...truncated)