Inorganic Nutrient Inputs from Precipitation, Throughfall, and Stemflow in Pinus densiflora and Quercus mongolica Stands in an Urban Forest Ecosystem

We measured the amount of precipitation, stemflow, and throughfall and concentrations of nine major inorganic nutrients ($H^+$, ${NH_4}^+$, $Ca^{2+}$, $Mg^{2+}$, $K^+$, $Na^+$, $Cl^-$, ${NO_3}^-$, and ${SO_4}^{2-}$) to investigate the nutrient inputs into soil from precipitation in Pinus densiflora and Quercus mongolica stands from September 2015 to August 2016. The precipitation inputs of $H^+$, ${NH_4}^+$, $Ca^{2+}$, $Mg^{2+}$, $K^+$, $Na^+$, $Cl^-$, ${NO_3}^-$, and ${SO_4}^{2-}$ into soil were 0.170, 15.124, 42.227, 19.218, 14.050, 15.887, 22.391, 5.431, and $129.440kg{\cdot}ha^{-1}{\cdot}yr^{-1}$, respectively. The P. densiflora stemflow inputs were 0.008, 0.784, 1.652, 1.044, 0.476, 0.651, 1.509, 0.278, and $9.098kg{\cdot}ha^{-1}{\cdot}yr^{-1}$, and those for Q. mongolica were 0.008, 0.684, 2.429, 2.417, 2.941, 1.398, 2.407, 0.436, and $13.504kg{\cdot}ha^{-1}{\cdot}yr^{-1}$, respectively. The P. densiflora throughfall inputs were 0.042, 21.518, 52.207 27.694, 20.060, 24.049, 37.229, 10.241, and $153.790kg{\cdot}ha^{-1}{\cdot}yr^{-1}$, and those for Q. mongolica were 0.032, 15.068, 42.834, 21.219, 20.294, 20.237, 24.288, 5.647, and $119.134kg{\cdot}ha^{-1}{\cdot}yr^{-1}$, respectively. Of the total throughfall flux (i.e., stemflow + throughfall flux) of the nine ions for the two species, ${SO_4}^{2-}$ had the greatest total throughfall flux and $H^+$ had the lowest. The net throughfall fluxes of the ions for the two species had various correlations with the precedent dry period, rainfall intensity, rainfall amount, and pH of precipitation. The soil pH under the Q. mongolica canopy (4.88) was higher than that under the P. densiflora canopy (4.34). The difference in the soil pH between the two stands was significant (P < 0.01), but the difference in soil pH by the distance from the stems of the two species was not (P > 0.01). This study shows the enrichments of inorganic nutrients by two representative urban forests in temperate regions and the roles of urban forests during rainfall events in a year.