Influence of Nitrogen in Compound Fertilizer on Soil CO2 Efflux Rates in Pinus densiflora S. et Z. Stands

Compound fertilizer is generally applied to alleviate multi-nutrient deficiency problems in forest stands, but research on the effect of fertilizer application on soil CO2 efflux (Rs) processes has focused on the role of single-nitrogen (N) application. This study evaluates the effects of N addition in compound fertilizer on the rates in Pinus densiflora S. et Z. (Korean red pine) stands. Compound fertilizer with N (N3P4K1 = 113:150:37 kg ha−1 yr−1) and without N (P4K1 = 150:37 kg ha−1 yr−1) was applied on the forest floor for three years. Rs rates were measured for four years, from April 2011 to March 2015. The mean annual Rs rates during the study period were 3.10 µmol m−2 s−1 in the N3P4K1, 3.08 µmol m−2 s−1 in the P4K1, and 3.08 µmol m−2 s−1 in the control treatment. The rates in all treatments were significantly lower in 2013 (2.73 µmol m−2 s−1) than in other sampling years (3.03–3.58 µmol m−2 s−1) when the mean soil water content was the lowest (15.7%) during the four sampling years (other sampling years: 23.0–24.1%). The exponential relationships between Rs and the soil temperature were slightly more significant in the fertilized (N3P4K1: R2 = 0.72–0.80; P4K1: R2 = 0.70–0.81) treatments compared to the control (R2 = 0.62–0.74) treatment. The mean Q10 values for the four years were similar between the N3P4K1 treatment (4.19), the control (4.23) treatment, and the P4K1 (4.24) treatment. The results demonstrate that mean annual Rs rates in Korean red pine stands were not affected by the increased N availability in compound fertilizer, whereas decreases in mean annual Rs rates may be strongly attributed to the soil water content.