Effects of throughfall exclusion and fertilization on soil co2 efflux and its components in a loblolly pine (pinus taeda) plantation

Abstract

Soil CO2 efflux (ES) and total below-ground carbon flux (TBCF) are two critical processes that determine the balance of soil carbon in forest ecosystems. Soil CO2 efflux is composed of root CO2 efflux (ER), heterotrophic CO2 efflux (EH) and ectomycorrhizal hyphae CO2 efflux (EM). However, little is known about how ES and its components and TBCF respond to decreasing soil moisture and changes in soil fertility. The experimental design was a 2 × 2 factorial combination of fertilization and precipitation replicated in four blocks. Heterotrophic CO2 efflux, ER and EM were separated using trenched and untrenched plots. Monthly we measured ES, EH, soil temperature and soil moisture along with leaf area index (LAI) and litterfall. Fertilization reduced ES and ER while the 30% throughfall reduction had no effect on EH, ER and EM. There were correlations between ES normalized to 15oC (E15) and soil moisture in all treatments except throughfall reduction. Heterotrophic CO2 efflux normalized at 15 oC (EH15) also increased with increasing soil moisture in control and fertilization treatments. Soil temperature and moisture and their interaction explained more variation in ES and EH than in ER and EM in the four treatments. In all treatments there was a positive relationship between EH and LAI. The average annual cumulative ES in the treatments was 544 g C m-2 yr-1 (fertilization), 645 g C m-2 yr-1 (fertilization and throughfall reduction), 665 g Cm-2 yr-1 (throughfall reduction) and 695 g C m-2 yr-1 (control). The contribution of EH to ES was from 50 to 58% and the contribution of EM to ES ranged from 10% to 12% in all treatments. Fertilization also had significant effects on annual ES, litterfall, and TBCF. Compared to the control, fertilization significantly decreased TBCF by 22%. Our results indicate that fertilization plays a critical role in carbon sequestration, increasing aboveground growth and decreasing ES and TBCF. The effect of reduced soil moisture on ES and its components was less than that of fertilization, and there were interactions between soil moisture, temperature and LAI affecting these fluxes.