Greenhouse gas fluxes from riparian forest soil depend on the responses of microbes to nitrogen and phosphorus additions

Abstract

Anthropogenic activities have drastically increased nitrogen (N) and phosphorus (P) input to the biosphere, which could potentially alter soil biogeochemical cycles. However, the responses of soil greenhouse gases (GHGs) to the interactions of N and P enrichment are not fully understood, especially for riparian forests. In this study, we measured soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes and their potential drivers, including soil properties and soil microbes. The measurements were made across a whole season in a riparian forest, where an on-going N and P addition experiment was conducted. The experiment included low N (LN, 30 kg N ha−1 year−1), high N (HN, 150 kg N ha−1 year−1), low P (LP, 30 kg P2O5 ha−1 year−1), high P (HP, 150 kg P2O5 ha−1 year−1), low N and low P (LNP), high N and high P (HNP), and a control (CK) treatment. The results show that the riparian forest soil was a significant source of CO2 and N2O and a sink for CH4. The nutrient additions significantly reduced CO2 emissions (ranging from −52.9% under HNP to −44.2% under LP treatment) across the whole season. A significant reduction in CH4 uptake was found under both N and P addition treatments and also under the combination of N and P addition during the growing season. A significant increase in N2O emissions was found under HN treatment, but decrease was found under P addition treatments (LP, HP and LNP) during the growing season. The CO2, CH4, and N2O fluxes were significantly correlated with the seasonal patterns of fungi, Gram-negative (GN) bacteria, and net nitrification (NetN) rates, respectively. Generally, N and P addition significantly reduced the amount of fungi and GN bacteria concentration, respectively; HN addition significantly increased while LP and HP decreased the NetN rates during the growing season. Our results also indicated the interactions of N and P showed antagonistic effects on all GHGs. Our findings indicate that soil GHG fluxes in this riparian forest depend on the responses of microbes to N and P additions and highlight the need to investigate the interactions of N and P in predicting the responses of soil GHGs to global change.