Contrasting effects of nitrogen addition on rhizosphere soil CO2, N2O, and CH4 emissions of fine roots with different diameters from Pinus tabulaeformis forest using laboratory incubation.

Nitrogen (N) addition has variable effects on chemical composition, function, and turnover of roots with different diameters. However, it is unclear whether N addition has variable effects on greenhouse gas (GHG) emission in rhizosphere soil. We performed N addition (0-9 g N m-2 y-1) experiment in a Pinus tabulaeformis forest and a lab-incubation experiment to determine the effects of N addition on carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) emissions in rhizosphere soils of roots with different diameters (very fine roots: <0.5 mm, intermediate fine roots: 0.5-1.0 mm, largest fine roots: 1.0-2.0 mm). Nitrogen addition significantly promoted CO2 emission and CH4 uptake, with maximum values (CO2, 623.15 mg C kg soil-1; CH4, 1794.49 μg C kg soil-1) in the 6 or 9 g N m-2 y-1 treatments (P < 0.05). Nitrous oxide emissions were inhibited, with the greatest inhibitory effect in the 9 g N m-2 y-1 treatment (48.63 μg N kg soil-1). Total phosphorus (TP) content significantly decreased and increased in rhizosphere soil and non-rhizosphere soil after N addition, respectively, while organic carbon (OC), total N (TN), ammonium (NH4+), and nitrate (NO3-) contents in rhizosphere soil increased. A greater change in chemical properties occurred in rhizosphere soil of largest fine roots than very fine roots. Carbon dioxide and nitrous oxide emissions in rhizosphere soil among root sizes exhibited similar responses to N addition. While CH4 uptake was more responsive to N addition in rhizosphere soil with very fine roots than with largest fine roots. Basically, OC, TN, NO3-, and NH4+ were key soil components driving GHG emissions; NO3- promoted CH4 uptake and N2O emissions, NH4+ inhibited CO2 emissions. GHG response to N addition varied greatly, particularly in rhizosphere soil with different root sizes mainly related to its chemical properties.