Nutrient Management Research Database

Plant diversity effects on ecosystem functioning can potentially interact with global climate by altering fluxes of the radiatively active trace gases nitrous oxide (N₂O) and methane (CH₄). We studied the effects of grassland species richness (1–16) in combination with application of fertilizer (nitrogen:phosphorus:potassium = 100:43.6:83 kg ha⁻¹ a⁻¹) on N₂O and CH₄ fluxes in a long-term field experiment. Soil N₂O emissions, measured over 2 years using static chambers, decreased with species richness unless fertilizer was added. N₂O emissions increased with fertilization and the fraction of legumes in plant communities. Soil CH₄ uptake, a process driven by methanotrophic bacteria, decreased with plant species numbers, irrespective of fertilization. Using structural equation models, we related trace gas fluxes to soil moisture, soil inorganic N concentrations, nitrifying and denitrifying enzyme activity, and the abundance of ammonia oxidizers, nitrite oxidizers, and denitrifiers (quantified by real-time PCR of gene fragments amplified from microbial DNA in soil). These analyses indicated that plant species richness increased soil moisture, which in turn increased N cycling-related activities. Enhanced N cycling increased N₂O emission and soil CH₄ uptake, with the latter possibly caused by removal of inhibitory ammonium by nitrification. The moisture-related indirect effects were surpassed by direct, moisture-independent effects opposite in direction. Microbial gene abundances responded positively to fertilizer but not to plant species richness. The response patterns we found were statistically robust and highlight the potential of plant biodiversity to interact with climatic change through mechanisms unrelated to carbon storage and associated carbon dioxide removal.

• This field study investigated the relationship between the diversity of plants present in a grassland and nitrous oxide and methane emissions, two potent greenhouse gases.
• Grassland plots with either 1, 2, 4, 8, or 16 different species were monitored from N2O and CH4 emissions, along with soil moisture and soil nitrogen levels. Half of the plots were fertilized with N, P, and K, while half were left unfertilized.
• The total number of nitrifying and denitrifying microbes was also measured.

• N2O emissions showed two distinct patterns with or without the presence of legumes. When legumes were present, there was no effect of plant diversity on N2O emissions, while fertilization resulted in a small increase in emissions. When legumes were  not present, fertilization had no effect on N2O emissions, while plant diversity increases suppressed N2O emissions.
• CH4 emissions were limited by increases in plant diversity.
• Plant diversity had little direct effects on the size of the nitrifying and denitrifying microbes.