Impact of fertilization on the abundance of nitrifiers and denitrifiers at the root–soil interface of plants with different uptake strategies for nitrogen

Abstract

Exploitative fast-growing plants have higher demands for nutrients compared to conservative slow-growing plants. We presume that these differences in nutrient uptake highly influence the microbial performance mainly in the rhizosphere of nutrient-poor soils. In order to investigate the influence of plants with contrasting exploitation types on microbial communities at the root–soil interface, we performed a greenhouse experiment in a N-poor, sandy soil using the fast-growing plant Dactylis glomerata and the conservative, slow-growing plant Festuca rubra. We applied four different amounts of the inorganic fertilizer ammonium nitrate (0, 50, 100, and 200 kg NH4NO3 ha−1). After 6 weeks, the abundance of nitrifiers and denitrifiers was investigated in the root–rhizosphere complex (RRC) based on the quantification of the marker genes amoA, nirK, nirS, and nosZ. Furthermore, soil chemical properties and the plant biomass were determined. Independent from the investigated plant species, fertilizer applications up to 100 kg ha−1 resulted in a clear depletion of ammonium and nitrate in the RRC, with ammonium and nitrate concentrations <1 mg kg−1 dry weight (dw). Only the highest fertilizer rate increased both ammonium and nitrate concentrations in the RRC of both plants reaching concentrations of 9.5 mg kg−1 dw for ammonium and 92.5 mg kg−1 dw for nitrate. The abundance of bacterial ammonia oxidizers (AOB) followed this trend (increase in abundance in response to the highest fertilizer rate), and copy numbers up to 3.2 × 107 copies g−1 dw were measured in the RRC of treatments with F. rubra where 200 kg N ha−1 was applied. As the archaeal ammonia oxidizers (AOA) did respond neither to plant species nor to the fertilizer application, the AOA/AOB ratio decreased from 10 in the non-fertilized treatments to 2 in treatments with 200 kg N ha−1. Also the abundance of microbes involved in denitrification strongly increased in response to higher fertilization rates in the RRC of both plant species, although higher gene copy numbers were detected in the rhizosphere of D. glomerata mainly for nitrous oxide reducers (up to 2.7 × 108 copies g−1 dw). Surprisingly, the highest fertilization rates resulted in a 50 % decrease in abundance of microbes involved in nitrite as well as nitrous oxide reduction.