Yield response of Lolium perenne swards to free air CO₂ enrichment increased over six years in a high N input system on fertile soil

Abstract

Summary After a step increase in the atmospheric partial pressure of CO2 (pCO2), the availability of mineral N may be insufficient to meet the plant's increased demand for N. Over time, however, the ecosystem may adapt to the new conditions, and a new equilibrium may be established in the fluxes of C and N. This would result in a higher dry mass (DM) yield response of the plants to elevated pCO2. The effect of elevated atmospheric pCO2 (60 Pa pCO2) was studied in Lolium perenne L. swards with two N fertilization treatments (14 and 56 g m−2 y−1) in a six‐year FACE (Free Air Carbon dioxide Enrichment) experiment. In the high N treatment, the input of N with fertilizer considerably exceeded the export of N with the harvested plant material in both CO2 treatments leading to an apparent net input of N into the ecosystem. Accordingly, the proportion of harvested N derived from 15N labelled fertilizer N, applied throughout the experiment (< 6 years), increased over the years. Under these high N conditions, the annual DM yield response of the Lolium perenne sward to elevated pCO2 increased (from 7% in 1993 to 25% in 1998). In parallel, the response of N yield to elevated pCO2 increased, and the initially negative effect of elevated pCO2 on specific leaf area (SLA) disappeared. The high N input system seemed to overcome in part an initially limiting effect of N on the yield response to elevated pCO2 within a few years. In contrast, there was no apparent net input of N into the ecosystem in the low N treatment, because N fertilization just compensated the export of N with the harvested plant material. Accordingly, the proportion of harvested N yield, derived from fertilizer N, which was applied throughout the experiment, remained low. At low N, the availability of mineral N strongly limited plant growth and yield production in both CO2 treatments; the low yields of DM and N, the low concentration of N in the plant material, and the low SLA reflected this. Although the plants grew under the same environmental conditions and the same management treatment as plants in the high N treatment, the response of DM yields to elevated pCO2 in the low N treatment remained weak throughout the experiment (5% in 1993 and 9% in 1998). The results are discussed in the context of the sizes of the different N pools in the soil, the allocation of N within the plant and the possible effects on temporal immobilization, and the availability of mineral N for yield production as affected by elevated pCO2 and N fertilization.

Nitrogen fertiliser rate affects the frequency of nitrate-dissimilating Pseudomonas spp. in the rhizosphere of Lolium perenne grown under elevated pCO2 (Swiss FACE)

“…In low N swards (14 g N m K2 y K1 ), nitrate was probably limiting for NDP in close proximity to roots because of plant uptake, and so competition would have arisen between NDP and Lolium plants for available N. Additionally, greater plant N demand (Daepp et al, 2000) or microbial immobilisation (Hungate et al, 1997) would have further reduced nitrate availability for NDP as indicated by lower available soil nitrate in the high N swards under elevated pCO 2 (Table 1).…”

Frequency and Diversity of Nitrate Reductase Genes among Nitrate-Dissimilating Pseudomonas in the Rhizosphere of Perennial Grasses Grown in Field Conditions

“…The C and T plots received 14 g m )2 y )1 N, NH 4 NO 3 , (at the beginning of the season, and then after each cut, except for the last). This amount is limiting for plant growth [14]. M. coerulea plants (noted M) originated from a littoral meadow located at the southern shore of Lake Neuchâtel (Cudrefin, Switzerland).…”

The impact of long-term elevated CO2 on C and N retention in stable SOM pools

“…These results suggest that PNL could have prevented soil C sequestration under elevated CO 2 in the Swiss FACE experiment. However, only the L. perenne system receiving low N fertilization rates and elevated CO 2 appeared limited by N availability (Daepp et al 2000). In the L. perenne systems receiving the high N fertilizer treatment, biomass production had increased by 28%.…”