Effects of elevated CO₂, temperature and N fertilization on nitrogen fluxes in a temperate grassland ecosystem

Abstract

Summary The soil nitrogen cycle was investigated in a pre‐established Lolium perenne sward on a loamy soil and exposed to ambient and elevated atmospheric CO2 concentrations (350 and 700 μL L−1) and, at elevated [CO2], to a 3 °C temperature increase. At two levels of mineral nitrogen supply, N– (150 kgN ha−1 y−1) and N+ (533 kgN ha−1 y−1), 15N‐labelled ammonium nitrate was supplied in split applications over a 2.5‐y period. The recovery of the labelled fertilizer N was measured in the harvests, in the stubble and roots, in the macro‐organic matter fractions above 200 μm in size (MOM) and in the aggregated organic matter below 200 μM (AOM). Elevated [CO2] reduced the total amount of N harvested in the clipped parts of the sward. The harvested N derived from soil was reduced to a greater extent than that derived from fertilizer. At both N supplies, elevated [CO2] modified the allocation of the fertilizer N in the sward, in favour of the stubble and roots and significantly increased the recovery of fertilizer N in the soil macro‐organic matter fractions. The increase of fertilizer N immobilization in the MOM was associated with a decline of fertilizer N uptake by the grass sward, which supported the hypothesis of a negative feedback of elevated [CO2] on the sward N yield and uptake. Similar and even more pronounced effects were observed for the native N mineralized in the soil. At N–, a greater part of the fertilizer N organized in the root phytomass resulted in an underestimation of N immobilized in dead roots and, in turn, an underestimation of N immobilization in the MOM. The 3 °C temperature increase alleviated the [CO2] effect throughout much of the N cycle, increasing soil N mineralization, N derived from soil in the harvests, and the partitioning of the assimilated fertilizer N to shoots. In conclusion, at ambient temperature, the N cycle was slowed down under elevated [CO2], which restricted the increase in the aboveground production of the grass sward, and apparently contributed to the sequestration of carbon belowground. In contrast, a temperature increase under elevated [CO2] stimulated the soil nitrogen cycle, improved the N nutrition of the sward and restricted the magnitude of the soil C sequestration.

Can we understand and predict the regulation of biological N2 fixation in grassland ecosystems?

“…With grass monocultures, elevated CO 2 induced parallel declines in the shoot N concentration, in the NNI, in the N yield and in NO 3 -leaching during winter Zanetti et al 1997). The N deprivation of the grass induced by CO 2 elevation occurred as a result of an increased N immobilisation by the litter and by the soil (Loiseau and Soussana 2000) during the first years after the start of the CO 2 enrichment. Over the 10 years of fumigation in the Swiss FACE, the most important change was the increasing response to elevated [CO 2 ] of the annual N yield (from -13 to ?29%) in the high but not in the low N treatment (Schneider et al 2004).…”

Effects of warming and nitrogen deposition on the coupling mechanism between soil nitrogen and phosphorus in Songnen Meadow Steppe, northeastern China

“…Currently, whether N deposition affects the net N mineralization rate is a topic of intense debate. Some believe that the net N mineralization rate accelerates with increasing N deposition (Loiseau and Soussana, 2000;Li et al, 2006;. The addition of N enhances the amount of available N in soil, and the amount of available N increases as more N is added, as the duration the addition increases, and as the rate of net mineralization increases (Robinson et al, 2004).…”

The role of plants and land management in sequestering soil carbon in temperate arable and grassland ecosystems

“…One of the unusual features of organic-C is that it provides a source of both nutrients and energy, since organic-C is required for both tissue construction and energy generation by organisms. Increased C supply in the rhizosphere can slowdown N cycling and produce a feed back on plant growth which has been termed the soil hypothesis (Loiseau and Soussana, 2000). The soil N supply can both limit C sequestration and interact with CO 2 to influence mineralisation and nitrous oxide release.…”