Plant effects on nitrogen retention in shortgrass steppe 2 years after 15N addition

Abstract

Ecosystems where plant-available nitrogen (N) is limited by constraints on decomposition may be quite capable of retaining additional N. However, there are many factors that will control the quantity of N retained, with potential implications for system carbon and nitrogen storage. We examined the retention and allocation of (15)N 2 years after labeling a semiarid, shortgrass steppe ecosystem in northeastern Colorado. The plant community of the study area is a patchy mixture of C3 (cool-season) and C4 (warm-season) graminoids; we hypothesized that differences in allocation patterns between the two plant types would lead to differing rates of N retention in this grazed system. We found that after three growing seasons (just over 2 years), an average of 28.3% of the original (15)N was retained in our plots, with nearly all of this N in soils (24.9%) rather than plants. Plots dominated by C3 plants had significantly less (15)N retained after 2 years than mixed C3-C4 plots. A high initial rate of retention by C3 plants, combined with a propensity for allocation to shoots rather than roots, likely led to this result in a system that typically has much of its aboveground tissue removed by grazers. In comparing our retention patterns to those of other studies in the shortgrass steppe, we found that the distribution of added (15)N to various ecosystem compartments (plants, mineral soil, soil organic matter) was similar across studies regardless of the experimental conditions, duration of study, and amount of (15)Nretained. We additionally observed the beginning of the formation of "resource islands," with (15)N being physically and biologically redistributed under plants from between plants.

Nitrogen uptake and turnover in riparian woody vegetation

“…The external plot experiment in Maine, USA and the Harvard forest chronic N addition experiments labeled their sites with treatments targeted at 344‰ and 1,000‰ respectively (Nadelhoffer et al 1995(Nadelhoffer et al , 1999. Indeed, most 15 N labeling experiments add tracers at concentrations greater than 2 atom% (Mugasha and Pluth 1994;George et al 1999;Epstein et al 2001). Studies labeled at this concentration of 15 N pay little heed to fractionations in δ 15 N, because the error introduced by fractionation is several orders of magnitude smaller than that caused by the experimental error.…”

The Nitrogen Cascade

“…Inputs of Nr in unpolluted regions are less than 1 g N per m 2 per year but can reach 5 g N per m 2 per year . The residence times of Nr in grasslands can be decades to centuries, because most biomass is subsurface and decomposition rates are slow (Blair et al 1998, Epstein et al 2001. Given the N-limited nature of most grasslands and the long residence times, there is a large potential for significant internal cycling (including redistribution by grazing animals) and for Nr accumulation.…”

Assessment of Nitrogen deposition effects and empirical critical loads of Nitrogen for ecoregions of the United States

“…To explore this, Epstein et al (2001) added 10 kg N ha -1 yr -1 of 15 N to three shortgrass steppe communities (C 3 , C 4 , and mixed communities) in northeastern Colorado, and sampled various pools over 3 years (background wet deposition not reported). Th ey found that, although the C 3 community initially retained more 15 N after the fi rst year (especially in plant material, Fig.…”

“…11.1), after the third year retention effi ciencies were fairly low (range: 20 to 35 percent). In addition, after 3 years, more 15 N was retained in the C 4 and mixed C 3 -C 4 communities primarily in the fi ne particulate organic matter fraction of the soil (Epstein et al 2001). Another study added 25 kg N ha -1 yr -1 to fi ve sites across the Great Plains (Table 11.1), and found after 3 years that 46 to 84 percent of the added N was retained primarily in the fi ne particulate organic matter of the top 20 cm of the soil, with increasing retention in soils with higher soil organic C and fi ner texture (Barret and Burke 2002).…”

“…Another study added 25 kg N ha -1 yr -1 to fi ve sites across the Great Plains (Table 11.1), and found after 3 years that 46 to 84 percent of the added N was retained primarily in the fi ne particulate organic matter of the top 20 cm of the soil, with increasing retention in soils with higher soil organic C and fi ner texture (Barret and Burke 2002). Th e lower retention effi ciencies found in Epstein et al (2001) compared with Barrett and Burke (2002; as well as others) was explained by higher losses to grazing in Epstein et al (2001), though it is unknown whether higher herbivory led to a redistribution of 15 N or actual losses (i.e., increased volatilization, denitrifi cation). Numerous tracer studies from forests corroborate the evidence that added N often resides in the soil organic or mineral layer rather than in plant tissue , Currie et al 2004, Nadelhoff er et al 2004.…”